Molecular Grafting of Fluorinated and Nonfluorinated Alkylsiloxanes

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Molecular grafting of fluorinated and non-fluorinated alkylsiloxanes on various ceramic membrane surfaces for the removal of VOCs applying vacuum membrane distillation Joanna Kujawa, Samer Al-Gharabli, Wojciech Kujawski, and Katarzyna Knozowska ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14835 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Molecular grafting of fluorinated and nonfluorinated alkylsiloxanes on various ceramic membrane surfaces for the removal of VOCs applying vacuum membrane distillation Joanna Kujawa1), Samer Al-Gharabli2), Wojciech Kujawski1)*, Katarzyna Knozowska1) 1)

Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7 Gagarina St., 87-100 Torun,

Poland 2)

Pharmaceutical and Chemical Engineering Department, German-Jordanian University, Amman

11180, Jordan KEYWORDS: ceramic functionalization, vacuum membrane distillation, ceramic membranes, hydrophobicity, volatile organic compounds ABSTRACT Four main tasks were presented i) ceramic membrane functionalization (TiO2 5kD and 300 kD), ii) extended material characterization (physicochemistry and tribology) of pristine and modified ceramic samples, iii) evaluation of chemical and mechanical stability and finally iv) assessment of membrane efficiency in VMD applied for VOCs removal from water. Highly efficient molecular grafting with four types of perfluoroalkylsilanes and one nonfluorinated agent was developed. Materials with controllable tribological and physicochemical

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 55

properties were achieved. The most meaningful finding is associated with the applicability of fluorinated and non-fluorinated grafting agents. The results of CA, HCA, SA, γcr as well as E, H and Fadh for grafting by these two modifiers are comparable. This provides insight into the potential applicability of environmental friendly hydrophobic and superhydrophobic surfaces. The achieved hydrophobic membranes were very effective in the removal of VOCs (butanol, methyl-tert-butyl ether and ethyl acetate) from binary aqueous solutions in vacuum membrane distillation. The correlation between membrane effectiveness and separated solvent polarity was compared in terms of material properties and resistance to the wetting (kinetics of wetting and in-depth liquid penetration). Material properties were interpreted considering Zisman theory and using Kao diagram. The significant influence of surface chemistry on the membrane performance was noticed (5 kD – influence of hydrophobic nanolayer and separation controlled by solution-diffusion model; 300 kD – no impact of surface chemistry and separation controlled by liquid-vapor equilibrium).

1. Introduction The surface and interfaces functionalization process is widely used in technical, engineering, or medical applications.1,

2

However, an improvement of advanced chemical methods for the

design and generation of surfaces with precise and controllable properties, is still challenging. To develop well controlled surfaces, the chemical modification methods should be supported by surface structuring.3,

4

The advantage of application of both types of surface modification

methods (i.e. physical and chemical) is to create and change the surface in a designed way, and this cannot be accomplished solely by structuring of the surface nor only by chemical functionalization.3


2

Page 3 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nowadays, the modification of ceramic materials e.g. ceramic powders, ceramic membranes, or thin layers is extensively investigated.2, 5 The observed tendency is related to the possibility of extent application of modified ceramic surfaces in comparison with polymeric materials. Ceramic materials possess much better thermal stability, non-swelling behavior, mechanical resistance, chemical inertness, and the possibility of an uncomplicated cleaning.6 Pristine ceramic materials are hydrophilic, which creates limitations in wider application of these materials. The hydrophobic or superhydrophobic layers created on the surface are usually formed via condensation reaction between hydroxyl groups present on the native ceramic and functional groups on the grafting agent (e.g. methoxy, ethoxy or chlorine atoms).2, 5, 7, 8 Hosseinabadi et al. 2 proposed a new generation of ceramics which are tuned using Grignard reactions with various alkyl modifiers and showing their potential application in organic solvent nanofiltration. Tsuru et al. comprehensively examined membranes based on silica–zirconia (Si/Zr molar ratio 9/1), in aqueous 9 and non-aqueous media 10. Authors prepared hydrophobic membrane by attachment of the silanol groups into the pores by a gas-phase reaction with trimethylchlorosilane at high temperature of 200oC.9, 10 Subsequently, these membranes were applied in nanofiltration process. The other research group also worked on the functionalization process at higher temperature (70−150

°C).1

They

modified

ceramic

surface

of

SiO2

by

1H,1H,2H,2H-

perfluoroctyltriethoxysilane and its non-fluorinated analogue creating alkylsilane−SiO2 hybrids.1 These types of grafting agents are extensively used for a very efficient hydrophobization of ceramics, metal oxide powders and ceramic membranes.

7, 8, 11

Kujawski et al.

7, 12

used

perfluoroalkylsilanes molecules (1H,1H,2H,2H-perfluorooctyltriethoxysilane and 1H,1H,2H,2Hperfluorodecyltriethoxysilane) for the ceramic membranes modification. These membranes were subsequently utilized in membrane distillation

12

and pervaporation


7

processes. 1H,1H,2H,2H-

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 55

perfluorotetradecyltriethoxysilane and 1H,1H,2H,2H-perfluorooctyltriethoxysilane were applied by Kujawa et al. for the functionalization of various ceramic membranes tested in volatile organic compounds (VOC) removal by vacuum pervaporation process

8

and water desalination

by membrane distillation processes.13, 14 The fabrication of efficient surfaces for VOCs removal is an important issue from environmental point of view. Nowadays, the application of organic solvents in industrial and technological processes is very common. However, the production of massive quantities of wastewaters is a destructive side-effect of this commercial activity. Wastewaters can be a source of health problems to humans as well as serious environmental issues.15 Taking into consideration various types of industry, it can be noticed that VOCs are extensively used in plastics and printing industry,

16

adhesives

17

and refrigerants

18

production and petroleum

processing.19 Determination of VOCs presence and their removal from environmental aqueous samples is essential due to their antagonistic effects on human health. According to World Health Organization (WHO), VOCs possessing boiling point in range from to 50–100oC can be classified as very volatile organic compounds (VVOCs).20 Therefore, butanol (Tb = 117.5oC), ethyl acetate (Tb = 77.1oC) and methyl-tert-butyl ether (Tb = 55.2oC) are classified in this group. These compounds are used at industrial scale, although they are classified hazardous ones.20 In order to recover or remove these chemicals from wastewaters, the conventional methods as distillation are usually used, regardless formation of azeotropes or high energy demand.7, 21, 22 The well-known methods utilized for VOCs removal, e.g. anaerobic/aerobic biological treatment, advance oxidation, distillation, adsorption, bioreactors or air stripping, possess limitations and shortcomings.21-23 These limitations are related to the costs, time,24 concentration of VOCs,25 and


4

Page 5 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


complexity of the examined system (possibility of the creation of more harmful products comparing to the original ones). Interesting solution to overcome these limitations is the utilization of membrane processes,18, 26 especially vacuum separation techniques (e.g. vacuum membrane distillation - VMD and vacuum pervaporation - VPV).7,

8, 21

VMD process is predominantly utilized for desalination,

removal of organics from water and for solvent concentration.13 VPV process is applied for separation of binary or multicomponent liquid mixtures.27 Both processes are in part similar, however main difference is related to the type of applied membrane and subsequently to the separation mechanism. In the case of VMD hydrophobic porous membranes are used, nevertheless the mechanism of separation is controlled by the liquid-vapor equilibrium. 28 In the case of VPV separation occurs due to a partial vaporization of components across a dense nonporous hydrophobic membrane and the mechanism is based on the solution-diffusion model.29 In both types of process, the utilization of hydrophobic membranes is required. For that reason, the functionalization and creation of stable hydrophobic ceramic membranes are extremely important. In our previous works, we highlighted that hydrophobization process by perfluoroalkylsilanes has a crucial influence on membranes properties

13, 29

as well as on

membrane efficiency of separation, e.g. VMD,8 air-gap membrane distillation,14 and direct contact membrane distillation.13 However, use of only two types of perfluoroalkylsilanes (1H,1H,2H,2H-perfluorooctyltriethoxysilane

and

1H,1H,2H,2H-perfluorotetradecyl-

triethoxysilane) was reported in the literature.13, 14 The aim of the presented work, is to functionalize the ceramic membranes by four perfluoroalkylsilanes varied in length of fluorinated part of the alkyl chain and possessing from 6 to 12 carbon atoms. Moreover, an additional non-fluorinated modifier, possessing 6 carbon


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 55

atoms, was used. The application of non-fluorinated molecules is important from the environmental point of view due to the need of elimination or reduction of the harmful fluorine compounds. The functionalization process of ceramics was followed by material and tribological characterization of obtained surfaces. A lot of effort was needed to find the correlation between type of grafting agent and quality of ceramics. Subsequently, the material and tribological properties were correlated to the membrane efficiency in VOCs removal. Such detailed characterization is required in designing ceramic materials with controlled properties.11, 30 This is the

first

example

where

extended

material

characterization

(including

chemistry,

physicochemistry and tribology) is correlated with effectiveness of the functionalized ceramic membranes in VOCs) removal from water by vacuum membrane distillation process. Furthermore, the chemical and mechanical stabilities of the membranes were evaluated. 2. Results and discussion 2.1. Non modified ceramic membranes Non modified, hydrophilic ceramic membranes planar and tubular were tested using various analytical methods. Planar membranes, both 5kD and 300kD, were tested using a contact goniometric technique, evaluating the alteration of water drop behavior on the ceramic surface. It was observed that wetting of the membrane depends directly on its morphology. 5kD membranes were characterized by longer time of water penetration inside the membrane due to denser and less porous structure. The porosity for 5kD titania membrane is in the range of 30%.30 On the other hand, the 300kD membrane possessing less dense and more porous structure (porosity ca. 40% 30) was penetrated by water molecules almost twice faster Knowing the physicochemistry of the soaking process, the kinetics of this phenomenon can be defined. Specifically, the evolution of water penetration into the porous membrane structure can be described by a zero-order


6

Page 7 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kinetics. It is worth mentioning that zero-order reaction is independent of the concentration of reactant. Calculated rate constant (k0) for water penetration was equal to 0.325 and 0.590 mol dm-3 s−1 for 5 kD and 300 kD ceramic membranes (Table 1), accordingly. Furthermore, considering the static contact angle measurements, it was noticed that CA value is independent of the membrane structure and for both types of non-grafted membranes it was equal to 40o ± 2o. Based on this information it can be concluded that no differences in the surface free energy will also be observed. However, due to the observed higher roughness of the 300 kD surface, slightly different values of tribological parameters like nanohardness (H), Young modulus (E) and adhesion force (Fadh) are noticed. The wettability behavior is one of the most significant parameters in the material characterization protocol. In the presented work, the membrane wettability by water was evaluated defining the profile of liquid penetration into the ceramic structure. Water molecules wet a ceramic sample and subsequently penetrate into the three dimensional mesh structure of the ceramics (Figure S1 – SEM photo of 5kD and 300kD). Equation 1 was used for the calculation of liquid depth penetration.31 The measurements were based on the dynamic contact angle measurements (Figure 1). χ =

d p γ LV t cos( CA ) 4η

(1)

[m ]

where: χ – depth of liquid penetration [m]; dp – pore size [m]; γLV – liquid vapor surface tension [mN m-1]; CA – contact angle [deg]; t – time [s]; η – solvent viscosity [Pa s]. In Figure 1 the relation between apparent water contact angle and depth of liquid penetration into ceramic structure of the membrane is presented. It was shown that in the case of 5 kD hydrophilic membrane, deposited water can penetrate through the whole membrane structure.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 55

Namely, after 120 seconds water come through 82 % of membrane thickness (total membrane thickness = 2.6 mm) and the contact angle reached value equal to 4o. This phenomenon can be linked to the structure of the membrane and presence of capillary forces during water transport across the dense structure of the membrane. However, in the case of 300 kD membrane, water is spread in a more open structure of ceramic (Figure 1). Considering transport of water through entire 300 kD membrane thickness, only 40 seconds were needed. The determination of wettability range possesses an important and practical meaning, e.g. in the selection of appropriate solvents for membrane cleaning procedure.20, 23, 32 One of the most accurate parameter for water resistance determination is critical surface tension (γcr).33, 34 In the presented research two various methods for critical surface tension calculation were implemented i.e., Zisman

33, 34

and Owens-Wendt’s

35

ones. The differences of the both

approaches are related to the type of surface that is characterized. Zisman’s method is recommended for non-polar surfaces.33-37 On the other hand, Owens-Wendt’s method 35 is based on a two component model (including dispersive and polar components).

35, 38

The polar

component describes dipole-dipole, dipole-induced dipole, hydrogen as well as other specific interactions (e.g. Debye and acid-base forces).1,

35, 37, 38

Those molecules possessed stable

inequity in the electron density because of moderated electronegativities of the bonding partners, having concurrently asymmetrical geometry (e.g. water). Molecules owning a dipole moment can create polar interactions between each other. In the case of more hydrophobic surfaces lower contribution of polar interaction will be observed.35,

37, 38

On the other hand, the dispersive

component is related to dispersive interactions, i.e. the temporary changes of the electron density not permanently localized on the molecule.1 The result is to produce temporary dipoles which further induce momentary dipoles in nearby molecules. Moreover, it should be pointed out that


8

Page 9 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dispersive interactions (London forces) are really weak.35,

37, 38

Within this work, the Owens-

Wendt’s approach gave much better results. The determined values of critical surface tensions (γcr) are equal to 29.2 ± 1.3 mN m-1 and 36.1 ± 1.5 mN m-1 m-1 for 5kD and 300kD samples, respectively (Table 1). The Owens-Wendt’s method for γcr determination is also applied for the calculation of total surface energy (SFE). For pristine membranes, values of surface free energy were equal to 140 ± 6 mN m-1, independently on the membrane morphology (Table 1). The pore size and the pore size distribution are essential parameters describing ceramic materials.39,

40

These parameters were calculated based on the adsorption-desorption nitrogen

isotherms and BJH model.40-42 The value of pore size (dp) for 5 kD membrane was in the range of 2 – 4 nm, whereas for 300 kD, dp parameter was equal to 200 nm (Table 1). In Figure 2, the adsorption-desorption nitrogen isotherms for chosen samples are presented. It can be seen that the functionalization by both non-fluorinated (Figure 2 B) and fluorinated grafting agents (Figure 2 C-F) changed the shape of isotherms. In the case of pristine sample and membranes functionalized by hydrophobic molecules possessing shorter alkyl chains, non-fluorinated (C6) as well as fluorinated (FC6 and FC8) ones, similar shapes of isotherm can be noticed. The observed shapes of the isotherms are characteristic for porous material with cylindrical pores having different cross-sections.43 Nevertheless, the modification with more hydrophobic compounds (FC10 and FC12) has different effect on the porous structure of ceramic membrane. The observed isotherms in these two cases (Figure 2 E and F) are typical for materials with spherical pores possessing additionally plenty of narrowings.40-42 Moreover, the observed hysteresis loop decreased when the more hydrophobic molecules with longer alkyl chains were used for quantifying. Comparing samples grafted by fluorinated and non-fluorinated molecules with the same length of alkyl chains, bigger hysteresis loop is observed for C6 than FC6. The


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 55

abovementioned alterations are caused by changes in membrane morphology, especially in the pores tortuosity. Moreover, the observed behavior can be explained by a reduction of C parameter (C is an equilibrium constant of adsorption) –Equation S2, determined from BET model.39,

41

The following features: surface chemistry, pore size diameter, adsorption capacity

and surface porosity have an important influence on the C factor.39-41 Smaller value of C factor is related to the lower capacity of adsorption on the surface.40, 41 Moreover, the differences in C parameter for modified samples are referred to the different interactions with the ceramic membranes associated with the dissimilar hydrophobicity level. The same phenomenon was observed for 5kD membranes. The C values for pristine 5 kD membrane was equal to 23.8. For modified samples smaller values were achieved. C factor for 5 kD membranes modified by fluorinated grafting agents FC6, FC8, FC10, FC12 and C6 were equal to 18, 17.5, 17 and 16.7, respectively. On the other hand, the value of C factor for functionalized samples with nonfluorinated silane was equal to 20.7. The established higher value for membrane grafted by nonfluorinates compounds is related to the presence of different interactions with the surface than those observed in the case of fluorinated compounds. Additionally, the presence of micropores was assessed by the t-plot method.41 For this purpose, according to the Harkins-Jura model, master isotherm relationship described by Lecloux–Pirrad was applied.39, 41 Based on that, the thickness (t) of adsorbed film was calculated.40, 41, 43 The aforementioned relation between thickness (t) of adsorbed film and relative pressure is shown in Figure 3. It can be seen that for 5 kD samples the t values are comparable for all membranes (Figure 3 A), independently on the surface character and surface chemistry (type of grafting agents). The lack of visible differences was observed in the most important part – BET region (in the range of the relative pressure between 0.05 and 0.35). Different observations can be made for


10

Page 11 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 kD membranes (Figure 3 B). Depending on the grafting compounds the thickness (t) of adsorbed film varies. With increase of the hydrophobicity level of silane molecules applied as modifiers (length of fluoro-carbon chains), the reduction of adsorbed t layer thickness is observed. On the other hand, the chemistry of grafting agent was irrelevant. The comparable data are achieved for fluorinated (FC6) and non-fluorinated compound (C6) possessing the same amount of carbon atoms in the alkyl chains. The results correlate well with the observed changes in membrane morphology, i.e. the decrease of pore size and formation of the less developed three dimensional mesh structure. These alterations in membrane morphology can be furthermore proved by the differences in the isotherm shapes (Figure 2). In the case of tubular pristine ceramic membranes, the hydraulic permeability of water was determined to assess the transport membrane properties. The calculations were performed based on the Equation 5. The pressure gradually increased and the permeate flux was measured. For both 5 kD and 300kD membranes, the linear relation between pressure and water flux was noticed with the correlation coefficient (R2) close to unity. The achieved values of hydrodynamic permeability coefficient (Lp) were equal to 0.23 x 103 kg h-1 m-2 bar-1 and 2.27 x 103 kg h-1 m-2 bar-1 for 5kD and 300kD, respectively. The observed differences are associated with the differences in membrane morphology (pore size, porosity, tortuosity). The determined higher value of Lp for 300 kD membrane will effect in the better transport through this membrane in comparison with 5 kD one. 2.1. Properties of functionalized membranes 2.1.1. Ceramic membranes with planar geometry The effectiveness of grafting of planar ceramic membranes was assessed by various techniques and methods. Contact angle (CA), sliding angle (SA) and hysteresis of contact angle (HCA) were


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 55

measured applying goniometric contact method. Moreover, the surface free energy (SFE) and its components (polar and dispersive), roughness parameters (RMS) and critical surface tension (γcr) were determined. The influence of ceramic membrane modification by four fluorinated (FC6, FC8, FC10 and FC12) and one non-fluorinated (C6) compounds on the physicochemical and tribological properties was evaluated. As an effect of ceramic membrane hydrophobization, the changes in membrane morphology as well as in material properties were noticed. The originally hydrophilic ceramic membrane with CA = 40o (Table 1) was turn into a highly hydrophobic one as a result of grafting process (Figure 4). All hydrophobized membrane samples possessed the value of contact angle higher than 120o. The aforementioned effect of grafting process was also observed

by

several

research

groups.

44-46

Yu

et

al.

44

used

1H,1H,2H,2H-

perfluorooctyltriethoxysilane for the formation of superhydrophobic (CA = 153o) ceramic membrane contactor. It was shown that grafted ceramic material possessed much better antifouling ability due to the high level of hydrophobicity and self-cleaning properties. The superhydrophobic hollow fiber membrane contactor shows potential in real industrial CO2 postcombustion capture due to the unique features - good anti-fouling and anti-wetting properties.44 1H,1H,2H,2H-perflourodecyltriethoxysilane was applied for functionalization of various substrates, e.g. zeolites – SAPO34

45

and wood.46 Junaidi et al.

45

worked on the formation of

water-resistant mix-matrix membranes (MMM) for the separation of wet gases such as biogas. The

membranes

were

created

by

incorporating

fluorocarbon

(1H,1H,2H,2H-

perflourodecyltriethoxysilane) functionalized SAPO-34 zeolite into polysulfone matrix. The purpose of grafting process was to avoid moisture adsorption on and/or inside the structure of zeolite. As a result of zeolite modification the contact angle value increased from 33o to 130o. Moreover, good adhesion between hybrid organic-inorganic filler and polymer phases was


12

Page 13 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed for MMM with 10 wt. % of modified SAPO-34 zeolite. Furthermore, the positive effect on the structure by the reduction of defect i.e. interface voids after hydrophobization was noticed.45 Generally, in dry mixed-gas separation, the reduction of more than 90% of separation performance was observed for reference polysulfone membrane with untreated SAPO-34 zeolite in comparison to MMM incorporated with 10 wt.% of hydrophobized SAPO-34.45 Gao and coworkers 46 modified wooden substrates by two-step method consisting of hydrothermal synthesis with TiO2 precursor following modification by 1H,1H,2H,2H-perflourodecyltriethoxysilane. Modified wood sample was characterized by a contact angle value of 153o, excellent water repellency as well as anti-acid resistivity. Moreover, the highly hydrophobic character was maintained (CA > 150o) after exposure to the UV light for 24 h, 10 h boiling at 150 °C and after one week of continuous contact with 0.1 M HCl solution. The developed method of wood hydrophobization was proposed for the protection of monumental structures.46 According to the data presented in Figure 4, in can be stated that type of modifier has an important influence on the resultant hydrophobicity level. Considering only the ceramic samples modified by the set of four fluorinated compounds, an interesting relation was noticed. The hydrophobicity increase in the following order CAFC10, CAFC8, CAFC6 and CAFC12 in the case of 5 kD membrane. The value of the contact angle was in the range of 123o – 133o (Figure 4). In the case of 300 kD membrane the smallest value of water contact angle was found for a membrane modified by FC8 (CA = 133o ± 2o), and slightly higher CA was observed for surface grafted by FC10 (CA = 135o ± 2o). However, similarly to the 5 kD membrane the most hydrophobic was the membrane modified with FC12 molecules (CA = 147o ± 2o) (Figure 4). The 300 kD membrane hydrophobized by FC12 can be classified as the superhydrophobic surface. 47, 48 In both cases of 5 and 300 kD samples, the changes of hydrophobic character were not related to the extension of


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 55

length of fluoro-carbon chains of grafting molecules. The same tendency was pointed out in our previous work related to the grafting of 1kD TiO2 ceramic membranes by the various fluorinated modifiers.49 Taking into account ceramic samples functionalized by non-fluorinated grafting agent, i.e. n-octyltriethoxysilane, a slightly smaller level of hydrophobicity was achieved; CA values were equal to 120o ± 2o and 135o ± 2o for 5 kD and 300 kD membrane, respectively. This behavior is associated with a bigger size (diameter) of fluorinated chains in comparison with non-fluorinated one. Differences in the size of molecules contribute to the creation of the larger value of energy penalty for hydration.1, 50 Considering the influence of fluorine atoms presence, the 6 and 5 % of increase was observed in the case of 5 and 300 kD membrane, respectively. To understand the abovementioned changes associated with more hydrophobic character of the fluorinated surface, the chemistry of the modifying molecules should be considered. The free energy of hydration per unit of surface possessing hydrophobic character is comparable for hydrocarbons as well as for fluorocarbons.50 Moreover, C-H bonds possessed lower value of dipole moment in comparison to C-F one. As a result, weaker linkage with dipolar water molecules could be expected. Furthermore, the C-F bonds polarizability of fluorine atoms is quite low. On the other hand, the value of hydration free energy for C-H bond is comparable with that for C-F.51 For this reason, the dispersion interactions of C-H with water molecules will be less attractive comparing with those for C-F one. Therefore, a surface grafted by nonfluorinated modifiers should be less hydrophilic comparing with the sample modified by fluorinated grafting agent.50,

52

However, due to higher value of lattice spacing of

fluoroalkylsilane chains, fluorinated surfaces will be more hydrophobic.50, 52 This phenomenon has been recently described by Soliveri et al. 1. The explanation of the observed behavior should be sought in the way of modifying molecules anchoring to the ceramic membrane. The


14

Page 15 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mechanism of attachment of fluorinated and non-fluorinated grafting agents was elaborated in our previous works.11, 49 Applying data from the contact angle measurements for water and glycerol, overall surface free energy (SFE) and its dispersive and polar components were calculated.37, 49 The achieved results were gathered and presented in Figure 4. It was clearly shown that functionalization process with alkylsilanes as well as with perfluoroalkylsilanes changed the value of surface free energy. The value of SFE for modified 5 kD and 300 kD ceramic sample was visibly reduced and values were in the range of 25.1 ± 1.1 mN m-1 to 36.2 ± 1.6 mN m-1 and 12.6 ± 0.7 to 24.8 ± 0.3 mN m-1, respectively. It should be remembered that the value of the overall surface free energy for pristine samples was equal to 140 ± 6.0 mN m-1, independently on the membrane pore size (Table 1). Comparing the SFE calculated data and measured CA, it can be noticed that these parameters are inversely proportional to each other (Figure 4). Considering ceramic sample modified by the set of four perfluorinated compounds, the highest values were observed for surfaces (5 kD and 300 kD) grafted by FC8, and in this particular case the smallest value of CA was also noticed. Considering ceramics functionalized by C6 and FC6, higher value of SFE is observed for the samples modified by non-fluorinated molecules. The observed differences were related not only to the type of silane coupling agents but also to the membrane morphology (pore size); 17% increase of total SFE was noticed in the case of the 5 kD membrane and 72% for the 300 kD one (Figure 4). Taking into account the Owens-Wendtąs method for the surface free energy determination, it should be stated that this parameter, describing physicochemistry of the modified surfaces, consists of two components, dispersive and polar.35,

49

In the case of

hydrophobic samples, lower input of polar part should be seen. 37 In the presented data, a polar component is around 21 % of overall SFE for 5 kD membranes and ca. 15 % for 300 kD sample.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 55

The observed differences are associated with the hydrophobicity level of the grafted surface. According to the scientific literature higher CA was found for more hydrophobic 300 kD sample. The collected experimental results possessed good compatibility with data presented elsewhere. For instance, Burnat-Hunek and Smarzewski

36

worked on the hydrophobization process of the

high-performance concrete that was modified by organosilicon. As a result of modification process the surface character was turned from hydrophilic (CA = 39o) to hydrophobic one (111o). At the same time, they observed the reduction of surface free energy, for these samples, from 138.4 mN m-1 to 9.4 mN m-1. Cichomski

53

presented results of titania support modification by

1H,1H,2H,2H-perfluorodecyltrichlorosilan. The reduction of surface free energy from 53.3 ± 4.0 mN m-1 to 25.2 ± 3.4 mN m-1 and at the same time an increase of the contact angle for water from 73±2o to 105±2o were observed. Moreover, the value of polar part of SFE accounted for 35% and overall SFE was equal to 38.4 ± 1.8 mN m-1. Based on the data gathered in Figure 4 it can be shown that smaller contribution of polar component is observed for 300 kD samples. Moreover, fluorinated 300 kD membranes possess lower value of polar component of SFE (range 1.8 – 3.8 mN m-1). The assessment of roughness parameters have an essential meaning in advanced characterization of physicochemical properties of the modified surfaces. The roughness of the ceramic membranes was expressed as a root mean square (RMS). Moreover, roughness is the right parameter to study advanced changes in the process formation of new surfaces (as an effect of modification). In Figure 4 the values of roughness parameters were presented for all modified 5 kD and 300 kD membranes. It can be noticed that there is a direct correlation between CA values and RMS. Generally, somewhat higher RMS values were noticed for 300 kD membranes comparing with 5 kD ones (Figure 4). Moreover, an impact of hydrophobization process is also


16

Page 17 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


clearly visible. For pristine samples RMS values were equal to 42.0 ± 1.3 nm and 61.2 ± 1.8 nm for 5 kD and 300 kD, accordingly. As a result of modification by silane compounds the RMS was measured in the range of 7.4 ± 0.2 nm – 23.1 ± 0.7 nm (Figure 4). The reduction of RMS value was associated with silanization of the ceramic surface. The coverage level of ceramic surface by silane grafting agents was affected by the grafting mechanism. The silanization occurs for single (single silanol structure), double (geminal silanol structure), and triple Si−O-ceramic surface bonds. Rougher surfaces were achieved for samples modified with C8 and C10 molecules than with C6 and C12. The detailed mechanism of the silane grafting agents attachment to the ceramic powders were presented elsewhere. 11,30,49 For the more detailed explanation of differences in the physicochemistry of modified surfaces, hysteresis of contact angle (HCA) and sliding angle (SA) were also calculated (Figure 5). HCA as well as SA are valuable and informative parameters in a solid–liquid interface characterization. It should be pointed out that for the surface possessing low value of HCA, small value of sliding angle will be observed, if the same testing liquid is applied.14, 54 According to the collected data (Figure 4 and 5), it can be stated that reduction of sliding angle as well as hysteresis of contact angle is directly associated with an increase in the hydrophobicity level (application of perfluoroalkylsilanes with longer fluoro-carbon chains) of the ceramic surfaces. However, it can be seen that the influence of fluorine atoms was marginal and value for HCA on the 5 kD surfaces functionalized by C6 and FC6 were equal to 40o and 38o, respectively. In the case of samples with bigger pore size (300 kD), smaller value of HCA was detected (HCAC6 = 22o, HCAFC6 = 19o), however, the same tendency was observed. In the terms of sliding angle (SA) (Figure 5), slightly higher value was noticed for membrane modified by C6 molecules than for FC6, for both 5 kD and 300 kD. The smallest SA was measured for surfaces having the


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 55

highest hydrophobicity level, grafted by FC12, both 5 kD (SA5kD-FC12 = 39o) and 300 kD (SA300kD-FC12 = 38o). Moreover, no impact of morphology (pore size) on the sliding angle was noticed (SA5kD-C6 = 53o, SA300kD-C6 = 50o, and SA5kD-FC6 = 42o, SA5kD-FC6 = 41o). Nevertheless, it should be highlighted that samples possessing small values of SA are the most interesting from practical and application points of view, e.g. as self-cleaning surfaces or in liquid flow applications (microchannels or nanochannels).47, 54 Another parameter describing physicochemistry of the modified surfaces is a critical surface tension (γcr). According to the achieved data (Figure 5) it can be seen that resistance to water was developed as a result of hydrophobization process. The improvement of resistance toward wettability depends slightly on the membrane morphology. In the case of 5 kD membrane the change of 38 % was observed, however, for the membrane with bigger pore size (300 kD) the alteration was ca. 56 %. The biggest differences were noticed after functionalization with the most hydrophobic grafting agent (FC12). The γcr was reduced from 29.2 ± 1.2 mN m-1 and 36.1 ± 1.4 mN m-1 for pristine membranes to 18.1 ± 0.7 mN m-1 and 15.7 ± 0.6 mN m-1 for 5 kD and 300 kD samples, respectively. Similarly to the other parameters (CA, RMS, SA and HCA), the values of critical surface tension for ceramic modified by FC8 and FC10 are slightly different. The source of explanation should be address to the chemistry of the hybrid organic-inorganic surface of functionalized ceramic membranes, i.e. in the mechanism of silane coupling agent bonding to the ceramic support.11,

49

Nevertheless, the established values of γcr parameters

pointing that samples grafted by FC8 and FC10 will be wetted by liquids possessing liquid surface tension at least 27 mN m-1 (5 kD samples) and 21 mN m-1 (300 kD samples). Furthermore, comparing results for samples modified by fluorinated and non-fluorinated molecules, the negligible differences were observed, that was included in the standard deviation


18

Page 19 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


range. For 5 kD membranes, γcr values were equal to 19.2 ± 0.8 mN m-1 and 22.5 ± 0.9 mN m-1 for modification with FC6 and C6, respectively. However, for 300 kD, this factor was equal to 16.3 ± 0.6 mN m-1 and 20.8 ± 0.8 mN m-1 for grafting with FC6 and C6, accordingly. Considering the applicability of the prepared hydrophobic membranes, it can be stated that samples would be suitable for water desalination by membrane distillation process. Salty water possesses the liquid surface tension around 79.1 mN m-1, and for this reason membrane modified by fluorinated or non-fluorinated molecules will not be wetted by feed solution. Moreover, it should be highlighted that type of silane coupling agent have a minor impact on the γcr parameter (Figure 5). One of the most important features of the new developed materials is water repellence and high stability. The Kao diagram utilizing Wenzel’s and Cassie-Baxter’s approaches can be used to correlate physicochemistry and wettability of the modified surfaces.3, 4, 55 The Kao diagram is a powerful tool in the material engineering combining the hydrophobicity/hydrophilicity of the surface with homo/heterogeneity as well as wettability properties (Figure 6). The general concept was presented by researchers from Kao Company.3, 4, 55 They discovered that two factors (i.e. chemical and geometrical) essentially impact the hydrophobicity. The aforementioned parameters can be tuned by chemical modification (chemical factor) or by introducing changes in nano- / microarchitecture of the surface (geometrical factor). The both surface modifications will lead to the reduction of surface energy.13, 30, 37 According to that, the group of Shibuichi

3, 4, 55

prepared a set of various surfaces possessing well-developed fractal surfaces. The samples were made from alkylketene dimer and were characterized by the contact angle value of 174°. Afterward, the formed highly heterogeneous, rough surfaces were compared with perfectly flat samples possessing the same chemistry. The highest CA value measured on the flat surfaces was


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 55

smaller than 109o. The collected data were gathered in a form of so called Kao diagram, presenting calculated cosine of apparent CA on a rough surface (θr) as a function of cosine of CA measured on perfectly smooth (so-called Young) surface (θs). In order to prepare such plot, one needs to perform the CA measurements for a broad spectrum of the testing liquids, possessing different liquid surface tension. According to the achieved data it is possible to compare at the same time physicochemical properties of modified materials at terms of wetting abilities and surface roughness (Figure 6). In the first quadrant of the coordinate system the data for hydrophilic and super-hydrophilic surfaces are gathered, regardless the surface roughness. In the presented work, the soaking of the solvents with low liquid surface tension (i.e. ethyl iodide, butyl acetate, butanol, methyl tert-butyl ether, n-hexane and perfluorohexane) into the material structure was observed (Figure 6). Furthermore, for the more detailed analysis of Kao diagram the models of Wenzel and/or Cassie-Baxter were taken into account.13,

30, 56, 57

The Wenzel

model56 is used for the description of wettability behavior on the homogeneous surface, whereas, the Cassie-Baxter

57

approach is applied for interpretation the heterogeneous surface possessing

hydrophobic or superhydrophobic character. Considering the testing liquids used during the measurements, determined values of roughness as well as critical surface tension, it was possible to foreseen the wettability of the evaluated ceramic membranes. For instance, applying water as testing liquid on the highly hydrophobic rough surface, air pockets between the surface heterogeneity will be observed (samples: 5kD grafted by FC12 and 300 kD modified with C6, FC6 and FC12). Therefore, it will lead to a composite solid–liquid–air interface formation and an increase of CA value. It can be concluded that pristine 5kD and 300 kD ceramic surfaces are placed in the wetting region (Figure 6). The hydrophilic, pristine sample was characterized by the lowest value of RMS as well as possessed the less hydrophobic character (Figure 4). In


20

Page 21 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


terms of functionalized samples, 5 kD membranes were practically located in the Wenzel’s region (Figure 6A). On the other hand, 300 kD hydrophobized ceramic membranes were placed in the Wenzel’s as well as in the Cassie-Baxter’s regions (Figure 6B). These observations are related to the fact that 300 kD surfaces possessed higher water contact angle as well as slightly higher roughness than 5 kD ones (Figure 4). Kao diagram proved relation between roughness, hydrophobicity and wettability resistance presented in the current study. Additionally, the established results are in a good agreement with the literature and predictions from Kao theory.3, 4, 13, 30, 55, 56

Discussing the collected data, it should be emphasized that it is possible to generate more hydrophobic surfaces by grafting molecules possessing shorter fluoro-carbon chains, e.g. FC6 instead of FC8 or FC10, independently on the utilized ceramics. Moreover, it is possible to prepare highly hydrophobic surface by modification with alkylsilanes coupling agent (C6) and eliminate the harmful fluorine compounds. The described findings are crucial from the potential application point of view i.e. in design and formation of a surface with controllable and predictable surface properties. Summarizing, it can be seen (Figure 4, 5 and 6) that grafting with alkylsilanes as well as perfluoroalkylsilanes was highly effective and clearly changed the physicochemistry and morphology of the ceramic membranes. 2.2.2.Ceramic membranes with tubular geometry Similarly to the planar ceramic membrane, tubular ones were also modified with high efficiency. Liquid entry pressure (LEPw) (Equation 6) was applied for the assessment of the grafting effectiveness for tubular 5 kD and 300 kD ceramic membranes. In the case of pristine ceramic membranes the value of LEPw is close to zero, independently on the membrane morphology.8, 28 This is associated with water penetration phenomenon into hydrophilic ceramic


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 55

membrane under nearly no pressure difference. It was observed that values of LEPw increased after functionalization of ceramics with perfluorinated as well as non-fluorinated grafting agents. For all modified tubular membranes LEPw values was measured in the range of 3 – 4 bar (Table 2). All 5 kD titania membranes were characterized by LEPw equal to 4 bar. In the case of 300 kD modified by C6, FC6 and FC8 LEPw value was equal to 3 bar. However, for 300 kD samples modified by two most hydrophobic molecules with the longest fluoro-carbon chains, FC10 and FC12, the LEPw was equal to 4 bar. The dissimilarity in the value of LEPw among membranes having different morphology (pore size) shows the influence of membrane material on the resultant level of hydrophobicity. The observed alteration are related to the way of grafting molecules attachment as well as accessibility of the hydroxyl groups on the ceramic.14,

58

Furthermore, the achieved marginally higher value of LEPw for 5 kD membranes can be explained by a fractional blockage of pores by perfluoroalkylsilanes or alkylsilanes. This observation was also made on the nitrogen isotherms (Figure 2). 2.2.3. Nanotribological study of hydrophobized ceramics by alkylsilanes and perfluoroalkylsilanes Tribological properties of the modified and pristine ceramics were established to obtain more detailed characterization of functionalized materials. For this purpose the AFM method was used to prepare the nanoindentation analysis. As a result, the following parameters were determined: adhesion force (Fadh), nanohardness (H) as well as Young modulus (E). It should be emphasized that friction forces of the surface are described and controlled by the interactions in low value of contact pressures on the adhesion force.59,

60

Furthermore, the

adhesion has the most important influence. Fadh depends on the following factors between two


22

Page 23 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surfaces being in the contact: capillary forces, chemical covalent bondings, electrostatic and van der Waals forces. 59, 60 According to the data gathered in Table 1 and 2, it can be seen that modification influenced in a significant way the nanotribological factors describing ceramic membranes. Adhesion forces for the samples grafted by fluorinated and non-fluorinated compounds were in the range of 2.99 ± 0.09 nN – 6.10 ± 0.18 nN and 2.84 ± 0.08 nN – 5.70 ± 0.16 nN for 5 kD and 300 kD, respectively. On the other hand, values of Fadh for pristine samples were equal to 26.5 ± 0.80 nN (5 kD) and 30.1 ± 0.90 nN (300 kD) (Table 1). Considering the collected data, it can be noticed that adhesion force reduced with increase of contact angle value as well as with formation of smoother surfaces (Figure 4 and 6). Applying more hydrophobic grafting agent from perfluorinated set of compounds contributed to the diminution of adhesion forces. However, comparing value of Fadh for fluorinated and non-fluorinated alkylsilanes, having the same length of alkyl chains, it can be seen that slightly higher values of Fadh for non-fluorinated (C6) surfaces were observed. Namely, ca. 17 % higher values of Fadh were measured for both 5 kD and 300 kD membranes. The achieved results are in a good accordance with the literature data. 53, 59, 60 Data obtained by Cichomski and co-workers

53, 60

show that for pristine titania support value of

adhesion force was equal to 25 nN. They also observed the reduction of this parameter (Fadh) to 10 nN and 22 nN after modification with 1H,1H,2H,2H-perfluorodecyltrichlorosilane and (3,3,3trifluoropropyl)trichlorosilane molecules. Another two parameters, Young modulus (E) and nanohardness (H) also change clearly as an effect of surface hydrophobization (Tables 1 and 2). Values of H and E for pristine 5 kD and 300 kD samples were equal to 4.9 ± 0.1 GPa (H for 5 kD), 4.5 ± 0.1 GPa (H for 300 kD) and 120 ± 2.4 GPa (E for 5 kD), 113 ± 2.3 GPa (E for 300 kD). The detected values of nanohardness for


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 55

functionalized samples were in the range of 8.1 ± 0.2 GPa – 10.0 ± 0.2 GPa for 5 kD samples and in the range of 8.0 ± 0.2 GPa – 9.1 ± 0.2 GPa for 300 kD ones. Moreover, it was observed that with higher level of hydrophobicity (application of grafting agents with longer fluoro-carbon chains) hardness of the sample evaluated in a nanoscale increased. It means that more resistant surface on the mechanical damages was achieved. Furthermore, interesting remark was noticed, namely value of H parameter for surface modified by C6 and FC6 was comparable and difference was in the range of standard deviation error. Considering Young modulus data, it was seen that marginally smaller values was established for the 300 kD ceramics (Table 2). However, the same tendency like in the case of nanohardness was noticed. The smallest values were calculated for C6 and FC6, but the differences were negligible. Subsequently, E increased with increased contact angle value (Table 2 and Figure 6). In the literature, comparable values of H and E for titania ceramics were found.

61

Nanohardness and Young modulus data were in

the range of 4 – 13 GPa and 115 – 210 GPa, respectively. The variance depends on the character of the tested samples, e.g. type of used grafting agent, chemistry of the surface as well as morphology of the titania support. The observed alteration in presented data of H and E in our work can be explained by bigger stiffness of the fluorocarbon chains. Moreover, it can be related to the fact that rotation of backbone structure for perfluorinated chains is significantly lower than that in the case of non-fluorinated chains, due to the smaller sizing of hydrogen atoms in a comparison to fluorine.62 Additionally, generated nanolayer of alkylsilanes modifiers is less rigid compared to one formed from perfluoroalkylsilanes. The bonds between carbon atoms in alkyl chain can rotate without hindrances comparing to fluoro-carbon chains 59. This phenomenon was pointed out by Bhushan et al.62, 63 They concluded that in the case of self-assembled monolayer with stiffer backbone structure, more energy is required for the elastic distortion. This remark


24

Page 25 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


can explain the observation of a smaller value of nanohardness and Young modulus parameter measured for ceramic modified by non-fluorinated (C6) molecules.62,

63

To explain this

observation more in detail, the AFM profiles and cross-sections of 5 kD (Figure 7) and 300 kD (Figure S2) samples were presented. Considering samples coupled by C6 and FC6 molecules only slight differences in depth and size of created cavities after nanoindentation analysis were observed. Results with the measured parameters are presented in Table 2. 2.3. Membrane efficiency in separation processes 2.3.1. Transport of pure water Transport and separation properties of all modified ceramic membranes were determined in vacuum membrane destination process applied for removal of volatile organic compounds (VOC) from water. Firstly, transport properties and permeability coefficients were established for samples contacting pure water as a feed. According to the gathered data (Table 3), it can be emphasized that morphology of ceramic material and type of utilized silane coupling agents possessed a visible influence on the transport properties of tested membranes (Table 3). Considering permeate flux for water (JH2O), it can be seen that comparable values of JH2O were noticed for the samples modified by grafting agents possessing the same length of alkyl chains, with or without fluorine atoms (Table 3). On the other hand, comparing permeate water fluxes for membranes functionalized with perfluorinated compounds, it can be seen that JH2O decreased when applying more hydrophobic grafting agents. Membranes hydrophobized by FC6 produced the highest water permeate flux, and shown the best transport properties in comparison with sample modified with FC12. The usage of FC12 agent can contribute to the partial pores clogging, especially in the case of 5 kD membranes, due to the size of grafting agent (≈ 2.2 nm) and that of pores (2 – 4 nm). The measured JH2O for 5 kD and 300 kD membranes were in the


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 55

range of 3.22 ± 0.13 kg m-2 h-1 - 5.32 ± 0.21 kg m-2 h-1 and 4.54 ± 0.18 kg m-2 h-1 - 5.77 ± 0.23 kg m-2 h-1, respectively. The explanation of diversification in water transport across the modified membranes can be sought in different permeability caused by the changed pore size after grafting. The permeance coefficient for water transport was calculated based on the Baker’s approach.64 According to the Baker et al. the membrane permeance is defined by the following Equation 2.64

J  Pi    = sat i p  l  pi − p

(2)

where pressure (pp) on the permeate side in terms of vacuum membrane distillation process is typically close to zero, and for this reason Equation 2 can be rearranged as follows (Equation 3):

 Pi  Ji   = sat  l  pi

(3)

The values of calculated permeance coefficients for water transport were gathered in Table 3. It can be seen that the type of grafting agent has an important impact on the permeance. Smaller values of permeance coefficients were determined for 5 kD membranes which was related to the limited transport due the pore size and resulted permeate flux (Table 3). The differences of permeance coefficients between 5 kD and 300 kD membranes grafted by the set of four fluorinated compounds increased with the application of more hydrophobic modifiers. Namely, the alteration of permeance coefficients for water transport between 5kD and 300 kD samples differs by 8.1 %, 9.6 %, 16.7 % and 29.5 % for samples modified with FC6, FC8, FC10 and FC12, respectively. Higher values of permeance coefficients were seen for 300 kD. Considering


26

Page 27 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between 300 kD and 5 kD membranes grafted by non-fluorinated C6 molecules, an increase of 7.1 % of permeance coefficient was observed. To characterize more in detail the transport of water across the ceramic hydrophobized membranes, an apparent activation energy (Eapp) of this process was calculated. For this purpose, measurements of water flux were determined at 5 different values of temperature (25, 35, 45, 55 and 65oC). Apparent activation energy (Eapp) was calculated based on Equation 4. 7, 29  E J = A exp − app  RT

  

(4)

where: J is flux, Eapp is an apparent activation energy, R is the gas constant and T is the applied temperature. The values of Eapp for all investigated membranes were calculated and collated in Table 3. It can be stated that higher value of Eapp, was observed for ceramics grafted by modifiers possessing higher hydrophobicity level. The observed tendency was related to higher hydrophobicity and resistances during water transport. The same behavior was seen during airgap and direct contact membrane distillation for 300 kD ceramic membranes hydrophobized by FC6 and FC12 molecules tested in desalination process. 13 The calculated Eapp

13

was equal to

49.2 kJ mol-1 and 58.8 kJ mol-1 for samples grafted by FC12 in direct contact membrane distillation as well as 43.1 kJ mol-1 and 56.4 kJ mol-1 in the case of membranes modified by FC6, respectively. Moreover, the value of Eapp depends on the membrane morphology and smaller value was noticed for 300 kD membrane. The observation was associated with bigger pore size and easier transport of vapors across these membranes. 2.3.2. Effectiveness of VOCs removal from binary mixtures The produced ceramic membranes were tested in vacuum membrane distillation process for the removal of volatile organic compounds from binary water–organic mixtures (H2O-EtAc, H2O-


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 55

MTBE and H2O-BuOH). The obtained data are presented in Figures 8, 9 and 10. During MD process, only vapors of solvents (water and VOCs) are transported across the porous structure of membrane. To ensure the efficient transport through the pores, membranes should not be wetted by transported liquid. During the material characterization of membranes the values of critical surface tension were determined for all investigated ceramic membranes (Figure 5). The measured values of γcr for modified samples were in the range of 18.1 ± 0.7 – 26.8 ± 1.1 mN m-1 and 15.7 ± 0.6 – 21.4 ± 0.8 mN m-1 for 5 kD and 300 kD samples, respectively. It means that liquid possessing value of liquid surface tension (γL) below 18.1 mN m-1 in the case of 5 kD samples and below 15.7 mN m1

in the case of 300 kD, will wet the membrane. The value of liquid surface tension of applied

feed solutions was in the range of 17.0 – 69.1 mN m-1. The values of liquid surface tension at the temperature of VMD experiment equal to 35oC for pure organic solvents were equal to 17.0 mN m-1, 21.7 mN m-1, 23.2 mN m-1, and 69.1 mN m-1 for MTBE, EtAc, BuOH and water, respectively. However, it should be noticed that during VMD process 3 wt. % of organic aqueous solutions were used. For this concentration the surface tension of aqueous solutions were equal to 62.8 mN m-1, 63.4 mN m-1, and 64.0 mN m-1, for MTBE, EtAc, BuOH, respectively. The presented values were calculated based on the Szyszkowski equation

65

as well as Meissner and

Michaels’s approach.66 According to these data it can be stated that all membranes modified by alkylsilanes and perfluoroalkylsilanes can be applied in VMD process, without the risk of wetting. In Figure S3, values of liquid surface tension (γL) and critical surface tension (γcr) for ceramic membranes in all configurations are presented. Transport and separation properties for the removal of VOCs in VMD process where the functionalized ceramic membranes were employed are gathered and presented in Figures 8, 9


28

Page 29 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and 10. All tested membranes were selective toward organic compounds. Considering 5 kD and 300 kD membranes, evident differences in separation properties were observed. In Figure 8, the McCabe-Thiele diagrams are shown. The noticed alterations were associated with membrane morphology (e.g. pore size) of 5 kD and 300 kD samples (Figures 2 and 3). Moreover, it was seen that the type of feed solution influenced the membrane selectivity (Figure 8). In this case, the differences should be related to the polarity of the applied solvents. The dielectric constant of the utilized solvents changed in the following order: εH2O =80.1 > εBuOH =17.9 > εMTBE =6.02 > εEtAc =4.5.7,

8

The separation effectiveness increases with polarity of applied feed solutions.

Membranes were more selective towards MTBE and EtAc separation (Figure 8). It can be observed that membranes grafted by C6 and FC6 molecules were the most selective where differences between these two molecules are marginal. Values of the separation factor beta (Equation 5) and PSI (Equation 6) for all investigated systems and membranes contacting 1 wt. % of organics are presented in Table 4. Considering only ceramic samples modified by perfluorinated compounds, it was observed that with the application of more hydrophobic modifiers (possessing longer fluoro-carbon chain) lower selectivity was observed (Figure 8). Very interesting phenomenon was observer when the separation properties of 5 kD and 300 kD membranes are compared. In the case of 5 kD membranes with an increase of hydrophobicity level of the surface, the significant changes of the separation ability were noticed. On the other hand, for 300 kD membrane, these variations were much smaller and the separation parameters were practically independent on the type of grafting molecules. It can be stated that in the case of 5 kD membranes (Figure 8 A, C, E) hydrophobic nanolayer can actively participate in the separation mechanism of VMD process. In that case the occurring separation results from the solution-diffusion mechanism across a dense brush layer.27, 29 The size of grafting agents (1.3 nm


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 55

(C6, FC6) to 2.2 nm (FC12)) is comparable with the pore size of the 5 kD membranes (2 – 4 nm). It can be suggested that attached hydrophobic brush resembles the dense selective layer in polymeric membranes. In the case of 300 kD membranes (Figure 8 B, D, F), a typical porous structure is observed (Figure S1) and the separation process is controlled exclusively by the liquid-vapor equilibrium.28, 29 In such case the ceramic membrane acts as a passive barrier during the transport of molecules across the membrane. In the case of 5 kD membrane the β separation factor, changes in the following order βFC6 > βC6 > βFC8 > βFC10 > βFC12 for all investigated systems (Table 4). Whereas, for membranes with bigger pores (300 kD) this order was different βFC6 > βC6 > βFC8 > βFC12 > βFC10 in every single case of separated mixtures. The least efficient membranes were those modified by FC10 molecules. The same observation was highlighted in our previous work focused on properties of membranes modified during the very short period of 5 – 15 min.11 The differences were referred to the mechanism of grafting molecules attachment to the ceramic support.11, 30 Concerning transport properties (Figure 9 and 10 and Table 4), it was noticed that in every single example of investigated mixture and membrane, water flux was much higher than flux of VOCs. It is related to the fact that feed solutions were diluted and subsequently the driving forces of organics were relatively low (Figure 10). Moreover, regarding data presented in Table 4 it can be seen the strong influence of the transport properties on the PSI value. The process separation index (PSI) is the most suitable factor for the comparison of these separation performance of different membranes. For 300 kD membranes modified by FC10 a very small value of PSI was observed for all tested systems (Figure 9 B, D, F). This is related to the limited transport across these membranes. Furthermore, the observed differences between separation properties of 5 kD and 300 kD membranes considered to be negligible comparing the transport


30

Page 31 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


properties. In view of transport through the 5 kD and 300 kD membranes, a slight influence of the type of grafting agent was noticed. The observed alterations were associated rather with the physicochemical properties of the separated organics than with the membrane morphology or properties of ceramic surface. 3. Conclusions Two types of ceramic membrane with molecular cut-off 5 kDa and 300kDa were efficiently functionalized and hydrophobized. Four perfluoroalkylsilanes and one non-fluorinated analogue with 6-12 and 6 carbon atoms in alkyl chain were selected as grafting agents, respectively. The hydrophobization of ceramic membranes was evaluated by various goniometric (CA, SA, HCA), microscopic (SEM, AFM) and surface (BET, BJH, γcr) techniques. Results indicate that, pore size of the membranes (5 kD and 300 kD) has an important impact on the effectiveness of the hydrophobization. In contrast to 5 kD, type of grafting agent has no influence on the value of CA observed for 300 kD membranes. Designed surfaces were characterized with high contact angle value (147o ± 2o), small hysteresis (HCA = 16o) as well as a low value of adhesive force (Fadh = 2.84 nN). On the other hand, modifications show significant effect on the nanotribological properties of prepared materials. In general, more mechanically-resistant surfaces with higher values of Young modulus and hardness were achieved. Hydrophobized membranes show high efficiency for the removal of VOCs (butanol, methyltert-butyl ether and ethyl acetate) from binary aqueous solutions in vacuum membrane distillation process. The highest performance was observed in separation of MTBE and EtAc. The competence of VOCs removal in membrane distillation process was correlated with material properties and resistance to wetting. Therefore, kinetics of wetting, depth of liquid penetration, permeance coefficients and apparent activation energy as well as other material properties were


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 55

evaluated. According to Zisman model and Kao diagram physicochemistry of membranes was presented in terms of wetting ability. The transport properties of functionalized membranes were discussed. For the 5 kD membranes, observation differences in selectivity depends on the type of silane grafting agent. On the other hand, 300 kD membranes, show no impact of surface chemistry upon variation in grafting molecules on the separation properties. Therefore, for the 5 kD membrane, the separation is according to solution-diffusion model. On the other hand, 300 kD hydrophobic membranes, are considered to be a passive barrier during the separation process and controlled by the liquid-vapor equilibrium (MD). Attachment of alkylsilane or perfluoroalkylsilanes derivatives to ceramic supports has great importance in controlled nano- or micro-architecture and chemistry of the tuned surfaces. Along with membrane separation technologies, results of this study can be employed in several technological areas including fabrication of microarrays, lab on a chip, microsystems and selfcleaning surfaces. 4. Experimental Section Materials. Ceramic titania (TiO2) membranes possessing tubular geometry (single-channel) were used. The molecular weight cut-off (MWCO) of ceramic membranes were equal to 5 kD and 300 kD. The 5 kD and 300 kD MWCO correspond to pore size of 2 – 4 nm and ~ 200 nm, respectively. The length of all tubular membranes was equal to 15 cm. In addition, planar TiO2 5kD and 300kD ceramic membranes were utilized. Planar membranes were used for the material and tribological characterization. Titania membranes were purchased from TAMI (France). Four types of perfluoroalkylsilanes grafting molecules were provided by SynquestLab (USA), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (CAS 51851-37-7) is referred thereafter as FC6;


32

Page 33 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1H,1H,2H,2H-perfluorodecyltriethoxysilane (CAS 101947-16-4) is referred thereafter as FC8; 1H,1H,2H,2H-perfluorododecyltriethoxysilane (CAS 146090-84-8) is referred thereafter as FC10; and 1H,1H,2H,2H-perfluorotetradecyltriethoxysilane (CAS 885275-56-9) is referred thereafter as FC12. Non-fluorinated grafting agent n-octyltriethoxysilane (CAS 2943-75-1) is referred as C6 was provided by Linegal Chemicals (Poland). All grafting agents and their solutions were kept under ambient atmosphere (Ar) to protect the modifiers from the selfcondensation process. Acetone, butanol (BuOH), butyl acetate, chloroform (stabilized by 1% ethanol), dimethyl sulfoxide, ethanol, ethyl acetate (EtAc), ethyl iodide, glycerine, n-hexane, methyl tert-butyl ether (MTBE), perfluorohexane, and pyridine were purchased from Avantor Performance Materials Poland S.A (Poland). RO deionized water (18 MΩ.cm) was utilized for the preparation of model VOCs aqueous solutions, used in VMD. All compounds and chemicals were used as received. Characterization of ceramic membranes. Contact angle, surface free energy and critical surface tension. Pristine and modified planar TiO2 membranes were characterized by following parameters: apparent contact angle (CA), sliding angle (SA), and contact angle hysteresis (HCA). During the measurements sessile drop (3 µl drop of pure liquid) and tilting plate methods were applied, described in detail elsewhere.67 The goniometric measurements were done at room temperature. Apparent CA values were calculated using Image J software (Image J, NIH – freeware version), with an accuracy of ± 2o. Presented results are the average values from 20-30 individual measurements. Surface free energy (SFE) values were determined based on the CA for water and glycerol.35, 37, 38 Critical surface tension (γcr) was measured according to Zisman and Owens-Wendt methods. 33 During γcr determination the chosen testing liquids at 20oC were applied: water (γ=72.7 mN m-1), glycerol (γ=63.4 mN m-1), dimethyl sulfoxide (γ=43.0 mN m-1),


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 55

pyridine (γ=38.0 mN m-1), ethyl iodide (γ=26.0 mN m-1), butyl acetate (γ=25.1 mN m-1), butanol (γ=24.2 mN m-1), methyl tert-butyl ether (γ=19.4 mN m-1), n-hexane (γ=18.4 mN m-1) and perfluorohexane (γ=11.9 mN m-1).32 Pore size distribution. For the determination of membrane pore size and a pore size distribution of pristine and functionalized ceramic membranes, nitrogen adsorption/desorption analysis (ASAP 20120) was used. During the measurements BET (Brunauer–Emmett–Teller) 37, 39, 41, 68

(Equation S1 and S2 in Supplementary Information) and BJH (Barrett-Joyner-Halenda)

39, 41, 68

models were applied. The aim of these experiments was to determine the influence of

various grafted hydrophobic molecules on the morphology of modified samples. Prior to the test, samples of ceramic were degassed at 90°C for 2h. The detailed experimental procedure is described elsewhere.58 Atomic force microscopy (AFM). Topography and the phase analysis of planar membranes were determined by AFM technique (machine - NanoScope MultiMode SPM System and NanoScope IIIa i Quadrex controller, Veeco, Digital Instrument, UK). Moreover, roughness parameters were determined by AFM images in a tip scanning mode.69 Roughness of the surface (scanning sample size 5µm x 5µm) was shown as RMS - root mean squared roughness. This factor, which describes the surface heterogeneity, is based on the average value of height deviations taken from the mean data plane.46, 48, 69 The RMS values for investigated samples were established according to the built-in mathematical algorithm in NanoScope Analysis Software (1.40, Build R3Sr5.96909, 2013 Bruker Corporation). The purpose of AFM analysis application was related to the wide characterization of hydrophobized ceramic membranes. To develop physicochemical properties of ceramics adhesive forces - Fadh, Young modulus – E, and hardness – H were determined.70 Moreover, the aim of these analysis was to examine the stability and


34

Page 35 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


quality of performed hydrophobic nanolayer (nanotribological analysis). This part of AFM analysis was performed in a contact mode. Fadh, H and E were determined from the load– displacement curves verified in the real time. Cantilever with three-sided pyramid diamond (cantilever spring constant 859 N/m; 60o apex angle) was applied for E and H determination. All tested ceramic samples were measured six times. As a result, an average value with ± 4% accuracy is presented. Fadh value, however, was achieved based on twenty-five measurements applying silicon nitride (Si3N4) probes NP-1 (spring constant equal to 0.58 Nm-1 was provided by the manufacturer) at the contact-mode. The ultimate loading force was 50–70 nN and tip velocity was equal to 7.88 µm s-1. All tests were done at an ambient temperature. Scanning electron microscopy (SEM). The imaging of ceramic membranes was done using the Quantax 200 with XFlash 4010 detector - Bruker AXS machine. The top view and crosssection of the membranes were imagined to determine the membrane morphology. Hydraulic permeability. Unmodified, hydrophilic ceramic membranes were characterized by hydraulic permeability measurements (Equation 5). The value of hydrodynamic permeability coefficients (Lp) were compared with each other for all testes membranes. This parameter depends on the membrane material, membrane morphology, and pore tortuosity.29 The permeability of all pristine membranes was determined from linear regression of the water flux (Jv) as a function of the transmembrane pressure ∆p (in the range of 1-9 bar) Equation 5. The hydraulic permeability Lp was determined utilizing a laboratory experimental rig described in detail elsewhere.14 Jv = L p ∆p

(5)

Liquid entry pressure (LEPw). The liquid entry pressure (LEPw) and (γcr) are essential limiting factors in a process design, e.g. in membrane process applications. The effectiveness of


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 55

tubular ceramic membrane modification was assessed by water liquid entry pressure measurements. LEPw is a pressure value required to transport the water across the hydrophobized/functionalized membrane structure.14 Based on the Equation 6, the following parameters can influence the resulting value of the liquid entry pressure: surface tension of entering liquid, membrane pore size, and contact angle. For hydrophobized samples liquid entry pressure is associated with hydrophobicity level of modified samples. LEPw values were assessed applying a laboratory rig presented elsewhere.14

LEPw =

2γ L cosθef rmax

(6)

where: LEPw is the pressure difference at the liquid - gas interface, γL is the surface tension of liquid, rmax is the maximal radius of the membrane pores, and θef is an effective contact angle. Membrane functionalization process. The modification process of ceramic membranes was performed according to the following experimental protocol.8, 14 The functionalization process of ceramic membranes is schematically shown in Figure 11. The pre-treated membranes were modified by immersing a sample in an appropriate 0.05M grafting solution (FC6, FC8, FC10, FC12 or C6) for 35h at room temperature kept under argon atmosphere. As soon as the modification process was completed, membranes were taken out from the modifying solution, and sequentially rinsed in pure acetone, ethanol, and water. Samples were subsequently dried at 110oC during ca. 12h and stored in the air. In case of tubular membranes, modified samples were tested in LEPw experiments and then examined in vacuum membrane distillation process for BuOH, EtAc, and MTBE from binary aqueous mixtures. Vacuum

membrane

distillation

(VMD).

Functionalized

ceramic

membranes

by

perfluoroalkylsilanes and non-fluorinated alkylsilanes were used in VMD process. Prior to the application in VOCs removal, the freshly functionalized membranes were conditioned by


36

Page 37 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contacting with pure water in the VMD experiment till the constant flux was achieved (usually after 5 – 7 hours).8 This step is needed to wash out of all non-grafted silanes chains that were adsorbed inside the membrane or on its surface.7, 8 The VMD experiments were carried out at 35oC, applying the experimental rig presented in Figure 12. Additionally, membranes were tested at the temperature range of 25 – 65oC (25, 35, 45, 55 and 65oC) what allowed to determine the value of an apparent activation energy (Eapp). During VMD experiments, the tubular membrane was placed in the stainless steel module (Figure 12). The feed solution circulated between module and feed tank at the constant flow rate (17 L min-1). Feed solution was in the direct contact with the tube side of tubular membrane, whereas permeate was collected from the shell side outside the module into two parallel permeate traps, chilled by liquid nitrogen. Thanks to that configuration of the setup, the continuous operation of the process was allowed. Vacuum on the permeate side was created by a vacuum pump (below 100 Pa). The stationary conditions of the system were reached after around 1 h from the outset of experiment. Experiments were carried out in contact with water and binary water-VOC mixtures. Firstly, the tests with pure water were done. Afterward, VMD experiments were performed with binary model aqueous solutions of butanol (BuOH), ethyl acetate (EtAc), and methyl tert-butyl ether (MTBE) as feed. The concentration of VOCs in aqueous solutions was kept in the range of 0 - 3 wt. %. Compositions of permeates and feeds was controlled by gas chromatography.8 The following equipment was used - Varian 3300 gas chromatograph with a TCD detector and PorapakQ packed column (injection port temperature at 200oC, detector temperature at 220oC, column temperature at 180oC). 0.2 - 0.4 µL of analyte was injected onto the column. For data processing and acquisition Borwin software (JMBS, France) was utilized. Limit of quantification (LOQ) and


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 55

limit of detection (LOD) for abovementioned analytical conditions were presented in details elsewhere.7, 29 The effectiveness of the separation process was assessed by the following parameters - total flux (J) – Equation 7, partial permeation fluxes (J1) – Equation 8, separation factor (β) – Equation 9, and Process Separation Index (PSI) – Equation 10.7, 29

J =

[

∆mt kg h-1 m-2 ∆t ⋅ A

[

J1 = J ⋅ Y1 kg h -1 m -2

β=

]

(7)

]

(8)

Y1 / Y2 x1 / x2

(9)

[

PSI = J (β − 1) kg h -1 m -2

]

(10)

where: ∆mt - total weight of compound in permeate [kg], A - area of membrane [m2], ∆t - time of permeation [h], Y, X - fractions of mass of 1 or 2 compound in permeate and feed.


38

Page 39 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. The relation between apparent water contact angle and depth of water penetration into ceramic structure of 5 kD and 300 kD membranes.

Figure 2. Nitrogen adsorption-desorption isotherm of non-modified (NM) (A) and functionalized 300 kD membranes by non-fluorinated C6 (B) and perfluorinated FC6 (C), FC8 (D), FC10 (E) and FC12 (F) compounds.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 55

Figure 3. Adsorbed layer (t) on the 5 kD (A) and 300 kD (B) pristine and modified ceramic membranes as a function of relative pressure.

Figure 4. Characterization of modified 5 kD and 300kD membranes, overall surface free energy, contact angle and roughness.


40

Page 41 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Characterization of modified 5 kD and 300kD membranes, critical surface tension, sliding angle and hysteresis of contact angle.

Figure 6. Kao diagrams for 5 kD (A) and 300 kD (B) pristine and modified membranes.


41


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 55

Figure 7. Nanoindentation analysis of pristine (A) and hydrophobized 5kD ceramic membranes by C6 (B) and FC6 (C) grafting agents.


42

Page 43 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. McCabe-Thiele diagrams for modified membranes in VMD process.


43


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 55

Figure 9. Transport of VOCs across modified membranes.


44

Page 45 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Transport of water across modified membranes.


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 55

Figure 11. The scheme of ceramic membranes functionalization by tri-functional perfluoroalkylsilanes molecules.

Figure 12. Scheme of VMD experimental rig. 1 - thermostated feed tank, 2 – circulating pump, 3 – thermostated membrane module with tubular membrane (4), 5 – cold permeate traps in liquid nitrogen, 6 – vacuum pump.


46

Page 47 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table. 1. Characterization of pristine membranes. Parameter

5 kD

300 kD

CA – contact angle [deg]

40 ± 2

40 ± 2

ko - constant rate [mol dm-3 s-1]

0.325

0.590

γcr - critical surface tension[mN 29.2 ± 1.3 m-1]

36.1 ± 1.5

SFE – surface free energy [mN 140 ± 6 m-1]

140 ± 6

dp - pore size [nm]

2-4

200

RMS – root mean square [nm]

45.0 ± 1.4

61.2 ± 1.9

Fadh – adhesion force[nN]

26.5 ± 0.8

30.1 ± 0.9

H – nanohardness [GPa]

4.9 ± 0.1

4.5 ± 0.1

E – Young modulus [GPa]

120 ± 2.4

113 ± 2.4

Table 2. Nanotribological characterization of modified 5 kD and 300 kD ceramics. Membrane

Adhesion [nN]

force Nanohardness [GPa]

5kD-C6

6.10 ± 0.18

8.1 ± 0.2

141.1 ± 2.8

5kD-FC6

5.03 ± 0.15

8.6 ± 0.2

151.0 ± 3.0

5kD-FC8

4.06 ± 0.14

8.9 ± 0.2

155.1 ± 3.1

5kD-FC10

3.84 ± 0.12

9.2 ± 0.2

161.0 ± 3.2

5kD-FC12

2.99 ± 0.09

10.0 ± 0.2

166.3 ± 3.3

300kD-C6

5.70 ± 0.16

8.0 ± 0.2

130.5 ± 2.7

300kD-FC6

4.75 ± 0.13

8.1 ± 0.2

134.0 ± 2.8

300kD-FC8

4.45 ± 0.12

8.3 ± 0.2

142.1 ± 2.9

300kD-FC10

3.61 ± 0.10

8.7 ± 0.2

154.2 ± 3.2


Young [GPa]

modulus

47


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300kD-FC12

2.84 ± 0.08

9.1 ± 0.2

Page 48 of 55

158.0± 3.3

Table 3. Modified 5 kD and 300 kD ceramic membranes characterization.

Membrane

JH2O [kg h-1 m-2]

LEP [bar]

Permeance coefficient of water -1

-2

Eapp [kJ mol-1] -1

[kg h m bar ] 5kD-C6

4

5.24 ± 0.21

92.44 ± 3.70

30.8 ± 1.2

5kD-FC6

4

5.32 ± 0.21

94.22 ± 3.77

31.3 ± 1.3

5kD-FC8

4

4.71 ± 0.19

83.55 ± 3.32

32.8 ± 1.3

5kD-FC10

4

4.03 ± 0.21

71.11 ± 2.81

34.4 ± 1.4

5kD-FC12

4

3.22 ± 0.13

56.89 ± 2.32

35.3 ± 1.4

300kD-C6

4

5.60 ± 0.22

99.55 ± 3.95

28.1 ± 1.1

300kD-FC6

3

5.77 ± 0.23

102.57 ± 4.11

28.6 ± 1.1

300kD-FC8

3

5.21 ± 0.21

92.44 ± 3.72

29.8 ± 1.2

300kD-FC10

3

4.80 ± 0.19

85.33 ± 3.45

31.3 ± 1.3

300kD-FC12

4

4.54 ± 0.18

80.71 ± 3.22

32.1 ± 1.3

Table 4. Efficiency of hydrophobized ceramic membranes in the separation of water–organic mixtures (1 wt. % of VOC). H2O-MTBE

H2O-EtAc

H2O-BuOH

Membrane β

PSI

β

PSI

β

PSI

5kD-C6

48

93

26

130

6

5

5kD-FC6

56

111

30

140

7

6

5kD-FC8

39

79

15

45

5

3

5kD-FC10

20

22

8

14

3

1


48

Page 49 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5kD-FC12

5

3

1

2

1

0.1

300kD-C6

94

950

42

324

11

45

300kD-FC6

109

1078

48

362

12

52

300kD-FC8

99

888

44

274

11

39

300kD-FC10

62

69

35

28

9

5

300kD-FC12

70

631

37

203

10

33

ASSOCIATED CONTENT Supporting Information. SEM of 5 kD and 300 kD pristine membranes; Nanoindentation analysis of pristine and hydrophobized 300kD ceramic membranes by C6 and FC6, FC8, FC10 and FC12 grafting agents; The evolution of liquid surface tension (γL) for pure solvents and their mixtures and critical surface tensions (γcr) for modified membranes. This material is available free of charge. All files in PDF. AUTHOR INFORMATION Corresponding Author * Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7 Gagarina St., 87-100 Torun, Poland, Tel: +48 56 611 43 15, Fax: +48 56 654 45 26, E-mail address: wojciech.kujawski@ umk.pl (W.Kujawski) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


49


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 55

This research was supported by 2012/07/N/ST4/00378 (Preludium 4) grant from the National Science Centre Poland. Partially research was supported by Statutory funds of Nicolaus Copernicus University in Toruń, Poland (Faculty of Chemistry, T-109 "Membranes and membrane separation processes - fundamental and applied research"). Notes The authors declare no competing financial interest ACKNOWLEDGMENT This research was supported by 2012/07/N/ST4/00378 (Preludium 4) grant from the National Science Centre Poland. Partially research was supported by the statutory funds of Nicolaus Copernicus University in Toruń (Faculty of Chemistry, T-109 "Membranes and membrane separation processes - fundamental and applied research"). Special thanks are due to Ms. Karolina Jarzynka for her kind assistance with the text editing. ABBREVIATIONS β, separation factor beta; BuOH, butanol; C, quilibrium constant of adsorption; C, noctyltriethoxysilane; CA, contact angle; d, kinetic diameter; dp, pore diameter; ε, solvent polarity E, Young modulus; Eapp, apparent activation energy; EtAc, ethyl acetate; Fadh, adhesion force; FC6,

1H,1H,2H,2H-perfluorooctyltriethoxysilane;

FC8,

1H,1H,2H,2H-

perfluorodecyltriethoxysilane; FC10, 1H,1H,2H,2H-perfluorododecyltriethoxysilane; FC12, 1H,1H,2H,2H-perfluorotetradecyltriethoxysilane; γcr, critical surface tension; γL, liquid surface tension; H, nanohardness; HCA, hysteresis of contact angle; Jt, total permeate flux; J1, partial permeate flux of 1 component; ko, constant rate; χ, depth of liquid penetration, LEPw, liquid entry pressure for water; MD, membrane distillation; MTBE, methyl tert-butyl-ether; MWCO,


50

Page 51 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecular weight cutoff; η, solvent viscosity; PFAS, perfluoroalkylsilanes; PSI, process separation index; RMS, root mean square; SA, sliding angle; SFE, surface free energy; t, time; Tb, boiling temperature; VMD, vacuum membrane distillation; VOC, volatile organic compound; VVOC, very volatile organic compounds.

REFERENCES 1.

Soliveri, G.; Pifferi, V.; Annunziata, R.; Rimoldi, L.; Aina, V.; Cerrato, G.; Falciola, L.; Cappelletti, G.; Meroni, D. Alkylsilane–SiO2 Hybrids. A Concerted Picture of Temperature Effects in Vapor Phase Functionalization. J. Phys. Chem. C 2015, 119, 15390-15400. 2. Hosseinabadi, S. R.; Wyns, K.; Buekenhoudt, A.; Van der Bruggen, B.; Ormerod, D., Performance of Grignard Functionalized Ceramic Nanofiltration Membranes. Separ. Sci. Technol. 2015, 147, 320-328. 3. Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. Super Water- and Oil-Repellent Surfaces Resulting from Fractal Structure. J. Colloid Interf. Sci. 1998, 208, 287-294. 4. Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Super-Water-Repellent Fractal Surfaces. Langmuir 1996, 12, 2125-2127. 5. Salehi, E.; Madaeni, S. S.; Shamsabadi, A. A.; Laki, S. Applicability of Ceramic Membrane Filters in Pretreatment of Coke-contaminated Petrochemical Wastewater: Economic Feasibility Study. Ceram. Int. 2014, 40, 4805-4810. 6. Dong, Y.; Lin, B.; Zhou, J.-e.; Zhang, X.; Ling, Y.; Liu, X.; Meng, G.; Hampshire, S. Corrosion Resistance Characterization of Porous Alumina Membrane Supports. Mater. Charact. 2011, 62, 409-418. 7. Kujawski, W.; Krajewska, S.; Kujawski, M.; Gazagnes, L.; Larbot, A.; Persin, M. Pervaporation Properties of Fluoroalkylsilane (FAS) Grafted Ceramic Membranes. Desalination 2007, 205, 75-86. 8. Kujawa, J.; Cerneaux, S.; Kujawski, W. Removal of Hazardous Volatile Organic Compounds from Water by Vacuum Pervaporation with Hydrophobic Ceramic Membranes. J. Membr. Sci. 2015, 474, 11-19. 9. Tsuru, T.; Wada, S.-i.; Izumi, S.; Asaeda, M. Silica–zirconia Membranes for Nanofiltration. J. Membr. Sci. 1998, 149, 127-135. 10. Tsuru, T.; Sudoh, T.; Yoshioka, T.; Asaeda, M. Nanofiltration in Non-aqueous Solutions by Porous Silica–Zirconia Membranes. J. Membr. Sci. 2001, 185, 253-261. 11. Kujawa, J.; Kujawski W. Functionalization of Ceramic Metal Oxide Powders and Ceramic Membranes by Perfluoroalkylsilanes and Alkylsilanes Possessing Different Reactive Groups: Physicochemical and Tribological Properties. ACS Appl. Mater. Interfaces 2016, 8, 7509-21. 12. Krajewski, S. R.; Kujawski, W.; Bukowska, M.; Picard, C.; Larbot, A. Application of Fluoroalkylsilanes (FAS) Grafted Ceramic Membranes in Membrane Distillation Process of NaCl Solutions. J. Membr. Sci. 2006, 281, 253-259.


51


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 55

13. Kujawa, J.; Cerneaux, S.; Koter, S.; Kujawski, W. Highly Efficient Hydrophobic Titania Ceramic Membranes for Water Desalination. ACS Appl. Mater. Interfaces 2014, 6, 1422314230. 14. Kujawa, J.; Kujawski, W.; Koter, S.; Jarzynka, K.; Rozicka, A.; Bajda, K.; Cerneaux, S.; Persin, M.; Larbot, A. Membrane Distillation Properties of TiO2 Ceramic Membranes Modified by Perfluoroalkylsilanes. Desalin. Water. Treat. 2013, 51, 1352-1361. 15. Dumanoglu, Y.; Kara, M.; Altiok, H.; Odabasi, M.; Elbir, T.; Bayram, A. Spatial and Seasonal Variation and Source Apportionment of Volatile Organic Compounds (VOCs) in a Heavily Industrialized Region. Atmos. Environ. 2014, 98, 168-178. 16. He, Z.; Li, G.; Chen, J.; Huang, Y.; An, T.; Zhang, C. Pollution Characteristics and Health Risk Assessment of Volatile Organic Compounds Emitted from Different Plastic Solid Waste Recycling Workshops. Environ. Int. 2015, 77, 85-94. 17. Girman, J. R.; Hodgson, A. T.; Newton, A. S.; Winkes, A. W. Emissions of Volatile Organic Compounds from Adhesives With Indoor Applications. Environ. Int. 1986, 12, 317-321. 18. Rodriguez, C.; Linge, K.; Blair, P.; Busetti, F.; Devine, B.; Van Buynder, P.; Weinstein, P.; Cook, A. Recycled Water: Potential Health Risks From Volatile Organic compounds and Use of 1,4-dichlorobenzene as Treatment Performance Indicator. Water Res. 2012, 46, 93106. 19. Cetin, E.; Odabasi, M.; Seyfioglu, R. Ambient Volatile Organic Compound (VOC) Concentrations Around a Petrochemical Complex and a Petroleum Refinery. Sci. Total Environ. 2003, 312, 103-112. 20. Indoor Air Quality: Organic Pollutants, Report on a WHO Meeting; World Health Organization: Berlin, 1987. 21. Gales, L.; Mendes, A.; Costa, C. Removal of Acetone, Ethyl Acetate and Ethanol Vapors from Air Using a Hollow Fiber PDMS Membrane Module. J. Membr. Sci. 2002, 197, 211222. 22. Urkiaga, A.; Bolaño, N.; De Las Fuentes, L. Removal of Micropollutants in Aqueous Streams by Organophilic Pervaporation. Desalination 2002, 149, 55-60. 23. Zheng, S.; Shen, C.; Alunbate, M.; Deng, J.; Wang, L.; Han, Z.; Tang, H. Discovery of VOC-compliant TEOS Sol and its Application to SiO2/Novolac Hybrid Coatings. Prog. in Org. Coat. 2013, 76, 425-431. 24. Jianping, W.; Yu, C.; Xiaoqiang, J.; Dongyan, C. Simultaneous Removal of Ethyl Acetate and Ethanol in Air Streams Using a Gas–Liquid–Solid Three-Phase Flow Airlift Loop Bioreactor. Chem. Eng. J. 2005, 106, 171-175. 25. Nikolajsen, K.; Kiwi-Minsker, L. Structured Fixed-Bed Adsorber Based on Zeolite/Sintered Metal Fibre for Low Concentration VOC Removal. Chem. Eng. Res. Des. 2006, 84, 562568. 26. Wijmans, J. G.; Kamaruddin, H. D.; Segelke, S. V.; Wessling, M.; Baker, R. W. Removal of Dissolved VOCs from Water with an Air Stripper Membrane Vapor Separation System. Sep. Purif. Technol. 1997, 32, 2267-2287. 27. Kujawski, W. Application of Pervaporation and Vapor Permeation in Environmental Protection. Pol. J. Environ. Stud. 2000, 9, 13-26.t 28. Matsuura, T.; Khayet, M. S. Membrane Distillation: Principles and Applications. Elsevier: 2011; p 512. 29. Kujawski, W.; Kujawa, J.; Wierzbowska, E.; Cerneaux, S.; Bryjak, M.; Kujawski, J. Influence of Hydrophobization Conditions and Ceramic Membranes Pore Size on their


52

Page 53 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30.

31.

32. 33. 34.

35. 36. 37. 38. 39.

40. 41.

42.

43. 44. 45.

46.

Properties in Vacuum Membrane Distillation of Water–Organic Solvent Mixtures. J. Membr. Sci 2016, 499, 442-451. Kujawa, J.; Cerneaux, S.; Kujawski, W.; Bryjak, M.; Kujawski, J. How To Functionalize Ceramics by Perfluoroalkylsilanes for Membrane Separation Process? Properties and Application of Hydrophobized Ceramic Membranes. ACS Appl. Mater. Interfaces 2016, 8, 7564-7577. Mu, J. E.; Shi, Y. T.; Yuan, F.; Liu, J. Mechanochemical Behavior of BaNd2Ti4O12 Powder in Ball Milling for High κ Microwave Applications. In Advances and Applications in Electroceramics II, John Wiley & Sons, Inc.: 2012, pp 135-146. Wypych, G. Handbook of Solvents, Volume 1, 2nd Edition, Properties. Elsevier: ChemTec Publishing, Ontario, Canada, 2014; Vol. 1. Fox, H. W.; Zisman, W. A. The Spreading of Liquids on Low Energy Surfaces. I. Polytetrafluoroethylene. J. Colloid Sci. 1950, 5, 514-531. Zisman, W. A. Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution. In Contact Angle, Wettability, and Adhesion, American Chemical Society: 1964; Chapter 1, pp 1-51. Owens, D. K.; Wendt, R. C. Estimation of the Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741-1747. Barnat-Hunek, D.; Smarzewski, P. Surface Free Energy of Hydrophobic Coatings of Hybrid-Fiber-Reinforced High-Performance Concrete. Mater. Technol. 2015, 6, 895-902. Chibowski, E. Surface Free Energy of a Solid From Contact Angle Hysteresis. Adv. Colloid Interf. Sci. 2003, 103, 149-172. Chibowski, E.; Jurak, M.; Holysz, L.; Szczes, A. Wetting properties of Model Biological Membranes. Curr. Opin. Colloid Interface Sci. 2014, 19, 368-380. Lecloux, A.; Pirard, J. P. The importance of standard isotherms in the analysis of Adsorption Isotherms For Determining The Porous Texture of Solids. J. Colloid Interf. Sci. 1979, 70, 265-281. Matějová, L.; Šolcová, O.; Schneider, P. Standard (master) Isotherms of Alumina, Magnesia, Titania and Controlled-Pore Glass. Micropor. Mesopor. Mat. 2008, 107, 227-232. Scherdel, C.; Reichenauer, G.; Wiener, M. Relationship Between Pore Volumes and Surface Areas Derived from the Evaluation of N2-Sorption Data by DR-, BET- and t-plot. Micropor. Mesopor. Mat. 2010, 132, 572-575. Khalfaoui, M.; Knani, S.; Hachicha, M. A.; Lamine, A. B. New Theoretical Expressions for the Five Adsorption Type Isotherms Classified by BET Based on Statistical Physics Treatment. J. Colloid Sci. 2003, 263, 350-356. Schneider, P. Adsorption Isotherms of Microporous-Mesoporous Solids Revisited. Appl. Catal. 1995, 129, 157-165. Yu, X.; An, L.; Yang, J.; Tu, S.-T.; Yan, J. CO2 Capture Using a Superhydrophobic Ceramic Membrane Contactor. J. Membr. Sci. 2015, 496, 1-12. Junaidi, M. U. M.; Leo, C. P.; Ahmad, A. L.; Ahmad, N. A. Fluorocarbon Functionalized SAPO-34 Zeolite Incorporated in Asymmetric Mixed Matrix Membranes For Carbon Dioxide Separation in Wet Gases. Micropor. Mesopor. Mat. 2015, 206, 23-33. Gao, L.; Lu, Y.; Zhan, X.; Li, J.; Sun, Q. A Robust, Anti-Acid, And High-Temperature– Humidity-Resistant Superhydrophobic Surface of Wood Based on a Modified TiO2 Film By Fluoroalkyl Silane. Surf. Coat. Technol. 2015, 262, 33-39.


53


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 55

47. Gogolides, E.; Ellinas, K.; Tserepi, A. Hierarchical micro and Nano Structured, Hydrophilic, Superhydrophobic and Superoleophobic Surfaces Incorporated In Microfluidics, Microarrays and lab on Chip Microsystems. Microelectron. Eng. 2015, 132, 135-155. 48. Ashraf, M.; Campagne, C.; Perwuelz, A.; Champagne, P.; Leriche, A.; Courtois, C. Development of Superhydrophilic and Superhydrophobic Polyester Fabric by Growing Zinc Oxide Nanorods. J. Colloid Sci. 2013, 394, 545-553. 49. Kujawa, J.; Rozicka, A.; Cerneaux, S.; Kujawski, W. The Influence of Surface Modification on the Physicochemical Properties of Ceramic Membranes. Colloid. Surface A 2014, 443, 567-575. 50. Dalvi, V. H.; Rossky, P. J. Molecular Origins of Fluorocarbon Hydrophobicity. P. Natl. Acad. Sci. 2010, 107, 13603-13607. 51. Miller, T. M. Atomic and molecular polarizabilities. In CRC Handbook of Chemistry and Physics, 96th Edition, Lide, D. R., Ed. CRC: Internet Version 2010 (CRC, Boca Raton, FL), 2010; pp 10193–10202. 52. Graupe, M.; Takenaga, M.; Koini, T.; Colorado, R.; Lee, T. R. Oriented Surface Dipoles Strongly Influence Interfacial Wettabilities. J. Am. Chem. Soc. 1999, 121, 3222-3223. 53. Cichomski, M. Tribological Investigations of Perfluoroalkylsilanes Monolayers Deposited on Titanium Surface. Mat. Chem. Phys. 2012, 136, 498-504. 54. Bhushan, B.; Jung, Y. C.; Koch, K. Micro-, Nano- and Hierarchical Structures for Superhydrophobicity, Self-Cleaning and Low Adhesion. Philos. Trans. R. Soc., A 2009; 367, 1631-1672. 55. Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. Super Water-Repellent Surfaces Resulting from Fractal Structure. J. Phys. Chem. 1996, 100, 19512-19517. 56. Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. Res. 1936, 28, 988-994. 57. Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. T. Faraday Soc. 1944, 40, 546551. 58. Kujawa, J.; Kujawski, W.; Koter, S.; Rozicka, A.; Cerneaux, S.; Persin, M.; Larbot, A. Efficiency of Grafting of Al2O3, TiO2 and ZrO2 Powders by Perfluoroalkylsilanes. Colloids Surf., A 2013, 420, 64-73. 59. Bhushan, B.; Cichomski, M.; Hoque, E.; DeRose, A.; Hoffmann, P.; Mathieu, J. Nanotribological characterization of Perfluoroalkylphosphonate Self-Assembled Monolayers Deposited on Aluminum-Coated Silicon Substrates. Microsyst. Technol. 2006, 12, 588-596. 60. Cichomski, M.; Kośla, K.; Pawlak, W.; Kozłowski, W.; Szmaja, W. Stability and tribological investigations of 1H, 1H, 2H, 2H-perfluoroalkyltrichlorosilane on Titania Surface. Tribol. Int. 2014, 77, 1-6. 61. Borgese, L.; Gelfi, M.; Bontempi, E.; Goudeau, P.; Geandier, G.; Thiaudière, D.; Depero, L. E. Young Modulus and Poisson Ratio Measurements of TiO2 thin Films Deposited With Atomic Layer Deposition. Surf. Coat. Technol. 2012, 206, 2459-2463. 62. Bhushan, B.; Kasai, T.; Kulik, G.; Barbieri, L.; Hoffmann, P. AFM Study of Perfluoroalkylsilane And Alkylsilane Self-Assembled Monolayers For Anti-Stiction in MEMS/NEMS. Ultramicroscopy 2005, 105, 176-188. 63. Bhushan, B.; Liu, H. Nanotribological Properties and Mechanisms of Alkylthiol and Biphenyl Thiol Self-Assembled Monolayers Studied by AFM. Phys. Rev. B 2001, 63, 245412.


54

Page 55 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


64. Baker, R. W.; Wijmans, J. G.; Huang, Y. Permeability, permeance and selectivity: A preferred way of reporting pervaporation performance data. J. Membr. Sci. 2010, 348, 346352. 65. von Szyszkowski, B. Experimentelle Studien über Kapillare Eigenschaften der Wasserigen Losungen von Fettsauren. Z. Physik. Chemie 1908, 64, 358-414. 66. Meissner, H. P.; Michaels, A. S. Surface Tensions of Pure Liquids and Liquid Mixtures. Ind. Eng. Chem. 1949, 41, 2782-2787. 67. Kwok, D. Y.; Neumann, A. W. Contact Angle Measurement and Contact Angle Interpretation. Adv. Colloid Interface Sci. 1999, 81, 167-249. 68. Li, W.; Amirfazli, A. A Thermodynamic Approach for determining the Contact Angle Hysteresis for Superhydrophobic Surfaces. J. Colloid Interf. Sci. 2005, 292, 195-201. 69. Janczewski, Ł.; Toboła, D.; Brostow, W.; Czechowski, K.; Hagg Lobland, H. E.; Kot, M.; Zagórski, K. Effects of Ball Burnishing on Surface Properties of Low Density Polyethylene. Tribol. Int. 2016, 93, Part A, 36-42. 70. Kaczmarek, H.; Gałka, P. Nano-Mechanical Properties of Modified Poly(Methyl Methacrylate) Films Studied by Atomic Force Microscopy. Tribol. Lett. 2011, 41, 541-554.

TOC


55

Molecular Grafting of Fluorinated and Nonfluorinated Alkylsiloxanes

Recommend Documents