How To Functionalize Ceramics by Perfluoroalkylsilanes for

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How to efficiently functionalize ceramics by perfluoroalkylsilanes for membrane separation processes ? - properties and applications of hydrophobized ceramic membranes. Joanna Kujawa, Sophie Cerneaux, Wojciech Kujawski, Marek Bryjak, and Jan K. Kujawski ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00140 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 2, 2016

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ACS Applied Materials & Interfaces

How to functionalize ceramics by perfluoroalkylsilanes for membrane separation process ? - properties and application of hydrophobized ceramic membranes Joanna Kujawa1), Sophie Cerneaux2), Wojciech Kujawski1)*, Marek Bryjak3), Jan Kujawski3) 1) Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7 Gagarina St., 87-100 Toruń, Poland 2) Institut Europeen des Membranes, UMR 5635, Place Eugene Bataillon, 34095 Montpellier cedex 5, France 3) Department of Polymer & Carbon Materials, Wrocław University of Technology, 27 Wyspianskiego St., 50-370 Wrocław, Poland KEYWORDS: hydrophobicity, ceramic membranes, perfluoroalkylsilanes, air-gap membrane distillation, desalination.

ABSTRACT The combination of microscopic (atomic force microscopy and scanning electron microscopy), goniometric (static and dynamic measurements) techniques, and surface characterization (surface free energy determination, critical surface tension, liquid entry pressure, hydraulic permeability) was implemented to discuss the influence of perfluoroalkylsilanes structure and grafting time on the physicochemistry of the created 1 ACS Paragon Plus Environment


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hydrophobic surfaces on the titania ceramic membranes of 5kD and 300kD. The impact of molecular structure of perfluoroalkylsilanes modifiers (possessing from 6 to 12 carbon atoms in fluorinated part of the alkyl chain) and time of functionalization process in the range of 5 to 35 h were studied. Based on the scanning electron microscopy with energy-dispersive X-ray spectroscopy it was found that the localization of grafting molecules depends on the membrane pore size (5kD or 300kD). In the case of 5kD titania membranes, modifiers are attached mainly on the surface and only partially inside the membrane pores. Whereas, for 300kD membranes, the perfluoroalkylsilanes molecules are present within the whole porous structure of the membranes. The application of 4 various types of PFAS molecules enabled for interesting observations and remarks. It was explain how to obtain ceramic membrane surfaces with controlled material (contact angle, roughness, contact angle hysteresis) and separation properties. The highly hydrophobic surfaces with low value of contact angle hysteresis and low roughness were obtained. These surfaces possessed also low value of critical surface tension what means that surfaces are highly resistant to wetting. This finding is crucial in membrane applicability in separation processes. The obtained and characterized hydrophobic membranes were subsequently applied in air-gap membrane distillation process. All membranes were very efficient in MD process showing good transport and selective properties (~99% of NaCl salt rejection). Depending on the membrane pore size and used modifiers, the permeate flux was in the range of 0.5 – 4.5 kg·m-2·h-1 and 0.3 – 4.2 kg·m-2·h-1 for 5kD and 300kD membranes, respectively.

1. Introduction The fabrication, design and characterization of materials with controlled properties e.g. hydrophobicity, hydrophilicity, wettability or micro- and nano-architecture are interesting and important in material science 1-5. In particular, a lot of attention is paid to the wetting control

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and to the practical applications of these surfaces, e.g. as anti-fogging, self-cleaning, antiicing, and antibacterial surface/materials 1, 5-10. Surface wettability depends directly on the character and physicochemistry of the surface. The level of water repellence can be govern by chemical mechanical/geometrical modification

9,

11-18

and/or the

3-6, 10, 19

. Depending on the physicochemical properties

(e.g. high or low surface energy), the examined materials can be classified basically as hydrophilic/superhydrophilic or hydrophobic/superhydrophobic 1, 4-6, 19. Water

contact

angle

(CA)

is

usually

used

as

a

factor

indicating

the

hydrophobicity/hydrophilicity level of a solid surface. In order to obtain complete characterization of the physicochemical properties of hydrophobic surfaces, both advancing and receding contact angles as well as hysteresis of CA should be also measured 4, 5, 16, 20, 21. There are several pathways to achieve hydrophobic surfaces, for instance by an artificial fabrication of highly rough surfaces or by chemical grafting with molecules possessing low surface free energy 5, 19. In order to manufacture rough surfaces, the following methods can be utilized: plasma etching and nanotexturing

2, 11

(AKD) 9, anodic oxidization of aluminium boiling water

, solidification of melted aklylketene dimer

22

or soaking of porous alumina-gel films in

10

. As a result, the ordered hierarchical, micro–nanostructured surfaces or

randomly nanostructured materials are obtained 5, 9-18. Functionalization process using low-surface free energy agents e.g. fluorine-containing such as fluorinated alkylsilanes (FAS), is often applied to achieve the surface hydrophobization

12, 13, 23

. Fluorinated alkylsilanes are frequently utilized for grafting of

ceramics 14, 16, 23, 24. In this particular case, the FAS modifiers are attached by covalent bonds to the hydrophilic, pristine surface rich in hydroxyl groups. As a result of such modification, the hydrophobic layer on the ceramic is created. The detailed discussion of the efficiency of



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such modification of metal oxide powders as well as ceramic membranes possessing different morphology has been recently presented by Kujawa et al. 13, 25. Ceramic membranes possess several advantages comparing to polymeric membranes, like high chemical, thermal and mechanical stability. Therefore ceramics are ideal materials for many applications in the chemical, biotechnological 26, food 27 and pharmaceutical industries 28

as well as in water treatment processes

29

including desalination

13, 25, 26

. Ceramic

membranes are hydrophilic by nature what limits the extended application of these membranes. To widen the usage of this type of membrane, the hydrophobization process can be utilized

5, 8, 12, 13, 23, 30, 31

. Various surface modification procedures using various

hydrophobization procedures with different grafting agents are known 13, 25, 32. However, there is very little works

12

, focused on the characterization of ceramic membranes grafted with a

homologues of perfluoroalkylsilanes possessing different molecular structure (from 6 to 12 carbon atoms in fluorinated part of the alkyl chain). It is worth to highlight that not enough attention has been paid to the description of the impact of type of grafting molecules (molecules from the same group perfluoroalkylsilanes) on the membrane properties. However, the ceramic functionalization by homologues of perfluoroalkylsilanes and subsequent detailed characterization of modified materials will deliver the crucial knowledge about the changes taking place on the ceramic surface upon functionalization. Membrane distillation (MD) is an example of thermally driven process that can be describe as a hybrid of thermal distillation and membrane process, utilized mainly for water desalination 29, 33-35. The featured characteristics of this process is that only vapours of solvent can be transported through the membrane pores. For this reason, the application of porous hydrophobic membranes is required. The driving force for the mass transport in MD is provided by the vapour pressure difference across the membrane created usually by difference of temperatures

33, 36, 37

. In the scientific literature, there are known following modes of


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membrane distillation: direct contact membrane distillation (DCMD), air-gap membrane distillation (AGMD), vacuum membrane distillation (VMD), sweeping gas membrane distillation (SGMD), and osmotic membrane distillation (OMD). Detailed and extensive reviews of membrane distillation have been presented by several authors

29, 34, 36, 38-40

. The

main advantage of MD over other separation processes is the utilization of lower temperature than in conventional distillation

41, 42

. Consequently, MD shows a potential to produce high-

quality drinking water using only low-temperature heat sources e.g. solar energy or waste heat from industrial processes 37, 43, 44. This study concentres on the fabrication of highly hydrophobic surfaces on 5kD and 300kD titania ceramic membranes (tubular and planar) by a chemical functionalization using 4 homologous of perfluoroalkylsilanes. The functionalization was followed by the comprehensive characterization of obtained surfaces. A special attention is paid on the impact of type of grafting molecules on the obtained hydrophobicity level as well as material properties of functionalized ceramics. In our previous works, it was stated that this parameter is particularly important and fundamental in design a surfaces with controlled properties 12, 23, 24, 45, 46

. Additional aim of this work was to present membrane efficiency in air-gap membrane

distillation used for water desalination. 2. EXPERIMENTAL SECTION 2.1. Materials Originally hydrophilic (water contact angle ≈ 40o), tubular (“inside CéRam” series) and planar titania ceramic membranes, both types with molecular weight cut-off of 5kD and 300kD (TAMI Industries, France) were applied. In the case of all utilized membranes, support and selective layer are made from titania. Planar membranes with 47mm of diameter possessed 1.7 x 10-3 m2 of active membrane area. Single-channel tubular membranes (5kD



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and 300kD) were characterized by outer and inner dimension of 10/5mm. 15 cm length of tubular membranes with 2.7 x 10-3 m2 of active membrane area were utilized. The following PFAS compounds were purchased from SynquestLab, USA: 1H,1H,2H,2H-perfluorooctyltriethoxysilane - C6F13C2H4Si(OC2H5)3 – C6 (CAS Number 51851-37-7),

1H,1H,2H,2H-perfluorodecyltriethoxysilane - C8F17C2H4Si(OC2H5)3 – C8 (CAS Number 101947-16-4),

1H,1H,2H,2H-perfluorododecyltriethoxysilane - C10F21C2H4Si(OC2H5)3 – C10 (CAS Number 146090-84-8),

1H,1H,2H,2H-perfluorotetradecyltriethoxysilane - C12F25C2H4Si(OC2H5)3 – C12 (CAS Number 885275-56-9). Modifiers were stored in the desiccator under the ambient atmosphere of argon and were

used as received without any further purification. Chloroform (stabilized by ethanol), acetone, and ethanol were supplied by Carlo Erba (France). Glycerol, dimethyl sulfoxide, pyridine, ethyl iodide, butyl acetate, butanol, methyl tert-butyl ether, n-hexane and perfluorohexane were purchased from Avantor Performance Materials Poland S.A (Poland). Sodium chloride was delivered by Fisher Scientific (UK). Deionized water (18 MΩ.cm) was applied for the preparation of all solutions used in MD experiments. 2.2.Membranes characterization Contact angle, surface free energy and critical surface tension. Pristine and functionalized ceramic membranes were characterized by measurements of apparent contact angle (CA), contact angle hysteresis (HCA) and sliding angle (SA) using a sessile drop (5 µl drop of pure liquid) and a tilting plate method, described in detail elsewhere

21, 47

. The both,

dynamic and static contact angles tests were performed. All experiments were performed at


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room temperature using goniometer PG-X (FibroSystem AB). The apparent contact angle values were established by Image J software (Image J, NIH – freeware version), with an accuracy of ± 2o. The results presented are the averages from 20-30 measurements. The following testing liquids at 20oC were used during the experiments: water (γ=72.7 mN m-1), glycerol (γ=63.4 mN m-1), dimethyl sulfoxide (γ=43.0 mN m-1), pyridine (γ=38.0 mN m1

), 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)

48

. Basing on the CA results obtained with aforementioned testing liquids

and applying the Zisman method

49, 50

the critical surface tension (γcr) was determined.

Dynamic measurements were done in order to characterize wettability behaviour of pristine, hydrophilic membranes. Whereas static contact angle measurements were performed to characterize the both, native and hydrophobized surfaces. Atomic force microscopy (AFM). An atomic force microscope (AFM) equipment was used to surface analysis (topography and phase analysis) of planar membranes (NanoScope MultiMode SPM System and NanoScope IIIa i Quadrex controller - Veeco, Digital Instrument, UK). Tip scanning mode was used for the surface roughness determination of samples. During the measurements, silicon nitride (Si3N4) probes (NP-1) (tip half angle 35o, tip radius 40nm) spring constant provided by the manufacturer (Veeco) 0.58 Nm-1 was used. Ambient temperature conditions were kept during all experiments. The root mean squared (RMS) roughness was used as a parameter describing heterogeneity of the samples. In this study, scan size areas were equal to 5µm x 5µm. The AFM analysis was applied to characterize the surfaces and to investigate an impact of grafting on the physicochemical properties of membranes. All samples were analysed at least five times and an average value was calculated (accuracy ± 4%).



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Scanning electron microscopy with energy-dispersive X-ray (SEM-EDX). The SEMEDX (Quantax 200 with XFlash 4010 detector – Bruker AXS) analysis was done to localize the place of modifiers molecules attachment on the ceramics. In this case, planar ceramic membranes were fractured after hydrophobization. Subsequently, the cross-section of the membrane samples were analysed by SEM-EDX technique. Pore size distribution. nitrogen adsorption/desorption analysis (ASAP 20120) using BET (Brunauer–Emmett–Teller)

17, 20, 51, 52

(Eqs. /1/,/2/) and BJH (Barrett-Joyner-Halenda)

17, 51, 52

models were utilized, in order to determine a membrane pore size and a pore size distribution. Samples were characterized before and after modification process. The purpose of this test was to illustrate how the hydrophobization by PFAS influences on the membrane morphology. Before the sorption tests, samples of membranes were degassed at 90°C for 2h. The experimental protocol of this analysis was presented elsewhere 13. a mC a=

p p0

/1/

 p  p  1 −  1 + (C − 1)  p0   p0 

where:

a – total adsorbed gas volume at p pressure p0 – saturation pressure of adsorbates at the temperature of adsorption p - equilibrium pressure of adsorbates at the temperature of adsorption am – capacity of monolayer (volume of gas adsorbed for monolayer)

C – BET constant of adsorption equilibrium:  E − EL   C = exp 1  RTa 

where:

/2/

E1 - heat of adsorption for the first layer,

EL - heat of adsorption for the second and higher layers and is equal to the heat of liquefaction.


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Hydraulic permeability. Transport properties of native tubular ceramic membranes were characterized by the hydraulic permeability tests (Eq. /3/) 53. The hydraulic permeability (Lp) depends on the type of membrane material, membrane pores geometry, porosity and thickness of selective layer 8, 18, 35. The permeability coefficient (Lp) of all pristine samples was obtained by a linear regression of water flux (Jv) as a function of increasing transmembrane pressure (∆p)

34, 40

. The transmembrane pressure in the range of 1 – 9 bar was applied. The obtained

experimental results were subsequently fitted to a straight line forcing a null intercept. The experimental rig used in the experiments was described in details elsewhere 25.

J v = L p ∆p

/3/

Liquid entry pressure (LEPw). The grafting efficiency of tubular ceramic membranes was determined by measurements of liquid entry pressure of water (LEPw) – Eq. /4/. LEPw is a pressure value, at which liquid penetrates through open pores of the membrane and is transported across the hydrophobic layer on the permeate side Laplace––Young equation (Eq. /4/)

7, 34, 40

. According to the

33, 34, 38, 39

, it can be seen that LEPw is related to the

surface tension, contact angle on membrane surface, and pore radius.

LEPw =

2γL cosθef r

/4/

The LEPw measurements were performed using a laboratory experimental rig described in detail elsewhere 25. LEPw values were determined for all tubular membrane samples, prior to the membrane characterization in membrane distillation process. 2.3.Grafting protocol Preceding the grafting operation, all ceramic membrane samples (tubular and planar) were cleaned according to the procedure presented in detail elsewhere 23, 25. Clean and dry ceramic materials were grafted by immersing samples in a grafting solution for a given period of time: 1st - total grafting time – 5h; 2nd - total grafting time – 15h and 3rd - total grafting time – 35h.



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The details of the hydrophobization process are described in our previous works

Page 10 of 37

12, 13, 23, 25

.

The obtained hydrophobized samples were characterized by methods described in section 2.2.

Figure 1. Scheme of preparation and modification protocol of ceramic membranes by PFAS molecules. Fig. 1. outlines the detailed protocol of modification and characterization of ceramics membranes hydrophobized using PFAS molecules. The hydrophobized tubular membranes were subsequently examined in the air-gap membrane distillation process (AGMD) of pure water and NaCl aqueous solutions. 2.4. Air-gap membrane distillation (AGMD)

Air-gap membrane distillation experiments were performed to evaluate membrane efficiency in the separation process. The AGMD is an exemplary application of the functionalized ceramic membranes. The AGMD experimental rig and the detailed experimental protocol are described in the previous work 23. The experiments were performed at temperature conditions of 5oC and 90oC for permeate and feed solution, respectively. The following feed concentrations of NaCl aqueous solutions were used 0 (pure water), 0.25, 0.5, 0.75, 1.0. NaCl solutions were prepared using deionized water and NaCl. Rejection coefficient of sodium chloride (Eq. /5/) was controlled by using the ion chromatography (Dionex DX-100 Ion Chromatograph).


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 Cp R NaCl =  1 −  Cf

  x100 [%] 

/5/

where: Cp and Cf stand for the NaCl concentration in permeate and feed solution. 3. RESULTS AND DISCUSSION 3.1. Characterization of pristine ceramic membranes

In the case of pristine and modified ceramics, determination of pore size or pore size distribution are very important 51, 52, 54, 55. Adsorption-desorption nitrogen isotherms for native and functionalized membranes were registered and examples of obtained isotherm for pristine and modified 300kD membranes are presented in Fig. 2. For the pristine sample (Fig. 2A) as well as for membrane grafted by C6 (Fig. 2B), shapes of isotherms are similar, typical for the cylindrical pores characterized by various cross-sections 56. However, after hydrophobization with C12 molecules, shape of the isotherm is different (Fig. 2C).

Figure 2. Nitrogen adsorption/desorption isotherm on pristine (A) and grafted TiO2 300kD membranes by C6 (B) and C12 (C) molecules. Grafting conditions: CPFAS = 0.05M, tmod = 35h, Tmod = room temperature. The obtained isotherm is typical for the material possessing spherical pores with a lot of narrowings

52, 54, 55

. It can be also seen that hysteresis loop is reduced after hydrophobization

with C12 molecules (Fig. 2C). It is a consequence of material functionalization by a grafting agent with longer fluoro-carbon chains as well as the change of the pores tortuosity. Changes



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can be also explained by a diminution of C parameter (C is an equilibrium constant of adsorption) – Eq. /2/, achieved from BET model 51, 52. This parameter depends on the surface porosity, pore size diameter, chemistry of the surface as well as adsorption capacity

51, 52, 55

.

Lower value of C is associated with lower adsorption capacity of the surface and dissimilar interactions with the ceramic surface that depend on the hydrophobicity level

52, 55

. The same

tendency was observed in the case of 5kD membranes. C value is reduced from 23.8 for a pristine sample to 18.0 and 16.7 for 5kD membranes modified by C6 and C12, respectively. The pore size distribution of ceramic membranes was established applying the nitrogen adsorption/desorption isotherm and BJH model

52, 54, 55

. The obtained pore diameter for 5kD

pristine membranes was in the range of 2-4nm, whereas for pristine 300kD ones it was equal to around 200nm ± 3nm. Furthermore, t-plot method was used to confirm the presence of micropores 52. The master isotherm relation presented by Lecloux–Pirrad

51, 52

used to calculate adsorbed film thicknesses (t)

and based on Harkins-Jura model was

52, 55, 56

. Relation between t and relative

pressure for chosen samples are shown in Fig. 3. It can be noticed that in case of 5kD adsorbed thicknesses were similar for all membranes, independently on the physicochemical properties and surface character. This observation was shown in the magnification of BET region (for relative pressure range from 0.05 to 0.35) (Fig. 3A). The lack of visible changes in the adsorbed thicknesses on the 5kD membranes is associated with a very small pore size (2-4nm) and low porosity level ~ 30%

57

. In the case of 300kD

membranes, the impact of grafting process on the t parameter is noticeable (Fig. 3B). It was found, that detected adsorbed thicknesses were lower for membranes modified by molecules with longer fluoro-carbon chains. This observation correlates well with the diminution of pore size and the obtained less open structure. The differences in the pore openness can be additionally confirmed by the changes in the shape of isotherm (Fig. 2A and 2C).


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Figure 3. Adsorbed layer (t) on the 5kD (A) and 300kD (B) membranes in the function of relative pressure. The pristine planar ceramic membranes were investigated using dynamic contact angle measurements. In that case, the changes in the deposited water drop behaviour were examined (Fig. 4A). It was noticed that membrane wetting depends on the membrane morphology. A membrane with denser structure, possessing smaller pores (5kD) is penetrated by water molecules slower than the membrane possessing bigger pores (300kD). Moreover, based on the obtained results (Fig. 4), it can be stated that the changes can be described by a zerothorder kinetic reaction. Consequently, the constant rate (k0) of zeroth order reaction was calculated 58. The values of the constant rates describing water penetration are equal to 0.325 and 0.590 mol dm-3 s-1 for 5kD and 300kD ceramic pristine membranes, respectively. On the other hand, it was also noticed that independently of the membrane morphology, water contact angle value is equal to 40o ± 2o for the both non-modified membranes (Fig. 4A). Material wettability and water repellence are very important properties from the possible practical point of view. The water resistance of material can be defined by value of critical surface tension (γcr). In this work, critical surface tension were determined according to the Zisman method

49, 50

for all investigated samples. In case of virgin ceramic membranes (Fig.

4B), γcr were equal to 34.1 ± 1.4 mN m-1 and 38.1 ± 1.6 mN m-1 m-1 for 5kD and 300kD 13 ACS Paragon Plus Environment


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samples, respectively. However, concerning the standard deviation of the obtained values, it can be concluded that γcr of pristine samples are independent of membrane morphology. Tubular native membranes were examined by determining the hydraulic permeability of water, according to the Eq. /3/. Based on these results, the transport properties of TiO2 pristine membranes were evaluated. The evolution of water flux through the native tubular ceramic membranes with increasing pressure is presented in Fig. 5.

Figure 4. A -Contact angle values evolution for non-grafted 5kD and 300kD TiO2 membranes. B – Zisman plot for pristine samples. All tested membranes exhibit linear relation with correlation coefficient (R2) close to unity (Fig. 5). The hydrodynamic permeability coefficient (Lp) for the membranes was equal to 0.232 x 103 kg h-1 m-2 bar-1 and 2.266 x 103 kg h-1 m-2 bar-1 for 5kD and 300kD pristine titania membranes, respectively. The higher value of permeability coefficient for 300kD membrane is related to the presence of bigger pores (~200nm) and higher porosity (~40%) than in the case of 5kD membrane (2-4nm pore size, ~30% of porosity). Smaller Lp for 5kD surface can be also associated with higher tortuosity of the ceramics. Moreover, smaller value of Lp will influence the lower transport across 5kD membranes comparing with 300kD.


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Figure 5. Hydrodynamic water flux as a function of transmembrane pressure for TiO2 membranes. 3.2.Hydrophobized ceramic surfaces Planar membranes. In case of planar ceramic membranes, the efficiency of functionalization process with PFAS molecules was expressed by a contact angle value of water. As a result of hydrophobization, the changes in physicochemistry of the membrane surface and morphology were observed. The substantial increase of water contact angle indicates, that planar titania ceramic membrane surfaces were efficiently modified, changing their hydrophilic character into hydrophobic one (Fig. 6). The obtained values of CA for all samples were higher than 90o, comparing to 40o for pristine samples, what additionally proves the achievement of hydrophobic surfaces. These facts are in the accordance with the results presented by various research groups

8, 30-32, 59

. Koonaphapdeelert and Li

12

modified alumina

hollow fibre ceramic membranes by 1H, 1H, 2H, 2H-perfluorooctylethoxysilane. After hydrophobization process an increase of contact angle from around 40o up to ~ 120o was observed

12

. As a result of the grafting process a slight decrease in gas permeability of the

membranes was noticed due to the additional resistance from a FAS layer on the membrane surface 8. Paterson et al. 30, 31 reported the method of anti-fouling coating by hydrophobization



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process. Additionally, they highlithed that these types of membrane modifiaction can be utilized in organic filtrations and in aqueous separation procedure as an anti-fouling coating. Krajewski et al.

32, 59

shown that it is possible to hydrophobize zirconia powders as well as

ceramic membranes with 1H,1H,2H,2H-perfluorodecyltriethoxysilane. Modified hydrophobic membranes were efficiently applied in membrane distillation process for water desaliation 48, 82

. Based on results presented in Fig. 6, it can be also concluded that modification conditions

(time and type of grafting agent) have an essential impact on the resulting hydrophobicity level. The extension of grafting duration contributed to an increase of apparent contact angle values for water (e.g. 110o ± 2o, 119o ± 2o, 130o ± 2o for 5kD-C6 modified by 5h, 15h and 35h). Moreover, hydrophobization by 4 types of PFAS modifiers allowed to observe an interesting relationship. The value of CA for samples changed in the following way: CAC12 > CAC6 > CAC8 > CAC10 (Fig. 6). The highest CA values were obtained for membranes grafted with C12 (148o ± 2o - 300kD membrane grafted 35h) and with C6 (145o ± 2o- 300kD membrane grafted 35h). These surfaces can be classified as superhydrophobic surfaces

1, 4

.

Surprisingly, an increase of CA is not related to the extension of length of fluoro-carbon chains of grafting molecules (CAC8 = 135 ± 2o and CAC10 = 137 ± 2o for 300kD membranes grafted 35h). The same observation was found in our previous work during modification of 1kD TiO2 ceramic membranes by the same modifiers

12

. The explanation of the noticed

phenomenon should be sought in the way of grafting molecules attachment to the ceramic surface. The comprehensive interpretation of these observations was presented in our previous works 12, 46.


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Figure 6. The apparent contact angle values as a function of total grafting time for 5kD and 300kD membranes. Grafting conditions: CPFAS = 0.05M, tmod = 5, 15, 35h, Tmod = room temperature. Furthermore, an influence of membrane morphology (membrane pore size and surface roughness) on the hydrophobicity level was notice. Membranes possessing bigger pore size were characterized by higher values of apparent CA (Fig. 6). This observation is directly correlated with the physicochemistry of the tested samples (e.g. roughness and surface free energy). The obtained results can be additionally discussed using Wenzel or Cassie-Baxter models

60, 61

. Wenzel models is used for the description of wettability behaviour on the

homogeneous surface. Cassie-Baxter model is utilized for the characterization of heterogeneous surface possessing hydrophobic or superhydrophobic character. Taking into 17 ACS Paragon Plus Environment


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consideration surface tension of testing liquid, critical surface tension of the solid and its roughness it is possible to predict the wetting behaviour of the examine surface. For instance, applying a non-wetting liquid, like water on highly hydrophobic rough surface, it will result in the formation of air pockets. Subsequently, it will lead to a composite solid–liquid–air interface creation and higher contact angle comparing with Wenzel less rough surface will be observed. For that reason, water drop behaviour on 300kD membranes is better explained by using Cassie-Baxter model. Considering 5kD membranes, Wenzel model is preferred for the description of water behaviour. In this latter case, a droplet in contact with a rough surface without air pockets, is referred to as a homogeneous interface 6. Moreover, measured contact angle values for 300kD membranes are higher than for 5kD ones (Fig. 6).

Figure 7. Presence of fluorine and silicon on the surface and inside the membranes structure from SEM-EDX analysis. Grafting conditions: CPFAS = 0.05M, tmod = 35h, Tmod = room temperature. SEM-EDX results allowed determining the location of PFAS molecules attachment (Fig. 7) to ceramic material. In case of microporous 5kD membranes, grafting molecules are located mainly on the membrane surface. This fact is related to the size of perfluoroalkylsilanes molecules (1.5 – 2.2 nm), which is comparable with the membrane pore size diameter (2-4 18 ACS Paragon Plus Environment

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nm). On the other hand, in the case of macroporous membranes possessing bigger pores (around 200 nm), it can be noticed that hydrophobic molecules are present inside the porous structure of the membrane and on the membrane surface. For 300kD membranes there are no limitations with perfluoroalkylsilanes location inside the porous structure due to the fact that pores are much bigger than even the longest grafting molecules. The measured CA values for water and glycerol as a testing liquids, were used for the calculation of surface free energy (SFE) for all samples

12, 20

. Modification process has a

strong impact on the SFE value (Fig. 8). It is clearly seen that after hydrophobization process, independently on the grafting conditions, the value of SFE is reduced. The values of SFE changed from 141 ± 6.1 mN m-1 for pristine surface (both 5kD and 300kD) to the values in the range of 1.8 ± 0.1 mN m-1 (300kD membrane grafted 35h by C12) – 68.5 ± 2.9 mN m-1 (5kD membrane grafted 5h by C10) for the functionalized samples (Fig. 8). The calculated SFE values are inversely proportional to the contact angle values, the highest SFE values were found for the least hydrophobic membranes i.e. grafted by C8 and C10 as well as membranes hydrophobized during the shortest time (Fig. 8). Taking into consideration the Owens-Wendt method of SFE calculation it should be noted that SFE consists of two components – polar and dispersive one

12, 62

. For the surface possessing more hydrophobic

character, the lower contribution of polar components should be observed 20.



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Figure 8. Surface free energy of the modified membranes. Grafting conditions: CPFAS = 0.05M, tmod = 5, 15, 35h, Tmod = room temperature. The obtained data are in a good accordance with the literature results. Burnat-Hunek

63

found that after hydrophobization with organosilicon compounds on the hybrid-fiberreinforced high-performance concrete surfaces, water contact angle changed from 55o to 113o. These samples were characterized by SFE dispersive component of 36.2 mN m-1 and polar one of 4.6 mN m-1. Cichomski

64

shown that as a result of grafting 1H,1H,2H,2H-

perfluorodecyltrichlorosilan on the Ti surface increase of water CA from 73±2o to 105±2o and reduction of SFE from 53.3 ± 4.0 mN m-1 to 25.2 ± 3.4 mN m-1 is observed. The contribution of dispersive component in total SFE for pristine and treated Ti was equal to 38.4 ± 1.8 mN m-1 and 24.8 ± 3.1 mN, respectively. From our results, it was found that 300kD membranes hydrophobized by C12 during 15h and 35h was characterized by the lowest value of polar contribution (i.e. 2 mN m-1). In Fig. 8 it can be also seen that not only duration of 20 ACS Paragon Plus Environment

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modification and type of grafting molecules have an impact on the SFE but also a size of membrane pores (5kD or 300kD). The extended duration of grafting process and the use of membrane with bigger pores diameter lead to the lower overall surface free energy 12, 45. For a better interpretation of the changes in surface physicochemistry related to modification process, the sliding angle (SA) and contact angle hysteresis (HCA) were also determined (Fig. 9A,B). Both parameters are important characteristic of a solid–liquid interface. Contact angle hysteresis occurs due to surface roughness and heterogeneity. In addition, surfaces with small HCA values are correlated to low SA values for the same testing liquid 6. Based on the obtained results, it can be noticed that diminution of the both contact angle hysteresis (Fig. 9A) and sliding angle (Fig. 9B) are directly related to the increase of hydrophobicity of modified TiO2 membranes (Fig. 6) 15, 25. Similarly to data presented in Figs. 4 and 5, it can be stated that experimental conditions influences the physicochemistry of the obtained hydrophobic surfaces, what is actually in a good agreement with literature data 12, 14, 16

. Thanks to the extension of modification duration from 5h to 35h, it was possible to

produce surfaces (5kD membranes) with low HCA around 20 ± 2o (5kD membrane grafted 35h by C6 and C12) comparing with samples grafted during 5h in the same modification conditions (HCA = 48 ± 2o). The higher hysteresis can be found for 300kD membranes, especially for samples modified by C8 and C10 molecules. According to the literature data for membranes characterized by the high HCA, bigger values of sliding angle were also observed 12, 14, 16

. For instance 5kD membrane grafted 35h by C12 was characterized by HCA = 28 ± 2o.

During analysis of the results in Fig. 9B it became clear that membrane pore size has a low impact on the sliding angle value. However, it should be emphasized that surfaces characterised by low values of sliding angle are particularly interesting from an application point of view in liquid flow applications, such as in micro- or nanochannels and self-cleaning surfaces 1, 6.



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In the discussion of physicochemistry of the hydrophobized surfaces expressed by CA, HCA and SA, it is extremely important to take into consideration a roughness of the examine samples. The roughness determined by AFM analysis is expressed as a root mean square (RMS)

3

of the surface and the results of these measurements are presented in Fig. 9C. In

general, lower values of RMS are observed for microporous 5kD membranes. For the nonmodified samples, RMS values were equal to 42 ± 1.8 nm and 62 ± 2.6 nm for 5kD and 300kD membranes, respectively. The RMS values found for hydrophobized samples were in the range of 7 ± 0.3 – 32 ± 1.4 nm in the case of 5kD membranes and in the range of 10 ± 0.4 – 48 ± 2.1 nm for 300kD membranes, respectively (Fig. 9). The reason of such significant changes in the RMS value was the presence of bonded molecules of PFAS on the surface as well as inside the membrane pores. This notification is consistent with results obtained from SEM-EDX analysis (Figure 7). The observed small RMS value for 5kD membranes verify the accurate choice of Wenzel model 61 for description of surface with low heterogeneity. In addition, the obtained smoother surface on the 5kD membranes explains lower values of contact angle hysteresis (Fig. 9A) and sliding angle (Fig. 9B) in comparison with 300kD membranes. Higher values of RMS found for 300kD membranes are directly correlated with a bigger surface heterogeneity and validate the application of Cassie-Baxter model

60

for the

surface description. Furthermore, it should be highlighted that with extension of hydrophobization duration, it was possible to produce smoother surface. This observation agrees well with the study of Li at el.

17

who also observed that the membrane surface

roughness decreases after longer membrane hydrophobization. Moreover, the rougher surfaces were found on the samples grafted with C8 and C10 molecules than with C6 and C12. This observation is also associated with the mode of grafting molecules attachment, which is one of the most important factor influencing the functionalization process

46

.

Namely, the extension of grafting time leads to the creation of smoother surface due to higher


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level of silanisation of ceramic surface. The silanisation occurs single (single silanol structure), double (geminal silanol structure) and triple Si-O-ceramic surface bonds 46. With the extension of grafting time, the fraction of geminal and siloxane connection is reduced. For that reason, longer hydrophobization results in the higher PFAS concentration on the ceramic surface. Referring to the obtained results, it can be stated that is possible to prepare more hydrophobic surfaces by grafting molecules possessing shorter fluoro-carbon chains, e.g. C6 instead of C8 or C10, independently on the utilized ceramics. The presented findings are essential from the potential application point of view i.e. they are crucial in design and creation a surface with controllable and predictable surface properties. In the evaluation of membrane resistance to water molecules, the values of critical surface tension were also determined. Based on the obtained results (Figs. 4B and 9D) it can be stated that the resistance toward wetting is improved as a results of modification process. The values of critical surface tension decreased from 34.1 ± 1.4 mN m-1 and 38.1 ± 1.6 mN m-1 for native membranes to 17.7 ± 0.7 mN m-1 and 15.4 ± 0.6 mN m-1 for the membranes grafted by C12 molecules (5kD and 300kD modified by 35h). The observed values for microporous membranes modified by C8 and C10 indicate that surface will be wetted by liquid characterized by surface tension value equal to 30 mN m-1or less. However, in the terms of membrane application e.g. to water desalination, all prepared membranes will be appropriate as surface tension of sodium chloride solution is ca. 79.1 mN m-1 and it could not wet the functionalized membranes. In addition, it should be emphasized that grafting conditions, especially duration of modification do not influence considerably the value of the critical surface tension. Differences in values of γcr, particularly between samples modified during 5h and 15h, were in the range of standard deviation (Fig. 9D).



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Summarizing, the modification process by functionalizing with perfluoroalkylsilanes was highly efficient and contributed to the changes in surface physicochemistry of the ceramics (Figs. 6,8,9).


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Figure 9. Characterization of hydrophobized ceramic membranes. The influence on the following parameters on the modified membranes: A – contact angle hysteresis; B – sliding angle; C – RMS; D – critical surface tension. Grafting conditions: CPFAS = 0.05M, tmod = 5, 15, 35h, Tmod = room temperature. Tubular ceramic membranes Tubular titania 5kD and 300kD ceramic membrane were also efficiently hydrophobized. The grafting efficiency in the case of tubular membranes was evaluated by the value of LEPw (Eq.1, Fig.10). It should be remembered that the LEPw value for pristine hydrophilic membranes is close to 0 bar

7, 24, 29, 36, 38

. This is associated with the fact that water can

penetrate hydrophilic ceramics and practically no pressure difference for water transport is needed. The determined LEPw values for hydrophobized membranes increased with an increase of the total grafting time from 5h to 35h and were changed from 1 to 4 bar (Fig. 10). The difference of LEPw values for 5kD and 300kD titania membranes indicates the impact of membrane materials on the resulting hydrophobicity level (Fig. 10). These results are associated with the mode of grafting process and amount of available hydroxyl groups on the surfaces 13, 25. Moreover, for 5kD-C12 and 300kD-C12 membranes, it was noticed that LEPw values became constant after 15h of modification. This fact can be explained by the saturation of the surface with C12 molecules and the utilization of all accessible hydroxyl groups

13

.

Moreover, the slightly higher values obtained for 5kD membranes can be explained by a partial clogging of pores by perfluoroalkylsilanes

31, 65

, what was detected from SEM-EDX

analysis (Fig. 7) and shown on the nitrogen isotherm (Fig. 2).



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Figure 10. The time evolution of LEPw for TiO2-5kD and TiO2-300kD membranes. Grafting conditions: CPFAS = 0.05M, tmod = 5, 15, 35h, Tmod = room temperature. 3.3. Properties of hydrophobized ceramic membrane in air-gap membrane distillation

The prepared hydrophobic titania tubular membranes were also tested in air-gap membrane distillation (AGMD) for desalination of aqueous NaCl solutions 29. Membrane distillation is a membrane separation process mostly suited for applications in which water is the main component present in the feed solution 66. In MD, it is essential that only solvent vapours are transported through the membrane pores and non-volatile compounds are retained. For this reason, the utilization of hydrophobic porous membranes is required. The hydrophobic character of the membrane avoids liquid from entering into pores, thanks to the surface tension forces. Thus, liquid/vapour interfaces are created at the entrances of the membrane pores23, 59. According to the presented data (Fig. 10), it can be stated that all membranes are suitable for application in membrane distillation process, due to their highly hydrophobic character and low values of critical surface tension (Figs. 9D, 10). Prior to the membrane characterization in desalination of aqueous NaCl solutions, the conditioned hydrophobized membranes were examined in contact with water as a feed solution. Under the experimental these conditions applied the driving force created in case of water feed was equal to 689 mbar

67

. The obtained data for 5kD and 300kD membranes 26

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grafted by C6, C8, C10 and C12 during extended grafting time of 5h, 15h and 35h are gathered in Fig. 11. In this initial test, the flux of pure water was determined and correlated with LEPw values. According to the data, it is seen that independently on the grafting conditions, type of PFAS molecules and the type of the membrane, the water permeate flux reduced exponentially with increasing LEPw value68. This outcome is very important if the practical utilization of hydrophobized membranes is taking into account. This is related to better applicability and higher efficiency in separation processes of the membranes possessing higher value of permeate flux.

Figure 11. Water permeate flux in the function LEPw for 5kD and 300kD membranes grafted with C6, C8, C10 and C12 molecules. Grafting conditions: CPFAS = 0.05M, tmod = 5, 15, 35h, Tmod = room temperature. In Fig. 12 the transport properties of functionalized tubular membranes in MD are presented. In Figs. 12 A-C, the data for 5kD membranes grafted with C6, C8, C10 and C12 during modification duration of 5h (Fig. 12A), 15h (Fig. 12B) and 35h (Fig. 12C) are shown. Similarly, results for the modified 300kD membranes are presented in Figs. 12 D-F.



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It can be seen that hydrophobization process has an important impact on the transport properties of tested membranes (Fig. 12). This observation is related to the change of membrane pores tortuosity and pore diameter after grafting processes and LEPw value (Figs. 2, 7, 10). Generally, 300kD membranes were characterized by higher permeate fluxes comparing with 5kD ones, what is related to the membrane morphology, i.e. bigger pore size diameter and higher porosity and lower LEPw value. As a consequence, the mechanism of the vapours transport will be different for 5kD and 300kD membranes 29, 68, 69. After extension of grafting time from 5h up to 35h, ca. 50% diminution of permeate flux was noticed, independently on the type of membrane as well as on the type of grafting agent (Fig. 12). Furthermore, the reduction of permeate flux values was also noticed as a result of NaCl concentration increase in feed. This behaviour is associated with diminution of driving force during the MD process with feed solution containing more non-vapour species. This phenomenon is explained by Raoult’s law describing that the vapour pressure above the solution containing more non-volatile component is reduced 29, 69. Considering the type of grafting molecules, it can be seen that length of PFAS molecules has also an important impact on the permeate flux. The lowest values of permeate flux were determined for membranes, grafted by C12 molecules (Fig. 12). This remark can be correlated with mode of PFAS chain attachment inside the porous structure membrane and then increase of membrane tortuosity and change of LEPw. In addition, the partial pores clogging by perfluoroalkylsilanes is possible 31, 65. Retention coefficient of salt (RNaCl) (Eq. /5/) is an important factor describing an efficiency of desalination process. It should be equal to 100% due to the fact that only solvent vapours are transport across the membrane. The obtained results are in good agreement with theory and values of RNaCl are very high (Fig. 13). Membrane character (e.g. hydrophobicity level) and


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condition of functionalization membranes have no impact on the selectivity. The significant influence on the selective properties has the composition of applying system, i.e. vapour pressure of the individual components of the solution.



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Figure 12. MD permeate flux vs. NaCl concentration of the feed for the 5kD and 300kD grafted membranes by PFAS. 5kD membrane grafted during 5h (A), 15h (B) and 35h (C) and 300kD membrane grafted during 5h (D), 15h (E) and 35h (F). Grafting time: 5h, 15 and 35h, CPFAS=0.05M, grafting temperature – room temperature. Temperature conditions of AGMD: Tf = 90oC; Tp = 5oC.

Figure 13. Rejection coefficient in MD process for hydrophobic ceramic membranes. 4. CONCLUSIONS The functionalization ceramic surfaces by perfluoroalkylsilanes were studied by a combination of surface, microscopic and goniometric techniques. The impact of molecular structure of perfluoroalkylsilanes (possessing from 6 to 12 carbon atoms in fluorinated part of the alkyl chain) and time of functionalization process in the range of 5 to 35 h were studied. The utilization of these 4 types of PFAS molecules, it was the source of interesting observation and remarks. Notwithstanding the different nature of the applied ceramics (planar and tubular membranes characterized by different morphology, i.e. microporous 5kD and macroporous 300kD), the utilized methods enabled a coherent view of the perfluoroalkylsilanes nanolayer as a function


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of the grafting conditions. Furthermore, in the work it was highlighted that the functionalization process conditions have an important impact on the physicochemistry of examine ceramic materials. All investigated ceramic materials were efficiently modified and as a consequence the changes of surface character from hydrophilic to hydrophobic one was observed. Based on the SEM-EDX analysis, it was shown that grafting molecules are attached on the surface as well inside the porous structure of the ceramic membranes. However, in the case of 5kD membranes, molecules attachment inside the pores is limited by the size of PFAS (1.5 – 2.2nm) comparable with membrane pores dimension (2-4nm). In case of planar membranes, the increase of CA was observed with an extension of grafting time from 5h to 35h as well as the reduction of SFE, HCA, SA and critical surface tension. Moreover, the hydrophobic layers obtained by a longer grafting duration were characterized by lower surface roughness expressed by RMS. The length of hydrophobic PFAS molecules has a significant impact on the hydrophobicity level. Nevertheless, the relation between length of PFAS chains and value of CA is not straightforward. The highest values of water CA were measured for surface modified by C12 (148o ± 2o) and C6 (146o ± 2o). Independently on the hydrophobization conditions (time and type of modifiers), higher values of water contact angle were noticed for the 300kD membranes. This important impact of the membrane morphology was also seen in terms of physicochemical properties e.g. SFE, HCA, SA, critical surface tension as well as roughness. The most important finding for planar membranes, was the fact that it is possible to create almost superhydrophobic surfaces (CA = 146o), possessing also a low value of contact angle hysteresis and low roughness by functionalization process with short fluorocarbon chains (C6 molecules).



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All tubular membranes were efficiently functionalized by PFAS molecules. It was shown that applied functionalization methods are sufficient for creation of ceramic membranes applicable in membrane distillation process. All membranes are highly efficient in AGMD process. The hydrophobization condition i.e. type of molecules and grafting time as well as membrane morphology have a visible impact on the transport properties in AGMD. Higher values of permeate flux were observed for 300kD titania membranes and samples modified within the shorter time. The hydrophobicity level has no effect on the selectivity process. It was shown that application of hydrophobized ceramic membranes for desalination process was very effective because of the salt rejection coefficient close to unity for all investigated membranes. AUTHOR INFORMATION Corresponding Author * 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 24 77, E-mail address: [email protected] (W.Kujawski) Funding Sources The research was supported by 2012/07/N/ST4/00378 (Preludium 4) grant from the National Science Centre Poland. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT


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This research was supported by 2012/07/N/ST4/00378 (Preludium 4) grant from the National Science Centre. Special thanks are due to Ms. Karolina Jarzynka for her kind assistance with the text editing. ABBREVIATIONS 5kD-C6, titania ceramic membrane (MWCO = 5kD) grafted by C6; 5kD-C8, titania ceramic membrane (MWCO = 5kD) grafted by C8; 5kD-C10, titania ceramic membrane (MWCO = 5kD) grafted by C10; 5kD-C12, titania ceramic membrane (MWCO = 5kD) grafted by C12; 300kD-C6, titania ceramic membrane (MWCO = 300kD) grafted by C6; 300kD-C8, titania ceramic membrane (MWCO = 300kD) grafted by C8; 300kD-C10, titania ceramic membrane (MWCO = 300kD) grafted by C10; 300kD-C12, titania ceramic membrane (MWCO = 300kD) grafted by C12; γcr, critical surface tension [mN m-1]; ∆p, transmembrane pressure [bar]; a, total adsorbed gas volume at p pressure [cm3]; AFM, atomic force microscopy; AGMD, air gap membrane distillation; AKD, aklylketene dimer; am, capacity of monolayer (volume of gas adsorbed for monolayer) [cm3]; BJH, Barrett-Joyner-Halenda model; C, constant of BET isotherm; C6, 1H,1H,2H,2H-perfluorooctyltriethoxysilane; C8, 1H,1H,2H,2H-perfluorodecyltriethoxysilane; C10, 1H,1H,2H,2Hperfluorododecyltriethoxysilane; C12, 1H,1H,2H,2H-perfluorotetradecyltriethoxysilane; CA, contact angle [o]; Cf, concentration of feed [M]; Cp, concentration of permeate [M]; DCMD, direct contact membrane distillation; E1, heat of adsorption for the first layer; EL, heat of adsorption for the second and higher layers and is equal to the heat of liquefaction; FAS, fluoroalkylsilanes; HCA, hysteresis of contact angle [o]; J, water flux [kg h-1 m-2]; LEPw, liquid entry pressure for water [bar]; Lp, permeability coefficient [kg h-1 m-2 bar-1]; MD, membrane distillation; MWCO, molecular weight cut off, OMD, osmotic membrane distillation; p0, saturation pressure of adsorbates at the temperature of adsorption [bar]; p, equilibrium pressure of adsorbates at the temperature of adsorption [bar]; PFAS, perfluoroalkylsilanes; t, thickness of adsorbed layer [nm]; R, gas constant [J K−1 mol−1]; RMS, roughness parameters – root mean square [nm]; RNaCl, rejection coefficient for sodium chloride [%];SA, sliding angle [o]; SEM-EDX, scanning electron microscope with energydispersive X-ray; SFE, surface free energy [mN m-1]; SGMD, sweeping gas membrane distillation; Ta, temperature of adsorption [oC]; Tf, feed solution temperature [oC]; Tp, permeate solution temperature [oC]; Tmod, temperature of modification [oC]; tmod, modification time [h]; VMD, vacuum membrane distillation.

REFERENCES (1) 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. (2) Lee, B. K.; Baek, I. B.; Kim, Y. Y.; Jang, W. I.; Yoon, Y. S.; Yu, H. Y., Fabrication of Large-Area Hierarchical Structure Array Using Siliconized-Silsesquioxane as a Nanoscale Etching Barrier. ACS Appl. Mater. Inter. 2015.



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