Characterization of Polymer Inclusion Membranes (PIMs) Containing

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Characterization of polymer inclusion membranes (PIMs) containing phosphonium ionic liquids as Zn(II) carriers Monika Baczy#ska, Micha# Waszak, Marek Nowicki, Dawid Prz#dka, S#awomir Borysiak, and Magdalena Regel-Rosocka Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04685 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Industrial & Engineering Chemistry Research

Characterization of polymer inclusion membranes (PIMs) containing phosphonium ionic liquids as Zn(II) carriers Monika Baczyńska1, Michał Waszak2, Marek Nowicki2, Dawid Prządka1, Sławomir Borysiak1, Magdalena Regel-Rosocka1* 1

Poznan University of Technology, Faculty of Chemical Technology, Institute of Chemical

Technology and Engineering, ul. Berdychowo 4, 60-965 Poznan, Poland 2

Poznan University of Technology, Faculty of Technical Physics, Institute of Physics, ul.

Piotrowo 3, 60-965 Poznan, Poland *e-mail: [email protected]

Abstract Phosphonium ionic liquids (IL), i.e. trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101), trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104) and tributyl(tetradecyl)phosphonium chloride (Cyphos IL 167), were used as ion carriers in CTA or PVC-based polymer inclusion membranes (PIM). Up to now morphology and structure of PIMs with phosphonium ILs have not been characterized in detail. Thus, the following techniques were proposed in this paper: contact angle measurements, FT-IR, XRD, DSC, AFM and SEM, to analyze influence of PIM morphology on the efficiency of Zn(II) transport. CTA-based membranes appeared to be more hydrophilic, with more expanded and rough surface that allowed better accessibility of the metal ions to the membrane. PVC-based PIMs were more hydrophobic and completely amorphous and their surface showed less diversity resulting in worse accessibility. Also, PIM stability after five cycles of transport processes was examined.

Keywords: Polymer inclusion membranes, Phosphonium ionic liquids, Membrane morphology, Zinc(II) transport, Diffusion coefficient

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1. INTRODUCTION In recent years, a remarkable progress concerning the removal and separation of various chemical species, such as organic compounds, metal ions and inorganic ions by liquid membrane systems is observed. Liquid membranes containing ion carriers can be an alternative to conventional liquid-liquid extraction because they permit separation of large amounts of organic diluents, which are usually volatile, flammable and toxic. Liquid membranes can occur in the form of bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs) and supported liquid membranes (SLMs). Although flat sheet SLMs are a very interesting option to overcome the solvent extraction downsides, they are not widely used on industrial scale, mainly due to their poor stability and short lifetime. A particular type of SLMs are polymer inclusion membranes (PIM) which differ from SLM in the way of immobilization of a carrier in the membrane. PIMs are formed by casting a solution containing a carrier (an extractant), a plasticizer, and a base polymer (e.g. CTA, PVC, PVDF).1,2 The carrier, which facilitates transport of ions by reversible reaction with them, is built into the structure of a polymer matrix in PIMs, while in SLMs the membrane pores only are impregnated with a carrier solution. Thus, PIMs are considered to be more stable than SLMs, and carrier loss from the former membrane type is negligible, and small amounts of carrier are required. It opens the possibility to use also expensive extractants as ion carriers. Generally, PIMs have been characterized as highly selective in metal transport as well as easy in setting up and operation. 1,2 PIMs with various carriers have been successfully applied for transport process of the target metals such as: alkali metals, 3 actinides, 4,5 lanthanides,6 precious metals (Au(III), Ag(I), Pd(II)),7-11 and heavy metals (Zn(II), Cr(III), Cd(II) and Cu(II)).10,12-15 Reduction of metal ion content in wastewater is an important issue from the environment protection point of view. For example, Zn(II) presence in the aqueous solutions is strictly limited by emission standards, e.g. to 2 mg/L Zn(II) in Europe. One of the main sources of Zn(II) effluents are spent pickling solutions from hot-dip galvanizing industry, composition of solutions from various pickling processes are presented in the previous work.16 The previous liquid-liquid extraction studies have led the authors to propose PIMs containing a group of phosphonium ionic liquids (ILs) as carriers to investigate Zn(II) removal from chloride aqueous solutions. 17,18 These ILs has been effectively applied in PIMs also to transport gold(III), 19,20 cadmium(II) and copper(II), 21 zinc(II) and iron(III) 22. 2 ACS Paragon Plus Environment

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Transport rate through PIMs is affected by many parameters such as: composition of the membrane (particularly concentration of the carrier, a type and concentration of the plasticizer and a kind of polymer matrix) and the mixing rate of the feed and the receiving phases.1,2,23 Our previous papers have been focused on Zn(II) transport properties of PIMs based on CTA or PVC matrix.17,18,24,25 It has been established that CTA membranes were capable of transporting even up to 90% of Zn(II) to the receiving phase, while PVC membranes have been inefficient and Zn(II) transport through them was negligible. The question to be answered is if the reason for such a difference in the transport behavior of the PIMs studied results from differences in their morphology and structure. The relation between the rate or efficiency of transport of various species through PIMs and their morphology has been already studied. Microstructure of the membranes containing various carriers determines the distribution of a carrier in the polymer matrix and finally affects the efficiency of ion transport, and is characterized by different techniques,7,10,15,26-32 e.g. atomic force microscopy (AFM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), contact angle measurements, impedance spectroscopy (IS). However, to the best of our knowledge, there are few reports (PVDF-based polymer inclusion membrane with Cyphos IL 104 as a carrier29,30) on the effect of the structure of PIMs containing phosphonium ILs on transport abilities of these membranes. As the membrane morphology can strongly affect the efficiency of the transport through it, this study was undertaken to determine the structure of PIMs containing trihexyl(tetradecyl)-phosphonium chloride (Cyphos IL 101), trihexyl(tetradecyl)phosphonium bis(2,4,4trimethylpentyl)phosphinate (Cyphos IL 104) or tributyl(tetradecyl)phosphonium chloride (Cyphos IL 167) as ion carriers by such techniques as AFM, SEM, FTIR, XRD, DSC, contact angle measurements, and to check a possible correlation between the morphology and structure of the membranes with the results of Zn(II) transport through them. Additionally, for the first time, Zn(II) diffusion coefficients are estimated for PIMs with various phosphonium IL carriers and stability studies of IL 104 containing PIMs are presented.



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2. EXPERIMENTAL 2.1. Reagents The following polymers were used as received: cellulose triacetate (CTA) (Fluka, Switzerland) and high molecular mass poly(vinyl chloride) (PVC) (Mw 80 000 g/mol; Mn 47 000 g/mol) (Sigma Aldrich, USA). Three phosphonium ionic liquids, i.e. trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101), trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104) and tributyl(tetradecyl)phosphonium chloride (Cyphos IL 167) supplied by Cytec Industry Inc. (USA) were applied as carriers for metal ions in PIMs. Chemical structures of the ILs are presented in Fig. 1. The plasticizer was o-nitrophenyl octyl ether (NPOE) (Sigma Aldrich). The density of NPOE was equal to 1.041 g/L. Dichloromethane (DCM) and tetrahydrofurane (THF) were used as received. Aqueous feed solutions contained 1.5.10-3 M Zn(II) (0.1 g/L), 0.58 M HCl, 5 M Cl- (NaCl was used to ensure a constant chloride content). 1 M H2SO4 was used as a receiving phase. The inorganic chemicals were of analytical grade and were purchased from POCh (Gliwice, Poland).

Cl-

C14H29 +

P H13C6

C6H13 C6H13

Trihexyl(tetradecyl)phosphonium chloride, Cyphos IL 101

C14H29

O

P+ C6H13

C6H13

-

CH3

H3C

O

CH3 CH3

P

H3C

H9C4

CH3

Trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, Cyphos IL 104

C14H29 P

CH3

H3C

C6H13

Cl-

+

C4H9 C4H9

Tributyl(tetradecyl)phosphonium chloride, Cyphos IL 167

Fig. 1. Structures of the phosphonium ionic liquids used as PIM carriers 2.2. Preparation of polymer inclusion membranes Solutions of CTA or PVC, the ion carriers (Cyphos IL 101, Cyphos IL 104 and Cyphos IL 167), and the plasticizer (NPOE) were dissolved in suitable solvents, i.e. DCM or THF for CTA or PVC, respectively. A portion of this solution was poured on a flat-bottomed glass Petri dishes (7.0 cm in diameter) and then covered with filter tissue. The organic solvent was allowed to 4 ACS Paragon Plus Environment

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evaporate over a period of 48 h. Finally, the membranes were carefully peeled from the glass ring and were soaked in deionized water for 24 h. The thickness of each membrane was measured by digital ultrameter (MG-401, ELMETRON, Poland) in at least 15 different locations of the membrane to calculate the average. 2.3. Transport experiments To transport Zn(II) ions across PIM, a sandwich type membrane module was used. Both aqueous phases (feed and receiving) were pumped with a peristaltic pump (ISMATEC), working at a speed of 20 mL/min throughout the experiment, from tanks containing the feed and receiving phases. The scheme of the system for Zn(II) transport is presented in our previous papers.17,25 The volumes of both phases were equal to 200 mL. The effective membrane area amounted to 15.9·10-4 m2. All experiments were performed at room temperature (22±1°C). Samples of 1 cm3 in volume of both aqueous phases were taken at regular time intervals. Zn(II) concentrations were analyzed with atomic absorption spectroscopy AAS (HITACHI Z-8200) at 213 nm in the airacetylene flame. 2.4. Characterization of PIMs The structures of blank CTA, PVC membranes and of PIMs were analyzed by means of wide angle X-ray scattering (WAXS) using Cu K radiation at 30 kV and 25 mA anode excitation. The X-ray diffraction (XRD) pattern was recorded for the angles from the range of 2theta=5-40° in a step of 0.04°/3 sec. Changes in the supermolecular structure of the polymer inclusion membranes were analyzed as a function of membrane composition. The sessile drop method was used to measure the contact angle of the prepared membranes. A 3 µL droplet of the feed phase was placed on the membrane surface and the contact angle was measured by Tracker (I.T. CONCEPT). The FT-IR spectra were acquired using Vertex 70 Spectrometer (Bruker Optics FT-IR) in the range of IR 400-4000 cm-1. AFM measurements were performed with Bruker Icon and NCLR (Nanosensors) cantilevers in tapping mode at room temperature (22±1°C). Images were analyzed with WSxM software.33 SEM images of the membrane samples were made using 6.0 kV scanning electron microscope Vega Tescan (cross sections) or Jeol 7001 TTLS (surface images). The thermal measurements of PIMs were carried out with differential scanning calorimeter Mettler DSC1 (Mettler Toledo). The samples (several pieces of membrane of the total mass of about 5 mg) were characterized in the 5 ACS Paragon Plus Environment


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temperature range from -80°C to 270°C, under nitrogen atmosphere at heating/cooling rate of 10°C/min. Viscosity measurements for carriers with NPOE mixutres were carried out using a rheometer (Rheotec RC30-CPS) with a cone-shaped geometry (C50-2) according to the procedure described elsewhere.34 2.5. Calculations The kinetics of transport through PIMs can be described by an equation analogous to the first order reaction to metal ion concentration: ln

c = −kt c0

(1)

where c0 (g/L) and c (g/L) are concentrations of metal ions in the feed at initial time and selected time, k is the rate constant (1/s), t is the time of transport (s).Values of the rate constant (k) are estimated from linear dependence of ln(c/c0) versus time. The transport parameters such as initial flux J0 (mol/s m2) and permeability coefficient were calculated according to the following equations: J0 =

V ⋅ k ⋅ c0 A

(2)

P=

V ⋅k A

(3)

where V is the volume of the aqueous phase equal to 2·10-4 m3, and A the membrane area equal to 15.9·10-4 m2. A parameter to quantify the amount of Zn(II) transported to the membrane at a given time is the percentage extraction (E), while the amount transferred to the receiving phase is described by the recovery factor (RF):

E=

c0 − c ⋅100% c0

(4)

cs ⋅100% c0

(5)

RF =

where cs (g/L) is the concentration of metal ions in the receiving solution after 48 h of membrane process.


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3. RESULTS AND DISCUSSION 3.1. Composition of the PIMs The content of the polymer matrix and carrier in PIM was changed to study the influence of their amount on the efficiency of Zn(II) transport through the membranes. The composition, thickness and permeability coefficient of the selected membranes used in Zn(II) transport (processes carried out for 48 or 55 hours) are given in Table 1.

Table 1. Composition and permeability coefficient characterizing Zn(II) transport across CTA and PVC membranes. Matrix Type

Carrier

NPOE

Thickness

P·106,

%

µm

m/s

0

0

24.4±1.3

1.06

60

40

0

33.0±3.5

1.95

75

20

5

30.4±1.6

4.76

Content, % 100

CTA

Type -

Content, %

65

IL

30

5

34.7±3.5

8.97

55

101

40

5

35.9±2.1

9.25

45

50

5

38.1±2.6

19.4

35

60

5

46.7±0.9

11.4

60

40

0

34.6±2.7

1.96

75

20

5

30.8±2.5

4.22

65

IL

30

5

35.6±1.9

5.68

55

104

40

5

40.5±0.9

5.84

45

50

5

44.3±2.4

18.7

35

60

5

49.7±0.6

10.8

60

40

0

24.3±0.8

1.44

20

5

19.7±1.8

1.28

30

5

22.7±1.6

2.75

40

5

26.8±0.7

6.41

50

5

27.0±1.2

20.2

75 65 55 45

IL 167



100 60 75 70 PVC

60 75 55

IL 101

IL 104

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0

0

88.0±.2.1

0.51

40

0

58.4±1.6

6.41

20

5

56.7±2.1

0.78

30

0

59.9±2.2

1.05

40

0

45.7±2.4

3.30

20

5

42.5±2.7

3.82

35

10

46.4±1.2

5.36

60

IL

40

0

43.1±0.8

2.07

75

167

20

5

47.7±1.8

0.94

The PIMs studied were prepared with various amounts of polymer matrix, carrier and plasticizers but the total mass of the components was kept constant. The membranes containing CTA as a polymer matrix were thinner than the PVC membranes, their average thickness was equal to 33 µm, while the PVC membranes were from 43 to 59 µm thick. The reason for this difference is the volume of the component solution used for membrane preparation: 6 and 8 mL for CTA and PVC-based membranes, respectively. Larger volume of component solutions used for preparation of PVC PIMs resulted from problems with thinner membranes which were too sticky and soft. Generally, it can be also indicated that with increasing content of a carrier in the membrane, PIMs become thicker. Few researchers also reported such phenomenon (at constant total mass of the components) – for example, the thickness of crown ether-containing PIMs increased with the quantity of the crown ether added and depended on the nature of the carrier.7 Some ambiguous differences in thickness of Aliquat 336-containing PIMs were reported by Wang et al.12 (81.7; 138 and 133 µm for 30; 40 and 50% A336, respectively). The values of permeability coefficients (Table 1) indicate that some CTA membranes transport Zn(II) much better than others, and the transport is enhanced by the increasing IL content (up to 50%). This phenomenon can be explained by better availability of the carrier and, thus, easier formation of Zn(II)-IL complexes. On the other hand, the viscosity of membrane phases increases also with the carrier concentration growth, thus, limiting diffusion of the Zn(II)IL complex through membranes of high IL content. Such limitation is likely to be responsible for a decrease in Zn(II) transport rate through the PIM containing 60% of IL 101 or IL 104. The best results of permeability coefficient were noted for PIMs containing 50% IL 167 (P=20.2·10-6 8 ACS Paragon Plus Environment

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m/s). PIMs containing 60% IL 167 appeared to be too soft and could not be used for transport of Zn(II) due to their damage (leaks were noted). In contrast, PVC membranes are less effective in Zn(II) transport than CTA ones. The highest value of permeability for PVC membranes is equal to 6.41·10-6 m/s, and decreases with increasing content of plasticizer in the membranes. These results of Zn(II) transport indicate differences between two types of polymer matrix used and composition of PIMs, therefore the membranes were fully characterized.

3.2. Characterization of PIMs 3.2.1. XRD measurements Characterization of PIMs morphology depending on their composition can bring important information to explain differences in transport through the membranes. The membranes were analyzed by X-ray diffraction to estimate the degree of crystallization of polymer material in the membranes. The XRD patterns for CTA and PVC-based PIMs are presented in Figs. 2 and 3.

450

400

350

Intensity

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

250

(a)

200

(b)

150

(c) 100

5

10

15

20

25

30

35

40

2 theta

Fig. 2. XRD patterns of the PIM containing a) CTA, b) 90% CTA, 10% NPOE, c) 55% CTA, 5% NPOE, 40% IL 101.



10 0

95

90

85

80

75

70

65

Intensity

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

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(c)

60

55

50

45

(b)

40

35

30

(a)

25

20

15

10

5

10

15

20

25

30

35

40

2 theta

Fig. 3. XRD patterns of the PIM containing a) PVC, b) 90% PVC, 10% NPOE, c) 55% PVC, 5% NPOE, 40% IL 101.

The XRD pattern of pure CTA shows two peaks at diffraction angles (2theta) of about 10 and 17° which is in agreement with the data given by other authors.35 These peaks are assigned to the crystalline domains of CTA. The intensity of these peaks decreases in the composite materials (PIMs), probably as a result of the increasing amount of amorphous compounds (NPOE and IL). The increase in the amorphousness of the PIMs related to the presence of NPOE and IL may be responsible for the difficulties in the nucleation process of CTA polymer. Moreover, for the three-component membrane (CTA-NPOE-IL), the peaks are shifted towards higher 2 theta values. It means that CTA chains are responsible for the observed decrease in the d-spacing of family of lattice planes as well as the rearrangement of the CTA chains. As follows from a comparison of the patterns for both polymer matrices, some crystallinity areas in CTA membranes and smaller spacing of lattice planes are likely to be responsible for better mobility of ions transported through CTA-based PIMs than PVC-based ones. PVC, as a typically amorphous polymer, is characterized by XRD pattern shown in Fig. 3. The influence of different modifying agents, e.g. TODGA (N,N,N’,N’-tetra-n-octyl diglycolamide),4 TOPO (trioctylphosphine oxide), DEHPA (bis(2-ethylhexyl)phosphoric acid),36,37 4,7,13,16,21,24-hexaoxa-1,10-diaza-bicyclo-[8.8.8]-hexacosane (222),38 or crown ethers (e.g. di-tert-butylbenzo-18-crown-6, DTBB18C6)39 on the loss of crystallinity of CTA has 10 ACS Paragon Plus Environment

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been observed also by other authors. It means that NPOE and phosphonium ILs dispersed in CTA hinder the crystallization. Arous et al.37,38 have emphasized that the amorphous structure of PIMs negates the hypothesis on the mechanism of ion successive jumping between carriers and complexing sites in the polymer structure. Thus, the presence of some crystalline domains in CTA membranes could support the fixed-site jumping mechanism of ion transport which seems to be more effective in comparison with slow diffusion in PVC amorphous membranes.

3.2.2. Differential scanning calorimetry DSC technique can be used to study chemical reactions and phase transitions in materials (e.g. crystallization, vaporization or glass transition). In this work, thermal measurements of PIMs were carried out only for CTA-based membranes because of a significantly higher permeability of this material than that of PVC-based PIMs. DSC measurements were performed in two cooling/heating cycles. In the first run, the sample was cooled to -80°C and kept at this temperature for 10 min, then heated to 270°C. In the second run, the sample was re-cooled to 80°C and again kept at -80°C for 10 min, then was re-heated to 270°C. DSC curves recorded on the first and second heating of exemplary CTA-based membranes are shown in Figs. 4 and 5, respectively. On the first heating the DSC curve revealed signals corresponding to vaporization of water, glass transition and cold crystallization. The large endothermic peak (with a maximum point at 72–74°C) is related to vaporization of water from CTA.40 The values of glass transition temperature Tg were determined from the curve recorded on the second heating (Fig. 5) as midpoint between onset and endset of the inflectional tangent. The Tg is by about 30°C lower than the temperature of cold crystallization Tc (Fig. 4).41 The presence of ionic liquid in the CTA-based PIMs caused a reduction in Tg of the material. Therefore, the mobility of the polymer chains increased because of the presence of ILs and NPOE in the CTA-based PIMs. It proves that IL101 and IL104 can act as plasticizers for CTA. This observation is consistent with the results obtained for CTA PIM containing quaternary ammonium salt by Pereira et al.23 and indicates that phosphonium cation can disrupt the CTA hydrogen bonded structure by strong dipole–dipole interactions with the polymer strands. Moreover, the addition of 5% NPOE in the presence of the IL results in only a slightly greater plasticization of the membrane. The endothermic peak at low temperatures (< -60°C) for PIMs is assigned to the glass transition of IL. 11 ACS Paragon Plus Environment


Heat flow, endo down

(e) o

TC=206 C o

TC=209 C

(d) (c)

o

TC=206 C

(b) o

TC=212 C

(a)

-80

o

TC=216 C

-40

0

40

80

120 160 200 240 280 o

T, C

Fig. 4. DSC thermograms – first heating of exemplary CTA-based membrane samples: (a) only CTA; (b) 60% CTA, 40% IL 104; (c) 55% CTA, 40% IL 104, 5% NPOE; (d) 70% CTA, 30% IL 101; (e) 65% CTA, 30% IL 101, 5% NPOE.

(e) o

Tg=175 C


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

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(d)

o

Tg=174 C

(c) o

Tg=175 C

(b)

o

Tg=178 C

(a)

o

Tg=183 C

-80

-40

0

40

80

120 160 200 240 280 o

T, C

Fig. 5. DSC thermograms - second heating of exemplary CTA-based membrane samples: (a) only CTA; (b) 60% CTA, 40% IL 104; (c) 55% CTA, 40% IL 104, 5% NPOE; (d) 70% CTA, 30% IL 101; (e) 65% CTA, 30% IL 101, 5% NPOE.


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Moreover, the cold crystallization temperature Tc of PIMs is lower than that of the pure CTA membrane (Fig. 4). The melting process of CTA occurs at temperatures higher than Tc. However, the melting point Tm could not be recorded in the temperature range tested. According to Kamide and Saito41, the Tm of CTA is 291ºC. For PIMs, an exothermic process is observed above Tc. The reasons for this phenomenon are probably the initial stage of decomposition of ILs and the exothermic reaction between the ionic liquid and NPOE.

3.2.3. FT-IR spectroscopy FT-IR spectroscopy is an important tool to investigate polymeric structure. It provides information about the interaction/complexation between various constituents in a polymeric film. The FT-IR spectra of neat CTA and IL 104 and CTA-NPOE are shown in Fig. 6 a. The spectra of CTA-NPOE-IL 104 membranes before and after Zn(II) transport are shown in Fig. 6 b. The spectra of PVC membranes are presented in Fig. 7. a) 100

80

%T

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


60

40

20

0 4000

3500

3000

2500

2000

1500

Wavenumber, cm

-1


1000

500


b) 100

%T

80

60

40

20

0 4000

3500

3000

2500

2000

1500

Wavenumber, cm

1000

500

-1

Fig. 6. FT-IR spectra of a) ( ̶ ) CTA, (- - -) IL 104, (· · ·) CTA-NPOE, b) ( ̶ ) 55% CTA, 40% IL 104, 5% NPOE (before the process of Zn(II) transport), (- - -) 55% CTA, 40% IL 104, 5% NPOE (after the process of Zn(II) transport).

a) 100

80

60

%T

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

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40

20

0 4000

3500

3000

2500

2000

1500

Wavenumber, cm

-1


1000

500

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b) 100

80

60

%T

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


40

20

0 4000

3500

3000

2500

2000

1500

Wavenumber, cm

1000

500

-1

Fig. 7. FT-IR spectra of a) ( ̶ ) PVC, (---) IL 104, (· · ·) PVC-NPOE, b) ( ̶ ) 75% PVC, 20% IL 104, 5% NPOE (before the process of Zn(II) transport), (- - -) 75% PVC, 20% IL 104, 5% NPOE (after the process of Zn(II) transport).

The main features of the spectra in Fig. 6 a and b are the absorption bands located around 3500-3300 cm-1 which can be attributed to the stretching vibrations of O-H groups in CTA. The absorption band of the O-H groups after addition of IL 104 to CTA-NPOE is deformed. The spectra before and after membrane extraction differ in the width of the bands attributed to the stretching vibrations of O-H groups. The bands around 3150-3050 and 2900 cm-1, very weak in the neat CTA spectra and strong after NPOE addition, are attributed to the stretching vibrations of C-H aromatic and aliphatic groups, respectively. According to Kaya et al.42 this change implies a strong bonding/connection between the polymer matrix and plasticizer. The band at around 1410 cm-1, is absent in the spectrum of CTA-NPOE because this band is attributed to the stretching vibrations the C-P group of IL 104. The absorption bands located around 1750 cm-1 and 1150 cm-1 correspond to the stretching modes of C=O groups, while these around 1470 cm-1 correspond to the stretching vibrations of C-N group. The less intense absorption bands at 1583 cm-1 are attributed to C=C bonds. The bands at 1531 and 1488 cm-1 correspond to the stretching modes of N=O and C=C bonds of NPOE, respectively, while the bands at 1192 and 1091 cm-1 are attributed to the C-O-C asymmetric and C-O-C symmetric vibrations, respectively. 15 ACS Paragon Plus Environment


In the spectrum of pure PVC (Fig. 7 a) the bands corresponding to C-Cl stretching appear at 840 and 700 cm-1 along with a strong band with a maximum at 1256 cm-1 which is attributed to CH2 wagging when the next C atom has a chlorine atom attached (-CH2Cl). The –CH2 stretching vibrations are observed at 2973 and 2898 cm-1. Moreover, the peaks at 1420 (scissors) and 1325 cm-1 (twisting) can be assigned to the –CH2 deformation modes. The C-H bands are observed at 944 cm-1 (trans CH wagging) and 691 cm-1 (cis CH wagging).36,43,44 The spectra of PVC PIMs almost do not change before and after Zn(II) extraction process (Fig. 7 b). Generally, it can be concluded that only weak physical interactions such as van der Waals ensure the mechanical stability of the membrane.39

3.2.4. SEM images The exemplary SEM images of PIMs containing CTA or PVC-IL 104 before and after the Zn(II) transport process are shown in Fig. 8.

a)

b)


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c)

d)

Fig. 8. SEM images of cross sections of PIMs containing: a) CTA-IL 104 before the process, b) CTA-IL 104 after the process, c) PVC-IL 104 before the process, d) PVC-IL 104 after the process.

The cross sections of PIMs containing CTA or PVC show a dense, nonporous and homogeneous structure at the magnification used. The images of membranes before and after Zn(II) transport processes indicate that there are no changes in the cross section resulting from the contact of the membranes with acidic aqueous solutions. In short, no change in the surface and cross section aspect after Zn(II) transport proves the stability of these membranes after one process.

3.2.5. Atomic force microscopy The AFM images of PIM containing CTA or PVC and phosphonium ILs studied are shown in Fig. 9.



Fig. 9. AFM images of PIMs containing CTA a) IL 101, c) IL 104 e) IL 167 and PVC b) IL 101, d) IL 104, f) IL 167. 18 ACS Paragon Plus Environment

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AFM images of PVC membrane show a smooth surface containing only small surface defects such as small and single cavities or nodules. Topography of the membranes made of CTA is more developed, but without irregular defects. Additionally, important differences in the surface roughness are noted. The membranes containing PVC as a polymer matrix show at least 50% less roughness than CTA – based membranes. It is noteworthy that CTA-based membranes have a wider range of height which evidences a non-homogeneous surface that can be conducive to effective transport of metal ions across the membranes containing CTA as a polymer matrix. For scanning scale of 3x3 µm, the difference in membrane surface heights ranges from 1.5 to 4 fold more than for PVC-based membranes. This increased roughness of the membrane results in enlargement of the area that can get in contact with the feed solution. The root-mean-square roughness (RMS) and mean roughness (RA) of PIMs were calculated as follows: −

(Z − Z ) 2 RMS = ∑ n N n=1 N

(6)

− N

Zn − Z

RA = ∑

(7)

N

n =1

where: N – is number of measurement points, Zn – the height of a single measuring point, −

Z – the height of the median plane.

Table 2. Roughness parameters for AFM images with scan size 3 µm of CTA and PVC membranes Carrier type

CTA

PVC

RMS, nm

RA, nm

RMS, nm

RA, nm

IL 101

4.53

3.50

1.58

1.17

IL 104

2.89

2.24

1.92

1.52

IL 167

3.55

2.74

0.84

0.67

Membranes of diverse surface structure show higher ability of ion transport than the more homogeneous, uniform ones.31 Roughness measurements indicate greater changes for CTA membranes than PVC matrix, the differences amount from several (for IL 167) to even 70% (for 19 ACS Paragon Plus Environment


IL 101) – Table 2. The roughness influence on transport properties of PIMs is consistent with the results of Zn(II) transport experiments that prove that CTA membranes better extract metal ions than the PVC-based ones.

Fig. 10. Comparison of phase contrast images for CTA membranes containing: a) 30% IL101, b) 30% IL 104, c) 40% IL 101, d) 40% IL 104. Color scale is 61, 68, 58, 56 deg respectively.

Phase contrast shown in Fig. 10 in the scanning range 500x500 nm reveals two mechanically different phases, represented by colors in the figure. There are some shapeless structures of ~15 nm to ~100 nm size surrounded with another material. Increase in the areas that are shaded darker is related to the increasing amounts of the ion carriers. This suggests that there


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is no complete mixing of the components and darker regions probably represent ion carriers. Membranes containing IL 104 are less homogenous in comparison to those containing IL101.

3.2.6. Contact angle measurements Contact angle measurements were made to investigate the hydrophilicity of the material surfaces. In this study, contact angle measurements were made at room temperature for PIMs containing CTA and PVC, carriers and NPOE or without plasticizer, before and after processes of Zn(II) transport. The results are shown in Table 3.

Table 3. Effect of the membrane composition on the contact angle of the PIMs before and after Zn(II) transport processes; membrane composition: 55% CTA, 40% IL, 5% NPOE; 60% CTA, 40% IL and 75% PVC, 20% IL, 5% NPOE; 60% PVC, 40% IL. Matrix

Carrier

Contact angle, °

NPOE, %

Before process

After process

5

38.3±2.1

48.6±1.4

0

40.1±1.6

48.4±2.3

5

36.9±1.2

45.6±2.6

0

41.5±1.4

48.9±1.1

5

37.7±0.7

43.2±1.3

0

39.6±0.8

48.7±2.4

5

53.2±1.7

59.8±2.8

0

57.7±1.9

60.3±2.2

5

48.3±3.1

55.7±2.5

0

53.1±1.5

59.7±2.4

5

47.8±3.3

52.2±2.2

0

49.4±2.4

54.8±1.7

Cyphos IL 101

CTA

Cyphos IL 104

Cyphos IL 167

Cyphos IL 101

PVC

Cyphos IL 104

Cyphos IL 167

For the membranes containing CTA and the plasticizer values of contact angle before transport processes are close to 38°, for PVC membranes these values are higher and reach values 21 ACS Paragon Plus Environment


close to 50°. Values of contact angles of PIMs without NPOE are higher than those of PIMs containing this plasticizer. It means that the presence of the plasticizer in the membrane improves wettability of the surface which results in better accessibility of the PIM surface to the hydrophilic chlorocomplexes of Zn(II). Though the ILs studied vary with anions (IL 101 and IL 104) or with cations (IL 101 and IL 167), the chemical structure of the carriers does not influence the wettability of PIMs because of small differences in hydrophilicity of these ILs (expressed as water solubility in the ILs).45 The obtained results show that both types of the polymer matrix are hydrophilic, however the PVC membranes are more hydrophobic than the CTA ones. It should be mentioned that the solid surface is considered as hydrophobic when the contact angle values are greater than 90°. Thus, the PIMs studied, generally, show hydrophilic character of the surface. However, the contact angle values after the process of Zn(II) transport, increase (Table 3), which means that the PIM surfaces are more hydrophobic. The change in surface wettability can result from slow degradation of the membranes in contact with acidic solutions, though SEM results (Fig. 8) do not show significant changes in the surface and cross section images.

3.3. Zn(II) transport through PIMs 3.3.1. The effect of matrix type on Zn(II) transport efficiency Transport experiments were carried out for the PIMs characterized previously to find if their morphology influences the transport properties. Extraction percentage and recovery factor presented in Fig. 11 illustrate the efficiency of Zn(II) transport to the membrane and to the receiving phase, respectively. Exemplary concentration profiles of Zn(II) in the feed and receiving phase are shown in Supporting Information (Figs. S1-S3). Higher values of extraction percentage and recovery factor were obtained for CTA –based membranes than for PVC ones. The recovery factor of Zn(II) with CTA-based PIMs ranged from 34 to 75% (membranes without NPOE) and from 48 to 90% (membranes containing 5% NPOE). It was shown that addition of this plasticizer increased the effectiveness of Zn(II) transport across PIMs, enhancing the mobility of the transported species by reduction of intermolecular attractive forces between polymer chains46 and increasing wettability of the surface. For PVC membranes the RF values were lower than 3%. It means that even if some Zn(II) was extracted to the


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membrane it was accumulated there, and did not strip to the receiving phase. Therefore, henceforth Zn(II) transport was studied in detail only for CTA-based membranes.

a)

b)

PVC-IL167

PVC-IL167

CTA-IL167

CTA-IL167

PVC-IL104

PVC-IL104

CTA-IL104

CTA-IL104

PVC-IL101

PVC-IL101

CTA-IL101

CTA-IL101 0

20

40

60

80

0

100

20

40

60

80

100

E or RF, %

E or RF, %

Fig. 11. The influence of the type of polymer matrix on the extraction efficiency (■) and recovery factor (□) of Zn(II) through membranes: a) without plasticizer (60% matrix, 40% IL) and b) containing 5% of plasticizer.

Differences in the structure and surface morphology related to the type of the base polymer are consistent with various Zn(II) transport efficiencies. According to some authors PVC membranes containing TODGA (N,N,N’,N’-tetraoctyl-3-oxapentanediamide) as a carrier could effectively transport As(V), Am(III).32,47 Though Kogelnig et al.22 described successful transport of Zn(II) from 5 M HCl through IL 101-PVC membranes to 1 M sulfuric acid, we were not able to achieve such good results. The differences in membrane permeability can result from the type of PVC used, which was not defined in Kogelnig’s work. The weak permeability of the PVC membranes used in this work can be attributed to the differences in the crystallinity of CTA membranes and amorphous characteristics of PVC (XRD patterns presented in Figs. 2 and 3). Additionally, non-homogeneity of Aliquat 336-PVC membranes can be a helpful hint to explain weak extraction of Zn(II) by the IL-PVC membranes.48 Abdul-Halim et al.48 have indicated that there are two individual phases, i.e. PVC rich and Aliquat 336 rich, in PVC/Aliquat 336 PIMs. To ensure ion transport through such



membranes, continuous channels of the carrier phase across the PIM are necessary (over 30 wt.% Aliquat 336). In the PVC-based PIMs tested in our work, despite more than 30% IL content in the membrane was used, Zn(II) extraction did not exceed 35%. On the other hand, Rahman and Brazel49 have reported that phosphonium ILs were effective PVC plasticizers. However, they noticed for Cyphos IL 101 no migration from flexible plastic to unplasticized PVC. It resulted probably from incorporating IL into the polymer network owing to chloride anion interactions. In the light of the results in the present work, the study of Rahman and Brazel suggests that Cyphos IL 101 can be bound strongly enough in PVC matrix to hinder Zn(II) transport across the PIM.

3.3.2. The effect of plasticizer amount on permeability coefficient To understand the effect of plasticizer content on the Zn(II) transport, membranes with a constant amount of carriers (40%) and different content of NPOE were prepared (Fig. 12).

10 8

P 106, m/s

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 57

6 4 2 0 0

5

10

15

20

NPOE, %

Fig. 12. Permeability coefficients of CTA-based PIMs containing various amounts of NPOE; IL 101 (■), IL 104 (●), IL 167 (▲).

The main role of a plasticizer is to separate polymer chains making them less rigid, thus, increasing diffusion coefficients. Also, the plasticizer improves the solubility of the extracted species in the membrane. The results presented in Fig. 12 indicated that the


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permeability coefficient of Zn(II) reached maximum at 5% plasticizer content. Further, in spite of higher NPOE content, transport of Zn(II) decreases. Nghiem et al.1 have indicated that the excess of plasticizer could migrate to the membrane surface and form a film there, which would form a barrier to the transport of metal ions across the membrane. However, Gyves et al.50 have demonstrated that the decrease in permeability with increasing plasticizer content in membranes was related to an increment in the viscosity of the membrane that limited the plasticization effect and also carrier movement. Values of viscosity measured for the mixtures of phosphonium ILs and NPOE (shown in Supporting Information, Table S1) do not indicate increase in this parameter with addition of the plasticizer (5, 10 and 20 wt%). Quite the contrary, viscosity of the mixtures decreases with increasing content of the plasticizer. On the one hand, presence of the plasticizer is necessary, as Fontas et al.51 emphasized, for formation of liquid micro-domains of the carrier solvated by the plasticizer which facilitates transport of metal ions. On the other hand, the increase in plasticizer content could enhance interactions between the plasticizer and the polymer chains reducing the occurrence of the liquid domains.51 Additional effect on the ion transport in such micro-domains can occur due to supramolecular aggregation of ILs, and formation of polar and nonpolar domains of various viscosity.52,53 Some crystallinity areas in CTA membranes and smaller spacing of lattice planes evidenced from XRD measurements (Fig. 2) support the idea of formation of liquid micro-domains in which ions transported through CTA-based PIMs are more mobile than in amorphous PVC-based membranes. Rodriguez de San Miguel et al.46 emphasized that in such micro-domains the transport of ions could be realized by various mechanisms (chained-carrier with reduced mobility, carrier-diffusion, and mobile-site jumping).

3.3.3. The effect of membrane thickness Transport of Zn(II) through the CTA-based PIMs of four different thicknesses was investigated, as the thickness of the membrane can significantly affect transport of the species regardless of the mechanism of transport (carrier-diffusion or fixed-site jumping). 37,38 The contents of the carriers and the plasticizer were equal to 40% and 5%, respectively. The obtained results show that the initial flux of Zn(II) decreases as the membrane thickness increases (Fig. 13) as the path of diffusion is longer. It is in accordance with the first Fick’s law:



J = − D0

∆c ∆x

(8)

where D0 is the diffusion coefficient.

3.0

2

2.5 2.0 1.5

5

1/(J0 10 ), mol/s m

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 57

1.0 0.5 0.0 15

20

25

30

35

40

45

50

55

6

d 10 , m

Fig. 13. The influence of membrane thickness on the initial flux of Zn(II) through the CTA-based PIMs containing 40% () IL 101, () IL 104, and () IL 167 and 5% NPOE. The inverse relationship between the initial flux of Zn(II) and membrane thickness (Fig. 13) is found to be a straight line with the correlation coefficients equal to 0.9989 (Cyphos IL 101), 0.9983 (Cyphos IL 104) and 0.9973 (Cyphos IL 167). The linear relationship confirms the existence of a rate-limiting transport due to the diffusion of a metal complex across the membranes. The relations shown in Fig. 13 are independent of the type of the IL used. Different molecular masses and relatively high viscosities of the ILs should greatly influence molecular diffusion, and lack of such effect could support fixed-site jumping mechanism. However, according to ion-exchange reactions of Zn(II) extraction given elsewhere,17,18,25 this is the phosphonium cation that is involved in reaction with anionic chlorocomplexes of Zn(II). Thus, there is no significant effect of the chemical structure of the ILs used. Additional argument supporting rather carrier-diffusion are observations of Fontas et al.51 that in PIMs the carriers are not bound chemically and as a consequence no typical fixed-site jumping transport occurred in such membranes. 26 ACS Paragon Plus Environment

Page 27 of 57

3.3.4. Stability of PIMs Finally, not only efficient transport but also stability of PIMs is important for their further application. In this study, extraction efficiency and recovery factor have been used as the parameters to evaluate the lifetime of PIMs.2 The exemplary transport experiments with the same membrane containing 55% CTA, 5% NPOE, 40% IL 104 were repeated five times (each cycle carried out 48 h), each time both aqueous phases (the feed and receiving) were renewed.

100

80

E or RF, %

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


60

40

20

0 I

II

III

IV

V

Number of cycle

Fig. 14. Stability of the PIM during the Zn(II) transport; membrane composition: 55% CTA, 40% IL 104, 5% NPOE; extraction efficiency (■), recovery factor (□).

The results presented in Fig. 14 indicate that the extraction efficiency and recovery factor of Zn(II) decrease by about 20% after five cycles of PIM use. The values of E and RF are equal to more than 95% and near 80%, respectively, in the first cycle. After the fifth cycle, the extraction efficiency and recovery factor are equal to 75% and 65%. The decrease in stability of the membrane may be caused by the partitioning of the carrier between the membrane and the aqueous solution.15 Some changes in PIM visible in the FT-IR spectra and SEM images after the first and the fifth cycle (Fig. 15) support the idea that the surface of the membranes is susceptible to the contact with acidic aqueous solutions.



a)

b)

c)

1 µm

10 µm

1 µm

d)

100

80

%T

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 57

60 40

20

0 4000

3500

3000

2500

2000

Wavenumber, cm

1500

1000

500

-1

Fig. 15. The SEM images of CTA-NPOE-IL 104 (a) before process, (b) after the 1st cycle, (c) after the 5th cycle, and (d) FT-IR spectra of (- - -) CTA-NPOE-IL 104 (after the 1st cycle), (· · ·) CTA-NPOE-IL 104 (after the 5th cycle). The SEM images of the PIMs before Zn(II) transport and after the processes (Fig. 15 a-c) show that after the fifth cycle some aggregates are present on the membrane surface. Compared with the PIM image after the first cycle, in which small nodules are visible, the image of the five times used membrane suggests that some components of the PIM have been washed out of the membrane surface. Also the FT-IR spectra (Fig. 15 d) confirm that there are changes in the chemical composition after the fifth cycle, resulting from loss of the IL carrier and the plasticizer. 28 ACS Paragon Plus Environment

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It is indicated for instance by the reduction in intensity of the band at 810 cm-1, assigned to IL 104 or in the intensity of the bands at 1470, 1488 and 1531 cm-1 attributed to C-N, N=O and C=C vibrations, respectively. In short, a decrease in the peaks intensity supports the SEM observations on reduction in the amount of some components, probably the plasticizer and IL. However, the loss of some components after five cycles is not significant because the membrane is still efficient in Zn(II) transport. To summarize, the reproducibility of the Zn(II) transport results with recycled PIMs might be increased by dissolving the membranes after some cycles and casting them once again, as proposed by Lamb et al.54

4. CONCLUSIONS Trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101), trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104) and tributyl(tetradecyl)phosphonium chloride (Cyphos IL 167) are proven to be effective Zn(II) ion carriers in CTA-based polymer inclusion membranes (PIM). A significant influence of polymer matrix type on the efficiency of Zn(II) transport is shown. The same phosphonium IL applied in PVC-based PIMs, is ineffective in transferring Zn(II) from the feed to the receiving phase. The techniques of material characterization have revealed some relationships between the composition of the membranes containing phosphonium ILs, their morphology and Zn(II) transport across them. Both CTA and PVC-based membranes are dense and non-porous, and the transport across these PIMs follows rather the diffusion mechanism than that involving successive fixed-site jumping between carriers. Though, it is difficult to indicate an exact mechanism of transport (chained-carrier with reduced mobility, carrier-diffusion, or mobile-site jumping), the results of XRD and DSC measurement, and transport data support the idea of formation of liquid micro-domains in which ions transported through CTA-based PIMs are more mobile than in amorphous PVC-based membranes. It is also stated that the difference in the effectiveness of the two polymer matrices results from their chemical character (polar O-H groups of CTA, non-polar PVC) and morphology of the PIMs. CTA-based membranes are more hydrophilic and their surface is more developed and rough that allows better accessibility of metal ions to the membrane. PVC-based PIMs are more hydrophobic, have less diverse and accessible surface and are completely amorphous.



Finally, PIMs stability after several cycles of transport processes is good as evidenced by small changes in FT-IR spectra and SEM images of the membranes resulting from slow washing out probably of the plasticizer and IL. To summarize, CTA-based PIMs show the properties of a good membrane material and offer a possibility of regeneration after several cycles of the use. Regeneration of the PIMs by dissolving them and casting them once again will be investigated in the future.

ACKNOWLEDGEMENTS The authors thank the NanoBioMedical Centre in Poznan for access to AFM equipment. The authors are grateful to Dr Karolina Wieszczycka for her help in FTIR analysis, and Dr Michał Niemczak for viscosity measurements. Monika Baczynska wishes to acknowledge the financial support within the project "Engineer of the Future. Improving the didactic potential of the Poznan University of Technology" POKL.04.03.00-00-259/12, implemented within the Human Capital Operational Programme, cofinanced by the European Union within the European Social Fund. This work was supported by the 03/32/DS-PB/0701 grant.

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Schow, A.J.; Peterson, R.T.; Lamb, J.D. Polymer Inclusion Membranes Containing Macrocyclic Carriers for Use in Cation Separations. J. Membr. Sci. 1996, 111, 291.

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Mahanty, B.N.; Raut, D.R.; Mohapatra, P.K.; Das, D.K.; Behere, P.G.; Afzal, Md. Comparative Evaluation of Actinide Ion Uptake by Polymer Inclusion Membranes Containing TODGA as the Carrier Extractant. J. Hazard. Mater. 2014, 275, 146.


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Supporting Information Additional concentration profiles of Zn(II) transport through CTA or PVC-based membranes studied in the paper and table with results of viscosity measurements of ILs and NPOE mixtures.


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Graphical abstract



Page 38 of 57

C14H29 P+

Cl-

C14H29 +

P H13C6

C6H13 C6H13

C6H13

C6H13

-

P

C14H29 +

P

CH3

H9 C4

CH3

C4 H9 C 4 H9

CH3

H3C H3C

Trihexyl(tetradecyl)phosphonium chloride, Cyphos IL 101

Cl-

CH3

H3C

O O

C6H13

CH3

Trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, Cyphos IL 104

Tributyl(tetradecyl)phosphonium chloride, Cyphos IL 167

Fig. 1. Structures of the phosphonium ionic liquids used as PIM carriers

ACS Paragon Plus Environment

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Fig. 2. 292x203mm (300 x 300 DPI)



Fig. 3. 292x203mm (300 x 300 DPI)


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Fig. 4. 204x143mm (300 x 300 DPI)



Fig. 5. 204x143mm (300 x 300 DPI)


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Fig. 6a. 206x145mm (300 x 300 DPI)



Fig. 6b. 206x145mm (300 x 300 DPI)


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Fig. 7a. 206x145mm (300 x 300 DPI)



Fig. 7b. 206x145mm (300 x 300 DPI)


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a)

b)

c)

d)

Fig. 7a-d



Fig. 8a-f


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Fig. 9a-d



Fig. 11a. 209x148mm (300 x 300 DPI)


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Fig. 11b. 201x142mm (300 x 300 DPI)



Fig. 12. 201x142mm (300 x 300 DPI)


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Fig. 13. 209x148mm (300 x 300 DPI)



Fig. 14. 206x145mm (300 x 300 DPI)


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a)

10 μm

b)

1 μm

c)

1 μm

Fig. 14a-c



Fig. 15d. 206x145mm (300 x 300 DPI)


Page 56 of 57

Page 57 of 57


PIM

XRD Intensity

SEM

CTA CTA+NPOE CTA+NPOE+IL 101

100

FTIR 5

80 60

10

15

before transport

20

25

30

35

40

2 theta

%T 40 20

after transport

0 4000 3500 3000 2500 2000 1500 1000 -1

Wavenumber, cm

AFM

DSC

500


1 2 Polymer 3 Inclusion 4 Membrane 5 6 7 8 9 10 11 12[R3R’P+][A-] 13 14 15 16 17 18 19 20 21 Zn(II) 22transport 23 24 25 26 27

o

TC=206 C o

TC=209 C o

TC=206 C o

TC=212 C o

TC=216 C

-80 -40

0


40

80 120 160 200 240 280 o

T, C

Characterization of Polymer Inclusion Membranes (PIMs) Containing

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