Sulfonic Membrane Sorption and Permeation Properties

Article pubs.acs.org/JPCB

Sulfonic Membrane Sorption and Permeation Properties: Complementary Approaches to Select a Membrane for Pervaporation C. Chappey,*,† K. Fatyeyeva,† E. Rynkowska,†,‡ W. Kujawski,‡ L. Karpenko-Jereb,§ A.-M. Kelterer,∥ and S. Marais† †

Normandie Univ, UNIROUEN, INSA Rouen, CNRS, PBS, 76000 Rouen, France Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7, Gagarina Street, 87-100 Toruń, Poland § Institute of Electronic Sensor Systems, Graz University of Technology, Inffeldgasse 10/II, 8010 Graz, Austria ∥ Institute of Physical and Theoretical Chemistry, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria ‡

ABSTRACT: In this contribution, the physical and chemical properties of the dense sulfonic membrane IonClad R4010 in the lithium form were studied to evaluate its potential application in pervaporation. To develop new membrane materials, it is necessary to know the influence of the membrane structure on the membrane equilibrium and transport properties. For this purpose, the sorption and permeation measurements of water and methanol in the liquid and vapor states were performed and correlated to the ion pairs/ solvent interactions analyzed by the infrared spectroscopy. The IonClad R4010 equilibrium and transport properties were found to be quite different depending on the permeant nature. The sorption and diffusion behavior of water and methanol was well described by means of the type II sorption model (BET theory). The swelling capacity of the IonClad R4010 membrane in methanol was found to be much lower than that in liquid water. In contrast to methanol, the total dissociation of the ion pairs in the IonClad R4010 membrane was obtained in the presence of water but only at high activity (∼0.8). Besides, the dispersion of the water molecules in the membrane was found to be homogeneous. The infrared spectroscopy results revealed that the methanol molecules had weaker interactions with the sulfonic groups of IonClad R4010 in agreement with the sorption data. The permeation properties were investigated by means of the sweeping gas and gravimetric methods in order to evaluate the membrane performance for pervaporation. The permeation results are in accordance with those obtained by sorption, thus confirming the complementariness of the two approaches. istics,11,12 namely, the small difference in the molecular size and low relative volatility (Table 1). Many polymers are extensively used in the PV processes and are of particular interest because of their relatively easy processability and low cost. However, their moderate permselectivity and swelling properties are major drawbacks for the practical application. The poly(acrylic acid), poly(vinyl alcohol), chitosan, and sodium alginate were the first generation of the hydrophilic PV membranes used for the dehydration of alcohols.13−16 However, their low mechanical properties and the excessive swelling in water encouraged development of the new polymer membranes with a higher water selectivity but also with improved thermal, mechanical, and chemical stability. The ion-exchange membranes have started to be frequently used in the pervaporation process. The advantage of the ion-

1. INTRODUCTION Pervaporation (PV) is known to be an effective and promising technology for the removal of organics from aqueous streams, for the separation of different mixtures of organic compounds with similar boiling points or azeotropic mixtures and for the dehydration of organic solvents, such as alcohols, ethers, esters, and ketones.1−3 This membrane separation process is used in many industrial fields such as biotechnology, food industry, and the chemical and petrochemical industry.4−6 The dehydration of alcohols is one of the most studied PV separation processes.4 Numerous papers related to the ethanol/water and isopropanol/water separations can be found in the literature, but only a few of them deal with the methanol/water separation.7−10 The interest in methanol is multiple. It is caused by the production of formaldehyde, acetic acid, and methyl tert-butyl ether. Methanol is also an intermediate product that can be used in the synthesis of the resins, foams, and plastics. Moreover, it is one of the most promising fuels for the fuel cells. However, it is a challenge for researchers to separate the methanol/water mixture, because of the solvents’ relatively similar character© 2017 American Chemical Society

Received: June 27, 2017 Revised: August 7, 2017 Published: August 9, 2017 8523

DOI: 10.1021/acs.jpcb.7b06305 J. Phys. Chem. B 2017, 121, 8523−8538

Article

The Journal of Physical Chemistry B Table 1. Physical and Chemical Characteristics of Water and Methanol solvent

molecular formula

molecular weight Msol (g/mol)

density ρS (g/cm3)

boiling temperature Tb (°C)

Hansen’s solubility parameter11 δ (MPa0.5)

dielectric constant12 ε at 25 °C

water methanol

H2O CH3OH

18 32

1.00 0.79

100 64.7

47.8 29.4

78.5 32.6

Table 2. Characteristics of the IonClad−Li+ Membrane

exchange membrane use is based on the balance between the membrane hydrophobicity and hydrophilicity, which can be rather easily controlled by the membrane structure. Since during pervaporation the interactions exist not only between the components of the liquid mixture but also between these components and the membrane material, the selectivity and transport properties of the ion-exchange membrane depend on (i) the nature of a polymeric backbone,17 (ii) the kind of ionexchange sites, and (iii) the kind of a counterion.18−20 In addition, the ion-exchange membranes present the strong interaction between water and the membrane ionic group and rich hydrophilic properties, thus enhancing the membrane permselectivity in the separation process. When exposed to water, most of the ion-exchange membranes undergo phase separation into the hydrophilic and hydrophobic domains. For example, the presence of the pendant chains terminated by the hydrophilic ion-exchange groups (i.e., the SO3H group) on the hydrophobic backbone causes the cluster vapor segregation within the membrane far from the polymer backbone.21 Therefore, it is clear that the hydrophobic/hydrophilic separation balance is the key parameter to develop the efficient PV membrane. In the case of the sulfonic ion-exchange membranes such as Nafion, it was shown that there is a trade-off between the selectivity and permeation properties if the alkali metal ions are used as the counterions.22 This fact was explained by the decreasing value of the solvation number of the counterion when the ionic form changed from lithium to potassium. For the membranes with the carboxylic ion-exchange sites, it was found that both the selectivity and permeability increased when the ionic form changed from lithium to potassium. Some chemical grafting reactions such as the radiationinduced grafting process allowed the separation factor to be enhanced and the membrane chemical resistance to be improved.23−25 Almost all grafted PV membranes are of the hydrophilic nature, and thus, they were found to be suitable for the removal of water from the organic mixtures.26 The hydrophilic IonClad R4010 membrane is composed of poly(styrene sulfonic acid) grafted to the polytetrafluoroethylene (PTFE)/perfluoropropylene copolymer (Figure 1). The hydrophobic fluorinated backbone provides the chemical, mechanical, and thermal resistance, whereas the grafted ionomer ensures a high ion-exchange capacity (IEC) (Table 2). The first major study of the IonClad membranes was provided by Tricoli et al.27 The proton conductivity as well as the methanol permeability of two IonClad membranes (R1010

thickness (μm) dry state wet state IEC (mmol/g) density ρp (g/cm3)

63 ± 3 71 ± 3 1.5028 1.88

and R4010) were compared with those of Nafion 117 in the temperature range 20−60 °C, and the obtained results showed that the IonClad membranes were less permeable (4 times lower) than the Nafion membrane, thus making them potential membranes for the direct methanol fuel cell (DMFC) application. The low methanol permeability of IonClad was used by Kujawski et al. to study the cationic IonClad membranes loaded with different counterions in contact with the alcohol vapors.28 It was found that ion pairs in the IonClad membrane remained undissociated in contact with pure alcohol. Also, the molecular modeling study showed that the cation-exchange membranes such as Nafion, IonClad, and M3 (a virtual ionomer, modeled with the same polymer matrix as IonClad (Figure 1) but with the carboxyl functional groups instead of the sulfonic groups) in the Li+ ionic form did not have the same interactions with water and methanol.29 The modelization has showed that the dissociation of the ion pairs occurs at the hydration level of seven water molecules in the case of the Nafion membrane, whereas no dissociation of the functional groups is observed for the IonClad and M3 membranes in the whole investigated range of the hydration level. During the contact with methanol, the solvation does not lead to any dissociation of the functional groups for all studied membranes. In the case of the binary mixtures, the dissociation of the ion pairs for the Nafion membrane is observed during the contact with water− and methanol−nonpolar binary mixtures, whereas the ion dissociation occurs only in the presence of water for the IonClad membrane.30 The enhanced dissociation of the carboxylic groups in the potassium form is the origin of the improved transport properties. It was also demonstrated that the diffusion coefficient of t-butanol was higher for the IonClad R4010 membrane in the potassium form compared to the lithium form and the electrostatic field created by the lithium ion was twice stronger than that created by the potassium cation. However, the PV flux was lower in the case of the sulfonic ion-exchange membrane in the potassium form than that in the lithium one.13 In the present paper, the choice of the lithium counterion is based on the influence of the nature of counterions on the formation of the water clusters, as it is shown that the smaller the counterion, the larger its solvation energy and the higher its mobility.15 Generally, the water cluster formation is easier when the counterion is H+ than when it is Li+. The obtained results of Jalani et al. clearly show the exception from the rule in the case of the methanol sorption, as both H+ and Li+ forms of the Nafion membrane exhibit similar sorption behavior.31

Figure 1. Chemical structure of the IonClad−Li+ membrane. 8524


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The Journal of Physical Chemistry B

such as swelling and clustering, which depend on the morphology and the membrane structure. The analysis of the experimental data obtained by the gravimetric and gas sweeping permeation measurements reveals the role played by the water molecules, namely, the plasticization effect.

The choice of an appropriate material as a PV membrane is crucial for the success of the whole process. The ideal membrane should possess several features such as excellent sorption capacity and good selectivity, high productivity, and high chemical and mechanical resistance, which ensure the high performance and the long-term stability. In this work, a comprehensive study of the IonClad R4010 membrane was performed in order to obtain a complete transport behavior; i.e., both permeation and sorption measurements for the whole activity range were realized, and the water and methanol diffusion mechanism was then deduced. In the dense polymer membranes, where the solution− diffusion mechanism is usually applied, the transport of the water or organic vapors can be investigated by the sorption, diffusion, and permeation measurements. Numerous studies of the diffusion and also the sorption and permeation of the organic components in various ion-exchange membranes can be found in the literature.32,33 Many authors are unanimous in affirming that the liquid or vapor solvent sorption measurements provide essential information to the understanding of the transport of the small molecules in the membrane despite the fact that the samples swollen in the pure liquid solvent reveal a higher mass gain than those in contact with the corresponding saturated vapor phase.21 Maldonado et al. have shown that the membrane thermal pretreatment greatly influences the sorption properties as the water uptake is higher for the membrane that is not heat pretreated before the sorption measurement.34 In addition, the sorption measurements allow determining the membrane selectivity and the membrane structural properties (i.e., the type of ion-exchange group, its ionic strength, and the type of counterion) from the membrane equilibrium and transport properties and this is irrespective of the intended application.35−37 However, a better insight in the membrane transport phenomena should be valuable for the development of more efficient and more selective membranes. The selective solvent sorption is used to determine the solvent permeability through the membrane, which is a fundamental parameter for the PV process. The sorption and permeation measurements allow determining separately the solubility, diffusion, and permeability coefficients. For the same vapor/polymer system, different behaviors can take place according to the sorption or permeation process used. The boundary conditions for the sorption and permeation measurements are different, as the polymer material is not in the same experimental conditions: an equilibrium state is obtained in the case of the sorption, whereas a stationary flux is reached for the permeation. In the present study, the correlation between the physical and chemical properties and the solvent sorption behavior of the IonClad R4010 membrane was established to determine the key factors influencing the selectivity. For this purpose, the state of the ion pairs (sulfonic groups and counterion) in the presence of the polar solvents (water and methanol) was analyzed by means of the swelling measurements. The solvation capacity of sulfonic ion-exchange groups as a function of the solvent used was investigated by describing the sorption isotherms by the Park model and by analyzing the infrared spectra performed at different relative humidities. In addition, the experimental sorption and permeation data were compared in order to understand the influence of the polymer structure on the solvent transport mechanism. The water and methanol diffusion coefficients were determined as a function of the solvent concentration to investigate the complex phenomena,

2. EXPERIMENTAL SECTION 2.1. Materials. Extra pure methanol (CH3OH, 99%, 0.2% max water) and phosphorus pentoxide (P2O5, 98%, extra pure) were purchased from Acros Organics, and hydrochloric acid (HCl, 37% ACS agent) and lithium hydroxide (LiOH, 98% reagent grade) were provided by Sigma-Aldrich. The saturated salt solutions were prepared with potassium carbonate sesquihydrate (Alfa Aesar) for 44% relative humidity (RH) at 20 °C and sodium chloride (Analar Normapur VWR) for 75.5% RH at 20 °C. Ultrapure water (H2O) was deionized by the Milli-Q process (18.2 MΩ/cm, Millipore). Glycerol (87%, GR for analysis, Merck) and diiodomethane (ReagentPlus, 99%, Sigma-Aldrich) were used as received. The dense IonClad R4010 membrane is produced by Pall Corporation (USA). The chemical structure and main characteristics of the IonClad R4010 membrane are presented in Figure 1 and Table 2, respectively. To facilitate the reading, the IonClad R4010 membrane is labeled as IonClad throughout the text. 2.2. Membrane Activation. The pristine membrane was annealed in hot water (80 °C) for 6 h. Thereafter, the membrane was immersed in 4 M hydrochloric acid solution for 24 h to exchange all counterions for the hydrogen form. Then, the membrane was washed with water up to neutral pH, and finally, the membrane was boiled for 24 h in 1 M solution of lithium hydroxide to obtain the corresponding cationic form. Before each measurement, the membrane was washed with water to remove the electrolyte excess and dried in a vacuum overnight at 110 °C. 2.3. Contact Angle Measurements and Surface Free Energy (SFE) Determination. The hydrophilic/hydrophobic character and the surface free energy (SFE) of the IonClad membrane were studied by means of the contact angle measurements using a Multiskop apparatus (Optrel, Germany) at ambient temperature (23 ± 1 °C) and humidity level (43 ± 4%). A dangling drop of 3 μL of a chosen probe liquid was carefully deposited on the IonClad membrane surface with a needle syringe. The contact angle θ was measured within 3 s after the drop deposition on the membrane surface by the sessile drop method with the help of CAM software. The contact angle measurements were performed for the dry and wet membrane states. For the dry state, the membrane was stored in a desiccator over P2O5 under a vacuum for 24 h. For the wet state, the membranes were placed in the different RH atmosphere (potassium carbonate sesquihydrate for 44% RH and sodium chloride for 75.5% RH) for 2 weeks. Three probe liquids (L) of different polarity were used water, glycerol, and diiodomethane. For each probe liquid, three drops were deposited uniformly on the membrane surface and the mean value of the contact angle was calculated. The contact angle measured by the sessile drop method can be described by Young’s equation (eq 1) assuming that the surface is smooth and homogeneous γSV = γSL + γLV cos θ (1) where γSV is the solid−vapor interfacial energy, γSL is the solid− liquid interfacial energy, γLV is the liquid−vapor interfacial energy, and θ is the contact angle value. Then, the Owens− 8525


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supplied a nitrogen stream which was split into a dry nitrogen flow and a solvent-saturated nitrogen flow at the atmospheric pressure with the total flow rate of the gas mixture of about 250 cm3/min. The water p(H2O) or methanol p(CH3OH) vapor pressure was gradually increased to the desired value and was maintained constant until the equilibrium state was reached. The vapor activity a is defined as the ratio of the vapor pressure pvapor to the saturated vapor pressure psat‑vapor (for water psat‑vap = 31.76 mbar and for methanol psat‑vap = 169.60 mbar at 25 °C41). Care was taken to avoid the vapor condensation in condition of the vapor pressure close to saturation. After a certain period of time, the sample weight reached a plateau. To plot the sorption isotherm, the resulting equilibrium solvent content Ceq was determined as a function of the vapor activity a. 2.6. Permeation Measurements. Sweeping Gas Method. The liquid and vapor water permeation measurements were performed at 25 °C in a thermostatic chamber using the homemade permeameter that consisted of the permeation cell, dry nitrogen supply, humidity generator (Bronkhorst, France), and a chilled mirror hygrometer (General Eastern Instruments, USA) used as a moisture sensor of the water vapors.42 Prior to the measurements, a membrane sample was sealed in the permeation cell and dried by circulating dry nitrogen (Air Liquide, France) on both sides of the membrane until a dew point lower than −67 °C was reached. Then, a stream of pure water or water vapors with the desired RH level was introduced into the upstream compartment while the downstream compartment was continuously swept out by the dry nitrogen stream. Because of the difference in the water activity on both sides of the membrane, the water molecules were transported through the membrane. The amount of vapors transported into the downstream compartment was monitored by using a chilled mirror hygrometer connected to the data acquisition system. The data were collected until the stationary state Jst was reached. The experiments were performed for liquid water (i.e., pervaporation mode as water activity is equal to 1) and for different water vapor activities in the feed stream. In addition, two different modes for the permeation of the water vapors were used. The first mode consisted of the continuous increasing of the RH level from 0% up to 97% without intermediate drying between the RH levels and the second onewhen the drying step was applied between each RH level. The water flux J(L, t) at the downstream side of the membrane was obtained from

Wendt method, which is a standard method for the SFE determination, was applied.38 The SFE value γS may be divided into two components, the surface energy caused by the polar interactions γpS and the surface energy caused by the dispersive interactions γdS: γS = γSd + γSp

(2)

The resulting equation may be written as follows: γL(1 + cos θ ) 2 γLd

=

γSd +

⎛ p⎞ γL ⎟ γSp ⎜⎜ d ⎟ ⎝ γL ⎠

(3)

2.4. Fourier Transform Infrared Spectroscopy (FTIR) Analysis. The FTIR measurements were performed using a FTIR Nicolet spectrometer (ThermoFischer, Avatar 360 Omnic Sampler) in ATR mode (Ge crystal) with a resolution of 8 cm−1 with 200 scans per spectrum in the range 4000−675 cm−1. The FTIR spectra of the membrane equilibrated with the methanol and water vapors at a given vapor activity (a) and liquid solvents were obtained by means of a homemade apparatus developed for this purpose. The spectra were interpreted according to the literature data.39,40 The dry membrane was positioned inside a special hood placed over the sample compartment. The vapor generator was connected to the hood for a few hours, ensuring that the membrane was in the equilibrium state with the vapor. Dry nitrogen and water− and methanol−nitrogen flows were controlled with an electronic gas flowmeter (Agilent Technologies, 5067 model). The FTIR spectra were recorded for the membrane equilibrated with the water and methanol vapors from a = 0 (i.e., the dry membrane) to a = 0.95 (in the case of water) and a = 0.70 (in the case of methanol). 2.5. Sorption Measurements. Liquid Sorption. The swelling behavior of the membrane was studied in two polar solventsultrapure water (H2O) and extra pure methanol (CH3OH). For this, the dry sample was immersed in the given solvent for 24 h at 25 ± 1 °C. Then, the sample was removed from the solvent and wiped out using KimWipes before weighing. This procedure was repeated until the equilibrium state was reached. The measurements were triplicated for each solvent for reproducibility. The molar swelling degree SM (in mol of solvent/g of dry membrane) of the membrane in the given solvent was calculated according to the following equation

SM =

(mS − md ) md Msol

J (L , t ) =

(4)

where ms is the weight of the swollen membrane sample (g), md is the weight of the dry membrane sample (g), and Msol is the solvent molecular weight (g/mol) (Table 1). Vapor Sorption. The water and methanol vapor sorption measurements at various partial pressures were performed by using an intelligent gravimetric analyzer sorption moisture (IGASorp, Mercer-Instruments, Passy, France) at 25 ± 1 °C. The samples were preliminarily dried for 1 week at 60 °C. Then, the sample was introduced in the IGAsorp module and additionally dried for 24 h by circulating a dry nitrogen gas (99.99%, Air Liquide, flow rate 250 cm3/min) until reaching a constant mass. A 100 mL portion of the given solvent (water or methanol) was placed in the humidifier. A gas flow controller

f −6 x out − x in 10 pt A RT

(5)

where f is the flow rate, A is the membrane surface area (3.6 cm2), xin and xout are the water concentration (ppmV) at the downstream membrane side at the entry and exit sides of the cell, respectively, R is the ideal gas constant, and T is the experimental temperature (in K). The water concentration x is deduced from the water vapor pressure p directly related to the dew point temperature (x ppmV = 106 p/pt, where pt is the total pressure equal to 1 atm). Initially, the nil flux increases progressively with time up to a limit value, which corresponds to the stationary flux Jst. The permeability coefficient P (mol·cm/s/cm2) is then determined as follows 8526


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P=

Jst L

is adsorbed in multilayers and starts to form aggregates. This last part corresponds to the additional swelling and the polymer matrix plasticization. Generally, the convex upper part of the curve may be correlated to the nature of the polymer matrix. When the matrix is hydrophobic, the end of the isotherm (in the activity range 0.8−0.95) is usually explained by the “clustering” or nonrandom aggregation of the vapor molecules. However, the BET model does not take into account the specific sorption. According to Rogers’s classification, five fundamental sorption modes may be distinguished.45 Type IV has a sigmoid shape and reflects a complex combination of several sorption modes. Sigmoidal isotherms are often attributed to the vapor sorption of the polar solvent in the hydrophilic polymers and ionomers.46 They are generally observed in the natural polymer systems and can be fitted by special models, i.e., BET type II equation, Guggenheim−Anderson−de Boer (GAB) equation, or Park’s equation. Taking into consideration that the sorption occurs preferentially around the hydrophilic ionic sites and that there is a negligible sorption elsewhere, the vapor sorption per sulfonic site reflects the intrinsic sorption properties of the sulfonic sites in their specific environment. The equilibrium vapor concentration Ceq is converted into the number of sorbed molecules per ion pair λ (eq 8), and the vapor sorption isotherm is represented as λ versus the vapor activity

(6)

Δa

where L is the membrane thickness (cm) and Δa is the activity difference. Gravimetric Method. The measurements were conducted using an IGASorp gravimetric system (IGAsorp, MercerInstruments, Passy, France) in the permeation configuration. Prior to the permeation measurements, the IonClad membranes were dried for 12 h over P2O5. In the beginning, the permeation cell and the dried membrane were weighed and the liquid solvent (water or methanol) was introduced into the lower compartment of the cell. The total weight of the system was determined and recorded. Then, the nitrogen flow controller generated a preset value of the solvent vapor activity (a1) by adjusting the ratio of the dry nitrogen flux ( fdry) and the solvent saturated nitrogen flux (fsolvent). The total flow rate of the gas mixture (fdry + fsolvent) was equal to ca. 100 cm3/min. The following values of a1 were chosen: 0, 0.25, 0.5, and 0.75. The transport of the solvent vapors through the membrane, caused by the activity difference (a0 = 1, Δa = a0 − a1), was proportional to the loss of the solvent mass in the lower part of the permeation cell. The mass of the cell was recorded as a function of time. For each a1 activity value, a new dried membrane sample was placed in the permeation cell. The vapor flux J (mol/s/cm2) at the given activity Δa was determined from the slope of the linear part of dC/dt J=

dC /dt Sa

λ=

(7)

(meq − m0)/M IECm0

(8)

where meq is the measured weight of the sorbate (g), m0 is the dry weight of the membrane, Msol is the sorbate molecular weight (g/mol), and IEC is the ion-exchange capacity (mol/g) (Table 2). In contrast to the most common models used in the literature to analyze the sorption isotherms, the well-known Park model offers the advantage to be able to describe correctly many sigmoidal sorption isotherms.47 The Park model describes the vapor sorption process as a combination of three sorption modes, i.e., the Langmuir mode, the Henry type sorption, and the cluster formation. This model is often used to analyze the water sorption in the polar system, as it gives a physical approach to the sorption phenomena over a wide range of activity.36,37,48 The equation of the Park model can be written as follows

2

where Sa is the active membrane surface (1.13 cm ). The permeability coefficient P was calculated according to eq 6.

3. THEORETICAL BACKGROUND 3.1. Sorption Isotherm. The vapor sorption isotherm is of special interest for the analysis of the membrane selectivity. The study of the vapor sorption mechanisms in an ion-exchange membrane requires a fundamental understanding of these processes via the mathematical modeling. In PV, the properties of the cation-exchange membranes depend mostly on the solubility and diffusivity of the permeant, which are, respectively, the thermodynamic and kinetic parameters that control the mass transfer. Different models can be found in the literature to describe the vapor sorption behavior of the polymer materials with various mechanisms of sorption, including the possible interactions between the polymer and vapors.43 Brunauer et al. have developed the Brunauer−Emmet−Teller (BET) theory which is based on a multilayer sorption mode.44 They identified five isotherms with different shapes (type I to type V). The sigmoidal isotherm referred to as BET type II has generally three zones. Each zone corresponds to a particular mode of the vapor sorption in the polymer matrix. Zone 1, concave to the abscissa at low activity, is usually attributed to the vapor sorption in the primary solvation shell of multiplets (ionic groups and counterions), taking into account that the ionic group reacts as Langmuir’s site. The further absorption of the vapor molecules allows forming a molecular monolayer covering all specific sites. The transition to the next area occurs when all specific sites are saturated. The molecule sorption in zone 2 takes place on the formed monolayer. The isotherm is linear in this region. In zone 3, convex at high activity, the vapor

λ=

AL bLa + KDa + K agan 1 + bL a

(9)

where λ and a are, respectively, the number of sorbed molecules per ion pair and the vapor activity; AL is the specific site capacity and bL is the affinity constant of the sorbed molecules on the specific sites; KD is the Henry coefficient; Kag is an aggregation parameter; and n is the number of solvent molecules per cluster. The aggregation parameter Kag is equal to

K ag = nKKDn

(10)

where K is the equilibrium constant of the clustering reaction. The nonlinear regression analysis was carried out using Table Curve 2D software. The ability of the Park model to fit the experimental points was evaluated by the average deviation modulus E49 8527


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The Journal of Physical Chemistry B E=

100 N

N

∑ i=1

⎛ 2 Δm(t ) ⎞ ⎟⎟ = −ln 82 − π Dt ln⎜⎜1 − Δmeq ⎠ L2 π ⎝

|mi − mpi| mi

(11)

where mi is the experimental value, mpi is the computed value, and N is the number of experimental points. An E modulus inferior to 10% was considered for a good fitting. 3.2. Sorption Kinetics. The study of the sorption kinetics is useful to validate the sorption analysis. When the diffusion is not time dependent, the mass transfer in the dense polymer membranes can be described by applying a mathematical approach based on Fick’s diffusion laws.50 To interpret the sorption kinetics, the moisture content Δm(t)/Δmeq retained by the membrane at a given vapor activity is measured over the time (Δm(t) is the sorbate content sorbed at time t, and Δmeq is the sorbate content sorbed at the equilibrium state). Generally, in the case of a Fickian behavior assuming D being constant, the sorption kinetics of the polymer/vapor system in the first half period is expressed by eq 12 (where n = 1/2 and k is a constant) and a typical curve can be obtained as shown in Figure 2. Δm(t ) = kt n Δmeq

These two simple equations (eq 14 and eq 16) allow calculating the diffusion coefficients D1 and D2, respectively, which are determined as a function of the sorption period. When the mass gain is lower than 50% (i.e., the first-half sorption), the D1 coefficient is determined, whereas the D2 coefficient is calculated for the mass gain higher than 50% (i.e., the second-half sorption). The D1 coefficient represents the diffusion through the surface, while the D2 coefficient is related to the diffusion inside the matrix. When D1 and D2 values are close enough, it appears that there is no difference between the surface absorption and diffusion inside the matrix, and thus, the diffusion coefficients are supposed to be constant. On the contrary, when D1 and D2 values are different, one can say that the diffusion coefficients are dependent on the solvent concentration. Generally, the initial increase of the diffusion coefficient at the low vapor activity until a certain maximum value may be observed due to the plasticization or swelling effect in the presence of the solvent vapor, so that D1 is found to be lower than D2.51 The decrease of the diffusion coefficient at the higher activity observed in most cases is caused by the penetrant clustering which makes the diffusive molecules less mobile and leads to the fact that D2 is found to be lower than D1.52 The decrease of D with the solvent concentration can also be observed when the polymer crystallizes during the water vapor sorption or when the internal rearrangement via the solvent/polymer interactions can take place that can be at the origin of the antiplasticization phenomenon.53 In general, the vapor sorption mechanisms are complex and result from a combination of different contributions such as the swelling, internal rearrangement, molecular interactions, and solvent sorption on specific sites. 3.3. Mean Cluster Size (MCS). In order to evaluate the mean cluster size (MCS), a method developed by Zimm and Lundberg54,55 and then by Starkweather56 can be applied to analyze the experimental isotherms. The MCS value represents the mean size of the solvent aggregates in the vicinity of the given penetrant molecule in excess of the penetrant concentration in the polymer. The general equation is written as

(12)

Figure 2. Sorption kinetics as a function of √t.

Until the equilibrium, the concentration profiles and sorbed content can be plotted by using the analytical solution. For the short time periods, the normalized mass can be calculated from ⎛ τ ⎞1/2 ⎡ Δm(t ) = 4⎜ ⎟ ⎢1 + 2(π )1/2 ⎝ π ⎠ ⎢⎣ Δmeq

∞

∑ (−1)n i erfc n=1

⎛ δ ln ΦS ⎞ ⎟ MCS = (1 − ΦS)⎜ ⎝ δ ln a ⎠ p , T

n ⎤⎥ 2(τ )1/2 ⎥⎦ (13)

D⎤ ⎥ t π ⎦

⎛ ρ ⎞−1 ⎟ ΦS = ⎜1 + ⎝ ΔM ⎠

∞

∑ n=0

e( −(2n + 1)2 π 2τ ) (2n + 1)2

and

ρ=

ρS ρp

where ΦS is the penetrant volume fraction, ρS and ρp are the volumetric mass density of the penetrant and polymer, respectively (Table 1 and Table 2), and ΔM is the vapor mass gain:

(14)

For the longer time periods, the normalized mass gain can be obtained from Δm(t ) 8 =1− 2 Δmeq π

(17)

with

Hence, the content of the sorbed solvent in the membrane during the first half sorption period S1 (Δm(t)/Δmeq < 0.5) can be expressed as Δm(t ) ⎡ 4 =⎢ ⎣L Δmeq

(16)

ΔM = (15)

Thus, the content of the sorbed solvent in the membrane during the second half sorption period S2 can be estimated by

(meq − m0) m0

(18)

To take the Park parameters into account, the MCS values were calculated as follows: 8528


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The Journal of Physical Chemistry B MCSPark =

(ρ)2

(

ΔM3 1 +

ρ ΔM

2

)

⎛ A b a ⎞ L L ⎜ + KDa + n2KKDn an⎟ 2 ⎝ (1 + bLa) ⎠

DM = D0eγCeq

where Ceq is the water concentration in the membrane at the stationary state and DM is the maximum diffusion coefficient determined for the concentration obtained at the stationary state.

(19)

3.4. Permeation. The analysis of the permeation kinetics allows us to determine how the polymer structure influences the membrane transport properties. Usually, water is chosen as a molecular probe, as the water molecules are able to interact with the polymer through the (anti)placticization effect. The diffusion coefficients are determined from the slope of the water flux curve during the transient state. By solving Fick’s diffusion equations, a dimensionless flux curve as a function of J Dt reduced time j = f(τ) is plotted, where j = J and τ = L2 (D is

4. RESULTS AND DISCUSSION 4.1. Contact Angle and Surface Free Energy (SFE) Characterization. The separation performance in PV highly depends on the membrane nature. One way of estimating the material hydrophilic/hydrophobic nature is by measuring its water contact angle. Hence, in order to characterize the surface hydrophilicity as well as to determine the SFE value, the contact angle values of the membrane in different states (i.e., dry and hydrated at different RH levels) were measured using a sessile drop method. The obtained contact angles are gathered in Table 3. From Table 3, it can be seen that the contact angle

st

the diffusion coefficient, which is assumed to be constant, and t is the measurement time). In order to determine if the diffusion coefficient depends on the permeant concentration, D is calculated at two characteristic points of the permeation curve, the inflection point I and the time-lag point L.57 From the theoretical dimensionless flux curve and considering D being constant, these two characteristic points were determined as follows: jI = J(τI ) = 0.24

(20)

jL = J(τL) = 0.62

(21)

τI = 0.09104 =

τL =

Table 3. Contact Angle Values of the IonClad Membrane as a Function of the Membrane State probe liquid water

DI tI L2

Dt 1 = L2 L 6 L

glycerol diiodomethane

(22)

(23)

(24)

where D0 is the diffusion coefficient when the permeant concentration is close to 0, γ is the plasticization factor, and C is the local concentration of the permeant molecules. From the obtained curves, it is possible to determine the mean integral diffusion coefficient ⟨D⟩ as follows ⟨D⟩ =

1 ΔC

∫C

C2

1

D(C) dC

(25)

where C1 and C2 are the water concentration in boundary conditions at the upstream and downstream sides of the membrane, respectively. In our case, C1 = 0 and C2 = Ceq and eq 25 may be written as ⟨D⟩ =

DM − D0 γCeq

102 86 78 43 68 93 74

± ± ± ± ± ± ±

2 2 3 1 1 1 2

of the dry IonClad surface is 102 ± 2° after 3 s of the water drop deposition. It should be noted that, in order to measure correctly the contact angle on the dry surface, a dry nitrogen sweep around the membrane surface was carried out during the measurement in order to maintain a constant dry state. In this case, the water contact angle measured as a function of the deposition time stays constant (up to 120 s), thus well confirming that the circulation of the dry nitrogen stream is efficient to maintain the membrane dry state. The water contact angle value (102 ± 2°) indicates the hydrophobic character of the dry IonClad membrane surface. The hydrophobic materials have little or no tendency to absorb water as low electrostatic interaction between the hydrophobic membrane and the water molecules is present, since the weak van der Waals force alone is not enough to hold the water molecules on the membrane surface. As a result, the water droplets tend to bead up. In the case of IonClad, the hydrophobic backbone structure (Figure 1) imposes the predominantly hydrophobic character, as the lithium sulfonated-terminated side groups do not have sufficient mobility to interact with the water molecules. For comparison, the hydrophobicity of the complete dry Nafion 117 membrane, i.e., perfluorinated proton conducting membrane (θ = 116° at 20 °C59) is close to that of the PTFE membrane. According to Zawodzinski et al., the water contact angle for the perfluorinated membranes varies with the hydration level.60 And really, the water contact angle values obtained for the IonClad membrane stored in the wet atmosphere at 44% RH, 75.5% RH and especially in liquid water are much lower than the water contact angle obtained for the dry membrane (Table 3). The high RH level facilitates the membrane wettability. Indeed, when the membrane surface is in contact with the

The experimental time periods noted t 0.24 and t 0.62 corresponding to j0.24 and j0.62, respectively, were determined from the curve j = f(t). The coefficients D0.24 and D0.62 were then calculated and compared. A difference in their values highlights the water concentration dependence of the diffusion. In the case of vapor permeants, it is usually observed that D increases with an increase in the permeant concentration. Such behavior is attributed to a plasticization phenomenon induced by the diffusing species, which leads to an increased free volume. This well-known behavior, considered as a Fickian process of type B,58 is usually fitted by the diffusion exponential law according to D = D0eγC

contact angle θ (deg)

membrane state dry 44% RH 75.5% RH stored in liquid water stored in liquid methanol dry dry

(26)

with 8529


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al.11 and was applied by Boakye et al.12 to the Nafion membrane. According to this model, three states of the ion pairing solvation exist in the ion-exchange membranes: (i) the counterion is in contact with the ion-exchange group, (ii) innersolvation layers with the ion pairs are formed, (iii) the ion pairs are totally dissociated during the contact with the solvent of high polarity. The solvent polarity can be expressed in terms of the Hansen solubility parameter and dielectric constant (Table 1). For the membrane in a dry state, the counterion is in contact with the ion-exchange group with none or residual solvent layer around the ion pairs.29 During the solvation step, the ion pairs with the inner-solvation layers are formed. If the membrane is equilibrated with the solvent of a high polarity (like water (ε = 78.5) or methanol (ε = 32.6), Table 1), the ion pairs should be totally dissociated and the solvation layers would be formed around each ion. The solvent swelling of the ion-exchange membranes is caused mainly by the solvation of the fixed ionic groups and the counterions.66 In addition, the swelling degree depends not only on the degree of the membrane crosslinking67 but also on the type of the counterions.68 The sulfonated ion-exchange membranes showed enhanced swelling in the water/organic mixtures (like water/ethanol, water/ isopropanol) compared to the swelling in the pure solvents.13,22 It was also found that the water swelling can be directly correlated with the hydration number of the counterions.69 The swelling data for the IonClad membrane in contact with two polar solvents are presented in Table 4, indicating that the

water vapors or liquid water, the hydrophilic moieties (−SO3Li groups, Figure 1) close to the surface reorient in such a way that they are exposed to water, thus hydrophilizing the membrane surface. Such surface reorganization would be more facile for a significant water uptake due to the plasticizing effect caused by the water molecules present in the polymer. He et al. have shown that the copolymerized sulfonated poly(ether ether ketone) (PEEK) membranes exhibit hydrophobic character during the contact with the water vapors and, on the contrary, the hydrophilic one in contact with liquid water.61 This change in the hydrophilicity/hydrophobicity balance was explained by the migration and reorientation of the sulfonic acid groups toward the membrane surface from the membrane bulk in a few seconds during wetting. Comparing the contact angle values obtained for the membranes stored in liquid water and methanol (Table 3), one can note that the value obtained for the membrane stored in liquid methanol (68 ± 1°) is higher than that obtained for the IonClad membrane stored in liquid water (43 ± 1°). It means that the water-swollen IonClad membrane significantly promotes the water impregnation, whereas the methanolswollen IonClad membrane is resistant to the water penetration. The presence of methanol within the membrane slows down the water penetration onto the membrane surface. Thus, the water absorption is easier at the IonClad surface compared to the methanol absorption. These results will be discussed further in detail together with the sorption results. The SFE value for the dry IonClad membrane was calculated using the Owens−Wendt method (eqs 2 and 3). The calculated SFE value γS is equal to 20.9 mN/m with 20.1 mN/m for the dispersive component γSd and 0.8 mN/m for the polar component γpS. The low SFE value obtained for the IonClad membrane is comparable with the values obtained for the most hydrophobic polymers; for example, 22.9 mN/m is obtained for low density PTFE.62 The fact that the IonClad membrane character is changed from the hydrophobic to hydrophilic one after the storage in liquid water can be useful for the PV processes, especially as the membrane hydrophilic surface is required for the dehydration of alcohols due to the membrane higher water selectivity. That is why the IonClad membrane can be used not only as an ionexchange membrane but also as the PV membrane. 4.2. Membrane Swelling in Liquid Solvents. The swelling of a membrane leads to an increase of the free volume within the membrane structure and, therefore, may greatly reduce the membrane selectivity. The swollen membrane can form a flexible structure, which facilitates the diffusion of the other components of the mixture through the membrane, resulting in the membrane poor selectivity, thus causing a detrimental effect on the PV process.63,64 Thus, the state of the sulfonic group and lithium counterion in the presence of a given solvent should be analyzed in order to better understand the behavior of the IonClad membrane in contact with the polar solvents. The nuclear magnetic resonance (NMR) study carried out for the Nafion membrane with the different counterions (Na, Li, and Cs) showed that the sodium spectrum had a large chemical shift with decreasing water content.65 The obtained data suppose that three or four water molecules are present around the sodium ions in the first hydration layer. The qualitative model of the ion pairing that takes into account the influence of the humidity on the dissociation of the ionexchange group/counterion pairs was proposed by Eigen et

Table 4. Swelling Degree of the IonClad Membrane in Contact with Water and Methanol solvent

swelling degree (mmol of solvent/g of dry membrane)

H2O CH3OH

10.0 ± 0.3 7.6 ± 0.2

sorption capacity is the highest in water. The higher membrane swelling degree in water can be explained by higher attractive interactions between the water molecules and IonClad, which improve the water solubility and sorption into the IonClad membrane. As clearly shown in Table 4, the affinity (i.e., the swelling degree) of the IonClad membrane with both studied solvents is of the same order as their polarity (Table 1). Some factors may explain the low swelling degree: the reduced free volume of the polymer, a denser polymer structure that limits the solvent diffusion/absorption into the membrane structure. Thus, the interactions between the polymer and the solvent are weak and the ion-exchange groups are inaccessible to the solvent, despite a high IEC value of 1.50 mmol/g (see Table 2). In order to better understand the solvent swelling in the IonClad membrane, a schematic representation of the ionic domains in IonClad is proposed in Figure 3. It should be noted that the polymer structure plays an important role in the swelling: sulfonic groups are numerous and located at the side chains (Figure 1). In addition, the hydrophobic side chains of IonClad are short, so that the sulfonic groups located near the hydrophobic areas are less accessible for the solvent (Figure 3). The fact that the IonClad membrane has a low swelling degree and is less permeable to methanol than to the water molecules might indicate that water/methanol separation should be possible. 4.3. Infrared Analysis. The FTIR analysis was conducted by many authors for the perfluorinated ionomers during the 8530


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result clearly indicates that the ion pair (sulfonic acid/lithium counterion) was dissociated only in the presence of the water molecules. As one can see in Figure 4, the dissociation of the ion pairs does not occur in the case of the methanol molecules whatever the solvent activity value; i.e., the vs band value remains practically constant in the whole activity region. In the case of the water swelling, the dissociation of the ion pairs in the IonClad membrane occurs at an activity higher than 0.5 (Figure 4), which corresponds to the formation of the ion pairs with the inner-solvation layers during the solvation step. This difference in the dissociation behavior for the studied membrane confirms the membrane applicability for the separation of the water/ methanol mixtures. 4.4. Water and Methanol Vapor Sorption. The vapor sorption isotherms are performed in order to evaluate the membrane sorption capacity under specific conditions of temperature and solvent activity. To the best of our knowledge, there are no reports in the literature on such a study for the IonClad membrane. Thus, in the present study, the modeling of the sorption isotherms by applying the Park model is carried out. This model allows well describing the interaction of the polymer chains and the sorbed molecules but also the interaction between the sorbed molecules. The vapor sorption isotherms for IonClad were measured and presented by the number of the sorbed molecules per ion pair λ (eq 8) as a function of the solvent activity (Figure 5). The values obtained from the IonClad sorption in contact with the liquid solvents (i.e., at a = 1) were also added to Figure 5. The inset to Figure 5 represents the vapor sorption kinetic measurements. As one can see, the water and methanol vapor sorption isotherms exhibit a sigmoid shape corresponding to type IV of the Rogers sorption mode classification. One can notice that the number of sorbed molecules per ion pair λ is found to be higher for water than for methanol over the whole solvent activity range (0 < a ≤ 1). Besides, the water dissociation ability is higher compared to that of methanol. This result reveals that the IonClad membrane is able to sorb a higher amount of water molecules per ion pair compared to methanol. Moreover, since water has a higher polarity than

Figure 3. Schematic representation of the ionic domains in the IonClad membrane.

solvation at room temperature.70−72 The dissociation of the sulfonic groups was interpreted in terms of different states of water in the membrane. The sulfonic ion-exchange membrane is characterized by the symmetric stretching vibration band S− O (vs) located at 1030−1070 cm−1, describing the behavior of the sulfonic anions in their environment.72 Several authors have shown that the vs (SO3) vibration shifts continuously from 1064 to 1057 cm−1 during hydration.73 Therefore, the state of the sulfonic ion-exchange groups can also be explained by the analysis of the symmetric S−O stretching band vs position, since it provides the information on the binding situation around the sulfonic group and its steric accessibility. The FTIR spectra of the dry IonClad membrane and for the membranes equilibrated with the studied solvents (water and methanol) in the vapor and liquid states were measured in the ATR mode. The symmetric stretching vibration band of the sulfonic group vs of the dry IonClad membrane is found to be located at vs = 1045 cm−1. The obtained results reveal a shift of the symmetric stretching vibration νs band of the sulfonic groups to the lower wavenumbers in the case of the contact with the water molecules (Figure 4). However, the position of the symmetric vibration band of the sulfonic group for the IonClad membrane in contact with the methanol vapors remains practically the same (vs = 1045 cm−1) as for the dry membrane, indicating that the ion groups in IonClad are not dissociated in this case. In addition, the vs band for IonClad equilibrated with liquid water is found to be much lower (vs = 1037 cm−1) compared to that for the dry membrane. This

Figure 4. Influence of water (▲) and methanol (△) activity on the sulfonic symmetric vibration band νs in the IonClad membrane at 25 ± 1 °C. 8531


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slightly higher for the water molecules than for the methanol ones (0.18 for water and 0.13 g for methanol), but this value stays still rather low. Legras et al.15 and Escoubes et al.46 have shown that, when Nafion (i.e., the polymer containing a perfluorinated backbone bearing sulfonic groups) is in the Li+ form, the first part of the water sorption isotherm is roughly linear, and on the contrary, when the membrane is in the H+ form, the first part of the water sorption isotherm is rather concave with respect to the abscissa axis at the low water activity (a < 0.3). In our case, the pseudolinear behavior is obtained for the water sorption of the IonClad membrane in the Li+ form. The strong interaction of the water molecules around the cations with the sulfonic groups (by the hydrogen bonding) induced a large number of Langmuir sites. The parameter AL represents the number of specific sites (hydrophilic groups) able to interact with the water molecules. On the contrary, the affinity constant of the sorbed molecules on the specific sites (bL parameter) is twice higher for the methanol sorption compared to the water sorption, as confirmed by the more pronounced Langmuir behavior (the first part of the methanol isotherm has a visible concave shape, Figure 5). The linear dependence of the vapor concentration at the medium water activity range (0.11 < a < 0.5) corresponds to the second term in eq 9, i.e., to the Henry sorption mode. The Henry law assumes a random sorption of the vapor molecules in the polymer matrix without specific interactions between the sorbed species and the polymer chains. The number of the sorbed molecules per the ion pair λ for the water molecules remains slightly higher than that for the methanol molecules (1.5 and 1.0 at a = 0.5, respectively). The water sorption data obtained for Nafion 117 have revealed the higher number of sorbed molecules per ion pairλ ∼ 3 at a = 0.5.27 Such a difference in the water sorption behavior between Nafion and IonClad indicates that the sulfonic sites are more accessible for the water molecules in the case of Nafion, keeping in mind that the IEC value of IonClad (1.50 mmol/g, Table 2) is higher than that of Nafion (0.8−0.9 mmol/g15). Such a water condensation phenomenon is often explained by a structural rearrangement of the polymer chains, thus leading to the formation of interconnected channels.74 Therefore, it seems that the Nafion membrane that consists of the ionic domains dispersed in a hydrophobic matrix allows a greater dissolution of the water molecules in comparison with IonClad, i.e., with the irradiated PTFE membrane grafted with the sulfonated styrene monomers (Figure 1), for which the dispersion of the ionic domains is more homogeneous (Figure 3). Jalani et al. have shown that the IEC effect on the water uptake can be explained by the fact that the lower the IEC value, the higher is the acid group density and more acidic is the membrane.31 This increase of the acid site density results in the increased water sorption and membrane swelling. The third term in the Park model (eq 9) represents the vapor cluster formation as it can be seen from the upward curve of the vapor sorption is observed at a > 0.75 (Figure 5). The calculated value of λ for water is twice higher than that for methanol (λ = 4.8 and λ = 2.9 at a = 0.93, respectively). These values are in good agreement with the values obtained in the case of the sorption in liquid solvents, i.e., at a = 1 (λ = 6.7 for water and λ = 5.0 for methanol). This result confirms the homogeneity of the IonClad polymer structure toward the vapor molecules. As observed for the other cation-exchange membranes such as Nafion, sometimes there is a significant difference between

Figure 5. Water (▲) and methanol (△) sorption isotherms of the IonClad membrane at 25 ± 1 °C. Lines represent the best fit by the Park model (eq 9). Inset: solvent sorption kinetics for the different activity values.

methanol (ε = 78.5 and ε = 32.6, respectively, Table 1), the capacity of the water vapor to condense in the polymer matrix is also higher. This result is in good agreement with FTIR data, indicating the dissociation of the ion pair (−SO3−−Li+) of IonClad in the presence of water but not in the case of methanol (Figure 4). Indeed, the obtained molar swelling degree of the IonClad membrane in contact with water and methanol (Table 4) shows that the water solubility is much higher than that of methanol. In the case of the ideal sorption, it is assumed that the free volume in the IonClad membrane is not changed significantly during the sorption, as the sorbed molecules only fill the existing free volume. When such interactions exist, they will promote the separation of the polymer chains and may result in an increase of the sorption capability. To understand the solvent sorption mechanism inside the membrane, a careful analysis of the organization (i.e., the way that the molecules are spatially arranged) of the solvent molecules inside IonClad was performed. For this purpose, the water and methanol vapor sorption isotherms were fitted by the Park model according to eq 9 (Figure 5). As was mentioned previously, the Park model is the association of three sorption mechanisms: the sorption on the specific sites (Langmuir’s mode) (a < 0.11), the nonspecific sorption (Henry’s law) (0.11 < a < 0.5), and the vapor molecule aggregation (clustering) at the high water activity (a > 0.7). The corresponding fitting parameters of this model are summarized in Table 5. The determined deviation modulus E (eq 11) does not exceed 1.4% which validates the good fit. It can be noted that, at the low activity (a < 0.11), the sorption capacity on the Langmuir sites (AL parameter) is Table 5. Calculated Water and Methanol Sorption Parameters of the Park Model (eq 9) parameter/solvent

H2O

CH3OH

AL bL KD Kag n E (%)

0.18 13.7 2.22 0.02846 4 1.0

0.13 25.5 1.72 0.00425 8 1.4 8532


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all studied activities. Therefore, the cluster formation should be more complex, taking into account that the nonrandom distribution of the water and methanol molecules (clustering) occurs preferentially in the vicinity of the ion pairs. The more pronounced difference in the MCS values between water and methanol at the higher activity (Figure 6) allows us to suppose a difference in the diffusivity and thus in the permselective properties of the IonClad membrane. It should be noted that the methanol and water concentration-dependent diffusivities in IonClad are quite different (Figure 7). It can be seen that both diffusion coefficients (D1 (eq 14) and D2 (eq 16)) strongly depend on the water activity a. At the beginning (when a < 0.25), D1 and D2 increase with the water concentration before reaching a plateau (at a ≈ 0.5) and then decrease at the higher water activity (a > 0.7). At the low water activity (a < 0.2), the first water molecules are mainly absorbed on the sulfonic sites due to the high water−ion interactions. With a increasing, the additional water uptake favors the water mobility, thus improving the swelling effect through the plasticization phenomenon increasing the free volume. For the higher water activity (a > 0.7), the diffusivity decrease (Figure 7) is due to the water molecule aggregation as the mobility of the water clusters is reduced by the steric effects. Legras et al. noted that such a decrease of the water diffusivity is due to a noninstantaneous sorption process as the kinetics of the water sorption at the membrane surface is slow.15 As for the water diffusion, a similar profile of the two diffusion coefficients (D1 and D2) was obtained for methanol (compare Figure 7a and Figure 7b). However, a shift of the plateau value to the higher activity (a ≈ 0.8) is observed in the case of methanol. Besides, one can see that the swelling process in methanol happens only when the methanol activity is higher than 0.4, whereas this process has been already observed at the lower activity (a < 0.1) during the water sorption. This result is in good agreement with the fact that the methanol molecules have weaker interactions with the ionic groups in comparison with the water molecules (Figure 4). Thus, the lower ability of methanol to separate the ion pairs limits its concentration in the ionic domains. Therefore, taking into account higher n value (Table 5), the methanol aggregates of greater size are located around the sulfonic groups and Li+ ions (Figure 1), whereas the water aggregates are additionally located between the sulfonic sites and Li+ cations (Figure 3). 4.5. Water and Methanol Permeation. The permeability is one of several key properties which are indicators of the material durability. The permeability itself is an intrinsic property, which is controlled by the other macroscopic properties such as the polymer structure, tortuosity, functional group uniformity, and distribution. In order to give an improved understanding of the transport mechanisms that take place during the PV process, one should combine the sorption and permeation results, as during PV a selective membrane is simultaneously exposed to vapor and liquid phases and both processes (i.e., sorption and permeation) will take place. In order to systematically study the effect of the water phase in contact with the membrane, the water permeation measurements through the IonClad membrane were performed by two methods: sweeping gas method and gravimetric method. In addition, the water vapor permeation kinetics

the water sorption behavior in the liquid and saturated vapor states.75 This fact is known as the Schroeder paradox and is still a subject of discussion. The solvent absorption from the liquid phase is quicker and more extensive as compared to that from the vapor phase. It is important to note that this phenomenon seems to be absent or moderate in the case of the IonClad membrane. It means that the dispersion state of the water molecules inside the IonClad matrix is different compared to the Nafion membrane, thus revealing a homogeneous distribution of the sorbed water molecules inside IonClad. The values of Kag (the equilibrium constant of the clustering formation)0.02846 and 0.00425 for water and methanol, respectively (Table 5)indicate a favorable water aggregation in the IonClad membrane, despite the fact that the water clustering process is usually considered as a slow phenomenon. For a better understanding of the sorption behavior, it is necessary to analyze the diffusivity analysis and to study the MCS evolution as a function of the solvent concentration. For this purpose, the analysis of the fitted parameters of the sorption model is usually performed using the MCS theory (eq 19). Sabard et al. have studied the water transport properties in the polyamide-6 film and determined the MCS values from the parameters of the BET II model.76 They have noted that the shape of the MCS curves corresponds to that observed for the sorption isotherms whatever the sorption model used. One can see from Figure 6 that the MCS values are the same for both solvents at a = 0.5. Surprisingly, for the higher vapor

Figure 6. Calculated MCS values (eq 19) for the water (▲) and methanol (△) vapor sorption of the IonClad membrane.

activity (0.5 < a < 0.9), the mean size of the methanol clusters becomes higher than that of water. The MCSPark value for the water molecules reaches a value of ∼2.3 at the higher activity, which is comparable with the MCS value found for Nafion 117 (2.146). At the same time, the determined MCS value for methanol is one molecule unit higher. This result agrees well with the tendency for the n parameter (Table 5), as the twice higher value is obtained for methanol (n = 4 and n = 8 for water and methanol, respectively). However, the fitted n values are found to be much higher than those of MCS (compare Table 5 and Figure 6). This result testifies to the fact that the interpretation of the clustering phenomenon based only on the Park model parameters, i.e., assuming n as the mean size of the aggregates which are formed between the molecules of the Henry population (random dispersion in the polymer matrix), is rather simplified, as it represents a unique adjustable value for 8533


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Figure 7. Evolution of the diffusion coefficients D1 (a) and D2 (b) as a function of the water (▲) and methanol (△) activity for the IonClad membrane at 25 ± 1 °C.

Figure 8. Solvent permeability coefficient of the IonClad membrane as a function of the solvent activity at 25 ± 1 °C: (a) gravimetric method (water (▲) and methanol (△)); (b) gas sweeping method (series 1 (●) and series 2 (▲)).

coefficient are higher when the solvent activity gradient is reduced. This result can be explained by the fact that, for Δa = 0.5, the permeation process occurs on both sides of the membrane, while only one side of the membrane is in contact with the liquid solvent at Δa = 1, so that the swelling effect is limited by the membrane thickness. For the gas sweeping method, the water permeation measurements were performed for water in the vapor and liquid states. In addition, two different ways of measurements were applied. The first way consists of the continuous increasing of the RH value up to 100%, i.e., up to liquid water, without an intermediate membrane drying step (series 1), whereas the membrane drying is performed after each steady state at the constant RH level during the second way of the measurements (series 2). The calculated water permeability coefficient P (eq 6) and different diffusion coefficients (eqs 24 and 25) are gathered in Table 6, and the water permeability coefficient as a function of the water activity is presented in Figure 8b. The plasticization phenomenon can be highlighted by the permeability measurements. It is difficult to give an unambiguous definition of the plasticization. Usually, the plasticization is defined as the increase of the permeant diffusivity as a function of the permeant concentration (or the

curves were measured at different RH levels and also for water in the liquid state. For the gravimetric method, the solvent permeation was performed with the weighed-cell technique, in which the polymer membrane is used to cap an open aluminum cup, thus functioning as a partition element between the interior and exterior of the cup. Under these conditions, a solvent activity difference (Δa = 0.5; 0.75; 1) is established between the inside and outside of the cup, so that the solvent is forced to permeate the membrane from inside (i.e., from the lower part of the permeation cell) to outside. In this case, the plasticization phenomenon is possible on both membrane sides. The vapor flux J at a given activity difference Δa was determined according to eq 7. The water and methanol permeability coefficient values (eq 6) of the IonClad membrane calculated at the different solvent activity values are presented in Figure 8a. As one can see from the obtained results, the methanol permeability is higher than the water permeability at Δa < 0.4. However, at the higher Δa value, this tendency is inversed; i.e., the IonClad membrane starts to be more impermeable toward methanol compared to the water molecules (Figure 8a). This results is in good agreement with the sorption measurements (Table 5 and Figure 5), namely, with the higher bL value for methanol. In all cases, it can be observed that the values of the permeability 8534


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(Table 6), assuming the same water transport mechanism in the IonClad membrane. The noted decrease of the diffusion coefficient D may be explained by the strong water molecule aggregation, which reduces the solvent mobility because of the larger cluster size (Table 6). In addition, the significant increase of the permeability coefficient P for the liquid water permeation can be explained by further swelling and strong plasticization of the IonClad membrane upon contact with permeant. This result leads to the percolation phenomenon accompanied by the creation of the preferential pathways for the water molecules, as was already observed for Nafion. Therefore, the higher swelling degree of the IonClad membrane and, thus, the higher water concentration in the polymer together with the increased mobility of the polymer chains increase the water permeability of the membrane.

Table 6. Calculated Water Permeability Parameters of the IonClad Membrane activity value/ P·107 parameter (mmol/cm/s) 0.30 0.50 0.70 0.93 1.00

0.29 0.32 0.33 1.33 86.3

0.28 0.50 0.84 0.97 1.00

0.21 1.43 1.83 1.36 75.8

D0·1010 (cm2/s) Series 1 2.28 2.84 3.69 2.19 Series 2 2.91 2.83 6.96 6.39

D0.24·1010 (cm2/s)

⟨D⟩·1010 (cm2/s)

19.3 24.6 48.6 37.8 0.48

67.4 98.4 180.0 145.0

5.0 5.1 5.6 6.0 >6

19.0 33.0 41.4 37.5 0.98

63.2 121.0 136.0 123.0

4.6 5.4 4.5 4.5 >6

γCeq

5. CONCLUSION In this work, the relation between the structure of the cation exchange membrane (IonClad) and its solvent sorption and permeation capacity was established. The swelling properties of IonClad in the polar solvents (water and methanol) were correlated with the polymer structure, and the possible percolation mechanism was proposed. It has been shown that the water and methanol sorption isotherms of IonClad exhibit a similar sigmoid shape which is successfully fitted by the Park sorption model. The Langmuir sorption is found to be very low at the low solvent activity (a < 0.11), whatever the solvent used. However, in the Henry law region (0.11 < a < 0.5), the water concentration is much higher compared to the methanol concentration. At the high vapor activity (a > 0.7), both water and methanol sorption are accompanied by the formation of the solvent aggregates but with different cluster size. The difference in sorption behavior between water and methanol is explained by the diffusivity mechanism which strongly depends on the solvent vapor activity, thus resulting in the different interactions of the solvent molecules with the IonClad chains. The analysis of the sorption data has shown that the first water molecules are mainly absorbed on the sulfonic sites due to the water−ion interactions. Then, the increase of the vapor amount leads to the water mobility increase due to the swelling effect. And at the high activity (a > 0.7), the observed diffusivity decrease is attributed to the formation of the water aggregates. This result indicates that the water vapors are able to condense in the polymer matrix. In the case of methanol, the diffusive behavior is found to be different, as the swelling process is shifted to the higher activity value compared to water. In other words, the methanol molecules have weaker interactions with the ionic groups in comparison with the water molecules because of the methanol lower ability to separate the ion pairs. In addition to the sorption measurements, the performed permeation measurements have shown that the IonClad membrane is less impermeable toward the methanol molecules as compared to the water molecules. Besides, the plasticization phenomenon was highlighted by the permeability measurements. The obtained results are in good agreement with the sorption and infrared spectroscopy analysis results. The infrared analysis of the membrane in the dry state and in contact with pure solvents in vapor and liquid phases reveals that the ion pairs (sulfonic groups/lithium counterions) are dissociated only in the presence of water. Besides, the total ion pair’s

pressure in the case of gases). An increase of diffusivity results from an increase of the polymer chain segment mobility that leads to the increase of the free volume due to the swelling effect. In addition, there is no general rule that can predict if a polymer will be plasticized by a penetrant. In our case, due to the boundary conditions, one can expect that the plasticization phenomenon occurs on both sides of the membrane for the gravimetric method, while in the case of the gas sweeping method this phenomenon occurs only on one membrane side, the one which is in direct contact with the solvent. In the case of the gas sweeping method, the increase of the permeability coefficient is observed with the water activity rising (Figure 8b). Such an increase of the water permeability coefficient with the water concentration can be easily explained by the role played by the water molecules that can highly plasticize the polymer matrix, inducing a strong swelling effect (γCeq value in Table 6). This plasticization phenomenon leads to an increase of the polymer free volume, so that the diffusivity is increased during the permeation process due to the raised number of percolation pathways. This fact is well confirmed by the ion pair dissociation observed in the presence of the water molecules at the high activity revealed by the FTIR analysis (Figure 4). The lower values of the permeability coefficient P measured over the water activity range by the gas sweeping method (Table 6) may be the result of the experimental conditions, as in this case the permeant molecules are in contact with the IonClad membrane only on one side, thus leading to the concentration gradient. Therefore, the water flux and the permeability values are lower. The slight difference of the permeability coefficient P values observed for the two measurement series in the case of the gas sweeping method (Figure 8b) may be explained by the different organization of the water molecules inside the IonClad membrane in each case. In the case of series 1, the continuous increasing of the water activity value was performed, leading to the progressive organization of the water molecules’ transfer inside the membrane. Thus, the water molecules have enough time to disperse progressively and to form the ionic domains and the percolation pathways inside the membrane (Figure 3). During the second measurement series, the intermediate membrane drying was performed (series 2), thus leading to the higher permeability P and diffusion D coefficient values (Figure 8b and Table 6). However, at the high water activity (a > 0.8) as well as in the case of liquid water (a = 1), the same coefficient values for both series of measurements are obtained 8535


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dissociation occurs only in contact with the water vapors at RH higher than 80%. Taking into account the obtained results, one can conclude that the IonClad membrane behaves differently with different sorption capacity and permeation performances toward water and methanol, which depend on the solvent activity. However, in order to confirm that this membrane is appropriate for the PV application, the permeation and especially pervaporation measurements should be performed for the water/methanol mixtures. Therefore, the influence of the solvent mixture composition in terms of the component concentration will be a subject of further study.

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AUTHOR INFORMATION

Corresponding Author

*Address: Laboratoire “Polymères, Biopolymères et Surfaces” Bd. Maurice de Broglie, Bat. Dulong 76 821 MONT SAINT AIGNAN Cedex, France. Phone: +33(0) 235 14 66 97. Fax: +33(0) 235 14 67 04. E-mail: [email protected]. ORCID

C. Chappey: 0000-0001-6196-8670 K. Fatyeyeva: 0000-0002-9241-8185 S. Marais: 0000-0002-1754-1183 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

A part of this research was performed in the frame of the bilateral project between Austria and Poland “Experimental and computational study of transport mechanism of organic solvents through ion-exchange membranes” (WTZ project PL 08/2012 of Ö AD) and a Lifelong Learning Program Erasmus. This project was supported by Hubert Curien’s Partnership Program “Polonium” (35501/2016).

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Sulfonic Membrane Sorption and Permeation Properties

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