Swelling and Stability of Polyelectrolyte Multilayers in Ionic Liquid

Article pubs.acs.org/Macromolecules

Swelling and Stability of Polyelectrolyte Multilayers in Ionic Liquid Solutions Nagma Parveen†,‡ and Monika Schönhoff*,† †

Institute of Physical Chemistry, University of Muenster, Corrensstr. 28/30, 48149 Münster, Germany NRW Graduate School of Chemistry, University of Muenster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

‡

ABSTRACT: Swelling of polyelectrolyte multilayers with aqueous solution of ionic liquid (IL) is examined with a dissipative quartz crystal microbalance. Multilayers are prepared with different combinations of polyelectrolyte, employing PEI(PSS/ PAH)4PSS and PEI(PSS/PDADMAC)4PSS. An enhancement of mass coverage and dissipation is found for films in contact with aqueous solution of IL, i.e., 1-hexyl-3-methylimidazolium chloride and 1-methyl-3-methylimidazolium chloride, suggesting incorporation of IL, accompanied by film swelling. Swelling increases with increasing IL concentration. It is strictly reversible up to a stability limit of IL concentration, above which irreversible layer decomposition starts. Each swelling step can be divided into a fast and a slow process, resulting from IL uptake and chain reorganization or layer decomposition, respectively. While absolute IL uptake and swelling limits differ, the concentration dependence is similar in all systems investigated. The swelling is attributed to hydrophobic interactions, since we find a correlation of the swelling with hydrophobicity of IL.

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step swelling behavior.15 Wong et al. showed the odd−even effect in the reversible swelling of PSS/PAH multilayers at different relative humidity.16 Kügler et al. observed a slow swelling dynamics of PEM at different relative humidity due to rigid matrix of polyelectrolytes in PEM.17 pH-sensitive and ionic strength induced swelling of exponentially growing multilayers made from biopolymers is also well studied by various groups.18−21 Mjahed et al. showed that PLL/HA films swell with increasing ionic strength of electrolyte solution up to a critical concentration and then dissolve under release of polyelectrolytes,19 and later on they showed the possible restructuring of such PEM.20 Multilayers from synthetic polyelectrolytes, such as PAA/PDADMAC, PSS/PDADMAC, or PSS/PAH films, also swell with increasing NaCl concentration.22 Jaber et al. could even show evidence of the presence of doping salt ions inside films like PSS/ PDADMAC.23 Furthermore, the dependence of salt induced swelling on the internal structure of PEM and anion-specific effects have been examined.24 Even a successful permeation or diffusion of various ions25−30 through PEM prepared from highly charged polyelectrolytes are observed. Though there is evidence of permeation or diffusion of organic molecules and ions in multilayers,31−33 the presence of ion pairs in PEM can inhibit permeation of organic molecules. Recently, structural modulation of PEM has also been observed with salt34 and other amphiphilic molecules like surfactants.35−37

INTRODUCTION Ion-pairing of oppositely charged polyelectrolytes was applied to assemble thin film membranes named as polyelectrolyte multilayers (PEM). Hong and Decher first proposed the layerby-layer (LbL) sequential assembly technique for PEM preparation. 1 In this technique a defined number of monolayers, each with a thickness in the nm range, are adsorbed, such that ultrathin films are obtained with a defined thickness in the nm to μm range, where electrostatic interaction between polyelectrolytes ensures chemical and mechanical stability of PEM. Variation of the polyelectrolytes and variation of preparation parameters like pH, salt concentration, temperature, and ionic strength can tune the physical and chemical properties of PEM. The same parameters can even act as external stimuli for postpreparative changes of film properties. The flexibility of PEM with respect to such wide physical and chemical stimuli/variables promotes their application potential in several fields like nanofiltration,2,3 chemical reactors,4 and as free-standing membranes.5,6 The versatility of PEM has been described in several review articles.7−10 Current issues of great interest are the swelling and permeability of these multilayers and their response to external stimuli. When multilayers are brought into contact with a solution differing in pH, solvent (e.g., water, ethanol), or electrolyte (type or concentration), typically swelling of the film is observed, which might be accompanied by chain rearrangement and uptake of water, solvent, or electrolyte. There is for example plenty of literature on pH-driven properties of PEM prepared from weak polyelectrolytes.11−14 In dry films in contact with air, Köhler et al. examined water uptake during swelling of dry PSS/ PDADMAC films in different relative humidity, yielding a two© XXXX American Chemical Society

Received: August 2, 2013 Revised: September 12, 2013

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dx.doi.org/10.1021/ma401625r | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Layer formation on a gold-coated quartz crystal: (a) PEI(PSS/PDADMAC)4PSS; (b) PEI(PSS/PAH)4PSS. The 3rd, 5th, 7th, 9th, and 11th overtones are displayed, normalized on the respective order, n. HMIM is a liquid, and EMIM is a yellowish-white solid at room temperature; both are water-soluble. Aqueous solutions of different molar concentrations of each ionic liquid are prepared by dissolving them in ultrapure water. To remove air bubbles, solutions are treated in ultrasound for 10 min prior to measurement. Quartz Crystal Microbalance with Dissipation (QCM-D). A quartz crystal microbalance with dissipation detection having four parallel measurement cells (E4, QSense, Göteborg, Sweden) is employed to study growth and swelling of polyelectrolyte multilayers. Gold-coated quartz crystals with a fundamental frequency of 5 MHz are used (purchased from QSense) for multilayer preparation and insitu swelling of multilayers with ionic liquid solutions. Prior to use, quartz crystals are cleaned with RCA solution, which is a mixture in a ratio of 1:1:5 of 25% ammonia solution (Acros), 35% hydrogen peroxide (VWR), and ultrapure water. In this solution quartz crystals are heated for 20 min at 70 °C and finally rinsed extensively with ultrapure water. The QCM-D cell temperature is fixed to 20 ± 0.02 °C. Solutions are flowed through the cells forming contact with the upper side of the crystal surface. Measurements of the frequency and dissipation change, Δf and ΔD, respectively, are started after establishing a constant baseline with ultrapure water. During multilayer preparation and swelling/deswelling the flow rate is 200 and 50 μL/ min, respectively. Typically, five different overtones (3rd to 11th overtone) are evaluated and the values of Δf and ΔD are normalized on the order of the respective overtone. Layer Buildup. Multilayers are prepared by alternating flow of polyanion and polycation solutions through the cell, forming PEI(PSS/PAH)4PSS and PEI(PSS/PDADMAC)4PSS. Thus, each layer system consists of 10 layers, always with a first layer of PEI and terminated with PSS. Layer adsorption is accomplished during flow of the corresponding polyelectrolyte solution through the cell for 15−25 min. After adsorption of each monolayer, a flow of washing solution through the cell is applied for 5−15 min. After 10 layers are adsorbed, multilayers are washed with ultrapure water (pH 6.5) at least for 100 min to remove excess salt. Swelling Experiments. Swelling measurements are done by exposing PEM to ionic liquid solutions. Since exchange of the total cell volume takes about 3−5 min, ionic liquid solution is flowed for 15 min, and then the flow is stopped, while further film swelling takes place at constant concentration of ionic liquid. The swelling is observed until frequency and dissipation reach a plateau, which is defined as an absolute change in frequency by less than ±2 Hz and in dissipation by less than ±10−6 within 1 h, which is mostly reached within the first 2 h of swelling. Otherwise, observation is continued for at least 8 h; only in some exceptional cases (especially during swelling with high ionic liquid concentration) no plateau is reached after this time. After each swelling step deswelling of the film is induced by a flow of ultrapure water through the cell until a plateau is reached and if not then is observed at least for 8 h. Such a swelling and deswelling step form one swelling cycle. A series of swelling cycles with successively increasing ionic liquid concentration are then performed on the same multilayer system. Such swelling cycles are performed for

Another potential application of PEM is ion conducting membranes in batteries, since PEM provide a thin, but yet very stable, membrane.38 This requires incorporating lithium salt, where an electrolyte solvent for lithium ions would help to dope PEM. A well-known electrolyte solvent is ionic liquid due to its high ionic conductivity, wide electrochemical window, and nonvolatility.39,40 There is an extensive number of studies on electrochemical and physical properties of mixtures of IL with polymers;41,42 recent advances particularly focus on mixtures of polyelectrolytes with IL.43 Ohno et al. developed a series of polymerizable ionic liquid and polymerized them to obtain ion conductive electrolyte.44 Furthermore, studies on such polymeric ionic liquids in combination with different monomeric ionic liquids are pursued.45−47 All the above studies focus on bulk materials or rather thick membranes (μm range) prepared thereof. So far there is no reported investigation on PEM doped with ionic liquid. Here, we study interactions of PEM with IL, employing aqueous solutions of IL in order to incorporate IL into PEM. Hence, we investigate swelling of PEM by aqueous solution of watersoluble ionic liquids with the help of the QCM-D technique, which yields the mass uptake. We report on the swelling of two different multilayers with two ionic liquids. The aim is to test the reversibility of the swelling process and gain guidelines for stability limits of multilayers. Quantitative analysis of QCM-D data helps to visualize a specific trend of swelling, and further comparison of the data reveals the influence of hydrophobic interactions between PEM and ionic liquid.

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MATERIALS AND METHODS

Polyelectrolytes. Branched poly(ethylenimine) (PEI) (MW = 25 000) and poly(diallyldimethylammonium chloride) (PDADMAC) (MW = 100 000−200 000) are purchased from Sigma-Aldrich. Poly(allylamine hydrochloride) (PAH) (MW = 120 000−200 000) is purchased from Polyscience. Poly(sodium styrenesulfonate) (PSS) (MW = 70 000) is bought from Acros, dialyzed by a semipermeable membrane to remove low molecular weight components, and finally lyophilized. Polyelectrolyte solutions are prepared by dissolving polymer in 0.25 M NaCl solutions in ultrapure water (specific resistivity 18 MΩ cm−1, purified by Milli-Q filtration). The concentration of each polyelectrolyte solution is 10 mM with respect to the monomer unit. All solutions used in layer buildup are adjusted to pH 6.5 by addition of either 0.1 mol/L NaOH or 0.1 mol/L HCl; both are purchased from Fluka. Washing solutions consist of 0.25 M NaCl in ultrapure water, also adjusted to pH 6.5. Ionic Liquids. 1-hexyl-3-methylimidazolium chloride (HMIM+ Cl−) (purity >98.5% and dry) and 1-ethyl-3-methylimidazolium chloride (EMIM+ Cl−) (purity >98% and dry) are purchased from Sigma-Aldrich and are abbreviated as HMIM and EMIM, respectively. B


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Figure 2. Bulk effect of ionic liquid solutions on gold-coated quartz crystal without polymer at varying concentration of (a) HMIM and (b) EMIM. Numbers denote ionic liquid concentration; “w” stands for flowing with water. The 3rd, 5th, 7th, 9th, and 11th overtones are displayed, normalized on the respective order, n.

Figure 3. Time-dependent frequency and dissipation changes during layer formation and swelling cycles with successively increasing ionic liquid concentration, given as numbers in the graphs in units of molarity: (a) PEI(PSS/PDADMAC)4PSS with HMIM; (b) PEI(PSS/PAH)4PSS with HMIM; (c) PEI(PSS/PDADMAC)4PSS with EMIM; (d) PEI(PSS/PAH)4PSS with EMIM. In (b) the concentrations in the range of 0.1−0.35 M are the same as in (a). where Edissipation and Estored are the energy dissipated and energy stored during one period of oscillation. The dissipation here is in the order of 10−6. Low dissipation suggests a rigid film, and high dissipation describes a soft film.

both multilayer systems not only with either of the two ionic liquids but also with NaCl solutions of varying concentration in the same described manner. All swelling experiments are repeated several times (at least three times) to check reproducibility. As quantitative results, average values of frequency and dissipation change, Δf and ΔD, respectively, are taken from the seventh overtone and displayed in the Results section along with their respective standard deviations as error bars. The frequency change is a measure of the change of mass coverage, while the dissipation value obtained in QCM-D is a measure of viscoelastic properties of adsorbed films on the quartz crystal and is defined as

ΔD =

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RESULTS 1. PEM Preparation. Alternating adsorption of PSS and PDADMAC or PSS and PAH polyelectrolytes yields PEM films. The frequency and dissipation changes obtained by QCM-D in situ are shown in Figure 1 and indicate layer growth. Here, black and red lines designate frequency and dissipation changes, respectively, displaying five different overtones. The total change of Δf amounts to −220 ± 25 Hz for PEI(PSS/PDADMAC)4PSS and −140 ± 10 Hz for

Edissipation 2πEstored

(1) C


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concentration is further increased. The initial decrease of Δf (see Figure 3a−d) during the first ∼400−600 min represents multilayer formation as described above. During the swelling cycles, a general behavior is the decrease of the frequency along with an increase in dissipation when the film is exposed to IL solution. Swelling cycles with increasing concentrations are continued until layer destruction occurs. For example, in Figure 3a during deswelling after contact with 0.75 M ionic liquid solution Δf increases to −150 Hz, which means that a part of the layer system is destroyed, as 10 layers of PSS/PDADMAC initially yielded a Δf value of −220 Hz. Therefore, the concentration range of swelling is different in all four cases depending on the different combination of multilayers and ionic liquid. During a typical swelling step we always observe a fast frequency decrease along with a slow frequency change until in most cases a plateau is reached. In some cases, a plateau is not reached and Δf continues to decrease over time, especially during swelling with high concentrations of ionic liquid solutions. Hence, for analysis the total effect of swelling or deswelling is divided in two processes (see Figure 4). The fast process is

PEI(PSS/PAH)4PSS. From here on we designate them as PSS/ PDADMAC and PSS/PAH in short. ΔD varies only by ±5 × 10−6 for both systems, indicating rather rigid films in both cases. The above Δf and ΔD values are comparable with those obtained by other groups for the same polyelectrolytes.48,49 Both films are thin and rigid as they have small Δf and low ΔD values. From Figure 1 it can be seen from the Δf values that PSS/PDADMAC shows a slightly nonlinear growth behavior and furthermore yields thicker films as compared to linearly growing PSS/PAH. 2. Bulk Effect of Ionic Liquid. The properties of a liquid in contact with an adsorbed film, particularly its viscosity, often have a significant influence on the measured Δf and ΔD response in QCM-D. This is called the bulk effect of liquid. For a quantification of swelling data it is thus necessary to take the bulk effect of the ionic liquid solutions in contact with the quartz crystal surface into account. The time trace graphs of Figure 2 show the decrease of Δf accompanied by an increase of ΔD upon injection of ionic liquid solutions. The experiment contains repeated cycles flowing ionic liquid solution and water through the cell, where the ionic liquid concentration increases in each cycle, in the same way it is done in swelling experiments. During washing with water, Δf and ΔD increase and decrease again, respectively, roughly yielding the initial values for the gold surface in contact with water after each cycle. Thus, the variations of Δf and ΔD are clearly an effect of the adjacent liquid. They are similar for both ionic liquid solutions and can be attributed mainly to the enhanced viscosity of the solutions. Interestingly, Δf and ΔD responses are not exactly reversible, especially at higher concentrations, which cause a small permanent change in frequency and dissipation. Recently, it has been observed that ionic liquids can adsorb to gold surfaces;50 hence, an adsorption of ionic liquid on gold-coated quartz crystal may be the reason for such changes. In case of HMIM adsorption occurs through a slow decrease of Δf over longer time while such a slow process is absent in the case of EMIM. For a quantitative analysis of the bulk effect, we determine the frequency change which occurs within the time scale required for filling the cell chamber of QCM. As the small arrow in the last cycle of Figure 2 indicates, the Δf change within 5 min of flow of solution into the QCM cell is considered as frequency change of a “fast” bulk process, expressed as δΔf b(IL)f. In case of the dissipation, the difference between the final change in bulk dissipation has been calculated as δΔDb(IL)f; see red arrow in Figure 2. For both HMIM and EMIM ionic liquids, δΔf b(IL)f and δΔDb(IL)f values show a more or less linear relationship with concentration. Hence, we can conclude that the bulk effect of ionic liquid has a linear relation with concentration. Since the viscosity is also linearly dependent on IL concentration (data not shown), the bulk effect directly scales with the viscosity of the solution. 3. Swelling of Polyelectrolyte Multilayers with Ionic Liquid Solutions. The time trace graphs in Figure 3 describe how multilayer swelling experiments are carried out with ionic liquid solutions. The four graphs of Figure 3 depict two different multilayer systems upon exposure to the two different ionic liquids HMIM and EMIM. Swelling of multilayers is investigated after exposure of the film to an IL solution. IL concentration is increased stepwise, where each swelling step is followed by deswelling in ultrapure water. This step is done in order to test reversibility of the swelling process, before the IL

Figure 4. Definition of fast and slow process during swelling and deswelling of polyelectrolyte multilayers.

defined as occurring within 5 min from the start of flow of ionic liquid solution. This process is described by δΔfswf, and the remaining change in frequency is a slow process expressed as δΔfsws. It is important to note the absence of a frequency plateau in the slow step in some cases, as this causes δΔfsws to depend on equilibration/swelling time. In these cases, δΔfsws values after 8 h are evaluated. Similarly, deswelling of multilayers is also divided in two processes, and the corresponding frequency changes are expressed as δΔfdef and δΔfdes (see Figure 4). For deswelling, the time scale of the fast process is defined as 15 min from the start of flow of water, since the fast deswelling process occurs slower than the fast swelling process. In dissipation data, slow processes do not play any role. Therefore, as depicted in Figure 4, only a final dissipation change is determined for swelling and deswelling, respectively; it is expressed as δΔDsw/defin. In some cases, for PSS/ PDADMAC, the dissipation value does not reach a plateau. In these cases, the dissipation change after 8 h is considered as δΔDdefin. 3.1. Swelling in Dependence of Concentration of HMIM and EMIM: Fast Process, Frequency Change. To analyze the fast swelling process in detail, the values of δΔfsw/def are D


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Figure 5. Frequency change −∂f fast during the fast process of swelling and deswelling of (a) PEI(PSS/PDADMAC)4PSS and (b) PEI(PSS/ PAH)4PSS multilayer with HMIM (squares) or EMIM (triangles) solutions of different concentration. Filled and empty symbols are for swelling and deswelling, respectively; solid lines are guides to the eye; and dashed lines are inverted solid lines.

extracted as described above. The δΔfsw/def response during swelling of either film with ionic liquid solution is quite high, but above we discussed that ionic liquid solution leads to a frequency and dissipation response even without polymer film due to a bulk effect. Thus, frequency and dissipation data of swelling and deswelling of multilayers need correction for the effect of bulk solution. Assuming the films are thin and rigid, the effect of film swelling and adjacent bulk solution can be approximated by a superposition. Then, a direct subtraction of the frequency values of the bulk effect from the measured swelling data yields the actual swelling and deswelling values of the film given by ∂ffast = δ Δf sw/de f − δ Δf b(IL/w)f

Table 1. Frequency Change −∂f fastsat Reached at the Saturation Limit of Swelling, csw, As Extracted from Figure 5 and Limiting Concentration for Layer Stability cstab As Extracted from Figure 6 PSS/PDADMAC:HMIM PSS/PDADMAC:EMIM PSS/PAH:HMIM PSS/PAH:EMIM

where C = 17.7ng/(cm 2 Hz)

csw/M

cstab/M

130 122 42 18

0.7 1.25 1.35 2

0.7 1.25 1.65 3

very well to the open symbols of the deswelling data points. This indicates a reversible fast process of swelling in this regime. Above csw, however, the data points scatter and reversibility is not valid anymore. 3.2. Swelling in Dependence on Ionic Liquid Concentration: Slow Process, Frequency Change. In Figure 4, we already explained how the slow process of swelling has been calculated. Unlike the frequency change during the fast process, we have not done any bulk effect correction in frequency change for the slow process. The reason is that in the bulk measurements shown in Figure 2 the fast process is the result of bulk properties of the solution, i.e., a viscosity change compared to pure water. The slow process in this reference experiment is attributed to a reaction with gold, which is neglected for gold surfaces covered with polymer. Furthermore, only HMIM solutions have a small slow step frequency response in bulk effect, not EMIM solutions. Hence, we express the slow step frequency change during swelling as ∂fslow = δΔfsw/des. Figure 6 depicts the −∂fslow data in dependence on concentration, where again the solid lines serve to point out trends. For all four systems shown in Figure 6, we observe a critical concentration for the swelling processes, above which −∂fslow has negative values. The mass decrease implies layer decomposition above this critical concentration, which we designate as cstab, the stability limit concentration. The values are given in Table 1. Interestingly, stability limits agree very well with the concentrations of swelling saturation. Thus, the onset of film decomposition can be identified with the limit of swelling in the fast process. The deswelling data tend to show negative values at high concentration as well, but their values are very small and scatter too much for further evaluation. 3.3. Dissipation Change. According to the above definition in connection with Figure 4, the value δΔDsw/defin, evaluated at the end of a swelling step, contains the fast swelling process and any potentially existing, but hardly detectable, slow processes. In analogy to the frequency data of the fast process, a bulk

(2)

Figure 5 presents the dependence of −∂f fast on the concentration of ionic liquid solutions. Note that here we have plotted the negative frequency change −∂f fast, such that positive values correspond to a mass increase, since in the case of thin, rigid films the Sauerbrey equation51 holds, i.e. Δm = −C Δf

−∂f fastsat/Hz

(3)

With the help of a guideline the values of −∂f fast, which are proportional to the mass enhancement into each film are depicted. In Figure 5a, full squares describe the swelling of PSS/ PDADMAC with HMIM. The solid guide line shows that −∂f fast initially weakly increases, which is followed by a steeper increase (up to 120 Hz) until it levels off at a concentration of 0.7 M. This behavior is a general trend observed for all four systems, even if the changes of slope are not in all cases very pronounced. The systems differ, however, very much in the value of −∂f fast reached in the plateau and in the concentration where this plateau is reached. These quantities are evaluated and are given as the saturation value of the fast process, −∂f fastsat, and the limiting concentration of the swelling, csw, in Table 1. They will be discussed further below. The reversibility of swelling by IL can be quantified by comparing frequency changes for swelling and deswelling. In Figure 5, the −∂f fast values for deswelling after exposure to the respective concentration of ionic liquid solutions are included as open symbols. They generally exhibit negative values, corresponding to a mass loss upon removal of IL solution. To check reversibility of the swelling process, a guide line (here dashed line) has been drawn, which is the inversion of the solid guide line in Figure 5, which describes the swelling process. For concentrations below the swelling limit, the dashed line agrees E


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Figure 6. Frequency change during the slow process of swelling and deswelling of polyelectrolyte multilayers with HMIM (squares) and EMIM (triangles) solutions: (a) PEI(PSS/PDADMAC)4PSS; (b) PEI(PSS/PAH)4PSS (filled and open symbols for swelling and deswelling, respectively; solid lines are guides to the eye).

Figure 7. Dissipation change during swelling and deswelling of polyelectrolyte multilayers with varying concentration of HMIM (squares) and EMIM (triangles): (a) PEI(PSS/PDADMAC)4PSS; (b) PEI(PSS/PAH)4PSS (fill symbols and open symbols represent swelling and deswelling, respectively).

PDADMAC and PSS/PAH the maximum of mass uptake amounts to 59% and 33% of the mass of the initially formed multilayer, respectively. From QCM data it cannot be deduced whether this mass consists of mainly ionic liquid or even water that enters the film. However, in systems where multilayers swell by pure water uptake, for example driven by a change of the internal layer charge density, not only −∂f but also the dissipation is typically enhanced upon swelling, suggesting softer films when the water content is enhanced.52 In our present systems, the dissipation is not significantly altered in spite of a very large mass uptake. It is thus likely that the mass enhancement is predominantly due to IL incorporation into the film. Because of its strong interactions with the polymer, uptake of IL may cause generally more rigid films than pure water uptake. A quantification of IL uptake by an ATR-IR study is under way and will be published in a forthcoming paper. In addition, it can be argued that IL uptake consists of an almost stoichiometric uptake of cations and anions; otherwise, again, an electrostatically swollen, overcharged film with enhanced dissipation would be expected. For IL incorporation, the limiting concentration marks the onset of saturation; however, in view of the agreement with the stability limit cstab extracted from the slow process, it can also be viewed as an onset of deconstruction. The agreement between saturation and stability limiting concentration is in any case remarkable. It seems that deconstruction occurs as soon as the capacity of the film for IL uptake is reached. While uptake and swelling are fast, the deconstruction process, however, is a slow process, its kinetics probably being limited by the slow dynamics of polymer chains in PEM.

effect correction of the dissipation data is done by subtraction of the dissipation values of IL solutions in contact with uncoated quartz from the dissipation values of the respective swelling or deswelling step, i.e. ∂Dfin = δ ΔDsw/de fin − δ ΔDb(IL/w)fin

(4)

This correction eliminates the effect of viscosity difference between IL solution and water. ∂Dfin has been plotted in dependence on concentration of ionic liquid solutions in Figure 7. The dissipation is hardly affected by film swelling, and only small values of ∂Dfin are obtained, which scatter around zero. Only PSS/PDADMAC at high concentrations of ionic liquid seems to have a trend of obtaining positive values upon deswelling for HMIM as well as EMIM solutions, which implies a slight softening of the films when ionic liquid is washed out. However, altogether, changes in dissipation are very small. This is also the reason why separate fast and slow swelling processes could not be identified in dissipation measurements.

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DISCUSSION The quantitative analysis of time-dependent QCM-D data helps us to analyze different aspects of swelling of PEM with ionic liquid solutions in detail, which is discussed in this section. 1. Swelling Concentration and Limiting Concentration. The most relevant parameter to characterize swelling is certainly the frequency shift −∂f fast, as it quantifies the mass uptake of the film. This −∂f fast response is fairly high in all four cases in Figure 5. This infers a good swelling behavior with strong mass uptake in multilayers. In the case of PSS/ F


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separation than less hydrophobic ones.26 This indicates hindered transport of more hydrophobic (larger) alcohols, which in this case was attributed to a limited pore size of the film. In another case, permeation through PEM was enhanced for a hydrophobic molecule when compared to a charged molecules of similar size, which is the same trend in our swelling study.31 To clearly visualize the dependence of swelling on hydrophobicity of swelling agent, we compare our swelling data with swelling experiments of both types of multilayers with an inorganic salt, i.e., NaCl. Swelling values −∂f fast for swelling with NaCl solutions are obtained in the same manner for films prepared as described above (data not shown). The mass uptake also increases with increasing salt concentration and is reversible up to a limiting concentration. Figure 8 shows

2. Hydrophobic versus Electrostatic Interactions. Basically, HMIM and EMIM ionic liquid are organic salts, having an imidazolium cation with an alkyl side chain. The alkyl side chains are hexyl and ethyl for HMIM and EMIM, respectively. The longer hexyl side chain causes HMIM to be more hydrophobic than EMIM. On the other hand, PEM are consisting of amphiphilic polyelectrolytes with a hydrophobic polymer backbone and charged side groups. The number of carbon atoms per charge for PDADMAC and PAH polyelectrolytes is 8 and 3, respectively, making PDADMAC the more hydrophobic compound. Consequently, PSS/ PDADMAC films are more hydrophobic than PSS/PAH films.26,53 The role of hydrophobic interactions during swelling of PEM with ionic liquid can now be analyzed by comparison of different systems. In both systems shown in Figures 5a and 5b, respectively, the swelling value −∂f fast is always larger with HMIM than with EMIM at the same concentration. The more hydrophobic cation thus causes more pronounced swelling. This promotes the idea of hydrophobic interactions as driving force of swelling. On the other hand, the limiting concentration csw and also the stability limit cstab are higher for EMIM than HMIM in both polymer systems. This is reasonable, considering that EMIM uptake is lower and assuming that film stability is governed by a critical amount being incorporated. Comparison between the two different multilayer systems shows that −∂f fast is generally larger in PSS/PDADMAC than in PSS/PAH films. This can again be attributed to hydrophobic interactions as driving forces for swelling. Here again, the limiting concentration of swelling is higher for the less strongly swelling system, PSS/PAH. In short, the more pronounced the swelling, the lower the limiting concentration for saturation and the limit for film deconstruction. Taking a quantitative look, PSS/PDADMAC has similar −∂f fast values, i.e., 130 and 122 Hz at the limiting concentrations of both ionic liquids, but PSS/PAH has more strongly differing values of −∂f fast, i.e., 42 and 18 Hz for HMIM and EMIM, respectively. It seems that PSS/PAH has a lower, but more distinctly different, uptake capacity for IL as compared to PSS/PDADMAC. Krasemann et al. found higher permeation rates of Na+ and Mg+ ions in PSS/PDAMAC than in PSS/PAH multilayers and the selectivity of permeation is higher in PSS/PAH.25 They anticipated that a high charge density of PSS/PAH multilayers causes a higher selectivity in permeation of inorganic ions. This is in parallel with our observation of a higher incorporation difference of HMIM over EMIM in PSS/PAH multilayers compared to PSS/PDADMAC multilayers. Thus, a higher charge density or a lower hydrophobicity of PSS/PAH causes less hydrophobic interaction of PSS/PAH with IL and hence higher differences in IL uptake. The differences between the four systems studied here can thus all be explained by hydrophobic interactions as driving force of a swelling process, where hydrophobic HMIM or EMIM is incorporated into PEM. Stronger interactions lead to film saturation with ionic liquid already at lower concentration. These findings are consistent with some aspects from the literature about swelling of PEM or permeation of ions through PEM: For example, swelling in alcohol solution is high in low charge density PEM.32 On the other hand, though, alcohol incorporation has even resulted in a deswelling of PLL/PAC multilayers.54 In pervaporation experiments through PEM membranes a more hydrophobic alcohol has a more efficient

Figure 8. Plot of normalized frequency change in the fast swelling process in dependence on hydrophobicity of the cation.

normalized values of −∂f fast, evaluated at the limiting concentration (csw) of each system. They are normalized on the respective value for HMIM. We note that csw increases in order of HMIM < EMIM < NaCl for both films; thus, swelling with NaCl solution follows the previous observation of less pronounced swelling being accompanied by a higher limiting concentration. The swelling is plotted in dependence of hydrophobicity of the cations of HMIM, EMIM, and NaCl. The hydrophobicity values, as measured by cation partitioning of HMIM and EMIM in reversed phase HPLC, have been taken from Ranke et al.55,56 As Na+ is a small inorganic cation, it must have a lower hydrophobicity than both of the large organic cations. In Figure 8, the −∂f fast values are shown. The data show that pure electrostatic swelling with NaCl yields only small mass increments as compared to the more hydrophobic cations EMIM and HMIM. For both multilayers the extent of swelling clearly increases with hydrophobicity of the salt. In case of NaCl solutions, the significant difference between swelling of PDADMAC and PAH containing films cannot be explained by hydrophobic interactions. There might be noncompensated excess charges of one of the two polymer components in the film, which attract foreign salt ions and lead to swelling. Such a mechanism might also have some relevance for IL incorporation; however, the differences between the systems studied here are clearly due to hydrophobic interactions. There are some studies in the literature that report on swelling of PEM with solutions of simple salts such as NaCl or KCl, and their trends match some of our observations. For example, Figure 5 shows an increase of the slope and thus an G


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for swelling of PEM with ionic liquid. Nevertheless, purely ionically driven uptake of simple salts like NaCl might be controlled by noncompensated intrinsic charges in PEM, yielding more pronounced swelling of PDAMAC-containing films as compared to PAH-containing PEM. When swelling with successively more hydrophobic ions, the degree of swelling is more and more enhanced, and the degree of swelling is directly proportional to the hydrophobicity of the salt. The swelling process is reversible up to the limiting concentration, which implies that it consist of a reversible uptake of IL and possibly water, while slow processes of chain reorganization play a minor role.

enhanced swelling process from a certain IL concentration on. Earlier, Sukhorukov et al. showed a reversible swelling behavior of PSS/PAH upon exposure to 0.1 M NaCl solution and found a similar nonlinear swelling enhancement for PSS/PAH within a certain concentration range of NaCl.57 Later, for different inorganic salt solutions, Salomäki found a reversible swelling behavior of PSS/PDADMAC upon changing the ionic strength or the specific anion.24 PEM from biopolyelectrolytes like hyaluronic acid/myoglobin films have a high frequency response in QCM upon swelling with increasing concentration of KCl salt solution, and the maximum mass increment is in the range of 1.4−7.4 μg cm−2 at different KCl concentrations.21 Our value of such maximum mass increment is in the range of 0.3−2.6 μg cm−2. It is a little less than the reported one because our multilayers systems are prepared from synthetic polyelectrolytes. 3. Reversibility and Chain Rearrangement. By the experimental procedure, involving swelling cycles with successively increasing IL concentration, reversibility of the swelling process is carefully monitored. Reversible swelling implies that ionic liquid ions entering the film during contact with IL solution can be completely removed during deswelling. It is quite remarkable that the results for the fast process show perfectly reversible swelling below the limiting concentration, as indicated by the dashed line in Figure 5. While for the fast swelling process, characterized by ∂f fast, a very clear picture emerges, the values of ∂fslow are more complex to analyze. We can assume that ∂f fast contains fast transport processes of the ions (and probably water) into the layer. The fast process is found to be reversible; thus, the ions and water can freely enter and leave the polymer matrix, and a new partitioning equilibrium between solution and film is rapidly (on the order of minutes) established. The slow process is more complex to understand as it can result from three possible mechanisms, i.e., (1) rearrangement of polymer chains during swelling, (2) precipitation of ionic liquid from solution onto gold-coated quartz crystal, or (3) desorption of polyelectrolytes, i.e., layer destruction. The first two mechanisms should yield a positive −∂fslow value, and the third one yields a negative −∂fslow value. As swelling with HMIM always has high positive −∂fslow (Figure 6), there might be such chain rearrangements or ionic liquid precipitation occurring during swelling with HMIM, which are not detectable with EMIM. Such chain rearrangements have been concluded from neutron reflectometry data, where a slow kinetic step after annealing PEM with inorganic salt was observed.58,59 Such chain rearrangements might also occur upon swelling with IL solution below the limiting concentration, but obviously they only have a minor effect. Chain rearrangements would be expected as irreversible; however, the effect in Figure 6 is too small to draw such conclusions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +49-251-8323419; fax +49-251-83-29138 (M.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NRW Research School “Molecules and Materials − A Common Design Principle”. REFERENCES

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CONCLUSIONS We have shown here that polyelectrolyte multilayers show a reversible and effective swelling behavior with ionic liquid solutions. We always find a mass increment of PEM in contact with ionic liquid solution, which is increasing up to a limiting concentration, where saturation with ionic liquid occurs. Correlated to this, at the same concentration, layer decomposition starts. The degree of swelling depends on the composition of the polyelectrolyte multilayers and on the chemical nature of the ionic liquid. Comparing different systems, hydrophobic interactions are claimed as driving force H


Macromolecules

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Swelling and Stability of Polyelectrolyte Multilayers in Ionic Liquid

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