Interactions between Polyelectrolyte Brushes and Hofmeister Ions

Sep 11, 2015 - We have investigated the interactions between the positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) ...
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Interactions between Polyelectrolyte Brushes and Hofmeister Ions: Chaotropes versus Kosmotropes Ran Kou, Jian Zhang, Tao Wang, and Guangming Liu* Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, P. R. China 230026 S Supporting Information *

ABSTRACT: We have investigated the interactions between the positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes and the Hofmeister anions and the interactions between the negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brushes and the Hofmeister cations using a combination of quartz crystal microbalance with dissipation and spectroscopic ellipsometry. A V-shaped anion series is observed in terms of the ion-specific interactions between the PMETAC brushes and the Hofmeister anions. We have found that the chaotropic and kosmotropic anions interact with the PMETAC brushes in different manners. The ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the direct interactions between the anions and the positively charged quaternary ammonium group via ion pairing mediated by ionic hydration strength or polarizability, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules between the anions and the brushes. The ion-specific interactions between the PMETAC brushes and the anions have significant influences on both the hydration and the conformation of the brushes. The cations exhibit weaker specific ion effects on the PSPMA brushes in comparison with the specific anion effects on the PMETAC brushes.

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INTRODUCTION Hofmeister effects have attracted extensive attention since such interesting phenomena were first observed by the Czech scientist Franz Hofmeister in 1888 in the experiments of precipitation of egg white protein by different types of salts in aqueous solutions.1−8 Typically, Hofmeister ions follow the series SO42− > HPO42− > CH3COO− > Cl− > Br− > NO3− > I− > SCN− for anions and Na+ > K+ > Rb+ > Cs+ > NH4+ > N(CH3)4+ for cations in terms of their ionic hydration strength.9 The Hofmeister ions are usually categorized as chaotropes or kosmotropes based on their perceived influences on water structure.10 The ions on the left side of the series, defined as kosmotropes, exhibit strong interactions with water molecules, whereas the ions on the right side of the series, defined as chaotropes, are weakly hydrated by water molecules. It has been widely recognized that chaotropes and kosmotropes exhibit distinct effects on the stability and solubility of proteins via the indirect water-mediated ion−protein interactions or the direct ion−protein interactions.4,7,11−16 During the past decades, several models have been proposed to interpret the mechanism of Hofmeister effects.17−20 The model of water matching affinities describes that only © XXXX American Chemical Society

oppositely charged ions with similar water affinities can form strong ion pairs, which dominates the ion-specific interactions.17,18 The polarizability of ions is also considered to be important for the specific ioneffects and is manifested through the ionic dispersion forces, when the dispersion potential is treated at the same level as the electrostatic forces.20 An alternative explanation relates the Hofmeister effects to the specific interactions between the ions and various surfaces such as hydrophobic solid−water and air−water interfaces, where the kosmotropes are repelled from the surfaces but the chaotropes are adsorbed onto the surfaces.21−28 In analogy with the remarkable ion specificity of proteins which are a kind of biopolyelectrolyte, the artificial polyelectrolyte systems such as polyelectrolyte brushes also have obvious specific ion effects. Previous studies have shown that the conformational behavior of polyelectrolyte brushes is significantly dependent on the type of ions dissolved in solution.12,29,30 Furthermore, the macroscopic properties of Received: July 22, 2015 Revised: September 7, 2015

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thicknesses of PMETAC and PSPMA brushes in air were ∼22 and ∼14 nm, respectively, as determined by the spectroscopic ellipsometry (M-2000 V, J.A. Woollam, USA) by treating the polymer layer as a single Cauchy layer, where the refractive index of the polyelectrolyte brushes was assumed to be 1.495.41,42 Combination of QCM-D and Ellipsometry Measurements. The ion specificity of polyelectrolyte brushes in aqueous solutions was investigated by the combination of QCM-D and spectroscopic ellipsometry (SE) with an ellipsometry-compatible QCM-D module (QELM 401) from Q-Sense AB.29,43,44 The quartz crystal resonator with a fundamental resonant frequency of 5 MHz was mounted in a fluid cell with one side exposed to the solution. The resonator had a mass sensitivity constant (C) of 17.7 ng cm−2 Hz−1.45 When a quartz crystal is excited to oscillate in the thickness shear mode at its fundamental resonant frequency (f 0) by applying a RF voltage across the electrodes near the resonant frequency, a small layer added to the electrodes induces a decrease in resonant frequency (Δf) which is proportional to the mass change (Δm) of the layer. In vacuum or air, if the added layer is rigid, evenly distributed, and much thinner than the crystal, then the Δf is related to Δm and the overtone number (n = 1, 3, 5, ...) by the Sauerbrey equation46

polyelectrolyte brushes such as surface wettability, lubricity, protein adsorption, and bacteria killing can also be modulated by the ion-specific interactions between the brushes and the Hofmeister ions.31−36 Obviously, there have been strong specific ion effects on both the microscopic and the macroscopic properties of polyelectrolyte brushes. The detailed understanding of the mechanism of the interactions between polyelectrolyte brushes and the Hofmeister ions should be the key factor for controlling ion-specific properties of polyelectrolyte brushes. However, the ion specificity of polyelectrolyte brushes has largely been overlooked in conventional theories,37−40 and the mechanism of how the Hofmeister ions interact with polyelectrolyte brushes remains to be clarified. Herein, positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) and negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brushes prepared by the surface-initiated atom transfer radical polymerization (SI-ATRP) method have been employed to investigate the interactions between polyelectrolyte brushes and the Hofmeister ions by using a combination of quartz crystal microbalance with dissipation (QCM-D) and spectroscopic ellipsometry. We intend to clarify how the chaotropes and kosmotropes interact with the polyelectrolyte brushes in different manners and how the ion-specific interactions influence the conformational behavior of the polyelectrolyte brushes.

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Δm =

ρq lq Δf f0 n

= −C

Δf n

(1)

where f 0 is the fundamental frequency and ρq and lq are the specific density and thickness of the quartz crystal, respectively. The dissipation factor is defined by43 D=

Ed 2πEs

(2)

where Ed is the energy dissipated during one oscillation and Es is the energy stored in the oscillating system. The measurement of ΔD is based on the fact that the voltage over the crystal decays exponentially as a damped sinusoidal when the driving power of a piezoelectric oscillator is switched off.43 By switching the driving voltage on and off periodically, we can simultaneously obtain a series of measures of the resonant frequency and the dissipation factor. In this work, the changes in Δf and ΔD due to the interactions between the Hofmeister ions and the polyelectrolyte brushes can be extracted by taking the frequency and dissipation of the blank resonator in the relevant salt solutions as the reference. The thickness of polyelectrolyte brushes in wet state was determined by numerical modeling of the ellipsometric data with a two-layer model using the software Complete EASE. The optical properties of the blank resonator were recorded at first as the background signal in the relevant salt solutions. The two layers represented the polymer and silica layers. The polyelectrolyte brushes were treated as a single layer, which was assumed to be transparent and homogeneous (Cauchy medium) with a wavelength-dependent refractive index, nbrushes = Abrushes + (Bbrushes/λ2), and a negligible extinction coefficient. Here, Abrushes, Bbrushes, and dwet were fitted simultaneously.44 All the experiments were conducted at 25 ± 0.02 °C. In this work, the wet thicknesses of PMETAC and PSPMA brushes in water are ∼50 and ∼57 nm, respectively, obtained from the ellipsometry measurements. By fitting the changes in frequency and dissipation at the third and fifth overtones with the Voigt model,47 the wet thicknesses in water obtained from the QCM measurements are respectively ∼103 and 108 nm for the PMETAC and PSPMA brushes. The deviation of the QCM wet thickness from the ellipsometry wet thickness may be due to the inaccurate consideration of viscoelastic properties of the polymer brushes during QCM fitting.

EXPERIMENTAL SECTION

Materials. 3-Sulfopropyl methacrylate potassium salt (SPMA) (96%) and 2-(methacryloyloxy)ethyltrimethylammonium chloride (METAC) (75 wt %, in H2O) were purchased from Alladin and used as received. 2,2′-Dipyridyl (≥97%), methanol (HPLC, ≥99.9%), N,N-dimethylformamide (HPLC, ≥99.9%), and ethyl 2-bromoisobutyrate were purchased from Aldrich and used as received. Copper(II) bromide (CuBr2, 99%) was purchased from Sinopharm and used as received. Copper(I) bromide (CuBr) was prepared from CuBr2 by reacting with sodium sulfite, then washed with glacial acetic acid, ethanol, and diethyl ether, and dried under vacuum for 12 h. 3-(2Bromoisobutyryl)propyltriethoxysilane (BPE) was synthesized by our lab. Triethylamine (NEt3) was stirred with KOH for 12 h at room temperature, refluxed with toluene-4-sulfonyl chloride, and distilled before use. Toluene was refluxed in the presence of Na wire and then distilled prior to use. All the salts (99.99%, metals basis) were purchased from Aladdin and used as received. When studying anion specificity, we employed sodium salts so that the influence of cations was constant; similarly, chloride salts were used when investigating cation specificity. The water used was purified by filtration through Millipore Gradient system after distillation, giving a resistivity of 18.2 MΩ cm. Preparation of PMETAC and PSPMA Brushes. The silica-coated resonator surface was activated with water plasma treatment at a power of 18 W for ∼3 min, rinsed with Milli-Q water, and blown dry with N2. Then, the activated resonator was immersed in a toluene solution of 10 mM BPE and 15 mM triethylamine for 24 h in a sealed flask. Afterward, the surface was washed with toluene and ethanol and dried with N2. PMETAC brushes were prepared by SI-ATRP as follows. METAC (7.2 mL, 28.0 mmol), 2,2′-bipyridine (0.03 g, 0.2 mmol), and ethyl 2-bromoisobutyrate (0.01 g, 0.05 mmol) were added to 20.0 mL of a water/methanol mixture (1:3, v/v). After the solution was stirred at 25 °C under N2 for 30 min, the initiator-modified resonator was placed inside the flask under the protection of N2 and then keep stirring for another 30 min. Subsequently, CuBr (0.015 g, 0.1 mmol) was added to initiate the SI-ATRP at 25 °C for ∼16 h. Then, the resonator was washed with water and methanol and blown dry with N2. PSPMA brushes were prepared in a similar procedure. The dry

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RESULTS AND DISCUSSION Figure 1 shows the shift in Δf of the PMETAC brushes in the presence of different types of anions as a function of salt concentration. Δf is indicative of a change in mass of the polyelectrolyte brushes induced by hydration/dehydration of the brushes.29 A V-shaped anion series SO42− > HPO42− > B

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should interact with the PMETAC brushes in a different manner. The hydration strength of the kosmotropic anions increases following the order CH3COO− < HPO42− < SO42−.9 In Figure 1, Δf also increases following this series, suggesting that the amount of coupled water of the brushes decreases as the hydration strength of the kosmotropic anions increases. The kosmotropic anion with a higher hydration strength would exhibit a stronger competition for the water molecules in the hydration layer of the brushes, making more water molecules unavailable in solvating the grafted chains, thereby leading to a lower extent of hydration of the brushes.17 Thus, the ionspecific interactions between the PMETAC brushes and the kosmotropic anions should be dominated by the competition for the water molecules in the hydration layer between the anions and the brushes. Interestingly, CH3COO− instead of Cl− locates at the borderline between these two types of interaction regimes. As the salt concentration increases, more chaotropic anions diffuse into the brushes and associate with the quaternary ammonium group via ion pairing. As a consequence, the interactions between the grafted chains and the surrounding water molecules are further weakened, resulting in a reduction in the extent of hydration of the brushes, as indicated by the large increase in Δf with increasing salt concentration for the same chaotropic anion. By contrast, only a slight increase in Δf can be observed with increasing salt concentration for the same kosmotropic anion, indicating the weak association between the strongly hydrated kosmotropic anion and the weakly hydrated quaternary ammonium group. This result further confirms that the kosmotropic anions interact with the PMETAC brushes mainly through the competition for the water molecules in the hydration layer of the brushes. Such a competitive interaction mainly occurs in the outer part of the brushes, which is weakly correlated with the diffusion of anions into the brushes and thus is only slightly dependent on the salt concentration. The distinct interaction modes between the kosmotropic and the chaotropic anions with the PMETAC brushes can be further verified by the overtone number dependence of change in Δf of the brushes in the presence of the anions (Figure S1, Supporting Information). In general, ΔD reflects the comparative softness or rigidity of polyelectrolyte brushes induced by the swelling or collapse of the brushes.49 In Figure 2, ΔD decreases following the series CH3COO− > Cl− > Br− > NO3− > I− > SCN− at the same salt concentration, suggesting that a more chaotropic anion can more effectively induce the collapse of the PMETAC brushes. As a more chaotropic anion has a stronger interaction with the quaternary ammonium group, a smaller amount of freely mobile anions within the brushes is resulted as the anion changes from CH3COO− to SCN− along the series. This gives rise to a lower osmotic pressure within the brushes and a stronger collapse of the brushes in the presence of a more chaotropic anion. For the same chaotropic anion, ΔD remarkably decreases as the salt concentration increases. This means that the PMETAC brushes strongly collapse with increasing salt concentration because the osmotic pressure within the brushes is inversely proportional to the external salt concentration in the salted brush regime.37 Furthermore, the difference in ΔD between the different salt concentrations becomes more obvious as the anion changes from CH3COO− to SCN−. This fact implies that a more chaotropic anion can more strongly interact with the quaternary ammonium group

Figure 1. Shift in frequency (Δf) of the PMETAC brushes in the presence of different types of anions as a function of salt concentration, where the overtone number (n) is 3 and Na+ is the common cation. Salt concentration: 0.001 M (open symbol), 0.01 M (half up-filled symbol), 0.1 M (half right-filled symbol), and 0.5 M (filled symbol).

CH3COO− < Cl− < Br− < NO3− < I− < SCN− is observed in terms of the change in Δf at the same salt concentration, which is inconsistent with the classical Hofmeister series.2 SO42− and HPO42− are typical kosmotropic anions, whereas Br−, NO3−, I−, and SCN− are typical chaotropic anions.2 CH3COO− and Cl− locate around the border between chaotropes and kosmotropes in the Hofmeister series.2 The quaternary ammonium associated with the grafted PMETAC chains is a weakly hydrated group.48 According to Collins’ concept of matching water affinities, the strength of interaction between the quaternary ammonium group and the anions should increase following the series SO42− < HPO42− < CH3COO− < Cl− < Br− < NO3− < I− < SCN− because the extent of hydration of the anions decreases from SO42− to SCN− along this series.9 Hence, the anions are expected to form stronger ion pair with the quaternary ammonium group from SO42− to SCN− along the series. A stronger interaction between the quaternary ammonium group and the anion would cause a weaker interaction between the grafted chains and the surrounding water molecules, thereby leading to a smaller amount of water molecules coupled with the grafted chains. As a result, Δf of the PMETAC brushes should increase following the series as the anion changes from SO42− to SCN−. Note that the anion series predicted from the ion-specific dispersion interaction should be the same as that based on the model of matching water affinities. More specifically, the ion-specific dispersion interaction between the anions and the quaternary ammonium group should increase following the series as the anion changes from SO42− to SCN− because the ionic polarizability is generally increased from kosmotropic to chaotropic anions.5,7,21 The interactions between the anions and the quaternary ammonium group would lead to a loss of hydration shell of the bound anions and a decrease in the extent of hydration of the grafted chains.22,27 This would also cause the Δf to increase following the series from SO42− to SCN−. However, the suggestions proposed above can only explain the increase of Δf from CH3COO− to SCN− but cannot interpret the decrease of Δf from SO42− to CH3COO−. That is, the ion-specific interactions between the PMETAC brushes and the chaotropic anions should be dominated by the direct interactions between the anions and the positively charged quaternary ammonium group via ion pairing mediated by ionic hydration strength or polarizability, but the kosmotropic anions C

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Figure 2. Shift in dissipation (ΔD) of the PMETAC brushes in the presence of different types of anions as a function of salt concentration, where the overtone number (n) is 3 and Na+ is the common cation. Salt concentration: 0.001 M (open symbol), 0.01 M (half up-filled symbol), 0.1 M (half right-filled symbol), and 0.5 M (filled symbol).

after diffusing into the brushes accompanied by a stronger collapse of the brushes. For the kosmotropic SO42− and HPO42−, the PMETAC brushes also collapse as the salt concentration increases, as reflected by the decrease in ΔD with the increase of salt concentration. This is also due to the reduction of osmotic pressure induced by the diffusion of kosmotropic anions into the brushes. Nevertheless, the difference in ΔD between the different salt concentrations for SO42− and HPO42− is much smaller than that for the chaotropic anions, suggesting the much weaker association of the quaternary ammonium group with the kosmotropic anions than that with the chaotropic anions. At the low salt concentrations, the ΔD for SO42− and HPO42− is much lower than that for the chaotropic anions. This simple experimental fact further suggests that the interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules in the hydration layer of the brushes. Such a kind of competition strongly weakens the interaction between the brushes and the water molecules, leading to a low energy loss due to the friction between the brushes and the surrounding water molecules and resulting in a small dissipation factor. At the high salt concentrations, the strong collapse of the PMETAC brushes makes the ΔD for the chaotropic anions is comparable to that for the kosmotropic anions. In addition, the ΔD for SO42− is smaller than that for HPO42−at the same salt concentration, which means that the SO42− can more effectively induce the collapse of the brushes than HPO42−. We will give a more detailed discussion on this later based on the ellipsometry results. The kinetic processes accompanying the conformational change of polymer brushes can be followed by the ΔD−Δf plot.29,50−54 For example, the conformational change during the formation of physically adsorbed polymer brushes can be well analyzed by the ΔD−Δf plot.52 For the chemically grafted polymer brushes, the cooperativity between hydration/ dehydration and swelling/collapse of the brushes can also be obtained from the ΔD−Δf plot.55 In Figure 3a, ΔD decreases with the increase of Δf for all the anions, implying the collapse of the brushes with the dehydration of the grafted chains as the salt concentration increases. Two kinetic processes are observed in the presence of chaotropic anions, as exhibited

Figure 3. (a) Dissipation shift (ΔD) versus frequency shift (Δf) of the PMETAC brushes as a function of the anion type. Inset: the ΔD−Δf plot for SO42− and HPO42−. (b) Slope (s) of the ΔD−Δf plot in the second kinetic process as a function of anion type for the chaotropic anions.

by a faster decrease in ΔD with Δf in the first process and a relatively slow decrease in ΔD with Δf in the second process. However, only one kinetic process can be observed in the presence of kosmotropic anions, which can be more clearly visualized from the inset of Figure 3a. The chaotropic anions can strongly interact with the quaternary ammonium groups associated with the grafted PMETAC chains. At the low salt concentrations, the external anions can merely diffuse into the periphery of the brushes; that is, the electrostatic screening effect only takes place in the outer part of the brushes.37 The collapse of the grafted chains in the outer part of the brushes has a stronger effect on the energy dissipation than that in the inner part of the brushes.50 Thus, the ΔD exhibits a more rapid decrease with Δf in the first kinetic process than that in the second kinetic process. The slope (s) of the ΔD−Δf plot in the second kinetic process decreases following the series CH3COO− > Cl− > Br− > NO3− > I− > SCN− (Figure 3b), indicating that a more chaotropic anion can more effectively induce the collapse of the brushes at the same extent of hydration of the grafted chains. For the cases of SO42− and HPO42−, the interaction between these two types of anions and the brushes is dominated by the competition for water molecules in the hydration layer of the brushes, which is only slightly dependent on the salt concentration. As the salt concentration increases, the diffusion of kosmotropic anions into the brushes reduces the osmotic pressure within the brushes, leading to a simultaneous occurrence of the collapse and dehydration of the brushes, which is reflected by the appearance of only one kinetic process in the ΔD−Δf plot. Moreover, the slope of the ΔD−Δf plot for SO42− is smaller than that for HPO42−, indicating that the SO42− can more effectively induce the collapse of the brushes than HPO42−, similar to the observation in Figure 2. D

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the decreased osmotic pressure within the PMETAC brushes. On the left side of CH3COO−, the thickness of the brushes decreases following the series CH3COO− > HPO42− > SO42−. This means that a more kosmotropic anion can more effectively induce the collapse of the PMETAC brushes, which is consistent with the QCM-D results in Figures 2 and 3. It is known that the hydrophobic interaction between the grafted chains is driven by an increase of entropy via the release of water molecules from the hydration shell of the hydrophobic chain segments.56,57 In the presence of a more kosmotropic anion, the water molecules would more easily leave the hydration shell assisted by the anionic competition for the water molecules. As a consequence, a stronger hydrophobic interaction between the grafted chains is resulted in the presence of a more kosmotropic anion, which leads to a stronger collapse of the brushes. Similar to the chaotropic anions, the salt concentration dependence of thickness of the brushes for the kosmotropic anions is also attributed to the decrease of osmotic pressure within the brushes with increasing salt concentration. It is worth noting that the Coulomb forces should also contribute to but may not dominate the interactions between the divalent anion and the PMETAC brushes. Otherwise, no obvious specific anion effects would be observed between HPO42− and SO42− since electrostatic Coulomb interactions are nonspecific. The weak salt concentration dependence of shifts in frequency and dissipation for HPO42− and SO42− in Figures 1 and 2 further suggest that the ion-specific interactions between the PMETAC brushes and the kosmotropic divalent anions are dominated by the anionic competition for water molecules in the hydration layer of the brushes.

Figure 4 shows the change in wet thickness of PMETAC brushes in the presence of different types of anions as a

Figure 4. Change in wet thickness of the PMETAC brushes in the presence of different types of anions as a function of salt concentration, where Na+ is the common cation. Salt concentration: 0.001 M (open symbol), 0.01 M (half up-filled symbol), 0.1 M (half right-filled symbol), and 0.5 M (filled symbol).

function of salt concentration. An inverted V-shaped anion series is observed as the anion changes from SO42− to SCN−. On the right side of CH3COO−, the thickness of the brushes decreases following the series CH3COO− > Cl− > Br− > NO3− > I− > SCN− at the same salt concentration, implying that a more chaotropic anion can more effectively induce the collapse of the brushes. For the same chaotropic anion, the thickness of the brushes decreases with increasing salt concentration due to

Scheme 1. Schematic Illustration of the Ion-Specific Interactions between the PMETAC Brushes and the Hofmeister Anions

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Figure 5. (a) Shift in frequency (Δf) of the PSPMA brushes in the presence of different types of cations as a function of salt concentration. (b) Shift in dissipation (ΔD) of the PSPMA brushes in the presence of different types of cations as a function of salt concentration. Here, the overtone number (n) is 3 and Cl− is the common anion. Salt concentration: 0.001 M (open symbol), 0.01 M (half up-filled symbol), 0.1 M (half right-filled symbol), and 0.5 M (filled symbol).

be converted to the Sauerbrey mass according to eq 1 (Figure S2). The molar masses of NH4+, K+, Na+, N(CH3)4+, Rb+, and Cs+ are 18, 39, 23, 74, 85, and 133 g mol−1, respectively. The cation series observed in Figure 5a is almost the same as the ion series ordered according to the molar mass of the cations with an exception of Na+. The reversal of the order between K+ and Na+ may be because the hydrodynamic molar mass of Na+ is larger than that of K+.18,60 Therefore, we suggest that the cation specificity observed in Δf is attributed to the different molar masses of the bound cations of the PSPMA brushes instead of the ion-specific interactions between the negatively charged sulfonate group and the cations. This suggestion is further supported by the fact that no obvious cation specificity is observed in ΔD (Figure 5b). Similar to the ΔD−Δf plot for the chaotropic anions, two kinetic processes are observed in the presence of cations as shown in Figure 6. In the first kinetic process at the low salt

The mechanism of ion-specific interactions between the PMETAC brushes and the Hofmeister anions is illustrated in Scheme 1. At the same salt concentration, the less hydrated chaotropic anion can more effectively induce the collapse and dehydration of the PMETAC brushes owing to the formation of stronger ion pair between the anion and the quaternary ammonium group. In contrast, the more hydrated kosmotropic anion can more effectively induce the collapse and dehydration of the PMETAC brushes due to the stronger anionic competition for water molecules in the hydration layer of the brushes. That is, the ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the ion pairing between the anions and the quaternary ammonium group, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules in the hydration layer of the brushes. As the salt concentration increases, the brushes collapse for both kosmotropic and chaotropic anions induced by the decrease of osmotic pressure within the brushes. We have also investigated the interactions between the negatively charged PSPMA brushes and the Hofmeister cations. As the salt concentration increases, Δf increases (Figure 5a) and ΔD decreases (Figure 5b) for the same type of cation. This result indicates that the brushes dehydrate and collapse with increasing salt concentration induced by the decrease of osmotic pressure within the brushes. Moreover, Δf decreases following the series NH4+ > K+ > Na+ > N(CH3)4+ > Rb+ > Cs+ at the same salt concentration, suggesting that the mass associated with the PSPMA brushes increases following this series. This cation series is inconsistent with either the classical Hofmeister series or the V-shaped ion series similar to the anions observed in Figure 1. Furthermore, in comparison with the specific anion effects on the PMETAC brushes, the cations exhibit weaker specific ion effects on the PSPMA brushes, as reflected by the smaller difference in Δf between the different types of cations at the same salt concentration. This is similar to our previous observation for the ion-specific conformational behavior of polyzwitterionic brushes.33 Manning’s theory of counterion condensation describes that strong polyelectrolytes are not 100% dissociated.58 That is, a portion of the counterions are condensed onto the polyelectrolyte chains in either territorial binding or site binding states (i.e., ion pairs).59 For the PSPMA brushes, the bound cations contribute to the coupled mass and can therefore be detected by QCM-D. The change in frequency in Figure 5a can

Figure 6. Dissipation shift (ΔD) versus frequency shift (Δf) of the PSPMA brushes as a function of the cation type.

concentrations, the rapid decrease in ΔD with Δf is attributed to the collapse of grafted chains in the outer part of the brushes with increasing salt concentration. In the second kinetic process at the high salt concentrations, the relatively slow decrease in ΔD with Δf, indicating that the collapse develops into the inner part of the PSPMA brushes due to the diffusion of external ions into the brushes. Again, no obvious cation specificity can be observed in the ΔD−Δf plot of the PSPMA brushes. Similarly, the change of wet thickness of the PAMPSA brushes also shows very weak specific cation effects (Figure 7), where the decrease in thickness of the PSPMA brushes with increasing salt concentration for the same type of cation is attributed to the reduction of osmotic pressure within the brushes. Interestingly, F

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ACKNOWLEDGMENTS The financial support of National Program on Key Basic Research Project (2012CB933800), the National Natural Science Foundation of China (21374110, 21574121, 91127042), and the Youth Innovation Promotion Association CAS is acknowledged.

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Figure 7. Change in wet thickness of the PSPMA brushes in the presence of different types of cations as a function of salt concentration, where Cl− is the common anion. Salt concentration: 0.001 M (open symbol), 0.01 M (half up-filled symbol), 0.1 M (half right-filled symbol), and 0.5 M (filled symbol).

the ion-specific interactions can be observed between the multivalent cations and the PSPMA brushes (Figures S3−S5). Similar to the divalent kosmotropic anions for the PMETAC brushes, the weak salt concentration dependence of shifts in frequency and dissipation for the multivalent cations (Figures S3 and S4) suggest that the ion-specific interactions between the PSPMA brushes and the kosmotropic multivalent cations should be dominated by the cationic competition for water molecules in the hydration layer of the brushes.

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CONCLUSION In this work, we have demonstrated that the chaotropic and kosmotropic anions interact with the positively charged PMETAC brushes in different manners. The ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the ion pairing between the anions and the positively charged quaternary ammonium group, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules in the hydration layer of the brushes. Such two distinct interaction modes give rise to a Vshaped anion series in terms of the interactions between the PMETAC brushes and the Hofmeister anions. In contrast, no obvious cation specificity can be observed in the interactions between the PSPMA brushes and the monovalent cations. The mechanism elucidated here forms a basis for understanding of the ion-specific interactions between polyelectrolyte brushes and Hofmeister ions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02698. Additional QCM-D and ellipsometry data about the PMETAC and PSPMA brushes (PDF)

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REFERENCES

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*E-mail: [email protected] (G.L.). Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.langmuir.5b02698 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b02698 Langmuir XXXX, XXX, XXX−XXX