Cation-Specific Conformational Behavior of Polyelectrolyte Brushes

Oct 9, 2014 - We have investigated changes in the cation-specific conformational behavior of poly(sodium styrenesulfonate) (PSS) brushes as the solven...
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Cation-Specific Conformational Behavior of Polyelectrolyte Brushes: From Aqueous to Nonaqueous Solvent Tao Wang,† Yunchao Long,† Lvdan Liu,† Xiaowen Wang,† Vincent S. J. Craig,‡ Guangzhao Zhang,§ and Guangming Liu*,† †

Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, PR China 230026 ‡ Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra ACT 0200, Australia § Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou, PR China 510640 S Supporting Information *

ABSTRACT: We have investigated changes in the cation-specific conformational behavior of poly(sodium styrenesulfonate) (PSS) brushes as the solvent changes from water to methanol using a quartz crystal microbalance with dissipation (QCM-D). A solvation to desolvation transition of the grafted chains accompanied by swelling to the collapse transition of the brushes is observed for Na+. In the case of Cs+, the brushes undergo solvation to desolvation to resolvation accompanied by swelling to collapse to reswelling transitions. The resolvation and reswelling transitions for Cs+ are induced by the charge inversion of the brushes via van der Waals interactions between Cs+ and the brushes. All of the transitions for monovalent cations become less obvious as the methanol content increases. For divalent Ca2+ and trivalent La3+, a solvation to desolvation to resolvation transition of the grafted chains accompanied by a swelling to collapse to reswelling transition of the brushes can be observed. The resolvation and reswelling of the brushes for the multivalent cations are induced by the charge inversion of the brushes via charge-image charge interactions. The extent of the transitions for the PSS brushes in the presence of multivalent cations is only slightly influenced by the methanol content.

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INTRODUCTION

It has been reported that the conformational behavior of polyelectrolyte brushes strongly depends on the nature of the salt present in aqueous solutions.14,23−25 That is, polyelectrolyte brushes exhibit ion-specific conformational behavior in the presence of different types of salts.24,26,27 Our recent studies have demonstrated that the specific ion effects on polymer conformation are also significantly influenced by the solvent composition in aqueous/nonaqueous solvent mixtures.28−31 For example, the specific ion effects on polymer conformation can be amplified by the addition of alcohols to water.28−31 In comparison to the ion-specific conformational behavior of polyelectrolyte brushes in aqueous solutions that is mainly dominated by the salt type, the specific ion effects on the conformational changes of polyelectrolyte brushes in aqueous/ nonaqueous solvent mixtures should be influenced by both the salt type and the solvent composition. However, the mechanism of how the ion-specific conformational behavior of polyelectrolyte brushes will be influenced by the solvent composition in aqueous/nonaqueous solvent mixtures is still unclear. In the present work, poly(sodium styrenesulfonate) (PSS) brushes have been grafted onto a resonator surface via surface-

Polyelectrolytes are macromolecules carrying ionic groups that can dissociate in polar solvents, forming a charged polymer backbone.1,2 When polyelectrolyte chains are attached to a solid surface with a high grafting density, the attached chains will form polyelectrolyte brushes.3−6 The conformational behavior of polyelectrolyte brushes usually plays a crucial role in their applications in a wide range of fields such as colloidal stability, protein adsorption, surface wettability, and tribology.7−12 It is known that the conformational behavior of polyelectrolyte brushes is strongly influenced by solvent quality, which can be tuned by either the addition of salts or the addition of organic solvents to aqueous solutions.6,13−19 For instance, polyelectrolyte brushes collapse with increasing salt concentration because of the electrostatic screening effect of the added salts.6,13,14 When organic solvents are added to aqueous solutions, the dielectric constant of solvent is reduced and the electrostatic attractions between the charged groups associated with polyelectrolyte chains and the counterions are strengthened.20 Therefore, the extent of counterion condensation increases upon addition of organic solvents to aqueous solutions.21,22 As a result, polyelectrolyte brushes also collapse with increasing content of organic solvents because the degree of charging of polyelectrolyte chains decreases with increasing extent of counterion condensation.15−19 © 2014 American Chemical Society

Received: August 20, 2014 Revised: October 5, 2014 Published: October 9, 2014 12850

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Figure 1. Shifts in frequency (Δf) and dissipation (ΔD) for the PSS brushes as a function of the volume fraction of methanol (xM) in the water/ methanol mixtures. (a) Frequency shift (Δf). (b) Dissipation shift (ΔD). The dashed lines are provided to guide the eye. PSS brushes in air and water were ∼9.0 and ∼27.0 nm, respectively, as determined by spectroscopic ellipsometry (M-2000 V, J. A. Woollam, U.S.A). QCM-D Measurements. QCM-D and the AT-cut quartz crystals were from Q-sense AB.32 The quartz crystal resonator with a fundamental resonance frequency of ∼5 MHz was mounted in a fluid cell with one side exposed to the solution. The resonator has a mass sensitivity constant (C) of 17.7 ng cm−2 Hz−1.33 When a quartz crystal is excited to oscillate in thickness shear mode at its fundamental resonance frequency ( f 0) by applying a RF voltage across the electrodes near the resonance frequency, a small layer added to the electrodes induces a decrease in resonance frequency (Δf) that 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 Δf is related to Δm and the overtone number (n = 1, 3, 5...) by the Sauerbrey equation,34

initiated atom-transfer radical polymerization (SI-ATRP). We have investigated the cation-specific conformational behavior of PSS brushes as the solvent changes from water to methanol using a quartz crystal microbalance with dissipation (QCM-D). Considering that both the cation type and methanol content have strong effects on the conformation of PSS brushes, we intend to clarify the influence of methanol content on the cation-specific conformational behavior of PSS brushes in water/methanol mixtures. Herein, the macroscopic phase separation that usually occurs for the free polyelectrolyte chains in organic solvent can be avoided such that we can examine the mechanism of the cation-specific conformational behavior of polyelectrolyte brushes in aqueous/nonaqueous solvent mixtures.

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EXPERIMENTAL SECTION

Δm = −

Materials. Sodium styrenesulfonate (SS) from Aladdin was recrystallized three times in ethanol/water mixtures. 2,2′-Dipyridyl (≥ 97%), methanol (HPLC, ≥ 99.9%), and ethyl 2-bromoisobutyrate were purchased from Aldrich and used as received. ω-Mercaptoundecyl bromoisobutyrate was purchased from Beijing HRBio Biotechnology Co. and used as received. Sodium chloride (NaCl, 99.9%, metals basis), cesium chloride (CsCl, 99.9%, metals basis), calcium chloride (CaCl2, 99.9%, metals basis), and lanthanum chloride (LaCl3, 99.9%, metals basis) were purchased from Aladdin and used as received. Copper(II) bromide (CuBr2, 99.0%) was purchased from Sinopharm and used as received. Copper(I) bromide (CuBr) was prepared from CuBr2 by reaction with sodium sulfite and then washed successively with glacial acetic acid, ethanol, and diethyl ether and dried under vacuum for 12 h. The water that was used was purified by filtration through a Millipore gradient system after distillation, giving a resistivity of 18.2 MΩ cm. Preparation of PSS Brushes. The gold-coated resonator was cleaned using piranha solution composed of one part H2O2 and three parts H2SO4 at 70 °C for ∼15 min, rinsed with Milli-Q water, and blown dry with N2 before use. The monolayer of ATRP initiator was prepared by placing the resonator in a 5.0 mM solution of ωmercaptoundecyl bromoisobutyrate in anhydrous ethanol for ∼24 h at room temperature. Then, the resonator was rinsed with ethanol, dried with N2, and used immediately for the following surface-initiated polymerization. PSS brushes were prepared by SI-ATRP as follows. SS (2.0 g, 10.0 mmol), 2,2′-bipyridine (0.06 g, 0.4 mmol), and ethyl 2bromoisobutyrate (0.02 g, 0.1 mmol) were added to 30.0 mL of a water/methanol mixture (1:1, 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. Afterward, CuBr (0.03 g, 0.2 mmol) was added to initiate the ATRP at 40 °C for ∼24 h, followed by washing with water and methanol. To remove the ligand, Cu2+, and unreacted monomers, the resonator was soaked in a 3.0 mg mL−1 EDTA−2Na solution overnight. Finally, the resonator was rinsed with water, blown dry with N2, and stored in N2 at 4 °C until used. The dry and wet thicknesses of

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 by32 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 sinusoid when the driving power of a piezoelectric oscillator is switched off.32 By switching the driving voltage on and off periodically, we can simultaneously obtain a series of changes in the resonance frequency and dissipation factor. Because the shifts in frequency and dissipation are also influenced by changes in solution density and viscosity during solution exchange, the shifts in Δf and ΔD due to conformational changes in polyelectrolyte brushes were extracted by subtracting the background response of the blank resonator. In the present study, all of the results obtained were from measurements of frequency and dissipation at the third overtone (n = 3). All experiments were performed at 25 ± 0.02 °C.

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RESULTS AND DISCUSSION Figure 1 shows shifts in frequency (Δf) and dissipation (ΔD) for PSS brushes as a function of volume fraction of methanol (xM) in the water/methanol mixtures. It is known that Δf is indicative of a change in mass of the brushes induced by the solvation/desolvation of the grafted chains and ΔD reflects the comparative softness or rigidity of the polymer layer induced by the swelling or collapse of the brushes.35 In Figure 1a, Δf gradually increases with increasing xM, indicating the gradual desolvation of the grafted chains with the addition of methanol. It is reported that the dielectric constant (ε) of water/methanol 12851

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Figure 2. Conformational behavior of PSS brushes in the presence of NaCl in the water/methanol mixtures. (a) NaCl concentration (CNaCl) dependence of the frequency shift (Δf) for the PSS brushes as a function of the volume fraction of methanol (xM). (b) NaCl concentration (CNaCl) dependence of the dissipation shift (ΔD) for the PSS brushes as a function of the volume fraction of methanol (xM). (c) Critical NaCl concentration (CNaCl) as a function of the volume fraction of methanol (xM) at which the transition occurs from the first to the second regime. (d) Dissipation shift (ΔD) versus the frequency shift (−Δf) for the PSS brushes as a function of the volume fraction of methanol (xM) in the presence of NaCl. The dashed curves are provided to guide the eye.

mixtures gradually decreases from 78.5 to 32.7 as xM increases from 0 to 100% at 25 °C.20 The Bjerrum length (lB), which reflects the strength of electrostatic interactions between monovalent ions, is defined as36

lB =

e2 4πεkBT

quality of the water/methanol mixtures for the PSS brushes becomes poorer as xM increases from 0 to 100%. Figure 2a,b respectively shows the NaCl concentration (CNaCl) dependence of Δf and ΔD for the PSS brushes as a function of xM in the water/methanol mixtures. There are three regimes in Figure 2a,b. For example, Δf only slightly decreases as CNaCl increases from 0 to 1.0 ×10−4 M, and then it exhibits a relatively rapid decrease as CNaCl increases from 1.0 ×10−4 to 1.0 ×10−2 M, followed by an increase in Δf with a further increase in CNaCl from 1.0 × 10−2 to 5.0 M at xM = 0%. Similarly, ΔD increases only slightly as CNaCl increases from 0 to 1.0 ×10−4 M and then rapidly increases as CNaCl increases from 1.0 ×10−4 to 1.0 ×10−2 M and finally decreases with increasing CNaCl from 1.0 × 10−2 to 5.0 M at xM = 0%. In the first regime, the slight shifts in Δf and ΔD reflect that the added ions have little effect on the hydration and conformation of the PSS brushes. This is because the added ions diffuse into the brushes with difficulty because of the relatively low concentration of external ions compared to that of the counterions trapped in the PSS brushes.6 In the second and third regimes, the decrease (increase) and the following increase (decrease) in Δf (ΔD) suggest that the extent of hydration of the PSS brushes first increases and then decreases with CNaCl. This is accompanied by the swelling and subsequent collapse of the brushes. Numerous studies have been conducted to investigate the salt effects on the conformational behavior of strong polyelectrolyte brushes.6,13,14,39 Most of the studies have shown that strong polyelectrolyte brushes are not influenced by the added salts in the low salt concentration regime, whereas strong polyelectrolyte brushes collapse upon the addition of salts in the high salt concentration regime.13−15 However, a few studies reported recently have demonstrated that the extent of hydration and thickness of strong polyelectrolyte brushes

(3)

where kB, T, and e are the Boltzmann constant, the absolute temperature, and the elementary charge, respectively. When the distance of two oppositely charged ions is less than lB, the two ions will form an ion pair.36 Therefore, the negatively charged sulfonate groups can more easily form ion pairs with counterions as lB increases because of the reduction of ε upon the addition of methanol. Manning’s counterion condensation theory states that the linear charge density of a rigid strong polyelectrolyte chain should be reduced to one charge per lB by condensed counterions.37,38 Though PSS chains are not perfectly rigid, the linear charge density should also decrease with increasing xM because of the increasing lB induced by the addition of methanol. Consequently, the extent of solvation of the grafted chains decreases with increasing xM because of the weakening interactions between the grafted chains and the solvent molecules via the charge-dipole interactions, as reflected by the gradual increase in Δf with xM. Meanwhile, the increasing extent of counterion condensation and the decreasing chain charge density reduce the osmotic pressure generated by the free counterions in the brushes and weaken the electrostatic repulsions between the grafted chains as xM increases, thereby leading to the collapse of the PSS brushes, as reflected by the gradual decrease in ΔD with xM (Figure 1b). From the discussions above, the solvent 12852

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Figure 3. Conformational behavior of PSS brushes in the presence of CsCl in the water/methanol mixtures. (a) CsCl concentration (CCsCl) dependence of the frequency shift (Δf) for the PSS brushes as a function of the volume fraction of methanol (xM). (b) CsCl concentration (CCsCl) dependence of the dissipation shift (ΔD) for the PSS brushes as a function of the volume fraction of methanol (xM). (c) Critical CsCl concentration (CCsCl) as a function of the volume fraction of methanol (xM) at which the transition occurs from the first to the second regime. (d) Dissipation shift (ΔD) versus frequency shift (−Δf) for the PSS brushes as a function of the volume fraction of methanol (xM) in the presence of CsCl. The dashed curves are provided to guide the eye.

regime become less obvious with increasing xM and eventually disappear at xM = 100%. As discussed in Figure 1, more ion pairs would be formed within the brushes with increasing xM. Consequently, the brushes adopt a more collapsed conformation as xM increases because of the strengthening of dipole− dipole interactions. If the external ions that enter the brushes can effectively break the dipole−dipole bonds between the grafted chains, then the solvation to desolvation and swelling to collapse transitions should become more obvious with increasing xM. However, the dipole−dipole interaction energy is inversely proportional to ε2.45 Thus, the dipole−dipole interactions become much stronger with decreasing ε as xM increases. Consequently, the cross-linked grafted chains are more difficult to separate with increasing xM, and the solvation to desolvation and swelling to collapse transitions for the PSS brushes become less obvious with the addition of methanol. This is further evidenced by the fact that the critical NaCl concentration required to induce the solvation and swelling of the PSS brushes increases with increasing xM (Figure 2c). Here, the critical NaCl concentration is determined by the intersection of the lines through the data during the transition (Figure S1, Supporting Information). It is known that the kinetic processes accompanying conformational changes in polymer brushes can be followed by the ΔD−Δf plot.46,47 Δf and ΔD only slightly change in the first regime in Figure 2a,b, which is reflected by the collection of all of the data points in one position in Figure 2d. Thus, there are only two kinetic processes in the ΔD−Δf plot for the conformational changes of the PSS brushes, which correspond to the second and third regimes in Figure 2a,b. In the first kinetic process, ΔD increases with increasing −Δf, indicating the solvation of the grafted chains and the swelling of the brushes upon addition of NaCl. In the second kinetic process,

increases with increasing salt concentration before entering the “salted brushes” regime in which the brushes collapse as the salt concentration increases,40,41 similar to the results shown in Figure 2a,b. The mechanism proposed to explain the swelling of PSS brushes in the second regime is that the penetration of added ions into the brush disrupts the multiplets formed by the ion pairs.40,42 More specifically, a portion of the confined counterions form ion pairs with the negatively charged sulfonate groups within the brushes in the salt-free solution. These formed ion pairs cause the grafted chains to associate or cross-link together to some extent because the dipole−dipole interactions are similar to those of polyzwitterionic brushes.40,42 Thus, the PSS brushes are in a partially collapsed state induced by the association of the grafted chains. The external ions can penetrate the outer part of the brushes even at low salt concentrations because the distribution of counterions is nonuniform within the brushes and the concentration of counterions in the outer part of the brushes is much lower than that in the inner part of the brushes.43 Because the electrostatic dipole−dipole interactions are weakened by the external ions that enter the brushes, the extent of both the hydration and swelling of the PSS brushes is expected to increase with salt concentration because of the disassociation of the cross-linked grafted chains.44 This is why Δf decreases and ΔD increases with CNaCl in the second regime. As the salt concentration increases further from 1.0 × 10−2 M in the third regime, Δf increases and ΔD decreases with CNaCl, indicating the dehydration of the grafted PSS chains accompanied by the collapse of the PSS brushes in the salted brushes regime.35 Similar results are also observed in water/methanol mixtures, but the solvation to desolvation and swelling to collapse transitions for the PSS brushes from the second to the third 12853

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Figure 4. Conformational behavior of PSS brushes in the presence of CaCl2 in the water/methanol mixtures. (a) CaCl2 concentration (CCaCl2) dependence of the frequency shift (Δf) for the PSS brushes as a function of the volume fraction of methanol (xM). (b) CaCl2 concentration (CCaCl2) dependence of the dissipation shift (ΔD) for the PSS brushes as a function of the volume fraction of methanol (xM). (c) Critical CaCl2 concentration as a function of the volume fraction of methanol (xM) at which the transition occurs. C1(CaCl2) indicates the critical CaCl2 concentration at which the transition occurs from the first to the second regime, and C2(CaCl2) indicates the critical CaCl2 concentration at which the transition occurs from the third to the fourth regime. (d) Dissipation shift (ΔD) versus frequency shift (−Δf) for the PSS brushes as a function of the volume fraction of methanol (xM) in the presence of CaCl2. The dashed curves are provided to guide the eye.

the decrease in ΔD with decreasing −Δf is indicative of the desolvation of the grafted chains accompanied by the collapse of the brushes with the further addition of NaCl. As xM increases, these two kinetic processes become less obvious and eventually disappear in pure methanol, which is similar to the observation in Figure 2a,b. Furthermore, the value of ΔD in the swelling process is always higher than that in the collapse process for the same −Δf, showing that for a given overall extent of solvation more shear energy is dissipated by the brushes undergoing swelling than those undergoing collapse. This phenomenon arises from the difference in the tails formed at the outer part of the brushes, which have strong effects on energy dissipation.16 In the swelling process, the brushes begin to swell from the outer part of the brushes because of the penetration of external ions, thereby forming some short tails on the surface of the brushes, giving rise to a high value of ΔD. In contrast, the collapse of PSS brushes from the swollen state does not generate as many flexible tails. Consequently, ΔD in the second kinetic process is lower than that in the first kinetic process for the same −Δf. Figure 3a,b shows the CsCl concentration (CCsCl) dependence of Δf and ΔD for the PSS brushes as a function of xM. In comparison to the conformational behavior of PSS brushes in the presence of NaCl, the PSS brushes exhibit similar conformational changes upon the addition of CsCl before CCsCl reaches ∼1.0 M. In the first regime, the hydration and conformation of PSS brushes show only slight changes, as indicated by the slight decrease in Δf and the increase in ΔD. From the second to the third regime, the decrease to increase transition of Δf and the increase to decrease transition of ΔD

imply the occurrence of the solvation to desolvation and swelling to collapse transitions of the PSS brushes. Again, the transitions of the PSS brushes become less obvious with increasing xM because the cross-linked grafted chains are more difficult to disassociate with the addition of methanol, which is further reflected by the fact that the critical CCsCl required to induce the swelling of the PSS brushes increases with increasing xM (Figure 3c). Interestingly, Δf decreases and ΔD increases again as CCsCl further increases after the third regime, indicating the resolvation of the grafted chains accompanied by the reswelling of the PSS brushes. That is, a fourth regime can be observed in the conformational changes of the PSS brushes in the presence of CsCl. Note that the absence of Δf and ΔD data at high CsCl concentrations at xM ≥ 80% is due to the low solubility of CsCl in these solutions. It has been reported that positively charged poly[4-vinyl (Nmethyl-pyridinium)] brushes can reswell from the collapsed brushes as induced by I− but not by Cl− and Br−.14 Zhao et al. have also shown that Cs+ can induce the reexpansion of the free collapsed PSS chains at high salt concentrations, but the exact mechanism is still unclear.48 Here, we propose that the reswelling of the collapsed PSS brushes by Cs+ in the fourth regime is attributed to the further adsorption of Cs+ to the brushes via van der Waals interactions. Before the reswelling of the brushes, the PSS brushes resemble a neutral polymer layer because of the screening effect of external ions. As CCsCl increases further, Cs+ may adsorb onto the brushes because of the van der Waals forces resulting from the high polarizability of Cs+.49 Although electrostatic repulsion between the adsorbed and incoming cations would prevent the 12854

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by the brushes.35 In the third regime, ΔD decreases with increasing CCaCl2, indicating the collapse of the PSS brushes induced by charge screening by the ions within the brushes. Meanwhile, Δf slowly decreases with increasing CCaCl2, implying that the mass associated with the brushes increases with the addition of CaCl2 even though the extent of solvation of the individual grafted chains may decrease with increasing CCaCl2. This fact implies that the brushes may form inhomogeneous structures during collapse.26 For example, some sulfonate groups in different grafted chains could associate and overlap via the cross-linking effect of Ca2+, but other charged groups still repel each other, thereby forming inhomogeneous structures.52 It is expected that some cavities would be formed in these inhomogeneous structures. The solvent molecules can be trapped in these cavities and can be sensed by QCM during the oscillation of the resonator. In doing so, the trapping of solvent molecules in these inhomogeneous structures dominates the desolvation of the individual grafted chains, giving rise to a slow increase in the mass associated with the brushes during the collapse of the brushes. Therefore, both Δf and ΔD decrease with increasing CCaCl2 in the third regime. In the fourth regime, Δf exhibits a rapid decrease accompanied by an increase in ΔD with increasing CCaCl2, suggesting the resolvation and reswelling of the PSS brushes. Previous studies have demonstrated that multivalent counterions can induce a charge inversion in the polyelectrolyte chain and a reexpansion of the chain conformation via the chargeimage charge attractions.48,53−56 For the PSS brushes in the presence of CaCl2, the brushes are almost neutralized by the external counterions at the end of the third regime. At the same time, the counterions could form a strongly correlated liquid layer at the surface of PSS brushes, which has a dielectric constant (ε1) that is larger than that of the solvent (ε2).51 Therefore, the counterions near the surface of the brushes would induce surface polarization under this layer and would produce image charges with opposite signs according to the equation51

continued adsorption of Cs+, the attractive van der Waals forces may overcome such repulsive forces and lead to the adsorption of Cs+ onto the brushes, similar to co-ion adsorption on protein surfaces.49,50 The adsorbed Cs+ would give rise to a charge inversion of the PSS brushes, leading to the resolvation and reswelling of the PSS brushes. Specifically, the PSS brushes would become positively charged when the amount of adsorbed Cs+ on the brushes is larger than that of negatively charged sulfonate group associated with the brushes. Such a charge inversion leads to the resolvation of the grafted chains, and the electrostatic repulsions between the grafted chains result in the reswelling of the PSS brushes. It is worth noting that the charge inversion induced by the monovalent ions is different from that of the multilvalent ions,51 which will be discussed below in more detail. In Figure 3d, there are three kinetic processes in the ΔD−Δf plot for the conformational changes of the PSS brushes in the presence of CsCl, which correspond to the second, the third, and the fourth regimes in Figure 3a,b. ΔD increases with increasing −Δf in the first kinetic process and decreases with decreasing −Δf in the second kinetic process, followed by a small increase in ΔD with increasing −Δf in the third kinetic process. These processes correspond to the swelling, collapse, and reswelling of the brushes, respectively. Similar to the observation in Figure 2d, the kinetic processes become less obvious with increasing xM, and the value of ΔD in the first kinetic process is always higher than that in the second kinetic process for the same −Δf because of the formation of some short tails at the surface of the brushes in the swelling process. The conformational behavior of the PSS brushes in the presence of CaCl2 is quite different from that in the presence of monovalent salts in the water/methanol mixtures (Figure 4). In Figure 4a,b, Δf and ΔD exhibit four different regimes with a change in CaCl2 concentration. In the first regime, Δf and ΔD change only slightly with CCaCl2 because the external ions do not easily diffuse into the brushes at low salt concentrations. Then, Δf decreases and ΔD increases with increasing CaCl 2 concentration in the second regime, which is indicative of an increase in the extent of solvation and swelling of the PSS brushes resulting from the weakening of dipole−dipole interactions and the disassociation of cross-linked grafted chains induced by the penetration of external ions. Unlike the conformational behavior in the presence of monovalent salts, the brushes swell in all of the solvent mixtures, even in pure methanol, suggesting that the divalent cation is more effective at weakening the dipole−dipole interactions than are the monovalent cations. Nevertheless, the salt concentration (C1(CaCl2)) at which the transition occurs from the first to the second regime increases with increasing xM (Figure 4c), so a higher salt concentration is required to induce the solvation and swelling of the brushes in the solvent mixtures with a higher xM. This result is similar to the observation for the monovalent cations (Figures 2 and 3) and is due to the strengthening dipole−dipole interactions within the brushes with the addition of methanol. As the CaCl2 concentration increases further, Δf exhibits a slow decrease and ΔD also exhibits a decrease in the third regime. This observation is quite different from that for the monovalent salts, where Δf increases but ΔD decreases with increasing salt concentration (Figures 2 and 3). Actually, Δf is related not only to the solvation and desolvation of the grafted chains but also to the trapping and release of solvent molecules

q′ = q

ε2 − ε1 ε2 + ε1

(4)

where q and q′ are the original charge and image charge, respectively. Meanwhile, the repulsion between the incoming counterions and the adsorbed counterions would create some correlation holes.51 The interactions between the incoming counterions and the image charges would produce further counterion adsorption on the PSS brushes through the correlation holes. Accordingly, charge inversion of the PSS brushes occurs in the fourth regime, and such a charge inversion leads to the resolvation and reswelling of the brushes. Similarly, the change in thickness of the PSS brushes with the CaCl2 concentration further confirms the charge-inversioninduced reswelling of the PSS brushes in the high CaCl2 concentration regime (Figure S2, Supporting Information). The critical CaCl2 concentration (C2(CaCl2)) required to induce the resolvation and reswelling of the brushes decreases with increasing xM, which is in contrast to the change in C1(CaCl2) (Figure 4c). That is, the added Ca2+ can more easily induce the resolvation and reswelling of the brushes with increasing xM. In eq 4, ε2 decreases as xM increases. Therefore, the attraction between the counterions and the image charges increases with increasing xM, so the resolvation and reswelling 12855

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Figure 5. Conformational behavior of PSS brushes in the presence of LaCl3 in the water/methanol mixtures. (a) LaCl3 concentration (CLaCl3) dependence of the frequency shift (Δf) for the PSS brushes as a function of the volume fraction of methanol (xM). (b) LaCl3 concentration (CLaCl3) dependence of the dissipation shift (ΔD) for the PSS brushes as a function of the volume fraction of methanol (xM). (c) Critical LaCl3 concentration as a function of the volume fraction of methanol (xM) at which the transition occurs. C1(LaCl3) indicates the critical LaCl3 concentration at which the transition occurs from the first to the second regime, and C2(LaCl3) indicates the critical LaCl3 concentration at which the transition occurs from the third to the fourth regime. (d) Dissipation shift (ΔD) versus frequency shift (−Δf) for the PSS brushes as a function of the volume fraction of methanol (xM) in the presence of LaCl3. The dashed curves are provided to guide the eye.

Figure 6. (a) Critical salt concentration (C1) as a function of the volume fraction of methanol (xM) at which the transition occurs from the first to the second regime in the presence of CaCl2 and LaCl3. (b) Critical salt concentration (C2) as a function of the volume fraction of methanol (xM) at which the transition occurs from the third to the fourth regime in the presence of CaCl2 and LaCl3.

penetration of external ions. In the second kinetic process, ΔD decreases with increasing −Δf, suggesting that the mass associated with the brushes increases during the collapse of the brushes due to the formation of inhomogeneous structures. In the third kinetic process, ΔD increases with increasing −Δf, which is indicative of the resolvation and reswelling of the brushes induced by the charge inversion of the brushes via attraction between the counterions and the image charges. Unlike the cases for the monovalent salts where the kinetic processes become less obvious with increasing xM, the kinetic processes for the PSS brushes in the presence of CaCl2 can be clearly seen in all of the solvent mixtures from water to methanol. We have also investigated the conformational behavior of PSS brushes in the presence of trivalent salt LaCl3. The shifts in Δf and ΔD are shown in Figure 5a,b, respectively. Four different regimes can be observed in the LaCl3 concentration

of the brushes can be more easily achieved by Ca2+ in the solvent mixtures with a higher xM. Besides, the reswelling of the brushes becomes more obvious with increasing xM, further indicating the enhanced attraction between the counterions and the image charges with the addition of methanol (Figure 4b). It should be noted that the reswelling of PSS brushes by Cs+ cannot be understood in this way because of the weak counterion−counterion correlation.51 In Figure 4d, there are three kinetic processes in the ΔD−Δf plot for the conformational changes of the PSS brushes in the presence of CaCl2, which correspond to the second, the third, and the fourth regimes in Figure 4a,b. Δf and ΔD change only slightly in the first regime in Figure 4a,b, which is reflected by the collection of all of the data points in one position in Figure 4d. In the first kinetic process, ΔD increases with increasing −Δf, indicating the solvation and swelling of the PSS brushes induced by the weakening of dipole−dipole interactions by the 12856

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Scheme 1. Schematic Illustration of the Cation-Specific Conformational Behavior of PSS Brushes in Water/Methanol Mixtures

(CLaCl3) dependence of Δf and ΔD, which is similar to that for CaCl2. In the first regime, Δf and ΔD change only slightly with CLaCl3 because the external ions do not readily diffuse into the brushes at low salt concentrations. The decrease in Δf and increase in ΔD in the second regime imply the solvation and swelling of the brushes due to the weakening of dipole−dipole interactions by the ions that enter the brushes. In the third regime, the decreases in Δf and ΔD suggest that the mass associated with the brushes increases during the collapse of the brushes due to the formation of inhomogeneous structures. The resolvation and reswelling of the brushes induced by charge inversion are observed in the fourth regime, as indicated by the decrease in Δf and increase in ΔD. The resolvation and reswelling of the brushes is absent in methanol because of the low solubility of LaCl3 in pure methanol. As the xM increases, La3+ is less effective at inducing the solvation and swelling of the PSS brushes in the second regime but can more easily induce the resolvation and reswelling of the brushes in the fourth regime, as indicated by the fact that C1(LaCl3) increases but C2(LaCl3) decreases with increasing xM (Figure 5c). Here, C1(LaCl3) indicates the critical LaCl3 concentration at which the transition occurs from the first to the second regime, and C2(LaCl3) indicates the critical LaCl3 concentration at which the transition occurs from the third to the fourth regime. In Figure 5d, three kinetic processes can be observed in the ΔD−Δf plot for the conformational changes of PSS brushes in the presence of LaCl3, which is similar to that for CaCl2 (Figure 4d). In Figure 6a, a higher critical LaCl3 concentration is required to induce the solvation and swelling of the brushes than for CaCl2 at the same xM, indicating that CaCl2 can more easily induce the solvation and swelling of the brushes compared to LaCl3 from the first to the second regime. The transition from the first to the second regime is related to the weakening of dipole−dipole interactions and the disassociation of the crosslinked grafted chains induced by the penetration of external ions into the brushes. Actually, the added multivalent cations may also cross-link the grafted chains again via cation bridging.26,52 In comparison to Ca2+, which carries two charges and at most can bind with two sulfonate groups, La3+ carries

three charges and at most can bind with three sulfonate groups from different chains. Therefore, the grafted chains can be more effectively cross-linked by La3+ than Ca2+. Thus, La3+ is less effective at inducing the solvation and swelling of the brushes than Ca2+ from the first to the second regime. Likewise, the rise in critical salt concentration for Ca2+ occurs at a lower methanol volume fraction than for Na+ (Figures 2c and 4c), further indicating that the cross-linking effect on the grafted chains induced by the multivalent cations is disadvantageous to the transition from the first to the second regime. In contrast, a higher critical CaCl2 concentration is required to induce the resolvation and reswelling of the brushes than for LaCl3 at the same xM, indicating that La3+ can more easily induce the resolvation and reswelling of the brushes compared to Ca2+ from the third to the fourth regime (Figure 6b). This is understandable because the attraction between the La3+ counterions and their image charges is stronger than that for Ca2+ according to eq 4. The cation-specific conformational behavior of PSS brushes is illustrated in Scheme 1. For the monovalent salts, the brushes undergo a swelling to collapse transition for Na+ and experience a swelling to collapse to reswelling transition for Cs+. These transitions become less obvious as the methanol content increases. For the multivalent salts, the brushes undergo a swelling to collapse (forming inhomogeneous structures) to reswelling transition for both Ca2+ and La3+. The extent of the transitions for the PSS brushes in the presence of multivalent salts is only slightly influenced by the methanol content.

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CONCLUSIONS Employing QCM-D, we have investigated changes in the cation-specific conformational behavior of PSS brushes as the solvent is changed from aqueous to nonaqueous. For Na+, the solvation to desolvation and swelling to collapse transitions of the brushes can be observed with salt concentration. For the case of Cs+, the brushes can redissolve and reswell from the collapsed state at high salt concentrations because of the charge inversion of the brushes via van der Waals interactions between Cs+ and the brushes. As the methanol content increases, the transitions for the monovalent cations become less obvious and the added ions are less effective at inducing the solvation and 12857

dx.doi.org/10.1021/la5033493 | Langmuir 2014, 30, 12850−12859

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swelling of the brushes. For all of the multivalent cations, the brushes exhibit solvation to desolvation to resolvation and swelling to collapse to reswelling transitions, which are influenced only slightly by the methanol content. The resolvation and reswelling transitions for the brushes in the presence of multivalent cations are induced by charge inversion of the brushes due to charge-image charge interactions. As the methanol content increases, the multivalent cations are less effective at inducing the solvation and swelling of the brushes but can more easily induce the resolvation and reswelling of the brushes.

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ASSOCIATED CONTENT

S Supporting Information *

Method used to determine the critical salt concentration and the ellipsometry results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Program on Key Basic Research Project (2012CB933800), the National Natural Science Foundation of China (21374110, 91127042, and 21234003), and the China Postdoctoral Science Foundation is acknowledged. We thank Mr. Ran Kou for help with spectroscopic ellipsometry measurements.

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