Ion Specificity at a Low Salt Concentration in Water–Methanol

Feb 20, 2013 - observed at low salt concentrations in organic solvents or ... methanol mixtures at a salt concentration as low as 2.0 mM, which is 2 o...
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Ion Specificity at a Low Salt Concentration in Water−Methanol Mixtures Exemplified by a Growth of Polyelectrolyte Multilayer Yunchao Long,† Tao Wang,† Lvdan Liu,† Guangming Liu,*,† and Guangzhao Zhang*,‡ †

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 ‡ Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou, P. R. China 510640 S Supporting Information *

ABSTRACT: By use of a quartz crystal microbalance with dissipation (QCM-D), we have investigated the specific ion effect on the growth of poly(sodium 2-acrylamido-2-me th ylpro panesulfon ate)/po ly (diallyldimethylammonium chloride) multilayer at a salt concentration as low as 2.0 mM in water−methanol mixtures. QCM-D results demonstrate that specific ion effect can be observed in methanol and water−methanol mixtures though it is negligible in water. Moreover, the specific ion effect is amplified as the molar fraction of methanol (xM) increases from 0% to 75% but is weakened again with the further increase of xM from 75% to 100%. Nuclear magnetic resonance measurements reveal that the counterion− polyelectrolyte segment interactions may not account for the observed ion specificity. By extending the Collins’ concept of matching water affinities to methanol and water−methanol mixtures, we suggest that the ion−solvent interactions and the resulted counterion−charged group interactions are responsible for the occurrence of the specific ion effect. The conductivity measurements indicate that water and methanol molecules may form complexes, and the change of relative proportion of complexes with the xM causes the amplification or weakening of the specific ion effect.

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mixtures.12−15 It is reported that the direct interactions of ions with the enzyme surface are crucial for ion-specific enzymatic activity in 2-methyl-2-butanol.13 This fact indicates that water may not be necessary for the occurrence of the specific ion effect. Nuclear magnetic resonance (NMR) measurements have demonstrated that ion pairing of monovalent anions to the tetramethylammonium ion in water−ethanol mixtures follows the Hofmeister series.14 On the other hand, polyelectrolyte multilayer (PEM) has received much attention due to its promising applications in a wealth of fields such as chemical sensors,16 optical devices,17 and biomedical coatings.18 Such a multilayer film not only can be fabricated in water but also can be constructed in organic solvents and water−organic solvent mixtures.19−24 Furthermore, PEM can also be treated as a model system to study the specific ion effect.19,25−30 Some previous studies show that the specific anion effect on the thickness, storage shear modulus, and swelling extent of PEM is related to the hydration entropy of anions.25−27 Other studies suggest that the ion-specific growth of PEM is correlated with hydrophobicity and affinity of counterions.19,28 To the best of our knowledge, all the previous studies on ion-specific growth of PEM have only been conducted in aqueous solutions, and no investigations have

INTRODUCTION In aqueous solutions, the specific ion effect or Hofmeister effect is ubiquitous in biological and chemical systems though its nature still remains elusive.1 Several models have been proposed to clarify the mechanism of the Hofmeister effect.2−10 Collins has proposed a concept that only oppositely charged ions with similar water affinities can form strong ion pairs, which dominates the ion-specific interactions in aqueous solutions.2,7 Ninham et al. have suggested that specific ion effect is due to the polarizability of ions and is manifested through the ionic dispersion forces.3,5,6 Kunz et al. have suggested that the nature of surfaces has significant influences on the Hofmeister effect.4 Recent studies on ion specificity in macromolecular systems indicate that both ion−solvent and ion−polymer interactions are responsible for the specific ion effect.8−10 Generally, the specific ion effect can only be observed at salt concentrations above ∼100 mM in aqueous solutions because the short-range ion-specific interactions are usually masked by the long-range nonspecific electrostatic interactions at low salt concentrations.11 Nonetheless, the specific ion effect might be observed at low salt concentrations in organic solvents or water−organic solvent mixtures because the addition of organic solvents to water can amplify the ion specificity.12 In fact, in comparison with the extensive attention on the Hofmeister effect in aqueous solutions, only a few studies have been paid to specific ion effect in organic solvents or water−organic solvent © 2013 American Chemical Society

Received: January 6, 2013 Revised: February 7, 2013 Published: February 20, 2013 3645

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related to Δm and the overtone number (n = 1, 3, 5, ....) by the Sauerbrey equation40

been performed to date in organic solvents or water−organic solvent mixtures. Organic solvents and water−organic solvent mixtures usually have a smaller dielectric constant than water. Thus, more ion pairs will be formed compared with that in aqueous solutions.31,32 Besides, the solvation of ions in water−organic solvent mixtures should be different from that in water because the solvent molecules can form complexes in the mixtures.33−36 Therefore, it is expected that the specific ion effect in organic solvents and water−organic solvent mixtures might be different from that in water. In the present work, we have investigated the specific ion effect on the growth of poly(sodium 2acrylamido-2-methylpropanesulfonate)/poly(diallyldimethylammonium chloride) (PAMPS/PDDA) multilayer in water−methanol mixtures using a quartz crystal microbalance with dissipation (QCM-D). We find that the specific ion effect can be observed in methanol and water− methanol mixtures at a salt concentration as low as 2.0 mM, which is 2 orders of magnitude lower than that in water. More interestingly, the strongest specific ion effect occurs at a certain solvent composition of the mixtures. We hope that the present study can help us to understand the mechanism of the specific ion effect in water−organic solvent mixtures.

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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 by38 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.38 By switching the driving voltage on and off periodically, we can simultaneously obtain a series of changes of the resonant frequency and the dissipation factor. The gold-coated resonator was cleaned by using piranha solution composed of 1 part H2O2 and 3 parts H2SO4 at ∼50 °C for ∼15 min, rinsed with Milli-Q water, and blown dry with a stream of nitrogen gas. A measurement of layer-by-layer (LbL) deposition was initiated by switching the liquid exposed to the resonator from water to PEI solution with a polymer concentration of 1.0 mg mL−1. PEI was allowed to adsorb onto the resonator surface for ∼20 min before rinsing with water to ensure a uniform positively charged coating, so that the effects of the substrate on the growth of multilayer were minimized.20 After water was replaced with a 2.0 mM salt solution with the water−methanol mixture as the solvent, 0.1 mg mL−1 PAMPS and PDDA solutions with the water−methanol mixture as the solvent in the presence of 2.0 mM salt were alternately introduced into QCM cell for ∼20 min with a pure solvent mixture with a salt concentration of 2.0 mM rinsing in between. It is worth noting that a higher salt concentration (>∼5.0 mM) would result in a precipitation of polyelectrolytes in methanol. The typical responses of frequency and dissipation during the LbL deposition are shown in Figure S1 (Supporting Information). Here, all the changes of Δf and ΔD were obtained from the measurements at the third overtone (n = 3), and all the experiments were conducted at ∼25 ± 0.02 °C. The PEI layer in the solvent mixtures with a salt solution of 2.0 mM is set as the zeroth layer (i.e., baseline), and the changes in frequency and dissipation induced by the multilayer growth can be extracted by using each corresponding PEI layer as the reference. The uncertainty for the QCM-D experiments mainly comes from the instrumental drifts, which are typically ∼2 Hz and ∼1 × 10−7 for frequency and dissipation, respectively, during the multilayer growth. Conductivity Measurements. The conductivity (k) of PDDA or PAMPS in the salt solutions was measured using a DDS-307 conductivity meter (CSDIHO, China) at 25 °C. The values of k are calculated by the equation21

EXPERIMENTAL SECTION

Materials. Diallyldimethylammonium chloride (DDA) and 2acrylamido-2-methylpropanesulfonic acid (AMPSA) were purchased from Aladdin and used as received. 4,4′-Azobis(isobutyronitrile) (AIBN, Aldrich) was recrystallized three times from ethanol. 4,4′Azobis(4-cyanovaleric acid) (ACVA) and poly(ethylenimine) (PEI, Mw ∼ 2.5 × 104 g mol−1) were purchased from Aldrich and used as received. NaF, NaCl, NaBr, LiCl, and KCl were purchased from Sinopharm or Aladdin and used as received. Methanol (HPLC grade), deuterated methanol (CD3OD) (99.8% D), deuterated water (D2O) (99.9% D), and sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) were purchased from Aldrich and used as received. The water used was purified by filtration through a Millipore gradient system after distillation, giving a resistivity of 18.2 MΩ·cm. When studying anion specificity, we employed sodium salts so that the influence of cation was constant; similarly, chloride salts were used when investigating cation specificity. Preparation of PAMPS and PDDA. PAMPS was obtained by the reversible addition−fragmentation chain transfer (RAFT) polymerization of AMPSA in a basic aqueous solution.37 The number-average molecular weight (Mn ∼ 1.3 × 104 g mol−1) and the polydispersity index (Mw/Mn ∼ 1.4) of PAMPS were determined by gel permeation chromatography (GPC) (Waters 1515) using monodisperse sodium polymethacrylate as the standard and a solution of 20% acetonitrile/ 80% 0.05 M Na2SO4 (v:v) as the eluent with a flow rate of 1.0 mL min−1. PDDA was synthesized by RAFT polymerization in a solvent mixture of water and methanol (2:1, v:v). The Mn (∼1.4 × 104 g mol−1) and Mw/Mn (∼1.6) were determined by GPC using monodisperse poly(ethylene glycol) as the standard and a 0.1 M NaCl solution acidified with 1% trifluoroacetic acid (v:v) as the eluent with a flow rate of 1.0 mL min−1. QCM-D Measurements. QCM-D and the AT-cut quartz crystals were from Q-sense AB.38 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 has a mass sensitivity constant (C) of 17.7 ng cm−2 Hz−1.39 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, Δf is

k=

kP − k S C

(3)

where kP, kS, and C are the conductivity of salt solution with polymer, the conductivity of salt solution without polymer, and the polymer concentration, respectively. In the conductivity measurements, the polymer concentration is fixed at 0.1 mg mL−1 and the salt concentration is fixed at 2.0 mM. Nuclear Magnetic Resonance (NMR) Measurements. 1H NMR spectra of PDDA and PAMPS were obtained on a 400 MHz NMR spectrometer (Bruker AV400) using D2O, CD3OD, and their mixtures as solvents in the presence of different ions with a salt concentration of 2.0 mM. To enhance the signal-to-noise ratio and improve the precision of NMR data, the polyelectrolyte concentration was fixed at 10.0 mg mL−1 in the NMR measurements. All the NMR spectra were externally referenced to DSS in pure D2O in NMR tubes adapted with coaxial inserts, while the polyelectrolyte samples were 3646

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factor, whereas a swollen and loose layer has a large one.42 In Figure 1, the small values of ΔD with the layer number for all the cases indicate the formation of rigid multilayers, which is probably due to the high extent of intrinsic charge compensation between the oppositely charged polyelectrolyte chains.19 Besides, the values of ΔD for the xM at 75% and 100% are smaller than those at the lower methanol contents, indicating that a denser multilayer structure is formed at the higher methanol contents.19,20 We will come back to this point later. Figure 2 shows the change of −Δf for the 8-bilayer multilayer as a function of xM for the different anions. For all the cases,

placed in the outer tubes. All the NMR measurements were conducted at 25 °C.

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RESULTS AND DISCUSSION Figure 1 shows the layer number dependence of frequency shift (−Δf) and dissipation shift (ΔD) for the growth of PAMPS/

Figure 2. Change in frequency (−Δf) for the 8-bilayer PAMPS/ PDDA multilayer as a function of the molar fraction of methanol (xM) in the presence of different anions, where the salt concentration is fixed at 2.0 mM. Figure 1. Layer number dependence of shifts in frequency (−Δf) and dissipation (ΔD) for the PAMPS/PDDA multilayer as a function of the molar fraction of methanol (xM) in the presence of 2.0 mM NaCl, where the odd and even layer numbers correspond to the deposition of PAMPS and PDDA, respectively.

−Δf increases with the xM from 0% to 75%, followed by a decrease of −Δf with the further increase of xM from 75% to 100%. This result is similar to the observation in Figure 1. It is known that the conformation of polyelectrolyte chains is influenced by the Debye length (lD) which can be described as43

PDDA multilayer as a function of the molar fraction of methanol (xM) in the presence of 2.0 mM NaCl, where the odd and even layer numbers correspond to the deposition of PAMPS and PDDA, respectively. For all the cases, the increase of −Δf with the layer number indicates that the PEM can be fabricated by the LbL deposition of polyelectrolytes in the solvents from water to methanol. As the xM increases, the multilayer growth gradually changes from a linear to a nonlinear manner. This result is similar to the previous observation for the growth of poly(styrenesulfonate sodium)/poly(allylamine hydrochloride) multilayer in the mixed solvents of water and ethanol.20 Also, the previous study shows that the change of −Δf for individual layers increases with the increasing ethanol content up to 70 wt %, and a higher ethanol content results in a precipitation of polyelectrolytes.20 The precipitation of polyelectrolytes can be avoided here since either PAMPS or PDDA can dissolve in water, methanol, and water−methanol mixtures. Thus, we can investigate the multilayer growth in the solvents from water to methanol. Interestingly, for the same layer number, −Δf increases with the increasing xM from 0% to 75% but decreases with the further increase of xM from 75% to 100%. We will discuss this in detail later. It is known that the dissipation factor of a film relates to its structure.41 A dense and rigid layer has a small dissipation

lD =

εkBT 2NAe 2I

(4)

where ε, kB, T, NA, e, and I are the dielectric constant of solvent, the Boltzmann constant, the absolute temperature, the Avogadro number, the elementary charge, and the ionic strength, respectively. Obviously, lD is influenced not only by I but also by ε. In other words, the increase of I and the decrease of ε have similar effects on lD. Note that the effective Debye length may have a smaller value than the classical lD by taking the polyelectrolyte into account (eq S1, Supporting Information). Considering that the I is fixed at 2.0 mM, the change of lD mainly comes from the variation of ε. It is reported that the ε of water−methanol mixtures gradually decreases from 78.5 to 32.7 as xM increases from 0% to 100% at 25 °C.31 That is, the addition of methanol to water results in a decrease of lD, giving rise to an increasing screening of electrostatic repulsions between the charges along the polyelectrolyte backbone. As a result, the polyelectrolyte chains would adopt a more coiled conformation with the increasing xM. As reported previously, the more coiled conformation is more favorable for the multilayer growth by increasing the chain interpenetration and the extent of surface charge overcompensation.30,41,44 This 3647

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may explain why −Δf increases with the xM from 0% to 75% but cannot explain why −Δf decreases as xM increases from 75% to 100% because the ε monotonously decreases with the increasing xM. On the other hand, a cation and an anion can form an ion pair when the distance between them is less than the Bjerrum length (lB) which is defined as lB = e2/4επkBT.45 The decrease of ε with the addition of methanol results in an increase of lB, causing more ion pairs formed between the counterions and the charged groups along the polyelectrolyte backbone at higher methanol contents. The counterions can screen the electrostatic repulsions more effectively when they form ion pairs with the charged groups.2,46 Thus, a more coiled polyelectrolyte conformation is generated at a higher xM. Obviously, the inference based on the change of lB still cannot explain the occurrence of the maximum −Δf at the xM of 75% since a more coiled conformation is more favorable for the multilayer growth. It should be kept in mind that the conformation of polyelectrolyte is also influenced by the chain charge density.47 To understand the ionization state of the polyelectrolyte chains, we have measured the conductivity of PAMPS and PDDA in the solvent mixtures in the presence of different anions at a salt concentration of 2.0 mM (Figure 3). For either PAMPS or PDDA, k decreases with the increasing xM from 0% to 75% but increases with the further increase of xM from 75% to 100% for all the anions. The facts indicate that the two types of polyelectrolytes have a minimum degree of ionization at xM of 75%, thereby resulting in the most coiled polyelectrolyte conformation and the fastest growth of multilayer at this methanol content. This is why −Δf has a maximum value at the xM of 75% for all the anions. The negative values of k for PDDA and PAMPS in the presence of F− might be due to the weak acid nature of hydrofluoric acid.48−50 The lowest degree of ionization of polyelectrolyte chains at xM of 75% is probably due to the formation of stable water/ methanol complexes at this methanol content.21 In general, the ionic dissociation is determined by the balance between ion− solvent, solvent−solvent, and ion−counterion interactions.51−53 In the dissolution of polyelectrolyte chains, the solvation of counterions and charged groups is an exothermic process due to the attractive charge-dipole interactions, whereas the separation of counterions and charged groups is an endothermic process because of the electrostatic attractions. Moreover, the solvent structure bridged via hydrogen bonds is broken during the solvation of counterions and charged groups, which is also an endothermic process. The ionic dissociation can be achieved only when the energy loss in the endothermic process is less than the energy gain in the exothermic process. With the addition of methanol, the electrostatic attractions between the counterions and the charged groups become stronger due to the decrease of ε. In other words, the counterions are more difficult to disassociate from the charged groups with the addition of methanol. Consequently, the degree of ionization should decrease with the increasing xM. This is obviously inconsistent with the fact that the degree of ionization has a minimum value at the xM of 75%. Actually, the solvent−solvent interactions also influence the degree of ionization. If the solvent molecules form more stable complexes, the breaking of solvent structure during the ionic solvation would cost more energy, leading to a lower degree of ionization. It is thought that water and methanol molecules can form complexes with a certain stoichiometry via hydrogen

Figure 3. Change in conductivity (k) of PAMPS and PDDA as a function of the molar fraction of methanol (xM) in the presence of different anions, where the polymer concentration is fixed at 0.1 mg mL−1 and the salt concentration is fixed at 2.0 mM. (a) PAMPS. (b) PDDA. The inset in (b) is the magnification of the change in k for PDDA in the range of xM between 50% and 100% in the presence of Br− and Cl−.

bonds where the relative proportion of the complexes changes with the methanol content.35,36,54 Raman spectra studies show that water and methanol molecules can form complexes with a stoichiometry of (H2O)2(CH3OH)5.54 That is, when xM is less than 75%, the methanol molecules might not be sufficient to complex with all the water molecules, and more complexes are formed in the solvent mixtures with the increasing xM. Further increasing xM from 75% to 100% leads the concentration of complexes to decrease because water molecules may not be sufficient to complex with all the methanol molecules. Thus, the complexes might have the highest concentration at xM of ∼75% where water and methanol molecules all form complexes.54 As discussed above, the higher the concentration of complexes is, the lower the degree of ionization would be resulted. Therefore, both PAMPS and PDDA exhibit a minimum degree of ionization at xM of 75%. It is interesting that the specific anion effect is observed in the growth of PEM. Namely, −Δf increases following the series F− < Cl− < Br− at the same xM. At the low xM, the differences between anions are small, indicating the weak specific anion effect. More interestingly, the specific anion effect is gradually amplified with the increasing xM from 0% to 75% but is weakened again as xM increases further from 75% to 100%. The changes of solvent properties (e.g., dielectric constant),14 3648

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counterion−polyelectrolyte segment interactions,55 and ion− solvent interactions2 with the addition of methanol might be responsible for the above-mentioned phenomena of the specific anion effect. At xM of 0%, no obvious specific anion effect is observed. This is understandable because lD is of ∼7 nm in the presence of 2.0 mM monovalent salt in water and the shortrange ion-specific interactions are masked by the long-range nonspecific electrostatic interactions. The effective Debye length may have a small decrease compared with the classical lD by taking the polyelectrolyte into consideration (see eq S1, Supporting Information). According to eq 4, lD decreases with the decreasing ε upon the addition of methanol, so that the anion-specific interactions and the specific anion effect should become more obvious with the increasing xM. This may explain the occurrence of specific anion effect in methanol and water− methanol mixtures but cannot explain why the strongest specific anion effect occurs at the xM of 75%. To clarify the influence of counterion−polyelectrolyte segment interactions on the specific anion effect, we have measured the 1H NMR spectra of PDDA as a function of xM in the presence of different anions at a salt concentration of 2.0 mM (Figure S4, Supporting Information). If the counterions interact with the chain segments in different strengths, the chemical shift (δ) for the same resonance peak of PDDA is expected to exhibit different values in the presence of different anions.55 Figure S4 shows that no obvious specific anion effect can be observed in the change of δ in the whole range of xM from 0% to 100%, indicating that the counterion−polyelectrolyte segment interactions might be not responsible for the occurrence of specific anion effect. It should be noted that the low salt concentration may lead the ion specificity in the chemical shift to be difficult to observe even though some specific interactions exist between the counterions and the polyelectrolyte segments or charged groups.55 From the discussions above, the occurrence of specific anion effect and the amplification or weakening of such an effect may not be attributed to the changes of ε and the counterion− polyelectrolyte segment interactions. Actually, the specific interactions between two oppositely charged ions can be explained by the concept of matching water affinities in aqueous solutions.2,7 A larger ion with a lower surface charge density interacts weakly with water molecules, leading to a weaker ionic hydration. In contrast, a smaller ion with a higher surface charge density interacts strongly with water molecules, resulting in a stronger ionic hydration. Collins suggests that only oppositely charged ions with similar water affinities can form strong ion pairs, giving rise to a strong charge screening effect on the electrostatic interactions.2 A small kosmotrope cannot form a strong ion pair with an oppositely charged large chaotrope because the electrostatic attractions between them are not strong enough to induce the loss of hydration shell of the kosmotropic ion.2,4 We may extend the concept of matching water affinities to methanol and water−methanol mixtures, i.e., matching solvent affinities. For the same solvent, the ionic solvation is mainly determined by the charge-dipole interactions between ions and solvent. Thus, it is reasonable to expect that two oppositely charged ions should have similar strengths of solvation in methanol and in water−methanol mixtures if they have similar strengths of hydration in water.56 In other words, two oppositely charged ions should have stronger interactions in methanol and in water−methanol mixtures if they interact more strongly in water. In aqueous solutions, the strength of

interactions between the weakly hydrated ammonium groups on PDDA chains and the anions increases following the order F− < Cl− < Br− because the extent of hydration of anions decreases from F− to Br−.57 Therefore, the strength of interactions between the ammonium groups and the anions is also expected to increase following the order F− < Cl− < Br− in methanol and in water−methanol mixtures. That is, the effectiveness of anions to screen the charges on PDDA chains increases following the series F− < Cl− < Br−, and PDDA chains would adopt a more coiled conformation as the anion changes from F− to Br−. A more coiled conformation is more favorable for the multilayer growth, so that −Δf increases following the order F− < Cl− < Br− at the same xM. In short, the anion− solvent interactions and the resulting counterion−charged group interactions may be responsible for the occurrence of specific anion effect. Additionally, the ionic dispersion forces may also have some influences on the observed specific anion effect. More specifically, the ion polarizability increases from F− to Br−; therefore, the ionic dispersion interactions between the anions and the PDDA should also increase from F− to Br−. This causes the effectiveness of anions to screen the charges on PDDA chains to increase following the series F− < Cl− < Br−. Thus, −Δf should increase following the order F− < Cl− < Br− at the same xM. Obviously, the conclusion derived from the theory of ionic dispersion forces is consistent with that obtained from the model of matching solvent affinities. The strength of charge-dipole interactions between the anions and the solvent complexes should decrease following the order F− > Cl− > Br− in the water−methanol mixtures as the anionic surface charge density decreases from F− to Br−. Relatively strong interactions between the anions and the solvent complexes lead to weaker interactions between the anions and the ammonium groups, whereas relatively weak interactions between the anions and the solvent complexes cause stronger interactions between the anions and the ammonium groups. As mentioned above, the concentration of solvent complexes increases with xM from 0% to 75%. The increasing concentration of water/methanol complexes would amplify the difference in the interactions between anions and solvent complexes, thereby enlarging the difference in the interactions between anions and ammonium groups, giving rise to an amplification of the specific anion effect. In contrast, as xM increases from 75% to 100%, the decrease of concentration of solvent complexes would weaken the specific anion effect. Thus, the change of relative proportion of the complexes in the water−methanol mixtures may be responsible for the amplification or weakening of the specific anion effect. Our previous study shows that the occurrence and the amplification of specific ion effect can also be observed in the water− methanol mixtures at a high salt concentration.12 That is, the occurrence and the amplification of specific ion effect might be only slightly dependent on salt concentration. In addition, a similar specific ion effect can also be observed in the presence of more chaotropic anions (e.g., SCN− and ClO3−) (Figure S5). Here, the divalent kosmotropes (e.g., SO42−) are not used in the experiments because they may cross-link the cationic polyelectrolyte chains.47 In Figure 4, ΔD remains almost constant between 0 and 6 × 10−6 as xM increases from 0% to 50% with the exception of Br− at the xM of 50%, followed by a decrease of ΔD as xM increases further from 50% to 100%. This result is similar to the observation in Figure 1. In the range of xM between 0% and 50%, the small values of ΔD indicate the formation of rigid 3649

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xM from 0% to 75% but decreases as xM increases from 75% to 100%. This is similar to the result in Figure 2 and can also be explained by the fact that the polyelectrolyte chains have the minimum degree of ionization at xM of 75% (Figure 6). More

Figure 4. Change in dissipation (ΔD) for the 8-bilayer PAMPS/ PDDA multilayer as a function of the molar fraction of methanol (xM) in the presence of different anions, where the salt concentration is fixed at 2.0 mM.

PEM due to the high extent of intrinsic charge compensation during the polyelectrolyte complexation. With the addition of methanol, the decrease of solvent quality leads to a collapse of PEM and a smaller ΔD. Meanwhile, a more coiled polyelectrolyte conformation with the addition of methanol would result in a thicker PEM and a larger ΔD. The resulting balance between these two opposite effects might make ΔD only slightly vary with the xM from 0% to 50%. The relatively low values of ΔD at xM of 75% and 100% compared with that at the lower methanol contents suggest that a denser multilayer structure is formed at the higher methanol contents, and the structure of PEM is dominated by the collapse of multilayer induced by the decrease of solvent quality. In addition, no single anion series can be observed in the change of ΔD regarding the specific anion effect. The change of ΔD in the presence of more chaotropic anions or in the salt-free solvent mixtures has similar results (Figure S6). The change in thickness of the PEM estimated by the Sauerbrey equation in the presence of different anions is shown in Figure S7. We have also examined the specific cation effect on the growth of PEM in the solvents from water to methanol. In Figure 5, for all the cations, −Δf increases with the increasing

Figure 6. Change in conductivity (k) of PAMPS and PDDA as a function of the molar fraction of methanol (xM) in the presence of different cations, where the polymer concentration is fixed at 0.1 mg mL−1 and the salt concentration is fixed at 2.0 mM. (a) PAMPS. (b) PDDA.

specifically, the polyelectrolyte chains adopt the most coiled conformation at the xM of 75%, which is the most favorable for the multilayer growth, resulting in the occurrence of the maximum −Δf at this methanol content. Generally, the specific cation effect is much weaker than that of anions. However, the cation specificity is observed in the present study; namely, −Δf increases following the series Li+ < Na+ < K+ at the same xM. Moreover, the specific cation effect is amplified as xM increases from 0% to 75% but is weakened again as xM increases further from 75% to 100%. This is also similar to the result shown in Figure 2. The 1H NMR spectra of PAMPS in the solvents from D2O to CD3OD in the presence of different cations at a salt concentration of 2.0 mM are shown in Figure S8. No obvious specific cation effect can be observed in the change of δ, indicating that the counterion− polyelectrolyte segment interactions might be not responsible for the occurrence of specific cation effect. On the basis of the concept of matching water affinities, the strength of interactions between the sulfonate groups on the PAMPS chains and the cations should increase following the series Li+ < Na+ < K+ in water because the alkyl sulfonate is a weakly hydrated group and the extent of hydration of cations decreases from Li+ to K+.57 It is expected that the strength of interactions between the sulfonate group and the cations in methanol or water−methanol mixtures also increases following the order Li+ < Na+ < K+ according to the extended concept of

Figure 5. Change in frequency (−Δf) for the 8-bilayer PAMPS/ PDDA multilayer as a function of the molar fraction of methanol (xM) in the presence of different cations, where the salt concentration is fixed at 2.0 mM. 3650

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thickness of the PEM estimated by the Sauerbrey equation in the presence of different cations is shown in Figure S7.

matching solvent affinities. Consequently, the effectiveness of cations to screen the charges on PAMPS chains increases following the series Li+ < Na+ < K+, and the PAMPS chains adopt a more coiled conformation as the cation changes from Li+ to K+. A more coiled conformation is more favorable for the multilayer growth, so that −Δf increases following the series Li+ < Na+ < K+ at the same xM. Also, the ionic dispersion forces between the cations and PAMPS should increase from Li+ to K+ since the ion polarizability increases from Li+ to K+. This would result in the same specific cation effect. The strength of charge-dipole interactions between the cations and the solvent complexes decreases following the order Li+ > Na+ > K+ in the water−methanol mixtures as the cationic surface charge density decreases from Li+ to K+. Relatively strong interactions between the cations and the solvent complexes would cause weaker interactions between the cations and the sulfonate groups, whereas relatively weak interactions between the cations and the solvent complexes would lead to stronger interactions between the cations and the sulfonate groups. The increasing concentration of water/ methanol complexes with the xM from 0% to 75% would amplify the difference in the interactions between cations and solvent complexes, thereby giving rising to an enlargement of the difference in the interactions between cations and sulfonate groups, so that the specific cation effect is amplified. As xM increases further from 75% to 100%, the specific cation effect is weakened again because of the decrease of the concentration of solvent complexes. In Figure 7, ΔD only slightly changes between 0 and 6 × 10−6 as xM increases from 0% to 50%. Afterward, ΔD decreases

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CONCLUSION In the present work, we have investigated the specific ion effect on the growth of PAMPS/PDDA multilayer in the water− methanol mixtures. Our study demonstrates that the specific ion effect can be observed in methanol and water−methanol mixtures at a salt concentration as low as 2.0 mM. The specific ion effect is amplified as xM increases from 0% to 75% but is weakened again as xM increases further from 75% to 100%. We suggest that the ion−solvent interactions and the resulted counterion−charged group interactions are responsible for the occurrence of specific ion effect. The amplification or weakening of the specific ion effect is determined by the variation of the concentration of water/methanol complexes.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data of the responses of frequency and dissipation during the LbL deposition, the change in thickness of the PEM, the H NMR spectra, and the calculation of effective Debye length. 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] (G.M.); [email protected] (G.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The financial support of National Program on Key Basic Research Project (2012CB933800), the National Natural Science Foundation of China (21004058, 91127042, 21234003), Scientific Research Startup Foundation of the Chinese Academy of Sciences, and the Fundamental Research Funds for the Central Universities (WK2060030008) is acknowledged.

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Figure 7. Change in dissipation (ΔD) for the 8-bilayer PAMPS/ PDDA multilayer as a function of the molar fraction of methanol (xM) in the presence of different cations, where the salt concentration is fixed at 2.0 mM.

as xM increases further from 50% to 100%. Similar results are also observed in Figure 4. The small ΔD indicates the formation of dense PEM. In the range of xM between 0% and 50%, the delicate balance between the densification and the thickening of PEM with the addition of methanol may make ΔD only slightly vary with xM. The relatively low values of ΔD at xM of 75% and 100% compared with that at the lower methanol contents suggest that the polyelectrolyte chains form a denser multilayer, and the structure of PEM is dominated by the collapse of multilayer at the higher methanol contents. Obviously, no single cation series is observed in the change of ΔD in terms of the specific cation effect. The change in 3651

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