J. Phys. Chem. B 2010, 114, 9987–9993
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Specific Anion Effects on the Growth of a Polyelectrolyte Multilayer in Single and Mixed Electrolyte Solutions Investigated with Quartz Crystal Microbalance Guangming Liu, Yi Hou, Xiao Xiao, and Guangzhao Zhang* Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, People’s Republic of China ReceiVed: March 1, 2010; ReVised Manuscript ReceiVed: June 4, 2010
The specific anion effects on the deposition of a multilayer formed by poly(sodium 4-styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) have been investigated by use of quartz crystal microbalance with dissipation (QCM-D) in single and mixed electrolyte solutions. In the case of single electrolyte solutions, the frequency change (-∆f) demonstrates that the multilayer in NaBr, NaClO3, and NaCl solutions grows in a nonlinear mode, which is dominated by the penetration of PSS. In NaF, CH3COONa, NaH2PO4, and Na2SO4 solutions, the multilayer growth mainly determined by the conformation of PDDA chains exhibits a linear mode. The growth shows a mode in between in NaHCO3 solution. The dissipation change (∆D) has slight dependence on anion species except in the cases of Br-, HCO3-, and SO42-. ∆D remarkably increases with layer number in NaHCO3 solution, indicating the formation of a relatively loose multilayer. In a mixed electrolyte solution containing chaotropic and kosmotropic anions, the multilayer growth is dominated by the chaotropic anions due to the anion competition effects. The effects of anions on the deposition of multilayer are nonadditive. Introduction In 1888, the German scientist Franz Hofmeister defined the series of cations and anions according to their relative ability to precipitate egg-white protein in aqueous solutions.1 Such specific ion effects were called Hofmeister effects later.2 Actually, Hofmeister effects are ubiquitous in biological and chemical systems though their nature still remains elusive.3 Generally, anions exhibit stronger specific effects than cations since the former are much larger than latter in diameter.4,5 The typical order of the anions in Hofmeister series is ClO4- > SCN> I- > ClO3- > NO3- > Br- > Cl- > HCO3- > CH3COO- > F> H2PO4- > SO42-.6-8 The anions were categorized as chaotropes and kosmotropes in the light of their effects on the viscosity of aqueous solutions.9 The anions on the left of Cldefined as chaotropes have weak interactions with water molecules, whereas those on the right of Cl- called kosmotropes are strongly hydrated by water molecules.10 Remarkable ion-specific effects have also been observed in synthetic polyelectrolytes aqueous solutions.11 In such a system, the molecular interactions are generally controlled by the longrange nonspecific electrostatic forces at low electrolyte concentrations. As the electrolyte concentration increases over a critical value (typically ∼0.1 M), the electrostatic interactions would be screened and the short-range ion-specific interactions become dominant.12 In regard to the specific ion effects in polyelectrolytes, it was proposed that the ionic dispersion forces between the counterions and the polyion surface play important roles in the ionic distributions and potentials, determining the conformational changes of polyelectrolyte.13 Such dispersion forces can result in the binding of ions onto the polymer surface, particularly for the chaotropes with large dispersion forces due to the high polarizability.14,15 On the other hand, based on the Collins’ concept of matching water affinities, it was also sug* To whom correspondence should be addressed. E-mail: gzzhang@ ustc.edu.cn. Tel: +86-551-3606763. Fax: +86-551-3606743.
gested that the charge density and the hydration of ions are critical for the interactions between the counterions and the oppositely charged groups on the polyelectrolyte chains.16,17 Specifically, the strongly (weakly) hydrated kosmotropic (chaotropic) anions and cations can form strong ion pairs, whereas the strongly hydrated kosmotropic anions (cations) and the weakly hydrated chaotropic cations (anions) only form weak ion pairs.18 In other words, only oppositely charged ions of equal water affinity can form inner sphere ion pairs, leading to the strong binding of counterions onto the oppositely charged groups on the polyelectrolyte chains. It has been reported that the ion-modulated growth of polyelectrolyte multilayer exemplifies the ion-specificity.19-24 Although the first observation of multilayer growth was the alternate deposition of oppositely charged colloidal particles,25 more extensive attentions had been focused on the layer-bylayer (LbL) deposition of polyelectrolytes due to its potential applications.26-31 So far, the nature of multilayer growth of polyelectrolytes is not well understood, particularly for that with specific ion effects. Saloma¨ki and co-workers19-21 have studied the specific anion effects on the thickness, storage shear modulus, and swelling extent of polyelectrolyte multilayers and revealed that the effects result from the hydration entropy of anions. After investigating the effects of various cations and anions on the multilayer growth, Dubas and Schlenoff22 proposed that the specific ion effects relate to the hydrophobicity and affinity of the counterions. Wong et al.23 found that the specific ion effects become important above the ionic strength of 0.1 M for anions and 0.25 M for cations in construction of polyelectrolyte multilayer. Haitami et al.24 demonstrated that the thickness and permeability of polyelectrolyte multilayer can be tuned by the supporting anions. On the other hand, regardless of the fact that the specific ion effects are usually observed in mixed electrolyte systems such as biological systems, one always likes to construct a polyelectrolyte multilayer in a single electrolyte solution instead of a
10.1021/jp1018263 2010 American Chemical Society Published on Web 07/21/2010
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mixed electrolyte solution for simplification. It is known that the distribution of ions in the interfacial region is quite complex even in single electrolyte solutions, some ions preferentially locate at the interface while other ions show affinity for the subsurface region.32-35 The ion distributions would become more complex in a mixed electrolyte solution due to the ion competition effects.36 Previous study demonstrated that the bubble coalescence inhibition in mixed electrolyte systems is consistent with the empirical rule observed in the single electrolyte systems, which depends upon the ion separation within the interfacial region.12 However, the situation is quite different for the polyelectrolyte systems. Since chaotropes and kosmotropes exhibit different interactions with the charge groups on polyelectrolyte chains,37 the ion-specificity in a mixed electrolyte solution containing both chaotropes and kosmotropes is quite different from that in a single electrolyte solution. Therefore, the investigations on the polyelectrolyte multilayer growth in a mixed electrolyte solution are expected to give more interesting information about the ion-specificity. It is reported that the chain interpenetration of polyelectrolytes plays a critical role in multilayer growth in either linear or nonlinear mode.38-42 The former occurs when no polyelectrolyte chains diffuse within the multilayer, but the latter happens with the chain diffusion throughout the multilayer.38,39 We have investigated the growth of a polyelectrolyte multilayer by use of quartz crystal microbalance with dissipation (QCM-D) and shown that the multilayer growth is dominated by chain conformation and interpenetration at low and high salt concentrations, respectively.41,42 In the present study, we report the layer-by-layer deposition of poly(sodium 4-styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) in single and mixed electrolyte solutions by use of QCM-D in real time. Considering that different anions have different interactions with the polyelectrolytes, the polyelectrolyte chains in multilayer should exhibit different penetration and conformational change in the presence of different anions. We hope that the present work can help to understand the specific anion effects on buildup and structure of the polyelectrolyte multilayer. Experimental Section Materials. PSS (Mw ∼ 1.0 × 106 g mol-1), PDDA (Mw ∼ 4.5 × 105 g mol-1) and poly(ethyleneimine) (PEI, Mw ∼ 2.5 × 104 g mol-1) from Aldrich were used as received. Sodium bromide (NaBr), sodium chlorate (NaClO3), sodium chloride (NaCl), sodium hydrogen carbonate (NaHCO3), sodium fluoride (NaF), sodium acetate (CH3COONa), sodium phosphate monobasic (NaH2PO4), and sodium sulfate (Na2SO4) were all AR grade (Sinopharm Chemical Reagent Co.) and used without further purification. Other regents were used as received. The water used was purified by filtration through Millipore Gradient system after distillation giving a resistivity of 18.2 MΩ cm-1. The small change of pH for the electrolyte solutions induced by the addition of electrolytes has slight effects on PSS and PDDA chains because they are strong polyelectrolytes. QCM-D Measurements. QCM-D and the AT-cut quartz crystals were from Q-sense AB.43 The quartz crystal with a fundamental resonant frequency of 5 MHz was mounted in a fluid cell with one side exposed to the solution. The crystal had a mass sensitivity constant (C) of 17.7 ng cm-2 Hz-1. The uncertainty for the experiments mainly comes from the instrumental drifts which are typically ∼3 Hz and ∼2 × 10-7 for the frequency and dissipation, respectively. The effects of surface roughness were neglected because the crystals were polished with a rms roughness less than 3 nm.44
Liu et al. TABLE 1: Characterization Data of the Polyelectrolyte Solutions PEI concentration solvent electrolyte
PSS -1
1.0 mg mL H 2O no electrolyte
0.1 mg mL H2O 0.5 M
PDDA -1
0.1 mg mL-1 H 2O 0.5 M
When a quartz crystal is excited to oscillate in the thickness shear mode at its fundamental resonant frequency (f0) 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 (∆f), which is proportional to the mass change (∆m) of the layer. In vacuum or air, if the added layer is rigid, evenly distributed, and much thinner than the crystal the ∆f is related to ∆m and the overtone number (n ) 1, 3, 5...) by the Sauerbrey equation45
∆m ) -
Fqlq ∆f ∆f ) -C f0 n n
(1)
where f0 is the fundamental frequency, Fq 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 ∆D 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 changes of the resonant frequency and the dissipation factor. The gold-coated resonator was cleaned by using Piranha solution composed of one part H2O2 and three 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 LbL deposition was initiated by switching the liquid exposed to the resonator from Milli-Q water to PEI solution with a concentration of 1.0 mg mL-1. PEI is 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 layers are minimized.46 The PEI layer was taken as the “0” layer of the multilayer. After water was replaced with pure electrolyte solution, 0.1 mg mL-1 PSS and PDDA were alternately introduced into QCM cell for ∼20 min with electrolyte solution rinsing in between. The detailed information about the polyelectrolyte solutions can be found in Table 1. Each monovalent electrolyte solution was prepared with a concentration of 0.5 M. For Na2SO4 solution, the concentration was varied so that the ionic strength was equal to those of the monovalent electrolyte solutions. The ionic strength for each mixed electrolyte solution was 0.5 M. Cations in all electrolytes were sodium (Na+) so that the cation effect can be neglected. All the measurements were conducted at 25 ( 0.02 °C. ∆f and ∆D values from the fundamental overtone were usually noisy because of insufficient energy trapping and thus discarded.47 The frequency and dissipation changes induced by the multilayer deposition can be extracted by using the PEI layer as the reference. From the changes in frequency and dissipation, one
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Figure 1. Changes in frequency (∆f) and dissipation (∆D) of LbL deposition of a PSS/PDDA multilayer, where NaCl concentration is 0.5 M.
can obtain the information about the layer thickness, adsorbed mass, conformational change of polyelectrolytes, and the penetration of polyelectrolyte chains between neighbor layers. Results and Discussion It is known that the sequential layer-by-layer deposition of oppositely charged polyelectrolytes on a solid surface results in a polyelectrolyte multilayer. When the first polyelectrolyte layer adsorbs on the solid surface, the introduction of polyelectrolyte chains carrying opposite charges leads to the second layer. Meanwhile, the surface charges of the first layer are overcompensated, making the adsorption of the third polyelectrolyte layer possible. One can construct a polyelectrolyte multilayer by alternate adsorption of two oppositely charged polyelectrolytes with this technique. Generally, the deposition of multilayer can adopt either linear or nonlinear mode.41,42 The former corresponds to the situation where none of polyelectrolytes diffuse within the multilayer, whereas the latter is related to the chain diffusion throughout the multilayer.38,39 In other words, the growth mode is controlled by the penetration of polyelectrolytes, which is correlated with the conformation and persistence length of polyelectrolyte chains.38 Since the ionpolyelectrolyte interactions have significant effects on the conformation and persistence length of polyelectrolytes,48 and each anion has a charge density and hydration different from another, their interactions with the charge groups on the polyelectrolyte chains are different. Thus, the conformation and persistence length of a polyelectrolyte in an electrolyte solution are greatly affected by the anions, and the growth mode of multilayer deposition and the penetration of polyelectrolytes are expected to be modulated by anions. Figure 1 shows the typical frequency (∆f) and dissipation (∆D) responses for the LbL deposition of PSS/PDDA in a 0.5 M NaCl solution. Before the other polyelectrolyte was introduced every time, a pure NaCl solution was added to rinse in case that polyelectrolytes form complexes in solution. The decrease in ∆f and increase in ∆D indicate that the polyelectrolytes gradually deposit onto the surface. The absolute value of ∆f induced by the polyelectrolyte deposition for a bilayer nonlinearly increases with time, indicating a nonlinear growth of the multilayer under the conditions.42,49 Figure 2 shows the layer number dependence of the changes in frequency (-∆f) and dissipation (∆D) for different anions, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively. As layer number increases, -∆f increases, indicating the sequential deposition of polyelectrolytes. For the same layer number, -∆f increases
Figure 2. The layer number dependence of frequency shift (-∆f) and dissipation shift (∆D) for different anions, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively.
in the order of SO42-, H2PO4-, CH3COO-, F-, HCO3-, Cl-, ClO3-, Br-, which is roughly consistent with the classical Hofmeister series.6-8 For example, -∆f induced by the deposition of polyelectrolytes for eight bilayers are 1161, 1430, 1707, 2453, 2788, 5457, 8518, and 12268 Hz for SO42-, H2PO4-, CH3COO-, F-, HCO3-, Cl-, ClO3-, and Br-, respectively. Hence, the deposition of polyelectrolytes is affected by the nature of the anions. Furthermore, the anions can be divided into two groups with HCO3- as the borderline. For SO42-, H2PO4-, CH3COO-, F- and HCO3-, -∆f linearly increases with layer number. In contrast, Cl-, ClO3-, and Br- lead the multilayer to grow nonlinearly. It is known that the dissipation change relates to the thickness and viscoelasticity of the layer on the resonator surface.50 Figure 2 shows ∆D gradually increases as layer number increases, further indicating the subsequent deposition of polyelectrolytes. Moreover, ∆D has slight dependence on anion species except in the cases of Br-, HCO3-, and SO42-. The relatively low ∆D observed in Na2SO4 solution indicates that SO42- leads to a thin and rigid multilayer, whereas the larger ∆D observed in NaBr solution reflects a thicker and more viscoelastic layer resulted. The multilayer deposited in NaHCO3 solution also exhibits a remarkable increase in ∆D as layer number increases. We will come back to this point later. The hydrodynamic thickness (t) and the adsorbed mass (m) of the PSS/PDDA multilayer in different electrolyte solutions as a function of layer number were obtained by fitting the data at n ) 3, 5, and 7 with the Voigt model (Figure 3), where the density of PSS/PDDA multilayer is taken to be 1000 kg m-3. Obviously, the fit results are similar to those in Figure 2. To clarify the anion-specificity, we will respectively discuss the specific anion effects on the multilayer growth in linear and nonlinear modes. Figure 4 shows the layer number dependence of frequency change (-∆f) in the solutions of Br-, ClO3-, Cl-, and HCO3-, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively. It is apparent that the multilayer growth is gradually dominated by the nonlinear mode as the anion changes from HCO3- to Br-. Figure 4 also shows the deposition of PDDA has a more remarkable contribution to the nonlinear increase of -∆f in comparison with PSS,
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Figure 3. The layer number dependence of hydrodynamic thickness (t) and adsorbed mass (m) for different anions obtained by fitting the data at n ) 3, 5, 7 using the Voigt model, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively.
Figure 4. The layer number dependence of frequency shift (-∆f) regarding Br-, ClO3-, Cl-, and HCO3-, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively.
Figure 5. The average frequency shift (-∆f) due to the deposition of PSS or PDDA for the last four layers as a function of anion species regarding Br-, ClO3-, Cl- and HCO3-.
indicating that the difference in nonlinear growth originates from the counter-anions. Namely, different counter-anions exhibit different interactions with the positively charged ammonium groups on the PDDA chains, leading to the difference in -∆f for the nonlinear multilayer growth. Nonetheless, for the multilayer deposited in NaHCO3 solution, the contribution to -∆f from the deposition of PSS approximates to that from PDDA. To look insight this phenomenon, we made a plot with respect to the average values of -∆f due to the deposition of PSS or PDDA for the last four layers as a function of anion species (shown in Figure 5). Figure 5 shows that the average value of -∆f caused by the deposition of PDDA for the 14th and 16th layers gradually increases as the anion changes from HCO3-, Cl-, ClO3- to Br-, whereas the average value of -∆f induced by the deposition of PSS for the 13th and 15th layers gradually decreases from HCO3- to Br- and eventually attains a negative value. For HCO3-, -∆f values caused by the deposition of PSS and PDDA
Liu et al.
Figure 6. The layer number dependence of frequency shift (-∆f) regarding HCO3-, F-, CH3COO-, H2PO4-, and SO42-, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively.
are almost equal to each other. As we reported before,42 for a certain PSS outer layer, when PDDA is introduced, it forms a layer on PSS surface via the electrostatic attraction. Thus, the layer thickness or mass increases, leading -∆f to increase. However, the subsequently introduced PSS chains would penetrate into PDDA layer, and they form complexes. Some water molecules are released from the multilayer during the complexation giving rise to the decrease in -∆f.41 At the same time, the surface charge changes from positive to negative, making the subsequent adsorption of PDDA chains on the surface possible. Accordingly, more chain interpenetration and complexation lead more water molecules to release out, so that more -∆f drop can be observed. The fact that -∆f for the deposition of PSS decreases from HCO3- to Br- indicates that the degree of chain interpenetration increases following the same anion order. When PDDA is introduced to PSS surface again, the penetrated PSS chains would diffuse out and interact with PDDA chains forming a new layer.38,39 Therefore, more PSS penetration results in more adsorption of PDDA, that is, a lower -∆f for the deposition of PSS gives rise to a higher -∆f for the deposition of PDDA. In other words, the anion-modulated chain interpenetration determines the nonlinear multilayer growth. According to the law of matching water affinity,16-18 a weakly hydrated chaotropic anion can form strong ion pairs with the weakly hydrated ammonium groups so that the chaotropic anions can screen the polyelectrolyte charges more effectively from Cl- to Br-, leading to a higher level of “extrinsic charge compensation”.40 As a result, they yield a more swollen PDDA layer, which facilitates the PSS chain penetration.38-42,51 Besides, the similar values of -∆f for the deposition of PSS and PDDA observed in NaHCO3 solution indicates a slight interpenetration between the two polyelectrolyte chains. It is known that two oppositely charged polyelectrolytes with a high level of chain interpenetration and complexation form a rigid multilayer, whereas those with a lower level of chain interpenetration result in a looser multilayer.41,42 This is why the multilayer deposited in NaHCO3 solution exhibits a dramatic increase in ∆D as layer number increases (Figure 2). Figure 6 shows the layer number dependence of frequency change (-∆f) for HCO3-, F-, CH3COO-, H2PO4-, and SO42-, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively. It can be seen that -∆f increases linearly with the layer number for all the anions. The most important event is that the deposition of PSS causes more increase in -∆f than that for PDDA except in the case of HCO3-. This implies that PSS chains form a swollen layer on PDDA surface. The subsequently introduced PDDA chains penetrate into such a swollen layer and form complexes with
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Figure 7. The average frequency shift (-∆f) due to the deposition of PSS or PDDA for the last four layers as a function of anion species regarding HCO3-, F-, CH3COO-, H2PO4-, and SO42-.
PSS chains. This is quite different from the situation about the nonlinear multilayer growth described above. The average values of -∆f due to the deposition of PSS or PDDA for the last four layers as a function of anion species are shown in Figure 7. The -∆f induced by the deposition of PDDA decreases from HCO3- to F-, and then holds constant at ∼0 from F- to SO42-. On the other hand, -∆f induced by the deposition of PSS increases from HCO3- to F-, followed by a gradual decrease from F- to SO42-. The increase of -∆f for PSS and the decrease of -∆f for PDDA from HCO3- to Freflect that the multilayer growth changes from a PSS penetration-dominated regime to a PDDA penetration-dominated regime. From F- to SO42-, -∆f for the deposition of PDDA remains almost a constant indicating that PDDA chains have a similar level of penetration for different anions. Therefore, the linear multilayer growth is not determined by the chain interpenetration. -∆f for the deposition of PSS gradually decreases from F- to SO42- indicating a gradual decrease of the amount of adsorbed PSS chains. Since ammonium is a weakly hydrated group, the charge screening effectiveness of the anions will decrease with the Hofmeister series from chaotropes to kosmotropes according to the Collins’ concept of matching water affinities.18,37,52 Therefore, the effectiveness of kosmotropic anions to screen the polyelectrolyte charges increases from SO42- to F-. At a more screened PDDA surface, PDDA chains adopt a more loopy conformation, yielding a higher surface charge density. As a result, the PDDA surface can adsorb more subsequently introduced PSS chains leading to a larger -∆f.53,54 Clearly, the linear multilayer growth is dominated by the anion-modulated conformation of PDDA chains on the surface instead of the chain interpenetration. It was suggested that the chain penetration is correlated with the chain persistence length of polyelectrolytes.38,39 The polyelectrolyte chains with long persistence length can penetrate into the layer formed by oppositely charged chains with shorter persistence length. The persistence length of polyelectrolyte (lp) is determined by the bare persistence length (l0) and the electrostatic persistence length (lp,e).55 It is known that PDDA has a longer bare persistence length (l0) than PSS.56,57 In the case of kosmotropic anions, the electrostatic interactions cannot be effectively screened due to the weak interactions between positively charged ammonium groups and kosmotropic anions, so that the persistence length of PDDA chains is longer than that of PSS chains. In the presence of chaotropic anions, however, the persistence length of PDDA chains is shorter than that of PSS chains because the anions can effectively screen the electrostatic interactions, reflecting in the fact that the persistence length (lp) of PDDA and PSS chains are ∼2.5 nm and ∼4.0 nm in a 0.5 M NaCl solution, respectively.58,59 This explains that PSS chains penetrate into PDDA layer in a
Figure 8. The layer number dependence of frequency shift (-∆f) in NaBr-NaF mixed solutions and NaClO3-NaF mixed solutions, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively.
nonlinear mode but PDDA chains penetrate into PSS layer in a linear mode. Figure 8 shows the layer number dependence of frequency change (-∆f) in NaBr-NaF mixed solutions and NaClO3-NaF mixed solutions, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively. It is expected that the multilayer growth would be gradually dominated by the nonlinear mode with the increasing molar percentage of chaotropic anion since the multilayer grows in linear and nonlinear modes in kosmotropic and chaotropic anion solutions, respectively. It can be seen that from Figure 8 the multilayer growth is gradually dominated by the nonlinear mode with increasing molar percentage of NaBr (or NaClO3) except in the solution of 80% NaBr (or 80% NaClO3) (mol/mol). In NaBr-NaF system, the growth of multilayer in the solution of 80% NaBr (mol/mol) is similar to that in the solution of 50% NaBr (mol/mol). The growth of multilayer in the solution of 80% NaClO3 (mol/mol) is similar to that in the solution of 40% NaClO3 (mol/mol) in NaClO3-NaF system. This interesting phenomenon should be attributed to the complex anionpolyelectrolyte interactions at a certain molar percentage of chaotropic anions on the surface, that is, it is related to the nature of F- since similar phenomena can be observed in either NaBr-NaF or NaClO3-NaF system. This can be further validated by the experiment where the multilayer is deposited in the mixed solutions of NaClO3-CH3COONa (shown in Figure 9). Clearly, the multilayer growth is gradually dominated by the nonlinear mode from 0% NaClO3 (mol/mol) to 100% NaClO3 (mol/mol). Yet, such a phenomenon was not observed for the solution of 80% NaClO3 (mol/mol). The facts further indicate that the phenomenon originates from F-. Additionally, the deposition of PDDA chains causes more increase in -∆f than PSS chains in the solution of 50% NaBr (or 50% NaClO3) (mol/mol) for all three different mixed electrolyte systems, similar to the deposition in chaotropic anion solutions. Therefore,
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Figure 9. The layer number dependence of frequency shift (-∆f) in NaClO3-CH3COONa mixed solutions, where the odd and even layer numbers correspond to the deposition of PSS and PDDA, respectively.
Liu et al. hydrated chaotropic anions can form strong ion pairs with the weakly hydrated ammonium groups on PDDA chains, the chaotropic anions would bind onto the polyelectrolyte chains more tightly in comparison with the kosmotropic anions.18,37,52 In the mixed electrolyte solutions, the chaotropic anions should prefer to bind onto the polyelectrolyte chains. Moreover, they can replace the already adsorbed kosmotropic anions.36 Thus, the anion competition effects lead the multilayer growth to be dominated by the chaotropic anions in the mixed electrolyte solutions. This conclusion is consistent with that shown in Figures 8 and 9. This is the reason that the theoretical values of -∆f induced by the deposition of multilayer are always higher than the experimental values in the mixed electrolyte systems. Conclusions
Figure 10. Comparison between experimental and theoretical values of the frequency shift (-∆f) for the eight bilayers in NaBr-NaF mixed solutions and NaClO3-NaF mixed solutions as a function of molar percentage of NaF.
We have investigated the anion-specific growth of a polyelectrolyte multilayer in solutions by using QCM-D. In single electrolyte solutions, the anions can be divided into two groups in terms of their specific effects on the multilayer growth. Br-, ClO3-, and Cl- lead to a nonlinear growth due to the anionmodulated chain interpenetration. A linear multilayer growth is observed in the cases of F-, CH3COO-, H2PO4-, and SO42-, where the anion-modulated conformation of polyelectrolyte chains plays a critical role. As the borderline of the two classes, HCO3- leads to a multilayer growth mode in between. Furthermore, due to the anion-modulated persistence length of PDDA chains, PSS chains penetrate into PDDA layer in a nonlinear growth mode, but PDDA chains penetrate into PSS layer in a linear growth mode. In a mixed electrolyte solution containing both chaotropes and kosmotropes, the multilayer growth is dominated by the chaotropic anions due to the anion competition effects, and the effects of anions on the multilayer growth are nonadditive. Acknowledgment. The financial support of the National Distinguished Young Investigator Fund (20725414), the ChinaAustralia Special Fund of the National Natural Science Foundation (NNSF) of China (51011120051), and Ministry of Science and Technology of China (2007CB936401) is acknowledged. We thank Professor Barry W. Ninham for helpful discussions regarding the multilayer growth in the mixed electrolyte solutions. References and Notes
Figure 11. Comparison between experimental and theoretical values of the frequency shift (-∆f) for the eight bilayers in NaClO3CH3COONa mixed solutions as a function of molar percentage of CH3COONa.
the chaotropic anions have stronger influences on the multilayer growth than the kosmotropic anions in the mixed electrolyte solutions. Note that if there were no interplay between the chaotropic anion-polyelectrolyte interactions and the kosmotropic anionpolyelectrolyte interactions in the mixed electrolyte solutions, the resulted frequency change (-∆f) by the deposition of multilayer would follow the additivity law. However, Figures 10 and 11 show that the resulted -∆f (for the eight bilayers) are always less than the theoretical values expected by the additivity law, that is, the specific anion effects on the multilayer growth in the mixed electrolyte solutions are nonadditive, implying the anion competition effects there.36 Since the weakly
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