Quantitative Evaluation of Long-Range and Cooperative Ion Effect on

Mar 19, 2019 - ... and Cooperative Ion Effect on Water in Polyamide Network ... a lower ion effect than the chloride salts of structure-breaking K+ an...
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Quantitative Evaluation of Long-Range and Cooperative Ion Effect on Water in Polyamide Network Ki Chul Park and Takehiko Tsukahara* Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, Tokyo 152-8550, Japan

J. Phys. Chem. B Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 03/25/19. For personal use only.

S Supporting Information *

ABSTRACT: Despite long-standing research efforts to elucidate the specific ion effect on the structure and dynamics of water, the spatial extent affected by ions and the cooperativity of ions and counterions are still controversial. Here, we demonstrate an undoubtable evidence of long-range and cooperative ion effect on water molecules in a polyamide network by using a precision ion sensor of photonic crystal hydrogel membrane. The ion effect was quantitatively evaluated by means of the osmotic work per unit cell volume change of photonic crystal, Wunit, required for the ion-inhibited dehydration, which means a suppressed migration of water molecules by the extensive effect of ions beyond their immediate hydration shells. It was found that Wunit required for 14 vol % contraction of the membrane sensor in LiCl aqueous solutions was 7.7 times larger than that in Sr(NO3)2 solutions. The combination of structure-making Ca2+ and Sr2+ with nitrate anions lowered the ion effect than the chloride salts of borderline Na+ and Ba2+. Furthermore, the nitrate salt of Sr2+ exhibited a lower ion effect than the chloride salts of structure-breaking K+ and Cs+. These results have revealed that the ion effect acts to water extensively, which is modulated by cooperative interactions of ions and counterions.

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INTRODUCTION The specific properties of electrolyte solutions have been a critical issue to be elucidated in chemical and biological fields over the past century since the Hofmeister effect of ions was discovered.1,2 The Hofmeister series of ions, which are ranked according to the ability of precipitating (salting-out) or solubilizing (salting-in) proteins, are generally classified as kosmotropes (structure makers) and chaotropes (structure breakers).1−3 It was assumed that kosmotropes stabilize hydrophobic interactions to lower the solubility of proteins, whereas chaotropes have the opposite effect. In a classical mechanism,4 it had been long accepted that the salting-out or salting-in of proteins would be related to the ordering or disordering of hydrogen-bond (HB) network of the surrounding water molecules induced by ions beyond their immediate hydration layers, which were attributed to kosmotropic or chaotropic nature of ions, respectively. The classical mechanism was derived from the inference based on the fact that hydrophobic interactions are solvent-induced phenomena in terms of the entropy and heat capacity changes of hydrophobic solvation5−12 and the degree of water structuring is determined by the kosmotoropicity/chaotropicity of ions linked to the increase/decrease in the viscosity of electrolyte solutions.13,14 However, it was pointed out that the kosmotropic/chaotropic notions were not necessarily consistent with the actual alterations in the structure or dynamics of water in molecular levels.15−19 Furthermore, some experimental and computational studies obtained the negative findings against the classical mechanism, leading to a recent consensus that the © XXXX American Chemical Society

specific effect of ions is caused by the interaction with macromolecules or their primary hydration layers.19−31 The ion-induced water structuring is also contradictory to the research results that ions have no long-range effect to alter the dynamics of the surrounding water molecules32 and the structure of water HB network due to the limited effect on the hydration shells in the vicinity of ions.17,33−36 In contrast, some studies suggested the ion-induced perturbation of water structures and dynamics beyond the first hydration shells of ions37−47 and the anionic effect on the water structures including peptide amide hydration.48 Therefore, the efficacy range of specific ion effect on water structures is left as a controversial issue. Another contentious issue in discussing specific ion effects includes interdependency between ions and counterions. The Hofmeister effect of cations is not as pronounced as that of anions, and the ranking order of cations on the solubility of uncharged peptides is altered according to counteranions in the salt solutions of low to medium concentrations.49,50 This suggests that the effects of anions and cations are not simply additive but cooperative. The ion cooperativity was also confirmed by a femtosecond and terahertz spectroscopic study of ion-hydration water.43,51 For example, the HB network and reorientation dynamics of water molecules around strongly hydrated ions are locked well beyond the first hydration shells by the cooperative interaction with the strongly hydrated Received: January 23, 2019

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DOI: 10.1021/acs.jpcb.9b00717 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B counterions.43 On the other hand, some previous studies found a negligible effect of ions on the dynamic behavior of water network52 and no indication of long-range cooperative ion effect other than a strong coupling of ions with their first hydration shells.33 Here, we report an experimental evidence of long-range and cooperative effect of ions on water molecules in a polymer network, which was quantitatively evaluated by ion sensing using a photonic crystal hydrogel sensor. The sensor module is comprised of nonclose-packed silica colloidal crystalline arrays (SiO2-NCP-CCA) immobilized by a poly(N-isopropylacrylamide) (PNIPAAm) hydrogel membrane.53 Photonic crystal hydrogels exhibit a high potential for the application to various sensors.54 PNIPAAm is a typical thermoresponsive polymer with a lower critical solution temperature (LCST), at which phase transition is caused by the thermal (de)stabilization of hydrophobic interaction of PNIPAAm in its aqueous solution. As a model of protein behavior, PNIPAAm has been adopted in studies of the mechanism underlying Hofmeister ion effects in terms that the salts induce the shift of LCST.31,55,56 The modulation of the phase transition behavior was attributed to the kosmotropic anion-induced destabilization of amide hydration,31 the surface tension increase in the water/ hydrophobic interface by chaotropic anions,31 the ion-binding interaction with the hydrophobic moiety by chaotropic I− anion,27 or the entropically driven binding of chaotropic SCN− anions on the amide group.57 On the other hand, previous studies utilizing the unique bulk and interface properties of charged PNIPAAm microgel particles revealed the presence of both mechanisms of ion accumulation/exclusion at the polymer/water interfaces and the specific ion interactions with the polymers and internal water molecules.55,58 However, the previous studies provided no quantitative findings on the specific ion effect. Moreover, the observed differences of the cation effects were not as clear as those of the anion effects. At a constant temperature below the LCST, the immersion of the PNIPAAm hydrogel membrane sensor in salt solutions causes its volume change due to the difference in osmolarity between the inside and outside of the membrane until the osmotic equilibrium is reached by ion diffusion and the osmolarity-driven migration of water. Important is the fact that the osmotic response is modulated by specific interactions between ions, polymers, and internal water molecules. Therefore, the osmotic pressure−volume work required for the dehydration of the hydrogel membrane provides a quantitative measure of long-range and cooperative ion effect on the abundant internal water molecules beyond the immediate hydration shells of the ions and polymers. Furthermore, the quantitative measure has an advantage to enable a direct comparison between ions of different valence because, in most cases, other parameters for describing the specific ion effects can scarcely be compared between ions of different valences.59

determined by dynamic light scattering (Delsa Nano HC, Beckman Coulter, Inc.) was 102 nm. The aqueous SiO2-NCPCCA was prepared by the deionization of silica suspension,60 where the mixed-bed ion-exchange resins (AG501-X8(D), 1.0 g of wet beads) were mixed with the silica suspension (5.0 mL) and shaken for 4 h. The resulting thickened suspension of clear pale purplish amber color was recovered and put into a regenerated cellulose membrane dialysis tube (Spectra/Pro 6, MWCO: 1 kDa, Spectrum Laboratories, Inc.). After sealing, the dialysis tube was immersed in DMF (500 mL) and stirred for 3 days (DMF was changed once a day). Hydrogel Immobilization of Photonic Crystals. The immobilization of SiO2-NCP-CCA by PNIPAAm gels was carried out by photoinitiation polymerization.61 NIPAAm monomer (1.45 mmol, 165 mg) and N,N′-methylenebisacrylamide cross-linker (0.0726 mmol, 11.2 mg, 5 mol % to NIPAAm) were dissolved in SiO2-NCP-CCA/DMF (1.0 mL). Then, 2,2-diethoxyacetophenone (1.5 μL) as a photoinitiator was mixed well. The resulting solution (60 μL) was dropped onto the quartz plate with a quadrangular template of polyimide film tapes (8 × 70 mm2, thickness: 63 μm) and sandwiched by another quartz plate. The liquid contact side of the quartz plates was pretreated by oxygen-plasma ashing (plasma cleaner PDC-32G, Harrick Plasma, Inc.) for 3 min just before use. The liquid membrane in the mold space was polymerized by irradiation with 365 nm UV light (ultrahigh pressure mercury lamp: 500 W) for 4 min. After polymerization, the cover plate was removed in ultrapure water (the characteristic color of photonic crystals immediately appeared). After 18 min, the hydrogel membrane was peeled off from the quartz plate and soaked in enough ultrapure water for exchanging DMF to water. The final color of the hydrogel membrane was dependent on the soaking time before peeling it off from the quartz plate. The soaking for more than 18 min provided metallic greenish blue, whereas the immediate peeling off made it metallic purplish blue. The strip-shaped hydrogel membrane prepared was cut into square-like pieces and used in ion-sensing experiments (cut size: about 7 × 7 and 3 × 3 mm2 for batch and flow experiments, respectively). Batch Experiments for Ion Sensing. The color of membrane sensors was observed before and after immersion in the salt solutions of alkali and alkaline earth metals. After immersion in 20 mL of 0.75 mol L−1 alkali salt solution (or 0.5 mol L−1 alkaline earth salt solution) at 18.0 °C for 10 min, the membrane sensor was transferred to the corresponding salt solution in a jacketed glass beaker controlled at 18.0 °C by a cooling/heating circulator. After standing for more than 10 min, the color of the membrane sensor was recorded by a digital camera. Prior to the salt solution experiments, the color of each membrane sensor immersed in ultrapure water was recorded in the same manner. Optical Measurements for Ion Sensing in Flow Experiments. The color variation of the membrane sensor was pursued by measuring the reflection spectrum. The membrane sensor was enclosed in the quartz cell with the sample room and the liquid flow channels for feed and drain. The salt solution was continuously supplied to the sample room at the flow rate of 50 μL min−1 by a peristaltic pump. The temperature of the quartz cell was controlled at 10 °C using a Peltier cooling device. The reflection spectra were recorded on the optical measurement system (Ocean Optics, Inc.) comprised of a light source (DH-200), a UV−vis spectrometer (USB2000+) and an optical fiber probe for

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EXPERIMENTAL METHODS Preparation of Photonic Crystals. SiO2-NCP-CCA in N,N-dimethylformamide (DMF) was prepared by the solvent exchange of aqueous NCP-CCA using dialysis. The aqueous suspension of silica nanoparticles (MP-1040, Nissan Chemical Industries, Ltd.) with the nominal silica content of 40 wt % and the particle diameter of 100 nm was used for preparation. The actual silica content determined from the weight change by dryness was 43.4 wt %, and the mean particle diameter B

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Figure 1. Ion-sensing results by photonic crystal hydrogel membrane sensors. (A) Color change of photonic crystal hydrogel membrane (size: 7 × 7 mm2, thickness: >63 μm) before and after immersion in alkali and alkaline earth chloride salt solutions. A pair of photographs, for example, 1 and 1′, represents the same membrane sensor immersed in ultrapure water and then in salt solution, respectively. The salt solutions of 1′−8′ correspond to 0.75 mol L−1 of LiCl (1′), NaCl (2′), KCl (3′), and CsCl (4′) and 0.50 mol L−1 of MgCl2 (5′), CaCl2 (6′), SrCl2 (7′), and BaCl2 (8′). Temperature was kept at 18.0 °C. (B) Typical reflection wavelength shift of the membrane sensor with varying the concentrations of salt solutions in the flow experiments. This figure was obtained using CaCl2 solutions. The peak denoted by number 1 originates from the initial state of photonic crystal hydrogel membrane in ultrapure water. The peaks of 2−7 correspond to the measurement results obtained using the salt solutions of 0.02, 0.10, 0.40, 0.50, 0.80, and 1.0 mol L−1. The reflection peak was red-shifted at the initial low concentration of 0.02−0.10 mol L−1 and then blue-shifted with the increase of the salt concentration. (C) Plot of reflection wavelength shift (Δλ) of photonic crystal hydrogel membrane (size: 3 × 3 mm2, thickness: >63 μm) against the concentration of salt solutions. The paucity of data points for CsNO3 and no data for Ba(NO3)2 are due to their low solubility.

that the hydrogel membrane was contracted by dehydration with increasing temperatures, which caused the successive color change from the initial metallic greenish blue to stronger blue, then purplish blue below LCST, and finally to colorless at LCST (i.e., 32 °C) by the shortening of the reflected light wavelength based on the reduction of d111 spacing (Supporting Information, Section 2). Also, in the batch experiments conducted at 18.0 °C, the color change of the membrane sensor was caused by immersing it in the solution of alkali salts (0.75 mol L−1) and alkaline earth salts (0.50 mol L−1). The observed color change (Figure 1A) showed a tendency that dehydration was more hindered especially by structure-making cations (the high charge densities of cations) in the order of Li+ > Cs+ > Na+ ≥ K+ and Mg2+ > Ca2+ > Sr2+ > Ba2+, which generally follow the order of hydration energies.62 This result suggests that the strongly hydrated cations enhance the interaction between water molecules abundant in the hydrogel membrane. However, only Cs+ exhibited a different trend from the order of hydration enthalpies (i.e., Li+ > Na+ > K+ > Cs+). The exceptional behavior of Cs+ will be caused by specific ion adsorption on the charged silica nanoparticles (see the Supporting Information Section 3 for the preferential adsorption of Cs+). The osmotic migration of water can be evaluated as the volume change per unit cell of photonic crystal. The silica CCA with well-controlled intervals of distance, which was assembled by the electrostatic repulsion among the surface-

illumination and detection (R200-7-UV−VIS). All reflection spectra were measured in an osmotic equilibrium after supplying the salt solution for 1 h (during which the wavelength shift ceased). The feed solution was changed in succession to the higher concentration of solution after each measurement. The optical fiber probe for illumination and reflected light detection was positioned perpendicular to the membrane sensor. The reflected light for a halogen light source was detected at the range of 380−900 nm.

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RESULTS AND DISCUSSION The crystal structure of silica CCA, which tends to take either face-centered cubic (fcc) or body-centered cubic (bcc) structures, was determined from the transmission spectrum of a SiO2-NCP-CCA/DMF liquid membrane for normal incidence (Supporting Information, Section 1).60 The volume percentage of silica CCA was 17.1%, and the mean particle diameter determined by dynamic light scattering was 102 nm. The transmission spectrum showed the diffraction peak at 390.7 nm. The diffraction wavelength calculated using the volume percentage and particle diameter of silica CCA was 389 nm for the fcc(111) diffraction and 378 nm for the bcc(110) diffraction. The former value agrees with the observed diffraction wavelength, indicating that silica CCA takes a fcc structure. Ion sensing was examined for the chloride and nitrate solutions of alkali and alkaline earth cations. It was confirmed C

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Figure 2. Ion-affected osmotic response in dehydration of photonic crystal hydrogel membrane sensors. (A) Plot of the osmotic pressure (Π) of bulk salt solutions against the volume reduction (ΔVfcc) of fcc unit cells. (B) Variation of Wunit with the reflection wavelength reduction (ΔλT) caused by contraction of photonic crystal hydrogel membrane.

ions between the two phases. In the membrane sensors of the present study, the fixed charges correspond to the deprotonated silanol groups (Si−OH) of the silica surface.65 With a typical surface density of 4.6−4.9 Si−OH nm−2 for amorphous silica,66 the estimated amount of silanol groups on the four silica nanoparticles contained per fcc unit cell was comparable to that of cations in the lowest concentration of the salt solutions considered (e.g., the molar ratio of Si−OH against the cations of 0.030 mol L−1 alkali salts in the fcc unit cell volume was 0.91−0.97). Therefore, depending on the pH of salt solutions, the specific ion adsorption on the silica surface will induce the swelling osmosis in the low salt concentration regions (see the Supporting Information Section 3 for the detailed discussion about the specific ion adsorption). After the swelling induced by the specific ion adsorption, the volume of the membrane sensor was reduced by hydrogel dehydration, whose degree was dependent on both anions and cations (Figure 2A). The diffuse layer thickness for the screening of silica surface charge (e.g., 1.77 nm for 0.030 mol L−1 of 1:1 electrolyte at 10 °C, which is compressed by the increase of salt concentrations) is far shorter than the lattice parameters of silica CCA (d111 spacing; 193 nm, lattice constant; 334 nm, the nearest interparticle distance; 236 nm). Therefore, the countercations (the electrical double layers with a small number of co-ions) surrounding the negatively charged surface of silica nanoparticles would have no influence on the distribution and interaction of ions in the peripheral major part of the hydrogel matrix. The swelling equilibrium of hydrogels with internal charges is described as Π = Πmix + Πel + Πion.64,67 The right side of the equation represents in turn the contribution of polymer/solvent mixing, elastic contribution due to a polymer−network deformation, and the contribution of ion/solvent mixing and electrostatic effects. For highly swollen charged hydrogels, the contribution of polymer/ solvent mixing term is usually small compared with the elastic

charged silica nanoparticles, enables to detect precisely the volume change of the hydrogel as a wavelength shift of reflected light on the photonic crystal lattice planes. In the flow experiments, the variation of reflection wavelength was monitored keeping the temperature at 10 °C. The reflection spectra showed that the wavelength shift was dependent on the concentrations of salt solutions (Figure 1B), which can be resolved at less than 1 nm scale. Furthermore, the extent of the wavelength shift (Δλ) was dependent on the kind of salts examined (Figure 1C). The high resolution of the reflection spectrum makes it possible to distinguish the imperceptible difference of ion effects. More specifically, in the crystal size of the present silica CCA, the reflection wavelength shift of 1.0 nm for the membrane sensor in a water medium is equivalent to the slight change of 2.1 × 10−4 μm3 per fcc unit cell (which corresponds to 0.57% of the fcc unit cell volume). The rather high sensitivity to the volume changes enables to clarify even the weak effect of cations. To quantify the specific ion effect, the osmotic pressure (Π) in each salt concentration was calculated using the van’t Hoff equation corrected with osmotic coefficients, ϕ (Supporting Information, Section 4), and the volume reduction (ΔVfcc) of photonic crystal fcc unit cell caused by the osmolarity-induced hydrogel dehydration was derived from the observed reflection wavelength by using the Bragg−Snell equation (Supporting Information, Section 5). The plots of Π against ΔVfcc exhibited a clear tendency of hydrogel swelling (ΔVfcc < 0) in the low salt concentration range (Figure 2A). In a swelling equilibrium, the total free energy change becomes minimum, or the chemical potentials of all mobile components become equal in the coexisting two phases.63,64 In charged hydrogels, according to Donnan equilibria,63,64 the counterions are held on the inside of the hydrogels to maintain the electrical neutrality for the fixed charges of the polymers and cause the swelling osmosis based on the difference in the concentration of mobile D

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Figure 3. Evaluation of anion−cation cooperativity in specific ion effects. (A) Comparison of Wunit in the alkali and alkaline earth chloride and nitrate salts (at the displacement of ΔλT = 30 nm from the most swollen state). (B) Relative ratios of Wunit to that of NaCl providing a relative measure of anion−cation cooperativity in specific ion effects. The relative ratios were derived from (A).

and ionic terms.67,68 The elastic modulus of PNIPAAm hydrogels with the cross-linker content of 5 mol %, which is the same as that of this study, is about 12 kPa much smaller than Π of the MPa order.69 Therefore, the specific interactions between ions, polymer segments, and water molecules, which are responsible for the salt dependence in hydrogel contraction, would be mainly reflected in the polymer/solvent mixing and ionic terms, and the specific ion effect can be evaluated by the pressure−volume work (WA) required for ioninhibited hydrogel contraction, according to the following equation: WA =

2B). In the alkali cations, Wunit was increased in the order of Li+ ≫ Na+ ≥ K+ > Cs+. The increase of Wunit means the enhancement of water−water interactions by long-range ion effects in the ion−water−polymer system, in terms that the number ratios (Rw/i) of water molecules to each ion in the salt solutions are sufficiently larger than the number of hydration water molecules on the ions (e.g., the Rw/i of NaCl solutions is 27.0 (at ΔλT = 30 nm) to 47.1 (at ΔλT = 15 nm), whereas the concentration-dependent hydration numbers18 of Na+ and Cl− are 4.5−5.3 and 6.1−5.6, respectively, and the Rw/i of LiCl solutions is 11.6 (at ΔλT = 30 nm) to 18.2 (at ΔλT = 15 nm), whereas the first hydration shell of Li+ is comprised of four water molecules70). Notably, Li+ showed the most remarkable ion effect than other alkali and alkaline earth cations in both chlorides and nitrates. The slopes of the Wunit variation with the progress of hydrogel contraction varied with the combination of anions and cations (Figure 2B). This result indicates that the strengthening efficiency of the salt effect is dependent on the ion cooperativity. The comparison of all cations examined (Figure 3A) exhibited the different ordering of Wunit (at ΔλT = 30 nm) between chloride and nitrate salts (i.e., Li+ > Mg2+ > Ca2+ > Sr2+ > Na+ ≈ K+ > Ba2+ > Cs+ for the chloride salts, Li+ > Mg2+ > Ca2+ ≈ Na+ > K+ > Sr2+ for the nitrate salts), which indicates that the dehydration is not monotonically inhibited in the order of hydration energies of cations in each of the chloride and nitrate salts. This result as well clearly demonstrates that the ion effect is based on not an additive but a cooperative action of anions and cations. Interestingly, compared with other cations, Na+ exhibited a closer value of Wunit in chloride and nitrate salts, which means less pronounced ion cooperativity. Therefore, the relative value of Wunit to that of Na+ becomes a useful measure for evaluating the cooperativity of anions and cations in the salt effect. The relative strength and cooperativity of salt effects were expressed as the relative ratios of Wunit to that of NaCl (Figure 3B). The overall ranking of alkali and alkaline earth chloride and nitrate salts based on the relative ratios of Wunit (the values in parentheses) is as follows: LiCl (3.17) > MgCl2 (2.69) > LiNO3 (1.99) ≈ CaCl2 (1.95) > Mg(NO3)2 (1.56) > SrCl2 (1.27) > KCl (1.05) ≈ NaCl (1.00) > BaCl2 (0.89) > NaNO3 (0.71) ≈ Ca(NO3)2 (0.68) > CsCl (0.63) > KNO3 (0.50) > Sr(NO3)2 (0.41). As a general trend, the nitrate

∫ (Π T − ΠS)dΔVfcc

Here, ΠS represents the osmotic pressure at a given displacement (i.e., ΔλS = 5.0 nm) that has reached from the most swollen state. To eliminate the contribution of the swelling caused by the specific ion adsorption on the charged silica surface, the osmotic work from the initial state to reach ΔλS was subtracted from the total work reaching the given state of ΠT at ΔλT from the most swollen state. Thus, WA corresponds to the osmotic work in the course of hydrogel contraction, which is modulated by the ion−water−polymer interactions depending on the kind of salts. For a direct intercomparison of specific ion effects, WA was normalized to the work amount per unit volume change, i.e., Wunit, by dividing with the ΔΔVfcc from the ΔVfcc at ΔλS = 5.0 nm to the ΔVfcc at ΔλT. The hydrogel contraction adopted for calculation corresponds to the blue shift of ΔλT = 15−30 nm from the most swollen state, for example, in LiCl solutions, which are equivalent to the small reduction of d111 spacing in the range of ca. 8.6−16.0 nm. The reduction of d111 spacing is comparable with ca. 12.4−22.5% decrease of Vfcc from the most swollen state and with 10.3−18.5 vol % decrease of internal solutions in the volume fraction of the photonic crystal hydrogel. On the other hand, the hydrogel contraction of ΔλS = 5.0 nm causes the d111 spacing reduction of 3.4 nm, the Vfcc decrease of 5.1%, and the internal-solution decrease of 4.2 vol %. Thus, the dehydration from ΔλS = 5.0 nm to ΔλT = 15−30 nm corresponds to 6.1−14.3 vol % decrease of the internal solution. The calculated Wunit of the alkaline earth cations was increased with the increase of the hydration energies (Mg2+ > Ca2+ > Sr2+ > Ba2+) for both chlorides and nitrates (Figure E

DOI: 10.1021/acs.jpcb.9b00717 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B anions lowered the salt effect, although the extent of the lowering was dependent on the combination with cations (the relative ratios of Wunit at ΔλT = 30 nm were 0.32 for Sr(NO3)2/SrCl2, 0.35 for Ca(NO3)2/CaCl2, 0.48 for KNO3/ KCl, 0.58 for Mg(NO3)2/MgCl2, 0.63 for LiNO3/LiCl, and 0.71 for NaNO3/NaCl). The relative position of salt effects in the ranking was determined by the combination of anions and cations. More specifically, the ion effects of chloride salts of Mg2+, Ca2+, Sr2+, and Li+ (which are classified as structuremaking ions by the transfer (solute, H2O → D2O) free-energybased parameter, ΔGHB, exhibiting the effect of solutes on the HB structure of water71) were higher than those of the chloride salts of Na+ and Ba2+ (which are classified as borderline ions between structure-breaking and structuremaking ions), whereas the nitrate salts of structure-making Ca2+ and Sr2+ exhibited the ion effect lower than the chloride salts of the borderline Na+ and Ba2+. Furthermore, the ion effect of nitrate salt of Sr2+ was lower than those of chloride salts of structure-breaking K+ and Cs+. The nitrate salt of Sr2+ exhibited the lower effect than the nitrate salts of Na+ and K+. These results have demonstrated that the extent of ion cooperativity is significantly dependent on the combination of anions and cations.

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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 present work was partially supported by “the Funding Program for Next Generation World-Leading Researchers (NEXT program)” of the Cabinet Office and “the Initiatives for Atomic Energy Basic and Generic Strategic Research 260402” from MEXT (Ministry of Education, Culture, Sports, Science and Technology).

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CONCLUSIONS The specific ion effect of alkali and alkaline earth chloride and nitrate salts in a polyamide network was quantitatively evaluated by the energy of work required for ion-induced osmotic dehydration of a photonic crystal hydrogel membrane sensor. Typically, the 14 vol % contraction of the membrane sensor required the normalized osmotic work for dehydration, Wunit, widely ranging from 5.3 × 10−13 J μm−3 for Sr(NO3)2 at a minimum to 4.1 × 10−12 J μm−3 for LiCl at a maximum (from the plot of Wunit at ΔλT = 30 nm in Figure 3A). The salt dependence of Wunit has disclosed that the dehydration of the hydrogel membrane containing more water molecules than the hydration numbers of the anions and cations is inhibited by the different strength of long-range ion effect on the HB network of water molecules in the polymer network. In comparison with the chloride anions, the nitrate anions were found to lower the salt effect. Furthermore, the intercomparison of the salt effects (Figure 3B) indicated that (i) the sodium salt was suitable as a reference of specific ion effect due to the less pronounced ion cooperativity and (ii) nitrate anions caused the ion effects of structure-making Ca2+ and Sr2+ to be lower than those of the chloride salts of borderline Na+ and Ba2+; furthermore, the ion effect of nitrate salt of Sr2+ was lower than those of chloride salts of structure-breaking K+ and Cs+ and (iii) the ion effect of Sr2+ was inverted with Na+ and K+ by counterions. The results of the present study have shown not only an undoubtable evidence but also quantitative information about the long-range and cooperative effect of anions and cations on water molecules in the model polyamide network, providing one clear answer to the long-standing controversial issue regarding the ion effect on water.

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equation corrected with osmotic coefficients; BraggSnell equation (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.9b00717. Transmission spectrum of photonic crystal; thermal response of sensor; specific ion adsorption; vant’t Hoff F

DOI: 10.1021/acs.jpcb.9b00717 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

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