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Reversal of Cation-Specific Effects at the Interface of Mica and Aqueous Solutions Zengqiang Jia, Xiong Li, Chang Zhu, Sen Yang, and Gang Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09956 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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The Journal of Physical Chemistry

Reversal of Cation-Specific Effects at the Interface of Mica and Aqueous Solutions Zengqiang Jia, Xiong Li, Chang Zhu, Sen Yang, Gang Yang* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China * To whom correspondence should be addressed: E-mail: [email protected]; Phone: 086-023-68251504; Fax: 086-023-68250444.

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ABSTRACT: Ion-specific effects are ubiquitous and have gained a renaissance over the past few decades while remain largely elusive. In this work, molecular dynamics simulations have been conducted to investigate the adsorption of different metal ions at the interface of mica and aqueous solutions, and cation-specific effects abide by the sequences of Na+ > K+ > Cs+ and Cs+ > K+ > Na+ for less and more charged surfaces, respectively. Mechanisms for cation-specific effects and reversal of Hofmeister series are then addressed at an atomic level. Hydration effect (i.e., interaction of metal ions with water) is the driving force for less charged surfaces while interaction of metal ions with mica plays a larger role for more charged surfaces, which further result in a reversal of Hofmeister series. Clay minerals generally carry an abundance of negative charges, and the finding that Hofmeister series can be reversed with no change for the sign of surface charges provides new insights about related processes and ion-specific effects. These results have significant implications because of the ubiquity and significance of charged systems, especially in biology, chemistry and colloid science.

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1. INTRODUCTION Franz Hofmeister1 in the late 1880s discovered that a series of electrolyte ions show consistent effects on the solubility of proteins and stability of their secondary and tertiary structures, which are collectively known as Hofmeister effects (or ion-specific effects). Over the past few decades, ion-specific effects have gained a renaissance and played an increasingly important role in a wide spectrum of chemical, physical and biological processes such as protein folding, enzyme activity, colloid stability, osmotic coefficient, surface tension and bubble coalescence.2-6 Owing to the ubiquity and significance, Kunz et al.7 claimed that ion-specific effects are as important in the scheme of things as was Mendel’s work to genetics, which has been gradually accepted by other researchers.2-6,8-11 As pointed out by Tobias and Hemminger,12 ion-specific effects continue to defy all-encompassing theories, and one of the fundamental topics is Hofmeister series. Zhang and collaborators13 demonstrated that at high electrolyte concentrations, effectiveness for the liquid-liquid phase transition of lysozyme declines as Cl- > NO3- > Br- > ClO4- > I- > SCN- while at low electrolyte concentrations corresponds to a nearly inverse Hofmeister series. Anion Hofmeister series is also affected by pH, buffer type, surface polarity and sign of surface charges.9,14 Schwierz et al.15 used the Poisson-Boltzmann theory to calculate the ion distributions onto the CH3-terminated self-assembled monolayers, indicating that Hofmeister series responds promptly to the change of surface properties: I- > Cl- > F- for negatively charged nonpolar surfaces, while F- > Cl- > I- when the surfaces get polar or positively charged, and the series restores back to I- > Cl- > F- for positively charged polar surfaces. In addition, Paterová et al.16 showed that in terms of hydration, anion-specific effects change from direct to inverse series upon uncapping the N-terminus of triglycine.

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As compared to those for anions, Hofmeister series for cations are less susceptible to condition changes.3,9,14 With respect to clay minerals, the experimental results clearly indicated the existence of cation-specific effects during ion adsorption,17-19 ion exchange20,21 and aggregation within the various alkali ion solutions.22,23 Abid and Ayadi18 found that Cd2+ and Cr3+ ions show disparate sorption capabilities at smectic clays, and the presence of Cd2+ promotes the uptake of Cr3+ while the presence of Cr3+ suppresses the uptake of Cd2+. On basis of selective ion sorption, an efficient sensor using the modified montmorillonite clays was developed that can detect Hg2+ in the trace concentration range.19 The cation-specific effects were determined to follow as Cs+ > Rb+ > K+ > Na+ > Li+ during ion exchange at montmorillonite and illite surfaces21 and aggregation of montmorillonite particles,22 and the sequence remains unaltered by change of electrolyte concentrations and when extended to multi-component clay systems.23 Computer simulations can help to interpret the experimental observations and also provide valuable information otherwise accessible about adsorption structure, dynamics and mechanisms.24-31 Meleshyn24 found that at the cleaved mica surfaces, Cs+ is preferentially adsorbed above the ditrigonal cavity whereas K+ can also appear above the substituted tetrahedrons, and he25 also determined that the adsorption affinities of different alkaline earth ions at the cleaved mica surfaces decline in the order of Mg2+ > Ca2+ > Sr2+ > Ba2+. Kobayashi et al.28 compared the adsorption affinities of the various alkali ions at mica surfaces and Hofmeister series therein was attained as Cs+ > K+ > Na+. Underwood et al.29 demonstrated that the cation specific effects at hydrated smectite surfaces follow as K+ > Na+ > Ca2+ > Cs+ > Ba2+. Very recently, Li et al.31 investigated the adsorption of Na+ and Cs+ at kaolinite surfaces with different charges and stated that the adsorption affinities are always presented in the order of Cs+ > Na+.

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In this work, molecular dynamics (MD) simulations were conducted to study the adsorption structure and dynamics of different metal ions at the interface of mica and aqueous solutions, finding that Hofmeister series follows as Na+ > K+ > Cs+ for less charged surfaces while is reversed for more charged surfaces (Cs+ > K+ > Na+), distinct from the results of kaolinite surfaces that are always presented in the order of Cs+ > Na+.31 The cation Hofmeister series remains unaltered by change of electrolyte concentrations. Currently, mechanisms for ion-specific effects are hotly debated and represent a challenging task.3,4,9 We demonstrated that the ion-water (i.e., hydration effect) and ion-mica interactions play a major role respectively for less and more charged surfaces, which further result in the reversal of Hofmeister series. The present results greatly promote the understanding of ion-specific effects, because charged systems are ubiquitous including biological systems where reversal of Hofmeister series occurs frequently.

2. COMPUTATIONAL DETAILS Models of mica, constructed from the crystallographic structure reported by Richardson et al.,32 are composed by 64 unit cells with the lateral dimensions of 41.59 Å × 36.10 Å (8 × 4 unit cells) and the thickness of 80.0 Å slab that was subsequently filled with 4018 water molecules to maintain a density of 1.0 g⋅cm-3. For each tetrahedral sheet, 1/16, 1/8, 3/16 and 1/4 Si4+ sites were substituted by Al3+ obeying the Löwensten’s rule, resulting in a wide range of surface charge densities (σ) of 0.08, 0.16, 0.24 and 0.32 C⋅m-2, respectively. Figure 1 illustrated the initial configuration of mica surfaces (σ = 0.32 C⋅m-2) in contact with 0.50 mol/L KCl solutions that were obtained by replacing certain numbers of water molecules with K+-Cl- ion pairs (the Cl- concentration was kept to 0.50 mol/L) in addition to some K+ ions balancing the 5



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negative charges due to Al3+/Si4+ substitutions. The other systems were prepared similarly and a total of 36 systems were produced considering different electrolytes (NaCl, KCl and CsCl), concentrations (0.15, 0.50 and 1.00 mol/L) and surface charge densities (σ = 0.08, 0.16, 0.24 and 0.32 C⋅m-2). MD simulations were conducted using the Gromacs package.33 The CLAYFF force field34 was employed to describe mica and ions (Table S1) and the flexible SPC model35 was engaged to account for water solvent, and such computational methods have been sufficiently validated to tackle the interfacial processes as presently investigated.29-31,36-39 Periodic boundary conditions (PBC) were used, and Ewald electrostatic summation and van der Waals (vdW) interactions were defined with the cut-off radii of 12.0 Å. Long-rang electrostatic interactions were handled by the Particle-Mesh-Ewald (PME) method. The equations of motion were integrated by the leapfrog algorithm using 2.0 fs time step.40 Temperature (T = 300.0 K) and pressure (p = 1.0 bar) were controlled by the V-rescale thermostats and Parrinello-Rahman barostats, respectively.41,42 20.0 ns MD simulations were run for each system, and all analyses were based on the final 5.0 ns MD trajectories, where all systems have already reached the equilibrium states. The free energy profiles for desorption of metal ions were estimated by potentials of mean force (PMF) via umbrella sampling43,44, and the procedures are as follows: Firstly, steered molecular dynamics (SMD) was used to generate a series of configurations. A representative steadily adsorbed metal ion was pulled away from mica surfaces until bulk solutions by conducting the harmonic potential (10000 kJ⋅mol-1⋅nm-2) with the pulling rate of 0.001 nm/ps. Remaining metal ions and water molecules were free to move during the pulling stage. The initial positions of metal ions corresponding to 0.08, 0.16 and 0.24, 0.32 C·m-2 are respectively at 1.5 and 1.0

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Å from mica surfaces, and the distance normal to mica surfaces was defined as reaction coordinate. Then a total of 46 windows are obtained with an interval of 0.4 Å and the overlapping of umbrella histograms of reaction coordinate from each window were examined to maintain the effectivity of umbrella sampling. Each window was simulated for 1.0 ns, with the coordinates and forces being stored every 1.0 ps. Finally, the weighted histogram analysis method (WHAM) was employed to extract PMF profiles.45,46 Models for calculating the dipole moments of inner-sphere metal ions were taken from the equilibrium configurations of MD simulations, see Figures S1 and S2. The water molecules around metal ions were retained, and the boundary O atoms of mica are saturated by H atoms directing along the bond vectors of what should have been the next lattice atoms. First-principles density functional theory (DFT)47 was used and dipole moments were derived from the Hirshlfeld population analyses.48 All elements were handled with the B3LYP/6-31+G(d,p) method,49,50 except Cs+ whose inner and outer electrons were described by LanL2DZ effective core potential (ECP) and LanL2DZ basis set, respectively.51

3. RESULTS AND DISCUSSION 3.1. Adsorption Behaviors. Snapshots of the equilibrium configurations for NaCl, KCl and CsCl solutions (0.15, 0.50 and 1.00 mol/L) in contact with mica surfaces (σ = 0.08, 0.16, 0.24, 0.32 C⋅m-2) are shown in Figures 1 and S3∼S14. Two independent peaks emerging in the atomic density profiles (Figures 2, S15 and S16) are respectively attributed to the inner-sphere and outer-sphere metal ions.24-31,52-56 Not that in some cases, especially for more charged surfaces, only one peak corresponds to the inner-sphere adsorption may be conspicuous. The distances of inner- and 7



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outer-sphere metal ions from mica surfaces remain essentially identical at different electrolyte concentrations while show obvious reductions with increase of surface charge densities. At σ = 0.08 C⋅m-2, the distances for inner- and outer-sphere Na+, K+, Cs+ ions are respectively centered at 2.2, 2.4, 2.6 Å and 4.0, 4.6, 5.6 Å, which show the same changing trend as those of ionic radii (Na+ < K+ < Cs+); When σ = 0.32 C⋅m-2, the distances for inner-sphere Na+, K+ and Cs+ ions fall at approximately 1.4, 1.6 and 1.9 Å and reduce substantially as compared to those of σ = 0.08 C⋅m-2, suggesting the reinforced interactions with mica surfaces due to increase of surface charge densities. In addition, the adsorption structures should be distinct for less and more charged surfaces as elaborated subsequently. The numbers of inner- and outer-sphere metal ions are calculated by integrating over the corresponding ranges of atomic density profiles, see Figure 3. The numbers of both inner- and outer-sphere metal ions increase with electrolyte concentrations while the outer-sphere mode corresponds to a more obvious augment. As compared to electrolyte concentrations, increase of surface charge densities causes a more striking enhancement on inner-sphere metal ions: At 0.50 mol/L, the inner-sphere Na+, K+ and Cs+ ions are counted at 2.4, 2.3 and 1.7 for σ = 0.08 C⋅m-2, 9.7, 9.2 and 7.6 for σ = 0.16 C⋅m-2, 23.1, 24.5 and 22.5 for σ = 0.24 C⋅m-2 and 34.9, 37.3 and 35.5 for σ = 0.32 C⋅m-2, respectively. For less charged surfaces (e.g., σ = 0.08 C⋅m-2), metal ions predominate in the outer-sphere mode, while elevation of surface charge densities causes the transformation to the inner-sphere mode and all metal ions exist principally as the inner-sphere species for σ = 0.24 and 0.32 C⋅m-2. 3.2. Reversal of Hofmeister Series. The PMF profiles used to estimate the free energies of adsorbed metal ions are presented in Figure 4. For less charged surfaces, there clearly exist two local energy minima in the PMF profiles corresponding to 8


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inner- and outer-sphere species, and relative energies of these two species are close to each other; e.g., at σ = 0.08 C⋅m-2, the energy barriers for desorption of Na+, K+ and Cs+ ions to enter into bulk solutions are calculated to be 8.1, 5.6 and 4.0 kJ⋅mol-1 for the inner-sphere species and 3.3, 2.5 and 2.3 kJ⋅mol-1 for the outer-sphere species, respectively. With increase of surface charge densities, the relative energies between the inner- vs. outer-sphere species are significantly magnified and a great deal of the outer-sphere species transform to the inner-sphere species, in good agreement with the results of atomic density profiles (Figure 2). As indicated in Figure 4, the energy barriers for desorption of inner-sphere Na+, K+ and Cs+ ions are calculated at 8.1, 5.6 and 4.0 kJ⋅mol-1 for σ = 0.08 C⋅m-2 and 16.4, 13.4, 11.8 kJ⋅mol-1 for σ = 0.16 C⋅m-2, suggesting the stronger interactions with mica due to increase of surface charge densities31 and agreeing with the results of time-evolution trajectories where metal ions of σ = 0.16 C⋅m-2 are more focused at mica surfaces than those of σ = 0.08 C⋅m-2 (Figures S14∼S25). For both 0.08 and 0.16 C⋅m-2, the sequence of ion-specific effects inferred from PMF profiles follow as Na+ > K+ > Cs+, in line with the stability ranking deduced from time-evolution trajectories (Figure S14∼S25). The energy barriers for desorption of inner-sphere Na+, K+ and Cs+ ions into bulk solutions continue to ascend with increase of surface charge densities and amount to 24.6 (or 20.5 as explained latter), 29.4, 33.8 kJ⋅mol-1 at σ = 0.24 C⋅m-2 and 34.6, 42.3, 49.6 kJ⋅mol-1 at σ = 0.32 C⋅m-2, respectively. In consequence, Hofmeister series is reversed due to increase of surface charge densities and follows in the order of Cs+ > K+ > Na+ at higher surface charge densities (σ = 0.24∼0.32 C⋅m-2), consistent with the analyses of time-evolution trajectories (Figures S14∼S25). As is known to us, Hofmeister series for cations is obviously less susceptible than for

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anions. Hofmeister series for anions will be reversed by changing the sign of surface charges, and anions are counterions at positively charged surfaces while transform to co-ions at negatively charged surfaces.57 However, in this work, mica remains negatively charged and metal ions always act as counterions, and the reversal of cation Hofmeister series is enforced merely through increase of surface charge densities rather than by change of the sign of surface charges. In addition to PMF profiles, the root-mean-square fluctuations (RMSF) of the last 3.0 MD simulations are also used to measure the stabilities of inner-sphere metal ions,31,39,58-61 see Table 1. The RMSF results indicate that the stabilities of inner-sphere metal ions are pronouncedly enhanced with increase of surface charge densities, in excellent agreements with those of PMF profiles and time-evolution trajectories. At σ = 0.08 C⋅m-2, the most stable metal ions have RMSFs of 0.1∼0.3 Å, and the numbers falling within this range decline as Na+ > K+ > Cs+. At σ = 0.16 C⋅m-2, only two Na+ ions have RMSF ≤ 0.1 Å indicating the pronouncedly enhanced stabilities due to increase of surface charge densities, while at higher surface charge densities, a considerable number with RMSF ≤ 0.1 Å are detected for all of Na+, K+ and Cs+ ions; e.g., at σ = 0.32 C⋅m-2, 18, 33 and 37 are counted for Na+, K+ and Cs+, respectively. According to the RMSF results, the sequences of cation-specific effects are Na+ > K+ > Cs+ for less charged surfaces (σ = 0.08 and 0.16 C⋅m-2) and Cs+ > K+ > Na+ for more charged surfaces (σ = 0.24 and 0.32 C⋅m-2). The trends are exactly the same as predicted by PMF profiles. 3.3. Adsorption Structures. According to time-evolution trajectories (Figures S14∼S25) and radial distribution functions (RDF, Figure 5), two types of inner-sphere metal ions emerge during the interfacial adsorption processes and are designated to be M1 and M2. As shown in the insets of Figure 5 (Enlarged in Figure S26), M1 species 10


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is situated in vicinity of substituted tetrahedral sites while M2 species lies above the center of ditrigonal cavities;24-26,52-56 in addition, the coordination environments of M1 and M2 species are disparate: metal ions (Na+, K+ and Cs+) of M1 type are directly coordinated to 3.4∼4.8 water-O (OW) atoms and 2.0∼2.7 surface-O (OS) atoms while metal ions of M2 type are directly coordinated with 1.5∼2.2 water-O molecules (OW) and 4.3∼5.8 surface-O (OS) atoms, see Figure S26 and Table 2. That is, M1 species forms more direct bonds with water molecules while M2 species forms more direct bonds with mica surfaces. For a specific surface charge density, the amounts of inner-sphere metal ions increase with electrolyte concentrations while the adsorption structures seem not affected by change of electrolyte concentrations. In contrast, the adsorption structures of inner-sphere metal ions show strong dependence on the choice of surface charge densities: For less charged surfaces (σ = 0.08 and 0.16 C⋅m-2), all inner-sphere metal ions exist exclusively as M1 type, while elevation of surface charge densities (σ = 0.24 and 0.32 C⋅m-2) drives metal ions substantially towards mica surfaces and results in the formation of M2 type, as inferred from the obviously shorter distances with mica surfaces in Figure 2 and the different coordination environments in Figures 5, S26 and Table 2 (More coordinated with OS atoms and less coordinated with OW atoms). K+ and Cs+ rather than Na+ are more affected by surface charge densities. At σ = 0.24 C⋅m-2, inner-sphere K+ and Cs+ ions have completely converted to M2 type, while only 45.1% Na+ ions transform to M2 type and the others remain as M1 type, see the fitted atomic density profiles in Figure S27. The desorption energies of M1and M2-type Na+ ions are calculated respectively at 20.5 and 24.6 kJ⋅mol-1 (Figure 4), implying the higher stability for M2 type. When the surface charge densities are further elevated to σ = 0.32 C⋅m-2, all inner-sphere metal ions including Na+ are 11



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exclusively presented as M2 type. 3.4. Mechanisms of Ion-Specific Effects. The hydration effects of different metal ions in bulk solutions are known to decline in the order of Na+ > K+ > Cs+,29,55,56 where the coordination numbers with water molecules (OW) are calculated to be 5.5, 6.7 and 7.6, respectively (Table 2). Figures 5, S26 and Table 2 indicate that the coordination numbers of metal ions with water molecules (OW) and surface-O atoms (OS) reduce as M0 (bulk solutions) > M1 > M2 and M2 > M1 > M0, respectively. The OS/OW ratios are 0 for M0 < 0.55∼0.59 for M1 K+ > Cs+ as in the condition of bulk solutions,29,55,56 in line with the changing trends predicted by PMF profiles, RMSFs and time-evolution trajectories. The elevation of surface charge densities drives inner-sphere metal ions to be obviously closer to mica surfaces forming M2 species, and as compared to M1 species, interactions of M2 species with mica are enhanced pronouncedly, at the expense of sharply reduced interactions with water (Table 2), which are consistent with the results of desorption energies for 12


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inner-sphere metal ions (Figure 4). Accordingly, for more charged surfaces (σ = 0.24∼0.32 C⋅m-2), interaction of metal ions and mica rather than water plays a more important role during the interfacial adsorption processes, coinciding with the local adsorption structures that M2 type is closer and more exposed to mica surfaces than M1 type. As indicated in Table 3, the dipole moments of all inner-sphere metal ions are substantially magnified due to the increase of surface charge densities, corroborating the more influences and enhanced interactions with mica. In addition, the dipole moments of the various alkali ions respond disparately to the increase of surface charge densities and the degree of influences descends in the order of Cs+ > K+ > Na+, as reflected by the ratios of dipole moments at 0.32 vs. 0.08 C⋅m-2 (Table 3).22,31 The calculated dipole moments are also in line with the results of adsorption structures: When the charge density increases to 0.24 C⋅m-2, Cs+ and K+ ions have completely converted to M2 type while 54.9% Na+ ions remain adsorbed as M1 type that has larger distances towards mica, corroborating the lagged response of Na+ towards the increase of surface charges. Accordingly, for more charged surfaces (σ = 0.24 and 0.32 C⋅m-2), interaction of metal ions with mica rather than water plays a larger role during the interfacial adsorption processes, and as a result, the ion-specific effects therein follow the sequence of Cs+ > K+ > Na+, in line with the results of PMF profiles, RMSFs and time-evolution trajectories. That is, mechanisms of ion-specific effects arising during the interfacial adsorption processes are distinct for different surface charge densities: Hydration effect (i.e., interaction of metal ions with water) is the driving force for less charged surfaces while interaction of metal ions with mica plays a more significant role for more charged surfaces. As a result, the sequences of cation-specific effects are disparate for less and more charged surfaces and correspond Na+ > K+ > Cs+ and Cs+ > 13



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K+ > Na+, respectively. That is, a reversal of Hofmeister series has been detected. Clay minerals generally carry an abundance of negative charges (i.e., always presented with the same sign of surface charges), and the finding that Hofmeister series can be reversed therein provides new insights about related processes and ion-specific effects.

4. CONCLUSIONS Molecular dynamics simulations have been conducted to study the adsorption of different metal ions at the interface of mica and aqueous solutions. For all metal ions, the adsorption numbers and strengths are enhanced in a direct proportion with surface charge densities. In this work, clear cation-specific effects have been observed during the interfacial adsorption processes, and a reversal of Hofmeister series is detected that results from the increase of surface charge densities (no change for the sign of surface charges). As is known to us, Hofmeister series for cations are less susceptible than for anions, and the reversal for anions can be caused by change for the sign of surface charges. Clay minerals generally carry an abundance of negative charges (i.e., always presented with the same sign of surface charges), and the finding that Hofmeister series can be reversed therein provides new insights about related processes and ion-specific effects. Cation-specific effects abide by the sequences of Na+ > K+ > Cs+ and Cs+ > K+ > Na+ for less and more charged surfaces, respectively. Meanwhile, the change of electrolyte concentrations exerts very limited influences on cation-specific effects. As indicated by time-evolution trajectories and radial distribution functions, the adsorption structures of inner-sphere metal ions show pronounced differences for less and more charged surfaces. Hydration effect (i.e., interaction of metal ions with water)

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is the driving force for less charged surfaces while interaction of metal ions with mica plays a larger role for more charged surfaces, which agree with the results of PMF profiles, RMSFs, time-evolution trajectories and adsorption structures. It thus provides satisfying interpretation for the reversal of Hofmeister series. The results obtained thus far have significant implications because charged systems are ubiquitous and important especially in biology, chemistry and colloid science; e.g., the surface charge densities of different proteins may vary significantly and the surface charges of specific proteins can be adjusted facilely by pH or other external conditions.

Supporting Information Parameters for the CLAYFF potential, Models for calculating dipole moments, Atomic density profiles and Time-evolution trajectories for aqueous solutions interacting interfacially with mica surfaces, Local adsorption structures for the inner-sphere metal ions as well as Peak-fitted atomic density profiles. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

Author Contributions Z.J. and X.L. contributed equally to the work.

Notes The authors declare no competing financial interest.

Acknowledgements This work was sponsored by the National Natural Science Foundation of China

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(21473137), the Fourth Excellent Talents Program of Higher Education in Chongqing (2014-03) and the Natural Science Foundation Project of CQ CSTC, China (cstc2017jcyjAX0145).

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References (1) Hofmeister F. Zur Lehre von der Wirkung der Salze. Archiv Exp. Pathol. Pharmakol. 1888, 24, 247-260. (2) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259-1281. (3) Nostro, P. L.; Ninham, B. W. Hofmeister Phenomena: An Update on Ion Specificity in Biology. Chem. Rev. 2012, 112, 2286-2322. (4) Jungwirth, P.; Cremer, P. S. Beyond Hofmeister. Nature Chem. 2014, 6, 261-263. (5) Jia, Z. Q.; Wang, Q.; Zhu, C.; Yang, G. In Advances in Colloid Science (Rahman M. M.; Asiri A. M. Eds.). InTech Publisher, Rijeka, 2016. (6) Okur, H. I.; Hladílková, J.; Rembert, K. B.; Cho, Y.; Heyda, J.; Dzubiella, J.; Cremer, P. S.; Jungwirth, P. Beyond the Hofmeister Series: Ion-specific Effects on Proteins and Their Biological Functions. J. Phys. Chem. B 2017, 121, 1997-2014. (7) Kunz, W.; Nostro, P. L.; Ninham, B. W. The Present State of Affairs with Hofmeister Effects. Curr. Opin. Colloid Interf. Sci. 2004, 9, 1-18. (8) Levin, Y.; dos Santos, A. P.; Diehl, A. Ions at the Air-water Interface: An End to a Hundred-year-old Mystery? Phys. Rev. Lett. 2009, 103, 257802. (9) Parsons, D. F.; Boström, M.; Nostro, P. L.; Ninham, B. W. Hofmeister Effects: Interplay of Hydration, Nonelectrostatic Potentials, and Ion Size. Phys. Chem. Chem. Phys. 2011, 13, 12352-12367. (10) Tian, R.; Yang, G.; Zhu, C.; Liu, X. M.; Li, H. Specific Anion Effects for Aggregation of Colloidal Minerals: A Joint Experimental and Theoretical Study. J. Phys. Chem. C 2015, 119, 4856-4864. (11) Moghaddam, S. Z.; Thormann, E. Hofmeister Effect on PNIPAM in Bulk and at an Interface: Surface Partitioning of Weakly Hydrated Anions. Langmuir 2017, 33,

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Page 24 of 33

Table 1. Numbers of inner-sphere metal ions falling with the specified RMSF (Å) ranges, for 0.50 mol/L NaCl/KCl/CsCl solutions in contact with mica surfaces that carry the various charges (σ , C⋅m-2) a σ 0.08

(C⋅m-2)

0.16

0.24

0.32

Na+

K+

Cs+

Na+

K+

Cs+

Na+

K+

Cs+

Na+

K+

Cs+

≤ 0.1

0

0

0

2

0

0

10

18

21

18

33

37

0.1∼0.3

2

1

0

7

5

1

11

3

0

14

0

0

0.3∼1.2

1

1

2

1

3

6

1

2

1

2

3

0

a

31, 39, 56

In line with previous works

, the RMSFs of inner-sphere metal ions are classified into several groups

whose stabilities decline as i (≤ 0.1 Å) > ii (0.1~0.3 Å) > iii (0.3~1.2 Å).

24


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2．．Numbers of the first-shell water-O (OW) and surface-O (OS) atoms for metal ions (M+) of the various conditions M1 (inner-sphere)

M2 (inner sphere)

M0 (bulk solutions)

Na+

K+

Cs+

Na+

K+

Cs+

Na+

K+

Cs+

OW

3.4

4.2

4.8

1.5

1.8

2.2

5.5

6.7

7.6

OS

2.0

2.3

2.7

4.3

5.1

5.8

0

0

0

Sum

5.4

6.5

7.5

5.7

6.9

8.0

5.5

6.7

7.6

OS/OW

0.59

0.55

0.56

2.9

2.8

2.6

0

0

0

25



Page 26 of 33

Table 3. Dipole moments calculated for inner-sphere metal ions at the interface of mica and electrolyte solutions a Charge density (C·m-2)

Na+

K+

Cs+

0.08

0.049

0.130

0.159

0.32

0.067 (1.367)

0.192 (1.477)

0.257 (1.616)

a

Units of dipole moments in Debye;

b

Ratios of dipole moments corresponding to two charge densities are given in parentheses.

26


Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Figure 1. Initial configurations (top) and snapshot of equilibrium configurations (bottom) of 0.50 mol/L KCl solutions in contact with mica surfaces (σ = 0.32 C·m-2). K+ and Cl- ions are presented as purple and green balls, respectively.

Figure 2. Atomic density profiles of Na+, K+ and Cs+ ions from 0.50 mol/L NaCl, KCl and CsCl solutions in contact with mica surfaces carrying the various charges (σ = 0.08, 0.16, 0.24, 0.32 C·m-2). The plane that passes through the bridging O atoms of the tetrahedral SiO4 surface is referred to as z = 0.

Figure 3. Numbers of Na+, K+ and Cs+ ions (Nad) from NaCl, KCl and CsCl solutions adsorbed at mica surfaces carrying the various charges (σ = 0.08, 0.16, 0.24, 0.32 C·m-2). Figure 4. Potentials of mean force (PMF) for Na+, K+ and Cs+ adsorption at mica surfaces carrying the various charges (σ = 0.08, 0.16, 0.24, 0.32 C·m-2). Figure 5. Radial distribution functions (RDF, g(r)) for M1-type (Left panel) and M2-type (Right panel) species, obtained from 0.50 mol/L electrolyte solutions in contact with mica surfaces with σ = 0.08 and 0.32 C·m-2, respectively. The local adsorption structures are plotted as insets.

27



Page 28 of 33

Figure 1

28


Page 29 of 33

4

16

0.08 C·m-2

3

Atomic density(nm-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Na+ 12 K+ 8 Cs+

2 1

4

0 32

0 64

0.24 C·m-2

24

Na+ K+ Cs+

16 8 0 0.0

0.16 C·m-2 Na+ K+ Cs+

0.32 C·m-2

48

Na+ K+ Cs+

32 16

0.2

0.4

0.6

0 0.8 0.0

0.2

0.4

0.6

0.8

Distance to the tetrahedral surface (nm)

Figure 2

29



0.15 mol/L Outer 30 Inner

Nad(Na+)

40

0.50 mol/L

1.00 mol/L

0.50 mol/L

1.00 mol/L

0.50 mol/L

1.00 mol/L

20 10 0 40

Nad(K+)

30

0.15 mol/L

Outer Inner

20 10 0

0.15 mol/L Outer 30 Inner 40

20 10 0

0. 08 0. 16 0. 24 0. 32 0. 08 0. 16 0. 24 0. 32 0. 08 0. 16 0. 24 0. 32

Nad(Cs+)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

Charge density (C·m-2) Figure 3

30


Page 31 of 33

5

5

0.08 C·m-2

0.16 C·m-2

0 0 -5

PMF(kJ·mol-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Na+ K+ Cs+

-5

Na+ K+ Cs+

-10 -15

-10

-20

0.24 C·m-2

0

0

-10

Na+ K+ Cs+

-20

0.32 C·m-2

Na+ K+ Cs+

-20

-40

-30 -40

0

2

4

6

8

10

12

-60 14 0

2

4

6

8

10

12

14

Distance to the tetrahedral surface (Å)

Figure 4

31



Na1-OS Na1-OW

Na1

g(r)

10

Na2-OS Na2-OW

Na2

8 6

8 6

4

CN

12

4 2

2 0

0

K1-OS K1-OW

K1

g(r)

10

K2-OS K2-OW

K2

8 6

8 6

4

CN

12

4 2

2 0

0

Cs1

10

Cs2-OS Cs2-OW

Cs2

Cs1-OS Cs1-OW

8 6

8 6

CN

12

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

4

4 2

2 0 2.0

0 2.5

3.0

3.5

4.0 2.0

2.5

r(Å)

3.0

3.5

4.0

r(Å)

Figure 5

32


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TOC Graphic

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Reversal of Cation-Specific Effects at the Interface ... - ACS Publications

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