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

Article Cite This: J. Phys. Chem. C 2018, 122, 5358−5365

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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* College of Resources and Environment & Chongqing Key Laboratory of Soil Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China

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ABSTRACT: Ion-specific effects are ubiquitous and have gained renaissance over the past few decades while remaining 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 on an atomic level. Hydration effect (i.e., interaction of metal ions with water) is the driving force for less charged surfaces, whereas 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 in 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. properties: I− > Cl− > F− for negatively charged nonpolar surfaces, whereas 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 Nterminus of triglycine. As compared to those for anions, Hofmeister series for cations is 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 exchange,20,21 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+, whereas the presence of Cr3+ suppresses the uptake of Cd2+. On the 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 cationspecific 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

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 ionspecific effects). Over the past few decades, ion-specific effects have gained renaissance and have 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−, whereas low electrolyte concentrations correspond 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 CH3terminated self-assembled monolayers, indicating that Hofmeister series responds promptly to the change of surface © 2018 American Chemical Society

Received: October 8, 2017 Revised: January 22, 2018 Published: February 21, 2018 5358

DOI: 10.1021/acs.jpcc.7b09956 J. Phys. Chem. C 2018, 122, 5358−5365

Article

The Journal of Physical Chemistry C concentrations and when extended to multicomponent 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 Meleshyn25 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+. 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, whereas it 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.

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.

tackle the interfacial processes, as presently investigated.29−31,36−39 Periodic boundary conditions were used, and Ewald electrostatic summation and van der Waals interactions were defined with a cutoff radius of 12.0 Å. Long-rang electrostatic interactions were handled by the Particle-meshEwald method. The equations of motion were integrated by the leapfrog algorithm using the 2.0 fs time step.40 Temperature (T = 300.0 K) and pressure (p = 1.0 bar) were controlled by the Vrescale thermostats and Parrinello−Rahman barostats, respectively.41,42 MD simulations (20.0 ns) 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 sampling,43,44 and the procedure is as follows: First, steered molecular dynamics 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. The 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 were, respectively, at 1.5 and 1.0 Å from mica surfaces, and the distance normal to mica surfaces was defined as the reaction coordinate. Then, a total of 46 windows were obtained with an interval of 0.4 Å, and the overlapping of umbrella histograms of the reaction coordinate from each window were examined to maintain the effectivity of the 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 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 were saturated by H atoms directing along the bond vectors of what should have been the next lattice atoms. Firstprinciples density functional theory47 was used, and dipole moments were derived from the Hirshlfeld population analyses.48 All elements were handled with the B3LYP/631+G(d,p) method,49,50 except Cs+ whose inner and outer

2. COMPUTATIONAL DETAILS Models of mica, constructed from the crystallographic structure reported by Richardson S. M. and Richardson J. W.,32 are composed of 64 unit cells with the lateral dimensions of 41.59 Å × 36.10 Å (8 × 4 unit cells) and a slab of 80.0 Å thickness 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 illustrates 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 number 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 negative charges due to Al3+/Si4+ substitutions. The other systems were prepared in a similar way, 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 simple point charge model35 was engaged to account for the water solvent, and such computational methods have been sufficiently validated to 5359


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The Journal of Physical Chemistry C electrons were described by LANL2DZ effective core potential 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, and 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

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, and 0.32 C·m−2).

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, and 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.

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), the metal ions predominate in the outer-sphere mode, whereas 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 inner- and outer-sphere species, and the relative energies of these two species are close to each other; for example, 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- versus outer-sphere species are significantly magnified and a great deal of the outersphere 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, and 11.8 kJ· mol−1 for σ = 0.16 C·m−2, suggesting the stronger interactions with mica because of 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

inner-sphere and outer-sphere metal ions.24−31,52−56 Note that in some cases, especially for more charged surfaces, only one peak corresponding to the inner-sphere adsorption may be conspicuous. The distances of inner- and outer-sphere metal ions from mica surfaces remain essentially identical at different electrolyte concentrations, whereas they 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+, and Cs+ ions are, respectively, centered at 2.2, 2.4, and 2.6 and 4.0, 4.6, and 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 because of 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, whereas 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 = 5360


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The Journal of Physical Chemistry C

agreement 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, whereas at higher surface charge densities, a considerable number with RMSF ≤ 0.1 Å are detected for all of Na+, K+, and Cs+ ions; for example, 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 (RDFs, Figure 5), two types of inner-sphere metal ions emerge

Figure 4. PMF for Na+, K+, and Cs+ adsorption at mica surfaces carrying the various charges (σ = 0.08, 0.16, 0.24, and 0.32 C·m−2).

with the stability ranking deduced from time-evolution trajectories (Figures 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, and 33.8 kJ·mol−1 at σ = 0.24 C·m−2 and 34.6, 42.3, and 49.6 kJ·mol−1 at σ = 0.32 C·m−2, respectively. In consequence, Hofmeister series is reversed because of 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 known to us, Hofmeister series for cations is obviously less susceptible than for anions. Hofmeister series for anions will be reversed by changing the sign of surface charges, and anions are counterions at positively charged surfaces, which transform to coions 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 in the sign of surface charges. In addition to PMF profiles, the root-mean-square fluctuations (RMSFs) 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

Figure 5. 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.

during the interfacial adsorption processes and are designated to be M1 and M2. As shown in the insets of Figure 5 (enlarged

Table 1. Number 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 0.08

0.16

0.24

0.32

σ (C·m−2)

Na+

K+

Cs+

Na+

K+

Cs+

Na+

K+

Cs+

Na+

K+

Cs+

≤0.1 0.1−0.3 0.3−1.2

0 2 1

0 1 1

0 0 2

2 7 1

0 5 3

0 1 6

10 11 1

18 3 2

21 0 1

18 14 2

33 0 3

37 0 0

a

In line with previous works,31,39,56 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). 5361


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The Journal of Physical Chemistry C Table 2. Number of First-Shell Water-O (OW) and Surface-O (OS) atoms for Metal Ions (M+) of Various Conditions M1 (inner-sphere) OW OS sum OS/OW

M2 (inner sphere)

M0 (bulk solutions)

Na+

K+

Cs+

Na+

K+

Cs+

Na+

K+

Cs+

3.4 2.0 5.4 0.59

4.2 2.3 6.5 0.55

4.8 2.7 7.5 0.56

1.5 4.3 5.7 2.9

1.8 5.1 6.9 2.8

2.2 5.8 8.0 2.6

5.5 0 5.5 0

6.7 0 6.7 0

7.6 0 7.6 0

adsorption processes. The hydration energies of metal ions (water molecules within 12.0 Å are selected) have been calculated for σ = 0.08 C·m−2, and the values of Na+ and K+ are obviously larger than that of Cs+, with their differences being approximately −113.9 and −44.9 kJ·mol−1, respectively. Accordingly, the hydration effect is attributed to arouse the cation-specific effects for less charged surfaces (σ = 0.08 and 0.16 C·m−2), and Hofmeister series therein should follow as Na+ > 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 the 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 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

in Figure S26), M1 species is situated in the vicinity of substituted tetrahedral sites, whereas 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, whereas 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, whereas M2 species forms more direct bonds with mica surfaces. For a specific surface charge density, the amounts of innersphere metal ions increase with electrolyte concentrations, whereas the adsorption structures seem not affected by change of electrolyte concentrations. By 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, whereas elevation of surface charge densities (σ = 0.24 and 0.32 C·m−2) drives metal ions substantially toward 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, whereas 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 M1-and 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 innersphere metal ions including Na+ are 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 surfaceO 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 ≪ 2.6−2.9 for M2. M1 species has obviously larger coordination numbers with water molecules, whereas M2 species has obviously larger coordination numbers with mica, suggesting that M1 and M2 species interact more strongly with water and mica, respectively. For less charged surfaces (σ = 0.08 and 0.16 C·m−2), M1 is the sole adsorption species, and hence interaction of metal ions with water plays a more important role during the interfacial

Table 3. Dipole Moments Calculated for Inner-Sphere Metal Ions at the Interface of Mica and Electrolyte Solutionsa charge density (C·m−2)

Na+

K+

Cs+

0.08 0.32

0.049 0.067 (1.367)

0.130 0.192 (1.477)

0.159 0.257 (1.616)

a Units of dipole moments in Debye. bRatios of dipole moments corresponding to two charge densities are given in parentheses.

moments of all inner-sphere metal ions are substantially magnified because of 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 versus 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, whereas 54.9% Na+ ions remain adsorbed as M1 type that has larger distances toward mica, corroborating the lagged response of Na+ toward 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 timeevolution trajectories. 5362


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The Journal of Physical Chemistry C 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, whereas 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 to Na+ > K+ > Cs+ and Cs+ > 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.

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adsorption structures for the inner-sphere metal ions as well as peak-fitted atomic density profiles (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 086-023-68251504. Fax: 086-023-68250444. ORCID

Gang Yang: 0000-0003-1032-6840 Author Contributions

Z.J. and X.L. contributed equally to the work. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS MD 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 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 known to us, Hofmeister series for cations are less susceptible than for anions, and the reversal for anions can be caused by the change in 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 RDFs, 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) is the driving force for less charged surfaces, whereas interaction of metal ions with mica plays a larger role for more charged surfaces, which agrees 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; for example, 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.

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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09956. 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, and local 5363


Article

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