How ions affect the structure of water

Abstract

We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. We perform (NPT) Monte Carlo simulations to explore how water molecules are organized around ions and around nonpolar solutes in salt solutions. The model gives good qualitative agreement with experiments, including Jones-Dole viscosity B coefficients, Samoilov and Hirata ion hydration activation energies, ion solvation thermodynamics, and Setschenow coefficients for Hofmeister series ions, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The two main ideas captured here are (1) that charge densities govern the interactions of ions with water, and (2) that a balance of forces determines water structure: electrostatics (water's dipole interacting with ions) and hydrogen bonding (water interacting with neighboring waters). Small ions (kosmotropes) have high charge densities so they cause strong electrostatic ordering of nearby waters, breaking hydrogen bonds. In contrast, large ions (chaotropes) have low charge densities, and surrounding water molecules are largely hydrogen bonded.

Figure 1

The MB-dipole model. (a) Two MB-dipole waters forming a hydrogen bond. (b) A cation and an MB-dipole water oriented in its most favorable orientation (180° with respect to the vector connecting the molecular centers). Also an anion and a water oriented in its most favorable orientation (0°).

Figure 2

Pair correlation functions of water around ions. (a) Cations and (b) anions. Smaller ions have tighter water shells, at reduced temperature T* = 0.20.

Figure 3

Angular distribution functions for waters in the first shell around an ion, for (a) cations and (b) anions at T* = 0.20. Large cations help promote hydrogen bonding of neighboring waters, leading to a single peak. For small cations, the electrostatic mechanism competes with the hydrogen bond mechanism for ordering waters. The reverse applies to anions. For small anions, the electrostatic mechanism dominates; for large anions, electrostatic and hydrogen-bonding mechanisms compete.

Figure 4

The average number of the water–water hydrogen bonds, 〈HB〉, per water molecule in the first shell around various ions at T* = 0.20.

Figure 5

Snapshots of waters in the first (shaded) and second shell (white) around an ion (black), showing likely configurations of water as inferred from statistics of pair distributions, angular orientations, and hydrogen bonding at T* = 0.20.

Figure 6

The MB-dipole model reproduces the dependence on ion radius of chaotropic and kosmotropic properties. The activation energy of Samoilov, ΔE_i (ref 15), changes in entropy, −ΔS_II (ref 17), and Jones–Dole B coefficients (ref 18), all at 298 K, are compared to the MB-dipole model liberation free energy, ΔG^lib, at T* = 0.20. σ for Mg⁺⁺, Ca⁺⁺, and Ba⁺⁺ was taken to be 0.26, 0.39, and 0.53l_HB, scaled from ionic crystal radii. Zero values indicate the transition between kosmotropes (greater than zero) and chaotropes (less than zero). Circles indicate relative ion radii.

Figure 7

A universal curve showing that chaotropic and kosmotropic properties depend on electrostatic potential at the ion. The activation energy of Samoilov, ΔE_i (ref 15), changes in entropy, ΔS_II (ref 17), and Jones–Dole B coefficients (ref 18), all at 298 K, as compared to the MB-dipole model liberation free energy, ΔG^lib, are shown for sets of cations and anions at T* = 0.20. A single distance shift was chosen to overlay the cations onto the anions, indicating that the physical basis for this asymmetry is the asymmetry of the dipole in water.

Figure 8

The MB-dipole model reproduces the temperature dependence of chaotrope and kosmotrope behavior. The temperature dependence of Jones–Dole viscosity coefficients, B(T), from experiments for (a) cations and (b) anions, and the temperature dependence of liberation free energy, ΔG^lib, from the MB-dipole simulations for (c) cations and (d) anions are shown.

Figure 9

The temperature dependence of electrostatic energy, E^elec, and the number of hydrogen bonds per molecule in the first shell, n_HB/n_shell, for cations (a and b) and anions (c and d).

Figure 10

Hofmeister effects in water and the MB-dipole model. (a) Experimental Setschenow coefficients for cation-chloride and sodium-anion salts as a function of ionic radii. (b) Perturbations to the free energy of transferring a hydrophobic solute into MB-dipole water with an ion, ΔΔG. (c) Experimental correlation between compression volumes for salts and Setschenow coefficients. Salts shown as in (a).

Figure 11

Water density around an ion affects the probability of hydrophobic solute insertion and the magnitude of Hofmeister effects. Shown are (top) most probable sites of hydrophobic solute insertion (colored black) in the first and second shell around different ions, measured in MB-dipole simulations. (middle) The average water density around different ions at T* = 0.20. (bottom) The potential of mean force between an ion and a hydrophobe at T* = 0.20 showing that solute insertion is favorable when first-shell water density decreases.

Figure 12

“Universal” charge density correlation for Hofmeister effects. Shown are (a) experimental Setschenow coefficients versus ionic radii, adjusting cation radii by 0.075 nm as in Figure 7, and (b) MB-dipole ΔΔG for ion effects on hydrophobic solute transfer free energies.