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.

Measuring the local electrical conductivity of human brain tissue

“…4,5 By contrast, the presence of a sodium cation fosters surrounding water molecules being directed toward the solvated cation. 6,7 In other words, the presence of a sodium ion alters the orientations of surrounding water molecules. Then, the rate with which water molecules surrounding a solvated sodium cation reorient is comparable to that with which a cation moves.…”

Ionic profiles close to dielectric discontinuities: Specific ion-surface interactions

“…As explained above, in a coarsegrained theory one averages over length-scales of the order of the ionic size, which is also the characteristic length-scale of the ionic specific potential, V ± [18,22]. In the future, it will be of merit to calculate from the ionic profiles, other macroscopic quantities (beside the surface tension), such as the differential capacitance [33], the solution dielectric constant [34] and the solution viscosity [35]. We elaborate here on the general formalism for calculating the electrostatic potential to one-loop order.…”

Influence of electric field on the hydrogen bond network of water

“…3 Despite substantial progress made in the past, what exactly is the internal structure of water in these situations, as well as what governs it, are still widely debated research topics. For example, while some studies [4][5][6] have supported the view of structure making and structure breaking by ionic solutes, others [7][8][9] have not. Similarly, in the case of water near charged surfaces, while experiments 10,11 reveal disruption of the H-bond network, simulations 12,13 show that the H-bond network is largely intact.…”