Pair correlations that link the hydrophobic and Hofmeister effects

Abstract

The Hofmeister effect describes how different ions make solutes more or less hydrophobic. The effect is thought to occur due to structural changes in the solvent induced by the ion's presence, particularly in water. In this study, the structural changes in water due to the presence of ions are investigated by molecular dynamics simulations of various monatomic ions in the SPC/E water model. Structural analyses reveal specific orientations of solvating waters around each of the ions studied. Using a new method, these orientations are quantified by a set of pair correlation functions that describe dipole-ion correlations in structure. These correlations are shown to contribute to the potential of mean force between waters and the ion of interest, and therefore to the free energy of the system. The magnitude of this free energy is found to result in a Hofmeister series for the various ions studied, therefore demonstrating a Hofmeister effect with respect to water's structure that is quantified by pair correlation functions. Most crucially, the pair correlations that lead to this Hofmeister effect also contribute to the hydrophobic effect (the entropy of hydrophobic solvation) [Liu et al., J. Chem. Phys., 2015, 142, 114117], and those which dominate the hydrophobic effect are modulated by an ion's presence, therefore demonstrating a mechanistic link between the two effects.

Long-range dipolar order and dispersion forces in polar liquids

“…Recently, the influence of various atomic ions on water's long-range dipolar correlations was reported, 62 where it was found that the presence of a single ion can modulate the pair correlations that contribute to the hydrophobic effect. 36 Furthermore, the ion's presence induces new correlations between permanent dipole moments and the ion itself; the free energy of which results in a Hofmeister series for different ions, 62 demonstrating specific ion effects with respect to dipole-ion correlations. Here, the interest is on the structure and lengthscale of the perturbation of water's structure induced by an ion's presence, as found in similar dipole correlations to those in φ MD (r).…”

“…Following the treatment by Besford et al, 62 the contribution of ψ MD (r) to the solvation free energy of the chloride ion may be given as…”

“…4 is evaluated to be 26 kJ/mol, in reasonable agreement for the same correlations for a chloride ion in the SPC/E water model (31 kJ/mol). Whilst a large portion of the solvation free energy of an ion, which for chloride in water is about 311 kJ/mol, 62 originates from the electrostatic forces related to the charge of the ion, 64 the portion originating from G ψ MD is still significant in magnitude. Crucially, the solvation free energy from G ψ MD is specific to the ion's perturbation of water's structure that varies in a Hofmeister series for different ions, 62 thus demonstrating a specific ion effect with respect to dipole-ion correlations in an ionic aqueous solution.…”

“…For example, femtosecond elastic second harmonic scattering (fs-ESHS) measurements have been used to probe structural changes in water induced by an ion's presence, which have suggested that specific ion effects are only found at high ionic concentrations (0.1M-1M). 22 The ion-induced changes in water structure are modulated differently depending on the ion, as observed through the ψ MD (r) correlations and the resulting free energy; however, the structural differences are subtle, 62 hence it is unclear at this point how this may relate to fs-ESHS measurements. From a purely structural perspective, pressure perturbation calorimetry measurements have shown that an ion can affect water structure beyond one or two molecule hydration layers, 65 consistent with the ψ MD (r) correlations.…”

The Attraction of Water for Itself at Hydrophobic Quartz Interfaces

“…We label φ M D (r) with a subscript "MD", to signify that it is calculated from simulated trajectories. The φ M D (r) interaction is always negative, so it produces an attractive force between pairs of polar molecules, which has been shown to contribute to the entropy of hydrophobic solvation, 59 to contribute to the long-range structure of polar liquids, including liquid water, 63 to be selectively modulated by different ions, 64 and to produce a stabilizing force at the water-vapor interface. 60 It is therefore of interest to compare the strength of this interaction when water molecules are constrained in their orientation by the quartz interface.…”