Effects of Translational and Rotational Degrees of Freedom on the Hydration of Ionic Solutes as Seen by the Popular Water Models.

Tomaž Mohorič

doi:10.17344/acsi.2014.1291

Abstract

We employed molecular dynamics simulations with separate thermostats for translational and rotational temperatures in order to study the effects of these degrees of freedom on the hydration of ions. In this work we examine how water models, differing in charge distribution, respond to the rise of rotational temperature. The study shows that, with respect to the distribution of negative charge, popular water models lead to different responses upon an increase of the rotational temperature. The differences arise in hydration of cations, as the negative charge distribution on the model solvent represents the determining factor in such cases. The cation-water correlation increases with the increasing rotational temperature if negative charge is placed in (or close to) the centre of the water molecule (a typical example is the SPC water model) and decreases, when the negative charge is shifted from the centre (as in the TIP5P model of water). Because all the water models examined here have similar distributions of positive charge, they all exhibit similar trends in solvation of anions. In contrast to above, the effect of translational temperature variation is similar for all water-solute pairs; any increase in translational temperature decreases the solute-water correlations.

Highlights

Properties of solutions are not determined solely by solute-solute interactions and by the ability of solvent to solvate the solute particles.[1,2,3,4] Hydration of molecules having hydrophobic and ionic groups, such as polyelectrolytes and proteins, is relevant for electrochemistry, geochemistry and especially for biology
Bren and Janei~ considered another approach to this problem,[14] employing the steadystate molecular dynamics simulations with separate thermostats for different degrees of freedom
The question we wish to answer here is the following: can we expect the same trends as obtained for SPC/E for other water models, or the results are model-dependent? For this reason we examined the behaviour of six popular water models, most of them are frequently used in the large-scale molecular-dynamics (MD) simulations of aqueous solutions,[16,17,18] in situations where the rotational temperature of the system is different than the translational one

Summary

Introduction

Properties of solutions are not determined solely by solute-solute interactions and by the ability of solvent to solvate the solute particles.[1,2,3,4] Hydration of molecules having hydrophobic and ionic groups, such as polyelectrolytes and proteins, is relevant for electrochemistry, geochemistry and especially for biology. A relevant physical situation may occur upon interaction of microwaves with aqueous solutions and has been studied by the non-equilibrium molecular dynamics simulation with the electric field modeled explicitly.[5,6,7,8,9,10,11,12,13] Bren and Janei~ considered another approach to this problem,[14] employing the steadystate molecular dynamics simulations with separate thermostats for different degrees of freedom They examined the hydration of hydrophobes and cations under conditions where the rotational temperature, TR, was different than the translational one TR. A brief summary of the most interesting results is placed at the end

Model and Simulation Details

Results and Discussion

Conclusions