Effect of simple solutes on the long range dipolar correlations in liquid water

Abstract

Intermolecular correlations in liquid water at ambient conditions have generally been characterized through short range density fluctuations described through the atomic pair distribution functions (PDF). Recent numerical and experimental results have suggested that such a description of order or structure in liquid water is incomplete and there exists considerably longer ranged orientational correlations in water that can be studied through dipolar correlations. In this study, using large scale classical, atomistic molecular dynamics (MD) simulations using TIP4P-Ew and TIP3P models of water, we show that salts such as sodium chloride (NaCl), potassium chloride (KCl), caesium chloride (CsCl) and magnesium chloride (MgCl$_2$) have a long range effect on the dipolar correlations, which can not be explained by the notion of structure making and breaking by dissolved ions. The relative effects of cations on dipolar correlations are observed to be consistent with the well-known Hofmeister series. Observed effects are explained through orientational stratification of water molecules around ions, and their long range coupling to the global hydrogen bond network by virtue of the sum rule for water. The observations for single hydrophilic solutes are contrasted with the same for a single methane (CH$_4$) molecule. We observe that even a single small hydrophobe can result in enhancement of long range orientational correlations in liquid water,- contrary to the case of dissolved ions, which have been observed to have a reducing effect. The observations from this study are discussed in the context of hydrophobic effect.