Orientational order as the origin of the long-range hydrophobic effect

TL;DR

This study reveals that orientational order, rather than density, underpins the long-range hydrophobic attraction between surfaces in water, with interference of orientational heterogeneity explaining the hydrophobic force law.

Contribution

It identifies orientation as the key order parameter responsible for the hydrophobic force law, challenging density-based explanations and linking it to orientational heterogeneity.

Findings

Orientation, not density, correlates with the hydrophobic force law.

Interference of orientational heterogeneity explains long-range attraction.

Hexagonal ice-like structures emerge near the attractive region.

Abstract

The long range attractive force between two hydrophobic surfaces immersed in water is observed to decrease exponentially with their separation -- this distance-dependence of effective force is known as the hydrophobic force law (HFL). We explore the microscopic origin of HFL by studying distance-dependent attraction between two parallel rods immersed in 2D Mercedes Benz model of water. This model is found to exhibit a well-defined HFL. Although the phenomenon is conventionally explained by density-dependent theories, we identify orientation, rather than density, as the relevant order parameter. The range of density variation is noticeably shorter than that of orientational heterogeneity. The latter is comparable to the observed distances of hydrophobic force. At large separation, attraction between the rods arises primarily from a destructive interference among the inwardly propagating oppositely oriented heterogeneity generated in water by the two rods. As the rods are brought closer, the interference increases leading to a decrease in heterogeneity and concomitant decrease in free energy of the system, giving rise to the effective attraction. We notice formation of hexagonal ice-like structures at the onset of attractive region which suggests that metastable free energy minimum may play a role in the origin of HFL.