Spectroscopic Signatures of the Dynamical Hydrophobic Solvation Shell Formation

TL;DR

This paper investigates the real-time formation of the hydrophobic hydration shell around a solute using a dynamic dielectric model, revealing spectroscopic signatures like frequency shifts and line width changes during solvation.

Contribution

It introduces a time-dependent dielectric continuum model to describe the dynamical buildup of the hydrophobic hydration shell and predicts measurable spectroscopic effects.

Findings

Blue shift in absorption frequency during hydration shell formation

Decreasing line width over time indicating solvent relaxation

Agreement with experimental data on iodine in water

Abstract

When a hydrophilic solute in water is suddenly turned into a hydrophobic species, for instance, by photoionization, a layer of hydrated water molecules forms around the solute on a time scale of a few picoseconds. We study the dynamic build-up of the hydration shell around a hydrophobic solute on the basis of a time-dependent dielectric continuum model. Information about the solvent is spectroscopically extracted from the relaxation dynamics of a test dipole inside a static Onsager sphere in the nonequilibrium solvent. The growth process is described phenomenologically within two approaches. First, we consider a time-dependent thickness of the hydration layer which grows from zero to a finite value over a finite time. Second, we assume a time-dependent complex permittivity within a finite layer region around the Onsager sphere. The layer is modeled as a continuous dielectric with a much slower fluctuation dynamics. We find a time-dependent frequency shift down to the blue of the resonant absorption of the dipole, together with a dynamically decreasing line width, as compared to bulk water. The blue shift reflects the work performed against the hydrogen bonded network of the bulk solvent and is a directly measurable quantity. Our results are in agreement with an experiment on the hydrophobic solvation of iodine in water.