Sub-diffusion and population dynamics of water confined in soft environments

TL;DR

This study uses molecular dynamics simulations to analyze water dynamics in soft, ionic surfactant environments, revealing sub-diffusive behavior mainly caused by interfacial water molecules, and offers a new framework for understanding dynamical heterogeneities.

Contribution

It provides the first detailed analysis linking interfacial water molecules to sub-diffusive dynamics in soft confining environments.

Findings

Water exhibits sub-diffusive behavior in confined environments.

Interfacial water molecules are key to the observed dynamics.

A new framework explains dynamical heterogeneities in soft confinement.

Abstract

We have studied by Molecular Dynamics computer simulations the dynamics of water confined in ionic surfactants phases, ranging from well ordered lamellar structures to micelles at low and high water loading, respectively. We have analysed in depth the main dynamical features in terms of mean squared displacements and intermediate scattering functions, and found clear evidences of sub-diffusive behaviour. We have identified water molecules lying at the charged interface with the hydrophobic confining matrix as the main responsible for this unusual feature, and provided a comprehensive picture for dynamics based on a very precise analysis of life times at the interface. We conclude by providing, for the first time to our knowledge, a unique framework for rationalising the existence of important dynamical heterogeneities in fluids absorbed in soft confining environments.