# Experimental Determination of the Dielectric Constant of Micellar Hydration Layer via Localized Surface Plasmon Resonance of Gold Nanoparticles

## TL;DR

This paper uses gold nanoparticles to measure the dielectric constant of a hydration layer near micelles, revealing how water molecules align differently in this structured environment.

## Contribution

The study experimentally determines the dielectric constant of a micellar hydration layer using localized surface plasmon resonance.

## Key findings

- The dielectric constant of the hydration layer decreases due to soft confinement between nanoparticles and micelles.
- Water molecules near the nanoparticle interface show in-plane alignment, reducing polarization in the out-of-plane direction.
- Micelles enhance the structuring of water molecules near the nanoparticle interface.

## Abstract

The localized surface plasmon resonance (LSPR) of plasmonic nanoparticles can be used both for measuring the dielectric constant in which they are dispersed and for determining changes in the structure of the medium around them. In this work, we explored the shift in the LSPR of gold nanoparticles (AuNPs), obtaining the out-of-plane dielectric constant of the hydration layer of nanoparticles dispersed in aqueous solutions of sodium dodecyl sulfate for concentrations below and above the critical micelle concentration. A reduction is observed, which is due to a soft confinement between the nanoparticle and the micelles. The confinement favors the in-plane alignment of the water’s molecular dipoles, hindering a rotation out-of-plane and reducing the tendency to align with an external electric field, i.e., diminishing the medium’s polarization.

The short penetration depth of the localized surface plasmon in a gold nanoparticle enables probing of the dielectric constant of a nanometer-scale layer near the interface, where water molecules are more highly organized than in bulk water. The presence of micelles further intensifies the structuring of water near the nanoparticle interface.