THz Dynamics of Nanoconfined Water by Ultrafast Optical Spectroscopy

TL;DR

This study explores the ultrafast vibrational and structural relaxation dynamics of nanoconfined water in porous silica using time-resolved optical Kerr effect spectroscopy, revealing hydration-dependent mobility and partial freezing phenomena.

Contribution

It provides new insights into how nanoconfinement and hydration levels influence water dynamics at ultrafast timescales, using advanced spectroscopic techniques.

Findings

Low hydration water remains mobile at all temperatures.

Partial ice formation occurs at full hydration around 248 K.

Surface water dynamics differ significantly from bulk water.

Abstract

We investigated the vibrational dynamics and the structural relaxation of water nanoconfined in porous silica samples with pore size of 4 nm at different levels of hydration and temperature. We used as spectroscopic technique the time-resolved optical Kerr effect, which enables to investigate the ultrafast water dynamics in a wide time (0.1-10 picosecond) or frequency (10-0.1 THz) window. At low levels of hydration, corresponding to two complete superficial water layers, no freezing occurs and the water remains mobile at all the investigated temperatures, while at the fully hydration we witness to a partial ice formation at about 248 K that coexists with the part of surface water remaining in the supercooled state. At low hydration, both structural and vibrational dynamics show significant modifications compared to the bulk liquid water due to the strong interaction of the water molecules with silica surfaces. Inner water, instead, reveals relaxation dynamics very similar to the bulk one.