Structure of Water at Hydrophilic and Hydrophobic Interfaces: Raman Spectroscopy of Water Confined in Periodic Mesoporous (Organo)Silicas

TL;DR

This study uses Raman spectroscopy to analyze how water's molecular structure varies at hydrophilic and hydrophobic silica interfaces under different humidity and temperature conditions, revealing distinct behaviors of confined water.

Contribution

It provides detailed insights into the structural differences of confined water at hydrophilic and hydrophobic interfaces using Raman spectroscopy, especially under low temperature and partial filling conditions.

Findings

Liquid water in capillary pores has a bulk-like structure with slight disruption.

Partially filled pores show no low-density amorphous ice signature even at -120°C.

Different hydroxyl groups indicate water's interaction with hydrophilic silica and hydrophobic organosilica.

Abstract

The temperature dependence of the structure of water confined in hydrophilic mesostructured porous silica (MCM-41) and hydrophobic benzene-bridged periodic mesoporous organosilicas (PMO) is studied by Raman vibrational spectroscopy. For capillary filled pores (75% relative humidity, RH), the OH-stretching region is dominated by the contribution from liquid water situated in the core part of the pore. It adopts a bulk-like structure that is modestly disrupted by confinement and surface hydrophobicity. For partially filled pores (33% RH), the structure of the non-freezable adsorbed film radically differs from that found in capillary filled pores. A first remarkable feature is the absence of the Raman spectral fingerprint of low density amorphous ice, even at low temperature (-120{\textdegree}C). Secondly, additional bands reveal water hydroxyls groups pointing towards the different water/solid and water/vapor interfaces. For MCM-41, they correspond to water molecules acting as weak H-bond donors with silica, and dangling hydroxyl groups oriented towards the empty center of the pore. For benzene-bridged PMO, we found an additional type of dangling hydroxyl groups, which we attribute to water at hydrophobic solid interface.