Structure of water at hydrophilic and hydrophobic interfaces: raman spectroscopy of water confined in periodic mesoporous (organo)silicas

Abstract

The temperature dependence of the structure of water confined in hydrophilic mesostructured porous silica (MCM-41) and hydrophobic benzene-bridged periodic mesoporous organosilicas (PMOs) 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 bulklike structure that is modestly disrupted by confinement and surface hydrophobicity. For partially filled pores (33% RH), the structure of the nonfreezable 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 a low temperature (-120 °C). Second, additional bands reveal water hydroxyl groups pointing toward 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 toward 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 the hydrophobic solid interface.