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.

Micro-Raman spectroscopic analysis of liquid–liquid phase separation

“…The two components at B2440 cm À1 and B2540 cm À1 correspond to the O-D stretch mode of HOD molecules having tetrahedral H-bonding structure and distorted H-bonding structure. 56,57 Fig. 5(c) and (d) show that the peak frequencies and relative fractions of the two components vary depending on the size of the droplet.…”

Structure of Water under Confinement in Periodic Mesoporous Organosilicas Investigated by X-ray Scattering

“…Studies considering the molecular structuring of water under nanoscale confinement have also previously been conducted using methods such as X-ray and neutron scattering 15−17 as well as Raman spectroscopy. 18,19 There are also a number of simulation studies on the topic. 10,12,14 Authors employing different methods arrive at varying conclusions, in particular with regard to the effect of the confinement on the amount of tetrahedral structuring.…”

“…34−36 Tuning the pore−water interaction and polarity of the pore wall has an influence on the structure and dynamics of water confined in PMOs. 19,27,29,37,38 For instance, using Raman spectroscopy to investigate the hydrogen bonding behavior in water confined in PMOs, a reduction of tetrahedrally H-bonded was found in both MCM-41 and a more hydrophobic, phenylene-bridged BTEB-PMO with the effect being more pronounced in the latter. 19 Another study by Jani et al 29 claims that the dynamics of water are slowed down in the case of PMO confinement.…”

“…19,27,29,37,38 For instance, using Raman spectroscopy to investigate the hydrogen bonding behavior in water confined in PMOs, a reduction of tetrahedrally H-bonded was found in both MCM-41 and a more hydrophobic, phenylene-bridged BTEB-PMO with the effect being more pronounced in the latter. 19 Another study by Jani et al 29 claims that the dynamics of water are slowed down in the case of PMO confinement. Here, the diffusivity of water molecules close to the PMO interface is slower in the case of hydrophilic groups in the pore wall compared to hydrophobic ones.…”

The Structure of Water in Silica Mesopores – Influence of the Pore Wall Polarity

“…The effect is related to the surface polarity, as a hydrophilic surface interacts more strongly with water molecules than a hydrophobic one. [26][27][28][29] For example, Fröba and coworkers studied water adsorption and mobility in periodic mesoporous organosilica materials and found that water adsorbing to bare (hydrophilic) silica and to organic (hydrophobic) moieties within the same mesopore can be discriminated, for example, by NMR and Raman [30] spectroscopy. In general, the strength of interaction between water molecules and (organo-) silica pore wall can be studied by H 2 O sorption experiments.…”