Influence of Pore Surface Chemistry on the Rotational Dynamics of Nanoconfined Water

Abstract

We have investigated the dynamics of water confined in mesostructured porous silicas (SBA-15, MCM-41) and four periodic mesoporous organosilicas (PMOs) by dielectric relaxation spectroscopy. The influence of water−surface interaction has been controlled by the carefully designed surface chemistry of PMOs that involved organic bridges connecting silica moieties with different repetition lengths, hydrophilicity, and H-bonding capability. Relaxation processes attributed to the rotational motions of nonfreezable water located in the vicinity of the pore surface were studied in the temperature range from 140 to 225 K. Two distinct situations were achieved depending on the hydration level: at low relative humidity (33% RH), water formed a nonfreezable layer adsorbed on the pore surface. At 75% RH, water formed an interfacial liquid layer sandwiched between the pore surface and the ice crystallized in the pore center. In these two cases, the study revealed different water dynamics and different dependence on the surface chemistry. We infer that these findings illustrate the respective importance of water−water and water−surface interactions in determining the dynamics of the interfacial liquid-like water and the adsorbed water molecules as well as the nature of the different H-bonding sites present on the pore surface.

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

“…40 Due to confinement effects, crystallization started below −40 °C in water-filled matrices, in agreement with previous dielectric spectroscopy and DSC studies. 24,25 Compared to bulk ice, the spectra of water-filled matrices measured below the freezing point, e.g., at T = −100 °C (cf. right bottom panel in Figure S1) present a blue-shifted excess of spectral intensity above 3400 cm −1 .…”

“…This indicates that in confinement, ice is defective and coexists with amorphous regions, in agreement with previous studies indicating the existence of an unfreezable interfacial layer. 25 In the present study focusing on the structure of the liquid state, we restricted the spectral analysis to temperatures above −15 and −40 °C for the bulk and capillary filled samples, respectively. The corresponding spectra are presented in Figure 3a for water-filled MCM-41.…”

“…24 Concomitantly, the DRS study showed that, unlike water in saturated pores, the layer of water adsorbed at a reduced relative humidity (RH) was most sensitive to water− surface interactions. 25 These findings illustrate the respective importance of water−water and water−surface interactions in determining the dynamics of the interfacial and confined water molecules. They also point out that understanding H-bond interactions is crucial to predict properties such as the molecular dynamics of confined molecules.…”

“…On cooling, crystallization temperatures were in agreement with previous dielectric spectroscopy and differential scanning calorimetry (DSC) studies, which indicated no significant heating of the sample induced by the excitation laser. 24,25 The scattered light was collected in backscattering configuration, and the Rayleigh scattering was removed by means of dielectric edge filters. To remove ripples generated by these filters, we have applied an intensity correction function on the experimental spectra.…”

Molecular-Level Insights into Unique Behavior of Water Molecules Confined in the Heterojunction between One- and Two-Dimensional Nanochannels

“…During such a short time, we believe that most of water molecules can stay the same subregion as the initial time. Furthermore, rotational properties of confined water have been studied by various experimental techniques such as dielectric relaxation spectroscopy (DRS), 82 nuclear magnetic resonance (NMR), 83 and quasielastic neutron scattering (QENS). 84 Recent 2 H NMR results by Kyakuno et al 83 have reported that the rotational correlation time of water in CNTs is about 10−20 ps, which is well comparable with our calculation results in Figure 8.…”