Dynamics of water confined in mesopores with variable surface interaction

Abstract

We have investigated the dynamics of liquid water confined in mesostructured porous silica (MCM-41) and periodic mesoporous organosilicas (PMOs) by incoherent quasielastic neutron scattering experiments. The effect of tuning the water/surface interaction from hydrophilic to more hydrophobic on the water mobility, while keeping the pore size in the range 3.5-4.1 nm, was assessed from the comparative study of three PMOs comprising different organic bridging units and the purely siliceous MCM-41 case. An extended dynamical range was achieved by combining time-of-flight (IN5B) and backscattering (IN16B) quasielastic neutron spectrometers providing complementary energy resolutions. Liquid water was studied at regularly spaced temperatures ranging from 300 K to 243 K. In all systems, the molecular dynamics could be described consistently by the combination of two independent motions resulting from fast local motion around the average molecule position and the confined translational jump diffusion of its center of mass. All the molecules performed local relaxations, whereas the translational motion of a fraction of molecules was frozen on the experimental timescale. This study provides a comprehensive microscopic view on the dynamics of liquid water confined in mesopores, with distinct surface chemistries, in terms of non-mobile/mobile fraction, self-diffusion coefficient, residence time, confining radius, local relaxation time, and their temperature dependence.Importantly, it demonstrates that the strength of the water/surface interaction determines the longtime tail of the dynamics, which we attributed to the translational diffusion of interfacial molecules, while the water dynamics in the pore center is barely affected by the interface hydrophilicity.

Structure and dynamics of water confined in cylindrical nanopores with varying hydrophobicity

“…At fixed temperature, water moving inside the hydrophobic pore is faster as compared to the other two pores with smaller contact angles. This behaviour is found to be in accordance with recent quasi-elastic neutron scattering experiments performed on porous silica and periodic mesoporous organosilicas [ 49 ]. Upon decreasing the temperature, water molecules slow down.…”

“…We find this behaviour for all the investigated pores, even for the highest hydrophilicity. In fact, in the case of water confined in hydrophilic confinement, the layers of water closer to the surface were shown to be immobile and even in a glassy state already at high temperature [ 26 , 27 , 49 , 50 ]. In silica pores, water of the contact layers can form hydrogen bonds directly with the pore wall [ 29 , 51 ] and the mobility of these molecules may vanish due to these strong directional hydrogen bonds [ 26 , 49 ].…”

“…In fact, in the case of water confined in hydrophilic confinement, the layers of water closer to the surface were shown to be immobile and even in a glassy state already at high temperature [ 26 , 27 , 49 , 50 ]. In silica pores, water of the contact layers can form hydrogen bonds directly with the pore wall [ 29 , 51 ] and the mobility of these molecules may vanish due to these strong directional hydrogen bonds [ 26 , 49 ]. The different behaviour observed in the present simulations seems related to the fact that the considered pore lacks the ability to form hydrogen bonds directly with water molecules; this feature contributes to the increase of the number of hydrogen bonds in the contact layer and the decrease in the inner layer.…”

An atomistic view on liquid phase adsorption

“…The number densities converges rapidly towards the density value at the pore center. The slowdown of water diffusion in confinement relative to the bulk phase can be compared with recent experimental data reported by Jani et al [72]. At a temperature of 300 K the experimen-tal self-diffusion coefficient of water in the bulk phase, 2.3 × 10 −9 m 2 s −1 , is reduced to 2.0 × 10 −9 m 2 s −1 in SBA-15 with a pore mean pore diameter of 6.6 nm and to 1.7 × 10 −9 m 2 s −1 in MCM-41 with a pore diameter of 3.8 nm, close to the simulated pore diameter of 4 nm.…”

Wafer‐Scale Electroactive Nanoporous Silicon: Large and Fully Reversible Electrochemo‐Mechanical Actuation in Aqueous Electrolytes

“…Note, however, that the silicon oxide in contact with aqueous electrolytes is hydroxylated. For such hydrophilic surfaces both experiments [ 61–64 ] and computer simulations [ 65–67 ] suggest the formation of highly viscous interfacial water layers of ≈1 nm thickness. In these layers the corresponding water self‐diffusivities is expected to exponentially increase toward the solid wall to values up to two orders of magnitude larger than in bulk water.…”