Fluid Behavior in Nanoporous Silica

We investigate dynamics of water (H₂O) and methanol (CH₃OH and CH₃OD) inside mesoporous silica materials with pore diameters of 4.0, 2.5, and 1.5 nm using low-field (LF) nuclear magnetic resonance (NMR) relaxometry. Experiments were conducted to test the effects of pore size, pore volume, type of fluid, fluid/solid ratio, and temperature on fluid dynamics. Longitudinal relaxation times (T₁) and transverse relaxation times (T₂) were obtained for the above systems. We observe an increasing deviation in confined fluid behavior compared to that of bulk fluid with decreasing fluid-to-solid ratio. Our results show that the surface area-to-volume ratio is a critical parameter compared to pore diameter in the relaxation dynamics of confined water. An increase in temperature for the range between 25 and 50°C studied did not influence T₂ times of confined water significantly. However, when the temperature was increased, T₁ times of water confined in both silica-2.5 nm and silica-1.5 nm increased, while those of water in silica-4.0 nm did not change. Reductions in both T₁ and T₂ values as a function of fluid-to-solid ratio were independent of confined fluid species studied here. The parameter T₁/T₂ indicates that H₂O interacts more strongly with the pore walls of silica-4.0 nm than CH₃OH and CH₃OD.