Highly concentrated LiCl methanolic solutions confined in a mesoporous silica: an NMR study on methanol self-diffusion

Self-diffusion of methanol in highly concentrated methanolic solutions of LiCl confined in a mesoporous silica gel was investigated using 1H Pulse Field Gradient (PFG) NMR and fast field-cycling (FFC) NMR relaxometry. The effect of both the salt and the porous matrix were evaluated by examining confined pure methanol and solutions characterized by salt concentrations ranging between 10 and 30 wt% as well as their bulk counterparts. Both the salt and the matrix slow down methanol self-diffusion, with the salt exerting a significantly stronger effect. The observed slowdown corresponds to the disruption of the hydrogen bonding network, as revealed by the temperature dependence of the 1H chemical shift of the methanolic hydroxyl group. The analysis of the PFG and FFC NMR data as a function of temperature (25–50 °C) revealed that self-diffusion is less activated for methanol molecules near the surface than for those further away and that the slowdown caused by silica is due to an entropic effect. The role of the matrix was also investigated in the most concentrated solution using 29Si and 7Li NMR, which allowed the presence of Si-O-Li+ bond to be excluded and a slowdown of the dynamics of confined Li+ to be revealed, respectively.