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

• Methanol self-diffusion was probed in LiCl solutions confined in a mesoporous silica. • Methanol self-diffusion was studied by nuclear magnetic resonance methods. • Effective methanol self-diffusion was obtained by pulsed field gradient techniques. • Methanol dynamics at the surface was studied by fast field-cycling relaxometry. • The activation energy for self-diffusion is lower at the surface than far from it. Self-diffusion of methanol in highly concentrated methanolic solutions of LiCl confined in a mesoporous silica gel was investigated using 1 H 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 1 H 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 29 Si and 7 Li 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.