Dynamics of DiPGME–Water Mixtures in Mesoporous Silica

Abstract

In this study, we have combined dielectric spectroscopy and 2H NMR to elucidate the molecular dynamics of aqueous solutions of dipropylene glycol monomethyl ether (DiPGME) confined into 2.8 nm pores of MCM-41. The results show that the concentration dependence of the dynamics is completely different compared to the corresponding bulk solutions, where a pronounced nonmonotonic concentration dependence was observed for the glass transition and its related α-relaxation. In the confinement, both the cooperative α-relaxation and the more local β-relaxation are almost unaffected by the water concentration. The main reasons for this seem to be that there is a preferential hydration of the inner pore surfaces, leading to a strong concentration gradient in the pores, as well as ice formation at higher water concentrations (45 wt % and above during heating), also leading to less water and a weaker concentration dependence in DiPGME-rich regions. The β-process is observed in the DS measurements even for confined DiPGME, without any water. This implies that the β-relaxation is strongly enhanced, compared to the α-relaxation, in the confinement, since it could not be clearly observed in the bulk liquid. A β-relaxation due to water was observed in the bulk solutions, but this process was rapidly speeding up with increasing water concentration, while it is basically concentration independent in the confinement. From the NMR measurements, it was also possible to conclude that the α-relaxation of the confined solutions is composed of a number of consecutive small-angle elementary rotational jumps, and that the β-process is related to a spatially restricted motion.