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.

2H NMR Studies on the Dynamics of Pure and Mixed Hydrogen-Bonded Liquids in Confinement

“…In Figure 10b and c, we display the correlation times τ p from the 2 H SLR and STE studies on bulk and confined PG-D 2 O and PGME-D 2 O mixtures. The time constants for 2PGME-D 2 O mixtures were reported in our previous study [40]. In the weakly supercooled regime, the bulk and confined mixtures share a common VFT behavior, Fig.…”

“…2 H STE experiments allow us to measure correlation functions of reorientation dynamics at lower temperatures. For 2PGME-D 2 O mixtures, detailed comparison with DS data revealed that the correlation functions decay in two steps, which can be related to the α and β processes [40]. Figure 10a shows cc 2 m ( ) F t for the PG-D 2 O mixture W45 in bulk and in confinement, which do not yield evidence for a clear bimodality.…”

“…Other alcohol-water mixtures are obtained by adding D 2 O in appropriate amounts to propylene glycol (PG, Acros Organics) [39], propylene glycol monomethyl ether (PGME, Sigma-Adrich), or dipropylene glycol monomethyl ether (2PGME, Sigma-Aldrich) [40]. These mixtures are confined to MCM-41 materials from the Buntkowsky group (C14, d = 2.8 nm) or from Sigma-Aldrich (d = 2.5-2.8 nm), as described in previous work [40].…”

“…These mixtures are confined to MCM-41 materials from the Buntkowsky group (C14, d = 2.8 nm) or from Sigma-Aldrich (d = 2.5-2.8 nm), as described in previous work [40]. No differences were found when using these silica matrices from different sources.…”

“…When cooling PG, PGME, and 2PGME [40] solutions with high water contents, 〈T 1 (T)〉 shows discontinuities, indicating that crystallization of excess water reduces the water concentration and affects the molecular dynamics in the remaining liquid fraction. As a consequence, at lower temperatures, the differences of the liquid compositions are reduced with respect to the nominal ones.…”