Revisiting the glass transition temperature of water-glycerol mixtures in the bulk and confined in mesoporous silica.

Abstract

In this work, we revisited the glass transition temperature (Tg) behavior of bulk and confined water-glycerol solutions as a function of the mixture composition and size of the confinement media, with the aim to shed some light on some controversies found in the literature. In the case of bulk mixtures, some discrepancies are observed due to the differences in the way of calculating Tg from the DSC experiments and differences in the protocols of cooling/reheating. However, unphysical behavior observed below the eutectic composition can be due to the crystallization of water during the cooling of the mixture. We also analyzed the effect of confinement on the glass transition of glycerol aqueous solutions, with glycerol mass fraction, wG, between 0.5 and 1.0, in silica mesoporous samples with pore diameters between 2 and 58 nm. Our results show that the the Tg dependence on pore size changes with the mixture composition. For glycerol-rich samples, Tg decreases with a decreasing pore size. This tendency changes with increasing water concentration below wG ∼ 0.6 for samples with dp between 2 and 8 nm, where two glass transition temperatures appear. We hypothesize that this effect is related to the existence of two liquid phases with different densities. The Tg composition dependence in confined glycerol-water mixtures was analyzed with the Gordon-Taylor equation modified for confined mixtures, which allowed us to calculate the Tg of the pure components as a function of the pore size. This analysis shows that for pores with dp > 20 nm, and for pure water and pure glycerol, Tg decreases with the pore size, attaining an almost constant value for samples with pore sizes between 2 and 8 nm. This Tg pore size dependence is explained considering the competition of two opposite effects: a reduction in Tg with a decreasing pore size given when the length scale of dynamics is comparable to the pore size, and an increment in Tg with a decreasing pore size as a result of increasing interactions of the confined liquid with the pore walls.