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 (T_g) 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 T_g 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, w_G, between 0.5 and 1.0, in silica mesoporous samples with pore diameters between 2 and 58 nm. Our results show that the the T_g dependence on pore size changes with the mixture composition. For glycerol-rich samples, T_g decreases with a decreasing pore size. This tendency changes with increasing water concentration below w_G ∼ 0.6 for samples with d_p 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 T_g composition dependence in confined glycerol–water mixtures was analyzed with the Gordon–Taylor equation modified for confined mixtures, which allowed us to calculate the T_g of the pure components as a function of the pore size. This analysis shows that for pores with d_p > 20 nm, and for pure water and pure glycerol, T_g decreases with the pore size, attaining an almost constant value for samples with pore sizes between 2 and 8 nm. This T_g pore size dependence is explained considering the competition of two opposite effects: a reduction in T_g with a decreasing pore size given when the length scale of dynamics is comparable to the pore size, and an increment in T_g with a decreasing pore size as a result of increasing interactions of the confined liquid with the pore walls.