Solid-liquid equilibrium and glass transition temperatures of tetraethylene glycol - water mixtures in bulk and confined mesoporous silica

The effect of confinement on the melting and glass transition (Tg) temperatures of tetraethylene glycol (TREG) aqueous solutions was studied in bulk and confined in mesoporous silica over a range of TREG weight fractions (wTREG) from 0.30 to 0.99. For bulk solutions Tg shows a constant value for wTREG = 0.90-0.99, which was explained considering that the addition of water to the polyol increases the size of the alpha relaxing entities. For wTREG = 0.50-0.90, Tg decreases with increasing water content due to the plasticizer effect of water. In contrast, for wTREG = 0.30-0.40, Tg values increase, reaching a constant value independent of the water content. This is explained by the fact that, in these dilute systems, water can crystallize during cooling the samples before the heating scan. Hence, the glassy phase attains the composition of the maximally freeze concentrated solution. The behavior of Tg with composition for samples confined in pores with diameters ranging from 8 to 58 nm is similar to that observed in bulk. However, in pores with 2 nm diameter, Tg exhibits a different behavior. Tg decreases with water content in the composition range wTREG = 0.90-0.99, indicating that the size of the alpha relaxing entities should be between 2 and 8 nm. For wTREG = 0.30-0.40, Tg also attains a constant value indicating that water also crystallizes in 2 nm pore samples when cooling them before the heating scans.