W-shape excess heat capacities, upper critical solution temperatures and non-randomness in oligomeric oxaalkane–dimethylsiloxane systems

Abstract

Excess heat capacities, C^E_p, of oxaalkane (glyme)–dimethylsiloxane systems at 25 °C are reported for 2,5-dioxahexane (monoglyme), 2,5,8-trioxanonane (diglyme), 2,5,8,11-tetraoxadodecane (triglyme) and 2,5,8,11,14-pentaoxapentadecane (tetraglyme) with hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane and octamethylcyclotetrasiloxane, except for tetraglyme with trisiloxane and tetrasiloxane, both incompletely miscible at 25 °C. In addition, C^E_p was obtained for diglyme mixed with a dimethylsiloxane oligomer corresponding to an average of 15 siloxane segments. W-shaped C^E_p–composition curves were found for all systems and were attributed to local non-randomness. UCST values were obtained for some systems. Non-randomness was quantified through the concentration–concentration correlation function, S_cc=[∂²(G/RT)/∂x²]^–1, where G is the solution molar free energy. UCST values were calculated using three models for G, corresponding to the Flory–Huggins combinatorial with interaction between molecular volumes or between molecular surfaces, and a combinatorial due to Prausnitz and collaborators with interaction between surfaces. The last model is best for these systems where there is a large difference in molecular surface/volume ratios. It gives reasonable UCST predictions and a quantitative correlation between the compositions of the maxima in S_cc and C^E_p, as well as a qualitative correlation between the shapes of S_cc and C^E_p curves.