Calorimetric study of dilute aqueous solutions of ethylene glycol oligomers

The enthalpies of mixing of water and five ethylene glycol oligomers (C0Ej; j = 1-5) were measured in an ideal dilute solution at seven temperatures from 283.15 to 313.15 K by using a high-accuracy isothermal titration calorimeter. The differential enthalpies of solution of C0Ej were evaluated by applying the thermodynamic equations to the experimental results. They were negative, and their absolute values increased with decreasing temperature and increasing oxyethylene chain length. It was shown that the dissolution of C0Ej is exothermic and that the interaction between water and C0Ej molecules is stronger than the average of interactions between the same species. Moreover, the partial molar heat capacity changes and the partial molar heat capacities at 298.15 K were calculated. It was shown that the partial molar heat capacities of C0Ej in an aqueous solution are increased from the molar heat capacities of the pure liquid state owing to hydration. It was found that all the partial molar quantities per oxyethylene are almost constant. The enthalpy of mixing of some protic solvents, i.e., methanol, ethanol, 1-propanol, and ethylene glycol, with C0E5 was also measured to examine the hydration of the oxyethylene group. Our experiment showed that the differential enthalpy of solution of C0E5 is very slightly negative or even positive in contrast to the large negative value of the water system. Furthermore, it was suggested by comparing the results of C0Ej with those of C8Ej that the hydration of the hydrophobic group is also exothermic at a lower temperature and strongly dependent on temperature.