Partial molar enthalpies of water and surfactant in solutions of pentaethyleneoxide dodecyl ether and water in hexadecane

Abstract

Partial molar enthalpies of the surfactant and water in the system pentaethyleneoxide dodecyl ether (C 12E 5)/hexadecane/water have been determined at 27 and 37°C and surfactant concentrations 1.5, 5, and 10 wt% in hexadecane. The partial molar enthalpy of water shows a pronounced minimum at a water content of 0.3–0.5 mole water per mole surfactant, indicating changes in the water interaction during the initial hydration of the surfactant as the water content increases. At very low concentrations of water the partial molar enthalpy of C 12E 5 becomes rapidly more exothermic as the polyethyleneoxide chain of the surfactant is hydrated. At higher concentrations the surfactant forms smaller aggregates or reversed micelles. The binding of water to these aggregates is associated with very small enthalpies of dissolution. The enthalpy of dissolution of the surfactant is dominated at all concentrations by the strong effects of hydration of the polyethylene oxide chain. The results are similar at both temperatures investigated (around and above the temperature of maximum solubilization) and also are quite similar in the system C 12E 4/decane/water. Measurements have also been made of the partial molar enthalpy of solution of C 12E 5 in hexadecane solution containing up to 10.5 wt% at 27°C and 5.1 wt% at 40°C. The results show that if cooperative formation of aggregates occurs in the binary or in the ternary solutions it is not accompanied by easily detectable changes in the enthalpies.