ENTHALPIC STUDIES ON THE FORMATION AND INTERACTION OF MICELLES OF A NONIONIC, SURFYNOL-465

The enthalpy of dilution DELTA //d//i//lH, of a nonionic surfactant, Surfynol 465, was measured in water at 25 degree C using a twin-type microcalorimeter. Both the amounts of water in the aqueous solution to be diluted and those in the diluent were kept constant, and the amount of Surfynol 465, n//2, was varied. The value of DELTA +ZZ//i//lH was always negative and decreased nonlinearly with n//2. The relative apparent molar enthalpy, ** phi L//2, increased sharply from nil, reached a plateau at 27. 8 kJ mole** minus **1, and increased again with the concentration of Surfynol 465. The relationship between DELTA //d//i//lH and n//2 was analyzed on the basis of an association equilibrium model and by using the optimal standard enthalpy of micellization at 23. 7 kJ mole** minus **1. In the analysis, an excess enthalpy of micelle in the concentration region higher than 0. 4 mole kg** minus **1 was found. An overlapping model was proposed for the excess enthalpy.