Thermodynamics of micellization and solubilization in the system water + sodium n-octanoate + n-pentanol at 25°c. Part 1.—Partial molar enthalpies

Abstract

The molar enthalpies of mixing have been determined for aqueous solutions of sodium octanoate (NaCs) and pentanol in the aqueous solution region as well as for solutions of water in pentanol. From these results, the partial molar enthalpies of the three components have been calculated. The behaviour of sodium octanoate, even at extremely high dilution, is different from that of a simple binary electrolyte, which can be attributed to the interaction between the hydrocarbon chain and water. Ionic solute effects have a dominating influence on the enthalpies in solution. The partial molar enthalpies of pentanol and octanoate increase steeply below the c.m.c. while that of water decreases. Above the c.m.c. there are slower changes in the opposite direction. The transfer of sodium octanoate or pentanol from infinite dilution to a micelle is an endothermic process involving heats of a few kJ mo1-I. When pentanol is added to a solution of NaCs below the c.m.c., micelle formation is induced. When it is added to NaC3 solutions with low concentrations of micelles, the pentanol dissolves almost ideally while the addition at high micellar concentrations appears to be accompanied by changes in the binding of water to the micelles.

Enthalpies of solution in sodium octanoate + water + alcohol mixtures

“…That of neat nbutanol, equally concentration independent in roughly the same concentration range [31], gives a AH~ value of-6.78 kJ mole -1 at 35 ~ the corresponding values [31,32] at 25 ~ being between -9.21 and-9.28 kJ mole -1. For n-pentanol, the literature data [15,32] for AH~ at 25 ~ differ, the two values reported being -6.33 and-7.65 kJ mole -1. Assuming that the temperature coefficient of AH ~ for pentanol is comparable to that of butanol, we estimate that the AH~ of pentanol in water is -5.15 kJ mole -1 at 35 ~ based on the more negative value reported [32] AH ~ values are less negative with increasing soap concentration, thus the enthalpies of transfer of propanol from water to aqueous sodium octanoate solutions are positive.…”

“…Our datum on the concentration of the soap in the saturated aqueous solution is in disagreement wkh the earlier [15,19] value of 40.65 wt% at 25 ~ since soap solubility in water usually increases with temperature [20]. The soap is sparingly soluble in anhydrous propanol at 35 ~ which is some 15 ~ below the Kraft point, but is dramatically increased on addition of water to the anhydrous propanol.…”

“…Two sets of calorimetric experiments were designed and their results are reported here: enthalpies of solution of sodium octanoate in water and in several of its mixtures with normal propanol, butanol, pentanol or hexanol; and that of the alcohols in aqueous solutions of sodium octanoate at various concentrations, at 35 ~ Some, and similar, ternary systems studied here have been looked at before. Mutual solubilities and phase boundaries between solid -liquid and liquidliquid phases, as well as some calorimetric enthalpy data have been investigated by the Scandinavian school [13][14][15][16] and their results are compared with ours whenever relevant.…”