Osmotic coefficients and enthalpies of dilution of aqueous sodium n-alkane-1-sulphonates at the temperatures (298.15, 313.15, 328.15, and 343.15) K

Abstract

Osmotic coefficients of aqueous solutions of sodium n-octane-1-sulphonate, sodium n-decane-1-sulphonate, sodium n-dodecane-1-sulphonate, and sodium n-tetradecane-1-sulphonate at the temperature T= 328.15 K and at molalities ranging from (0.01 to 0.30) mol·kg -1were estimated from vapour pressure osmometer measurements. Integral enthalpies of dilution for aqueous solutions of the same surfactants from T= 298.15 K to T= 343.15 K and at final molalities ranging from about (0.1 to 3.0)· m( cmc) were also measured, where m( cmc) denotes the critical micelle molality. Data for each surfactant were treated using a mass-action model that includes cation-anion interaction terms in the activity coefficient expressions for one anionic micellar species and the monomeric surfactant species. Ion-ion interaction parameters were estimated from the osmotic coefficients, and the temperature coefficients of these parameters were estimated from the enthalpies of dilution. Treatment of the enthalpies of dilution with the model also led to estimated values of Δ r H m ofor the formation reaction of micelles and to the corresponding values m= m( cmc). A simultaneous fit of all the enthalpies for each surfactant yielded numerical values of a small number of parameters for simple equations that describe the rather complex behaviour of aqueous surfactant solutions in terms of an equilibrium constant and activity coefficient equations and their temperature derivatives.