VOLUMES AND HEAT-CAPACITIES OF ANIONIC-NONIONIC SURFACTANT MIXTURES

Density, heat capacity and surface tension measurements of sodium decylsulfate (NaDeS)-dodecyldimethylamine oxide (DDAO)-water mixtures were carried out as functions of the surfactants total molality m(t) at fixed stoichiometric mixture compositions X(NaDeS). From the surface tension data, the critical micelle concentration of NaDeS-DDAO mixtures as a function of X(NaDeS) were obtained. From density and heat capacity data, the apparent molar volume V-Phi,V-2 and heat capacity C-Phi,C-2 of NaDeS-DDAO mixtures in water were calculated, respectively. At a given mole fraction, V-Phi,V-2 and C-Phi,C-2 monotonically increases and decreases, respectively, with increasing m(t). However, anomalies were observed at X(NaDeS) = 0.1 and 0.3 for both V-Phi,V-2 and C-Phi,C-2 vs. m(t) curves. The nonideal contributions to the thermodynamic properties for the formation of surfactant-surfactant mixed micelles in water by mixing aqueous solutions of pure NaDeS and DDAO micelles were calculated at 0.3 mol-kg(-1) for the micellized surfactants mixture. The excess volume V-exc and heat capacity as functions of X(NaDeS) are concave and S-shaped curves, respectively. All the properties are compared to those for sodium dodecylsulfate-DDAO mixture. In addition, to clarify the effect of the change in the hydrophobicity of the surfactants mixtures V-exc for the dodecyltrimethylammonium bromide-decyltrimethylammonium bromide mixture were calculated from literature data.