VOLUMES AND HEAT-CAPACITIES OF ANIONIC-NONIONIC SURFACTANT MIXTURES

Density, heat capacity and surface tension measurements of sodium decy lsulfate (NaDeS)-dodecyldimethylamine oxide (DDAO)-water mixtures were carried out as functions of the surfactants total molality m(t) at fi xed stoichiometric mixture compositions X(NaDeS). From the surface ten sion data, the critical micelle concentration of NaDeS-DDAO mixtures a s 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 g iven mole fraction, V-Phi,V-2 and C-Phi,C-2 monotonically increases an d decreases, respectively, with increasing m(t). However, anomalies we re 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 prop erties for the formation of surfactant-surfactant mixed micelles in wa ter by mixing aqueous solutions of pure NaDeS and DDAO micelles were c alculated at 0.3 mol-kg(-1) for the micellized surfactants mixture. Th e excess volume V-exc and heat capacity as functions of X(NaDeS) are c oncave and S-shaped curves, respectively. All the properties are compa red to those for sodium dodecylsulfate-DDAO mixture. In addition, to c larify the effect of the change in the hydrophobicity of the surfactan ts mixtures V-exc for the dodecyltrimethylammonium bromide-decyltrimet hylammonium bromide mixture were calculated from literature data.