Thermodynamics of mixed micellar systems of sodium octanoate and ethoxylated alcohols in water at 25°C

Thermodynamic volume and compressibility properties of binary aqueous systems of sodium octanoate (C₈Na) and polyoxyethylene butyl ether compounds (ethoxylated alcohols), with one to three oxyethylene groups (C₄EO_X), and ternary systems of these compounds, have been determined as a function of surfactant and alcohol concentrations at 25°C from density and ultrasonic velocity measurements. Values of standard state apparent molar volume and apparent molar adiabatic compressibility properties of transfer of ethoxylated alcohols from water to aqueous C₈Na solutions were obtained. The observed trends in the properties of the aqueous binary C₈Na systems and of the transfer functions for C₄EO_x, at a given low concentration of 0.05m, were analyzed using theoretical models. Good agreement between the simulated results and the experimental data was achieved. The results obtained for the distribution coefficients of the alcohols and the thermodynamic properties of both components of the mixed micelle show that the hydrophobicity of the ethoxylated alcohols is enhanced by introducing more oxyethylene groups into the alcohol. The distribution behavior of these compounds between the C₈Na micelles and the aqueous phase also depends on the difference between the hydrophobicity of the surfactant and the alcohols.