Surface and thermodynamic properties of sodium dodecyl sulphate and sodium dodecyl sulphonate

Determinations of the surface tension of aqueous solutions of sodium dodecyl sulphate and sodium dodecyl sulphonate were made in the presence of a neutral salt (NaCl; 0.1 M) at 20 degrees C. On the basis of these determinations the isotherms and the standard thermodynamic parameters of adsorption were determined The equation of state for a monolayer film of the studied surfactants was also discussed. A linear dependence was found between log Gamma and log c in the range of low surfactant concentrations The limiting value of Pi (A-A(0)) was found to be 1 kT in contrary to the case in which the solutions were made in the absence of neutral salt. It has been suggested that there are not only attractive forces of the Lifshitz-van der Waals type brit also acid-base forces which can affect the Pi parameter.