Behavior of Anionic Surfactants and Short Chain Alcohols Mixtures in the Monolayer at the Water-Air Interface

Measurements of the surface tension of aqueous solution of mixtures of sodium dodecyl sulfate (SDDS) with methanol and ethanol in SDDS concentration range from 10(-5) to 10(-2) M and mixtures of sodium hexadecyl sulfonate (SHS) with methanol and ethanol at SHS concentration from 10(-5) to 8 x 10(-4) M and for methanol and ethanol from 0 to 21.1 and, 11.97 M, respectively, were carried out at 293 K. Moreover, the surface tension of aqueous solution mixtures of SDDS with propanol in the concentration range from 0 to 6.67 M taken from the literature was also considered. The results obtained indicate that it is possible to describe the relationship between the surface tension and molar concentration or molar fraction of alcohol by Szyszkowski and Connors equations. However, the Fainerman and Miller equation allows us to predict the isotherm of the surfactant tension at constant anionic surfactants concentration at which their molecules are present in the solution in the monomeric form if the molar area of surfactants and alcohols can be determined. Based on the surface tension isotherms, the Gibbs surface excess of anionic surfactants and alcohols concentration at water-air interface was determined and then recalculated for Guggenheim-Adam surface excess concentration of these substrates, and next the molar fraction of alcohols and surfactants in the surface layer was determined. These molar fractions were discussed with regard to surfactant and alcohol standard free energy of adsorption at the water-air interface determined from Langmuir and Aronson and Rosen equations. The standard free energy of adsorption determined in these ways was compared to that deduced on the basis of pC(20) and Lifshitz van der Waals-components of the anionic surfactant and alcohol tails.