The Adsorption of the Ampholytic Surfactant and the Anionic Surfactant at the Nitrogen–Solution Interface from the Mixed Aqueous Solution

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Abstract
The amounts of adsorption of an ampholytic surfactant, N-dodecyl-β-alanine(NDA), and an anionic surfactant, sodium alkylsulfate(SAS), at nitrogen-solution interface from aqueous equimolar mixtures of them were measured directly by the radiotracer method, using tritiated and non-tritiated samples of NDA and 35S-labeled and non-labeled samples of SAS: sodium tetradecylsulfate(STS), sodium dodecylsulfate(SDS), and sodium decylsulfate(SDeS). The adsorption of both components of the mixed systems was observed at concentrations lower than those for the systems containing each component alone. A stronger adsorption of NDA than that of SAS was observed for all mixed systems. The overall area per alkyl chain at the maximum adsorption was about 26 Å2 for each system, showing the adsorbed layer to be in a highly condensed state. Electrophoresis measurements, together with surface tension measurements of NDA-SDS systems and the elemental analysis of the precipitates from concentrated NDA-SDS solutions, suggested the presence of a 1 : 1-complex formed between NDA and SDS. The Gibbs adsorption isotherm for the binary system was derived. The calculated amounts of adsorption agreed with the observed values. A linear relationship was found to hold between the logarithm of the constants of complex formation and the number of carbon atoms of SAS. A free energy of the complex formation of −0.50 kcal per mole of the methylene group was found.

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