Interpretation of surface-tension isotherms of n-alkanoic (fatty) acids by means of the van der Waals model

• Published in: Journal of colloid and interface science 2006-08, Vol.300 (2), p.809-813
• Main Authors: Danov, K.D., Kralchevsky, P.A., Ananthapadmanabhan, K.P., Lips, A.
• Format: Article
• Language: English
• Subjects: Adsorption; Chemistry; Exact sciences and technology; General and physical chemistry; n-Alkanoic acids; Normal fatty acids; Surface (Gibbs) elasticity; Surface physical chemistry; Surface pressure; Surface-tension isotherms
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2006.04.026

Here we apply the two-dimensional van der Waals model to interpret surface-tension isotherms of aqueous solutions of n-alkanoic (fatty) acids. We processed available experimental data for a homologous series of eight acids, from pentanoic to dodecanoic (lauric). Only three adjustable parameters have been varied to fit simultaneously all experimental curves. Excellent agreement between the theoretical model and the experiment has been obtained. The determined parameter values comply well with the molecular properties and allow one to calculate the surfactant adsorption, surface elasticity, and the surface pressure vs area isotherms. For the dodecanoic acid, the van der Waals model indicates the existence of a surface phase transition. By means of the two-dimensional van der Waals model, experimental surface-tension isotherms of eight normal alkanoic acids are simultaneously fitted, and the respective adsorptions, surface pressures and elasticities are determined.