ISSN 1061933X, Colloid Journal, 2012, Vol. 74, No. 2, pp. 172–185. © Pleiades Publishing, Ltd., 2012. 172 1 1. INTRODUCTION The standard free energy of surfactant adsorption, ΔG °, is a surface excess of the Gibbs thermodynamic potential [1] and is widely used as a basic thermody namic characteristic of surfactants [2–4]. The depen dence of ΔG ° on the surfactant chainlength for a ho mologous series enables one to distinguish between the contributions of the surfactant headgroups and tails into the adsorption energy [5–7]. The dependence of ΔG ° on the temperature, T, along with the definition of Gibbs free energy, ΔG ° = ΔH ° – TΔS °, allows one to determine also the standard adsorption enthalpy, ΔH°, and entropy, ΔS°. These thermodynamic pa rameters not only provide a quantitative characteriza tion of surfactants, but also bring information about the molecular processes accompanying their adsorp tion. For example, the analysis of experimental data indicates that TΔS ° |ΔH °| for both ionic and non ionic surfactants, which means that the increase of en tropy rather than the gain of energy determines the driving force of adsorption. This fact can be explained with the orientation of water molecules around the hy drocarbon chains in the solution that lowers the entro py of the system. Consequently the drawing of these chains out of the aqueous phase upon adsorption is ac companied by a rise of entropy [2, 3]. 1 The article is published in the original. * The article is dedicated to Academician Anatoly I. Rusanov on the occasion of his 80th birthday.  For nonionic amphiphiles, ΔG° can be determined from the slope of the plot of surface pressure, π s , vs. the surfactant concentration, c, at low concentrations, at which this plot is linear (Henry region) [5]. This ap proach often encounters difficulties due to the slow adsorption kinetics at c → 0. Because of that, Rosen and Aronson [8] proposed an empirical definition of adsorption free energy, which is easily deter mined by a linear fit of surfacetension data at higher concentrations. However, has unclear physical meaning. Alternatively, the use of a theoretical model of adsorption allows one to determine the true value of ΔG° by a nonlinear fit of a surfacetension isotherm, which is carried out numerically. The respective sys tems of equations have been derived and computation al procedures have been developed for both nonionic and ionic surfactants (see below). Here, our goal is to compare the theoretical and empirical approaches to the determination of ΔG °, ΔH ° and ΔS °, and to discuss the advantages and dis advantages of these approaches. In addition, our goal is to check whether the determined ΔG °, ΔH ° and ΔS ° are sensitive to the kind of the used theoretical model. Here, we compare the applicability of the adsorption models of Frumkin, van der Waals and Helfand–Frisch–Lebowitz, which have found numerous applications for the interpretation of sur facetension data [9–15]. Generalizations of these models to the cases of ionic surfactants and mixed sys R , G ° Δ R G ° Δ The Standard Free Energy of Surfactant Adsorption at Air/Water and Oil/Water Interfaces: Theoretical vs. Empirical Approaches 1, * Krassimir D. Danov and Peter A. Kralchevsky Department of Chemical Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Blvd., Sofia, 1164 Bulgaria Received October 7, 2011 Abstract—The standard free energy of surfactant adsorption represents the work of transfer of a surfactant molecule from the bulk of solution to an infinitely diluted adsorption layer. This quantity can be determined by nonlinear fits of surfacetension isotherms with the help of a theoretical model of adsorption. Here, the models of Frumkin, van der Waals and Helfand–Frisch–Lebowitz are applied, and the results are compared. Irrespective of the differences between these models, they give close values for the standard free energy. The results from the theoretical approach are compared with those from the most popular empirical approach. The latter gives values of the standard free energy, which are considerably different from the respective true values, with c.a. 10 kJ/mol for nonionic surfactants, and with c.a. 20 kJ/mol for ionic surfactants. These dif ferences are due to contributions from interactions between the molecules in dense adsorption layers. It is concluded that the true values of the standard free energy can be determined with the help of an appropriate theoretical model. For the processed sets of data, the van der Waals model gives the best results, especially for the determination of the standard adsorption enthalpy and entropy from the temperature dependence of sur face tension. The results can be useful for the development of a unified approach to the thermodynamic char acterization of surfactants. DOI: 10.1134/S1061933X12020032