The standard free energy of surfactant adsorption at air/water and oil/water interfaces: Theoretical vs. empirical approaches

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.

Sulfonated methyl esters of fatty acids in aqueous solutions: Interfacial and micellar properties

“…Table 4 in Ref. [36]. Here, this model is applied to fit the surface tension isotherms for C14-and C16-SME in Fig.…”

Effect of Ionic Correlations on the Surface Forces in Thin Liquid Films: Influence of Multivalent Coions and Extended Theory

“…All these parameters have been determined from the fits of surface tension isotherms at various salt concentrations [28]. Equation (3.16) corresponds to the van der Waals adsorption model, which turns out to be adequate for describing surfactant adsorption at a fluid interface [38]. In addition, the counterion binding can be described by the Stern isotherm [27]: $\frac{{\mathrm{\Gamma }}_2}{{\mathrm{\Gamma }}_1}=\frac{K_{\text{St},2}{\gamma }_{2\infty }{c}_{2\infty }\exp false({\mathrm{\Phi }}_{\mathrm{s}}false)}{1+K_{\text{St},2}{\mathrm{\gamma }}_{2\infty }{c}_{2\infty }\exp false({\mathrm{\Phi }}_{\mathrm{s}}false)}$…”

Thermal and mechanical behavior of ZrTiO 4 -TiO 2 porous ceramics by direct foaming

“…The area under a DTA peak is the enthalpy change and is unaffected by the heat capacity of the sample [24]. In cations occupying the available octahedral site [13,14].…”

“…On the other hand, Hertzian indentation mechanics have been used extensively for the analysis and characterization of the fracture and deformation properties of brittle ceramics, including porous ceramics [12,13]. Hertzian fracture is commonly associated with energy dissipation by internal friction at sliding grains, platelet or whiskers, or other microstructural elements that bridge the crack wake [14,15].…”