Ellipsometry is a possibility to measure directly the adsorption of surfactants at liquid interfaces. For pros and cons and range of applicability, please look into literature (H. Motschmann and R.Teppner, Ellipsometry, in “Novel Methods to Study Interfacial Layers”, Studies in Interface Science, Vol. 11, D. Möbius and R. Miller (Eds.), Elsevier, Amsterdam, 2001, p. 1). Easier accessible is the surface tension of a solution. Via thermodynamic models it is possible to determine the adsorbed amount from so-called surface tension – concentration isotherms g(c). The equivalent to the above Langmuir isotherm is the von Szyszkowski isotherm:
(gamma)0 is here the surface tension of the pure solvent, and R and T are the gas low constant and the absolute temperature.
The classical isotherms, for example Langmuir and also the more advances Frumkin isotherms, do not describe perfectly all experimental data. In recent time, better models have been developed, which are summarised in a book (V.B. Fainerman, D. Möbius and R. Miller (Eds.), Surfactants – Chemistry, Interfacial Properties and Application, in Studies in Interface Science, Vol. 13, Elsevier, 2001 ). As an example, the following graph shows the surface tension isotherms of five members of the homologous series of fatty acids, a classical model surfactant.
