3. Equilibrium adsorption properties of single and mixed surfactant solutions

Abstract

This chapter presents systematized experimental results for equilibrium surface (in some cases interfacial) tensions for several homologous series of non-ionic and ionic surfactants. In this chapter, attention is paid to the selection of experimental equilibrium data. The two main principles employed in the chapter are (1) high purity of the surfactants and solutions used, (2) control of experimental conditions under which the equilibrium was established. For surfactants with high adsorption activity, e.g., oxyethylated alcohols or ethers, the concentration in the solution is low, and the time necessary to attain the equilibrium is usually tens of hours. New theoretical models are used to analyze the experimental dependencies of equilibrium surface tension on concentration (or activity for ionic surfactants) for several homologous series of surface active substances. To process the experimental data, a problem-oriented software was developed in the form of fitting programs. These are used to establish the relations between the adsorption equilibrium constants, intermolecular interaction parameters, molar areas of molecules corresponding to their different orientations in the monolayer etc., and the number of methylene groups and the type of the polar group specific to the surfactant molecule. It is also shown in this chapter that the most reliable conclusions about the correspondence between the theoretical model and the experimental data can be provided by a comparison between the equilibrium tensiometry data and results obtained in dynamic surface (interfacial) tension studies, and also with data from other experimental techniques (ellipsometry, neutron and X-ray reflection, Brewster angle microscopy, and other optical methods).