Purity of surfactant solutions

The term surface-chemically pure , defined by Lunkenheimer Miller (1979), is based on the finding that small amounts of highly surface active contaminations can control the properties of an adsorption layer. The study of the ability of surfactants to change interfacial properties requires an adsorbed layer consisting mainly of the surfactant molecules under study. Therefore, any information about the percentage of a potential impurity, given for commercially available samples, is useless. The reason can be understood easily from the situations shown in Fig. 5.1. 

Although, there is a very small number of molecules of type B, compared to type A, in the solution, the equilibrium adsorption layer is formed by almost the same number of both molecule types (Fig. 5.1. a). In this case, the surface activity of the contaminant (substance B) is much higher than that of the main component (substance A). In contrast. Fig. 5.1. b demonstrates the situation where both components have a similar surface activity. Here, the adsorbed layer at the interface is mainly controlled by molecules of type A. This solution is in a state of sufficiently high grade of purity for interfacial studies. 

Investigations of the solution, demonstrated in part (a) could lead to very confusing results, depending on the experimental conditions. For example, the change of temperature could change the surface activity of the two components in different way and the composition of the adsorbed layer can change with temperature. If dynamic interfacial studies are performed, the results will depend strongly on the time windows of the methods used. The three states of a dynamic adsorption layer (Fig. 5.2., a-c) demonstrate such a situation schematically. 

In the present case, at time t = t, the adsorption layer has reached an intermediate coverage and consists of almost only molecules of type A. After a time t = tj, equilibrium has not yet been reached, but component B occupies already a remarkable part of the interfacial area. The final equilibrium state of the adsorbed layer is established at time t tj, and the interfacial properties are mainly controlled by the contaminant B. The evolution of the adsorbed layer composition with time is on a logarithmic scale. The absolute time ranges are a function of the absolute concentration, the time differences and concentration ratios at the interface and the surface activities of the two components. In practical cases, surfactants are not only contaminated by one but often by several components of different surface activity. This complicates the analysis of purification procedures and the grade of purity of prepared surfactant solutions. 

A thermodynamic criterion for judging the grade of purity of a surfactant solution was developed by Lunkenheimer Miller (1987,1988). It is based on any purification procedure to remove impurities from a surfactant solution. If is the variable which denotes the extent of removal of surface active molecules from a solution, it was shown that the relation 7 (4) is 

---