Procedures to purify surfactant solutions

The purification of surfactants has to be performed in a way that addresses their specific effects on interfacial properties. Thus, chemical purification procedures, such as recrystallisation, distillation, flocculation, and washing with organic solvents, are important steps (Czichocki et al. 1981) but usually do not lead to a sufficient purity, i.e. surface-chemical purity. Therefore, efficient purification methods are based on interfacial principles. 

One possibility consists in the use of dispersed solids, such as latex particles. Surface-chemically active impurities are preferentially adsorbed at the rather large surface area of these particles, and after removal of the disperse phase by filtration, a purified solution is obtained (Rosen 1981, Carroll 1982, Lunkenheimer et al. 1984). This method has two disadvantages there is a large remarkable and unknown loss of the main surfactant, and the dispersion can introduce impurities into the surfactant solution. 

A second purification method based on interfacial principles is foam flotation (Brady 1949, Elworthy Mysels 1966, Lemlich 1968, Somasundaran 1975, Gilanyi et al. 1976). A column is filled with the surfactant solution to be purified. Bubbles of purified gas are formed at the bottom of the column and rise up in the solution, carrying an adsorption layer to the top. This adsorption layer is formed by the main surfactant and the contamination. Due to the higher surface activity of the impurity, it is enriched in the contaminant in the top layer and in any resulting foam. Liquid in the column is progressively purified by removing the foam or the top layer. The same procedure may be successfully used for the purification of water samples from uiy surface active material (Loglio et al. 1986,1989). 

A further and very efficient method of purifying aqueous surfactant solution consists in a cyclic removal of the adsorption layer. The general idea is to establish an almost equilibrium adsorption layer in which the highly surface active impurity is enriched with respect to the main surfactant. If the adsorption layer is removed, the ratio of surfactant/impurity is changed (cf. Eq. (5.6)). The idea has been realised in an apparatus which automatically performs the following steps (Lunkenheimer et al. 1987a)  

formation of an adsorption layer at a large surface during time tj  

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