The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory [1,2] has made it possible to discuss the stability of lyophobic colloids quantitatively. This theory is based on the interaction due to overlapping of electrical double layers and London–van der Waals forces between colloid particles.
Let us consider two immiscible bodies in contact to form an interface. The highly polar body is positively charged, and the other is negatively charged (Koehn’s rule). However, the interface may have the adsorption of ions, dissociation of functional groups, orientation of dipoles, breaking down of chemical bonds, and so on. These affect the real charge densities at the interface, and hence the potential difference at the interface is not explained in terms of such simple dielectric properties of the bodies.
The desire for cleanliness and other cosmetic benefits (fragrance, beauty, skin care) has been part of the human condition since early written history. Modem cleansers are based on fats, oils, and their derivatives, which have been utilized for cleansing in various forms for over two millennia (Fig. 1). Soap, generally defined as the alkali salt of a fatty acid, is a member of the general class of compounds called surfactants (surface-active agents). These molecules are characterized by having a hydrophilic head group and a hydrophobic 308tail. Synthetic detergents (or syndets) are another class of surfactant with similar structure and mechanisms of soil removal. However other key properties—for example, lather, bar feel, and sensitivity to water hardness—can be quite different.
Sophorolipids are glycolipidic biosurfactants extracellularly produced by several yeast species. Owing to the nonpathogenic character of the production host and the high yields, there is quite some commercial interest in these molecules. Indeed, to date sophorolipids are available in the market and nd applications in real-life products. Further research is currently being conducted in order to broaden their application range, either by evaluating their behavior in new applications or by modifying the sophorolipid chemical structure.
