Structural requirements of the binding of nonionic surfactants to sodium dodecylsulfate

The interaction of 24 nonionic surfactants (6 ethoxylated stearic acid derivatives, 10 ethoxylated fatty alcohols, and 8 ethoxylated anhydrosorbite derivatives) with the anionic surfactant sodium dodecylsulfate (SDS) was studied by reversed-phase chromatography, and the relative strength of interaction has been calculated. Principal component analysis (PCA) was used for the determination of molecular substructures and physicochemical parameters accounting for the interaction. Each nonionic surfactant interacted with SDS; the relative strength of interaction depended considerably on the molecular structure of surfactants. In each instance the SDS - surfactant complex was less hydrophobic than the surfactant itself. The result of PCA points to the involvement of both hydrophobic and hydrophilic forces in complex formation. The data also indicate that the apolar alkyl chain of SDS is probably bound to the hydrophobic moiety of surfactants, and the polar head group interacts with the hydrophilic ethylene oxide chain of nonionic surfactants. The strong interaction between SDS and nonionic surfactants suggests that this interaction may have a marked influence on the biological efficacy of any pharmaceutical, agro-chemical formulations and cosmetics containing simultaneously both types of surfactants.