Surface activity of cationic surfactants, influence of molecular structure

An interfacial behavior of aqueous solutions of cationic surfactants—dodecyl trialkyl ammonium bromides, di(do)decyldimethyl-ammonium bromides and gemini cationic surfactants having two dodecyldimethylammonium groups connected at the level of head-groups by a (–CH2–)n spacer group were investigated. The equilibrium surface tension isotherms of investigated ammonium salts solutions were measured by a pendant drop shape analysis method. The experimental findings were described by means of the “surface quasi-two-dimensional electrolyte” (STDE) model, which takes into account adsorption of multivalent surfactant ions and counterions within the Stern layer. It was found, according to the expectations, that monovalent cationic surfactants having two hydrophobic tails are several orders of magnitude more surface active than their counterparts with single hydrocarbon chain. The surface activity of gemini surfactants is lower than ones with single head-group and double hydrophobic chain of the same length due to the electrostatic effect of divalent charge. Furthermore the extension of the head-group in dodecyl trialkyl ammonium bromides by hydrophobic side chains, i.e. replacement of methyl by ethyl, propyl and butyl increases the surface activity to much smaller extent than the addition of CH2 groups to the hydrophobic chain.