In mixed solutions of anionic and cationic surfactants, called catanionics, ion pairs are
formed which behave like non-ionic surfactants with a much higher surface activity than the single
components. In equimolar mixtures of NaCnSO4 and CmTAB, all surface-active ions are paired.
For mixtures with n + m = const, the interfacial properties are rather similar. Catanionics containing
one long-chain surfactant and one surfactant with medium chain length exhibit a strong increase in
surface activity as compared with the single compounds. In contrast, catanionics of one mediumand one short chain surfactant have a surface activity similar to that of the medium-chain surfactant
alone. Both the Frumkin model and the reorientation model describe the experimental equilibrium
data equally well, while the adsorption kinetics of the mixed medium- and short-chain surfactants
can be well described only with the reorientation model
