Equilibrium and dynamic surface tension properties of salt-free catanionic surfactants with different hydrocarbon chain lengths

The equilibrium and dynamic surface tension properties of a series of 1:1 salt-free catanionic surfactants (alkyltrimethylammonium decanoate) were investigated; and, the influence of the length of hydrocarbon chain of cationic part (carbon number of 10, 12, 14, 16, 18, respectively) on the surface properties was discussed. The plot of the logarithm of the critical aggregation concentration (cac) against the carbon number showed a linear decrease with an increase in chain length. On the basis of the slope of this plot, it was found that the variation of cac with the chain length is smaller for salt-free catanionic surfactants (C+n-C−10) than that for single anionic or cationic surfactants. The surface tension at the cac was quite low, much lower than that of single cationic or anionic surfactants at cmc. At dilute surfactant concentration, the adsorption process is diffusion-controlled adsorption; at higher concentration (above cac), it became the mixed diffusion-kinetic mechanism. The diffusion rate of the monomer for the salt-free catanionic surfactants was lower than that of conventional cationic or anionic surfactants, due to a larger molecular size. Furthermore, it was confirmed that the kinetic energy of adsorption at the gas/solution interface decreased with an increase in the hydrocarbon chain length.