Equilibrium and dynamic surface tension of quaternary ammonium salts with different hydrocarbon chain length of counterions

Series of cationic surfactants didecyldimethylammonium carboxylate were synthesized with didecylmethylammonium, dimethyl carbonate and different carboxylic acids, also the equilibrium and dynamic surface tension of them were measured. Then the influence of the hydrocarbon chain length of anionic parts (carbon number of 2, 4, 6, 8, 10, respectively) on the surface properties was discussed. The well known Ward-Tordai equation was employed to analyze the adsorption of didecyldimethylammonium carboxylate. The parameters of dynamic surface tension and effective diffusion coefficients (Deff) were calculated and analyzed. On the basis of the experiments and dates, the plot of the logarithm of cmc against the carbon chain length for didecyldimethylammonium carboxylate show a linear decrease with an increase in the chain length. It is also found that the equilibrium surface tension parameters show some regular changes with the variation of the carbon chain length of counterions for the hydrophobicity, “shielding effect” and steric hindrance. From the conductivity, the conduction properties of didecyldimethylammonium carboxylate transform from ionic type to nonionic type when the carbon number of counterions is greater than or equal to 6. Furthermore, it is confirmed that the diffusion coefficient of adsorption at the interface increase with the carbon chain length of counterions which is different from the conventional surfactants, also, an appropriate theory that the adsorption mechanism of these surfactants is controlled by not only diffusion-controlled but also activated-diffusion can be given.