Equilibrium and dynamic surface tension properties of aqueous solutions of sulfonated cationic-nonionic fluorocarbon surfactants

The equilibrium and dynamic surface tension properties in aqueous solutions of nonionic and cationic fluorinated surfactants bearing a sulfonated group are reported. Both surfactants show low critical micellar concentration and very low equilibrium surface tension values, but the cationic surfactant is more effective at decreasing surface tension, which is attributed to a small surface area per molecule. In dynamic surface tension measurements both surfactants show an induction time, which is longer for the nonionic surfactant with a higher molecular weight, slower diffusion rates, and higher molecular surface area. The dynamic behavior of surface tension is modeled by a series of exponentials from which a relaxation time for surface tension decay can be estimated. This relaxation time is faster for the cationic surfactant and decreases with both surfactant concentration and temperature. In mixtures of both surfactants, the values of the relaxation time are between those of the pure surfactants, indicating neither synergism nor antagonism.