Dynamic surface excesses of fluorocarbon surfactants

Dynamic adsorption and surface tension behavior of aqueous fluoro-surfactant solutions has been investigated using a range of experimental techniques, including neutron reflection (NR). Equilibrium tensions, gamma(eq), have been measured by drop volume tensiometry (DVT), and dynamic surface tensions, gamma(dyn), have been determined using the nonperturbative method, surface light scattering (SLS). Dynamic conditions for NR and SLS measurements were established using an overflowing cylinder (OFC). The OFC provides a dynamic surface on the 0.1-1 s time scale and offers a large (similar to50 cm(2)), near flat surface for interrogating interfacial properties. To exploit these techniques effectively, a fluorocarbon anionic surfactant, sodium bis(lH, 1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) has been specifically selected. Molecular structure effects have been explored with the C6 analogue sodium bis(l-H, 1H,7H-dodecafluoro-n-heptyl) sulfosuccinate (di-HCF6). Using OFC-NR, dynamic surface excesses, Gamma(dyn), have been measured directly, and these values have been compared to equilibrium coverages, Gamma(eq), determined by DVT and NR. Close to the critical micelle concentration (cmc) of di-CF4 (1.58 mmol dm(-3)), Gamma(dyn) and Gamma(eq) are very similar, and the ratio phi = Gamma(dyn)/Gamma(eq) is unity to within the precision of the experiment. For moderate differences in surface tension, up to Deltagamma = gamma(dyn) - gamma(eq) less than or equal to 15 mN m(-1), phi remains close to 1. At concentrations of 0.2-0.7 mmol dm(-3), the dynamic surface excess Gamma(dyn) is measurably different from the equilibrium value Gamma(eq) (phi < 1). This concentration range coincides with the largest differences in surface tension, Deltagamma. For both di-CF4 and di-HCF6, the maximum values of Deltagamma and DeltaGamma occur around the same bulk concentration, similar to0.7 mmol dm(-3).suggesting that dynamic surface behavior is determined mainly by mass transport (which is related to the bulk concentration), rather than surfactant properties such as cmc, for these surfactants.