| Abstract
| - Dynamic adsorption and surface tension behavior of aqueous fluoro-surfactant solutions has beeninvestigated using a range of experimental techniques, including neutron reflection (NR). Equilibriumtensions, γeq, have been measured by drop volume tensiometry (DVT), and dynamic surface tensions, γdyn,have been determined using the nonperturbative method, surface light scattering (SLS). Dynamic conditionsfor NR and SLS measurements were established using an overflowing cylinder (OFC). The OFC providesa dynamic surface on the 0.1−1 s time scale and offers a large (∼50 cm2), near flat surface for interrogatinginterfacial properties. To exploit these techniques effectively, a fluorocarbon anionic surfactant, sodiumbis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) has been specifically selected. Molecular structureeffects have been explored with the C6 analogue sodium bis(1H,1H,7H-dodecafluoro-n-heptyl) sulfosuccinate(di-HCF6). Using OFC−NR, dynamic surface excesses, Γdyn, have been measured directly, and these valueshave been compared to equilibrium coverages, Γeq, determined by DVT and NR. Close to the critical micelleconcentration (cmc) of di-CF4 (1.58 mmol dm-3), Γdyn and Γeq are very similar, and the ratio φ = Γdyn/Γeqis unity to within the precision of the experiment. For moderate differences in surface tension, up to Δγ= γdyn − γeq ≤ 15 mN m-1, φ remains close to 1. At concentrations of 0.2−0.7 mmol dm-3, the dynamicsurface excess Γdyn is measurably different from the equilibrium value Γeq (φ < 1). This concentration rangecoincides with the largest differences in surface tension, Δγ. For both di-CF4 and di-HCF6, the maximumvalues of Δγ and ΔΓ occur around the same bulk concentration, ∼0.7 mmol dm-3, suggesting that dynamicsurface behavior is determined mainly by mass transport (which is related to the bulk concentration),rather than surfactant properties such as cmc, for these surfactants.
|
