Four nonionic fluoro-surfactants have been synthesized and their surface adsorption and micellization properties investigated. The compounds were perfluoroalkyl triethyleneoxide methyl ethers X-(CF2)m-CH2-O-(C2H4O)3-CH3, with X either H or F, and m either 4 or 6 (H4EO3, F4EO3, H6EO3, and F6EO3). A strong structure-activity relationship was observed, depending on the nature of the hydrophobic chain. Initial surface tension measurements, using long-time dynamic drop volume tensiometry (DVT), were consistent with trace hydrophobic impurities, which could be eliminated by vacuum distillation to yield surface chemically pure surfactants. Neutron reflectivity (NR) measurements were performed to determine surface excess as a function of bulk concentration, and there was good agreement with tensiometrically derived coverages. Switching the terminal group H-CF2to CF3-reduced the cmc by a factor of 4, lowered the limiting molecular areas acmc by ∼ 10 Å 2 , and reduced the cmc surface tension γcmc by 9 mN m -1 . Increasing the chain length also gave rise to significant changes. Therefore, surfactants of this kind are unusual in that physicochemical properties can be controlled over a wide range, but with only minor variations in chemical structure. Furthermore, it is shown that with high purity surfactants tensiometric measurements are able to distinguish variations in surface coverage which arise from such subtle structural changes.

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Advances in techniques for investigating dynamic adsorption of surfactants at the air± water interface are discussed. Of particular interest are the maximumbubble-pressure method, and the interrogation of adsorption layers on an overowing cylinder (OFC) cell using ellipsometry, surface-light scattering, laser Doppler velocimetry and neutron re¯ection. Recent studies described here, with a model di-chain anionic¯uorocarbon surfactant sodium bis(1H,1H-nona¯uoro-n-pentyl) sulphosuccinate (di-CF4) demonstrate that the OFC presents an ideal exible platform for investigating dynamics of adsorption from solutions. Under certain conditions the behaviour is consistent with an activated-diOE usion mechanism, as has been found previously for simple alcohols and various other non-ionic and ionic 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, 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 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 (∼50 cm 2 ), near flat surface for interrogating interfacial properties. To exploit these techniques effectively, a fluorocarbon anionic surfactant, sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) has been specifically selected. Molecular structure effects have been explored with the C6 analogue sodium bis(1H,1H,7H-dodecafluoro-n-heptyl) sulfosuccinate (di-HCF6). Using OFC-NR, dynamic surface excesses, Γ dyn, have been measured directly, and these values have been compared to equilibrium coverages, Γeq, determined by DVT and NR. Close to the critical micelle concentration (cmc) of di-CF4 (1.58 mmol dm -3 ), Γdyn and Γeq are very similar, and the ratio φ ) Γdyn/Γeq is unity to within the precision of the experiment. For moderate differences in surface tension, up to ∆γ ) γdynγeq e 15 mN m -1 , φ remains close to 1. At concentrations of 0.2-0.7 mmol dm -3 , the dynamic surface excess Γdyn is measurably different from the equilibrium value Γeq (φ < 1). This concentration range coincides with the largest differences in surface tension, ∆γ. For both di-CF4 and di-HCF6, the maximum values of ∆γ and ∆Γ occur around the same bulk concentration, ∼0.7 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.