Fluorinated nonionic surfactants bearing either CF3- or H-CF2- terminal groups: Adsorption at the surface of aqueous solutions

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-CF2- to CF3- reduced the cmc by a factor of 4, lowered the limiting molecular areas αcmc by ∼ 10 Å2, 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.