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 ad sorption and micellization properties investigated. The compounds were perf luoroalkyl 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 na ture 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 t o yield surface chemically pure surfactants. Neutron reflectivity(NR) measu rements were performed to determine surface excess as a function of bulk co ncentration, and there was good agreement with tensiometrically derived cov erages. Switching the terminal group H-CF2- to CF3- reduced the cmc by a fa ctor of 4, lowered the limiting molecular areas alpha (cmc) by similar to 1 0 Angstrom (2), and reduced the emc surface tension gamma (cmc) by 9 mN m(- 1). Increasing the chain length also gave rise to significant changes. Ther efore, surfactants of this kind are unusual in that physicochemical propert ies can be controlled over a wide range, but with only minor variations in chemical structure. Furthermore, it is shown that with high purity surfacta nts tensiometric measurements are able to distinguish variations in surface coverage which arise from such subtle structural changes.