Effects of hydrophobic chain structure on adsorption of fluorocarbon surfactants with either CF3- or H-CF2-terminal groups

Tensiometry and neutron reflection (NR) have been used to investigate the a dsorption at the air/water interface of four fluorinated surfactants. These compounds were two single-chain carboxylates-sodium perfluorononanoate (Na PFN) and sodium 9H-perfluorononanoate (HNaPFN)-and two double-chain sulfosu ccinates-sodium bis(LH, 1H-perfluoropentyl)-2-sulfosuccinate (DCF4) and sod ium bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (DHCF4). The replacemen t of a terminal fluorine for hydrogen creates a permanent dipole in the hyd rophobic chain tip which significantly affects the surfactant behavior. Thi s change in structure leads to an increase in the critical micelle concentr ation (cmc) which was 10.0, 40.0 1.6, and 16.0 mmol dm(-3), respectively.;P t their cmc's the interfacial area per molecule was 41 and 44 Angstrom(2) f or NaPFN and HNaPFN and 63 and 66 Angstrom(2) for DCF4 and DHCF4 as determi ned from NR. From tensiometry the values obtained were 43, 51, 56, and 65 A ngstrom(2), respectively. For all four compounds the surface excess measure d by tensiometry, using a prefactor of 2 in the Gibbs equation, was in reas onable agreement with that obtained from NR. Partial structure factors of t he adsorbed layers were determined for all four surfactants. The widths of the surfactant distributions were found to be significantly broadened by ro ughness at the interface. The widths of the solvent distribution were about 5 Angstrom for the carboxylates and essentially 6 Angstrom for the sulfosu ccinates. In all cases the measured separations of the water and surfactant distributions were 5 Angstrom, indicating a greater penetration of water i nto the sulfosuccinate layers. The contribution of capillary waves to the s urface roughness was examined in order to assess the static disorder of mol ecules in the adsorption layer. The presence of the chain dipole does not a ppear to cause any significant structural differences normal to the interfa ce.