The state of fluorocarbon surfactant monolayers at the air-water interface and on mica surfaces

Abstract

Monolayers of a cationic, double-chained fluorocarbon surfactant (N-(α-trimethylammonioacetyl)-O-O′-bis(1H, 1H, 2H, 2H-perfluorodecyl)- l- glutamate chloride) were studied at the air-water interface and adsorbed on the muscovite mica basal plane. The stability of the monolayer at the air-water interface was considerably higher on a 10 −2 M KBr subphase than on pure water. Fluorocarbon surfactant-coated mica surfaces were obtained either by Langmuir-Blodgett (LB) deposition or by adsorption from a polar solvent. The adsorbed layer was characterized by means of ESCA, transfer ratio and contact angle measurements. A more densely packed layer could be obtained by LB deposition than by adsorption from solution. For this surfactant the preferential conformation of the molecule in layers adsorbed from solution is different from that in LB layers. At low adsorption densities the advancing contact angle increases rapidly with the adsorbed amount. The receding angle is hardly affected until the area per molecule is less than about 140 Å 2. At higher adsorption densities, the receding contact angle is more sensitive to the adsorbed amount than the advancing angle. The reason for the low salt stability, the low adhesion force in water, and the numerous cavities forming around the contact region when surfaces coated by this fluorocarbon surfactant are brought together in water is proposed to be due to (molecular size) heterogeneities in the surfactant layer.