Adsorption kinetics in binary surfactant mixtures studied with external reflection FTIR spectroscopy

Abstract

The adsorption kinetics of mixtures of soluble surfactants at the air-water interface was studied on an overflowing cylinder. Vibrational spectra of the adsorbed monolayers were acquired by external reflection Fourier transform infrared spectroscopy, and target factor analysis was used to determine the compositions of mixed monolayers. Laser Doppler velocimetry was employed to measure the surface expansion rates, and hence the effective surface age, which was in the range 0.1-1 s. Three surfactant mixtures with different interactions were investigated. Blends of the cationic hydrocarbon surfactant hexadecyl trimethylammonium bromide (CTAB) and the nonionic hydrocarbon surfactant octaethylene glycol monodecyl ether (C10E8) were found to mix ideally in the monolayer over a wide range of subsurface compositions. Combinations of C10E8 and the anionic fluorosurfactant ammonium perfluorononanoate (APFN) exhibited nonideal mixing that could be described with regular solution theory. APFN adsorbed to the interface at a rate limited by monomer diffusion but the adsorption of C10E8 appeared to be under kinetic control. The adsorption behavior of the cationic-anionic mixture of CTAB and APFN was dominated by the interaction of the oppositely charged headgroups. Either side of equimolar bulk composition only the species in excess was found to adsorb, which is rationalized by the presence of aggregates in the bulk that act as a sink for free monomer. Prior evidence in the literature suggests these aggregates may be vesicles. At equimolar compositions both species were found to coadsorb; this unexpected result may be explained by adsorption of vesicles to the uncharged interface. A semiquantitative model based on the interaction between the electrical double layers of the oppositely charged monolayer and vesicle explains the absence of adsorption of vesicles away from equimolar compositions. The combination of the overflowing cylinder, to generate a steadily expanding surface with well-defined hydrodynamics, and FTIR spectroscopy, to quantify the composition of the adsorbed monolayer, can be used to study a broad range of mixed monolayers at the air-water interface under nonequilibrium conditions.