Adsorption dynamics with interfacial reaction
Faculty of Sciences. Chemistry
Revue roumaine de chimie. - Bucarest
, p. 539-553
University of Antwerp
A quantitative theoretical model is developed for the surfactant adsorption from oil solution to the oil/water interface in a time scale from seconds to hours. The kinetic model takes into consideration the diffusion of surfactant molecules in the oil phase to the oil/water interface and the surface transformation between two adsorption conformations namely from conformation 1 to conformation 2 of surfactant molecules. Conformation 1 is supposed soluble in the oil phase. but conformation 2 is assumed to be insoluble in oil. Further, this model is based on the diffusion penetration theory associated with a reversible surface reaction of the first order that describes the surface transformation from conformation 1 to conformation 2. Both conformations are assumed to be insoluble in the aqueous phase. A general analytical expression is derived for the relaxation of interfacial tensions that is convenient for both. numerical calculations and for asymptotic analysis. At short and long diffusion relaxation times the extrapolated equations are deduced. To illustrate the applicability of-this model, the obtained equations are applied to the dynamic interfacial tensions generated by two carotenoids (i.e., ethyl ester of beta-apo-8'-carotenoic acid and beta-carotene, all trans isomers), that independently adsorb from the hexane phase to the hexane/water interface. The model calculations show the characteristic times for the carotenoid adsorption in terms of diffusion relaxation and kinetic relaxation times at the hexane/water interface. The surface reaction rate constants yield to mean values of 8.2 10(-4) s(-1) for ethyl ester of beta-apo8'-carotenoic acid and of 1.3 10(-3) s(-1) for beta-carotene. This model describes very well the experimental interfacial tension isotherms for the two biosurfactants studied. The calculated results are in good agreement with the molecular structure of the two carotenoids adsorbed at the hexane/water interface.