A kinetic study of the formation of beta-cyclodextrin complexes with monomolecular films of fatty acids and glycerides spread at the air/water interface

The kinetics of formation of inclusion complexes between beta-cyclodextrin and monolayers of one-, two- and three-chained lipid molecules, namely, oleic acid (OA), monoolein (MO), diolein (DO) and triolein (TO), was investigated at various pH using three independent dynamic methods. The formation and solubilization of soluble inclusion beta-CD/OA and beta-CD/MO complexes was detected by measuring the decrease of the surface area and surface pressure of the OA and MO monolayers in the presence of beta-CD within a wide range of concentrations. A third approach, describing the dilatational properties of the monolayers, influenced by the formation and solubilization of the complexes, was developed. Using the three above-mentioned independent methods, the rate constants of formation (k1) and dissociation (k2) of beta-CD/OA and beta-CD/MO, were determined. We observed that solubilization flux i s for OA monolayer increases with pH and at pH 11 reached a value, which is closed to the diffusion flux iD and the process thus becomes diffusion controlled. For MO monolayer no significant effects of pH was observed above pH 6. The surface pressure (Deltapi)--area per molecule (A) and surface potential (DeltaV)--area per molecule (A) isotherms and rheological properties of DO and TO monolayers were measured in the presence or absence of beta-CD. DO and TO form water-insoluble complexes with beta-CD, as visualized by AFM images.