ABSORPTION BEHAVIORS OF SURFACTANT MOLECULES ON A LIPID-COATED QUARTZ-CRYSTAL MICROBALANCE - AN ALTERNATIVE TO EYE-IRRITANT TESTS

The absorption behaviors of 13 kinds of surfactant molecules in a lipid matrix were studied by using a synthetic dimethyldioctadecylammonium poly(styrene-4-sulfonate) (2C18N+-2C1/PSS-) microbalance or a naturally occurring dipalmitoylphosphatidylethanolamine (DPPE) multibilayer film-coated quartz-crystal microbalance (QCM) in distilled water. The resonance frequency of the lipid-coated QCM decreased linearly with increasing the mass of substances deposited in the lipid matrix on the QCM, and the partition coefficients (P) and diffusion rate constants (D) were obtained quantitatively. There existed a good correlation between the log P values of the 13 surfactants in the lipid matrix on the QCM and the eye-irritant value (the Draize score) obtained from animal experiments. The surfactant molecule showing the higher Draize score (higher eye irritancy of rabbits) gave the larger partition to the lipid matrix. Significant correlations were not observed between the Draize score and partition coefficients in protein layers on the QCM or the hydrophilic-lipophilic balance (HLB) of surfactants. Thus, the eye irritancy could be explained by the absorption behaviors in the lipid matrix of the biological membranes, but not by the simple hydrophobicity. P and D values increased drastically at the phase transition (T(c)) from the solid state to the liquid-crystalline state of the 2C18N+2C1/PSS- multibilayer film. Surfactant molecules absorbed and penetrated deeply into the lipid matrix as a monomeric form but not an aggregated micellar form.