Hydronium ion sensitivity of surface stabilized stearic acid membranes prepared by Langmuir-Blodgett monolayer transfer

Alteration of the physical and electrostatic structure of fatty acid membranes may be monitored using the emission intensity of the fluorescent probe nitrobenzoxadiazole dipalmitoylphosphatidylethanolamine (NBD-PE). Stearic acid/NBD-PE monolayers were deposited onto quartz wafers by Langmuir-Blodgett dipcasting from the air-water interface. The membranes were investigated spectrofluorimetrically and by fluorescence microscopy. Modulation of the pH at the surface of these membranes resulted in alterations of the fluorescence intensity. The results are consistent with an electrostatic mechanism of perturbation of the structure of the monolayer. Alteration of local pH gradients resulted in changes of the extent of ionization and affected the structure of the hydration layer and/or the hydrogen bond network at the membrane-solution interface. Fluorescence microscopy showed that the structures present on dipcast wafers were altered drastically during titrations as a result of mechanical agitation from stirring.