DESORPTION OF FATTY-ACIDS FROM MONOLAYERS AT THE AIR WATER INTERFACE TO BETA-CYCLODEXTRIN IN THE SUBPHASE/

In this study the cyclodextrin (CD)-induced desorption of fatty acids from monolayers at the air/water interface was examined. Desorption ra tes were determined while maintaining the monolayer at constant surfac e pressure. Of the three fatty acids examined (tetra-, penta-, and hex adecanoic acids), tetra- and pentadecanoic acids displayed a liquid-ex panded (LE) to liquid-condensed (LC) phase transition at ambient tempe rature. We examined the effect of the phase transition on desorption r ates with pentadecanoic acid monolayers. The cyclodextrin-induced deso rption of pentadecanoic acid was a linear function of time (zero-order kinetics) when the monolayer surface pressure was kept constant durin g the course of the experiments, With different fatty acid monolayers, the desorption rates (with the monolayers in the LC state) were obser ved to decrease as the fatty acid chain length increased (from 14 to 1 6 carbons). The desorption rates with pentadecanoic acid monolayers in creased curvilinearly with increasing temperature (the monolayer was i n the LC state at all temperatures examined; between 9 and 29 degrees C). To studs the effect of the LE --> LC phase transition on desorptio n kinetics, pentadecanoic acid monolayers were prepared at 30 degrees C, At this temperature, the LE --> LC transition region (plateau) was between 13 and 17 mN/m. The desorption rate versus surface pressure fu nction increased linearly with increasing surface pressure throughout the LE --> LC transition region. At pressures above 20 mN/m (when the pentadecanoic acid monolayer was altogether in the LC state), however, the rate of desorption appeared to increase faster as the lateral sur face pressure increased.