Interactions between amphotericin-B and sterols in monolayers. Mixed filmsof amphotericin B-cholesterol

pi-A isotherms of mixed monolayers composed of cholesterol and amphotericin B (AmB) spread on aqueous buffers of various pH and temperatures show the existence of interactions between the two components, which are more pronou nced when the mole fraction of AmB is 0.7. As a consequence of the interact ions, the excess areas and excess free energies of mixing are negative at l ow surface pressures and positive at high surface pressures. These results suggest that negative deviations of the additivity rule are due to the form ation of a hydrogen-bonded AmB-cholesterol complex in which AmB molecules a re oriented horizontally at the interface and cholesterol molecules lie ver tically. The positive excess areas of mixing at high surface pressures coul d be due to AmB being less desorbed in the substrate by composition-depende nt van der Waals interactions between the apolar moieties of the components , bath oriented in this situation in a vertical position at the A/W interfa ce.