Influence of ethanol on the thickness and free energy of film formation ofDMPC foam films

Foam films prepared from 1,2-dimirystoil-sn-glycero-3-phosphorylcholine (DM PC) dispersions in water-ethanol mixtures were investigated. Their thicknes s and contact angle (foam film/meniscus) were measured. Experimental result s show that an increase of EtOH concentration in the him forming dispersion s leads to a decrease in the film thickness. At EtOH concentrations above 4 0% v/v the foam films have a bilayer structure without any noticeable core of solvent. The film thickness remains constant with the further increasing of the EtOH concentration up until 50% v/v. This behaviour is corroborated by the strong increase in the contact angles (a decrease in the free energ y of him formation) with increasing EtOH concentration. Ellipsometric measu rements on the thickness of adsorbed DMPC monolayers and surface pressure i sotherms of DMPC spread on the water-ethanol subphases show that the effect ive area per lipid molecule decreases, resulting in a larger monolayer thic kness, when the EtOH concentration in the subphase is increased. An increas e of the EtOH concentration leads to a dehydration of the DMPC molecules an d a reduction in strength and range of the repulsive hydration force betwee n the film monolayers. The film thickness and the free energy of film forma tion are governed by the balance of the van der Waals attraction and the re pulsive hydration force between the foam him surfaces. (C) 1999 Elsevier Sc ience B.V. All rights reserved.