Effect of the charged lipid DMPG on the thickness and contact angle of foam films

The interactions in foam films stabilized by mixed monolayers of the zwitterionic lipid DMPC and the charged lipid DMPG were investigated. The film thickness and the contact angle between the film and the meniscus were measured. The free energy of interaction was determined from the contact angle, An increase in the film thickness with increasing content of DMPG in the lipid mixture at constant ionic strength and with decreasing concentration of sodium chloride at constant DMPC/DMPG ratio indicates an influence of the electrical double layer interaction in the system. The dependence of the film interaction energy on the content of DMPG or salt in the film forming lipid suspensions confirms this conclusion. The double layer potential phi of films made from mixture with constant DMPC/DMPG ratio was estimated, It results that phi in the case of the mixture increases with increasing concentration of NaCl, This confirms the results of Grigoriev et al. (J. Phys. Chem. B 1999, 103, 1013) about an increasing dissociation of DMPG in a surface monolayer on increasing concentration of sodium ions in the subphase. The measured film thicknesses are smaller than those estimated from a model which includes the electrical double layer interaction and the dispersion interaction. This indicates an additional unknown attractive contribution to the total disjoining pressure.