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 zwitte rionic lipid DMPC and the charged lipid DMPG were investigated. The film th ickness and the contact angle between the film and the meniscus were measur ed. The free energy of interaction was determined from the contact angle, A n increase in the film thickness with increasing content of DMPG in the lip id mixture at constant ionic strength and with decreasing concentration of sodium chloride at constant DMPC/DMPG ratio indicates an influence of the e lectrical double layer interaction in the system. The dependence of the fil m interaction energy on the content of DMPG or salt in the film forming lip id suspensions confirms this conclusion. The double layer potential phi of films made from mixture with constant DMPC/DMPG ratio was estimated, It res ults that phi in the case of the mixture increases with increasing concentr ation 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 m easured film thicknesses are smaller than those estimated from a model whic h includes the electrical double layer interaction and the dispersion inter action. This indicates an additional unknown attractive contribution to the total disjoining pressure.