THICKNESS TRANSITIONS IN LYSOLECITHIN FOAM FILMS

Microscopic foam films have been obtained from aqueous palmitoyl lysophosphatidylcholine (lyso PC) solutions and the effect of the monovalent Na+ and divalent Ca2+ on their equilibrium thickness has been studied. At low NaCl concentrations. silver-coloured films have been obtained. A thickness transition zone is found with increase in NaCl concentration, leading to the formation of Newtonian black films (NBF). These results are compared with previous results obtained for microscopic films of non-ionic surfactant solutions. A remarkable effect of divalent Ca2+ on the properties of the films is found. In the interval 0.0001-0.0008 mol dm-3 CaCl2, NBF are obtained. When the electrolyte concentration (C(el)) is increased to 0.001 mol dm-3 CaCl2 a transition from NBF to silver films is observed. The thickness of these films decreases with further increase in C(el) until transitions to common black films, and subsequently again to NBF, are observed. This result is interpreted as due to specific interaction of Ca2+ ions with the intrinsically uncharged phospholipid head groups. In the case of lyso PC foam films with CaCl2 added, direct measurements of the disjoining pressure vs thickness (PI (h)) isotherms have been performed and a barrier-like transition to NBF is found. The results are discussed in connection with the predictions of the DLVO theory. In the case of NaCl, the diffuse electric layer potential (phi-0 and charge density (sigma-0) are low, similar to the results obtained for non-ionic surfactant films. In the case of CaCl2, an increase in C(el) leads to an increase in phi-0 and sigma-0, which is interpreted as due to increased divalent ion binding. The comparison between the experimental PI (h) isotherms and the theoretical predictions shows that in the cases of low phi-0 and sigma-0 studied, at constant C(el) electrostatic repulsion under the conditions of constant charge is operative. The results are considered in connection with the possibilities of using the microscopic foam film model for biological studies.