ELECTROSURFACE PROPERTIES OF LYSOPHOSPHATIDYLCHOLINE FOAM FILMS - EFFECT OF PH AND CA2+

The electrosurface properties of microscopic foam films stabilized with the zwitterionic soluble phospholipid palmitoyl-glycero-3-phosphocholine (lyso PC) are studied by measuring the effect of pH on the equilibrium film thickness at a constant ionic strength (NaCl + HCl) of the solutions for different lyso PC concentrations. It is found that a decrease in the pH has a strong effect on the thickness of the films. At low lyso PC concentration, an isolectric point (pH*) corresponding to pH 3.6 is found at which all the films rupture. In the case of higher lyso PC concentration, a critical pH (pH(cr)) value of 4.1 is found at which a silver-to-Newtonian black film transition occurs. In both cases, metastable zones are outlined where in the first case occasional rupture of the films is observed, while in the second case, Newtonian black films are formed. Using the equations of the Derjaguin-Landau-Verwev-Overbeek (DLVO) theory, the diffuse electric layer potential and surface charge density are computed. The results obtained corroborate the assumption that the surface charge and potential at the air/film interfaces originate from the preferential adsorption of OH- ions. Experimental data obtained with CaCl2 added to the lyso PC solutions are in accordance with this mechanism of the formation of electrostatic interactions in the case of films stabilized with uncharged substances. It is shown that the specific adsorption of the positive Ca'' ions leads, at low CaCl2 concentrations, to a decrease in the initially negative surface charge and the diffuse electric layer potential, and to a recharging of the film interfaces at higher CaCl2 concentrations.