Surface potentials and ion sorption of dimyristoyl-, dipalmitoyl- and distearoyl phosphatidylcholine films on NaCl and CaCl₂ solutions

Abstract

When either dimyristoyl-, dipalmitoyl-, or distearoyl phosphatidylcholine was spread on the aqueous phase, the electrical potential at collapse pressure on 75 mM CaCl₂ was about 70 mV higher than on 150 mM NaCl; the difference between Ca²⁺ and Na⁺ fell gradually to 15-25 mV at the lower pressures. When, however, the electrolyte was injected under the dipalmitoyl phosphatidylcholine film preformed on distilled water, the surface potential at any film pressure was the same on water, NaCl or CaCl₂ in the first 4 min; but, the presence of ≥ 1 mol% acidic phospholipid (dicetyl phosphate or dipalmitoyl phosphatidic acid), together with dipalmitoyl phosphatidylcholine in mixed films, caused the surface potential to increase rapidly in the order CaCl₂ > NaCl > water, by a quantity that was proportional to the concentration of the acidic lipid in the film and Na⁺ or Ca²⁺ in the aqueous phase. It is suggested that, in the absence of acidic phospholipids (contaminants), the modest rise in surface potential is due to penetration of the electrolyte's, as well as water's, ion pairs. Measurements of surface radioactivity (⁴⁵Ca²⁺) and infrared spectra of the interfacial material led to the discovery of a new species, Ca(OH)₂, at the airwater interface of CaCl₂ solutions. A direct correlation exists between adsorption of Ca²⁺ and Ca(OH)₂ formation, both of which are enhanced by the presence of dipalmitoyl phosphatidylcholine at the interface; no specific correlation, however, can be established between either surface potential and ion binding of the neutral phospholipid or surface potential and ion pair penetration, since it is not yet possible to identify the electrically significant ion pair in the ionic maze of the interfacial lattice.