ON THE SIGN AND ORIGIN OF THE SURFACE-POTENTIAL OF PHOSPHOLIPID MONOLAYERS

A method for the measurement of monolayer surface potential is described, where the interracial potential change is directly sensed by the reference electrode itself; not derived from the trough electrode, as in the conventional methods. In this way, a negative surface potential is observed for phosphatidylcholine (PC) monolayers. In general, the new surface potential curves show two distinct features: (1) An initial sudden change with the monolayer gaseous-liquid expanded (G-LE) phase transition. This electrical potential appears to be a characteristic of the lipid polar group and is -350 mV for various PC films. (2) A gradual surface potential change which is a function of both the physical state and compressibility of the film. This potential can reach a maximum value of -650 mV depending on the fatty acid composition of PC monolayers. The relevance of the results to the origin of electrical potential of phospholipids is discussed.