Ionic properties of phospholipids at the oil/water interface

Abstract

Surface pressures and surface viscosities of dipalmitoylphosphatidylethanolamine and its polar group analogues were measured at the hexane/water interface as a function of bulk pH. The monolayers expanded as the bulk pH was shifted from neutral to alkaline and gave rise to an increase in surface pressure at a constant area, and the surface viscosity was simultaneously reduced at high pH. For the increase in surface pressure, theoretical values were calculated using the Gouy-Chapman equation for the electrical double layers produced by shifting pH, and good agreements were obtained with the measured ones from which a simple mechanism was deduced for the increase in pressure. The ionic dissociation characteristics of amino groups of the lipids were discussed taking p K a values given in the above calculations into account. The reduction of the surface viscosity was thought to be attributable to disintegration of zwitter ionic structure in the condensed monolayers.