Phospholipid monolayers at non-polar oil/water interfaces. Part 1.-Phase transitions in distearoly-lecithin films at the n-heptane aqueous sodium chloride interface

Using compression techniques, surface pressure (II) against area (A) isotherms are measured as a function of temperature for insoluble monolayers of 1,2-distearoyl-sn-glycero-3-phosphorylcholine spread at the n-heptane/aqueous sodium chloride interface. Problems of spreading are resolved and accurate results at moderate and high II are presented. The results show an almost first order phase change which is very temperature sensitive. Various forms of the Clapeyron analysis are applied to the data to calculate the heats of the phase change and to assess variations arising from the choice of a close-packed area. These heats vary with temperature but not with salt concentration or pH over the range studied. Results on 1,2-dioleoyl-sn-glycero-3-phosphorylcholine show no such phase changes.