Search

Following a short historical survey of the field of insoluble lipid monolayers, the relevance of such surface studies to membrane structure and behavior is briefly discussed. Some of the main findings on phospholipid monolayers at both air—water and oil—water interfaces are then described with particular emphasis on the two-dimensional phase transitions found at high monolayer densities. Finally new surface pressure and potential data for phosphatidyl ethanolamine monolayers spread over a wide range of areas at the oil—water interface are presented and discussed in molecular terms.

Contact angles have been measured on smooth polyethylene, polytetrafluoroethylene, nylon, and polyester plates as a function of the surface pressure of insoluble monolayers at an air/salt solution interface. In all cases, surface tensions that gave zero contact angle at the solid were much lower than critical surface tensions established previously for the same solids using surfactant solutions or pure liquids. An analysis of the results shows transfer of monolayer to the more hydrophobic solid surfaces under conditions depending on the nature of the monolayer.

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.

Surface pressure against area isotherms for dilute monolayers of a homologous series of 1,2-diacyl-lecithins spread at the oil/water interface are presented as a function of temperature and salt concentration. Comparative results for di-C14-cephalin monolayers are used to distinguish head-group effects. Over the whole range of areas studied the isotherms are independent of chain length and salt concentration but vary with head-group, with lecithins giving the higher pressures. From conventional Amagat-type plots the monolayers are non-ideal. Apparent co-areas for both lecithins and cephalins are large with the lecithins showing a marked positive temperature coefficient. Negative entropies of compression calculated from the lecithin data are larger than those for single chain ionised molecules spread at the oil/water interface. These phospholipids show little evidence of cluster formation at areas higher than the phase transitions reported in part 1, but undergo complex configurational changes on compression which depend on medium range dipolar interactions and steric interaction of the chains.

Isotherms of surface pressure (π) against area per molecule (A) are reported for a homologous series of pure synthetic saturated 1,2-di-acyl glycerophosphocholines (lecithins)(C14 to C22) spread at n-heptane/aqueous electrolyte interfaces. The lecithins show second-order phase transitions which for a given temperature move to higher π and lower A as the chain length is decreased until for di-C12 lecithin no phase transition can be distinguished.The calculations of Clapeyron heats in Part 1 are extended to the new data using a single value of the reference area for the solid state for all temperatures and chain lengths. These heats vary with chain length and decrease linearly with temperature. The high heat capacity calculated in Part 1 for di-C18 lecithin at low temperatures is shown to be an artefact. Entropies of compression have been calculated from the free energies obtained by integrating the full isotherms between a reference area in the expanded region above any phase transition and another area in the solid-condensed region. These entropies vary linearly with chain length, having a slope given by the configurational entropy term R ln 3 per methylene group, where R is the gas constant. The entropy change for each phospholipid is approximately 2(n–1)R ln 3, where n is the chain length. This finding, taken together with a comparison of the heats of the monolayer phase transitions with the calorimetric heats of melting of phospholipid chains, and with related published data on the heats of fusion and the heats of solution of the alkanes, suggests that the chains of the phospholipids are fully flexible at low monolayer densities and very restricted on the condensed side of the phase transitions.

Surface pressure against area isotherms for phospholipid monolayers at the oil/water interface display second-order phase transitions which resemble the ones between “condensed” and “expanded” monolayers at air/water interfaces. Behaviour at lower surface densities indicates a strong repulsive interaction between the phospholipid molecules at the oil/water interface. A statistical mechanical model is developed in which the monolayer molecules occupy sites on a two-dimensional lattice and adopt one of two possible orientational states. By appropriate choice of the interaction energies of the molecules in these states and the use of order–disorder statistics the features characteristic of a second-order phase transition are obtained for isotherms at both interfaces. From the point of view of our present theory what is termed the “expanded” state is in fact a fluid phase whose degree of order is lower than that of the “condensed” state. The choice of a large overall attractive interaction energy between nearest-neighbours yields the liquid–vapour phase transition which has been observed in experiments at the air/water interface whereas with an overall repulsive interaction the liquid–vapour transition is absent, as in the oil/water case.