Phospholipid monolayers at non-polar oil/water interfaces. Part 3.—Effect of chain length on phase transitions in saturated di-acyl lecithins at the n-heptane/aqueous sodium chloride interface

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Abstract
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.

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