Hysteresis in compression-expansion cycles of distearoylmonogalactosylglycerol monolayers

Abstract

Surface pressure (π) versus molecular area ( A) isotherms of distearoylmonogalactosylglycerol spread at the air-water interface have been examined in compression and expansion cycles. The high pressure portion ( π > Mn/m) of the isotherm in films compressed to just below the collapse pressure is identical in compression and expansion modes, but marked and reproducible hysteresis in pressure-area relationship is found in the region of low molecular densities ( A > 0.45 ( nm( 2/ molecule ). Compression of monolayers above the collapse pressure (49 mN/m) results in the creation of bulk phase lipid domains which reorient at the interface on subsequent expansion of the film leading to the creation of a high pressure hysteresis loop, and in the low molecular density region the same hysteretic behaviour is observed as above. An analysis of the π— A isotherms of the monolayers, according to the Ehrenfest theory of transition order indicates a discontinuity in the relationship between the surface compressibility and surface pressure at a pressure of 11.5 mN/m. This is interpreted, at a macroscopic level, as a second-order transition between liquid-expanded and liquid-condensed states. On the basis of molecular models and surface characteristics derived from compression and expansion isotherms, the molecular arrangements in the monolayer are related to the conformation of saturated monogalactosyldiacylglycerol formed in aqueous dispersions, obtained by X-ray diffraction.