PHASE-TRANSITIONS IN DIGLYCERIDE MONOLAYERS STUDIED BY COMPUTER-SIMULATIONS, PRESSURE-AREA ISOTHERMS AND X-RAY-DIFFRACTION

1,2-sn-diglyceride monolayers exhibit unique and complex phase transit ions as a function of surface pressure. The dynamical response of the layer on expanding the film has been investigated by computer simulati ons, (ii-A) isotherms and grazing-incidence X-ray diffraction. Good ag reement is found between the simulations and experiments. The Langmuir film undergoes two phase transitions occurring at 38.3 and 39.8 Angst rom(2)/molecule. The transition at low surface density is associated w ith a tilt of approximate to 14 degrees in the direction close to near est neighbour. The first transition is unique for the diglyceride mole cules and has not been observed for other amphiphilic molecules. It is driven by a competition of hydrophobic//hydrophilic and intra/intermo lecular forces and can be pictured as a ''seesaw'' mechanism. Due to t he close packing of the chains at high surface pressure, the sn-l este r group aligns with the alkyl chain, and only the sn-2 ester group is favoured to lay at the aqueous interface. Hydrophilic forces attract t he sn-l ester group, but its motion towards the aqueous subphase is hi ndered by intra- and intermolecular chain interactions. On expansion, the intermolecular interaction decreases, and at the first transition, the intramolecular interaction between the two chains is strong enoug h to cause a ''swelling'' of the molecules. The diffraction pattern de termined in the different mesophases reveals that the layer maintains its hexagonal structure up to the second phase. Concomitant with the t ilt, the structure relaxed from a hexagonal to a distorted hexagonal l attice.