Chain-Melting Phase Transition in a Lamellar Film of Dimyristoyl-Phosphatidylserine on the Surface of a Silica Hydrosol

The structure dynamics in the temperature range of the melting phase transition of a dimyristoyl-phosphatidylserine multilayer on the surface of a colloidal silica solution with a particle diameter of 5 nm has been investigated by X-ray reflectometry and grazing diffraction of 71-keV photons. The joint model and model-free analysis of the reflectometry data revealed a structure consisting of a surface lipid monolayer and a set of lamellar bilayers sandwiched between water layers, with a period of ~150 Å. With an increase in temperature above the critical value one can observe a surface monolayer transition from a crystalline phase with a minimum area per lipid molecule of 40 ± 1 Å² to a disordered (liquid) phase with a calculated area per molecule of 52 ± 2 Å². At low temperatures, the data indicate that from five to eight H₂O molecules are tightly bound to the PS fragment of the lipid in both the monolayer and the bilayer structures. However, above the transition temperature, approximately 14 water molecules are attached to the headgroups of the bilayer: this is almost twice as many molecules as the eight H₂O molecules-per-headgroup in the surface monolayer.