Chain-melting phase transition in a lamellar film of dimyristoyl-phosphatidylserine on the surface of a silica hydrosol

Structural dynamics of multilayer of dimyristoyl-phosphatidylserine formed on the surface of silica sol with 5 nm nanoparticles size has been investigated by X-ray reflectometry and grazing-incidence diffraction at 71 keV photon energy. Combined model-based and modelless analysis of reflectometry data revealed the structure consisting of a surface monolayer and a stack of lamellar bilayers sandwiched between water layers, with a spatial period of ~ 150 Å. With increase in temperature above the chain-melting point the surface monolayer is observed to transition from a surface crystal phase with minimal area-per-lipid value of (40 ± 1) Å2 to a disordered liquid phase with estimated area-per-lipid value of (52 ± 2) Å2. Under low temperatures both monolayer and bilayer slabs contain 5 to 8 H2O molecules bound to lipid PS-fragment; however, above the melting point the amount of contained water rises to about 14 molecules per bilayer headgroup.