Quantification of the layer of hydration of a supported lipid bilayer.

Dual polarization interferometry (DPI) and quartz-crystal microgravimetry (QCM-D) were used to investigate the adsorption of DOPC vesicles to a solid hydrophilic surface. The layer of hydration formed between a self-assembled DOPC bilayer and a silica solid support was probed in assemblies constructed using H(2)O and D(2)O buffers. We used QCM-D to measure the mass of the bilayer, including the mass contribution of the coupled solvent that resides between the membrane-solid interface. The mass of only the DOPC in the bilayer was resolved using DPI. By comparing these two measurements, and also accounting for the bulk phase effects on mass, we have been able to determine the mass of water below the bilayer. The thickness of this hydration layer, calculated by relating its mass to the density of the layer, was determined to be 10.46 A +/- 0.15 A for trapped D(2)O and 10.21 A +/- 0.40 A for trapped H(2)O, in agreement with measurements obtained by other methods. This work establishes the feasibility of concurrently using DPI and QCM-D to gauge the extent of hydration in thin films.