Surface-energy dependent spreading of lipid monolayers and bilayers

Abstract

Spontaneous spreading of phospholipids following hydration by water is directly compared on hydrophilic and hydrophobic surfaces using planar, composite substrates exhibiting binary patterns of surface energies. The use of patterned substrates—in conjunction with real-time, quantitative applications of imaging ellipsometry and epifluorescence microscopy—affords a side-by-side comparison of spreading kinetics and equilibrium morphologies following the hydration of a single parent-lipid source. We find that for fluid phospholipids (T > T_m), substantially contiguous bimolecular and mono-molecular lipid layers spread away from a source on hydrophilic and hydrophobic surfaces, respectively. For bilayers and monolayers, the advancing lipid sheets exhibit square-root-of-time dependent kinetics reflecting a balance between a spreading force and a resistive drag. Furthermore, monolayers advance at only a slightly faster rate (by a factor of 1.7 ± 0.2) than bilayers despite a substantially higher available spreading energy. This apparently counter-intuitive observation can be reconciled in terms of a corresponding rise in the competing drag energy—consistent with the differences in slip-planes associated with mono- and bilayer spreading.