Order in phospholipid Langmuir-Blodgett layers and the effect of the electrical potential of the substrate

The ordering in dipalmitoylphosphatidylcholine (DPPC) Langmuir-Blodgett monolayers and bilayers on a semiconducting indium tin oxide (ITO) surface has been investigated at the equilibrium potential of the interface and at various externally applied potentials. Second- and fourth-rank order parameters of a diphenylhexatriene (DPH) containing phospholipid probe were derived from total internal reflection fluorescence measurements, and orientation distributions were calculated using the maximum-entropy method. Generally, bimodal orientation distributions were obtained, suggesting that only part of the probes is aligned with the DPPC molecules. The effect of applied potentials is small for DPPC layers on unmodified (hydrophilic) ITO; with decreasing potential the ordering changes slightly to more random distributions, possibly because of the onset of hydrogen evolution at the substrate surface. For monolayers on hydrophobized ITO, where the phospholipids are initially with their tails directed toward the surface, the changes are more significant. At the highest positive potential applied, the derived order parameters indicate that nearly all probes are flat on the surface. This can be understood as a result of enhanced competition between headgroups and tails for access to the surface as it becomes more polarized. On unmodified ITO the electrochemistry of Fe(CN)6(3-/4-) and Ru(bipyridyl)3(2+/3+) is hardly hindered by the presence of DPPC monolayers or bilayers. On hydrophobized ITO a DPPC monolayer enhances the redox reactions.