Abstract
The Langmuir-Blodgett technique was used for deposition of monolayer lipid films of dipalmitoyl phosphatidylcholine (DPPC) on glass with a variable surface density of lipid molecules. In a certain range of surface density these surfaces oriented homeotropically the highly polar nematic liquid crystal of 4-cyano-${4}^{\ensuremath{'}}$-$n$-pentylbiphenyl (5CB). Optical transmission curves of liquid crystal layers sandwiched between orienting lipid monolayers were recorded versus magnetic field with high accuracy. They were further converted into optical retardation and fitted by a complete theory of the transition containing anchoring strength as a parameter. Independent measurements of refractive indices of 5CB were also performed by the prism method. Anchoring energies thus obtained showed a nonmonotonic behavior with increasing packing density of DPPC monolayers and a pronounced minimum around $0.82 {\mathrm{nm}}^{2}$ per molecule. This is explained by a complete theory of surface anchoring including steric, electric, flexoelectric, and notably surface polarization coupling mechanisms. The last one stands for surface polarized layers at the liquid-crystal interfaces interacting with surface electric field due to lipid molecules. The competition of these mechanisms leads to an initial dehancement followed by an enhancement of the anchoring strength at monotonically increasing DPPC packing density. Surface polarization evaluated from the experimental data is in good correspondence with molecular parameters.
Published Version
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