Measurements of anchoring energy of a nematic liquid crystal, 4-cyano-4 '-n-pentylbiphenyl, on Langmuir-Blodgett films of dipalmitoyl phosphatidylcholine

The Langmuir-Blodgett technique was used for deposition of monolayer Lipid films of dipalmitoyl phosphatidylcholine (DPPC) on glass with a variable su rface density of lipid molecules. In a certain range of surface density the se surfaces oriented homeotropically the highly polar nematic liquid crysta l of 4-cyano-4'-n-pentylbiphenyl (5CB). Optical transmission curves of Liqu id crystal layers sandwiched between orienting lipid monolayers were record ed versus magnetic field with high accuracy. They were further converted in to optical retardation and fitted by a complete theory of the transition co ntaining anchoring strength as a parameter. Independent measurements of ref ractive indices of 5CB were also performed by the prism method. Anchoring e nergies thus obtained showed a nonmonotonic behavior with increasing packin g density of DPPC monolayers and a pronounced minimum around 0.82 nm(2) per molecule. This is explained by a complete theory of surface anchoring incl uding steric, electric, flexoelectric, and notably surface polarization cou pling mechanisms. The last one stands for surface polarized layers at the l iquid-crystal interfaces interacting with surface electric field due to lip id molecules. The competition of these mechanisms leads to an initial dehan cement followed by an enhancement of the anchoring strength at monotonicall y increasing DPPC packing density. Surface polarization evaluated from the experimental data is in good correspondence with molecular parameters. [S10 63-651X(99)08701-2].