Contact Angles and Alignment of Liquid Crystals on Lecithin Monolayers

The mechanism of aligning liquid crystals by surface action of solid substrates was explained in literature among others by a thermodynamical model. This model claims the difference of surface energies of the solid substrate and the liquid crystal to determine the orientation of the liquid crystal. In this letter, contact angle measurements found with two different nematics on different lecithin monolayer coated substrates are reported. Decreasing area per molecule of the lecithin monolayer from 0,8 nm2 to 0,4 nm2 resulted in an increase of contact angle and simultaneously in a loss of homeotropic orientation, which is contradictious to the proposed thermodynamical arguments.