Alignment of Liquid Crystals by Amphiphilic Monolayers

The action of solid surfaces coated with monomolecular films of amphiphilic compounds (especially lecithins) on adjacent liquid crystal phases resulting in an uniform homeotropic orientation has been investigated. The quality of orientation was found to be effected by molecular structure of the amphiphilics as well as the liquid crystals, the packing density of the film, and the temperature. The experimental results can be understood in terms of the following model: Lecithins form brush-like films with the hydrophobic alkyl chains pointing off the glass surface. Under suitable conditions holes of molecular dimensions exist in the films into which nematogenic molecules can penetrate. By means of elastic interaction forces between these anchored molecules and the adjacent nematic bulk the homeotropization is induced.