The Anisotropy of the Surface Tension of Polar Liquids: The Case of Liquid Crystals

Abstract The contact angle of the liquid crystal (LC) 4-phentyl, 4′-cyano biphenyl (5CB) on glass or polyethylene does not correspond to the value expected from its surface tension measured by the “du Nouy” method (29 × 10−3 JM−2). The value deduced from the Young-Dupre law is 40 × 10−3 JM−2. Both these values have already been reported in the literature. Their apparent discrepancy is explained by the LC surface tension anisotropy. We show that 5CB as many other LCs orient perpendicular to the free surface but parallel to glass or polyethylene Thus both values of the surface tension correspond to two different molecular orientations. The LC oriented perpendicularly has a surface tension of 29 × 10−3 JM−2 and 40 × 10−3 JM when it lies parallel to the surface. We suggest that the anisotropy between the perpendicular and parallel state of LCs also exists between two perpendicular orientations. Furthermore, such anisotropy associated with any polar molecules explains the difference of the interfacial energy of similar polar or non polar (i.e., octanol-octane) compounds and water.