Thin films (0,2 – 10μm) of a nematic liquid crystal NLC: 4-4′-pentylcyanobiphenyl (5CB) have been spread on large surfaces of water.
The orientation of the molecules has been examined between crossed polarizer und analyzer und by studying the average retardation of white light as a function of film thickness. Ath T ≌ 2.2μm a Frederiksz transition takes place owing to the balance of elastic and surface forces acting on the film and determining the molecular orientations in bulk at the surface as a function of films thickness. From this variation andh T , an anisotropy of surface tension of 5CB of the order of 10−6 Jm−2 is found. This weak anisotropy determines also the variation of the structural disjoining pressure und the film stability which is discussed at length. A thermodynamic potential for this films is suggested.