Influence of an external field on the surface tension of free-standing smectic films

We have carried out a theoretical study of the effect of the electric field E on the thermodynamical properties and surface tension gamma of free-standing smectic films. Calculations, based upon the extended McMillan approach with anisotropic forces, show a stepwise reduction and increase of the values of the Helmholtz free energy f and gamma, respectively, per partially fluorinated 5-n-alkyl-2-[4-n-(perfluoroalkyl-metheleneoxy)phenyl] (H10F5MOPP) molecule, as the temperature is raised above that for the bulk smectic-A-isotropic transition. Calculations show that E may not only affect the layer-thinning transition sequence, but also change the first multilayer jump in the thickness and increase the value of gamma per H10F5MOPP molecule. Reasonable agreement between the theoretically predicted and the experimentally obtained data on gamma of the partially fluorinated H10F5MOPP film has been obtained.