On the applicability of certain approximations of the Lifshitz theory to thin films

In an attempt to evaluate the attractive interaction energies between cells, the van der Waals forces between two semi-infinite media composed of substance A and separated by a thin film of substance B are considered on the basis of the Lifshitz theory. An improved approximation is proposed, whereby some previously neglected terms are included in the final expression for forces and interaction energies. The symmetry behavior of forces under interchan e of media is treated. Numerical investigations show that the proposed corrections become important in certain situations. The problem of a correct representation of dielectric coefficients for biological systems for the purpose of calculating dispersion forces is also discussed. A proposed representation of dielectric permeability for the case of liquid water is shown to well represent experimental data. Calculations of the van der Waals forces with the improved representation of dielectric permeabilities for a water-hydrocarbon system show significant changes from previously obtained results.