The disjoining pressure (DP) concept was introduced as an explanation for the excess pressure that must be applied to offset the tendency of the film phase to separate or disjoin from the confining bulk phases. It is considered to be a function of the separation distance between the confining phases with expressions for dispersion, electrostatic, structural and steric components. These component expressions permit DP isotherms, as a function of separation, to be calculated. In this paper an alternative approach is adopted whereby the energy of the film is allowed to possess a greater functional dependence (i.e. both separation and radial distance dependence). A variational approach to obtain an Euler-Lagrange equation for the DP isotherm and its local and global equilibrium structure shows that the isotherm has specific functional forms that are limited when subjected to geometric constraints (i.e. solid boundary).