The solar f modes are a branch of oscillations characterized by a close correspondence between their measured dispersion relation and that predicted for a pure surface gravity wave: omega^2=gk where g is the surface gravity of the Sun. However, there is now substantial evidence for deviations from this simple behaviour. We consider the hypothesis of Rosenthal & Gough that the f modes are characterized better as an interfacial wave propagating at the chromosphere-corona transition. Using a standard solar interior model, a semi-empirical atmospheric model, and a parametrized transition region model as our equilibrium state, we solve the linearized oscillation equations for the interfacial f mode. We find that the frequencies of the interfacial f mode differ from those of the classical f mode only at very high degrees. We conclude that the interfacial f-mode theory may be the correct explanation for the very high-degree data, but that some other mechanism is required to explain the lower degree data.