Solid core photonic bandgap fibers (PBGFs) incorporate a microstructure lattice of high index rods in a low index matrix surrounding a defect core formed by one or several missing rods. Liquids, which can have a wide variety of absorption, gain, nonlinear, and thermal properties, have been used as the high index medium in such fibers. The modal interaction with the liquid is thus an important consideration in the design of solid core PBGFs. We numerically investigate the modal overlap with the high index rods and show that it strongly depends on the core size, and that it has only a weak direct dependence on other lattice properties such as fill fraction, number of rings, or index contrast. We apply our results to calculating the effect of material absorption in the fluid on the transmission properties. We present experimental data which quantitatively confirm our numerical predictions.