Two-dimensional Monte Carlo simulations for adsorption on a columnar structured substrate were made to explore the steric effects of pore width and depth on molecular adsorption under adsorbate-adsorbate repulsive interactions. Simulation adsorption data fit a Frumkin-type isotherm for each set of pore width, depth and top column dimensions. As pore depth increases and pore width approaches the adsorbate molecular size, the surface coverage by adsorbed particles decreases sharply. Therefore, the adsorbate-adsorbate interaction parameters derived from the simulated isotherms exhibit a marked dependence on the substrate roughness factor (R). Phenomenologically, the influence of R on the isotherms is smoothed down by introducing a correction factor (f), which allowed us to collapse all simulated isotherms into a single isotherm. Results from the model are qualitatively compared to earlier observations about the influence of columnar structured gold surface roughness on the adsorption of 1,10-phenantroline in acid solution (oph) at constant potential (E > E-pzc).