Protein adsorption in an aqueous solution is known to be strongly controlled by the interacting electrostatic potential energy in the solution phase because protein molecules themselves have electrical charges on their surfaces. Nonetheless, conventional electrostatics, for example the Gouy-Chapman theory, does not account for the existence of adsorbed protein molecules but rather assumes that there is an imaginary charged surface formed by the ions in the solution. However, a realistic calculation of the electrostatic potential experienced by a colloidal particle in an aqueous solution, means that the adsorbed charged molecular surfaces in the solution should be accounted for. Accordingly, a model was formulated under the postulation of two different layers representing the adsorbed molecular layer and the bulk solution phase. A modified Poisson-Boltzmann equation describing the existence of the two unique electrical layers was solved numerically. The electrostatic potential was then calculated in terms of the distance from the air-water interface using parameters such as the ionic strength of the solution, molecular charges of the adsorbed layer, thickness of the molecular layer etc. The potential change in the molecular layered region was shown to be concave-shaped, thereby indicating a slower decay of the potential near the solution interface compared to the drastic decay in the conventional case with an ideal charged surface. The larger thickness of the molecular layer, which exceeded 2 nm, presumably double-layered or larger, led to a lowering of the maximum potential.