Noncovalent interactions between the polyoxometalate [PMo12O40]3– and acryloyloxyundecyltrimethyl ammonium bromide surfactant, used during membrane preparation, were evaluated in the frame of density functional theory. The electronic solvation energy of [PMo12O40]3 and bromide anions was also evaluated, at the same level of theory, in order to predict a probable exchange on the polymeric surface between these anions at the water/polymer interface. Energy balances were theoretically assessed, showing that the bromide cannot be exchanged with this nanosized polyanion in large extent. In order to validate this theoretical conclusion, ad hoc and accurate measurements were carried out by using homemade polymeric membranes and by dipping them in an ca. 0.4 mM solution of Na3[PMo12O40] for 4 days. The Br– concentration, released in a polyoxometalate solution, was followed at different times during the test period by gravimetrical analysis. The agreement between the theoretical prediction and experimental data was remarkable, as the quantum calculations correctly accounted for the short-range intermolecular interactions involved in this phenomenon. Bearing in mind that the achieved conclusion is based on an ab initio quantum approach, the findings of this study can be considered rather general and then exploitable for other similar systems.