pH 5.0 [@Malmsten1997]. By considering a pH 5.0, we minimize the charge interaction for Alb. Hence, the interaction of Alb with the surface is modeled with a short range attraction, that can be thought as mainly due to the entropic gain for water exclusion at the interface, $$V_{A,S}(z)\equiv
 4\epsilon_{A,S}\left(\left(\frac{\sigma_{A}}{z}\right)^{24}-\left(\frac{\sigma_{A}}{z}\right)^{12}\right)
\label{1}$$ where $z$ is the distance between the center of mass of the protein and the surface, $\epsilon_{A,S}$, related to the binding affinity and the dissociation constant, is the attractive energy between Alb and the surface. Here $\sigma_{A}\equiv R_A/2^{1/12}$, with $R_A$ radius of the Alb, takes into account that Alb is a globular protein whose conformation may become distorted on interaction with the surface [@Roach2005].

The IgG structure resembles a $\gamma$ that can be roughly approximated with a spherical shape. We model protein-protein interaction for the two spherical proteins as $$V_{i,j}(r)\equiv\epsilon_{i,j}\left(\frac{\sigma_{i,j}}{r}\right)^{24}$$ where $r$ is the protein-protein distance, $\epsilon_{i,j}$ the characteristic interaction energy between protein $i$ and protein $j$, where each index can be $A$ for Alb or $I$ for IgG, and $\sigma_{i,j}\equiv R_i+R_j$, with $R_i$ radius of protein $i$. Attraction among proteins is not included at this level of description, as it is small compared to protein-surface interaction and the protein 