In some cases proteins are attracted to surfaces by an excessive surface charge.  When a surface in a fluid has a net charge, ions in the fluid will adsorb to the surface.  Proteins also have charged surfaces due to charge amino acid residues on the surface of the protein. The surface and the protein are then attracted by Coulombic forces.
The attraction a protein feels from a charged surface () depends exponentially on the surface's charge, as described by the following formula:
Where 
*  is the potential felt by the protein
*  is the actual potential of the surface
*  is the distance from the protein to the surface, and
*  is the Debye length.
A proteins surfaces potential is given by the number of charged amino acids it has and its isoelectric point, pI.