The first level of concern of interactions between proteins and synthetic surfaces deals with non-specific adsorption: that is, adsorption reflecting hydrophobic or electrostatic interaction. Chemically modified Scanning Force Microscope (SFM) probes allow it to study such interactions with surfaces separately and hence to mimic the different contributing forces to protein-surface interaction independently. Poly(ethylene glycol) (PEG) has been recognized for a long time for its outstanding protein resistant properties. The force between oligo(ethylene glycol) terminated (self-assembled) monolayers and proteins, however, depends on the conformation of the adsorbed molecules and parameters of the environment. We have studied the interaction between differently functionalized (charged and hydrophobic) SFM probes and oligo(ethylene glycol) terminated surfaces depending on the length of the ethylene glycol part, the cation in the electrolyte solution and the ion concentration of the aqueous environment.