Interaction between a hydrophobic probe and tri(ethylene glycol)-containing self-assembled monolayers on gold studied with force spectroscopy in aqueous electrolyte solution

Methoxy-tri (ethylene glycol)-terminated self-assembled monolayers of alkanethiols adsorbed on gold were investigated by force spectroscopy with hydrophobic probes under electrolyte solution. Various ions from the Hofmeister series dissolved in the electrolyte, which are known to have different influence on water structure and to be able to react specifically with ethylene glycol based molecules, were employed in order to test the hypothesis of a structured long-range (several nanometers) water interface and to elucidate the type of (repulsive) force that is experienced by hydrophobic probes. This force is expected to play an important role in the resistance of these films to protein adsorption. Further analysis by photoelectron and mass spectroscopic techniques was employed to test for a specific interaction between the ions and the organic film. Our experiments support the significance of an electrostatic component in the interaction between hydrophobic probes and the organic film and corroborate earlier studies.