Direct observation of ruthenium complex adsorption at the glass/water interface studied by optical second harmonic generation

Adsorption behaviors of an amphiphilic ruthenium (II) tris(2,2′-bipyridine) derivative (Ru-complex) at the glass/water interface have been investigated by using the in situ optical second harmonic generation (SHG) technique. A clear SHG signal was observed when an alkaline-treated (hydrophilic surface) glass substrate was immersed into an aqueous Ru-complex solution, but the signal decreased as the electrolyte concentration increased. The SHG signal also depended on pH of the solution; the pH-profile tended to be correlated with acid–base equilibria of surface silanol groups. It was suggested that the SHG response was ascribed to electrostatic adsorption of the hydrophilic head group of the complex to the hydrophilic glass surface. In the case of an octadecyldimethylsilyl (ODDS)-derivatized (hydrophobic surface) glass substrate, on the other hand, no appreciable SHG signals were observed at first. But the signal increased with increasing NaCl concentration, probably due to adsorption of the complex by hydrophobic interactions between alkyl groups of the complex and the ODDS groups, with its head group pointing toward the bulk.