Frequency changes observed with the EQCM resulting from hydrophilic/hydrophobic transitions

The processes responsible for the altered frequency response of the EQCM when the electrode surface changes from a hydrophilic to a hydrophobic condition have been investigated using silver electrodes in ethyl xanthate solution. The frequency accurately tracks the voltammetric charge when silver ethyl xanthate is formed, but moves in the opposite direction when xanthate is chemisorbed; chemisorption is known to result in the surface becoming hydrophobic. The characteristics of the EQCM in a xanthate solution that had been completely degassed were the same as those in one in equilibrium with nitrogen at atmospheric pressure. This is in agreement with slippage occurring at the hydrophobic quartz crystal/electrolyte interface as a result of a change in water structure but inconsistent with the formation of nanobubbles. Similar voltammograms were observed for specular reflective silver surfaces to those for electrodeposited ones, but the alteration in frequency response during change in wettability was much greater. This is also explained in terms of water structure.