Effect of the diffuse double layer on the interpretation of EQCM results in dilute NaClO4 solutions

The response of the electrochemical quartz crystal microbalance (EQCM) in d ilute NaClO4 solutions was studied with gold and iron electrodes during a s tepwise increase of the perchlorate concentration. In the range from 10(-4) M to 7.8 x 10(-2) M, the quartz resonant frequency of the 10 MHz AT cut cr ystals increased by about 700 Hz, indicating a mass loss on the electrode. A model was developed in which the diffuse double layer and the oscillating bulk electrolyte layer, characterised by the velocity decay length of the damped shear wave in solution, are treated as two independent, superimposed sheets. By assuming a characteristic thickness of the diffuse double layer according to the Gouy-Chapman theory and by treating the diffuse double la yer as a rigid sheet, the measured mass loss could be simulated qualitative ly. The viscosity changes in the diffuse double layer as well as in the sen sed electrolyte bulk layer were found to be negligible in the concentration range investigated. In dilute solutions, the frequency shift following a c oncentration change is entirely due to thinning of the diffuse double layer with increasing concentration. The results demonstrate the importance of d iffuse double layer effects for EQCM measurements in dilute electrolytes. ( C) 2001 Elsevier Science B.V. All rights reserved.