Liquid density response of a quartz crystal microbalance modified with mesoporous titanium dioxide

Abstract

A new method for the determination of liquid densities with a surface-modified 5-MHz quartz crystal microbalance (QCM) is described. Modifier is a thin mesoporous TiO2 him coated on the crystal of the QCM (50 < porosity (P) < 70%; 0.1 < film thickness (d(T)) < 6.0 pm). Modification is accompanied by a resonant frequency shift (1 < Delta f(T) < 60 kHz) reflecting the TiO2 film thickness (d(T)), its porosity (P), and the density of the nanocrystals (rho(T)). Immersion of the QCM in a liquid of density rho(1) leads to a further resonant frequency shift (Delta f(1)) of 0.8-40 Wt depending mainly on d(T), P, and rho(1). The liquid in the pores behaves as a rigid mass. In the case of thick films, the contribution from the bulk of the film to Delta f(1) is so important that surface phenomena related to the viscosity of the solvent and to the roughness of the film/liquid interface become negligible; i.e., a crystal with a 6-mu m TiO2 film can be used to measure the density of a liquid of unknown viscosity with an average error of /-0.02 g/cm(3). Model considerations lead to the differential d Delta f(1)/d Delta f(T) = rho(1)P/rho(T)(100 - P); i.e., the accurate solvent density becomes available from Delta f(1) differentiated with respect to Delta f(T) without any interference from viscosity and surface roughness. Experimentally, an average error of /-0.005 g/cm(3) was observed. Scope and limits of the new technique are presented.