Improved Accuracy in Dynamic Quartz Crystal Microbalance Measurements of Surfactant Enhanced Spreading

Abstract

Our previously reported quartz crystal microbalance (QCM) method for measuring dynamic wetting on solid substrates has been modified slightly to obtain more accurate spreading rates of aqueous dispersions of nonionic surfactants. Spreading rates are determined from analysis of frequency changes that occur as a result of an aqueous film containing dispersed surfactant spreading radially over the resonator surface following introduction of a small droplet to the center of QCM. The modification involves larger QCM electrodes, which increase the area over which the spreading event is measured compared to previously used smaller electrodes, and a frequency counter capable of more rapid data acquisition. The larger electrodes permit spreading to be measured over a larger area relative to the initial drop size and for longer time, while faster data acquisition provides more data immediately following introduction of the droplet to the QCM surface. This obviates overweighting of data acquired in the later stages of spreading when the aqueous film approaches the electrode edges and advances along the electrode tabs. These wetting rates were corroborated by real-time video microscopy of the wetting processes on identical surfaces. Comparison of the wetting rates on gold electrodes modified with various organosulfur monolayers reveals that the dependence of rates on surface energy is identical for the different electrode sizes, the rates being systematically larger for the larger electrodes.