IMPROVED ACCURACY IN DYNAMIC QUARTZ-CRYSTAL MICROBALANCE MEASUREMENTSOF SURFACTANT ENHANCED SPREADING

Our previously reported quartz crystal microbalance (QCM) method for m easuring dynamic wetting on solid substrates has been modified slightl y to obtain more accurate spreading rates of aqueous dispersions of no nionic surfactants. Spreading rates are determined from analysis of fr equency changes that occur as a result of an aqueous film containing d ispersed surfactant spreading radially over the resonator surface foll owing introduction of a small droplet to the center of QCM. The modifi cation involves larger &CM electrodes, which increase the area over wh ich the spreading event is measured compared to previously used smalle r electrodes, and a frequency counter capable of more rapid data acqui sition. The larger electrodes permit spreading to be measured over a l arger area relative to the initial drop size and for longer time, whil e faster data acquisition provides more data immediately following int roduction of the droplet to the QCM, surface. This obviates overweight ing 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 microsc opy of the wetting processes on identical surfaces. Comparison of the wetting rates on gold electrodes modified with various organosulfur mo nolayers reveals that the dependence of rates on surface energy is ide ntical for the different electrode sizes, the rates being systematical ly larger for the larger electrodes.