Examining soft particulates using an atomic force microscope and a quartz crystal microbalance

The adsorption of a simple surfactant [cetyltrimethylammonium chloride (CTAC)], in relatively high concentration salt solutions has been studied using atomic force microscopy (AFM) and a quartz crystal microbalance (QCM). Two oppositely charged surfaces were investigated: silica and alumina. The AFM images demonstrated clear adsorption of spherical CTAC micelles on silica but not on alumina, as expected. However, AFM images indicated the presence of rod and worm-like micelles of CTAC on both surfaces in agreement with what was expected from the solutions being used. This was explained in terms of the much greater stability of these larger structures in the vicinity of the AFM tip compared to the much smaller spherical micelles, even though no bonding will take place at the alumina surface. The QCM data on these solutions also shows a significant difference in the behavior for the spherical micelle system when compared with the rod or worm-like systems. From the QCM data we infer that a slip plane is present for the worm-like solutions and that the effect of slip on the data increases as the salt concentration increases.