Dissolution on cholesterol monohydrate single-crystal surfaces monitored by in situ atomic force microscopy

The mechanism(s) and dissolution rates on the (001) surface of cholesterol monohydrate (ChM) in aqueous solutions containing 10-50% ethanol were monitored using in situ atomic force microscopy (AFM). Dissolution was found to proceed mechanistically via the layer-by-layer retreat of 34 A bilayer or multilayer steps and/or the creation and expansion of bilayer and multilayer etch pits. In general, the dissolution rate is very strongly dependent on the local surface topography, which is highly variable on (001) ChM. Since the overall dissolution on the surface is by necessity a function of the relative density of different types of surface features, dissolution on both typical "smooth" and "rough" micron-sized areas was examined. For areas exhibiting low topographical relief, the rate of solute loss from isolated surface features was monitored by tracing the position of particular step fronts in sequential images over time. Although the absolute local dissolution rates are variable, the loss of solute molecules from isolated bilayer islands and/or pits occurred on the order of similar to 10(5) molecules/min. The critical interstep distance for nondissolving features (observed in multiple experiments) appeared to decrease with increasing ethanol concentration, suggesting that surface diffusion effects likely influence this process. Dissolution in regions exhibiting much greater topographical relief tended to occur most readily by the rapid expansion of multilayer pits rather than by bilayer step retreat. In such areas, the alternative method of roughness analysis provided a more reliable means to track surface changes over time. The relatively high frequency with which such pit features were observed presumably speaks to the significantly large defect density in conventionally grown ChM crystals.