Gypsum growth in the presence of background electrolytes studied by Scanning Force Microscopy

Abstract

Background electrolytes in aqueous solutions can strongly influence the growth rate of gypsum, which can be easily shown from bulk growth experiments. However, such experiments cannot be used to decipher the actual mechanism unambiguously. We have used Scanning Force Microscopy in order to characterize the microtopography of the gypsum (010) surface, to determine the dominate step-generating process, and to investigate the influence of NaNO3 and NaCl as background electrolytes on the growth kinetics of monolayer steps. Due to surface defects, the step density can be inhomogeneous, resulting in variability in bulk growth rate data. Two-dimensional nucleation (birth and spread) has been found to be the dominant process that forms new steps on the (010) surface. Surface nuclei may be bounded by [100] and [001] steps, judging from the shape and orientation of the smallest observed surface islands during growth. In addition, ab initio molecular orbital calculations show that [100] and [001] steps on the (010) surface of gypsum have nearly identical energies. Nevertheless, well beyond the nucleation stage, growth occurs on the (010) surface via the advance of monolayer steps (representing one growth slice, 7 A in height) parallel to [101] and [001]. Steps parallel to [101] migrate faster than [001] steps, in agreement with ab initio calculations which show that [101] steps have a considerably higher energy than [001] steps. The step displacement velocity of the fast growth direction parallel to [101] is increased in the presence of NaCl, relative to NaNO3, as background electrolytes, whereas the slow growth direction parallel to [001] shows no background electrolyte effect. The different growth kinetics for the two major growth directions in the presence of different electrolytes can be explained by the difference in step energies. On the basis of our data, it appears very likely that a specific interaction between [101] steps and Cl− in solution results in an increased dehydration frequency for hydrated Cat2+ near [101] step-edge regions relative to NO3− containing supersaturated solutions.