Influence of organic dyes on potassium sulfate crystal growth: a joint morphological and atomic force microscopy analysis

In situ atomic force microscopy (AFM) was used to study potassium sulfate crystal growth in the presence of acid fuchsin and pyranine. These polysulfonated dyes are able to adsorb onto the {1 1 0} and {0 1 0} (pyranine only) crystal faces producing strong habit modifications. Using AFM, we analyzed the changes in surface micromorphology induced by the additives on advancing steps for the {1 1 0} and {0 1 0} surfaces. Evolution of micromorphology with time showed that at low supersaturations, layers grow by step flow at preexisting steps by the addition of growth units at the step edges. During growth, the presence of dye molecules at concentrations as low as ∼2×10−6 and ∼4×10−4 M for pyranine and acid fuchsin, respectively, produces significant changes in the step morphology and growth rates. Indeed, the additive molecules attach to the terraces pinning the growing front; as a consequence, the edges of the growing steps become jagged. At critical dye concentrations, crystal growth is heavily encumbered or even blocked along certain crystallographic directions. Concurrently, on a macroscopic scale strong habit modifications are observed. Comparison of our in situ AFM experiments with previous habit modification studies showed that acid fuchsin is also able to enter the {0 1 0} surfaces, a previously unnoticed phenomenon.