Critical Evaluation of Dyes as Crystal Growth Inhibitors

Between 1933 and 1951, H. E. Buckley conducted an extensive investigation of the manner in which commercial dyes modify the habits of simple salt crystals. While he and others reasonably assumed that the mechanism by which dyes inhibited the growth of crystal surfaces involved the substitution of anionic functionalities for anion position in the lattice, the stereochemical details of these processes were never articulated. In light of the recent explanation, by Davey and coworkers,1 of the inhibition of alkaline earth sulfate growth by the stereospecific two-point attachment of diphosphonates, we felt that the time was ripe for a reinvestigation of Buckley's data. A conformational analysis, using the AM1 Hamiltonian, was carried out on nine constitutionally isomeric azo dyes whose habit modifying effects for five salts were recorded by Buckley. No stereochemical relationships were found which could explain the stated data. We argue that the action of unspecified impurities, often comprising 50% of commercial dye samples, may render a structural interpretation of Buckley's data virtually unattainable.