Crystal packing and layered morphology relationships in naphthalene sulfonate compounds

The crystal structure of sodium naphthalene 2-sulfonate (Na2-NS) is reported. This compound raised the attention as a pollutant, being widely used in industry, and its intercalation in inorganic matrices, such as layered double hydroxides (LDH), could be a suitable removal strategy. The crystal structure of the title compound, despite its simplicity, is not known in the literature, so we looked for a strategy to grow crystals suitable for a single crystal study. Although many attempts were made to recrystallize it, Na2-NS grows in bunches of very thin laminae, with a high degree of mosaicity and intergrowth, explaining the absence of a reported crystal structure. The crystal structure shows layers of Na+ cations with the organic part arranged in between. The crystals grow easily in the layer plane, whilst the growth perpendicular to the layers is driven by weak non-bonding interaction and thus unfavored. The crystal packing features were related to the density of charges in the cationic layer with respect to the size of the anion. By comparing the crystal structures of 2-NS salts with different cations, and with or without an amino substituent in different positions, it was possible to find the relationship between the density of the positive charges and the deepness of interdigitation of the 2-NS moieties. We exploited this information to shed light on the structural features of 2-NS and related compounds intercalated into LDH. The X-ray powder diffraction pattern of 2-NS intercalated LDH (V. Toson, E. Conterosito, L. Palin, et al. Facile intercalation of organic molecules into hydrotalcites by liquid-assisted grinding: yield optimization by a chemometric approach. Cryst. Growth Des. 2015, 15, 5368) resulted consistent with a crystal packing characterized by the partial interdigitation of the 2-NS anions.