Abstract
A facile aqueous medium crystallization strategy has been found for this water insoluble title compound. Two crystalline forms, differing in the number of lattice water molecules (mono hydrated; form I & di-hydrated; form II), have been obtained, depending on the duration of crystallization. X-ray structural studies reveal that the compound crystallizes in the zwitterionic state in both the forms, due to the protonation of one imidazole nitrogen by – $$\hbox {SO}_{3}\hbox {H}$$ group, are engaged in hydrogen bonded interactions, involving lattice water molecule(s), and thus the supramolecular arrangements of the forms are influenced by lattice waters. Additional water molecules in the form II make it more hydrophilic, as confirmed using contact angle measurements. Procedures were optimized for better crystalline yields, and the bulk purity were ascertained by TGA, elemental and PXRD analysis. AC impedance measurements prove that both the crystalline forms are indeed proton conducting electrolytes, with magnitudes of $$1 \times 10^{-5}\,\hbox {S cm}^{-1}$$ and $$5 \times 10^{-5}\,\hbox {S cm}^{-1}$$ for form I & II, respectively under humidified conditions at $$25\,^{\circ }\hbox {C}$$ . Proton conducting performance of II has been found to be greater than I and could be correlated with its higher hydrophilicity and enriched hydrogen bonds for Grotthuss mechanism. Two lattice water dependent supramolecular forms and their influence in the spectroscopic and proton conductivity results of the materials are discussed. SYNOPSIS Simple crystallization strategies for this water-insoluble title compound were identified. X-ray studies show occurrences of two lattice water dependent forms. Crystalline forms are distinguishable using spectroscopic techniques. Form II has been shown to be a better humidity based proton electrolyte than I.
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