Solvent-Driven Transformation of Zn/Cd²⁺-Deoxycholate Assemblies

Deoxycholic acid (DOC) is a unique, biologically derived surfactant with facial amphiphilicity that has been exploited, albeit minimally, in supramolecular assembly of materials. Here, we present the synthesis and structural characterization of three hybrid metal (Zn²⁺ and Cd²⁺)-DOC compounds. Analysis by single-crystal X-ray diffraction reveals the many interactions that are possible between these facial surfactants and the influence of solvent molecules that drive the assembly of materials. These structures are the first metal-DOC complexes besides those obtained from alkali and alkaline earth metals. We isolated polymeric chains of both Cd and Zn (Zn_poly-DOC and Cd_poly-DOC) from water. Major interactions between DOC molecules in these phases are hydrophobic in nature. Cd_poly-DOC exhibits unique P1 symmetry, with complete interdigitation of the amphiphiles between neighboring polymeric chains. Zn₄-DOC, obtained from methanol dissolution of Zn_poly-DOC, features the OZn₄ tetrahedron, widely known in basic zinc acetate and MOF-5 (metal organic framework). We document a solvent-driven, room-temperature transition between Zn_poly-DOC and Zn₄-DOC (in both directions) by scanning and transmission electron microscopies in addition to small-angle X-ray scattering, powder X-ray diffraction, and infrared spectroscopy. These studies show the methanol-driven transition of Zn_poly-DOC to Zn₄-DOC occurs via an intermediate with no long-range order of the Zn₄ clusters, indicating the strongest interactions driving assembly are intramolecular. On the contrary, water-driven solid-to-solid transformation from Zn₄-DOC to Zn_poly-DOC exhibits crystal-to-crystal transformation. Zn_poly-DOC is robust, easy to synthesize, and comprised of biologically benign components, so we demonstrate dye absorption as a proxy for water treatment applications. It favors absorption of positively charged dyes. These studies advance molecular level knowledge of the supramolecular assembly of facial surfactants that can be exploited in the design of organic–inorganic hybrid materials. This work also highlights the potential of solvent for tuning supramolecular assembly processes, leading to new hybrid materials featuring facial surfactants.