Assessing the role of interfacial electrostatics in oriented mineral nucleation at charged organic monolayers

Charged organic-aqueous interface electrostatics are assessed in order to clarify potential electrostatic contributions to oriented mineral nucleation at planar organic surfaces. Fatty acid and acidic phospholipid monolayers contacting aqueous solutions of sparingly soluble salts serve as model systems. Monolayer electrostatic parameters and ion distributions in the interfacial aqueous solution are calculated using a Gouy-Chapman-Stern electric double-layer model that incorporates the multiple, multivalent cations and anions found in mineralizing solutions. The calculations provide insight into the relationship between surface-directed crystallization and factors such as ion binding, surface charge density, and interfacial in concentrations. Variations in crystal nucleation, phase, and morphology observed at charged monolayers arl: interpreted in terms of calculated departures from lattice ion stoichiometry and pH lowering within the interfacial aqueous solution. Lattice matching is addressed in terms of Stem layer calculations of Ca2+ ion binding at carboxylic acid monolayers. Arguments are presented suggesting that near negatively charged surfaces, soluble ionic species concentrations are enhanced relative to free anion concentrations, and a link between interfacial anion speciation and oriented mineral nucleation is proposed. Implications for biomineralization and template-directed crystallization at planar surfaces are discussed.