Factors Controlling Adsorption Equilibria from Solution onto Solid Surfaces:  The Uptake of Cinchona Alkaloids on Platinum Surfaces

Zhen Ma, Ilkeun Lee, and Francisco Zaera*
Contribution from the Department of Chemistry, University of California, Riverside, California 92521
J. Am. Chem. Soc., 2007, 129 (51), pp 16083–16090
DOI: 10.1021/ja076011a
Publication Date (Web): November 29, 2007
Copyright © 2007 American Chemical Society
*

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, zaera@ucr.edu

Abstract

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The room-temperature adsorption of four closely related cinchona alkaloids and three reference quinoline-based compounds from CCl4 solutions onto a polycrystalline platinum surface was characterized by in situ reflection−absorption infrared spectroscopy (RAIRS). The adsorption equilibrium constants (Kads) were found to follow the sequence cinchonine > quinidine > cinchonidine > quinine > 6-methoxyquinoline > lepidine > quinoline. Some of this ordering can be explained by differences in solubility, but quinidine displays a much larger Kads than expected on the basis of its large relative solubility; bonding to the surface must also play a role in determining its behavior. It was determined that each alkaloid binds differently on Pt at saturation coverages. While the quinoline ring of cinchonidine tilts along its long axis to optimize π−π intermolecular interactions, in cinchonine it tilts along the short axis and bonds through the lone electron pair of the nitrogen atom instead, and both quinine and quinidine exhibit additional bonding via the methoxy oxygen atom at intermediate concentrations. Perhaps a more surprising result from this work is the fact that cinchonine displays a higher Kads than cinchonidine, quinine, or quinidine even though, according to previous work, it can be easily displaced from the surface by any of those other cinchona alkaloids. A full explanation of these observations requires consideration of the solvent above the adsorbed species.

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History

  • Published In Issue December 26, 2007
  • Received August 9, 2007

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