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Issue 20, 2012
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What drives the precipitation of long-chain calcium carboxylates (soaps) in aqueous solution?

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Abstract

The interaction of sodium octanoate, decanoate or dodecanoate with calcium(II) in aqueous solutions has been studied using turbidity, conductivity and potentiometric measurements. These show a marked alkyl chain length dependence on the behaviour. At the calcium concentration used (1.0 mM), there is little interaction with the octanoate, the decanoate shows initially formation of a 1  1 complex, followed by precipitation, while the dodecanoate precipitates at low surfactant concentrations. The solid calcium carboxylates were prepared, and show lamellar, bilayer structures with planes of calcium(II) ions coordinated to carboxylate groups through bidentate chelate linkages. Thermogravimetry and elemental analyses indicate the presence of coordinated water with the calcium decanoate but not with longer chain carboxylates. The results of both the solution and solid state studies suggest that precipitation of long-chain carboxylates depends on a balance between hydration effects and hydrophobic (largely van der Waals') interactions. Electrostatic effects make little energetic contribution but play the important structural role of ordering the carboxylate anions before precipitation. Differences are observed in the interactions between calcium(II) and long chain alkylcarboxylates and alkylsulfates, and are interpreted in terms of stronger binding to the metal of the carboxylate group. A general mechanism is proposed for calcium carboxylate precipitation from aqueous solution based on this and previous studies.

Graphical abstract: What drives the precipitation of long-chain calcium carboxylates (soaps) in aqueous solution?

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Publication details

The article was received on 30 Dec 2011, accepted on 28 Mar 2012 and first published on 29 Mar 2012


Article type: Paper
DOI: 10.1039/C2CP24152H
Citation: Phys. Chem. Chem. Phys., 2012,14, 7517-7527
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    What drives the precipitation of long-chain calcium carboxylates (soaps) in aqueous solution?

    R. F. P. Pereira, A. J. M. Valente, M. Fernandes and H. D. Burrows, Phys. Chem. Chem. Phys., 2012, 14, 7517
    DOI: 10.1039/C2CP24152H

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