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

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.

The interaction of long chain sodium carboxylates and sodium dodecylsulfate with lead(II) ions in aqueous solutions

“…This justifies the observation of interactions between octanoate and lead(II), whereas no interaction is observed for the same carboxylate with the more ionic Ca 2+ [1]. This is in agreement with stability data on the metal acetates, where the stability constant for the 1:1 complex is higher with Pb 2+ than with Ca 2+ [61].…”

“…As with the other carboxylates, at higher molar ratios, the electrical conductance of octanoate-lead(II) solutions increases upon increasing sodium octanoate concentration, since above the mic, sodium octanoate is in excess and the neutral lead(II) complexes do not contribute significantly to the overall ionic conductivity. As was previously observed with calcium(II), [1] these results show that although interactions between alkanoates and lead(II) are probably driven by electrostatic interactions through charge neutralization [26], and by changes in ion hydration, the effect of the alkyl chain length cannot be neglected. However, Pb 2+ is a much softer Lewis acid than Ca 2+ [57], and there is evidence from X-ray crystallography [58,59], and 207 Pb NMR spectroscopy [40,60] that the lead(II)…”

“…This complements our previous work on the precipitation of carboxylates by the more ionic calcium(II) ion [1]. The effects of metal ion hydration, covalent (hard/soft) character and of surfactant alkyl chain length on the interaction process will be discussed.…”

“…With sodium octanoate and lead(II), a rather weaker interaction is seen, and complexation is only observed at metal:carboxylate ratios > 0.5. However, in contrast to our previous work on octanoate and calcium(II) in aqueous solutions [1], precipitation does occur at higher concentrations. This difference between the behaviour of the metal ions is probably due to the more covalent nature of the bonds of the carboxylate with Pb 2+ than with Ca 2+ .…”

“…We have previously shown that the interaction between divalent or trivalent metal ions and alkanoates in aqueous solutions is quite different from that observed with sodium dodecylsulfate (SDS) [1,50]. With SDS, the maximum interaction concentration is not reached at charge neutralization, but at higher r values.…”