Solubilization of copper(II) complexes of N-alkyl-β-alanines in their micellar solutions

Abstract

The solubilization of the copper(II) complexes of N-alkyl-β-alanines, NAA (butyl, NBuA; hexyl, NHeA; octyl, NOA; decyl, NDeA), in aqueous micellar solutions of NAA has been investigated. It was found that a crystalline coordination compound of composition Cu(I);NAA(2), the complex trans-[Cu(NAA)₂(H₂O)₂], was formed by direct mixing of NAA with copper(II) chloride in an aqueous solution. The amount of the insoluble stoichiometric 1:2 complex increased with increasing concentration of NAA in the concentration region below the critical micelle concentration (CMC) of NAA, but the 1:2 complex began to redissolve above the CMC and disappeared at a certain concentration of NAA, i.e. the insoluble 1:2 complex was solubilized in the micelles of NAA.

Visible spectroscopic data suggested that the dielectric environment around the coordination sphere of the copper(II) ion in the solubilized 1:2 complex molecule was similar to that in the crystalline 1:2 complex molecule. Lamellar structures of NAA micelles solubilizing the 1:2 complexes were characterized by small-angle X-ray diffractometry. A model for the solubilization was proposed: two hydrocarbon chains in a 1:2 complex molecule of trans form were interlinked with the NAA bilayers. The free energy of solubilization was estimated from the solubilized amounts of the 1:2 complexes for the NDeA and NOA systems. It was found that the change in the free energy of solubilization per methylene group was −1.04 kJ mol⁻¹, which is roughly half of the corresponding value for soap micellization.