Thermodynamic and solubilization properties of a polynuclear copper complex in ionic surfactants media

Abstract

The solubilization behavior of hexanuclear copper complex, [Cu6(η1 :μ2 -C4H10NO)4(η1 :η1 :μ2 -C5H9O2)4(η1 :μ1 -C5H9O2)2(μ 3-OH)2] (1) (where, C4H10NO = N,N-dimethylethanolamine and C5H9O2 = pivalic acid) ionic micellar media has been explored in this study. The interaction of copper cluster under investigation (1) was experimented with an anionic surfactant (sodium lauryl sulfate; SLS) and a cationic surfactant (cetyltrimethyl ammonium bromide; CTAB). Solubilization was observed in aqueous solutions of ionic surfactants at their pre-micellar, micellar and post-micellar concentrations ranges using UV–Visible spectroscopy and electrical conductivity. The thermodynamic attributes of micellization including standard entropy (ΔSmic ), enthalpy (ΔHmic ) and free energy (ΔGmic ) were estimated from electrical conductivity measurements. The negative values of ΔGmic and ΔHmic indicated the stability of micellization, in addition to the enthalpy and entropy-driven nature of the process. Thermodynamic studies demonstrate increases in the CMC of both surfactants due to incorporation of the copper complex in the micelles. Partitioning and binding parameters were revealed by UV–Vis spectroscopy to quantitatively measure the interaction of the copper cluster with the surfactant in terms of partition constant (Kc ), partition coefficient (Kx ), binding coefficient (Kbind ), and free energies of free energy of partition (ΔGpart ) and binding (ΔGbind ). The presence of cationic CTAB led to greater red-shift in UV–Vis study of 1 as compared to SLS due to stronger ionic interactions. The strong interaction and high solubility have been seen in 1 with CTAB as compared to SLS.