Effect of cyclodextrins’ cavity on the kinetics of alkaline hydrolysis of tris(1,10-Phenanthroline)Fe(II) in presence of surfactant

Abstract

The kinetics of alkaline hydrolysis of tris(1,10-phenanthroline)Fe(II) has been explored spectrophotometrically in the presence of cyclodextrins (CDs), quaternary amphiphile (CTAB), and their mixed systems. All kinetics run in pseudo-first order conditions with excess concentration of alkali at 298 K (± 0.1 K). The reaction rate accelerates in the presence of CTAB. However, in the presence of CDs, the observed rate constant decreases continuously and also the catalytic properties of amphiphile diminishes, indicating the inclusion of both tris(1,10-phenanthroline)Fe(II) and surfactant into the CD cavity in the hydrolysis reaction. The cavity size of γ-CD among all of the CDs favors the incorporation of both Fe(II) complex and CTAB, which significantly reduces the micellar surface catalysis of CTAB. The decreasing sequence of the rate in CTAB-CDs mixed systems is observed as γ-CD > β-CD > α-CD. All the experimental kinetic results have been well explained based on the pseudo-phase model (PP model) of micelles and CD-inhibited model of CD systems. The calculated theoretical rate constant values are well consistent with the experimental data. In the absence and presence of CTAB, the calculated values of equilibrium binding constant between Fe(phen)3 2+ and γ-CD () are 109.17 (±1.03) and 414.74 (±3.63) (L mol−1), respectively, which support the diminishing of the catalytic properties of CTAB in the presence of γ-CD in the hydrolysis reaction. The experimental results also corroborate that fact. Finally, the effect of temperature on the reaction rate has been observed and the thermodynamic activation parameters have been evaluated. The linear plot of standard activated enthalpy with standard activated entropy reveals an iso-kinetic relationship that confirms a common mechanism for the reaction in different environments.