Correlation between rheological properties of zinc carboxylate liquids and molecular structure

Abstract

Using Fourier transform infrared spectroscopy (FTIR), the viscosity of zinc 2-ethylhexanoate liquid has been found to correlate with the intensity of an asymmetric COO stretching resonance at 1632 cm-1. This is consistent with the presence of the zinc carboxylate polymer, catena-2-ethylhexanato-(O, O′)-tri-μ-2-ethylhexanato-(O,O′)-dizinc(II) as the origin of the viscosity, a conclusion that is further supported by theoretical predictions. Density functional theory has been used to assign the IR spectra of the zinc carboxylate dimer, catena-2-ethylhexanato-(O,O′)-di-(tri-μ-2- ethylhexanato-(O,O′)-dizinc(II)-formic) acid, and the model of the molecular liquid, μ-4-oxo-hexakis-(μ-2-ethylhexanato)-tetrazinc(II). The predicted spectra indicate that the decreased symmetry of the polymer relative to the zinc 2-ethylhexanoate liquid increases the intensity of the asymmetric carboxylate stretch at 1632 cm-1 and leads to the observed correlation. © 2007 American Chemical Society.