Dimerization of 2-Ethylhexylphosphonic Acid Mono-2-ethylhexyl Ester (HEH[EHP]) as Determined by NMR Spectrometry

The aggregation state of extractant molecules is an important consideration in characterizing or modeling solvent extraction systems used in hydrometallurgy. For example, previously reported efforts to measure organic-phase dimerization constants have required the presence of an additional phase, either aqueous phase or gas phase. NMR spectroscopy can be used to probe organic-phase inter-molecular interactions without requiring the presence of an additional phase, making it possible to study the simplest organic system, that contain only extractant and diluent. In this report, the dimerization of 2-ethylhexylphosphonic acid mono-2-ethylhexl ester (HEH[EHP]) in n-dodecane and in toluene was investigated by two different NMR-based methods: chemical shifts and Diffusion Ordered SpectroscopY (DOSY). The chemical-shift analysis requires monitoring the chemical shift of the acidic proton as the concentration of HEH[EHP] changes. DOSY is a 2D NMR technique used to probe the size of molecules. The size of diffusing species was related back to the average aggregate molecular weight via a calibration curve. Because the DOSY method had not been used in this manner before, a validation of the method using the interaction constant between HDEHP and CMPO was performed. After this validation, DOSY was applied to HEH[EHP] dimerization. DOSY results demonstrated that the chemical shift of the acidic proton was the peak most affected by the dimerization state of the HEH[EHP]. All other peaks, including that of ³¹P, were affected more significantly by the changing dielectric constant of the solution. The dimerization constants determined were significantly lower than those reported in prior literature, possibly due to the effect of water in the literature values.