(204t) Measurement of Diffusion Coefficients of Chromium(III) Acetylacetonate in Supercritical Carbon Dioxide at High Temperatures

Recently, supercritical fluid deposition (SFD) has become focus of attention as well as various chemical processes with supercritical CO₂ such as extraction, dyeing, chemical reaction etc. SFD is useful for plating with metals on high-aspect-ratio substrates due to negligible surface tension of scCO₂. These methods are expected for materials of nano-devices instead of using aqueous solutions which results in destructing trenches on the substrates when evaporating solvents having high surface tension such as an aqueous solution. Thus, SFD could be a promising process, and the physical properties, especially diffusion coefficient, are required for design reactors at high temperatures [1, 2]. While the Taylor dispersion method is common to measure diffusion coefficients in scCO₂, it is not suitable with high accuracies because of peak tailing at high temperatures. Consequently, measurements of diffusion coefficients in scCO₂ at high temperatures were scarcely reported.
In this work, diffusion coefficients of chromium(III) acetylacetonate, Cr(acac)₃, together with retention factors were measured above 70 °C using the chromatographic impulse response method, which is a kind of transient response methods and two parameters: diffusion coefficient and retention factor are simultaneously determined [3]. Validating a hydrodynamic equation [4] as a predictive correlation for determined diffusion coefficients of Cr(acac)₃, average absolute relative deviation (AARD) increased with increasing temperature. The diffusion coefficients at higher temperatures deviate from the reference values in low temperature scCO₂. It is implied that the hydrodynamic equation based on the Stokes-Einstein equation, derived from those in liquid phases, is no longer valid in supercritical fluids at high temperatures and low pressures. Moreover, solubilities of the solutes in scCO₂ can be estimated using an Ekart’s equation [5] from retention factors determined. The estimated solubilities provided high accuracies with AARD < 10% at all temperatures.

Bibliography
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[2] S. Yoda et al., J. of Supercritical Fluids, 123 (2017) 82.
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[4] C.Y. Kong et al., J. Chromatogr. A, 1035 (2004) 177.
[5] M. Ekart et al., AIChE J., 39 (1993) 235.