Measurement and modelling of tracer diffusivities of gallic acid in liquid ethanol and in supercritical CO2 modified with ethanol

- Measurement of tracer diffusivities (D12) of gallic acid by Taylor-Aris method.
- Solvents: compressed liquid ethanol and SC-CO2 modified with 16 wt% ethanol.
- Experimental conditions studied: 303.15-333.15 K and 1250 bar.
- Diffusive behaviour interpreted in terms of free volume and hydrodynamic theories.
- Accurate modelling with DHB, TLSMd and Magalhães et al. correlations.

The tracer diffusion coefficients, D12, of gallic acid in liquid ethanol and in supercritical carbon dioxide (SC-CO2) modified with 16 mol.% ethanol were measured using the chromatographic peak broadening technique of Taylor-Aris. The diffusivity in ethanol ranged 0.485 × 10−5−0.905 × 10−5cm2·s−1 (303.15-333.15 K and 1-100 bar) and in SC-CO2/ethanol 3.256 × 10−5−4.978 × 10−5cm2⋅s−1 (313.15-333.15 K and 150-250 bar). The dependencies of D12 upon temperature, pressure and Stokes-Einstein coordinates were examined in detail. With respect to modelling, lower deviations were achieved in both systems by: four 2-parameter correlations of Magalhães et al. (error 0.9-3.0%;); the 1-parameter TLSMd model (5.8% and 2.9% of error); and the 2-parameter Dymmond-Hilbrand-Batchinski (DHB) expressions (5.7% and 1.6% of error). The Wilke-Chang correlation cannot be applicable, due to consistent overpredictions of 133.0% (gallic acid/ethanol) and 114.0 % (gallic acid/SC-CO2/ethanol).

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