Diffusion of Ethanol in Supercritical Carbon Dioxide—Investigation of scCO2-Cosolvent Mixtures Used in Pharmaceutical Applications

Abstract

Diffusion coefficients, D, for ethanol in supercritical carbon dioxide (scCO2) were measured in the temperature range 306.15–331.15 K and along the 10.5 MPa isobar, using the Taylor dispersion technique. The obtained diffusivities ranged from 1.49 × 10−8 to 2.98 × 10−8 m2 s−1, an order of magnitude higher than in usual liquids. The dependence of D on temperature and solvent density was examined. Various correlation models based in the hydrodynamic theory were assessed to estimate the diffusion coefficients, with reasonable results obtained for the Wilke–Chang and Lai–Tan models.

Binary Diffusion Coefficients for Short Chain Alcohols in Supercritical Carbon Dioxide—Experimental and Predictive Correlations

“…The maximum of mobility (in terms of the inverse of the kinematic viscosity) for pure CO 2 at 10.5 MPa and the inflection point for density curves, calculated from the data obtained from NIST [ 18 ], take place at around T ≈ 322 K. This is also in accordance with isobaric MD simulations [ 39 ], experimental results obtained for CO 2 [ 44 ] and similar studies in the literature [ 10 , 19 , 37 , 40 ]. Inflection points obtained from the fitting of the dependence of the diffusion coefficients with temperature, for each of the systems studied here, are observed at temperatures between 319 K (for the smaller molecule methanol) and 325 K (for butanol).…”

“…The Taylor dispersion technique (TDT) here employed has been optimized for the measurement of high pressure and supercritical fluids [ 19 , 45 ], namely scCO 2 , and is represented in Figure 4 . It operates upon the fundamental basis of TDT: a pulse of a solute that is injected into a solvent stream, flowing under a laminar regime through a circular cross section capillary tube, will widen due to the joint action of convection occurring in the longitudinal axis and molecular diffusion occurring in the radial direction.…”

“…Calculation of the diffusion coefficients from the absorbance–response curve of the solute is performed using the same procedure described for conventional TDT [ 19 , 45 , 47 , 48 , 49 ], with the assumption that small changes in concentration C are directly proportional to variations in absorbance. Diffusion coefficients are extracted from the experimentally measured signal by fitting the response curve to: where the three first terms A 0 + A 1 t + A 2 t 2 reflect the drift and curvature of the baseline due to small concentration and temperature variations; t R is the retention time of the peak, R = (∂A/∂C)λ is the sensitivity of the detector (directly dependent on the wavenumber at which the measurements are carried out) and Δ A is the peak height relative to the baseline.…”

“…The procedure to select the working wavenumbers for each solute and the experimental protocol have been explained in detail previously [ 19 , 45 ], so only the most relevant points are mentioned below.…”

“… Notes: a From Ref. [ 19 ]; S D stands standard deviation of the mean. Standard uncertainties are u c ( T ) = 0.01 K and u c ( p ) = 0.005 MPa.…”