Apparent molar volumes of methanol, ethanol, and 2-propanol in dense phase CO2

Abstract

Despite the increasing importance of CO₂ processing and the use of aliphatic alcohols as cosolvents in supercritical fluid extraction, there is an apparent lack of densimetric or volumetric data for dilute alcohols in near-critical CO₂ within the literature. To this end, apparent molar volumes of binary solutions for methanol, ethanol, and 2-propanol in CO₂ were calculated from measured density differences (δΔρ = 0.01–1.60 kg m⁻³) determined from T = 298–313 K and p = 8–13 MPa using a flow densimeter modified for high pressure measurement. This temperature and pressure range was chosen for the proximity to the critical point of pure CO₂, where volumetric changes are very sensitive and allow for better calibration of binary parameters. The apparent molar volumes of the mixtures were then used to optimize mixing coefficients with reference quality pure component Helmholtz equations-of-state and Fluctuation Solution Theory. Apparent molar volumes of these alcohol solutes resulted in poorer fits of the binary mixing coefficients for reference quality reduced Helmholtz Equations of State when compared to Fluctuation Solution Theory; however, excess functions with more parameters may be developed by future researchers. With the Fluctuation Solution Theory equations, Krichevskii parameters were determined for each alcohol in CO₂. These values were similar to a previous study of H₂O in CO₂; however, variance was noted from literature values with different methods of extrapolation.