Analysis of volume expansion mechanism of CO2–acetate systems at 40 °C

We measured the density and liquid phase CO2 mole fraction (xCO2)(xCO2) of CO2-expanded acetates (methyl acetate, ethyl acetate, propyl acetate, butyl acetate, i-propyl acetate, and t-butyl acetate) at 40 °C and carried out molecular dynamics (MD) simulations. The pressure dependence of xCO2xCO2 was almost the same for all measured acetates. The expansion coefficient and the partial molar volume estimated using the Peng–Robinson equation of state was found to have regions: a nearly constant region and a rapidly changing region that seem to be caused by the interspaces. When the length of the alkyl chain increased, the interspaces became larger. CO2 molecules existed in the interspaces while the volume remains nearly constant in the lower xCO2xCO2 region. However, there were no interspaces in the higher xCO2xCO2 region where volume expanded rapidly and these trends were supported by the MD simulations. The fraction from the center of mass of CO2 to the carbonyl oxygen atom was highest in regions of lower xCO2xCO2, while the distance from the center of mass of CO2 to the carbonyl oxygen atom was shortest regardless of region or mixture. The results show that CO2 molecules tend to aggregate around the carbonyl oxygen in the acetate.

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