Evaluation of cluster formation using the infinite dilution partial molar excess entropies and enthalpies of 1-alkanols in supercritical carbon dioxide

The present study examined the criteria of cluster formation around an 1-alkanol molecule in CO2 using the infinite-dilution partial molar excess entropies (si,CO2E,∞) and enthalpies (hi,CO2E,∞) of 1-alkanols in CO2. The excess properties were calculated from the infinite dilution activity coefficient of the 1-alkanol in CO2, which was calculated from the fugacity using an equation of state approach. The EoS interaction parameters were determined using the experimental data at infinite dilution. At high pressures above the CO2 critical point, si,CO2E,∞≪0 and hi,CO2E,∞≪0 hold, clusters are formed with a high order and strong attractive forces. The temperature-pressure trajectory providing the highest order and the strongest attractive force of clusters overlaps with the vapour pressure curve of pure CO2, and the condensation trend of CO2 results in the formation of clusters in the CO2. The criteria of cluster formation explain the contradicting density regions identified in the literature.