Investigating the Effect of the Zwitterion/Lactone Equilibrium of Rhodamine B on the Cybotactic Region of the Acetonitrile/scCO2 Cosolvent

We investigated the effect of adding acetonitrile to supercritical carbon dioxide (scCO2) in the presence of rhodamine B. This spectroscopic investigation of the scCO2/acetonitrile, rhodamine B/scCO2, and rhodamine B/acetonitrile interactions revealed that rhodamine B, which possesses a temperature dependent equilibrium between a zwitterionic form and a neutral form, had a strong affect on the cybotactic region. To confirm that this effect was only dependent upon the rhodamine B/acetonitrile interactions and not merely due to the bulk-phase behavior of the scCO2, we measured the compressibility of the scCO2/acetonitrile mixture and found it to be independent of the acetonitrile concentration to less than ∼0.047 mol fraction. We fit the compressibility data using the Peng−Robinson equation of state because it is most appropriate for fluids in the region between 1.72 and 12.45 MPa and between 313 and 333 K.