Solvent Strength Characterization of Carbon Dioxide Expanded Fluorinated Solvents

CO₂-expanded solvents have been investigated as media for chemical reactions, separation, and materials processing. The tunability of CO₂-expanded fluorinated solvents was examined by two complementary approaches: solvatochromic characterization, which elucidates specific solute/solvent interactions, and Hansen solubility parameter estimation, which describes bulk solvent properties. The π* (dipolarity/polarizability) of CO₂-expanded ethoxynonafluorobutane (ENB), as determined from solvatochromic measurements, decreased with increasing CO₂ expansion pressure, in agreement with its volume expansion behavior. Analogous to cosolvent tuning in supercritical fluids, the solvent strength of ENB was manipulated with the addition of strong hydrogen-bond-accepting and -donating cosolvents, 2,2,2-trifluoroethanol (TFE; α = 1.51) and N-methyl-2-pyrrolidinone (NMP; β = 0.78), respectively. The Hansen solubility parameters of ENB and CO₂-expanded ENB (at 8-20 bar) were estimated from fluorous/organic miscibility studies. The experimental Hansen solubility parameters were used to guide the further selection of immiscible organofluorous pairs that become a homogeneous mixture under mild CO ₂ pressure.