Quantification of Dipolar Contribution and Modeling of Green Polar Fluids with the Polar Cubic-Plus-Association Equation of State

Water, alcohols, ketones, esters, and ionic liquids are usually ranked as green solvents. There is still no consensus how to model these substances in the best manner due to their strong polarities. In this work, with a newly developed polar Cubic-Plus-Association equation of state (PCPA EoS), the influence of the dipole-dipole interaction on thermophysical properties is investigated for associating fluids (water and alcohols), polar nonassociating fluids (hydrogen sulfide, ketones, and esters), and ionic liquids. Special attention is paid to the heat capacity, which is difficult to model as a so-called second-order derivative property. The correlation capabilities of PCPA and CPA are first compared for vapor pressure, liquid density, and heat capacity for a primary evaluation of the dipole-dipole interaction. Then the contributions to residual heat capacity are estimated and compared for the individual terms of PCPA reflecting different molecular interactions. It is found that the contributions from the dipole-dipole interaction are negligible for associating compounds, while they are very important for both polar nonassociating fluids and ionic liquids as accurate heat capacities can be correlated only after they are counted in the model.