Molecular design and engineering for affinity separation processes using isothermal titration calorimetry (ITC) and molecular modeling (MM)

A molecular design approach based on isothermal titration calorimetry (ITC) and molecular modeling (MM) was investigated for the development of new extractants in liquid-liquid extraction (LLX) systems. The molecular designs were validated in LLX experiments. Key in the regeneration of a solvent by temperature-swing back-extraction is the temperature dependency of the complexation equilibrium, which is related to the enthalpy of complexation ∆H. The relation between the molecular structure of extractants and the thermodynamics of complexation in LLX was studied for extraction of acetic acid by basic extractants and 4-cyanopyridine by phenols. For the basic extractants longer alkyl chains and additions of rings decreased ∆H, whereas for the phenols the acidity of the phenolic proton, and hence the complexation enthalpy, could be increased by electron withdrawing substituents. Using ∆H as determined with ITC, the temperature dependency of the liquid-liquid equilibrium could indeed be described. Also enthalpy-entropy compensation (EEC) could be observed in the data obtained by ITC, i.e. for extractants of the same family with a larger enthalpy, an opposite effect on the entropic contribution was observed.