How to parameterise the association term in SAFT models? Insights from the I-PC-SAFT Equation of State

An extended version of the I -PC-SAFT equation of state (EoS) which incorporates the association term is presented, and a parameterisation protocol is proposed for water and three well-known homologous series of self-associating compounds: linear alcohols (from C 1 to C 18 , except C 14 ), linear carboxylic acids (from C 2 to C 17 ) and linear amines (from C 1 to C 12 ). The protocol follows the same constraints used in the original non-associative I -PC-SAFT (NonA- I -PC-SAFT), i.e., an exact reproduction of the critical temperature and pressure, acentric factor as well as the saturated liquid density at a reduced temperature of 0.8. Seven (1A, 2A, 2B, 3A, 3B, 4A and 4B) and eight classical association schemes (1A, 2A, 2B, 3A, 3B, 4A, 4B and 4C) were examined for the three homologous series and water respectively, in addition to the reference variant without the association term (NonA- I -PC-SAFT). For each component, the optimal reduced association parameters ( κ A B , ε A B / ε ) were determined by minimising an objective function that prioritises the vapour pressure, followed by liquid density and finally thermal properties. Compared to the NonA- I -PC-SAFT EoS, the association term substantially improved predictions for strongly associating fluids. For alcohols, the mean absolute percentage error (MAPE) in vapour pressures decreases from ∼15 % to ∼2 %, heat capacity and heat of vaporisation errors decrease from ∼16 % and ∼10 % to <5 %. Similar improvements are observed for carboxylic acids: from ∼8 % to less than ∼1 % for the vapour pressures, from ∼14 % to 9 % for the heat of vaporisation, while the errors of the heat capacities were barely reduced from ∼8 % to ∼7 %. For water, the association term reduces MAPE for vapour pressures from ∼5 % to ∼1 %, for heat of vaporisation from ∼4 % to ∼2 % and for heat capacity from ∼24 % to ∼4 %. In contrast, minimal improvements are observed for the amines that are weakly associating. The liquid densities errors remained close to ∼5 % for all systems. In the case of alcohols, amines and water, all association schemes gave comparable results. The optimal association scheme was found to be 2B for both amines and alcohols, 1A for the acids and 4B for water. These results demonstrate that the association term is essential for strongly associating alcohols, carboxylic acids and water, whilst the NonA- I -PC-SAFT EoS offers a simpler alternative for amines without significantly compromising the accuracy of the predictions.