Fourier Transform Infrared Spectroscopy Applied to Association Modeling of Binary Alcohol plus Cyclohexane Mixtures

Accurate modeling of associating mixtures requires a thermodynamic term for the contributions of molecular aggregation to nonidealities. Alcohols are frequently modeled with the TPT-1 variation of Wertheim's theory by using one association strength. Spectroscopic data reveal the need for two strength parameters to simultaneously represent the dilute and concentrated regions. This work applies a recently developed attenuation function scaling technique to over 500 spectra for 10 alcohols in cyclohexane over a range of 30 K, interprets the spectra for unbonded hydroxyls, and fits the unbonded fraction using a resummed thermodynamic perturbation theory (RTPT) association term. The association contribution obtained from IR measurements is transferred to the NRTL-PA activity coefficient model, resulting in an improved simultaneous representation of the phase equilibria and excess enthalpy compared to TPT-1. The RTPT variant of NRTL-PA correctly represents the shape of the approach to infinite dilution for activity coefficients and partial molar excess enthalpies of alcohol, where the slope magnitude of the property decreases as infinite dilution is approached. Association parameters for primary alcohols increase modestly with the chain length between ethanol and 1-hexanol. At 318.15 K, the association strength for the n-mer is about 10-12 times the association strength for the dimer for the primary alcohols studied. The NRTL contribution for all binary primary alcohol systems is similar in magnitude for all systems after accounting for the association contribution via spectroscopy. The TPT-1 parameters are obtained from the infinite dilution association activity coefficients of the RTPT model for use in the more widely available TPT-1 software.