Fourier Transform Infrared Spectroscopy Applied to Association Modeling of Binary Alcohol + 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.