Modeling Dipolar Nonprotogenic Solvents with PC-SAFT-Type Equations of State: Pure Substance Properties

Abstract

Dipolar nonprotogenic solvents (DNSs) are interesting and important substances widely used in various applications, including organic syntheses, sustainable fuels, and organic electronics. They exhibit intriguing molecular and interactional properties, characterized by large dipole moments, strong dipole–dipole interactions, hydrogen bond (HB) acceptor ability, and the formation of dimer complexes. The latter aspect, along with the traditional view of DNSs as having a weak or negligible HB donor ability, has led to debates about the origin (dipolar vs HB) of these complexes. Therefore, modeling and predicting the thermodynamic properties of DNSs is challenging and requires sophisticated approaches. The PC-SAFT-type equations of state are powerful tools for describing the macroscopic thermodynamic properties of fluid systems, including DNSs. In this work, we explore and compare the performance of various modeling strategies within PC-SAFT for pure DNSs, including nonpolar, explicitly dipolar, and pseudo-associating approaches. These strategies differ in the treatment of the strongly dipolar character of DNSs. The PC-SAFT parameter sets for each DNS and strategy were determined de novo by fitting them to reliable reference data on the liquid density and vapor pressure. A comprehensive computational evaluation of the results for the fluid-phase thermodynamic properties of six DNSs in their pure form is provided, and the merits and drawbacks of the considered strategies are discussed. The pure-compound parameter values are also analyzed. The best results are obtained from the pseudo-association strategy, which considers DNSs to be self-associating with both HB acceptor and donor sites, followed by an explicitly dipolar approach with optimized dipole moment values. Surprisingly, a nonpolar strategy without any explicit dipolar treatment provides results comparable to those of the above models. It is also demonstrated that optimized or gas-phase dipole moments of DNSs are significantly better for use within PC-SAFT in the context of pure DNSs than those related to the liquid phase calculated quantum-mechanically using a polarizable continuum model. Possible explanations for these observations are provided.