Applications of Spectroscopy in Thermodynamic Investigations of Fluid Mixtures

Thermodynamic models of fluid mixtures are widely used in chemical engineering to calculate phase equilibria for designing thermal separation processes. This work discusses how spectroscopic information can be incorporated into such models, combining microscopic insight from spectroscopy with macroscopic information from classical thermodynamic data, especially phase-equilibrium measurements. Because many current models cannot reliably predict mixture phase behavior from pure-component data alone, adjustable parameters are typically fitted to mixture measurements, yet parameter identifiability from VLE data is limited and predictions—particularly of temperature dependencies—are often unsatisfactory. These difficulties are especially pronounced for systems with specific chemical interactions such as hydrogen bonding and for chemically reacting mixtures where new species form, both of which increase model complexity and parameter count. Spectroscopic methods (e.g., IR and NMR) can help by determining the “true” composition of mixtures, including associating species and reaction products, which can support separating chemical from physical contributions to macroscopic behavior and improve model robustness. The work also highlights solvatochromic parameters, often derived from spectroscopy, as descriptors of molecular properties (polarity/polarizability, acidity, basicity) that can correlate equilibria, reaction rates, or partition coefficients and may serve as an alternative to group-contribution approaches. In addition, spectroscopy can probe local mixture properties such as local composition around probe molecules, which is relevant because local–bulk differences often control phase behavior and are explicitly represented in many models. Laser light scattering is presented as a complementary technique—particularly for polymer solutions—providing quantities such as osmotic virial coefficients that augment classical thermodynamic measurements. The discussed concepts and methods are illustrated with examples from the author’s work and recent literature, with enough background included to be accessible to readers familiar with mixture thermodynamics without requiring extensive spectroscopy expertise.