Structure and thermodynamics of associating solutions: Prediction of phase equilibria

The nature and type of interactions among the components of an associating liquid mixture dictate its phase behavior. Modeling the thermodynamics of such solutions is non-trivial due to (a) the difficulty in reconciling a partition function arising due to the specific interactions, with that due to purely physical forces based on a mean field approximation and (b) the difficulty in the accurate estimation of the interaction parameters. The primary objective of this work is to obtain a thermodynamic description of the phase behavior of such non-ideal systems.; Simple lattice-based association models are formulated to quantitatively predict LLPS in small molecular mixtures of a self-associating and a non-associating component. The parameters of the model are obtained through independent 1H NMR experiments and do not require experimental measurements of phase equilibria. Predictions of liquid-liquid phase equilibria and vapor-liquid equilibria for a series of alcohol-alkane mixtures match experimental data quite well.; A lattice-based association model for polymer solutions is developed for the case in which solvent molecules can associate at multiple sites on a polymer segment. A special case of the model—association at two possible sites on a segment—is implemented in this work. The model predicts LCST in PIPA-water and PEO-water systems. The quantitative agreement with experimental data is reasonable for PIPA-water and poor for PEO-water. The neglect of self-association in water in this formulation is one of the important factors responsible for the failure of the model. Vapor-liquid equilibria predictions in PEO-CHCl ₃, CDA-acetone and PVME-CHCl₃ match experimental data very well for the first two systems. When the same set of parameters used for PEO-CHCl ₃ is transferred to the PVME-CHCl₃ case, the model underestimates the activity of chloroform. However it is possible to obtain a good correlation with experimental data using a lower value for the PVME-CHCl₃ interaction energy than that for PEO-CHCl₃.; A model that takes into account the possibility of multiple solvent molecules being associated with a polymer segment is developed. This formulation also takes into account self-association in the solvent. Predictions of LLPS in PMMA-butanol and PMMA-cyclohexanol mixtures are reasonably close to experimental data. The model correlates the LCST data in the PEO-water system very well and also captures the effect of molecular weight on phase equilibria quite reasonably. However, it cannot predict the closed loop phase behavior of PEO-water for a PEO molecular weight of 3000 gm/mol.; Molecular modeling offers a new way to compute interaction parameters that are required by models for predicting phase equilibria. In this work, a methodology to determine such parameters is developed and applied to liquid methanol, butanol, methanol-water mixtures, butanol-water mixtures and methanol-hexane mixtures. However, the results of the simulations do not agree quantitatively with those obtained from 1H NMR. Recommendations for improved simulation procedures using other methods and force fields are provided.; Finally, the phenomenon of cold-solutioning in cellulose triacetate—acetone mixtures is investigated. An insoluble mixture of CTA in acetone exhibits partial miscibility upon quenching to about ∼78°C and slowly warming to room temperature, with the concentration of the clear phase dependent on the initial concentration. The implications of such a process are discussed and some experiments to explain the observed phenomena in terms of the interplay between the kinetics and thermodynamics of the process and degree of substitution in the polymer are discussed.