Prediction of the thermodynamic properties of associating polyatomic fluids
Ghonasgi, Dhananjay Bhimrao
Chapman, Walter G.
Doctor of Philosophy
Thermodynamic properties of associating polyatomic fluids are studied using molecular simulation and theory. The overall goal of this study was to develop a theory to predict the properties of fluid mixtures with intermolecular and intramolecular association. Intermolecular association is well understood mainly due to a theory developed by Wertheim for spherical associating molecules. In this work we performed the first tests of Wertheim's theory and its extensions for a wide variety of model fluids. Predictions from Wertheim's theory for associating hard sphere (HS) and Lennard Jones (LJ) molecules with four bonding sites were tested against simulation results. We found that the first order perturbation form of Wertheim's theory is in good agreement with simulation results. The theory was also tested for associating diatomic, triatomic, and longer chain molecules. The approximations used to simplify the theory are explained and we show how a spherical reference can be used to predict the properties of an associating chain fluid. A hard chain equation of state was previously obtained by extending Wertheim's theory to mixtures. In this work, a new chain equation of state is derived which predicts properties in much better agreement with molecular simulation, especially for long chains ($>$8 segments). The chain equation of state was also extended to LJ chains and mixtures of LJ chains. The results from this extension are in good agreement with simulation results. Prior to this work, no theory could accurately predict the properties of intramolecularly associating fluids. Such a theory is essential to the prediction of the properties of associating polymer solutions. A theory for intramolecular association in the absence of intermolecular association was developed. This theory is in good agreement with simulation results for tetramers with association sites on the terminal segments which can only bond intramolecularly. To develop a theory for the competition between inter and intramolecular association we combined Wertheim's theory for intermolecular association with our theory for intramolecular association. This combined theory gives good agreement with simulation results for tetramers with association sites that can bond intermolecularly and intramolecularly. The theory also explains some of the anomalous behavior of these systems.
Chemical engineering; Physical chemistry; Molecular physics