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Title: Ring formation in a statistical associating fluid theory framework
Author: Febra, Sara
ISNI:       0000 0004 7659 0537
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Hydrogen bonds (HB) form, most commonly, between independent molecules (intermolecular HB), leading to the formation of linear or branched chain-like networks, which can extend in open form and can include ring-like networks (closed loops). In addition, hydrogen bonds may involve atoms in different parts of the same molecule (intramolecular HB), on occasion leading to bent X-H...X conformations in smaller molecules (e.g., Schiff bases) where strong steric conditions apply, or from within large macromolecules (polymers [1,2,3,4,5] and proteins [6,7,8]) with little constraint from the covalent bonds otherwise binding the atoms. The formation of HB leads to long-lived molecular aggregates and the macroscopic manifestation of these two types of HB bond can be rather different and striking [9, 10, 11, 12, 13]. The direct result of the formation of hydrogen bonds is the decrease in the availability of the donor/acceptor sites of the chemical groups involved to associate. In the development of equations of state (EOSs) based on Wertheim's thermodynamic perturbation theory (TPT), including the statistical associating fluid theory (SAFT), the formation of any ring clusters, that are formed by inter- or intramolecular hydrogen bonding, is typically neglected. As a consequence, the applicability of SAFT-like EOSs is limited to systems where the anomalies arising from ring formation are insignificant. Previous attempts to extend the TPT formalism to account for rings have not provided all the answers. The issue is addressed here whereby the TPT treatment is extended to account for ring formation, under the approximation that the appropriate many-body distribution function of the ring aggregate can be expressed as a power of a (pair) radial distribution function in a homogeneous fluid. The theory developed in this thesis constitutes an improvement to the existing theories of association in that it can be used to account for the competition between free monomers, linear-chain, branched-chain, inter- and intramolecular ring aggregates by hydrogen bonding. The theory requires as input the size of rings formed and one extra parameter per ring type to capture the probability associated with the two sites in a chain molecule/aggregate meeting each other. The resulting generic framework is applicable to mixtures with an arbitrary number of association sites and ring types. The newly developed treatment is then compared to the standard framework for reference, to examine the impact of ring formation on the phase equilibria of model systems and to model the solubility of ring-forming statins in simple alcohols. The formation of both inter- and intramolecular rings is favoured by the increase of the association energy, low temperature and low density. For fixed parameters, the formation of rings may result in either the enhancement or the decrease of the solubility of statins, depending on the nature of the solvent.
Supervisor: Galindo, Amparo ; Adjiman, Claire ; Jackson, George Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral