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.