Rational design of heterogeneous nucleants for crystal polymorph control

Crystallisation is widely used for purification of commodity and speciality chemicals and pharmaceuticals and for making advanced materials for catalysis, separations and sensing applications. One of main challenges in developing robust and efficient crystallisation processes is to control nucleation of desired crystalline phases as many systems tend to crystallise in multiple structural forms of the same composition called polymorphs. For example, polymorphism is extremely important in the development of pharmaceutical products as polymorphs have different properties, such as solubility, dissolution rate and thus bioavailability in resulting dosage forms. Control of polymorph nucleation is a particular challenge for the development of continuous crystallisation processes when nucleation in bulk solution is not forthcoming or undesirable polymorphs tend to nucleate spontaneously.

Crystallisation of competing polymorhps is strongly dependent on kinetic effects and is highly pathway dependent. A variety of heuristic approaches are commonly used to control the kinetic pathway, such as solvent effects, quenching and heterogeneous nucleation approaches through various interfaces deliberately introduced into supersaturated solutions.
Interfaces that are designed to induce nucleation are commonly called nucleants and numerous strategies have been proposed for their design under three broad areas: epitaxy, surface chemistry and surface topology. While epitaxy matching is suitable for some specific systems, its general use is limited. Using surface chemistry is typically based on trial and error and there is a lack of general underlying principles. However, previous approaches tend to overlook the role of electrostatic effects at crystal/nucleant interfaces.

This aim of this project is to develop a new approach to designing heterogeneous nucleants based on tunable interfacial electrostatics using surfaces functionalised with monolayers of molecules with a range of dipole moments and functional end groups. This will allow us to independently vary both surface chemistry and surface electrostatics in order to direct formation of desired polymorphs. In particular, the surface electrostatics (adjustable through monolayer dipole moment) will be crucial in controlling solid forms with unit cells with a net dipole moment. There are many such systems, ranging from the smallest amino acid (glycine) to pharmaceuticals to functional polymers.

This project will take a combined experimental and simulation approach to understand how to manipulate surface chemistry and surface electrostatics in tandem to direct formation of a particular polymorph. In the experimental part, we will systematically investigate the effect of tunable monolayers on heterogeneous nucleation of representative organic compounds relevant to the pharmaceutical and fine chemical industries in order to explore the design space of novel heterogeneous nucleants. Characterisation of functionalised surfaces and crystals grown on them will be performed with a suite of advanced characterisation techniques available in the CMAC National Facility housed in TIC, including AFM, SEM, Raman microscopy and GI-SAXS. In the simulation part, we will gain a molecular level insight using a combination of quantum mechanical calculations and classical molecular dynamics simulations, which will enable calculation of relative energetics of competing polymorphs on functionalised surfaces spanning the range of surface chemistry and surface electrostatics corresponding to systems investigated experimentally.