CBET-EPSRC Molecular Engineering of Inhibitors to Self-Assembly: Fundamental structure informing in silico design

Polyaromatic hydrocarbons (PAHs) are complex organic molecules which have the unique trait of including in their molecular structure more than one carbon rings. Everyday examples include naphthalene and some household solvents, however they are more common as chemical feedstocks and materials. Chemically, these compounds are unique both in terms of the physical properties and in terms of the way they interact with other compounds. PAHs have a strong propensity to self-associate, which must be either carefully controlled to obtain optimum material properties or appropriately inhibited to avoid unwarranted behaviour. The crux of the matter is that the association of PAHs in mixtures of organic solvents is central to a diverse range of contemporary engineering challenges including the fabrication of organic photovoltaics, design of high-performance discotic liquid crystals, and prevention of petroleum asphaltene aggregation and fouling.

The problem faced by us is that the association of PAH's is misunderstood. It is a complex problem that involves not only the chemical nature of the molecules but the collective behaviour of molecules forming solid structures from solution. We are uniquely placed to study this problem, as we will obtain detailed information from X-ray and neutron experiments, where high energy beams scatter off pairs and clusters of these molecules giving us direct information on the type, shape and size of the clusters formed. In parallel, we will study these systems through molecular simulations, where we solve by numerical methods the time evolution of a model of the fluid at the level of the atoms forming the molecules. These simulations intimately depend on the description of the intermolecular forces, which we will validate against the scattering experiments.

The disordered (as opposed to crystalline) multiscale structure of petroleum asphaltenes (aromatic aggregates of 4-8 molecules and diffuse clusters of radii ~5-20 nm) will serve as a benchmark case. Their association is driven by a collection of interactions, including, but possibly not limited to, a) phase separation due to the large difference in average molecular size between molecules and the surrounding solvents, b) enhanced interactions between the cores of the PAH cores that form a significant part of the molecules and c) polar interactions arising from the presence of heteroatoms (S, N, O, etc.). Of these three contributions, the latter is much less studied and is the focus of this study.

In a final stage of our integrated approach we will consider coarse-grained simulations, where molecules are modelled by larger units (of several atoms each). This strategy, which we will fine tune to our rigorous experiments and fine-grained simulations, will allow us to perform extremely large simulations and explore time scales that are relevant to the association of PAH's. Our ultimate objective is to develop a set of guidelines that could inform the computer design of inhibitors to self-assembly. This will open an incredibly powerful research area where one could envision engineering molecules on a computer to satisfy industrial requirements.

This proposal is very timely, as it is fully in line with these perceived high-priority needs and most importantly aligns with EPSRC's Energy Efficiency portfolio. It will immediately benefit the energy industry, but has potential to impact UK competitiveness and its results will be academically relevant.

The choice of asphaltenes as a prototypical associating system stems from its topical nature in the energy industry and as such the possibility of providing guidelines towards the design of asphaltene deposition inhibitors is key. The single blockage of a production pipe by asphaltenes can cause losses of over 70M$ per incident, hence the prospect of being able to intelligently design inhibitors (as opposed to the empirical trial-and-error search) is appealing. A large energy company such as BP, which actively supports this proposal, sees this project as a chance to enhance their international competitiveness in the area of energy efficiency.

On a broader scale, the methods designed here will provide unparallel support to the UK neutron source. The recently completed second target station (TS2) at STFC is optimized for long wavelength neutrons for the study of larger scale structures within materials. The Near and InterMediate Range Order Diffractometer (NIMROD) on TS2 uses this broad spectrum of neutron wavelengths, with wide and small angle detectors to provide a unique Q-range allowing simultaneous determination of atomistic and mesoscale structure within disordered materials. These capabilities are unique to the UK and we strive to obtain the maximum profit from their availability. An underlying theme of this proposal is the development of methods for comparison (and refinement) of CG simulations to this type of experimental scattering data. NIMROD provided an uncommonly large Q-range, which will allow the extension of "traditional" EPSR approaches (which use atomistic simulation), to coarse-graining modelling. Such a capability does not have a mirror either in the EU or the US, posing a significant advantage in the continuous advancement in measurement science, protocols, and standards. The development of these capabilities is crucial if emerging materials are to be developed to meet the challenges of new technologies, the risks of changing requirements, and the opportunities of potential markets that are ahead. Furthermore, it is an opportunity to tailor the knowledge to the current and future UK needs.

On a more fundamental scale, the general principles and procedures that stem our proposed work; on hand the benchmarking and development of force fields relevant to larger polycondensed molecules and on the other the development of procedures to analyse and understand the results from scattering experiments; are of general interest to many collateral disciplines.