Crystallisation in the Real World: Delivering Control through Theory and Experiment

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry

Abstract

Crystallisation is a fascinating process. From common observations such as the formation of ice on a window or scale in a kettle, crystallisation is important to virtually every area of science, and lies at the heart of processes as varied as the production of ceramics, pharmaceuticals, fine chemicals, nanomaterials and biominerals. Equally important is the prevention of unwanted crystallisation in the form of weathering, scale or kidney stones. Only by understanding how materials crystallise can we hope to control these processes.

Despite the importance of crystallisation, we still have a poor understanding of many of the mechanisms that underlie this fundamental phenomenon. This is due to the fact that crystallisation is governed by molecular scale processes that are very difficult to study experimentally. For example, while experiments can identify reaction conditions that generate specific crystal polymorphs, they cannot alone explain why this occurred.

This Programme Grant will couple experiment and theory to address this challenge. Our experimental programme brings to the fore such frontier analytical techniques as liquid-phase TEM and functional scanning probe microscopies that will allow us to study the changes in solid and solution during crystallisation as never before. With recent advances in modelling we shall be able to perform simulations of nucleation and growth processes on comparable time- and length-scales, providing a unique opportunity to fully understand crystal nucleation and growth at the nanoscale. These studies will be linked to simpler bulk experiments to provide a holistic view of crystallisation in the real world.

We will use this approach to address six major challenges in the crystallisation of inorganic compounds. Each challenge, as well as being of fundamental importance, is ultimately significant to industry and has practical applications as varied as scale prevention in dishwashers, dental remineralisation and tailoring particle shape for paper coatings. Investigations of homogeneous crystallisation in bulk solution will lay the foundation for our nucleation studies, revealing how we can direct nucleation pathways by varying solution and environmental conditions. We will then build on this work to explore the fascinating question of polymorphism, giving us predictive understanding of conditions which deliver specific crystal polymorphs. Turning then to the ubiquitous phenomenon of surface-directed crystallisation, both theory and cutting-edge analytical methods will bring new understanding of how surfaces - and the changes they cause in the adjacent solution - govern crystallisation. This naturally leads us to a search for effective nucleating agents, which, despite the promises of classical nucleation theory, are known for only a small number of systems. Control of crystal growth to generate particles with defined shapes and sizes is another topic of great industrial importance, and soluble additives are widely used to achieve this goal. By understanding crystal/ additive interactions we aim to pre-select additives to grow crystals with target properties, or to inhibit unwanted crystallisation. Finally, we will study crystallisation within confined volumes; this will ultimately enable us to use confinement to control crystallisation.

These ambitious objectives can only be met within the framework of a Programme Grant, which provides the flexibility and long-term funding to bring together the very different disciplines of theory and experiment. While each of the individual tasks focuses on a distinct problem in crystallisation, they are intimately linked over the entire project by common methods and understanding, and developments in one task will drive advances in others.

Planned Impact

This project will deliver major new capability in understanding and controlling inorganic crystallisation under conditions relevant to real world applications. This will be achieved through a combined experimental and theory approach, led by innovations in experimental methods and theory. The impact will therefore be extremely broad, encompassing all who research, manufacture or use crystalline materials in sectors ranging from the Chemical Industry, to Environment, Healthcare, Formulated Products, Oil and Gas, Water, Mining and Advanced Materials.

Industry: As indicated by our industrial collaborators in their letters of support, a greater understanding of crystallisation processes is required in a huge range of applications. The ability to generate specific crystal polymorphs is required in the field of nanomedicine (eg vaterite as soluble drug-delivery agents), while papers coated with aragonite rather than calcite exhibit superior brightness, opacity and strength. Nucleants in washing detergent can reduce crystal deposition on clothing, while the remediation of buildings from weathering relies on crystallisation in porous media. We need to enhance the mechanical properties of building materials such as gypsum, while controlling crystallisation on surfaces is critical to issues as diverse as scale inhibition in heating systems and oil wells, and bone and tooth regeneration. These are all topics addressed in the research programme, where direct collaboration with our industrial partners Proctor and Gamble, Unilever, BP, Lubrizol and Saint Gobain will ensure that technological and economic impact are achieved. Members of the consortium have had considerable success in translating research results to end-users and this embeds valuable experience in the Programme and provides a model that could be followed.

Training: The training and mentoring of early-stage researchers is an essential part of the project. They will acquire a wide range of technical and transferable skills, which will be of value both during this project and in their subsequent careers in industry and academia. We also place a high priority on training in communication and engagement with end users. The project will improve staff skills (science and team-working) via collaboration, through visits and secondments to our academic collaborators and through interactions with industry partners.

Dissemination: Industry and interest groups will be engaged via multiple channels, including our annual "Crystallisation day", specialist workshops, reports to learned societies and policy-makers, through social media, briefing documents and our web presence. We will promote new opportunities for academic-academic and academic-industry partnerships, beyond the consortium and partners. In this way we will seek to build a network for the benefit of the UK as a whole in the short, medium and long-terms. Our results will be published (after IP protection if necessary) in leading journals and presented at national and international scientific meetings. Papers will be available on the open access portals of our institutions and we will archive data in accordance with RCUK best practice.

Outreach and Advocacy: Crystallisation lends itself to impact through public engagement and outreach. All our institutions have well-established outreach programmes including annual Festivals of Science and presentations for teachers/sixth formers and all have dedicated academic outreach officers. We will also exploit all opportunities available to promote advocacy for the Engineering and Physical Sciences within the UK. As the project embodies fundamental interdisciplinary science whilst also being strongly linked to issues of major public interest such as healthcare and the environment, it will be an excellent vehicle for engaging decision makers. We will also empower our industrial partners to act as advocates for the promotion of the project's science and activities.

Publications

10 25 50
 
Description Additive-Directed Crystallisation
Our goal is to generate a new framework for understanding additive-directed crystal growth, where this dynamic phenomenon is governed by both kinetics and thermodynamics. This will ultimately enable us to make informed choices about additives and reaction conditions that will deliver crystals with the desired properties.

(1) We are developing improved models for both speciation and nucleation (including inert ions, ion ratios, temperature and solvent effects) informed by complementary state-of-the-art experiments. On the experimental side, at Warwick a multiplexed, nanoscale (30 nm) nucleation experiment has been developed using electrokinetic mixing to repetitively trigger nucleation and growth. The data provide statistical measurements of nanoparticle induction times. By making measurements in both H2O and D2O, a previously unknown kinetic isotope effect has been found, with a pronounced increase in induction time in D2O compared to H2O solutions at lower pD/pH. Supporting solution characterisation (Raman, IR) and path integral quantum calculations (Scott Habershon, Warwick) are currently underway, and complementary modelling has been performed at Sheffield (CLF, JH). Wider application of this approach to peptide crystallisation has also been demonstrated in collaboration with FCM's group at Leeds.

Molecular simulations have used new functionality in the LAMMPS package to explore the effect of latent heat transport (and resulting inhomogeneous temperature) on rates predicted by classical nucleation theory. We are now in the process of propagating the resulting change in cluster growth rates into a calculated uncertainty on the homogeneous nucleation rate. Using lattice gas models, we are exploring the equivalent problem in solution crystallisation, specifically the effect of solute depletion in finite size simulations. A forthcoming paper will propose a protocol based on partial path transition interface sampling which mitigates the resulting systematic error. Experimental work under this theme is progressing via commissioning of two liquid-cell TEM holders for use at Leeds and Liverpool (partner) and work is commencing on these. MEng final project student is undertaking some preliminary studies and initiating a collaboration with Biology at Leeds on time resolved cryo-freezing of crystallising carbonate solutions and a new PhD student (Afzali) has started in early March.

(2) The driving forces for the production of the different calcium carbonate polymorphs under ambient conditions are poorly understood. We have developed new analysis methods based around Manhatten distance alogorithms to identify the possible phases forming in calcium carbonate solutions prior to nucleation. The results are indicating that there may be kinetic differences between aragonite and calcite formation which could explain their significant differences in terms of formation which are at odds with thermodynamic data. We are now expanding this to explore the effect of solution conditions.

(3) We are characterising the solution adjacent to surfaces, emergent nuclei and growing crystals using scanning probe techniques such as scanning ion conductance microscopy (SICM). Initial work has focused on the calcium carbonate system. Initially the probe response to the solution conditions of interest (pH, pCa) has been investigated, supported by laser scanning confocal microscopy and finite element method simulations. This has enabled measurements of the spatially resolved calcite/water interfacial conductivity as a function of pH and ion concentration. Preliminary results suggest higher pH leads to an accumulation of positive ions at the calcite surface. Further experiments and MD simulations (CLF, Sheffield) are underway to support this. Additional surface speciation analysis will be available from an infrared spectro-electrochemical cell which has been designed and 3D printed to enable characterisation of both surface and solution species as a function of potential. Electrochemical impedance spectroscopy of the graphite/NaCl(aq) interface is planned to complement constant chemical potential molecular dynamics (MS,UCL).

(4) The ability to control nucleation would have a significant impact on many areas of science and technology. Our aim here is to identify the key features of nucleants that can be used to induce nucleation in mineral salt systems with the ultimate aim of designing active agents for future applications. Experimental progress has been in generating crystal surfaces with different topological features e.g. microcracks to observe how these influence the nucleation process. The results have shown, in certain cases, extreme control over preferential nucleation at sites with no clear structural variation to other non-nucleating sites. This suggests extremely small-scale effects may influence nucleation in these regions. Within the simulations we have begun by building up models of pre-nucleated structures that could be used for exploring chemical species at the interface of nucleants.

(5) We have used a combination of experiment and modelling to investigate how additives interact with a crystal surface, and how this can lead to their occlusion within the bulk crystal. Experiments were conducted by precipitating calcite in the presence of combinations of additives - namely amino acids and coloured dye molecules. These demonstrated that strongly coloured calcite crystals only form in the presence of Brilliant Blue R (BBR) and four of the seventeen soluble amino acids, as compared with almost colourless crystals using the dye alone. The active amino acids are those which themselves effectively occluded in calcite, suggesting a mechanism where they can act as chaperones for the dye molecules. Modelling work showed that this property can only be explained by additive/ crystal interactions. Aspartic acid appears to promote BBR occlusion by disrupting the first hydration layer and allowing its intimate association with the crystal surface. These results provide new insight into crystal-additive interactions and suggest a novel strategy for generating materials with target properties.

We have also combined simulations and experiments to understand the relationship between occlusion and crystal morphology change. Using gold nanoparticles functionalised with proteins and synthetic homopolymers as additives, we have shown that high concentrations of glycoprotein-functionalised nanoparticles can be incorporated within calcite single crystals. Notably, this can be achieved without any change in morphology. Hydroxylated polymers were even more effective, yielding dark red crystals containing 37 wt% nanoparticles. The nanoparticles are perfectly dispersed within the host crystal and so closely apposed that they exhibit plasmon coupling. Simulations were performed to gain insight into the interactions between the nanoparticles and the calcite surface and suggested that the interactions of the functional groups with the kink sites are primarily responsible for the experimental observations. This simple and scalable occlusion approach opens the door to a novel class of single crystal nanocomposites.
Exploitation Route We expect our results to have an impact on both academic and industrial researchers.
Sectors Agriculture, Food and Drink,Chemicals,Environment,Manufacturing, including Industrial Biotechology,Culture, Heritage, Museums and Collections

URL https://realworldcrystals.leeds.ac.uk/
 
Description Quorum Cryo-liftout 
Organisation Quorum Review- Independent Review Board
Country United States 
Sector Private 
PI Contribution Collaboration with company Quorum on in-situ liftout of frozen TEM samples
Collaborator Contribution Collaboration with company Quorum on in-situ liftout of frozen TEM samples
Impact none so far
Start Year 2017
 
Description iCASE Award on corrosion 
Organisation TATA Steel
Country India 
Sector Private 
PI Contribution Project to commence in Oct 2019
Collaborator Contribution Project to commence in Oct 2019
Impact To commence
Start Year 2019
 
Description Be Curious Science Outreach event 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Microscopy Talk, Activities and Display at Be Curious Festival at University of Leeds, March 2019.
Year(s) Of Engagement Activity 2019
URL http://www.leeds.ac.uk/info/4000/around_campus/460/be_curious_festival-about_leeds_and_yorkshire
 
Description Exhibition of scientific images 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact 6 week exhibition of images of crystals at the North Bar Leeds
Year(s) Of Engagement Activity 2018,2019
 
Description Presentation (Goldschmidt): Why no Aragonite? Polymorph selection in the early stages of calcium carbonate nucleation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk given at international conference resulting in debate/discussion and collaboration
Year(s) Of Engagement Activity 2018
 
Description Presentation (MRS): Simulating the effect of organic molecules on clustering in calcium carbonate and phosphate 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presentation given at international meeting (MRS) giving rise todebate/discussion
Year(s) Of Engagement Activity 2018
 
Description Presentation of research at the East Midlands Big Bang Fair on Saturday 9th February. 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Presentation of research at the East Midlands Big Bang Fair on Saturday 9th February, with over 4000 people attending. PM used the crystallisation of cocoa butter (aka chocolate) to explain polymorphism while IM discussed the importance of crystals in nature using the Meldrum group's impressive collection of biological crystals and scanning electron micrographs.
Year(s) Of Engagement Activity 2019
 
Description Workshop on Forcefield Development 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Workshop on theory of simulation methods to develop and use atomic forcefields.
Year(s) Of Engagement Activity 2019
URL https://www.ccp5.ac.uk/node/305