Mineral-fluid reactivity: The action is at the interface

For more than 30,000 years, humans have been able to characterise materials, so they could choose the best for making tools, jewellery, shelter and clothing. For more than 300 years, scientists and craftsmen have known enough about fluids to characterise them and use them, even if they never succeeded in precipitating gold from solutions of lead. However, it has only been the last 30 years that we have been able to characterise the composition and structure of the mineral-fluid interface with techniques that can "see" at the nanometre scale and to simulate interactions from first principals. Once one can understand the mechanisms that control the solid-fluid interface, one can control the material’s properties and its behaviour. This is the key to designer materials and solving challenges in nature.

My journey into the nanometre scale world of interfaces began with demonstrating that the surface of a calcite crystal is not simply a termination of the bulk structure. Instead, it is a defect, where the atoms at the interface are restructured. In response to fracture in a vacuum or in contact with gas, water or organic compounds, the atoms of the mineral surface rearrange and the molecules in the fluid in contact organise themselves to delocalise charge differences between each other and with the surface. This plays a role in the behaviour of adsorbates. In turn, ion and organic compound adsorbates can modify surface properties, dramatically changing behaviour, even at tiny fractions of a monolayer. Understanding mineral-fluid-organic compound interactions gives us a powerful tool, the ability to predict behaviour and to control reactions.

Work in my group shows that the character of simple organic compounds, i.e. their functional group(s), size and branching, determine their adsorption energy on calcite. Density functional theory simulations match very well with adsorption energy determined with X-ray photoelectron spectroscopy. If we can define such relationships for other mineral systems, it could lead to a whole new conceptual framework for describing and predicting mineral-water-organic compound reactivity.