The Impact of Local Structure and Dynamics on the Free Energy of Aqueous Mineral Interfaces

The strong interactions between mineral ions and water molecules can profoundly alter the structure and properties of water molecules at aqueous interfaces[1]. Strong ordering produces density fluctuations, particularly for very flat interfaces (such as mica and calcite)[2]. Diffusion rates of water molecules are greatly reduced from the bulk, leading to the hypothesis that ice-like structure is present. The water often stabilises the mineral surface and configurational interfacial energies obtained from classical molecular dynamics simulations are used to interpret this, even though this ignores entropic effects. Our new methods for calculating interfacial free energies[3], based on using Einstein crystals as a reference state, demonstrate that these effects contribute significantly to the thermodynamics of these interfaces and cannot be ignored.<br/><br/>In this work we examine the aqueous surfaces of calcium sulphate (gypsum, bassanite) and calcium carbonate (calcite, aragonite). We explore the structure and dynamics of water at these interfaces and quantify the enthalpic and entropic contributions to the total free energy. New insights into the competing growth mechanisms of different polymorphs may also be obtained[4]. We will also discuss effects due to the presence of solute ions close to the interface which can alter the activity of interfacial water, induce electrical space charge layers or change the relative stability of different surfaces by disrupting the ordering of the solvent. Our simulations will examine the potential mechanisms of attachment at these surfaces and how the interfacial energy and solution affects these growth processes.<br/><br/>[1] Y.S. Ranawat, Y.M. Jaques and A.S. Foster; Nanoscale Adv. 3 (2021) 3447<br/>[2] H. Söngen, S.J. Schlegel, Y.M. Jaques, J. Tracy et al; J. Phys. Chem. Lett. 12 (2021) 7605<br/>[3] S.R. Yeandel, C.L. Freeman and J.H. Harding; J. Chem. Phys. 157 (2022) 084117<br/>[4] M. Ilett, H.M. Freeman, Z. Aslam, J.M. Galloway et al; J. Microsc. 288 (2022) 155