Computer simulations of phase equilibria, dynamics and solvation in ionic and polar fluids

The proposal is concerned with carrying out computer simulations of model ionic and polar fluids, in order to study systematically the physics of condensation, molecular dynamics, and solvation. The four main objectives are of equal importance.

1. To employ finite-size scaling computer simulations in determining the vapour-liquid coexistence curves, and the associated critical points, of a simple molecular model that can encompass ionic (Coulombic) and polar limits.

2. To determine at what point between the ionic and polar limits condensation either disappears, or switches over to an unusual, defect-driven mechanism.

3. To study and characterise the molecular-scale mechanisms that may be responsible for complex and/or anomalous physical phenomena in dense ionic fluids.

4. To carry out a systematic study of solvation in ionic and polar fluids, and to identify the key structural characteristics that dictate a solvent's ability to solvate particles (including nanoparticles).

Fluids are amongst the most common and functional materials encountered in Nature, and are central to a large number of different disciplines including biochemistry, chemical engineering, chemical synthesis, soft condensed-matter physics, and materials science. The interactions between the constituent particles in a fluid dictate its thermodynamic properties (e.g. boiling point), its dynamical characteristics (e.g., viscosity, response to alternating electric fields), and its ability to solvate other particles. In this research, computer simulations will used to explore the intimate links between "molecules" and "matter".
The project will begin with an investigation of the roles of electrostatic interactions between particles in dictating whether a substance possesses a boiling point, delineating the boundary between gas and liquid. It is known that Coulomb's law interactions between charged particles can help a substance form distinct gas and liquid states, but it is not yet known whether electric dipole-dipole interactions can as well. A molecular model will be chosen that can be varied between the ionic and dipolar extremes, and its ability to condense will be simulated on a computer as a function of its "ionicity" and "dipolarity". This will enable us to make a detailed link between the microscopic characteristics of an ionic or polar fluid, and its bulk behaviour.
The next phase of the project will be concerned with the way ions move in a liquid, and the resulting bulk dynamical properties such as viscosity and diffusion. The dynamical properties of ions dictate how the liquid will respond to an alternating electric field, and this response has many possible applications, such as in microwave heating, and in microwave chemistry. Our computational "experiments" will yield a unique insight on the way charged molecules translate and rotate, and hence effect charge transport through the liquid.
Finally, the abilities of ionic and polar fluids to dissolve other, larger particles will be examined. This is of utmost importance in chemistry where the majority of new compounds are synthesised in solution, and in biology where proteins may fold up to minimise their contact with surrounding water.
The results of this research will advance our fundamental understanding of fluids, and may find application in diverse areas of physical and biological science.

The project was focused on the computer simulation of ionic and polar fluids. It led to four main outcomes.
(1) We explored the crossover in behaviour from that of 'ionic' fluids (such as salt solutions, and molten salts) to 'polar' fluids (such as water, or acetone). We achieved this by designing molecular models that possess the electrostatic charges found in such systems, and studying the properties of the fluid as a function of the charge distribution. We focused on how such liquids boil to form a vapour, and we studied how the boiling point and the microscopic structures of the vapour and liquid states vary with charge distribution. Our results yield insight on a very broad range of 'ionic' fluids, including room-temperature ionic liquids. [Journal of Chemical Physics 126, 191104 (2007)]
(2) The 'polar' limit referred to above can be realized in the laboratory using dispersions of magnetic nanoparticles, each one of which possesses a magnetic dipole moment. Such dispersions ('ferrofluids') find application as switchable lubricants, drug-delivery systems, biomedical contrast agents, and in many other areas; the key feature is that the liquids can be manipulated with magnetic fields. Despite their utility, some basic questions remain, such as whether such dispersions are stable or not with respect to separation in to dilute and concentrated states. We have performed the most in-depth study of this particular, vexed question since it was first posed in the 1970s. Our study has highlighted some characteristic structural features, and extremely subtle thermodynamic signatures, which should be considered in any complete description of the separation phenomenon; we have also delineated the boundary between stable and unstable regimes in terms of the various particle interactions, which include the magnetic dipole-dipole forces. [Physical Review E 77, 013501 (2008); Europhysics Letters 84, 26001 (2008); Molecular Physics 107, 403 (2009)]
(3) In the course of this work, we were motivated to develop and apply advanced computer simulation techniques for the calculation of phase transitions, such as the separation phenomenon highlighted above. In particular, we developed a highly efficient Monte Carlo simulation technique, which allows direct computation of free energies across wide ranges of fluid concentration. We tested the technique on a wide variety of systems, including atomic fluids, ionic fluids, polar fluids, liquid crystals, and a model strongly-correlated electronic system. [Journal of Chemical Physics 127, 154504 (2007); Physical Review E 78, 036703 (2008)]
(4) Finally, we studied the dynamical properties in simple models of room-temperature ionic liquids. Simple models are required because the natural dynamics of such materials are very slow (for instance, these liquids are extremely viscous) and it is therefore difficult to perform detailed, atomic-scale simulations over sufficiently long times. We studied simple models which mimic the dynamical properties of real room-temperature ionic liquids, and yield insight on the molecular mechanisms which give rise to the bulk behaviour. For instance, we observed a 'conduction by molecular rotation' effect. The results of our work will help inform and direct the design of new ionic liquids for chemical applications. [Condensed Matter Physics 14, 33602 (2011)]
In this project international collaborations were established, and papers coauthored, with prominent academics in Russia, Ukraine, and Canada. The results have been presented at the Universities of Cambridge, Manchester, Oxford, Utrecht, and Latvia (Riga), Marie Curie Sklodowska University (Lublin), the Institute of Chemical Processes in Prague, a CECAM workshop on phase transitions (Lyon), a ScotCHEM Computational Chemistry Symposium (Glasgow), a German Ferrofluid Workshop (Mainz), a Computational Molecular Science conference (Cirencester), and a CECAM workshop on room-temperature ionic liquids (Dublin).