Angus Gray-Weale's Research

Electrostatic screening is the damping of electrostatic interactions by charge correlations [1], important in nanostructured materials [2, 3], in colloid science [4] and in biomolecules [5]. The screening length is the fundamental quantity. It is the distance over which these correlations decay, and the effective range of the screened electrostatic interaction. The Poisson-Boltzmann equation describes screening [6], and is popular for its simplicity and elegance. It has flaws, and cousins in whom the flaws have been fixed or avoided [7]. We now know that the media in which charged particles move are not always passive. Their structure and correlations are often important to the distributions of charge [8, 9]. Further developments will require the distributions of other properties, like mass, disorder or polarisation, to be coupled to the charge so that we can study screening beyond the dilute charge-carrier limit. Examples of such systems include defects in elastic or plastic crystals, polyelectrolyte structures, or solvated ions carrying fluctuations or distortions.

Molten salts, or if they are liquid under ordinary conditions, ionic liquids, are interesting for the special properties unique to fluids of charged particles and for applications of ionic liquids as environmentally friendly solvents. I worked out the theory of the structure of fluids of polarisable ions (the paper is available here). Most theories of fluid structure assume that atoms and ions remain spherical. The reason for this simplifying assumption is that if the particles are rigid, it's easy to work out the force on each particle: you just add up one contribution from each other particle within range. But real atoms, molecules and ions are polarisable, i.e. their electron clouds are distorted by interactions or applied electric fields. You need to know how polarised each particle is before you can find the force, but the forces depend on the electric fields which depend on how polarised the particles are . . . you go round in circles. It is possible to reduce the full problem to the simpler case and so solve it. Others had studied polarisable particles before, but always classically fluctuating dipoles. I used the correct, induced dipole.

The more practical use for this work was the study of the screening lengths in the molten salt (or ionic liquid). The screening length tells you how far two particles have to be from each other before they are 'unaware' of each other. This is not just the range of the interparticle force - that's what it would be if there were only two particles, but particles in between the two interrupt their influence on each other.

The screening length is important for studying the thermodynamics of the ionic liquid, or for the chemical environment of a solute in the ionic liquid solvent or, perhaps most importantly, for studying the structure of an interface between the ionic liquid and another material.


[1] Debye, Hückel, Physikalische Zeitschrift 24 186-206 (1932).
[2] Shekibi, Gray-Weale, Macfarlane, Hill, Forsyth. J. Phys. Chem. C 111 11463-11468 (2007).
[3] Sata, Eberman, Eberl, Maier. Nature 408 946 (2000); Maier. Nature Materials 4 805 (2005).
[4] Löwen, Hansen, Madden. J. Chem. Phys. 98 (4) 3275 (1993).
[5] Baker, Sept, Joseph, Holst, McCammon. Proc. Natl. Acad. Sci. 98 (18) 10037-41 (2001).
[6] Honig, Nicholls. Science 268 (5214) 1144-49 (1995).
[7] Borukhov, Andelman, Orland. Phys. Rev. Lett. 79 435-8 (1997).
[8] Gray-Weale. Faraday Discuss. 134 297-334 (2007).
[9] Gray-Weale and Madden. Mol. Phys. 101 (11) 1761-1779 (2003); Gray-Weale, D.Phil. thesis, Oxford, 2003.