Particle and surface interactions in electrolytes underpin the behavior of many systems in soft and biological matter, electrochemical devices, and materials science. Of particular recent interest are electrolytes at very high concentration, and the extreme case of ionic liquids: solvent-free salts in the liquid state under ambient conditions. Recent work has illuminated several important gaps in our understanding of interactions in concentrated electrolytes, together presenting fascinating puzzles for those interested in the nature of the (ionic) liquid state.
In my laboratory we perform high-resolution measurements of the force between uniform charged surfaces across liquids using a Surface Force Balance (SFB), and in this seminar I will present some examples of recent experiments with highly concentrated electrolytes and ionic liquids. Measurements of structure in the confined liquid show a switch in wavelength at a characteristic ion density indicating a transition from solvent- to ion-determined correlation length. Beyond the structural region an anomalously long electrostatic screening length is observed which increases with increasing volume fraction of ions in direct contrast to predictions of standard theories of dilute electrolyte[2,3].
To finish, I will outline new procedures under development in my laboratory to measure properties and interactions in liquids and soft matter confined to nanoscopic films between two atomically smooth electrodes[4,5]. This leads to the possibility of varying the surface potential or charge on a nano-slit and recording the concomitant change in mechanical and other properties of a confined film. Alternatively, by polarizing the two graphene electrodes with opposite sign, an electric field can be applied across nano-films of liquid. Some early results will be presented, indicating an unexpectedly slow timescale of response in ionic liquids.
Susan Perkin is Associate Professor of Physical Chemistry and Fellow of Trinity College, University of Oxford. She studied for her DPhil with Jacob Klein , working both in Oxford and at the Weizmann Institute. She held a Junior Research Fellowship at Merton College, Oxford (2005-2008), then moved to UCL in London (2007-2012, as RCUK Fellow then Lecturer). In London she set up a laboratory and research group to work on ionic liquids and liquid interfaces. In 2012 she returned to the Faculty of Chemistry at Oxford. Current interests include electrostatics in concentrated electrolytes and ionic liquids, molecular mechanisms of friction and lubrication, field effects on confined liquids, and controlling surface properties through the design of switchable thin films. In the past few years Susan has been awarded a Starting Grant from the European Research Council, the Harrison-Meldola Memorial Prize from the Royal Society of Chemistry, and a Philip Leverhulme Prize from The Leverhulme Trust.
1. A. Smith, A. Lee, and S. Perkin. Phys. Rev. Lett. 118, 096002 (2017).
2. A. Smith, A. Lee, and S. Perkin, J. Phys. Chem. Lett. 7, 2157 (2016).
3. A. Lee, C. S. Perez-Martinez, A. Smith, S. Perkin. Phys. Rev. Lett. 119, 026002 (2017).
4. J. Britton et al., Langmuir 30, 11485-11492 (2014).
5. C. van Engers et al., Nano Letters 17, 3815-3821(2017).