• Smith A.M.; Lee A.; Perkin S.
    Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition
    Phys. Rev. Lett. 118, 096002, 2017

     The structure and interactions in electrolytes at high concentration have implications from energy storage to biomolecular interactions. However, many experimental observations are yet to be explained in these mixtures, which are far beyond the regime of validity of mean-field models. Here, we study the structural forces in a mixture of ionic liquid and solvent that is miscible in all proportions at room temperature. Using the surface force balance to measure the force between macroscopic smooth surfaces across the liquid mixtures, we uncover an abrupt increase in the wavelength above a threshold ion concentration. Below the threshold concentration, the wavelength is determined by the size of the solvent molecule, whereas above the threshold, it is the diameter of a cation-anion pair that determines the wavelength.


  • Smith, A.M.; Lee, A.A.; Perkin S.
    The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
    J. Phys. Chem. Lett., 2016, 7, 2157-2163.

    Abstract: According to classical electrolyte theories interactions in dilute (low ion density) electrolytes decay exponentially with distance, with the Debye screening length the characteristic length scale. This decay length decreases monotonically with increasing ion concentration due to effective screening of charges over short distances. Thus, within the Debye model no long-range forces are expected in concentrated electrolytes. Here we reveal, using experimental detection of the interaction between two planar charged surfaces across a wide range of electrolytes, that beyond the dilute (Debye–Hückel) regime the screening length increases with increasing concentration. The screening lengths for all electrolytes studied—including aqueous NaCl solutions, ionic liquids diluted with propylene carbonate, and pure ionic liquids—collapse onto a single curve when scaled by the dielectric constant. This nonmonotonic variation of the screening length with concentration, and its generality across ionic liquids and aqueous salt solutions, demonstrates an important characteristic of concentrated electrolytes of substantial relevance from biology to energy storage.


  • Britton, J.; Cousens, N.E.A.; Coles, S.W.; Van Engers C.D.; Babenko, V.; Murdock, A.T.; Koos A.; Perkin S.; Grobert, N.
    A Graphene Surface Force Balance.
    Langmuir 201430, 11485-11492.

    Abstract: We report a method for transferring graphene, grown by chemical vapor deposition, which produces ultraflat graphene surfaces (root-mean-square roughness of 0.19 nm) free from polymer residues over macroscopic areas (>1 cm2). The critical step in preparing such surfaces involves the use of an intermediate mica template, which itself is atomically smooth. We demonstrate the compatibility of these model surfaces with the surface force balance, opening up the possibility of measuring normal and lateral forces, including friction and adhesion, between two graphene sheets either in contact or across a liquid medium. The conductivity of the graphene surfaces allows forces to be measured while controlling the surface potential. This new apparatus, the graphene surface force balance, is expected to be of importance to the future understanding of graphene in applications from lubrication to electrochemical energy storage systems.


  • Smith, A. M.; Lovelock, K. R. J.; Gosvami, N. N.; Welton, T.; Perkin, S.
    Quantized friction across ionic liquid thin films.
    Phys Chem Chem Phys 2013, 15, 15317-15320.

    Abstract: Ionic liquids – salts in the liquid state under ambient conditions – are of great interest as precision lubricants. Ionic liquids form layered structures at surfaces, yet it is not clear how this nano-structure relates to their lubrication properties. We measured the friction force between atomically smooth solid surfaces across ionic liquid films of controlled thickness in terms of the number of ion layers. Multiple friction–load regimes emerge, each corresponding to a different number of ion layers in the film. In contrast to molecular liquids, the friction coefficients differ for each layer due to their varying composition. 


  • Perkin, S.; Crowhurst, L.; Niedermeyer, H.; Welton, T.; Smith, A. M.; Gosvami, N. N.
    Self-assembly in the electrical double layer of ionic liquids.
    Chem Commun 2011, 47, 6572-6574.

     We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation–anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.


  • Perkin, S.; Kampf, N.; Klein, J.
    Long-range attraction between charge-mosaic surfaces across water.
    Phys Rev Lett 2006, 96, 038301

  • Abstract: We have measured directly the forces across water between hydrophilic surfaces covered with a random mosaic of positive and negative charged domains. We find a strong, long-ranged attraction between them at a surface separation comparable with the charge domain size (many tens of nanometers). This attraction persists at higher salt concentration, but its range then becomes comparable to the Debye screening length. We attribute the attraction to correlation between negative and positive regions on opposing surfaces, facilitated by the lateral mobility of the charge patches on the surfaces.