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Now showing items 1 - 16 of 8217

  • Forces between solid surfaces in aqueous electrolyte solutions

    Smith, Alexander M.   Borkovec, Michal   Trefalt, Gregor  

    This review addresses experimental findings obtained with direct force measurements between two similar or dissimilar solid surfaces in aqueous electrolyte solutions. Interpretation of these measurements is mainly put forward in terms of the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). This theory invokes a superposition of attractive van der Waals forces and repulsive double layer forces. DLVO theory is shown to be extremely reliable, even in the case of multivalent ions. However, such a description is only successful, when appropriate surface charge densities, charge regulation characteristics, and ion pairing or complexation equilibria in solution are considered. Deviations from DLVO theory only manifest themselves at distances of typically below few nm. More long-ranged non-DLVO forces can be observed in some situations, particularly, in concentrated electrolyte solutions, in the presence of strongly adsorbed layers, or for hydrophobic surfaces. The latter forces probably originate from patch-charge surface heterogeneities, which can be induced by ion-ion correlation effects, charge fluctuations, or other types of surface heterogeneities. (C) 2019 Elsevier B.V. All rights reserved.
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  • Measuring Inner Layer Capacitance with the Colloidal Probe Technique

    Smith, Alexander M.   Maroni, Plinio   Borkovec, Michal   Trefalt, Gregor  

    The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica.
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  • Long range electrostatic forces in ionic liquids

    Gebbie, Matthew A.   Smith, Alexander M.   Dobbs, Howard A.   Lee, Alpha A.   Warr, Gregory G.   Banquy, Xavier   Valtiner, Markus   Rutland, Mark W.   Israelachvili, Jacob N.   Perkin, Susan   Atkin, Rob  

    Ionic liquids are pure salts that are liquid under ambient conditions. As liquids composed solely of ions, the scientific consensus has been that ionic liquids have exceedingly high ionic strengths and thus very short Debye screening lengths. However, several recent experiments from laboratories around the world have reported data for the approach of two surfaces separated by ionic liquids which revealed remarkable long range forces that appear to be electrostatic in origin. Evidence has accumulated demonstrating long range surface forces for several different combinations of ionic liquids and electrically charged surfaces, as well as for concentrated mixtures of inorganic salts in solvent. The original interpretation of these forces, that ionic liquids could be envisioned as "dilute electrolytes,'' was controversial, and the origin of long range forces in ionic liquids remains the subject of discussion. Here we seek to collate and examine the evidence for long range surface forces in ionic liquids, identify key outstanding questions, and explore possible mechanisms underlying the origin of these long range forces. Long range surface forces in ionic liquids and other highly concentrated electrolytes hold diverse implications from designing ionic liquids for energy storage applications to rationalizing electrostatic correlations in biological self-assembly.
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  • Underscreening in concentrated electrolytes

    Lee, Alpha A.   Perez-Martinez, Carla S.   Smith, Alexander M.   Perkin, Susan  

    Screening of a surface charge by an electrolyte and the resulting interaction energy between charged objects is of fundamental importance in scenarios from biomolecular interactions to energy storage. The conventional wisdom is that the interaction energy decays exponentially with object separation and the decay length is a decreasing function of ion concentration; the interaction is thus negligible in a concentrated electrolyte. Contrary to this conventional wisdom, we have shown by surface force measurements that the decay length is an increasing function of ion concentration and Bjerrum length for concentrated electrolytes. In this paper we report surface force measurements to test directly the scaling of the screening length with Bjerrum length. Furthermore, we identify a relationship between the concentration dependence of this screening length and empirical measurements of activity coefficient and differential capacitance. The dependence of the screening length on the ion concentration and the Bjerrum length can be explained by a simple scaling conjecture based on the physical intuition that solvent molecules, rather than ions, are charge carriers in a concentrated electrolyte.
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  • Attractive non-DLVO forces induced by adsorption of monovalent organic ions

    Smith, Alexander M.   Maroni, Plinio   Borkovec, Michal  

    Direct force measurements between negatively charged colloidal particles were carried out using an atomic force microscope (AFM) in aqueous solutions containing monovalent organic cations, namely tetraphenylarsonium (Ph4As+), 1-hexyl-3-methylimidazolium (HMIM+), and 1-octyl-3-methylimidazolium (OMIM+). These ions adsorb to the particle surface, and induce a charge reversal. The forces become attractive at the charge neutralization point, but they are stronger than van der Waals forces. This additional and unexpected attraction decays exponentially with a decay length of a few nanometers, and is strikingly similar to the one previously observed in the presence of multivalent ions. This attractive force probably originates from coupled spontaneous charge fluctuations on the respective surfaces as initially suggested by Kirkwood and Shumaker.
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  • Attractive Non-DLVO Forces Induced by Adsorption of Monovalent Organic Ions

    Smith, Alexander M.   Maroni, Plinio   Borkovec, Michal  

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  • Demand response: A carbon-neutral resource?

    Smith, Alexander M.   Brown, Marilyn A.  

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  • Influence of Lithium Solutes on Double-Layer Structure of Ionic Liquids

    Smith, Alexander M.   Perkin, Susan  

    The ionic liquid electrode interface has attracted much recent interest owing to its importance for development of energy storage devices; however, the important step of adding electro-active ions is not yet well understood at the molecular level. Using direct force measurements across confined electrolyte films, we study the effect of added lithium-ion solute on the double-layer structure of an ionic liquid electrolyte with molecular resolution. We find anionic clusters involving lithium can persist adjacent to the surfaces, and in many cases, this inhibits direct adsorption of lithium ions to the negative surface. Two apparently similar ionic liquid solvents show diverging properties, with one facilitating and the other preventing direct Li-ion adsorption onto the negative surface. The results have implications for the selection of ionic liquids as electrolytes in lithiumion batteries.
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  • Influence of Lithium Solutes on Double Layer Structure of Ionic Liquids

    Smith, Alexander M.   Perkin, Susan  

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  • Policy Considerations for Adapting Power Systems to Climate Change

    Smith, Alexander M.   Brown, Marilyn A.  

    Risks of maladaptation, efforts to integrate local knowledge, and considerations for other policy priorities will help ensure a more robust adaptation process for power systems. Existing modeling tools can be used to provide an assessment of adaptation measures that moves toward incorporating these insights, although future work is still necessary to incorporate factors like cost and risks for imposition of path-dependency.
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  • Quantized friction across ionic liquid thin films

    Smith, Alexander M.   Lovelock, Kevin R. J.   Gosvami, Nitya Nand   Welton, Tom   Perkin, Susan  

    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.
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  • Molecular Friction Mechanisms Across Nanofilms of a Bilayer-Forming Ionic Liquid

    Smith, Alexander M.   Parkes, Michael A.   Perkin, Susan  

    The prevailing paradigm in boundary lubrication asserts, in essence, that surfaces coated in amphiphiles slide past each other by way of the "slippery" exposed alkyl chains while the polar head group remains anchored at the surface. Here we show, for ionic liquid boundary lubricants, that the molecular mechanism of shearing is more subtle; while a monolayer on each surface gives rise to alkyl plane shearing, a bilayer on each surface shears at the ionic (nonalkyl) interface. The incorporation of water from the environment dramatically alters the shear at ionic interfaces but leaves alkyl plane shearing unaffected. Our experiments involve shearing two identical and atomically smooth surfaces past one another with films of an ionic liquid between, with subnanometer control of the film thickness and ultrasensitive shear stress resolution. With this, we uncover molecular mechanistic details relevant to boundary lubrication in general and the development of ionic liquid lubricants in particular.
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  • Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition

    Smith, Alexander M.   Lee, Alpha A.   Perkin, Susan  

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  • Self-assembly in the electrical double layer of ionic liquids RID D-5539-2009

    Perkin, Susan   Crowhurst, Lorna   Niedermeyer, Heiko   Welton, Tom   Smith, Alexander M.   Gosvami, Nitya Nand  

    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.
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  • Structure and dynamics of mica-confined films of [C(10)C(1)Pyrr][NTf2] ionic liquid

    de Freitas, Adilson Alves   Shimizu, Karina   Smith, Alexander M.   Perkin, Susan   Canongia Lopes, Jose Nuno  

    The structure of the ionic liquid 1-decyl-1-methylpyrrolidinium bis[(trifluoromethane)sulfonyl] imide, [C(10)C(1)Pyrr][NTf2], has been probed using Molecular Dynamics (MD) simulations. The simulations endeavour to model the behaviour of the ionic liquid in bulk isotropic conditions and also at interfaces and in confinement. The MD results have been confronted and validated with scattering and surface force experiments reported in the literature. The calculated structure factors, distribution functions, and density profiles were able to provide molecular and mechanistic insights into the properties of these long chain ionic liquids under different conditions, in particular those that lead to the formation of multi-layered ionic liquid films in confinement. Other properties inaccessible to experiment such as in-plane structures and relaxation rates within the films have also been analysed. Overall the work contributes structural and dynamic information relevant to many applications of ionic liquids with long alkyl chains, ranging from nanoparticle synthesis to lubrication. Published by AIP Publishing.
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  • Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids

    Smith, Alexander M.   Lovelock, Kevin R. J.   Gosvami, Nitya Nand   Licence, Peter   Dolan, Andrew   Welton, Tom   Perkin, Susan  

    Ionic liquids can be intricately nanostructured in the bulk and at interfaces resulting from a delicate interplay between interionic and surface forces. Here we report the structuring of a series of dialkylpyrrolidinium-based ionic liquids induced by confinement. The ionic liquids containing cations with shorter alkyl chain substituents form alternating cation-anion monolayer structures on confinement to a thin film, whereas a cation with a longer alkyl chain substituent leads to bilayer formation. The crossover from monolayer to bilayer structure occurs between chain lengths of n = 8 and 10 for these pyrrolidinium-based ionic liquids. The bilayer structure for n = 10 involves full interdigitation of the alkyl chains; this is in contrast with previous observations for imidazolium-based ionic liquids. The results are pertinent to these liquids' application as electrolytes, where the electrolyte is confined inside the pores of a nanoporous electrode, for example, in devices such as supercapacitors or batteries.
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