77 research outputs found
Surface forces generated by the action of electric fields across liquid films
We explore the force generation and surface interactions arising when
electric fields are applied across fluid films. Using a surface force balance
(SFB) we measure directly the force between two electrodes in crossed-cylinder
geometry across dielectric and electrolytic fluids. In the case of dielectric
films the field between the electrodes exerts a force which can be well
explained using classic expressions and with no fitting parameters. However
when the electrodes are separated by a film of electrolyte, an alternating
electric field induces a force which diverges substantially from the calculated
static response of the electrolyte. The magnitude of the force is larger than
predicted, and the interaction can switch from attractive to repulsive.
Furthermore, the approach to steady state in electrolyte takes place over
-- ~s which is very slow compared to both the charging and viscous
timescales of the system. The non-trivial electrolyte response in AC electric
fields, measured here directly, is likely to underlie several recent reports of
unexpected and bifurcating forces driving colloids in AC fields. Our
measurements suggest ways to control colloidal and soft matter using electric
fields, as well as providing a direct measure of the length- and time-scales
relevant in AC electrochemical and electrokinetic systems
The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
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-Huuckel) 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 non-monotonic
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.Comment: This document is the unedited authors' version of a Submitted Work
that was subsequently accepted for publication in the Journal of Physical
Chemistry Letters, copyright American Chemical Society, after peer review. To
access the final edited and published work see
http://pubsdc3.acs.org/articlesonrequest/AOR-EW6FuIC6wIh6D9qqEeH
Unravelling Nanoconfined Films of Ionic Liquids
The confinement of an ionic liquid between charged solid surfaces is treated
using an exactly solvable 1D Coulomb gas model. The theory highlights the
importance of two dimensionless parameters: the fugacity of the ionic liquid,
and the electrostatic interaction energy of ions at closest approach relative
to thermal energy, in determining how the disjoining pressure exerted on the
walls depends on the geometrical confinement. Our theory reveals that
thermodynamic fluctuations play a vital role in the "squeezing out" of charged
layers as the confinement is increased. The model shows good qualitative
agreement with previous experimental data, with all parameters independently
estimated without fitting
Scaling analysis of the screening length in concentrated electrolytes
The interaction between charged objects in an electrolyte solution is a
fundamental question in soft matter physics. It is well-known that the
electrostatic contribution to the interaction energy decays exponentially with
object separation. Recent measurements reveal that, contrary to the
conventional wisdom given by classic Poisson-Boltzmann theory, the decay length
increases with ion concentration for concentrated electrolytes and can be an
order of magnitude larger than the ion diameter in ionic liquids. We derive a
simple scaling theory that explains this anomalous dependence of the decay
length on ion concentration. Our theory successfully collapses the decay
lengths of a wide class of salts onto a single curve. A novel prediction of our
theory is that the decay length increases linearly with the Bjerrum length,
which we experimentally verify by surface force measurements. Moreover, we
quantitatively relate the measured decay length to classic measurements of the
activity coefficient in concentrated electrolytes, thus showing that the
measured decay length is indeed a bulk property of the concentrated electrolyte
as well as contributing a mechanistic insight into empirical activity
coefficients.Comment: To appear in Physical Review Letter
Quantized Friction across Ionic Liquid Thin Films
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
The surface force balance: direct measurement of interactions in fluids and soft matter
Over the last half-century, direct measurements of surface forces have been instrumental in the exploration of a multitude of phenomena in liquid, soft, and biological matter. Measurements of van der Waals interactions, electrostatic interactions, hydrophobic interactions, structural forces, depletion forces, and many other effects have checked and challenged theoretical predictions and motivated new models and understanding. The gold-standard instrument for these measurements is the \textit{surface force balance}, or \textit{surface forces apparatus}, where interferometry is used to detect the interaction force and distance between two atomically smooth planes, with 0.1~nm resolution, over separations from about 1~\unit{\um} down to contact. The measured interaction force \textit{vs.} distance gives access to the free energy of interaction across the fluid film; a fundamental quantity whose general form and subtle features reveal the underlying molecular and surface interactions and their variation. Motivated by new challenges in emerging fields of research, such as energy storage, biomaterials, non-equilibrium and driven systems, innovations to the apparatus are now clearing the way for new discoveries. It is now possible to measure interaction forces (and free energies) with control of electric field, surface potential, surface chemistry; to measure time-dependent effects; and to determine structure \textit{in situ}. Here, we provide an overview the operating principles and capabilities of the surface force balance with particular focus on the recent developments and future possibilities of this remarkable technique
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Zwitterions fine-tune interactions in electrolyte solutions
Cellular organisms regulate electrolyte composition in the cytosol to optimize intracellular molecular interactions at the same time as balancing external osmotic pressure. While osmotic pressure can be tuned using multiple ionic, zwitterionic, and nonionic solutes, interactions between proteins and other macromolecules are sensitive to the precise composition of the medium. Nonetheless, the roles of individual ions and nonionic solutes in mediating cellular interactions remain relatively unexplored, and standard buffer solutions used in laboratory studies often contain only a few simple salts. Here, we report on model experiments investigating the combined effect of ionic and zwitterionic solutes on interaction forces across electrolytes, revealing a clear role for zwitterions in modifying interactions compared to simple salt solutions. First, we find that zwitterions act to disrupt water layering at interfaces, leading to smoothed interaction potentials. Second, we find that zwitterions strengthen electrostatic repulsions by enhancing effective surface charge. Third, zwitterions enhance the effective dielectric permittivity of the solution, and this “dielectricizer” effect extends the range of electrostatic repulsions compared to solutions without zwitterion present. The latter two effects are likely important in stabilizing proteins and other macromolecules when external osmotic and mechanical pressure are very high and simple ionic solutes alone would lead to collapse
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Hydrogen bond donors dictate the frictional response in deep eutectic solvents
Deep eutectic solvents (DESs) show promise as boundary lubricants between sliding surfaces, taking advantage of their physical stability, chemical stability, and tunability. Here, we study friction forces across nanofilms of two archetypal DES mixtures: choline chloride + ethylene glycol and choline chloride + glycerol. Using a surface force balance, we control the film thickness (to subnanometer precision) and determine the friction force simultaneously. Measurements are made at different mole fractions of the choline chloride salt and the molecular solvent, allowing us to determine the role of each species in the observed behavior. We find that the nature of the molecular solvent is dominant in determining the lubrication behavior, while the fraction of ChCl is relatively less important. By analyzing the steps in friction and the gradient of friction with load as the layers squeeze away from between the surfaces, we learn various mechanistic aspects of lubrication across the DES nanofilms of relevance to design and optimization of these promising fluids
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Preparation and characterisation of high-density ionic liquids incorporating halobismuthate anions
A range of ionic liquids containing dialkylimidazolium cations and halobismuthate anions ([BiBrxClyIz]− and [Bi2BrxClyIz]−) were synthesised by combining dialkylimidazolium halide ionic liquids with bismuth(III) halide salts. The majority were room temperature liquids, all with very high densities. The neat ionic liquids and their mixtures with 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide were characterised using Densitometry, Viscometry, NMR Spectroscopy, Electrospray Ionisation Mass Spectrometry (ESI), Liquid Secondary Ion Mass Spectrometry (LSIMS), Matrix-assisted Laser Desorption/Ionization Mass Spectrometry (MALDI), X-Ray Photoelectron Spectroscopy (XPS) and Thermogravimetric Analysis (TGA), to establish their speciation and suitability for high-temperature applications
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