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We combine direct surface force measurements with thermodynamic arguments to demonstrate that pure ionic liquids are expected to behave as dilute weak electrolyte solutions, with typical effective dissociated ion concentrations of less than 0.1% at room temperature. We performed equilibrium force–distance measurements across the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C ₄mim][NTf ₂]) using a surface forces apparatus with in situ electrochemical control and quantitatively modeled these measurements using the van der Waals and electrostatic double-layer forces of the Derjaguin–Landau–Verwey–Overbeek theory with an additive repulsive steric (entropic) ion–surface binding force. Our results indicate that ionic liquids screen charged surfaces through the formation of both bound (Stern) and diffuse electric double layers, where the diffuse double layer is comprised of effectively dissociated ionic liquid ions. Additionally, we used the energetics of thermally dissociating ions in a dielectric medium to quantitatively predict the equilibrium for the effective dissociation reaction of [C ₄mim][NTf ₂] ions, in excellent agreement with the measured Debye length. Our results clearly demonstrate that, outside of the bound double layer, most of the ions in [C ₄mim][NTf ₂] are not effectively dissociated and thus do not contribute to electrostatic screening. We also provide a general, molecular-scale framework for designing ionic liquids with significantly increased dissociated charge densities via judiciously balancing ion pair interactions with bulk dielectric properties. Our results clear up several inconsistencies that have hampered scientific progress in this important area and guide the rational design of unique, high–free-ion density ionic liquids and ionic liquid blends.

Particle acceleration in magnetic reconnection is a critical area that demands investigation for a better understanding of energization processes in space plasma. NASA's Magnetospheric Multiscale Mission detected multiple separatrix crossings during the asymmetric magnetic reconnection at Earth's subsolar magnetopause. This unique observation revealed a highly field‐aligned beam of electrons and the coexistence of strong parallel electric fields greater than −23 mV/m along with bipolar electric field structures. For the first time, we report the observation of multiple plasma double layers (DLs) in asymmetric magnetic reconnection. Based on observations, we have identified that DLs are generated through the current‐driven Buneman instability along the reconnection separatrix layer, a finding well supported by existing theoretical and simulation studies. Such local electric fields can accelerate thermal electrons up to the keV range, which can undergo further energization downstream of the reconnection.

Ion distributions play a central role in various settings—from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength Γ of ion-ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameterfree density functional theory that includes ion-ion correlations and ion-solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.

Soft, ionic hydrogels provide transparent, compliant electrodes that could be used in electrostatically controlled artificial muscles. [Also see Report by Keplinger et al. ] Development of actuator technologies with capabilities that can match or exceed those found in biology represents a topic of long-standing interest within the advanced robotics community. One promising and remarkably simple class of such an “artificial muscle” exploits a dielectric elastomer (an electrical insulator) sandwiched between a pair of mechanically compliant electrodes ( 1 , 2 ). Electrostatic force generated by an applied voltage deforms the dielectric and causes rapid, controlled displacements with large amplitudes. On page 984 of this issue, Keplinger et al. ( 3 ) describe an important advance in this dielectric elastomer actuator (DEA) technology, in which the authors replace the electrodes with soft, ionic hydrogels. The result provides a clever solution to a daunting materials challenge; it enables delivery of high voltages for fast, effective operation without any mechanical constraint on the motions of the dielectric, in a form that also provides almost perfect optical transparency.

Valves are prone to wear under harsh environments, such as high temperatures and reciprocating impacts, which has become one of the most severe factors reducing the service life of engines. As a lightweight ceramic, CrN is considered an excellent protective material with high-temperature strength and resistance to wear. In this study, a CrN coating was applied onto the valve cone surface via double-layer glow plasma surface metallurgy technology. The formation process, microstructure, phase composition, hardness, and adhesion strength were analyzed in detail. Impact wear tests were conducted on the valve using a bench test device. The SEM and EDS results showed that the CrN coating evolved from an island-like form to a dense, cell-shaped surface structure. The thickness of the coating was approximately 46 μm and could be divided into a deposition layer and a diffusion layer, from the outer to the inner sections. The presence of element gradients within the diffusion layer proved that the coating and substrate were metallurgically bonded. The adhesion strength of the CrN coating measured via scratch method was as high as 72 N. The average Vickers hardness of the valve cone surface increased from 377.1 HV0.5 to 903.1 HV0.5 following the plasma alloying treatment. After 2 million impacts at 12,000 N and 650 °C, adhesive wear emerged as the primary wear mode of the CrN coating, with an average wear depth of 42.93 μm and a wear amount of 23.49 mg. Meanwhile, the valve substrate exhibited a mixed wear mode of adhesive wear and abrasive wear, with an average wear depth of 118.23 μm and a wear amount of 92.66 mg, being 63.7% and 74.6% higher than those of the coating. Thus, the CrN coating showed excellent impact wear resistance, which contributed to the enhancement of the service life of the valve in harsh environments.

The physical concept and the model of the construction of the plasma source of charged particles in crossed E × H fields are presented in which, due to the special configuration of the electrode structure, conditions are created for the formation of stationary double electric layers. It is shown that the formation of such layers promotes the growth of the source perveance and provides the possibility of simultaneous or alternating formation of flows of charged particles of different signs. The electrode structure and the main characteristics of the developed source are given, and the mechanism of its operation is proposed.

We have measured the repulsive force between B-form double helices in parallel packed arrays of polymer-condensed DNA in the presence of 0.005-1.0 M ionic solutions. Molecular repulsion is consistently exponential with a 2.5-3.5 angstrom decay distance, when the separation between DNA surfaces is 5-15 angstrom. Only weakly dependent on ionic strength and independent of molecular size, this intermolecular repulsion does not obey the predictions of electrostatic double-layer theory. Rather, it strongly resembles the ``hydration force'' first recognized and quantified between phospholipid bilayers. Only beyond 15 angstrom separation between molecules is there evidence of electrostatic double-layer forces. The quantitative failure of electrostatic double-layer theory seen here must gravely affect accepted analyses of other polyelectrolyte systems. Because the packing of condensed DNA resembles the spacings of DNA in many bacteriophages, our results permit estimation of the ``DNA pressure'' in phage heads.

Cuticles were isolated enzymatically from the leaves of two maple species (Acer saccharum Marsh and A. platanoides L.) and from orange (Citrus aurantium L.). The cuticles were placed in a plastic cuvette and different concentrations of KCl were perfused over the physiological inner and outer surfaces while the electrical potential (Eio) that developed across the cuticles and was caused by ion diffusion was measured. Eio was always positive, indicating that the permeability of K+ was always greater than that of Cl-. Measured Eio in cuticles did not fit the Goldman equation, whereas, Eio measured during KCl diffusion across selected artificial membranes fit the equation. The magnitude of Eio in cuticles and artificial membranes also was dependent on ionic strength, decreasing as ionic strength increased. These observations are explained by combining classical transport equations with equations that describe the equilibrium ion distribution between ionic double layers in the cuticle or membranes and the bathing solution

Electrical double layer theory is used to predict ionic transport coefficients in the fluid between two parallel charged plates, which represent clay particles. The compaction of a finite quantity of mud (clay suspended in water), to form a clay filter cake, is then modelled, using double layer theory to predict the forces between the clay particles. The path followed by the water and ions is taken to be a smoothly varying channel, which is widest at the upper (least-compacted) surface of the cake, and narrowest at the (most-compacted) cake base. There is a constant surface charge density over the walls of the channel. The electrical double layers surrounding the clay particles overlap one another at the base of the cake, and act as an ion-selective membrane. When filtration commences, the ionic concentration in the filtrate is initially lower than that in the clay suspension. The mean ionic concentration of the suspension increases during the compaction: that of the filtrate first increases and then decreases. Such variations have been previously reported, and may be responsible for the observed time dependence of the clay concentration at the base of clay filtercakes. Predictions are also made of the streaming potential generated across the cake by the motion of the filtrate: these are of the same order of magnitude as those obtained experimentally.