Surface Nanocrystallization of an Ionic Liquid

Jeon, Young-Deuk; Vaknin, David; Bu, Wei; Sung, Jaeho; Ouchi, Yukio; Sung, Woongmo; Kim, Doseok

doi:10.1103/physrevlett.108.055502

Cited by 43 publications

(44 citation statements)

References 30 publications

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“…This contrasts molecular dynamics simulations of a model of BMIM + PF − 6 on graphite electrodes that indicates a voltage driven structural transition and divergent capacitance [5]. Experimental indications of similar emergent long-ranged correlations have been observed in many systems, including spontaneous two-dimensional ordering of PF 6 on gold [13] and free surfaces [14], as well as observations of hysteresis upon voltage cycling of C 9 MIM + Tf 2 N − on epitaxial graphene [3] with observed structural bistability [15].To explain these observations of spontaneous interfacial ordering, I consider the implications of two competing interactions: 1) short range repulsions that arise from packing constraints and can favor spontaneous phase separation, and 2) long-ranged attractions that arise from oppositely charged species and frustrate phase separation. For a symmetric solution, the lowest-order expansion around a uniform charge density yields an effective Hamiltonian,…”

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confidence: 74%

“…The truncation to second order in φ(z = 0), restricts the analysis to weak interactions between the liquid and the electrode [28], where a 1 and h are both order 1, accommodating non-bond interactions like van der Waals forces, and small applied potentials. Previous observations of ordering near free interfaces in ionic liquids suggest that this approximation is sufficient [14]. The field h is related to the chemical potential difference for positive or negative charge density at the interface.…”

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confidence: 99%

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Interfacial Ordering and Accompanying Divergent Capacitance at Ionic Liquid-Metal Interfaces

Limmer

2015

Phys. Rev. Lett.

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A theory is constructed for dense ionic solutions near charged planar walls that is valid for strong inter-ionic correlations. This theory predicts a fluctuation-induced, first-order transition and spontaneous charge density ordering at the interface, in the presence of an otherwise disordered bulk solution. The surface ordering is driven by applied voltage and results in an anomalous differential capacitance, in agreement with recent simulation results and consistent with experimental observations of a wide array of systems. Explicit forms for the charge density profile and capacitance are given. The theory is compared with numerical results for the charge frustrated Ising model, which is also found to exhibit a voltage driven first-order transition.Recently, experimental observations and molecular simulations have suggested a link between long-range structural correlations and the electrochemical response of a double layer capacitor composed of an ionic liquid electrolyte [1][2][3]. Specifically, these observations have alluded to a possible singular response of the differential capacitance to changes in the applied electric potential [4]. It has been postulated on the basis of molecular simulations that this response results from a competition between entropic effects of packing and local constraints of electric neutrality within the ionic liquid near a planar, constant potential electrode [5]. Using general arguments, I construct an effective field theory for a symmetric solution of dense ionic media that validates this proposal. This theory explains the observed anomalous capacitance as a result of a first-order interfacial transition associated with spontaneous charge ordering at the electrode surface.The interface between a dense ionic solution and a metal electrode has been the subject of much recent study, due to the development of ionic liquid-based supercapacitors that exploit charge separation to create high power energy storage devices [6,7]. Such concentrated electrolyte solutions exhibit inter-ionic correlations that render typical mean-field theories developed for dilute solutions, such as Gouy-Chapman-Stern theory [8], not applicable. Extensions of these theories to account for excluded volume have been developed [9][10][11][12], which are capable of capturing interfacial layering and a nonmonotonic capacitance as a function of applied potential. However, such extensions typically assume a linearly responding charge density, which necessitates that their predicted response functions are bounded. This contrasts molecular dynamics simulations of a model of BMIM + PF − 6 on graphite electrodes that indicates a voltage driven structural transition and divergent capacitance [5]. Experimental indications of similar emergent long-ranged correlations have been observed in many systems, including spontaneous two-dimensional ordering of PF 6 on gold [13] and free surfaces [14], as well as observations of hysteresis upon voltage cycling of C 9 MIM + Tf 2 N − on epitaxial graphene [3] with observed structur...

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confidence: 74%

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confidence: 99%

Interfacial Ordering and Accompanying Divergent Capacitance at Ionic Liquid-Metal Interfaces

Limmer

2015

Phys. Rev. Lett.

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“…More extreme cases where interfacial boundary conditions induce an ordered surface, crystallization for instance, were manifested as the coexistence of surface alkane chains in crystalline state and bulk alkane chains in liquid state. 41–43 …”

Section: Resultsmentioning

confidence: 99%

Structure of Biodegradable Films at Aqueous Surfaces: X-ray Diffraction and Spectroscopy Studies of Polylactides and Tyrosine-Derived Polycarbonates

et al. 2013

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Three representative polymers of increasing modulus, poly(D,L-lactic acid), PDLLA, poly(desaminotyrosyl-tyrosine ethyl ester carbonate), PDTEC, and the same polymer with iodinated DTE segments, PI2DTEC, were characterized by surface-pressure versus area (Π-A) isotherms and surface sensitive X-ray diffraction techniques. 10–100 Å thick films were prepared for these studies by spreading dilute polymer solutions at air-water interfaces. The general properties of the isotherms and the Flory exponents, determined from the isotherms, vary in accordance with the increasing modulus of PDLLA, PDTEC, PI2DTEC, respectively. The analysis of in-situ X-ray reflectivity and grazing incidence X-ray diffraction (GIXD) measurements from films at aqueous surfaces provides a morphological picture that is consistent with the modulus of the polymers, and to a large extent, with their packing in their dry-bulk state. Large absorption of X-rays by iodine enabled X-ray spectroscopic studies under near-total-reflection conditions to determine the iodine distribution in the PI2DTEC film and complement the structural model derived from reflectivity and GIXD. These structural studies lay the foundation for future studies of polymer-protein interactions at aqueous interfaces.

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“…Previous studies have mainly focused on ion distributions normal to the electrode surface; however, several recent publications highlight the importance of understanding the 3D structure of the double layer in ionic liquids [24,25]. Previous experimental and simulation results have shown in-plane ordering of the ions at the surface which has been linked to electrochemical response [24,26,27]. Therefore, it is important to have techniques capable of determining the interfacial structure of ionic liquids at the molecular level in a 3D nature.…”

Section: Introductionmentioning

confidence: 97%

Topological defects in electric double layers of ionic liquids at carbon interfaces

et al. 2015

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The structure and properties of the electrical double layer in ionic liquids is of interest in a wide range of areas including energy storage, catalysis, lubrication, and many more. Theories describing the electrical double layer for ionic liquids have been proposed; however, a full molecular level description of the double layer is lacking. To date, studies have been predominantly focused on ion distributions normal to the surface; however, the 3D nature of the electrical double layer in ionic liquids requires a full picture of the double layer structure not only normal to the surface, but also in plane. Here we utilize 3D force mapping to probe the in plane structure of an ionic liquid at a graphite interface and report the direct observation of the structure and properties of topological defects. The observation of ion layering at structural defects such as step-edges, reinforced by molecular dynamics simulations, defines the spatial resolution of the method. Observation of defects allows for the establishment of the universality of ionic liquid behavior vs. separation from the carbon surface and to map internal defect structure. These studies offer a universal pathway for probing the internal structure of topological defects in soft condensed matter on the nanometer level in three dimensions.Please cite this article as: J.M. Black, et al., Topological defects in electric double layers of ionic liquids at carbon interfaces, Nano Energy (2015), http://dx.

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Surface Nanocrystallization of an Ionic Liquid

Cited by 43 publications

References 30 publications

Interfacial Ordering and Accompanying Divergent Capacitance at Ionic Liquid-Metal Interfaces

Interfacial Ordering and Accompanying Divergent Capacitance at Ionic Liquid-Metal Interfaces

Structure of Biodegradable Films at Aqueous Surfaces: X-ray Diffraction and Spectroscopy Studies of Polylactides and Tyrosine-Derived Polycarbonates

Topological defects in electric double layers of ionic liquids at carbon interfaces

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