Ionic Fluids Research Articles

Liquid–vapour transition in screened Coulomb binary mixtures

The smooth cut-off hierarchical reference theory (HRT) is applied to a simple model of charged colloid mixture: an equimolar binary fluid of hard-core particles with a Yukawa potential which is repulsive between particles of the same species, and attractive between particles of different species. The critical behaviour of this Yukawa restricted primitive model (YRPM) is determined, and shown to belong to the Ising universality class for every value of the inverse Yukawa screening length z. This remains true even in the limit z → 0 of unscreened Coulomb potential, when the YRPM reduces to the restrictive primitive model (RPM) of ionic fluids, thereby providing a microscopic justification to the occurrence of Ising-like criticality in the RPM. A simple closure for the pair correlations based on the generalized mean spherical approximation (GMSA) is adopted. Within such an approximation scheme, the fluid–fluid phase diagram and critical point are obtained for two representative values of the inverse screening length: z = 2 and z = 0.1.

Editorial: Special issue on ionic fluids and its biological application

Ionic fluids have ever been of central interest in physical chemistry, but in recent years have also permeated in biology and resulted in numerous biological applications. It has been well recognized that life occurs in ionic fluids, thus the properties and behavior of ionic fluids are critical to understand many biological phenomena. There have been tremendous progresses during the past a few decades in the theories of ionic fluids, from the cluster expansion theory of McMillan-Mayer, integral equation approaches with the hypernetted chain (HNC)/mean spherical approximation (MSA) and Percus-Yevick (PY) equations, classical density functional theory to recent variational field theory that includes both structure and flow. Theoretical studies have begun to consider more realistic interactions between various components in ionic fluids. However, two properties render ionic fluid theories difficult, namely the long-range correlation of Coulomb interactions and the size/shape effect of various components in ionic fluids. These interactions have distinct consequences on the structure of ionic fluids. Additionally, the long-range nature of the Coulomb interactions not only causes technical problems in theory, but also challenges numerical simulations.

Potential of mean force between identical charged nanoparticles immersed in a size-asymmetric monovalent electrolyte

In a previous theoretical and simulation study [G. I. Guerrero-García, E. González-Tovar, and M. Olvera de la Cruz, Soft Matter 6, 2056 (2010)], it has been shown that an asymmetric charge neutralization and electrostatic screening depending on the charge polarity of a single nanoparticle occurs in the presence of a size-asymmetric monovalent electrolyte. This effect should also impact the effective potential between two macroions suspended in such a solution. Thus, in this work we study the mean force and the potential of mean force between two identical charged nanoparticles immersed in a size-asymmetric monovalent electrolyte, showing that these results go beyond the standard description provided by the well-known Derjaguin-Landau-Verwey-Overbeek theory. To include consistently the ion-size effects, molecular dynamics (MD) simulations and liquid theory calculations are performed at the McMillan-Mayer level of description in which the solvent is taken into account implicitly as a background continuum with the suitable dielectric constant. Long-range electrostatic interactions are handled properly in the simulations via the well established Ewald sums method and the pre-averaged Ewald sums approach, originally proposed for homogeneous ionic fluids. An asymmetric behavior with respect to the colloidal charge polarity is found for the effective interactions between two identical nanoparticles. In particular, short-range attractions are observed between two equally charged nanoparticles, even though our model does not include specific interactions; these attractions are greatly enhanced for anionic nanoparticles immersed in standard electrolytes where cations are smaller than anions. Practical implications of some of the presented results are also briefly discussed. A good accord between the standard Ewald method and the pre-averaged Ewald approach is attained, despite the fact that the ionic system studied here is certainly inhomogeneous. In general, good agreement between the liquid theory approach and MD simulations is also found.

Ionic Liquid Based Aqueous Biphasic Systems with Controlled pH: The Ionic Liquid Cation Effect

This work addresses the evaluation of the ability of ionic liquid cations on the formation of aqueous biphasic systems (ABSʼs), with K2HPO4 or a mixture of inorganic salts, K2HPO4/KH2PO4, aiming at controlling the pH values of the coexisting aqueous phases. Using chloride-based ionic liquids, the effects of the cation core, the length of the alkyl side chain, and the positional isomerism on ABS formation ability were investigated. All binodal curves were determined by the cloud-point titration method at 298 K. From the obtained phase diagrams it is shown that the biphasic area increases with the cation side chain length, from ethyl to hexyl chains, although for longer chains, an inversion on the binodal curves sequence appears due to the self-aggregation of the longer chain ionic liquids in aqueous solutions. The influence of the cation core and the positional isomerism of the ionic liquids on their ability to form ABS closely correlates with the ionic fluid affinity for water.

Thermophysical Characterization of Ionic Liquids Able To Dissolve Biomass

Among new potential solvents for lignocellulosic materials, ionic liquids (ILs) are attracting considerable attention. Hence, the knowledge of the thermophysical properties of such fluids is essential for the design of related industrial processes. Therefore, in this work, a set of thermophysical properties, namely, density, viscosity, and refractive index, as a function of temperature, and isobaric thermal expansivity and heat capacities at a constant temperature, were determined for eight ionic liquids with the 1-ethyl-3-methylimidazolium cation combined with the following anions: acetate, methylphosphonate, methanesulfonate, trifluoromethanesulfonate, dicyanamide, thiocyanate, tosylate, and dimethylphosphate. Imidazolium-based ILs were chosen since these are the most studied ionic fluids in biomass dissolution approaches, while a large array of anions was investigated because it was already demonstrated that it is the IL anion that mainly governs the dissolution.

A molecular Debye-Hückel theory and its applications to electrolyte solutions

In this report, a molecular Debye-Hückel theory for ionic fluids is developed. Starting from the macroscopic Maxwell equations for bulk systems, the dispersion relation leads to a generalized Debye-Hückel theory which is related to the dressed ion theory in the static case. Due to the multi-pole structure of dielectric function of ionic fluids, the electric potential around a single ion has a multi-Yukawa form. Given the dielectric function, the multi-Yukawa potential can be determined from our molecular Debye-Hückel theory, hence, the electrostatic contributions to thermodynamic properties of ionic fluids can be obtained. Applications to binary as well as multi-component primitive models of electrolyte solutions demonstrated the accuracy of our approach. More importantly, for electrolyte solution models with soft short-ranged interactions, it is shown that the traditional perturbation theory can be extended to ionic fluids successfully just as the perturbation theory has been successfully used for short-ranged systems.

A model of electrodiffusion and osmotic water flow and its energetic structure

We introduce a model for ionic electrodiffusion and osmotic water flow through cells and tissues. The model consists of a system of partial differential equations for ionic concentration and fluid flow with interface conditions at deforming membrane boundaries. The model satisfies a natural energy equality, in which the sum of the entropic, elastic and electrostatic free energies is dissipated through viscous, electrodiffusive and osmotic flows. We discuss limiting models when certain dimensionless parameters are small. Finally, we develop a numerical scheme for the one-dimensional case and present some simple applications of our model to cell volume control.

Integral Equations in the Study of Polar and Ionic Interaction Site Fluids

In this review article we consider some of the current integral equation approaches and application to model polar liquid mixtures. We consider the use of multidimensional integral equations and in particular progress on the theory and applications of three dimensional integral equations. The IEs we consider may be derived from equilibrium statistical mechanical expressions incorporating a classical Hamiltonian description of the system. We give example including salt solutions, inhomogeneous solutions and systems including proteins and nucleic acids.

Gas–liquid coexistence in asymmetric primitive models of ionic fluids

The gas–liquid coexistence of z:1 asymmetric primitive models is studied using the collective variable based theory. Coexistence-curve data are examined in terms of the corresponding-state variables τ ∗ = | T* − T c*|/ T c* and Δρ ∗ = | ρ ∗ − ρ c*|/ ρ c*. Calculations are made for 1:1, 2:1, 3:1 valences and for different values of ion size ratio λ = σ +/ σ − ( λ ≥ 1 and λ ≤ 1). We analyze a dependence of the critical amplitude of the coexistence curve and the asymptotic slope of the coexistence-curve diameter on the ion size ratio at a fixed valence. It is shown that the both characteristics take maximum values in the equisize case and demonstrate a nonmonotonous behavior with λ at z ≥ 2. Besides, the critical amplitude depends on λ very weakly. As expected, the approximation considered in this work predicts the mean-field critical behavior for size- and charge-asymmetric primitive models in the vicinity of the critical point. A comparison with the available theoretical and simulation results is given.

Publisher's Note: “What makes ionic fluids characteristically ionic? A corresponding-states analysis of the surface tension of an ionic model fluid with variable dispersion interactions” [J. Chem. Phys. 134, 094703 (2011)

Publisher's Note: “What makes ionic fluids characteristically ionic? A corresponding-states analysis of the surface tension of an ionic model fluid with variable dispersion interactions” [J. Chem. Phys. 134, 094703 (2011)

Effect of Diode-Laser and AC Magnetic Field of Bovine Serum Albumin Nanospheres Loaded with Phthalocyanine and Magnetic Particles

This study reports on the development and characterization of bovine serum albumin (BSA) nanospheres containing Silicon(IV) phthalocyanine (NzPc) and/or maghemite nanoparticles (MNP), the latter introduced via ionic magnetic fluid (MF). The nanosized BSA-loaded samples were designed for synergic application while combining Photodynamic Therapy and Hyperthermia. Incorporation of MNP in the albumin-based template, allowing full control of the magnetic content, was accomplished by adding a highly-stable ionic magnetic fluid sample to the albumin suspension, following heat denaturing. The material's evaluation was performed using Zeta potential measurements and scanning electron microscopy. The samples were characterized by steady-state techniques and time-resolved fluorescence. The in vitro assay, using human fibroblasts, revealed no cytotoxic effect in all samples investigated, demonstrating the potential of the tested system as a synergistic drug delivery system.

Non-equilibrium molecular dynamics simulation of electrokinetic effects on heterogeneous ionic transport in nano-channel

The presence of the electric double layer (EDL) near the solid/liquid interface has a great impact on the liquid flowing through nano channels. In this paper, a non-equilibrium molecular dynamics method (NEMD) model is developed to investigate the transport characteristics of the heterogeneous ionic fluid flowing in a 4.672 nm-depth channel due to the electrokinetic effect induced by the EDL. An external perturbing velocity is introduced on the liquid instead of a constant force to study the electrokinetic effects on the flow. The effect of charges adsorbed on channel surfaces on flow resistance is especially considered. Two different charged surfaces with discrete or uniform charge distributions are compared, respectively. Besides Lennard-Jones (L-J) potential energy and Coulomb force, the ion–dipole interaction between the charged particle and the water molecule in the liquid is especially taken into account. According to the simulation results, the liquid density reaches the peak value in the EDL. However, unlike that predicted by the Poisson–Boltzmann theory, the liquid density profile is not exactly an anti-parabola curve, which has a significant fall before the density reaching the peak value in the EDL. It is shown that the charges absorbed on the channel surface have little influence on the liquid density distribution, but they play an important role in the fluid velocity distribution. Meanwhile, the liquid velocity slippage in the EDL emerges, which is well in agreement with the prior experimental data available. Moreover, though the non-dimensional velocities are nearly the same under different external velocity fields, the velocity slippage for the uniformly charged wall is much larger than that for the discretely charged wall. All of these results provide a probable reason why the flow behavior in a nano-channel is particularly different from that in a macro duct.

Selol-loaded magnetic nanocapsules: A new approach for hyperthermia cancer therapy

Polylactic-co-glycolic nanocapsules, loaded with nanosized magnetic particles and Selol (a selenium-based anticancer drug), were successfully prepared by the precipitation method. Maghemite (γ-Fe2O3) nanoparticles were incorporated into the nanocapsules using a highly stable ionic magnetic fluid sample. The obtained nanocapsules presented no agglomeration, negative surface charge while revealing a narrow monomodal size distribution. All the nanocapsule formulations exhibited a good physical stability at 4 °C during 3 month storage period. The in vitro antitumoral activity of Selol-magnetic nanocapsules was assessed using a murine melanoma cell line. The influence of nanocapsules on cell viability was investigated by spectrophotometric assay. The results demonstrated that Selol-loaded magnetic nanocapsules (at 100 μg/ml/5 × 109 particle/ml) showed antitumoral activity of 50% on melanoma cells (absence of magnetic field). These results clearly indicate that the loaded nanocapsules represent a novel and promising magnetic drug delivery system suitable for cancer treatment via the active drug and magnetohyperthermia.

What makes ionic fluids characteristically ionic? A corresponding-states analysis of the surface tension of an ionic model fluid with variable dispersion interactions

Inorganic molten salts, such as NaCl, are known to show characteristically lower values of Guggenheim's corresponding-states surface tension γ(red) at a given reduced temperature T∕T(c) than simple or aprotic polar fluids. Recently, the corresponding values of γ(red) for (some) room temperature ionic liquids (RTILs) were found in the same region as those for weakly polar fluids, that is, markedly above the values typical of inorganic molten salts despite the ionic character of RTILs. Here, we present the results of simulations of an ionic model fluid in which the strength of attractive dispersion interactions among the ions is varied relative to the Coulomb interactions. For weak dispersive interactions, the behavior known for real inorganic molten salts is found. If the attractive dispersion energy of two unlike ions at contact exceeds 20% of the Coulombic attraction in such an isolated ion pair, γ(red) increases markedly and approaches the region of values for simple and polar fluids. Rough theoretical estimates of the relative strengths of dispersive and Coulombic attractions in molten inorganic salts and in RTILs support our conclusion that the dispersion interactions in RTILs are strong enough for their corresponding-states surface tension to behave regularly and, thus, to deviate from the values one would expect for strongly ionic systems.

Electrodiffusion and Osmotic Water Flow and its Variational Structure

We introduce a model for ionic electrodiffusion and osmotic water flow through cells and tissues. The model consists of a system of partial differential equations for ionic concentration and fluid flow with interface conditions at deforming membrane boundaries. The model satisfies a natural energy equality, in which the sum of the entropic, elastic and electrostatic free energies are dissipated through viscous, electrodiffusive and osmotic flows. We discuss limiting models when certain dimensionless parameters are small. Finally, we develop a numerical scheme for the one-dimensional case and present some simple applications of our model to cell volume control.

Contact Angle Hysteresis on Superhydrophobic Surfaces: An Ionic Liquid Probe Fluid Offers Mechanistic Insight

Silicon/silicon dioxide surfaces containing 3 μm (width) × 6 μm (length) × 40 μm (height) staggered rhombus posts were prepared using photolithography and hydrophobized using a perfluoroalkyl-containing monofunctional silane. These surfaces exhibit water contact angles of θ(A)/θ(R) = 169°/156°. Water drops come to rest on a carefully aligned horizontal sample but roll when the surface is tilted slightly. No visible trail or evidence of water "left behind" at the receding edge of the drop is apparent on surfaces that water drops have rolled on or on samples removed from water through the air-water interface. When dimethylbis(β-hydroxyethyl)ammonium methanesulfonate (N(+)S(-), a nonvolatile ionic liquid) is used as the liquid probe fluid (instead of water), contact angles of θ(A)/θ(R) = 164°/152° are observed and ∼3-μm-diameter sessile drops are visible (by scanning electron microscopy - SEM) on the top of every post of a sample drawn out of this liquid. We interpret the formation of these sessile microdrops as arising from microcapillary bridge failure that occurs during receding events and emphasize that the capillary bridges rupture in primarily a tensile failure mode. Smaller sessile drops could be prepared using mixtures of water and N(+)S(-). Microdroplets of N(+)S(-) were also observed to form selectively at particular features on surfaces containing square holes separated by ridges. This suggests that pinning sites can be identified using microscopy and this ionic liquid probe fluid.

Silicon field effect transistors as dual-use sensor-heater hybrids.

We demonstrate the temperature mediated applications of a previously proposed novel localized dielectric heating method on the surface of dual purpose silicon field effect transistor (FET) sensor-heaters and perform modeling and characterization of the underlying mechanisms. The FETs are first shown to operate as electrical sensors via sensitivity to changes in pH in ionic fluids. The same devices are then demonstrated as highly localized heaters via investigation of experimental heating profiles and comparison to simulation results. These results offer further insight into the heating mechanism and help determine the spatial resolution of the technique. Two important biosensor platform applications spanning different temperature ranges are then demonstrated: a localized heat-mediated DNA exchange reaction and a method for dense selective functionalization of probe molecules via the heat catalyzed complete desorption and reattachment of chemical functionalization to the transistor surfaces. Our results show that the use of silicon transistors can be extended beyond electrical switching and field-effect sensing to performing localized temperature controlled chemical reactions on the transistor itself.

Transfer Layers: A Comparison across SWNTs, DWNTs, Graphite, and an Ionic Fluid

Lubrication is the science of friction at moving interfaces. Nanomaterials acting as interfacial modifiers can minimize friction and thereby improve energy efficiency. To test this hypothesis, single- (SWNT) and double-walled (DWNT) carbon nanotubes and an ionic fluid are tested individually and compared to SWNTs and graphite as additives within the ionic fluid. The minimum coefficient of friction is correlated with the longest lifetime using a ball-on-disc tribometer, in air, at atmospheric pressure. Results are interpreted in terms of the nanotubes' mechanical properties and the formation of transfer layers upon the tribosurfaces.

The liquid-vapor interface of the restricted primitive model of ionic fluids from a density functional approach

We investigate the liquid-vapor interface of the restricted primitive model for an ionic fluid using a density functional approach. The applied theory includes the electrostatic contribution to the free energy functional arising from the bulk energy equation of state and the mean spherical approximation for a restricted primitive model, as well as the associative contribution, due to the formation of pairs of ions. We compare the density profiles and the values of the surface tension with previous theoretical approaches.

Electrospun nanosized cellulose fibers using ionic liquids at room temperature

Aiming at replacing the noxious solvents commonly employed, ionic-liquid-based solvents have been recently explored as novel non-volatile and non-flammable media for the electrospinning of polymers. In this work, nanosized and biodegradable cellulose fibers were obtained by electrospinning at room temperature using a pure ionic liquid or a binary mixture of two selected ionic liquids. The electrospinning of 8 wt% cellulose in 1-ethyl-3-methylimidazolium acetate medium (a low viscosity and room temperature ionic liquid capable of efficiently dissolving cellulose) showed to produce electrospun fibers with average diameters within (470 ± 110) nm. With the goal of tailoring the surface tension of the spinning dope, a surface active ionic liquid was further added in a 0.10 : 0.90 mole fraction ratio. Electrospun cellulose fibers from the binary mixture composed of 1-ethyl-3-methylimidazolium acetate and 1-decyl-3-methylimidazolium chloride ionic liquids presented average diameters within (120 ± 55) nm. Scanning electron microscopy, X-ray diffraction analysis, nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric assays were used as core methods to evaluate the structural integrity, morphology and crystallinity of the raw, electrospun, and regenerated samples of cellulose. Moreover, the photoluminescence spectra of both raw and electrospun fibers were acquired, and compared, indicating that the cellulose emitting centers are not affected by the dissolution of cellulose in ionic liquids. Finally, the use of non-volatile solvents in electrospinning coupled to a water coagulation bath allows the recovery of the ionic fluid, and represents a step forward into the search of environmentally friendly alternatives to the conventional approaches.