Water-Ionic Liquid Interfaces

The 18C6 Molecules and their Strontium Complexes at the Ionic Liquid / Water Interface... 

Sieffert, N. and G. Wipff, The [BMI][Tf2N] ionic liquid/water binary system A molecular dynamics study of phase separation and of the liquid-liquid interface, J Phys Chem B, Vol. 110, (2006) p. 13076. 

Silvester DS, Arrigan DWM (2011) Array of water vertical bar room temperature ionic liquid micro-interfaces. Electrochem Commun 13(5) 477-479. doi 10.1016/j.elecom.2011.02.025... 

Laforge FO, Kakiuchi T, Shigematsu F, Mirkin MV (2004) Comparative study of electron transfer reactions at the ionic liquid/water and organic/water interfaces. J Am Chem Soc 126 15380-15381... 

Although this technique has not been used extensively, it does allow structures of adsorbed layers on solid substrates to be studied. Liquid reflectivity may also be performed with a similar set-up, which relies on a liquid-liquid interface acting as the reflective surface and measures the reflectivity of a thin supported liquid film. This technique has recently been used to investigate water-alkane interfaces [55] and is potentially useful in understanding the interaction of ionic liquids with molecular solvents in which they are immiscible. 

Thermodynamics of adsorption at liquid interfaces has been well established [22-24]. Of particular interest in view of biochemical and pharmaceutical applications is the adsorption of ionic substances, as many of biologically active compounds are ionic under the physiological conditions. For studying the adsorption of ionic components at the liquid-liquid interface, the polarized liquid-liquid interface is advantageous in that the adsorption of ionic components can be examined by strictly controlling the electrical state of the interface, which is in contrast to the adsorption studies at the air-water or nonpolar oil-water interfaces [25]. 

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

Hesleitner, P. Babic, D. Kallay, N. Matijevic, E. (1987) Adsorption at solid/solution interfaces. 3. Surface charge and potential of colloidal hematite. Langmuir 3 815-820 Hesleitner, P. Kallay, N. Matijevic, E. (1991) Adsorption at solid/liquid interface. 6. The effect of methanol and ethanol on the ionic equilibrium at the hematite/water interface. Langmuir 7 178-184... 

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

Recently [111], the ET was probed at the interface between water and a hydrophobic ionic liquid (EL), l-octyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide. Ferrocene was dissolved in an ionic liquid, and ferrocyanide—in the aqueous phase. The tip was immersed in the aqueous phase. Ferricyanide, electrogenerated at the tip, diffused toward the interface where it was reduced by ferrocene... 

The interfacial potential drop at the nonpolarizable ITIES was controlled by varying the concentration of either the cation or the anion of the ionic liquid in the aqueous phase. The kinetics of interfacial ET followed the Butler-Volmer equation, and the measured bimolecular rate constant was much larger than that obtained at the water-1,2-dichloroethane interface. In the second publication, Laforge et al. [112] developed a new method for separating the contributions from the interfacial ET reaction and solute partitioning to the SECM feedback. 

Danielsson et al. [25] have studied the synthesis of PEDOT in ionic liquids that utilize bulky organic anions, l-butyl-3-methylimidazolium diethylene glycol monomethyl ether sulfate and l-butyl-3-methylimidazolium octyl sulfate, the latter of which is a solid at room temperature and thus requires the addition of either monomer or solvent (in this case water) to form a liquid at room temperature. Polymerization in a water-free ionic liquid was only possible in the octyl sulfate species, but the polymerization of EDOT was successful in aqueous solutions of both the ionic liquids (0.1 M). The ionic liquid anions appear to be mobile within the polymer, exchangeable with chloride ions at a polymer/KCl(aq) interface, but it is interesting that when the PEDOT is in aqueous solutions of the ionic liquid, at higher concentrations (0.01-0.1 M) the imidazolium cation can suppress this anion response. The ion mobility in both the ionic liquid and in the polymer film in contact with the solution is significantly increased by addition of water. 

In this chapter we present a few selected results on the nanoscale electrodeposition of some important metals and semiconductors, namely, Al, Ta and Si, in air- and water-stable ionic liquids. Here we focus on the investigation of the electrode/electrolyte interface during electrodeposition with the in situ scanning tunneling microscope and we would like to draw attention to the fascinating... 

In Table 2 we summarize some of their findings. The table shows the amount of water in the ionic liquid, the amount of ionic liquid in the water, and the width of the interface separating the two phases at the end of the simulation for the different force fields. It is seen that the popular TIP3P model gives quite different results from those of the other force fields and in the simulation of the mixing process there is not even a well defined interface. The other two force fields give similar results. All simulations support that the ionic liquid is essentially water-free, as should be. 

Table 2 Molar fraction of water in the ionic liquid and of the ionic liquid in water, as well as width of the interface as found in the molecular-dynamics simulations with different force fields. (IL) describes the effective charge of the ions in the ionic liquid used in the force field for this system, and FF(H20) describes the force field for the water. The results for the mixing simulations were obtained by considering a system where the ionic liquid and the water initially were separated, whereas demixing marks results where the two systems initially formed a mixture. All results are from ref 29... 

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