Ionic liquids charge density distribution

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

The calculations were based on the Guoy-Chapman model for an electric double layer at the interface, a modified Stem model for the inner layer, and experimental input data for predicting the most likely cation-anion arrangement at the surface as shown below in Figure 7.15. The surface potential values 0 were measured and derived for the three ionic liquids mentioned above that had prositive values in the order of [BMIM][Bp4] > [BMIM][DCA] > [BMIM][MS] with potentials of 0.42, 0.37, and 0.14 V respectively. These surface potential values confirm that ionic liquids have a high charge density and different behavior at the interface versus the isotropically distributed molecules in the bulk. The surface potential at the interface includes ions in the Stern layer as well as the dipole contributions. The ion composition of the outer diffuse layer is assumed to give electroneutrality. 

The direction of F anion attack is consistent with the distribution of the density of positive charge [64,65]. The soft electrochemical fluorination very soon attracted attention in the West and is extensively used and developed [70]. Ionic liquids [71], alkali metal fluorides in poly(ethylene glycol) [72], etc., were proposed to be used instead of organic solvents. Just as the hard electrochemical fluorination was called the Simons process the soft fluorination should be referred rightfully to as the Rozhkov—Knunyants reaction ... 

Fig. 10 shows the radial particle densities, electrolyte solutions in nonpolar pores. Fig. 11 the corresponding data for electrolyte solutions in functionalized pores with immobile point charges on the cylinder surface. All ion density profiles in the nonpolar pores show a clear preference for the interior of the pore. The ions avoid the pore surface, a consequence of the tendency to form complete hydration shells. The ionic distribution is analogous to the one of electrolytes near planar nonpolar surfaces or near the liquid/gas interface (vide supra). 

The effect of electrolyte concentration on the transition from common to Newton black films and the stability of both types of films are explained using a model in which the interaction energy for films with planar interfaces is obtained by adding to the classical DLVO forces the hydration force. The theory takes into account the reassociation of the charges of the interface with the counterions as the electrolyte concentration increases and their replacements by ion pairs. This affects both the double layer repulsion, because the charge on the interface is decreased, and the hydration repulsion, because the ion pair density is increased by increasing the ionic strength. The theory also accounts for the thermal fluctuations of the two interfaces. Each of the two interfaces is considered as formed of small planar surfaces with a Boltzmannian distribution of the interdistances across the liquid film. The area of the small planar surfaces is calculated on the basis of a harmonic approximation of the interaction potential. It is shown that the fluctuations decrease the stability of both kinds of black films. 

A mmiolayer of ionic species can be adsorbed at the interface with a solid substrate (a Helmholtz monolayer), Fig. 10.10a. A diffuse layer of ions of the opposite sign with density p(z) provides the overall electrical neutrality. This mechanism is not specific for liquid crystals, it takes place in the isotropic liquids as well. However, in liquid crystals the surface field E = 47cPs rf can interact with the director and change orientation of the latter. Qualitatively, the ionic polarization can be estimated as Psurf = where n is the number of charges q and is a characteristic (Debye) length for the charge distribution. 

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