Electrostatic Forces in Nonpolar Media

The charging of surfaces in nonpolar liquids is relevant not only when oil is transported but also to understand surface forces and the stabilization of dispersions in nonpolar liquids. In this section, we describe electrostatic forces in media with low dielectric constant (reviewed in Ref. [456]). 

A major difference between nonpolar liquids and water is that ions do not easily dissociate because the work required to separate two charges is higher. Let us, for example, take the dissociation of the carboxylic group on benzoic acid QHsCOOHi iCGHsCOO + H +. The pfCof the dissociation is 4.24 in water with 8 = 78.4 at 25 °C. In a 1 1 water-efhanol mixture with e = 50.4, it increases to pK= 5.87 [457]. To estimate the electrostatic work required to separate two charges. 

It decreases with increasing dielectric permittivity. For example, the work to create an ion pair with both ions having a radius J = 0.2 nm so that d = 2Ri at 25 °C is 

4 ke T. In acetone, it requires 6.9 ke T. Experimental values for the solubility of salts confirm this prediction. For example, 360 g 1 of sodium and potassium chloride are soluble of both salts in water. In methanol, the solubilities are 30.2 and 53 gl, in acetone the values are 0.00042 and 0.00091 g 1 , respectively. In less polar media, the solubility decreases further and below e 5, it is negligible. Ion dissociation in nonpolar liquids has been extensively studied [458, 459, 460, 461], in particular by conductivity measurements. 

Equation (4.72) not only shows how important the dielectric permittivity for ion dissociation is but also tells us that ion dissociation requires less energy if the ions are large. If we take the distance of closest approach d to be the sum of the two ionic radii, the dissociation energy decreases for large ions. 

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Both the Gouy-Chapman and Debye-Hiickel are continuum theories. They treat the solvent as a continuous medium with a certain dielectric constant, ignoring the molecular nature of the liquid. Also, the ions are not treated as individual point charges but as a continuous charge distribution. For many applications, this is sufficient and the predictions of continuum theory agree with experimental results. Before we finally calculate the free energy of an electric double layer and force between two double layers, we discuss the limitations and problems of the continuum model. At the end of this chapter, electrostatic forces in nonpolar media are described. 

The long range of double-layer forces in nonpolar media is demonstrated by force measurements. The first direct force experiments were carried out with the SFA [479]. More recently, forces in nonpolar liquids have been measured with optical techniques [464, 478]. Optical techniques are more suitable for such long-range interaction potentials. One example is shown in Figure 4.12. Sainis et al. [464] measured the force between two PMM A particles by first bringing both particles into close proximity by optical tweezers. Then, the particles are released and they start to diffuse apart, driven by the electrostatic force. Their movement is hindered by Stokes friction. Their position is tracked by optical microscopy. From the trajectories of many events, the force versus distance was calculated. 

In nonpolar media due to the low ionization of the solute species, electrostatic attractive or repulsive forces can be ruled out as a major mechanism for adsorption. However, polar interactions have to be considered especially when polar surfaces such as oxides are involved. Recent work has shown acid-base interactions between the surface species and the solute molecules to be responsible for adsorption in nonaqueous media. Fowkes has suggested that the interaction between a solid surface and an uncharged adsorbate can be divided into two parts, dispersive interactions and polar interactions. The dispersive interactions are due to the fluctuating dipole moments created by the movement of electrons in any atom or molecule and thus occur between all atoms and molecules. Polar interactions refer to specific interactions between hydrophilic surface groups and functional groups in the adsorbate molecules. 

Micellar solutions are sometimes called ordered media [12]. The chemical order in a micellar solution seems to be greater than in a classical solution. Equation 2.9 shows ftiat the micellization of surfactant molecules obeys the second principle of thermodynamics. It seems that the surfectant hydrocarbon chains have a much higher freedom of motion inside the micelle core than in the water bulk [13]. The micelle structure minimizes the molecule energy. The large entropy increai of water molecules associated with the removal of nonpolar surfactant tails from the aqueous solution (hydrophobic effect) is the main micelle driving force. Electrostatic forces tend to separate the polar heads that bear the same charge. The whole micelle is an equilibrium between these forces. This equilibrium is very sensitive to any chemical additive or parameter that can act on any of the forces, such as salts, polar or nonpolar solutes, temperature and/or pressure. 

Concerning more complex systems such as colloidal suspensions of nanoparticles, Santini and co-workers reported studies on the influence of the size of the nanostructures in ionic liquids and the size distribution of ruthenium nanoparticles synthesised therein by reduction of a nonpolar organometallic complex. The stabilisation mechanism proposed is based on a tenplate effect linked to the structure of the ionic liquids [66, 75, 76], not to electrostatic stabilisation (due to DLVO-type forces as observed in colloidal suspensions in aqueous electrolytes). Watanabe and co-workers also observed that silica nanoparticle colloids couldn t be stabilised in ionic liquids without surface-grafted polymer chains, again indicating that DLVO-type forces are insufficient for effective stabilisation, as expected from the short screening length in these media composed mainly of ions [77]. 

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