Ionic fluctuation forces surfaces

This is valid only in the case of an effectively infinite medium in which no walls limit the flow of charges. Conductors must be considered case-by-case under the limitations imposed by boundary surfaces. See, for example, the treatment of ionic solutions (Level 1, Ionic fluctuation forces Tables P.l.d, P.9.C, S.9, S.10, and C.5 Level 2, Sections L2.3.E L2.3.G and Level 3, Sections L3.6 and L3.7). 

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

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. 

The last effect to be described here is film elasticity In case of ionic surfactants the aqueous phase in the double layers contain dissolved counter ions of the surfactants. When the ionic density increases, the repulsive forces of equally charged ions become substantial, see Fig. 11. The repulsive forces are also responsible for a certain elasticity of double layers. The thickness of double layers in the well-known coloured air bubbles lies between 1,000 and 10,000 A. It can be determined by the order of interferential colours The process is very dynamic and fluctuates over the surface area. Under certain conditions the drainage reaches an end at a metastable state (so called black films ) giving the lamella or bubble a limited time of existence ... 

Interfacial surfactant density fluctuations (see Figure 21.14(b)) are another phenomenon not accounted for in a classical DLVO description of thin-film forces. For ionic surfactants, these fluctuations induce charge fluctuations which in turn can influence the height of the DLVO barrier, AH max- Applying a standard statistical thermodynamic approach to the interface provides a simple method for investigating which physical properties influence surfactant density fluctuations at the air-water interface. Analogous to bulk density fluctuations, surface density fluctuations can be expressed by the following ... 

In polar liquids, a polar solute experiences an additional friction, called the dielectric friction, produced by a lag in the electrostatic forces as the solute dipole rotates away from its equilibrium orientationT " " ° " " " " " ° The reduced polarity at the liquid-vapor and water-organic liquid interfaces is thus expected to slow energy relaxation and speed up reorientation. However, surface roughness, capillary fluctuations, and the ability of an ionic solute to keep its hydration shell can complicate this picture. 

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