Electrostatic interaction forces

If a substance is to be dissolved, its ions or molecules must first move apart and then force their way between the solvent molecules which interact with the solute particles. If an ionic crystal is dissolved, electrostatic interaction forces must be overcome between the ions. The higher the dielectric constant of the solvent, the more effective this process is. The solvent-solute interaction is termed ion solvation (ion hydration in aqueous solutions). The importance of this phenomenon follows from comparison of the energy changes accompanying solvation of ions and uncharged molecules for monovalent ions, the enthalpy of hydration is about 400 kJ mol-1, and equals about 12 kJ mol-1 for simple non-polar species such as argon or methane. 

The charge delocalization or the polarizability difference explains the selectivity behaviour in cases of high polarizability differences (complex versus aqueous metal ion) or in a homologous series of ions (either inorganic cations or ammonium cations). The smaller hydration status of all types of interlamellarly adsorbed cations is ascribed to the mutual stabilization by charge delocalization over the planar oxygens and exchangeable cations and is caused by the electrostatic interaction forces. 

When two similarly charged colloid particles, under the influence of the EDL, come close to each other, they will begin to interact. The potentials will detect one another, and this will lead to various consequences. The charged molecules or particles will be under both van der Waals and electrostatic interaction forces. The van der Waals forces, which operate at a short distance between particles, will give rise to strong attraction forces. The potential of the mean force between colloid particle in an electrolyte solution plays a central role in the phase behavior and the kinetics of agglomeration in colloidal dispersions. This kind of investigation is important in these various industries ... 

The purpose of the present chapter is to introduce some of the basic concepts essential for understanding electrostatic and electrical double-layer pheneomena that are important in problems such as the protein/ion-exchange surface pictured above. The scope of the chapter is of course considerably limited, and we restrict it to concepts such as the nature of surface charges in simple systems, the structure of the resulting electrical double layer, the derivation of the Poisson-Boltzmann equation for electrostatic potential distribution in the double layer and some of its approximate solutions, and the electrostatic interaction forces for simple geometric situations. Nonetheless, these concepts lay the foundation on which the edifice needed for more complicated problems is built. 

The basic point is that the mass action laws of chemistry ([A][B]/[AB] = constant) do not work for ions in solution. The reason they do not work puzzled ehemists for 40 years before an acceptable theory was found. The answer is based on the effects of electrostatic interaction forces between the ions. The mass aetion laws (in terms of concentrations) work when there are no charges on the partieles and hence no long-range attraction between them. When the particles are charged. Coulomb s law applies and attractive and repulsive forces (dependent on 1/r where r is the distanee between the ions) come in. Now the particles are no longer independent but puU on each other and this impairs the mass action law, the silent assumption of which is that ions are free to act alone. 

The electrostatic interaction force Pe(h) per unit area between the two charged brush layers at separation h when they are separated (h > Ido) is given by the osmotic pressure at the midpoint between the plates x = hH minus that in the bulk solution phase, namely,... 

The potential distribution can be calculated with Eqs. (18.34) and (18.36) with the help of the values of y(—d) and y. The electrostatic interaction force acting between membranes 1 and 2 per unit area can be calculated from... 

FIGURE 8.25 The stability of a sol (a suspension of colloidal particles) may be evaluated from the balance of repulsive (electrostatic) interaction forces and attractive (dispersive) interaction forces, e.g., by applying the DLVO theory (Equation 8.103). If a potential energy barrier exists the system is stable (left). If the barrier is removed, the coagulation of the particles is contolled by diffusion alone. (Courtesy of Jean Le Bell.)... 

In addition to these electrokinetic phenomena, electrostatic interactions among the microparticles due to their induced dipole are also observable [23]. The electrostatic interaction force, Fdipoie = r Sm Q fcM E [44], can make the particles form a structure like perl chain by attractive forces in the direction of an electric field, and a crystalline structure with a regular distances among the particles by repulsive forces in the plane perpendicular to the electric field. These electrostatic attractive and repulsive interactions can interfere with the precise control of microparticles using lab-on-a-display. On the other hand, we can utilize these phenomena for several applications such as a manufacture of self-assembled micropattem structures, a study about interactions between two cells, and a bead-based immunoassay. 

Figure 5. Change of distances among 3-pm-diameter microparticles concentrated within the illuminated area. Combination of several physical mechanisms including frequency-dependent electrokinetics and electrostatic interaction forces affect the behavior of particles. (Reproduced with permission from Ref [30] Copyright 2009, American Chemical Society.)...

These observations indicate that besides a difference in the amount of protein actually incorporated in the gel network, there is also a difference in the spatial structure of the gel network between low and high pH. The difference in network structure may have a clear effect on mechanical properties of the gels.32,33 Moreover, electrostatic interaction forces in and between the protein molecules will vary with pH. 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

Surface forces also include electrostatic interaction forces arising from the overlap of the double layers (DL) of a particle and a bubble, which usually have equal charges (Huddleston Smith 1975), i.e., the electrostatic component of the disjoining pressure of an interlayer between them (Derjaguin 1934), which may be positive. In the case of large particles, the positive disjoining pressure of the double layer is overcome by an inertia impact on the bubble surface. The small particles do not undergo such an impact the approach occurs in an inertialess way and can be hampered by electrostatic repulsion (second peculiarity). 

Electrostatic interaction forces (3) can contribute to the adsorption of polar molecules on polar adsorbents. The energy of interaction Eg of an adsorbate dipole of moment in a surface field of intensity F is given as... 

Molecular and Electrostatic Interaction Forces Acting on Drops... 

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