Emulsion particle-electrolyte interaction

When the electrostatic stabilization of the emulsion is considered, the electrolytes (monovalent and divalent) added to the mixture are the major destabilizing species. The zeta potential of the emulsion particles is a function of the concentration and type of electrolytes present. Two types of emulsion particle-electrolyte (ions) interaction are proposed non-specific and specific adsorption.f H non-specific adsorption the ions are bound to the emulsion particle only by electrical double-layer interactions with the charged surface. As the electrolyte concentration is increased, the zeta potential asymptotes to zero. As the electrostatic repulsion decreases, a point can be found where the attractive van der Waals force is equal to the repulsive electrostatic force and flocculation of the emulsion occurs (Fig. 9A). This point is called the critical flocculation concentration (CFC). 

Determination of the Electrophoretic Mobility, To evaluate the equation for the double-layer interaction (eq 5), the zeta potential, must be known it is calculated from the experimentally measured electrophoretic mobility. For emulsions, the most common technique used is particle electrophoresis, which is shown schematically in Figure 4. In this technique the emulsion droplet is subjected to an electric field. If the droplet possesses interfacial charge, it will migrate with a velocity that is proportional to the magnitude of that charge. The velocity divided by the strength of the electric field is known as the electrophoretic mobility. Mobilities are generally determined as a function of electrolyte concentration or as a function of solution pH. 

Electrosteric stabilization causes the colloidal particles to become resistant against both electrolytes (based on steric repulsion) and temperature (based on the electrostatic interaction). Such stabilization is frequently used industrially by the application of mixed stabilizers (one electrostatic and one steric). Likewise, polyelectrolyte chains - either adsorbed or covalently bound - impart an extraordinary stability against electrolytes [63, 64]. This effect may contribute to stability in emulsion polymerizations containing a small percentage of ionic monomers in the monomer mixture. 

A manifestation of surface forces (either between particles, bubbles, and emulsion droplets or in the vicinity of the three-phase contact line (Fig. 1)) is the disjoining pressure. Let us consider the interaction of two thick plain parallel surfaces divided by a thin layer of liquid of thickness h (e.g., aqueous electrolyte solution) in Fig. 2. The surfaces are not necessarily the same, as shown by two examples (i) 1 is air, 3 is a liquid film, and 2 is solid support (ii) 1 and 2 are air or a liquid, and 3 is a liquid film (of a different liquid). Example (i) is as a liquid film on a solid support and models a liquid layer in the... 

In this case, it is the steric stabilization mechanism that protects the interactive particles from coagulation. In addition, the use of non-ionic t)rpes improves the stability of latex product against electrolytes, freeze-thaw cycles, water and high shear rates. As a result of them, in many emulsion pol3merization recipes (particularly in industry), mixtures of anionic and non-ionic emulsifiers have been widely used together in a s)mergistic manner to control the particle size and to impart enhanced colloidal stability [33-35]. The cationic and zwitterionic emulsifiers are used infrequently in emulsion pol3metizafion applications. 

There are three scenarios for the behavior of two colliding particles in a dispersion (e.g., emulsion) depending on the properties of the films (Fig. 1) (1) When the film formed upon particle collision is stable, floes of attached particles can appear. (2) When the attractive interaction across the film is predominant, the film is unstable and ruptures this leads to a coalescence of the drops in emulsions or of the bubbles in foams. (3) K the repulsive forces are predominant, the two colliding particles will rebound and the colloidal dispersion will be stable. In some cases, by var3ring the electrolyte concentration or pH, it is possible to increase the repulsion between the particles in a flocculated dispersion and to cause the inverse process of peptization [1]. 

---