Non-flocculated Emulsions

The stability of w/c emulsions, defined as the time required for the volume of the emulsion to settle from 100% to 90% based upon visual observation, has been measured for PFPE-COO"NH4" surfactants with molecular weights ranging from 667 to 7500 [17]. Figure 2.4-10 shows the stability of emulsions formed by the above microfluidizer for equal weights of water and CO2 and 1.3 wt% of 2500g/mol PFPE-COO NH4. For each experiment where non-flocculated emulsions were present during shear, the specific conductivity was less than O.lpS/cm, indicating water droplets in a CO2 continuous... 

Note that the minimum flocculation degree value is observed at the same surfactant ratios at which the minimum interfacial tension value is achieved. This makes clear the previously known facts about the increase of the emulsion stability at low y. This occurs, first of all, due to an appreciable decrease in the flocculation degree up to its complete suppression. These phenomena result in an increase in the sedimentation stability both for non-flocculated emulsions (dispersity increase) and partly flocculated emulsions (decrease in flock size and flocculation degree). 

The presence of dispersed particles may significantly affect the value of dielectric constant of disperse system. In some cases, e.g. in non-aggregated (non-flocculated) inverse emulsions (Chapter VIII,3), the dielectric constant is related to the volume fraction of droplets in the emulsion, VKi, by the Bruggerman relationship... 

Figure 5.6 Dependence of emulsion viscosity on droplet concentration for non-flocculated and flocculated emulsions. The viscosity increases with increasing droplet concentration.

Another method of reducing creaming or sedimentation is to induce weak flocculation in the emulsion system. This may be achieved by controlling some parameters of the system, such as electrolyte concentration, adsorbed layer thickness and droplet size. These weakly flocculated emulsions are discussed in the next section. Alternatively, weak flocculation may be produced by addition of a free (non-adsorbing) polymer. Above a critical concentration of the added polymer, polymer-polymer interaction becomes favourable as a result of polymer coil overlap and the polymer chains are squeezed out from between the droplets. This results in a polymer-free zone between the droplets, and weak attraction occurs as a result of the higher osmotic pressure of the polymer solution outside the droplets. This phenomenon is usually referred to as depletion flocculation [59] and can be applied for structuring emulsions and hence reduction of creaming or sedimentation. 

The equation that we gave in 6.4.2 for the velocity of a cloud of spheres moving in a liquid under the influence of gravity, is sometimes applicable to the stability of non-flocculated suspensions or emulsions, viz. 

Even simply for the purpose of classifying rheological behaviour, it is convenient to distinguish between flocculated and non-flocculated systems. This is because, firstly, the theoretical position is much more well developed as a function of oil volume fraction for dispersed (non-flocculated) systems than for flocculated ones, an4 secondly, the experimental behaviour of many emulsion systems can be interpreted most effectively at the mechanistic level from a detailed consideration of the type and extent of flocculation. Much of the experimental work published recently on emulsion rheology has been concerned with the role of water-soluble polymers in controlling the structure and stability of flocculated systems. Of particular importance in such systems is the viscoelasticity of the polymer-containing aqueous continuous phase and the nature of the interaction between polymer and emulsion droplets. 

The application of these considerations on the interface between water and an organic liquid (oil) leads to the conclusion that only a weak double Ia5 er is present in the water phase, which explains the lack of stability against flocculation of non-stabilised emulsions (c/ chapter VIII 12a, p. 338). 

Rheological Property Determination. The rheology of an emulsion is often an important factor in determining its stability. Any variation in droplet size distribution, degree of flocculation, or phase separation frequently results in viscosity changes. Since most emulsions are non-Newtonian, the cone-plate type device should be used to determine their viscosity rather than the capillary viscometer. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

Non-sulfonated lignins find utility as emulsifiers and stabilizers in water-based asphalt emulsions, as coreactants in phenolic binder applications, as negative plate expanders in lead acid storage batteries, as protein coagulants in fat rendering, and as flocculants in waste water systems. 

Ion bridging is a specific type of Coulombic interaction involving the simultaneous binding of polyvalent cations (e.g., Ca, Fe, Cu ) to two different anionic functional groups on biopolymer molecules. This type of ionic interaction is commonly involved in associative self-assembly of biopolymers. As a consequence it is also an important contributory factor in the flocculation (via bridging or depletion) of colloidal particles or emulsion droplets in aqueous media containing adsorbed or non-adsorbed biopolymers (Dickinson and McClements, 1995). 

These calculations also demonstrate the general theoretical principle, which has been confirmed in practice for various dairy-type emulsions, that the depletion interaction is of insufficient magnitude to induce flocculation when the non-adsorbed protein species are too small (e.g., individual protein molecules) or too large (e.g., native casein micelles). 

Radford, S.J., Dickinson, E. (2004). Depletion flocculation of caseinate-stabilized emulsions what is the optimum size of the non-adsorbed protein nano-particles Colloids and Surfaces A Physicochemical and Engineering Aspects, 238, 71-81. ... 

Depletion flocculation has also been induced in oil-in-water emulsions by adding different concentrations of a non-adsorbing biopolymer (xanthan) to the aqueous phase. At low frequencies, the attenuation coefficient of the emulsions decreased with increasing... 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

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). 

Fig. 9 Zeta potential and flocculation rate of a parenteral emulsion in the presence of a non-specifically adsorbing electrolyte (A) and a specifically adsorbing electrolyte (B).

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