Particle concentration, emulsions flocculation

Creaming or sedimentation occurs when the dispersed droplets or floccules separate under the influence of gravity to form a layer of more concentrated emulsion, the cream. Generally a creamed emulsion can be restored to its original state by gentle agitation. This process, which inevitably occurs in any dilute emulsion if there is a density difference between the phases as a consequence of Stokes law, should not be confused with flocculation which is due to particle interactions resulting from the balance of attractive and repulsive forces. Most oils are less dense than... 

Occurrence of flocculation may be explained if secondary minima aggregation is assumed. This is confirmed by the V/KT vs (H (A)) curves, which indicate that at higher values of H, repulsion becomes negligible and attraction predominates and emulsion flocculates. It is also observed that the depth of secondary minimum is more at higher concentration, 5-20 KT, which is deep enough for reversible aggregation, while at low concentrations, the depth of the secondary minima is too shallow to trap the particles. 

Macromolecular colloid solutions also play an important role in ensuring the stability of disperse systems (e.g. suspensions, emulsions). In the case of emulsions the polymer decreases the rate of separation by increasing viscosity on the one hand, and it has an enthalpy stabilizing effect by adsorption on the surface of the droplets on the other hand [3, 4, 7]. Depending on the concentration of the polymer, a protecting and flocculating effect can be observed during the interaction between suspensions and polymers. If the polymer concentration is low, the polymer adsorbed on the surface of the particles connects the particles into loose floccules. Thereby, the rate of... 

The colloidal properties of emulsions are responsible for the quality of many foods. Ultrasound is sensitive to most of the properties of interest and can be used as both a research and a process-control tool by food scientists. As a research tool, ultrasonic measurements are particularly powerfid as they can be used to generate information not readily available by other methods - importantly, physical state, particle size, concentration, and flocculation in concentrated and optically opaque emulsions. In a process environment, ultrasonic measurements can be effected noninvasively in process lines and are therefore compatible with the stringent hygiene and cleaning requirements of food production. 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

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]. 

Goebel et al. [107] have reported that the particle concentration reaches a maximum at very low conversion (below 1%). This agrees well with the concept of an emulsifier-free emulsion polymerization of conventional monomers in which the maximum number of particles below 5% appears. Beyond this critical conversion, the number of particles starts to decrease due to the particle association. The strong association of particles results from the low stability of small particles. The same results were found by other groups [76,108,109] in which a strong flocculation of latex particles led to a decrease of particle concentration with conversion. 

Since the emulsions stabilities for this oil-phase only were not sufficiently high to achieve reliable results, asphaltenes precipitated from crude A were added to the model emulsions in the same amount as in the actual crude oU. Figures 12.7 and 12.8 show the viscosity ratio as a function of particle concentration for the model emulsions stabilized with very hydrophobic and mildly hydrophilic silica particles, respectively. Model oil and crude oil emulsions show many similarities. For all model emulsions the stability in the absence of particles is lower, because the absolute viscosity is much lower than the one encountered in crude oil emulsions. Consequently, the effect of the shear rate on the destabilization is more pronounced. However, as particles are added in larger amounts, crude oil and model oil emulsions behave in the same way. Nevertheless, in Figure 12.8 the viscosity ratios are shifted towards lower values since a strong flocculation phenomenon enhanced by the low viscosity of the samples brings coalescence. 

Increased depletion attraction. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet. This attractive force increases as the concentration of colloidal particles increases, until eventually, it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (68-72). This type of droplet aggregation is usually referred to as depletion flocculation (17, 18). 

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). 

Beyond the minimum viscosity, the increase in viscosity with addition of NaCl is believed to be related to further reduction in the net (negative) charges on the dispersed droplets. Reduction of the net charge on the oil particles promotes particle-particle interaction leading to flocculation which (as discussed below) will promote viscosity increase. Hence the total Na+ content from NaOH and from NaCl acts to cause an increase in viscosity above the minimum. The location of the minima varied -with the initial NaOH content of the emulsion but occurred at almost the same total Na+ concentrations (Fig. 10). 

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