Physical barriers to coalescence

Stable emulsions often form during industrial processing. On the microscopic scale, the reasons that the droplets remain dispersed fall into two broad categories (1) physical barriers to coalescence and (2) electrical repulsion between droplets. An example of a physical barrier is the presence of finely divided solids at the oil-water interface. Of primary concern, however, is the consideration of electrical forces because their influence is significant at relatively longer distances. Electrical repulsive forces arise... 

During the stabilization process, particles (e.g., fat crystals) must collect at the emulsion droplet interface and provide a physical barrier to coalescence. The key factors that determine the influence of fat crystals on emulsion stabilization are (1) the wettability of the crystals at the interface, (2) interfacial film rheology, (3) particle microstructure (polymorphism and morphology), and (4) location of fat crystals (in the O/W dispersed emulsion or in the continuous W/O emulsion phase) (Johansson et al., 1995 Johansson and Bergenstahl, 1995 Rousseau, 2000). 

Film formation of PUDs involves coalescence of the particles and the formation of a continuous film. A PUD particle has a hydrophilic shell and a hydrophobic core. Upon film formation, there is a very slow process of coalescence on a molecular level whereby there is a physical barrier to polymer chain mixing. The resultant coatings have hydrophilic and hydrophobic regions, which create a more water-permeable film (Chainey etaL, 1985). 

Emulsions have been widely used as vehicles for oral, topical, and parenteral delivery of medications. Although the product attributes of an emulsion dosage form are dependent on the route of administration, a common concern is the physical stability of the system, in particular the coalescence of its dispersed phase and the consequent alteration in its particle-size distribution and phase separation. The stabilization mechanism(s) for an emulsion is mainly dependent on the chemical composition of the surfactant used. Electrostatic stabilization as described by DLVO theory plays an important role in emulsions (0/W) containing ionic surfactants. For 0/W emulsions with low electrolyte content in the aqueous phase, a zeta potential of 30 mV is found to be sufficient to establish an energy maximum (energy barrier) to ensure emulsion stability. For emulsions containing... 

The formation of an interfacial layer consisting of surface-active material present in erude oil (asphaltenes and resins) produces a physical barrier for droplet—droplet coalescence. Numerous researehers have noted the presence of an interfacial skin in oil-water systems with these surface-active components present (3,5,9,19-26). Mohammed et al. (21), using a Langmuir film balance, found the inter-faeial dilatational modulus to be dependent on the presence of asphaltenes and resins. Fordedal et al. (20) have shown... 

In addition to particle breakup, the coalescence process may be affected as well. It has been speculated that exfoliated clay platelets or well-dispersed nanoparticles may hinder particle coalescence by acting as physical barriers [19,22]. Furthermore, it has been suggested that an immobilized layer, consisting of the inorganic nanoparticles and bound polymer, forms around the droplets of the dispersed phase [50]. The reduced mobility of the confined polymer chains that are bound to the fillers likely causes a decrease in the drainage rate of the thin film separating two droplets [44]. If this is the case, this phenomenon should be dependent on filler concentration this is shown in Figure 2.8, which shows the effect of nanoclay fillers on the dispersed particle size of a 70/30 maleated EPR/PP blend [19]. 

The surfactant must also operate against the coalescence of drops to form a separate oil layer. When this happens, the grease loses its ability to perform its sealing function, leading to a loss of the lubricant. The Davies equation [49] for drop coalescence, eq. (15.1), shows a relation of drop coalescence to the energy barrier of coalescence. A surfactant structure that will increase this barrier will result in better grease performance. This barrier can result from both physical and electrostatic means. 

The predictions of different quantitative criteria for stability-instability transitions were investigated [461], having in mind that the oscillatory forces exhibit both maxima, which play the role of barriers to coagulation, and minima that could produce flocculation or coalescence in colloidal dispersions (emulsions, foams, suspensions). The interplay of the oscillatory force with the van der Waals surface force was taken into account. Two different kinetic criteria were considered, which give similar and physically reasonable results about the stability-instability transitions. Diagrams were constructed, which show the values of the micelle volume fraction, for which the oscillatory barriers can prevent the particles from coming into close contact, or for which a strong flocculation in the depletion minimum or a weak flocculation in the first oscillatory minimum could be observed [461]. 

Reactive surfactants can covalently bind to the dispersed phase and as such have a distinct advantage over conventional surfactants that are only physically adsorbed and can be displaced from the interface by shear or phase changes with the subsequent loss of emulsion stability. Furthermore, if the substrate is coalesced to produce decorative or protective films, the desorption can result in, e.g. reduced adhesion, increased water sensitivity and modification of the hardness, barrier and optical properties of the film. 

The surfactants used for the preparation of disperse systems are seldom effective in maintaining the long-term physical stabihty (absence of flocculation and/or coalescence) of the formulation. This is due to their weak and reversible adsorption and lack of the presence of a high-energy barrier that prevents flocculation as a result of van der Waals attractions. For this reason, dispersants and emulsifiers of polymeric nature that are strongly and irreversibly adsorbed at the interface are required. In addition, these polymeric dispersant provide effective repulsive forces (referred to as steric repulsion) that overcomes the van der Waals attractions. The criteria for an effective dispersant are [1, 2] ... 

The rate at which the droplets of a macroemulsion coalesce to form larger droplets and eventually break the emulsion has been found to depend on a number of factors (1) the physical nature of the interfacial film, (2) the existence of an electrical or steric barrier on the droplets, (3) the viscosity of the continuous phase, (4) the size distribution of the droplets, (5) the phase volume ratio, and (6) the temperature. 

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