Dispersed particles, emulsions

Sprays. Aerosol spray emulsions are of the water-in-oil type. The preferred propellant is a hydrocarbon or mixed hydrocarbon—hydrofluorocarbon. About 25 to 30% propellent, miscible with the oil, remains in the external phase of the emulsion. When this system is dispensed, the propellant vaporizes, leaving behind droplets of the w/o emulsion (Fig. 2b). A vapor tap valve, which tends to produce finely dispersed particles, is employed. Because the propellant and the product concentrate tend to separate on standing, products formulated using this system, such as pesticides and room deodorants, must be shaken before use. 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Content uniformity and long-term stability of a pharmaceutical product are required for a consistent and accurate dosing. Aggregation of dispersed particles and resulting instabilities such as flocculation, sedimentation (in suspensions), or creaming and coalescence (in emulsions) often represent major problems in formulating pharmaceutical disperse systems. 

Although they are a relatively small volume product—approximately 75,000 tons produced in 1949 (126)—interest in asphalt emulsion has continued at a high level. Abraham (6) has reviewed the patent literature relative to the types of emulsifying agents used, while commercial practice has been discussed by Day (16). The most common emulsifiers are sodium or potassium soaps of tall oil, abietic acid, or Vinsol resin, or colloidal clays such as bentonite for adhesive base emulsions. Lyttleton and Traxler (53) studied the flow properties of asphalt emulsions, and Traxler (122) has investigated the effect of size distribution of the dispersed particles on emulsion viscosity. A decrease in particle size uniformity was found to be accompanied by a decrease in consistency because particles of various size assume a more loosely packed condition than do those of the same size. 

If the surfactant is ionic and imparts a charge to the interface, then the dispersed particle will be surrounded by an ion atmosphere. We see in Chapters 11 and 13 how an ion atmosphere surrounding a particle may slow down the rate at which such particles come together. This is one of the ways by which an emulsion may achieve some degree of kinetic stability. 

Heterogeneous Copolymerization. When copolymer is prepared in a homogeneous solution, kineiic expressions can be used to predict copolymer composition Bulk and dispersion polymerization are somewhat different since the reaction medium is heterogeneous and polymeri/aiion occurs simultaneously in separate loci. In bulk polymerization, for example, the monomer swollen polymer particles support polymerization within the particle core us well as on the particle surface, lit aqueous dispersion or emulsion polymeri/aiion the monomer is actually dispersed in two or three distinct phases a continuous aqueous phase, a monomer droplet phase, and a phase consisting of polymer particles swollen at Ihe surface with monomer. This affect the ultimate polymer composition because llie monomers are partitioned such that the monomer mixture in the aqueous phase is richer in the more water-soluble monomers than the two organic phases. 

The easiest model to treat theoretically is the sphere, and many colloidal systems do, in fact, contain spherical or nearly spherical particles. Emulsions, latexes, liquid aerosols, etc., contain spherical particles. Certain protein molecules are approximately spherical. The crystallite particles in dispersions such as gold and silver iodide sols are sufficiently symmetrical to behave like spheres. 

Figure 2.3 shows an optical micrograph obtained with reflected fluorescent light. This technique clearly shows the presence of a multiple, O/W/O/W, emulsion which would have been extremely difficult to detect by other means. Combined techniques can be used to observe different dispersed species. For example, reflected white light can make dispersed particles easy to observe (Figure 2.4a) whereas reflected light on... 

Polymer colloids involve dispersions containing polymer particles having sizes greater than about 1 nm. If dispersed in aqueous solution, such a polymer dispersion is called a latex. These are usually synthetic polymer particles formed by free radical polymerization [784], Many kinds of polymerization systems exist, involving almost all of the possible kinds of colloidal dispersion, including emulsion polymerization, hence the more general term heterophase polymerization is sometimes used. Several reviews are available [785-789]. Emulsion polymerization provides a convenient means of controlling the polymerization of monomers and is used to make, for example, synthetic rubber which is mostly a co-polymer of butadiene and styrene. 

Consider first the effect of a dispersed phase, of volume fraction continuous phase of viscosity D0 and dispersed particles (droplets) which do not attract. At low volume fractions the Einstein equation should apply to a suspension of solid particles at constant temperature,... 

Electrophoresis — Movement of charged particles (e.g., ions, colloidal particles, dispersions of suspended solid particles, emulsions of suspended immiscible liquid droplets) in an electric field. The speed depends on the size of the particle, as well as the -> viscosity, -> dielectric permittivity, and the -> ionic strength of the solution, and it is directly proportional to the applied electric field. In analytical as well as in synthetic chemistry electrophoresis has been employed to separate species based on different speeds attained in an experimental setup. In a typical setup the sample is put onto a mobile phase (dilute electrolyte solution) filled, e.g., into a capillary or soaked into a paper strip. At the ends of the strip connectors to an electrical power supply (providing voltages up to several hundred volts) are placed. Depending on their polarity and mobility the charged particles move to one of the electrodes, according to the attained speed they are sorted and separated. (See also - Tiselius, - electrophoretic effect, - zetapotential). 

Liquid formulations can be used as pastes or slurries, dispersions, or emulsions (in aqueous or other solvents) to physically separate the compound to be protected. Dispersions, pastes, or slurries are not commonly used but are a feasible approach when highly concentrated products are needed. For instance, many early analytical enzymes were sold as crystal slurries to preserve stability. Dispersions can be prepared from dry materials. Most of the dispersions are visually cloudy in appearance due to the particle size of the dispersed compounds and are thus undesirable in most endproducts. 

Attempts to describe the unlimited increase of the viscosity of dispersions and emulsions observed when their concentrations approach the maximum values (tPmax) meet great theoretical difficulties. Various approaches were developed to overcome these difficulties. Thus, for example, Russel et al. [58] suggested that account should be taken of the Brownian motion of particles in colloidal dispersions in the form of a hydrodynamic contribution. They showed that this contribution which is to be taken into account in considering a slow flow (with slow shear rates y), increases considerably with increasing dispersion concentration. For a description of the dependence of viscosity on concentration the above authors obtained an exact equation only in the integral form. At low shear rates it gives the following power series ... 

The most widely used emulsion solvent evaporation method for preparation of nanoparticles using PLGA requires surfactants to stabilize the dispersed particle [23]. This method often has a problem that the surfactant remains at the surface of the particles and is then difficult to remove when PVA is used as surfactant. Other surfactants such as the span series or tween series, PEO, etc. are also used... 

Physical-chemical features of WIW emulsions compared to oil-in-water (OIW) and water-in-oil WIO) emulsions are low interfacial tension, interfacial layers of low biopolymer concentration, interfacial adsorption of lipids and high deformability of aqueous dispersed particles. Low interfacial tension in WIW emulsions reflects similar compositions of coexisting phases, where water and the biopolymers are partially cosoluble. The low-density interfacial layer is due to a trend of incompatible biopolymers to have surroundings of the same type. One more feature of WIW emulsions is a great difference in concentration between coexisting phases. This is due to the competition between the biopolymers for space in solution. The competition can be characterized (Figure 3.5) by the angle made by the tie-line with one of the concentration axes. 

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