Emulsions unstable

The most important characteristic of a water-in-oil emulsion is its stability . The reason for this importance is that one must first characterize an emulsion as stable (or unstable) before one can characterize the properties. Properties change very significantly for each type of emulsion. Until recently, emulsion stability has not been defined (77). Therefore, studies were difficult because the end points of analysis were not defined. This section of the chapter summarizes studies to measure the stability of water-in-oil emulsions and to define characteristics of different stability classes. Four states in which water can exist in oil will be described. These include stable emulsions, mesostable emulsions, unstable emulsions (or simply water and oil), and entrained water. These four states are differentiated by visual appearance as well as by rheolo-gical measures. 

Hydrocarbon groups have the opposite effect because they are hydrophobic so a lot of hydrocarbon (as, for example, in a polybutadiene resin) will help in increasing the film resistance. This cannot be taken too far. Too much hydrocarbon will make the emulsion unstable, thus it will be impossible to produce a practical system. Thus, a balance needs to be found between the hydrophilic and hydrophobic portions of the polymer molecule. 

Emulsification involves the joining of two mutually insoluble materials, such as petroleum oil and water. The surfactant, which usually has a hydrophilic or water-soluble end and a hydrophobic or oil-soluble end, holds the oil and water together in much the same manner that a fastener holds two pieces of material. Often, the emulsion which forms is unstable, subsequently breaking up and releasing the oil from the water. Break-up is actually preferred, because the oil then floats to the surface, whereas the surfactant is free to emulsify more oil. 

Most tanks store Hquid rather than gases or soHds. Characteristics and properties such as corrosiveness, internal pressures of multicomponent solutions, tendency to scale or sublime, and formation of deposits and sludges are vital for the tank designer and the operator of the tank and are discussed herein. Excluded from the discussion are the unique properties and hazards of aerosols (qv), unstable Hquids, and emulsions (qv). A good source of information for Hquid properties for a wide range of compounds is available (2). 

Polymerization. Emulsion polymerization is used, but the latex is too unstable for use and all the latex is coagulated to dry mbber. The molecular weight range is 100,000—200,000 with a Mooney viscosity of 50—70. 

A laboratory check is normally desirable to assess the amounts of organic and mineral acids present, and a check on hardness is usually necessary in any case. Hardness in water is due to dissolved salts - mainly of calcium, magnesium and iron, and occasionally of aluminum. Softening may be required if the water is extremely hard, because the salts react with the emulsifier in the soluble oil to form an insoluble scum that floats on the surface of the emulsion. The scum may not in itself be harmful, but its formation uses up some of the emulsifier and causes the emulsion to be unstable. 

There are many variations on this theme. Fed-batch and continuous emulsion polymerizations are common. Continuous polymerization in a CSTR is dynamically unstable when free emulsifier is present. Oscillations with periods of several hours will result, but these can be avoided by feeding the CSTR with seed particles made in a batch or tubular reactor. 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

A study by Environment Canada and the U.S. Minerals Management Service attempted to develop a standard test for emulsion breaking agents [586]. Nine types of shaker test methods were tried. Although the results are comparable with different tests, a stable water-in-oil emulsion must be used to yield reproducible results. Tests with unstable emulsions showed nonrepro-ducible and inconsistent results. 

Formulations of tetraethyl pyrophosphate as an emulsive concentrate proved to be relatively complex, considering the apparent ease of formulation. Because of the unstable nature of the chemical, dust formulations were considered impossible. It was found after extensive research work that a dust which would be stable for 10 days to 2 weeks could be made with a specially selected and processed filler. 

Emulsions are thermodynamically unstable systems. However, using appropriate emulsifying agents... 

Many drugs are too unstable—either physically or chemically—in an aqueous medium to allow formulation as a solution, suspension, or emulsion. Instead, the drug is formulated as a dry powder... 

An acetone powder of P. homomalla was subsequently used to generate allene oxide 64 from exogenous 8P-HPETE [91]. This highly unstable compound (t1/2 = ca. 15 s at 0 °C, pH 7.4) was obtained by performing the biosynthesis at low temperature (0 °C) for 2 min in a vortexed emulsion of pH 6 buffer and pentane. Under these conditions, the allene oxide partitioned into the pentane where it was relatively protected from hydrolysis. HPLC analysis of the... 

Towards high concentrations of the electrolyte, the microemulsion changes to an emulsion containing normal monomer droplets. With a further increase in the electrolyte concentration, the emulsion becomes unstable and breaks down ( salting out ). 

An emulsion that is, for instance, stable over many years at low droplet volume fraction may become unstable and coalesce when compressed above a critical osmotic pressure 11. As an example, when an oil-in-water emulsion stabilized with sodium dodecyl sulfate (SDS) is introduced in a dialysis bag and is stressed by the osmotic pressure imposed by an external polymer solution, coarsening occurs through the growth of a few randomly distributed large droplets [8]. A microscopic image of such a growth is shown in Fig. 5.1. 

Porous or sintered plates are the ideal and are used in small-scale studies of fluidized bed behaviour (Kunii and Levenspiel, 1991) and form a highly expanded unstable gas-solid dispersion directly above the distributor which rapidly divides into a large number of small bubbles plus an emulsion phase. Bubbles grow rapidly thereafter by coalescence. Kunii and Levenspiel (1991) also suggest that other... 

As mentioned earlier, ordinary emulsions as prepared by mixing oil, water, and emulsifier are thermodynamically unstable. That is, such an emulsion may be stable over a length of time, but it will finally separate into two phases (the oil phase and the aqueous phase). They can also be separated by centrifugation. These emulsions are opaque, which means that the dispersed phase (oil or water) is present in the form of large droplets (more than a micrometer and thus visible to the naked eye). 

It was found that if less soap were employed the emulsions were unstable. The experimental results agree with the conclusion that the soaps exert their emulsifying powers on oil-water mixtures by going to the interface and coating the interface with a layer of soap one molecule deep. 

---