Water-continuous emulsions, determining dispersed phase

Oil-Water Versus Water-Oil Emulsions. If oil and water are vigorously shaken, they form a dispersion of water droplets in oil and oil droplets in water. When shaking is stopped the phases start to separate small water drops fall toward the interface, and oil drops rise. The emulsion quickly breaks. Adding an emulsifier to the system changes the outcome after standing, one phase becomes continuous, while the other remains dispersed. The nature of the emulsion is determined by the emulsifier. As a general rule, the continuous phase is the one in which the emulsifier is soluble. Thus sodium stearate promotes an oil-in-water (o/w) emulsion, while zinc distearate promotes a water-in-oil (w/o) emulsion. Several qualitative theories have been advanced to explain this empirical mle. 

The ratio of the oil to water alone is not sufficient to determine which is the dispersed phase because the presence of emulsifiers or solids can significantly affect the amount of dispersed phase distributed in a given amount of continuous phase. Figure 2 shows an example of a fire flood emulsion that is water-in-oil, although the emulsion contains 63% (by weight) water. Explosives are often water-in-oil emulsions with up to 92% water phase (J6, 17). 

Figure 2. Scanning electron micrographs (at three magnifications) of a fire flood emulsion illustrating a case in which, although the water-oil ratio is 2.5 1, water is the dispersed phase. The composition of this emulsion is 63% water, 11% solids, and 26% oil. The compositions of the dispersed and continuous phases were determined from the X-ray signal excited in the electron microscope. The size of the dispersed water phase ranges from less than 0.1 pm up to about 10 pm. The large features labeled O are regions of oil phase that can be described as oil emulsified in a continuous phase of a water-in-oil emulsion. These complex systems are difficult to characterize with anything but microscopic methods.

Some of the more sophisticated techniques offer detailed information or levels of accuracy that are not required in day-to-day operations. However, when operational upsets cannot be handled by normal methods, details of the emulsion properties have to be understood. For example, subtle changes in the size distribution of the dispersed phase (while total oil, water, and solids remain constant) can be important in determining process performance. An oil-in-water or water-in-oil emulsion can invert during processing as one or the other phase is removed, and the point in the process when this inversion occurs can have implications for the efficiency of the operation. The addition of diluent to reduce oil-phase viscosity, for instance, is much more efficient if oil is the continuous phase. 

Emulsions are characterized in terms of dispersed / continuous phase, phase volume ratio, droplet size distribution, viscosity, and stability. The dispersed phase is present in the form of microscopic droplets which are surrounded by the continuous phase both water-in-oil (w/o) and oil-inwater (o/w) emulsions can be formed. The typical size range for dispersed droplets which are classified as emulsions is from 0.25 to 25 p (6). Particles larger than 25 p indicate incomplete emulsification and/or impending breakage of the emulsion. Phase volume ratio is the volume fraction of the emulsion occupied by the internal (dispersed) phase, expressed as a percent or decimal number. Emulsion viscosity is determined by the viscosity of the continuous phase (solvent and surfactants), the phase volume ratio, and the particle size (6). Stroeve and Varanasi (7) have shown that emulsion viscosity is a critical factor in LM stability. Stability of... 

The emulsifying effect of a copolymer can be characterized by determining the type of emulsion (DMF in hexane or hexane in DMF), its stability, its viscosity, and the particle size of the dispersed phase. These characteristics of oil-in-oil emulsions obtained with PS-PI block copolymers were studied as functions of solvent volume ratio, molecular weight, composition, and structure of the copolymer (5). Although Bancroft s rule was established for conventional oil-water emulsions, it appears to apply also to oil-in-oil emulsions—the continuous phase of the emulsion is preferentially formed by the solvent having the best solubility for the emulsifier (6, 7). Thus, block or graft copolymers can be prepared giving hexane/DMF, DMF/hexane, or both types of emulsions. 

Inverse (or water-in-oil) emulsions (315, 401) are emulsions in which an aqueous phase is dispersed within a continuous organic phase. This system is essentially the inverse of a conventional emulsion, hence the name inverse emulsion. The organic phase is typically an inert hydrocarbon (such as mixed xylenes or low-odour kerosenes), and the aqueous phase contains a water-soluble monomer such as acrylamide (268). The aqueous phase may be dispersed as discrete droplets or as a bicontinuous phase (335), depending upon the formulation and conditions of the inverse emulsion. The hydrophilic-lipophilic balance (HLB) value of the stabiliser determines the form and stability of an inverse emulsion, with HLB values of less than 7 being appropriate for inverse emulsions. Steric stabilisers such as the Span , Tween , and Plutonic series of nonionic surfactants are usually used in preparing inverse emulsions. Inverse emulsions, suspensions, miniemulsions (199), and microemulsions have been prepared, primarily as a function of the stabiliser concentration. Commercial products produced by inverse emulsion polymerisation include polyacrylamide, a water-soluble polymer used extensively as a thickener. 

In many surface separation processes there will occur three distinct phases or process streams, an oil product stream, which may contain emulsified water, an aqueous tailings stream, which may contain emulsified oil, and an interface or rag layer emulsion stream, which may contain emulsified oil and/or water. The interface emulsion layer may build up to a certain level in a process, continuously reform and break in the separator and never cause operational problems. On the other hand, the interface emulsion layer may build to such an extent that it requires removal and treatment. Knowledge of the nature of the dispersed phase will be required to determine an effective treatment. Figure 3 Ulustrated the simultaneous presence of W/O and O/W/O emulsion. Mikula shows (Figure 1 in reference [132]) a photomicrograph of a quite stable interface emulsion (rag layer emulsion) in which one can clearly observe the simultaneous occurrences of both O/W and W/O emulsions in different regions of the same sample. 

Several techniques determine whether the continuous phase is oil or water. The simplest is the dilution method, in which a drop or two of the emulsion is added to water. If it is an oil-in-water emulsion it will spread and disperse. If it is water-in-oil it will remain as a drop 18). The dilution test can be effective, but care must be taken that sampling the emulsion does not itself determine the continuous phase. For instance, drawing a water-in-oil emulsion up through the capillary of a dropper can cause the emulsion to... 

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