Methods of Emulsion Formation

A similar conclusion was reached when investigating microemulsions (as described in the 

FIGURE 1.58 Different phase equilibria in a water-surfactant (emulsifier)-oil mixture system (schematic). 

Total force between two bodies = repulsion forces + attraction forces 

The nature of the total force thus determines whether 

The attraction force arises from van der Waals forces. The kinetic movement will finally determine whether the total force can maintain contact between two (or more) particles. 

If one shakes oil and water, the oil breaks up into drops. However, they will quickly coalesce and return to the original state of two different phases. The longer one shakes, more drops reduce in size. In other words, the energy put into the system makes the drops smaller in size. 

Near the surfactant region the crystalline or lamellar phase is found. This is the region one finds in hand soaps. The ordinary hand soap is mainly the salt of fatty acid (coconut oil fatty acids or mixtures [85%] plus water [15%] and some salts. X-ray analyses have shown that the crystalline structure consists of a layer of soap separated by a water layer (with salts). The hand soap is produced by extruding under high pressure. This process aligns the lamellar crystalline structure lengthwise. If the degree of expansion versus temperature is measured, the expansion will be found 

The actual procedure is as follows. A suitable number of test samples (more than 50) are prepared by mixing each component in varying weights to represent a comparable number of regions (around 50 samples). The test samples are mixed under rotation in a thermostat over a few days to reach equilibrium. The samples are then centrifuged and the phases are analyzed. From these analyses, the phase structures are determined. They are then investigated using a suitable analytical method. 

It is obvious that studies of multicomponent systems will lead to a very large numbers of phases. However, by analyzing a typical system, some trends can be found that can be used as guidelines. 

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The method of emulsion formation must be standardized and reproducible. The equipment used, the shear rate, the time and temperature of emulsification are the main factors in emulsion formation. 

To make an emulsion, oil, water, surfactant and energy are needed. The composition of the system and the way of processing then determine emulsion type (oil-inwater or water-in-oil), droplet volume fraction ( ), droplet size and composition of the layer of surfactant around the droplets. These variables determine most emulsion properties, notably physical stability. Consequently, knowledge of emulsion formation is of considerable importance. In this chapter, a review is given, with some emphasis on newer developments. Some aspects are left out, because they have been sufficiently discussed in earlier reviews. " For the convenience of the reader, however, important general points are recalled. Some aspects are not discussed, such as the preparation of high-internal phase emulsions, double emulsions, microemulsions and emulsions with very coarse drops. Typically, the emulsions considered have droplets of, say, a micrometre in diameter. Some specialized methods of emulsion formation will also be left out. 

An experimental study was performed to determine the applicability of the theory. Oil-in-water (o/w) emulsions, stabilised with anionic surfactants, were prepared, with known quantities of added electrolyte, and were creamed by either gravitation or centrifugation. The results can be summarised as follows at low electrolyte concentrations, where h would have a finite value, <(> was less than 0.74. Over a range of concentrations, where it was assumed that both 0 and h were negligible, = 0.74 ( 0.02). The emulsions were found to be polydis-perse, so this did not appear to affect the volume fraction to a great extent. In addition, < > was found to be independent of the method of cream formation. 

Usually, no emulsions are formed within the petroleum layer. Emulsion formation begins during the movement of petroleum to the mouth of the oil well and intensifies during further transport of petroleum in pipes (i.e. emulsions are predominant where there is the potential for continuous mixing of petroleum and water). The intensity of emulsion formation in an oil well depends on the method of petroleum extraction. This, in turn, is defined by the character of the oil wells, time of its operation and physical-chemical properties of the petroleum. 

Clearly, the process of selecting the best surfactant or surfactants for the preparation of an emulsion has been greatly simplified by the development of the more or less empirical but theoretically based approaches exemplified by the HLB, solubility parameter, and PIT methods. Unfortunately, each method has its significant limitations and cannot eliminate the need for some amount of trial-and-error experimentation. As our fundamental understanding of the complex phenomena occurring at oil-water interfaces, and of the effects of additives and environmental factors on those phenomena, improves it may become possible for a single, comprehensive theory of emulsion formation and stabilization to lead to a single, quantitative scheme for the selection of the proper surfactant system. 

In that quest, several variations to the standard method of emulsion manufacture have been introduced, in order to improve aspects of film formation, e.g. 

Emulsions resulting from the emulsion polymerisation of acrylic or vinyl monomers are unique compared to other resins used for surface coating applications. As such they have properties which are totally different to a conventional solution acrylic, polyester or alkyd resins. Their mechanism of film formation is totally different to other types of resins. Because particles are present it is necessary for them to coalesce to film form and pigmentation is also different to conventional solution polymers. Consider first the unique properties and test methods of emulsion polymers. 

This parameter is also called neutral oil, or, in the case of higher molecular weight materials, wax. The determination is most often performed by extraction from water with a nonpolar solvent, such as petroleum ether. The nonpolar hydrocarbon will be extracted, while the surfactant and salts remain in the aqueous phase. Sulfones, if present, are included with the neutral oil. Some surfactants are not amenable to analysis by extraction because of emulsion formation. These are analyzed by ion exchange or adsorption chromatography procedures. The chromatography methods are more amenable to automation. 

Petroleum sulfonates contain much more unsulfonated material than other surfactants, often 25-50%. The liquid-liquid extraction methods suitable for removal of oil from other anionics give less than quantitative results for petroleum sulfonates because of emulsion formation. While extraction is suitable for special cases, column chromatography separation of the oil is universally applicable. It is prudent to check the effectiveness of the separation by inspecting the IR spectrum of the oil fraction for the absence of sulfonate absorption at 1050 and 1200 cm" (95). There is no similar method to quickly check the purity of the sulfonate fraction, although an HPLC procedure could presumably be developed. 

The multiple emulsion technique includes three steps 1) preparation of a primary oil-in-water emulsion in which the oil dispersed phase is constituted of CH2CI2 and the aqueous continuous phase is a mixture of 2% v/v acetic acid solution methanol (4/1, v/v) containing chitosan (1.6%) and Tween (1.6, w/v) 2) multiple emulsion formation with mineral oil (oily outer phase) containing Span 20 (2%, w/v) 3) evaporation of aqueous solvents under reduced pressure. Details can be found in various publications [208,209]. Chemical cross-linking is an option of this method enzymatic cross-linking can also be performed [210]. Physical cross-linking may take place to a certain extent if chitosan is exposed to high temperature. 

Kiel, O.M. "Method of Fracturing Subterranean Formations Using Oil-In-Water Emulsions," US Patent 3,710,865(1973). 

Izquierdo, P., Feng, J., Esquena, J.,Tadros, T.F., Dederen, J.C., Garcia, M.J., Azemar, N. and Solans, C. (2005) The influence of surfactant mixing ratio on nano-emulsion formation by the PIT method. Journal of Colloid and Interface Science, 285 (1), 388-394. 

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