Stability, emulsion

Emulsions are stable as long as the drops are separated from each other. Flocculation of an emulsion or dispersion takes place upon collision of the droplets, which is related to the Brown motion, convective stirring, or gravitational forces. Actually, any emulsion can be separated into oil and water phases by suitable centrifugation treatment. 

The dispersion force of attraction between two bodies i and j (molecules, particles, drops), Ejj, is dependent on following parameters  

Hm has been shown to be always positive, which suggests that, in two-phase systems (such as oil-water), the particles will always be attracted to each other. This means that even air bubbles will attract each other, as is also found from experiments. A linear relation is found between H1216/n and yLD, as expected from Equation Experimental values of Am as determined from flocculation kinetics showed that this agreed with the theoretical relation. 

As discussed in the suspensions section, the driving force for creaming (whereby the droplets have a density lower than the medium) or sedimentation is gravity. When the gravity force exceeds the Brownian diffusion, creaming or sedimentation will occur. With macroemulsions (droplets 1 pm), the creaming or sedimentation rate is very fast and it may be completed in a matter of hours or days. 

The most common procedure for eliminating creaming or sedimentation is to use a thickener in the continuous phase, e.g. a high molecular weight polymers such as hydroxyethyl cellulose or xanthan gum. These thickeners produce a gel in the continuous phase that has a yield value ( 0.1 Pa) and a high zero shear viscosity ( 1000 Pa s), thus preventing any creaming or sedimentation. 

Flocculation of emulsions resembles that of suspensions and the driving force is the van der Waals attraction, which becomes significant at short distances of separation. To prevent flocculation, one needs a strong repulsive force that operates at intermediate separations, thus preventing the close approach of the droplets. Two main repulsive forces can be distinguished Electrostatic and steric (discussed in detail in Chapters 6 and 7). 

Several methods may be applied to reduce Ostwald ripening in emulsions  

The driving force for coalescence is the thinning and disruption of the liquid film between the droplets. This can occur in a floe , in a cream or sedimented layer or during Brownian collision. This is described in detail in Chapter 6. 

Several breakdown processes may occur on storage depending on (i) Particle size distribution and density difference between the droplets and the medium, (ii) Magnitude of the attractive versus repulsive forces which determine flocculation, (iii) Solubility of the disperse droplets and the particle size distribution which determine Ostwald ripening, (iv) Stability of the liquid film between the droplets that determines coalescence. (v) Phase inversion. The various breakdown processes are illustrated in the Fig. 3.32. This is followed by a description of each of the breakdown processes and methods that can be applied to prevent such instability. 

Emulsion creaming or sedimentation is the result of gravity, when the density of the droplets and the medium are not equal. For small droplets ( 0.1 p, i.e. nanoemulsions) the Brownian diffusion -i- kT (where k is the Boltzmann constant and T is the absolute temperature) exceeds the force of gravity (mass x acceleration due to 

For emulsions consisting of monodisperse droplets with radius 1 pm, the emulsion separates into two distinct layers with the droplets forming a cream or sediment leaving the clear supernatant liquid. This situation is seldom observed in practice. For a polydisperse (practical) emulsion, the droplets will cream or sediment at various rates. In the latter case, a concentration gradient builds up with the larger droplets staying at the top of the cream layer or the bottom, 

C(h) is the concentration (or volume fraction cji) of droplets at height h, whereas Cq is the concentration at zero time which is the same at all heights. 

For very dilute emulsions (cfi 0.01), the rate Vq can be calculated using Stokes law which balances the hydrodynamic force with gravity force, 

The creaming velocity of an isolated rigid spherical particle suspended in a Newtonian liquid obeys Stokes Law  

It should be stressed that Stokes law is inappropriate for accurately predicting gravitational separation in many industrial emulsions because they do not exist as dilute suspensions of non-interacting rigid spheres suspended in a Newtonian fluid. For this reason, the theory has been extended to take into account various other factors, such as droplet fluidity, droplet concentration, particle-particle interactions, and non-Newtonian continuous phases [3]. 

Water soluble polymers can influence the stability of emulsions to gravitational separation in a variety of ways. Non-adsorbed polymers may either increase or decrease stability depending on their effective size and concentration in solution. Increasing the concentration of a polymer in solution causes an increase in continuous phase viscosity (and may even lead to gelation), which should slow down droplet movement. On the other hand, the presence of non-adsorbed polymer also increases the magnitude of the depletion attraction between droplets, which may cause flocculation and therefore accelerate droplet movement. The presence of an adsorbed polymer may also influence stability to gravitational separation in a number of ways. For example, the size of the droplets produced 

Coalescence requires that the molecules of liquid within two or more emulsion droplets coming into direct contact. Droplets therefore need to be in close proximity, which is for example the case in highly concentrated emulsions, flocculated emulsions, or creamed layers. In a subsequent step, a disruption of the interfacial membrane must occur to allow the liquid molecules to come into direct contact. The rate at which coalescence proceeds, and the physical mechanism by which it occurs, is thus highly dependent on the nature of the emulsifier used to stabilize the system. Improving the stability of an emulsion to coalescence may 

Partial coalescence. Partial coalescence occurs when two or more partially crystalline oil droplets come into contact and form an irregularly shaped aggregate [ 11 ]. It is initiated when a fat crystal from one partially crystalline droplet penetrates into the liquid portion of another partially crystallize droplet. Consequently, the lipid crystal is surrounded by lipid molecules instead of water molecules which is thermodynamically favored, that is the fat crystal is better wetted by liquid oil rather than water. Over time the droplets may continue to merge to further reduce the surface area of Hpid that is exposed to water. Nevertheless, the aggregates partly retain the shape of droplets from which they were formed due to the low mobility of molecules in fat crystal networks. 

How can we understand and manipulate colloidal sta-bUity No theory can explain all phenomena, perhaps though the most successful one is the DLVO theory, which is built on an understanding/balance of the attractive van der Waals forces and the repulsive 

Rate of movement can be predicted via stokes equation at idealised conditions 

As mentioned, steric stabilization can be achieved with polymers, including polyelectrolytes, e.g. proteins and biopolymers, and also with (ionic or not) oligomers. (Block-) copolymers are used much more than homopolymers, as explained previously, since one part of the copolymer adsorbs on the surface and the other is soluble with the solvent and is thus extended in the solution. Natural polymers like the protein casein in milk and many synthetic polymers like PEG (poly(ethylene glycol)s) are used to stabilize emulsions and colloidal dispersions. Many block 

The polymers adsorb on the interface (and may obtain different configurations). They enter into the hq-uid, and when particles come close together their polymer layers will interfere with each other, causing steric repulsion . This type of repulsion depends not oidy on the properties of the polymer but also of the solvent. In a good solvent the layer will be more swollen, and more effective, than in a bad solvent. If the polymers are charged, you must also expect electrical effects. 

Enhanced double-layer repulsions for Ionised stabilizers e.g. anionic surfactants 

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Agar occurs as a cell-wall constituent of the red marine algae Rho ophyceae, from which it is extracted by hot water, and marketed as a dry powder, flakes, or strips. It dissolves in hot water and sets on cooling to a jelly at a concentration as low as 0-5%. Its chief uses are as a solid medium for cultivating micro-organisms, as a thickener, emulsion stabilizer in the food industry and as a laxative. 

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

C. Long-Range Forces as a Factor in Emulsion Stability... 

The repulsion between oil droplets will be more effective in preventing flocculation Ae greater the thickness of the diffuse layer and the greater the value of 0. the surface potential. These two quantities depend oppositely on the electrolyte concentration, however. The total surface potential should increase with electrolyte concentration, since the absolute excess of anions over cations in the oil phase should increase. On the other hand, the half-thickness of the double layer decreases with increasing electrolyte concentration. The plot of emulsion stability versus electrolyte concentration may thus go through a maximum. 

An important industrial example of W/O emulsions arises in water-in-crude-oil emulsions that form during production. These emulsions must be broken to aid transportation and refining [43]. These suspensions have been extensively studied by Sjoblom and co-workers [10, 13, 14] and Wasan and co-workers [44]. Stabilization arises from combinations of surface-active components, asphaltenes, polymers, and particles the composition depends on the source of the crude oil. Certain copolymers can mimic the emulsion stabilizing fractions of crude oil and have been studied in terms of their pressure-area behavior [45]. 

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

There have been some studies of the equilibrium shape of two droplets pressed against each other (see Ref. 59) and of the rate of film Winning [60, 61], but these are based on hydrodynamic equations and do not take into account film-film barriers to final rupture. It is at this point, surely, that the chemistry of emulsion stabilization plays an important role. 

Emulsions, asphaltic Emulsion stabilizers Emulsion steam drive... 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

Petroleum and Goal. The alkanolarnines have found wide use in the petroleum industry. The ethanolamines are used as lubricants and stabilizers in drilling muds. Reaction products of the ethan olamines and fatty acids are used as emulsion stabilizers, chemical washes, and bore cleaners (168). Oil recovery has been enhanced through the use of ethan olamine petroleum sulfonates (169—174). OH—water emulsions pumped from wells have been demulsifted through the addition of triethanolarnine derivatives. Alkanolarnines have been used in recovering coal in aqueous slurries and as coal—oil mix stabilizers (175—177). 

The sodium salt of CS [9005-22-5] is prepared by reaction of cellulose with sulfuric acid in alcohol followed by sodium hydroxide neutrali2ation (20). This water-soluble product yields relatively stable, clear, and highly viscous solutions. Introduced as a thickener for aqueous systems and an emulsion stabilizer, it is now of no economic significance. 

Na[AuClJ, per mole of silver haHde. Coordination compounds are used as emulsion stabilizers, developers, and are formed with the weU-known thiosulfate fixers. Silver haHde diffusion transfer processes and silver image stabilization also make use of coordination phenomena. A number of copper and chromium azo dyes have found use in diffusion transfer systems developed by Polaroid (see Color photography, instant). Coordination compounds are also important in a number of commercial photothermography and electrophotography (qv) appHcations as weU as in the classic iron cyano blueprint images, a number of chromium systems, etc (32). 

Product Stability and Emulsion Stability. These properties are not necessarily related, but are both highly prized in the selection of a carrier. The first refers to the storage or shelf stabiUty of the product. Many carrier preparations are not properly balanced, or unsuitable emulsifiers have been used. Upon storing, these products separate in layers, particularly when exposed to temperature changes. 

Emulsions stabilized with a nonionic surfactant and catalyzed with a monomer soluble initiator were found to foUow kinetics dependent on initiator concentration (17). 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Emulsions and Emulsion Stability, edited by Johan Sjoblom... 

Proper control of the properties of drilling mud is very important for their preparation and maintenance. Although oil-base muds are substantially different from water-base muds, several basic tests (such as specific weight, API funnel viscosity, API filtration, and retort analysis) are run in the same way. The test interpretations, however, are somewhat different. In addition, oil-base muds have several unique properties, such as temperature sensitivity, emulsion stability, aniline point, and oil coating-water wettability that require other tests. Therefore, testing of water and oil-base muds will be considered separately. 

Triazolotriazines 711 were prepared (89EGP273834, 89EGP273835) by treating triazole 710 in methanol with potassium cyanide followed by acetic acid. These compounds act as intermediates for photographic emulsion stabilizers (Scheme 149). 

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