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So it becomes necessary to understand how to develop an emulsion system to be stable or unstable, depending on the needs of the industry. To understand emulsion stability, it is important be cognizant that there are five types of breakdown processes which can occur in emulsions. These are listed in Table 1-2, along with factors that influence that type of breakdown.14 Coalescence and phase inversion When two oil drops approach each other, a thin film of the continuous water phase is trapped between the drops. The behavior of the thin film determines the degree of stability of the emulsion, and the rate of thinning of the film determines the time required for the two drops to coalesce (i.e., coalescence rate). When the film has thinned to a critical thickness, it ruptures, and the two drops unite or coalesce to form one larger drop.15, 16 The rate of film thinning depends on the surface viscosity of the surfactant film adsorbed at the oil/water interface. The film may drain evenly or unevenly depending on the interfacial tension gradient due to adsorbed surfactant.17 The factors that influence the rate of film thinning between droplets therefore influence the emulsion stability. A summary of all of the factors influencing coalescence of droplets is given in Table 1-3.18 Phase inversion can occur in emulsions due to a number of factors. For a given emulsifier concentration, the viscosity of an emulsion gradually increases as the phase volume of the dispersed phase is increased. However, at a certain critical volume fraction φc, there is a sudden decrease in viscosity, which corresponds to the point at which the emulsion inverts. φc was found to increase with increasing emulsifier concentration.19 The sudden decrease in viscosity is due to the sudden reduction in dispersed phase volume fraction. Often φc is in the range of 0.74, so that upon inversion, the dispersed phase volume fraction reduces from 0.74 to 0.26, thus reducing the viscosity significantly. φc should theoretically be in the range of 0.74 for spheres of equal radii to be at the maximum packing,20 but φc=0.99 was found for paraffin oil/aqueous surfactant solutions21 and φc=0.25 was found for olive oil/water emulsions.22 The phase inversion of emulsions can be brought about by several parameter changes, listed in Table 1-4. Demulsification As discussed previously in this section, it is not always desirable to have a stable emulsion.

Often an emulsion is present in a system in which it is undesirable. One example is the presence of aqueous emulsion droplets dispersed in crude oil. Crude oil is always associated with water or brine in oil reservoirs and also contains natural emulsifying agents, such as resins and asphaltenes.

These emulsifying agents form a thick, viscous interfacial film around the water droplets, resulting in a very stable emulsion. Therefore, demulsification is very important in the crude oil industry. Many physical methods have been developed for demulsification, depending on the industrial application. A wide variety of chemical additives for demulsification have been developed in recent years. These additives are all relatively high molecular weight polymers capable of being adsorbed at the O/W interface and displacing the film. The primary advantage of these additives is that they can be added to the system even before emulsion formation, so that they act as inhibitors.

In the petroleum industry, demulsifiers have been considered for breaking the common fuel oil emulsions. In this area, chemical demulsifiers that have been investigated include ultrahigh molecular weight polyoxiranes23 and micellar solutions containing petroleum sulfonates, electrolytes, and cosurfactants.24 It is evident that there are many methods for demulsification.

The nature of the emulsion to be separated is the key factor in determining which method(s) is best for each particular demulsification problem. Surfactant selection for emulsification Often the selection of surfactants in the preparation of either O/W or W/O emulsions is made on an empirical basis. However, in 1949, Griffin 25 introduced a semi-empirical scale for selecting an appropriate surfactant or blend of surfactants. This scale, termed the hydrophilelipophile balance (HLB), is based on the relative percentage of hydrophilic to hydrophobic groups in the surfactant molecules and ranges from 1 to 40. An HLB of 1 represents a surfactant that is highly oil-soluble and an HLB of 40 represents a highly water-soluble surfactant.

Surfactants with a low HLB number normally form W/O emulsions, whereas those with a high HLB number often form O/W emulsions.26 A summary of the HLB range required for various purposes is given in Table 1-5.

The calculation of the HLB number for a given surfactant, as developed by Griffin,25 is quite laborious and requires a number of trial and error procedures. Simplification methods were later developed by Griffin25 that applied to certain surfactants. Davies26 developed a method for calculating the HLB values of surfactants directly from their chemical formulas, using empirically determined numbers. The HLB number can also be determined experimental through several correlations that have been developed. These correlations relate the HLB number to such parameters as the cloud point,27 water titration value for polyhydric alcohol esters,28 and the heat of hydration of ethoxylated surfactants.29 Another method that may be used to select a surfactant suitable for forming an emulsion is by using the phase inversion temperature (PIT) method. The phase inversion temperature (PIT) is the temperature at which an emulsion experiences phase inversion, as described in a previous section. The PIT of non-ionic emulsifiers has been shown to be influenced by the surfactant HLB number, so the PIT can be used similarly to the HLB number in selecting an emulsifier.22 The primary distinction is that the PIT is a characteristic property of the emulsion, not of the emulsifying agent.22 Due to this, the PIT includes the effect of additives on the solvent, the effect of mixed emulsifiers or mixed oils, etc. In other words, the HLB number is actually a function of all of these properties, but only the PIT completely analyzes a given emulsion system. The PIT method is useful because the PIT is a measurable property which is related to the HLB number. A summary of effects of PIT and droplet stability from different investigations is given below:22

• The size of emulsion droplets depends on the temperature and the HLB of emulsifiers

• The droplets are less stable toward coalescence close to the PIT

• Relatively stable O/W emulsions are obtained when the PIT of the system is some 20 to 65°C higher than the storage temperature

• A stable emulsion is obtained by rapid cooling after formation at the PIT

• The optimum stability of an emulsion is relatively insensitive to changes of HLB value or PIT of the emulsifier, but instability is very sensitive to the PIT of the system

• The stability against coalescence increases markedly as the molar mass of the lipophilic or hydrophilic groups increased

• When the distribution of hydrophilic chains is broad, the cloud point is lower and PIT is higher than when there is a narrow size distribution.

The PIT can be measured by the following methods: 1) direct visual assessment,30 2) conductivity measurement,31-33 3) Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC),34 and 4) viscosity measurement.35, 36 Both the HUB and PIT methods for selecting an emulsifier in a system have been widely used and adapted to meet industry needs. Applications of emulsions Emulsions are desirable for many different applications because they provide a system having a large interfacial area. Historically, cosmetic emulsions are the oldest class of manufactured emulsions.7 Emulsions are desirable for cosmetic applications because: 1) they increase the rate and extent of penetration into the skin, 2) they open up the possibility of applying both water- and oil-soluble ingredients simultaneously (e.g. deodorants), and 3) they provide for efficient cleansing.

Emulsions are also widely used for pharmaceutical applications in the form of creams or ointments and as drug delivery vehicles. They are also ideal for use as polishes (e.g. furniture polishes, floor waxes, etc.) paints, and agricultural sprays. Many foods are manufactured in the form of emulsions including mayonnaise, salad dressings, milk, and margarine. Another industry where emulsion technology is important is the asphalt industry when the principal requirement is the production of water-repellent surfaces. Emulsions are also used as polymerization vehicles to aid in the production of high polymeric materials such as plastics, synthetic fibers, and synthetic rubbers. These are just a few of the many applications of emulsions.

1.1.3 Microemulsions A microemulsion is a thermodynamically stable, isotropic dispersion of oil and water containing domains of nanometer dimensions stabilized by an interfacial film of surface-active agent(s).37 The term “microemulsion” originated from Jack H. Schulman and coworkers in 1959,38 although Hoar and Schulman originally described water-in-oil microemulsions, which they referred to as transparent water-in-oil dispersions, in 1943.39 As implied above, microemulsions may be of the oil-in-water (O/W) (see Figure 1-5)) or the water-in-oil (W/O) type depending on conditions of the system and system components.

According to Bancroft,40-42 phase volume ratios are less important in the determination of the microemulsion type that will be formed (i.e., W/O or O/W) than the surfactant characteristics (e.g. HLB). As previously mentioned, the Bancroft rule states that whichever phase the surfactant has a greater affinity for will typically be the continuous phase.

The creation of a microemulsion entails the generation of a huge interfacial area, which, according to the following equation,43 requires a significant lowering of the interfacial tension (usually 1 mN/m):44

–  –  –

where W is the work performed, γ is the surface or interfacial tension at the air/water or oil/water interface and ΔA is the change in surface or interfacial area. This ultra-low interfacial tension in spontaneously formed microemulsions is achieved by the incorporation of surfactant(s) (typically a surfactant + a cosurfactant, especially when ionic surfactants are used).45 Figure 1-6 shows the thermodynamic explanation for the behavior of macro- and microemulsions. As can be concluded from the graph, there is an optimum radius for microemulsion systems where the free energy of dispersion becomes negative, thereby making the microemulsion stable and its formation energetically favorable.46 Schulman and others first noticed microemulsion systems in 1943 when they observed that the addition of a medium chain-length alcohol made a coarse macroemulsion that was stabilized by an ionic surfactant become transparent.39 Even then, Hoar and Schulman recognized the important role of a very low interfacial tension in causing spontaneous emulsification of the added water in oil.39 They concluded that the role of the alcohol is as a stabilizer against the repulsive electrostatic forces that the ionic surfactant head groups would experience.

Schulman and others used a variety of experimental techniques (e.g. X-ray diffraction,38 ultra-centrifugation,47 light scattering,38 viscosimetry,48 and nuclear magnetic resonance (NMR)49, 50) to elucidate some of the characteristics of these microemulsion systems following the groundbreaking work of Schulman and Hoar. These studies were instrumental in providing them with information about the structure, size, and interfacial film behavior of microemulsions.

They were able to determine the size of the droplets and they found that the presence of the alcohol within the system led to greater interfacial fluidity.

Later, in 1967, Prince38 proposed a theory that the formation of microemulsions was due to the negative interfacial tension that results from high surface pressure of the film. Prince explained this negative interfacial tension based on the depression of the interfacial tension between the oil and water phase that occurs when surfactant is added. The principle behind this theory is described by the series of equations that follow. The surface pressure of the film at the air/water interface, πaw, is defined as:38, 45

–  –  –

where γo is the surface tension of the pure surface (without surfactant) and γs is the surface tension of the surface with surfactant. In the case where oil is the second phase (oil/water system), the surface pressure of the surfactant film at the oil/water interface, πow, can be defined


–  –  –

where (γo/w)o is the interfacial tension of the “pure” oil/water interface (i.e., in the absence of surfactant) and (γo/w)s is the interfacial tension of the oil/water interface in the presence of

surfactant film. Rearrangement of Equation (1-3) gives:

–  –  –

Based on Equation (1-3), for a surfactant film that can generate a very high surface pressure (πow), the interfacial tension of the surfactant film at the oil/water interface (γo/w)s becomes negative. This is only a transient phenomenon because generation of a negative interfacial tension leads to a negative free energy of formation of the emulsion, which is an

unstable situation. This is illustrated by the following equation:

–  –  –

where ΔA is the increase in interfacial area, ΔSconfig is the configurational entropy of the droplets of the liquid that are formed and T is the absolute temperature.51 The negative interfacial tension accounts for the spontaneous increase in interfacial area that occurs in the formation of microemulsions. When a transient (unstable) negative interfacial tension is experienced, the system will seek to stabilize by spontaneously generating new interfacial area, thereby raising the interfacial tension back to acceptable, stable limits. As previously mentioned, in order to form microemulsions, it is required that the concentration of surfactants be greater than that required to reduce the oil/water interfacial tension to zero and to cover the total interfacial area of all dispersed droplets. The transient negative interfacial tension that is generated facilitates the spontaneous break-up of droplets.

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