EMULSIFIER CHEMISTRY

II. EMULSIFIER CHEMISTRY

Schuster (1985) has written a comprehensive text on emulsifiers with extensive discussions on the current theories of emulsion formation and emulsifier chemistry, function, and analysis. He included discussions covering all the emulsifiers used in Germany and the United States, plus applications in a wide variety of products.

A. Synthesis and Structure

Food emulsifiers can be categorized (Table 1) on the basis of several characteristics includ- ing origin, either synthetic or natural; potential for ionization, nonionic versus ionic; hydrophilic/lipophilic balance (HLB); and the presence of functional groups.

1. Lecithin and Lecithin Derivatives The primary source of lecithin, the only naturally occurring emulsifier used in any signifi-

cant quantity in the food industry, is soybeans. Soybean oil contains anywhere from 1–

Table 1 Food Emulsifier Categories Lecithin and lecithin derivatives

Glycerol fatty acid esters Hydroxycarboxylic acid and fatty acid esters Lactylate fatty acid esters Polyglycerol fatty acid esters Ethylene or propylene glycol fatty acid esters Ethoxylated derivatives of monoglycerides Sorbitan fatty acid esters Miscellaneous derivatives

3% phospholipids in the crude oil (Haraldsson, 1983). Other, less significant, sources include corn, sunflower, cottonseed, rapeseed, and eggs. Lecithin is obtained by an aque- ous extraction of the oil extracted from soybeans. Phase separation occurs upon hydration of the phospholipids and the two phases are separated by centrifugation (Flider, 1985). The crude extract, after water removal, contains about 35% triglycerides and smaller amounts of nonphospholipid materials. Extraction with acetone is used to produce an oilfree lecithin. The term ‘‘lecithin’’ has been used to describe both phosphatidylcholine and mixtures of phospholipids. Current recommendations by IUPAC-IUB (1977) (Inter- national Union of Pure and Applied Chemistry–International Union of Biochemistry) suggest the use of 3-sn-phosphatidyl-choline rather than lecithin to describe 1,2-diacyl- sn-glycero-3-phosphatidylcholine. However, a commercial soybean-derived lecithin preparation contains several different phospholipids, primarily phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol (Hurst and Martin, 1984). The struc- tures are shown in Fig. 1. Over 90% of the phospholipids in soybean lecithin are these three (Scholfield, 1981). Several papers from a symposium on lecithin presented at the 1980 American Oil Chemists’ Society meeting in New York were published in the 1980 October issue of the Journal of the American Oil Chemists’ Society. The composition of

Figure 1 The primary phospholipids reported in commercial lecithin, where R 1 and R 2 are fatty Figure 1 The primary phospholipids reported in commercial lecithin, where R 1 and R 2 are fatty

Commercial lecithin preparations can be treated or modified chemically to provide

a product with altered functional characteristics. Treatment with either hydrogen peroxide or benzoyl peroxide will produce a lighter colored product. The chemical modification of lecithin by reaction with hydrogen peroxide plus lactic or acetic acid and water will pro- duce a hydroxylated product. Hydroxylation occurs at the double bonds (Schmidt and Orthoefer, 1985), altering lecithin such that its hydrophilic character is increased. The result is a product with improved oil-in-water (O/W) emulsifying properties relative to unmodified lecithin (Prosise, 1985).

Triglycerides are soluble in acetone, whereas phospholipids are not. Therefore, the greater the percentage of acetone-insoluble material, the greater the phospholipid content in crude lecithin (Prosise, 1985). Because of this, one of the primary criteria for the evalua- tion of lecithin is the percentage of acetone-insoluble material. Lecithin is also evaluated on the basis of several other parameters (Food Chemicals Codex, 1981) including acid value (an indication of free fatty acids), hexane insoluble matter (an indication of fibrous material), water, peroxide value, and metallic impurities. Individual phospholipids in soy lecithin can be quantitated using HPLC (Hurst and Martin, 1984). The emulsifying proper- ties of native, purified, and modified lecithin can be determined based on a method devel- oped by Von Pardun (1982). First, an emulsion is formed utilizing lecithin. Next, after placing the emulsion under moderate stress, the half-life of the system is measured. Alter- natively, the rate of phase separation, creaming, flocculation or coalescence can be deter- mined based on a visual observation combined with droplet size determination (Rydhag and Wilton, 1981).

The Food Chemicals Codex (1981) contains information on nearly all the food emul- sifiers discussed in this chapter. Included in this information is a description of the emul- sifier, the requirements in terms of specifications for each emulsifier and a detailed de- scription of each assay used to evaluate either the emulsifier specifications or quality parameters. The information contained in the Food Chemicals Codex is a result of the work by the Committee of Codex Specifications of the Food and Nutrition Board of the National Research Council. The Food and Nutrition Board is an advisory group in the area of food and nutrition. It is not a regulatory agency of any government even though some governments may adopt the Food Chemicals Codex for their use.

2. Mono- and Diglycerides Mono- and diglycerides are the most commonly used food emulsifiers ( Fig. 2 ). They con-

sist of esters synthesized via catalytic transesterification of glycerol with triglycerides, with the usual triglyceride source of hydrogenated soybean oil. Mono- and diglycerides are also synthesized directly from glycerol and fatty acids under alkaline conditions. Mo- lecular distillation is used to prepare a purified product containing up to approximately 90% monoglyceride. Zlatanos et al. (1985) prepared monoglycerides from the reaction of glycidol (2,3-epoxy-1-propanol) and carboxylic acids with a yield in excess of 90%. Advantages of the process included the synthesis of difficult-to-produce monoglycerides and a good potential for continuous processing. Campbelltimperman et al. (1996) have prepared mono- and diglycerides from a butterfat fraction by chemical glycerolysis, while Pastor et al. (1995) reported the enzymatic preparation of mono- and distearin by glycerol- ysis of ethylstearate and direct esterification of glycerol in the presence of a lipase from

Figure 2 Mono- and diglycerides.

Several tests are used for characterizing commercial sources of mono- and diglycer- ides, including total monoglycerides, hydroxyl value, iodine value, and the saponification value (Food Chemicals Codex, 1981). With the monoesters, the fatty acid can be attached at either the alpha or beta positions, likewise with the diglycerides.

3. Hydroxycarboxylic and Fatty Acid Esters To produce an emulsifier with increased hydrophilic character relative to monoglycerides,

small organic acids are esterified to monoglycerides (Fig. 3). Some of the acids used are acetic, citric, fumaric, lactic, succinic, and tartaric. Succinylated monoglycerides are synthesized from succinic anhydride and distilled monoglycerides (Larsson, 1976). They are used by the baking industry as dough conditioners and crumb softeners. Acetic acid esters of mono- and diglycerides are synthesized from fatty acids plus acetic anhydride or by transesterification. The product is lipid soluble and water insoluble. Functions in food include control of fat crystallization and improvement of aeration properties of high- fat foods. They are often added to shortenings or cake mixes.

Figure 3 Organic acid ester of monoglyceride, where at least one R is a short chain organic acid,

Figure 4 Sodium stearoyl-2-lactylate, where n normally averages 2 and R is a fatty acid moiety.

To synthesize other acid esters, citric acid esters of mono- and diglycerides, glycerol is esterified with a mixture of citric acid and fatty acids. It can also be prepared by the direct esterification of citric acid with glyceryl monooleate (Food Chemicals Codex, 1981). The product is hot water and lipid soluble. Functions in food include emulsification, anti- spattering agent in margarine, improvement of bakery product characteristics, a fat replace- ment in high fat foods, and a synergist and solubilizer for antioxidants.

Diacetyl tartaric acid esters of monoglycerides (DATEM) are synthesized from di- acetyl tartaric acid anhydride and monoglycerides (Krog and Lauridsen, 1976). The emul- sification properties of DATEM depend primarily upon the type of fatty acid and the percentage of esterified tartaric acid. Emulsifier quality is based upon results from the analyses for tartaric and acetic acid, acid value, total fatty acids, saponification value, and metallic residue (Food Chemicals Codex, 1981).

Lactic acid esters of mono and diglycerides consist of a mixture of lactic and fatty acid esters of glycerin. The emulsifier is dispersible in hot water. Important qualitative parameters include the percentage of monoglycerides, total lactic acid, acid value, free glycerin, and the amount of water (Food Chemicals Codex, 1981).

4. Lactylate Fatty Acid Esters Polymeric lactic acid esters of monoglycerides (Fig. 4) are also available, commonly

known as sodium or calcium stearoyl-2-lactylates. Typically, there are two lactic acid groups per emulsifier molecule. To produce the emulsifier a mixture of the fatty acid, polylactic acid, and calcium or sodium carbonate is heated at about 200 °C for about 1 h with agitation in an inert atmosphere (Krog and Lauridsen, 1976). The calcium salt is less dispersible in water than sodium stearoyl-2-lactylate.

5. Polyglycerol Fatty Acid Esters Polyglycerol esters of fatty acid (Fig. 5) are also used in food products, primarily in baked

goods. They consist of mixed partial esters synthesized from the reaction of polymerized glycerol with edible fats. Polyglycerols will vary in degree of glycerol polymerization with an average specified. The source of fatty acids as well as the degree of polymerization

Figure 5 Polyglycerol esters of fatty acids, where R 1 ,R 2 , and R 3 are each either a fatty acid

Figure 6 Propylene glycol esters of fatty acids, where R 1 and R 2 represent either a fatty acid and/or a hydrogen and where at least one R represents a fatty acid.

can vary, providing a wide range of emulsifiers, from hydrophilic to very lipophilic (Food Chemicals Codex, 1981).

6. Polyethylene or Propylene Glycol Fatty Acid Esters Fatty acids can be esterified directly to polyethylene glycol ethers (Fig. 6) (Meffert, 1984)

or by enzymatic preparation, which allows better control of the reaction (Bhattacharyya et al., 1984). Shaw and Lo (1994) reported the production of propylene glycol fatty acid

(C 12 ,C 14 ,C 16 ,C 18 , and C ) monoesters by lipase catalyzed reactions. Propylene glycol 18 : 1 monoesters of docosahexaenoic acid and eicosapentaenoic acid, which are water-in-oil (W/O) emulsifiers useful in the food industry, have been synthesized by lipase-catalyzed esterification (Liu and Shaw, 1995). The HLB of the emulsifier is altered by adjusting the degree of ethoxylation. Fatty acid polyglycol esters are good O/W emulsifiers (Maag, 1984).

7. Ethoxylated Derivatives of Monoglycerides Ethoxylated mono- and diglycerides are produced from the reaction of several moles of

ethylene oxide and mono- or diglycerides under pressure (Meffert, 1984). Ethoxylation of monoglycerides results in a product that is much more hydrophilic relative to monoglyc- erides (Rusch, 1981).

Polyoxyethylene monoglycerides may contain as many as 40 moles of ethylene ox- ide per mole of monoglyceride (Schuster, 1985). The end product of the synthesis is actu- ally a mixture with a distribution range and peak (Becher, 1967); therefore, lots often vary among manufacturers.

8. Sorbitan Fatty Acid Esters Polyoxyethylene sorbitan esters are synthesized by the addition, via polymerization, of

ethylene oxide to sorbitan fatty acid esters. These nonionic hydrophilic emulsifiers ( Fig.

7 ) are very effective antistaling agents and, thus, are used in a wide variety of bakery products (Schuster and Adams, 1984). These emulsifiers are much more widely known as the polysorbates, e.g., polysorbate 20, 60, and 80. Polysorbate 20, 60, and 80 utilize lauric, stearate, and oleate, respectively, for the fatty acid portion of the molecule (Food Chemicals Codex, 1981). Polysorbate 60 is a monostearate, while polysorbate 65 is a

Figure 7 Polysorbates, where w ⫹ x ⫹ y ⫹ z ⫽ 20 (approximately) and Rs represent a single fatty acid and hydrogens for polysorbate 20, 40, 60, and 80. For polysorbate 65, each R represents

a stearic acid moiety. The fatty acids are lauric, palmitic, stearic, and oleic acid for polysorbate 20, 40, 60, and 80, respectively.

9. Miscellaneous Derivatives Fatty acids can be esterified directly to compounds other than glycerol, for example, sugar

alcohols, like sorbitol, mannitol, and maltitol, and sugars, like sucrose, glucose, fructose, lactose, and maltose (Torrey, 1983).

Sorbitol or sorbitan esters are formed from 1,4-anhydro-sorbitol and fatty acids (Mef- fert, 1984). Typically, the emulsifier consists of a mixture of stearic and palmitic acid esters of sorbitol and its mono- and dianhydrides (Fig. 8). Ethoxylated derivatives can also be prepared by the addition of several moles of ethylene oxide to the sorbitan monoglyceride ester, and, depending on the number of moles of ethylene oxide added, have a wide range in HLB.

Figure 9 Sucrose fatty acid esters, where at least one of either R 1 ,R 2 , or R 3 represents a fatty acid and the reminder may represent a fatty acid or a hydrogen; the degree of substitution is 1–3.

Lactitol (the hydrogenation product of lactose) palmitate is synthesized by di- rect esterification at a temperature of approximately 160 °C (van Velthuijsen, 1979). The product mixture can be separated with silica gel thin layer chromatography (TLC) with chloroform–acetic acid–methanol–water (80 : 10 : 8 : 2, v/v) as the eluent. The esters con- taining one or two fatty acid groups are effective emulsifiers. Lactitol esters containing at least 4 moles of fatty acid per mole of lactitol could potentially be used as a low-calorie fat substitute, since hydrolysis occurs to only a minor degree in humans.

Akoh (1994) has reported the enzymatic synthesis of acetylated glucose fatty acid esters. Two immobilized lipases from Candida anarctica (SP 382) and Candida cylindra- ceae catalyzed the synthesis of novel acetylated glucose fatty acid esters with glucose pentaacetate and Trisun 80 (80% oleic) vegetable oil or methyl oleate as substrates in organic solvents. The incorporation of oleic acid onto the glucose ranged from 30–100%. It was possible to catalyze the synthesis of glucose fatty acid esters with free glucose as the sugar substrate. Other researchers have reported the synthesis of a novel nonionic surfactant, dialkyl glucosylglutamate from delta-gluconolactone, glutamic acid, and alkyl alcohols (Tsuzuki et al., 1993).

Sucrose fatty acid esters (Fig. 9) can be synthesized using a variety of solvents or by direct esterification (Wei, 1984). The first description of a practical commercial process for the preparation of sucrose esters of fatty acids was reported by Osipow et al. in 1956. Enzymatic synthesis of carbohydrate esters of fatty acids has also been reported for the esters of sucrose, glucose, fructose, and sorbitol with oleic and stearic acid (Seino et al., 1984) and fatty acid esters of fructose (Arcos et al., 1998). The report by Arcos et al. indicates that these workers were able to enzymatically prepare three different 1,6-diacyl fructofuranoses. At low temperatures (5 °C) the synthesis produces quantitative yields of the diesters by simple addition of the original sugar to a solution of the fatty acid in a solvent (acetone) which is accepted by the European Economic Commission (EEC) for use in the manufacture of additives. By varying the degree of esterification, the HLB, and hence the functionality, can be controlled. Sucrose monoesters have an HLB value greater than 16, while the triesters have an HLB value less than 1. Monoesters are particularly useful for the stabilization of O/W emulsions (Maag, 1984), whereas diesters are best for W/O emulsions. With esterification equal to or greater than 5 moles of fatty acid per mole of sucrose, the emulsification properties of sucrose fatty acid esters are lost (Wei, 1984). But, at that degree of esterification, the sucrose fatty acid polyester can be used as a low- calorie fat replacement since it is neither digestible nor absorbable.

The consistency of both O/W and W/O emulsions can be affected with the addition of ethylene or propylene glycol monostearate. The most common ethylene and propylene glycol esters used as emulsifiers are the monostearate and monopalmitate.

B. Analysis B. Analysis

Dieffenbacher and Bracco (1978) developed a thin layer chromatographic method for the detection of a mixture of several emulsifiers. They utilized a chloroform–methanol extraction in combination with a column chromatography cleanup. Thin layer chromatog- raphy was used for separation and quantitation. Three different solvent systems were used

along with a variety of spray reagents for detection. With a combination of R f values plus specific and nonspecific detection sprays, 14 different emulsifies were detected. Gas–liquid chromatography (GLC) analysis for the silylated derivatives of both the polyglycerols and their fatty acid esters (Sahasrabudhe, 1967) and the lactylated mono- glycerides (Neckerman and Noznick, 1968) are available. Soe (1983) separated several different emulsifier groups using GLC. Emulsifiers separated included monoglycerides, lactic acid esters of monoglycerides, acetic acid esters of monoglycerides, and propylene glycol esters of fatty acids. Each of the emulsifier groups analyzed contained esters of several different fatty acids. The AOAC INTERNATIONAL has developed a gas chro- matographic technique for the analysis of monoglycerides and diglycerides, which has been adopted as an IUPAC/AOCS/AOAC method (Firestone, 1994).

Sudraud et al. (1981) separated mixtures of emulsifiers with high-performance liquid chromatography (HPLC). Analysis of acetylated tartaric acid esters of mono- and diglycer- ides indicated a complex mixture containing at least 12 different components. Garti and Aserin (1981) utilized partition chromatography HPLC to separate and quantitate poly- glycerol esters of fatty acids. Schlegelmilch et al. (1995) have developed an HPLC method for the quantitation of free polyglycerol in polyglycerol caprate emulsifiers, while Martin (1995) has reported the use of an HPLC having an evaporative light scattering detector for the analysis of nonionic emulsifiers.

Sucrose fatty acid esters can by determined with HPLC using a reverse phase column and either a methanol–water solvent or a methanol–isopropanol solvent (Kaufman and Garti, 1981). Tsuda and Nakanishi (1983) and Gupta et al. (1983) have developed methods for the analysis of sucrose mono- and diesters using GLC. The AOAC recommends an HPLC method for the quantitation of sucrose mono- and diesters, but it may be difficult to obtain good peak separation when fatty acid esters of glycerol are present (Gupta et al., 1983).

Daniels et al. (1982) developed an analysis for a mixture of polyoxyethylated stearic acid esters of sorbitol and its anhydrides (polysorbate 60) in salad dressings. The procedure utilized a combination of colorimetric analysis and silica gel TLC plus column chromatog- raphy.

Buschmann and Hulskotter (1997) have developed a titration procedure for low- ethoxylated nonionic emulsifiers. An ionic group is introduced into the molecule by a derivatization reaction. The reaction product is then determined by conventional titration methods for anionic emulsifiers without any modification. This method has been used for the analysis of other nonionic emulsifiers such as sorbitan esters and ethoxylated fatty acid amides.

Wheeler (1979) developed an analysis for the determination sodium stearoyl-2-lac- tylate in flour and flour blends utilizing a chloroform extraction, thin layer chromatography for purification, and a colorimetric procedure for quantitation.

Artz and Myers (1994) developed a supercritical fluid method for extraction, sepa- Artz and Myers (1994) developed a supercritical fluid method for extraction, sepa-

chromatography on a 25% cyanopropyl stationary phase with a mobile phase of CO 2 at 100 to 150 °C. Samples of acetylated monoglycerides were placed in a supercritical fluid extraction cell on a glass bead bed and extracted for 15 min at 50 °C at 340, 408, and 680

atm with CO 2 . Acetylated monoglycerides added during twin-screw extrusion of corn starch were extracted from the extrudate for 15 and 45 min at pressures of 544 and 646 atm with 0–5% methanol in supercritical carbon dioxide. The percent acetylated mono- glyceride extracted after 45 min at 120 °C and 646 atm was 60%. Recently, fatty alcohol ethers, nonionic surfactants which are used as general purpose emulsifiers, were separated by use of water-modified carbon dioxide mobile phase in capillary supercritical fluid chro- matography (Pyo et al., 1996). Much greater peak intensities were observed in the chro-

matogram with on-line modified mobile phase than with pure CO 2 . Berchter et al. (1997) reported the use of the matrix assisted laser desorption and ionization time of flight mass spectrometry (MALDI-TOF-MS) for the analysis of nonvol- atile highly polar or high-mass substances, especially emulsifiers.

C. Kinetics of Emulsification

This section briefly discusses the kinetics of emulsifiers with respect to the adsorption and desorption kinetics of emulsifiers at hydrophobic and hydrophilic surfaces, as well as micelle lifetimes. Whereas nonionic emulsifiers are adsorbed as submonolayers or monolayers at hydrophobic surfaces, surface micelles or bilayers are formed at hydrophilic surfaces (Tiberg, 1996). Thus, the need for a better understanding of the adsorption/de- sorption phenomena of emulsifiers and macromolecules at surfaces becomes evident in the desire to control and affect surface properties in a range of technical and biological processes. Depending upon whether the hydrophilic or the hydrophobic segment of the emulsifier has stronger tendency to be adsorbed at the surface, different molecular arrange- ments are expected at the surface. If the water-soluble component interacts more favorably with the surface and the hydrophobic segments are sufficiently large to trigger self-associa- tion, surface micelles or various bilayer type aggregates can be expected at the surface. If, on the other hand, the hydrophobic part is adsorbed more strongly at the surface, it should result in the formation of monolayer or submonolayer structures at the surface.

The behavior of nonionic emulsifiers has been studied using ellipsometry. This tech- nique is used to measure polarization changes that occur using oblique reflection of a polarized light beam from a surface. The polarization changes are very sensitive to the presence of a thin film or a layer of adsorbed molecules at the surface. The difference in the polarization state between the incident and the reflected light is described by the measured ellipsometric angles. From these measurements, adsorption isotherms of emulsifiers can

be plotted and determined. Dynamic interfacial tension is another widely used technique for probing emulsifier adsorption/desorption kinetics interfaces. After the interface is dis- turbed, the interfacial tension is measured as a function of time to monitor emulsifier adsorption at the interface.

1. Nonionic Emulsifiers at Hydrophilic Surfaces When the emulsifier concentration is substantially less than the critical micelle concentra-

tion (CMC), the emulsifiers are adsorbed as unimers to the surface. However, as the emul- tion (CMC), the emulsifiers are adsorbed as unimers to the surface. However, as the emul-

a very narrow region. The bulk concentration at which the aggregation process starts is called the critical surface aggregation concentration (CSAC) (Levitz et al., 1984; Lind- heimer et al., 1990; Tiberg et al., 1994a). The cooperative isotherm obtained is characteris- tic of emulsifiers with weak interaction energies between the head groups and the surface. Below the CSAC, the adsorbed-layer thickness is always very small. Above CSAC, the adsorbed-layer thickness jumps to stable plateau values roughly corresponding to twice the emulsifier length, while the adsorbed amount generally increases more slowly. This indicates that the emulsifiers form discrete surface aggregates, with a well-defined thick- ness. The whole adsorption (or surface aggregation) process occurs below the CMC, since the surface aggregates have a lower free energy than the corresponding bulk structures. The surface excess increases as the ratio of lipophilic to hydrophilic groups of the emulsi- fiers increases. This effect is mainly related to the fact that emulsifiers assemble into surface aggregates of different in-plane dimensions. Thus, surface aggregates adsorbed on hydrophilic surfaces exhibit properties that are closely related to the corresponding bulk structures. The adsorbed layer thickness reaches steady state values at low surface coverages. The thickness remains unaffected during desorption indicating that surface micelles are present at hydrophilic surfaces from relatively low to high coverages. The adsorption process is controlled by diffusive transport of monomers and micelles to the interface through a stagnant layer. The absorption process can be described quantitatively using the following equation, which describes the rates of adsorption and desorption under steady state conditions (Tiberg et al., 1994b):

d D mon ND Γ/dt ⫽ mic

c mic, b ⫺c δ mic, s

c mon, s ⫺c mon, s ⫹

where N is the mean aggregation number of the micelles, D mon and D mic are the diffusion coefficients (in m 2 /s) of emulsifier monomers and micelles, respectively, δ is the stagnant layer thickness, and Γ is the emulsifier adsorption (in kmol/m 2 ); c mon, b ,c mon, s ,c mic, b , and

c mic, s , are the concentrations (in kmol/m 3 ) of monomers and micelles in the bulk and just outside the surface, respectively. The thickness of the stagnant layer δ can be calculated from the equation

D mon ⫽⫺

δ The emulsifier activities, both in bulk solution and just outside the adsorbed layer, are

(CSAC) dt

approximately constant, the latter through a local equilibrium existing between the ad- sorbed aggregates and the solution in the immediate vicinity of the adsorbed aggregates.

2. Nonionic Emulsifiers on Hydrophobic Surfaces The adsorption on hydrophobic surfaces gives a simple Langmuirian type isotherm, where

the saturation adsorption is reached around the CMC. The maximum plateau adsorption increases with the length of the hydrocarbon chain and/or a reduction in the size of the hydrophilic group. The isotherm is fairly well represented by a Langmuir isotherm with the general form

Γ p C b Γ⫽ Γ p C b Γ⫽

m 2 ). The adsorption on a hydrophobic surface is a noncooperative process involving the formation of surface aggregates. As with the case on hydrophilic surfaces, the adsorbed layer thickness on hydrophobic surfaces increases in a steplike fashion with concentration as the CSAC is exceeded. The hydrocarbon region of the adsorbed layer is free of water. Thus, the emulsifier head has roughly the same volume independent of the surface cover- age, indicating an increase of the emulsifier concentration, and hence surface coverage, resulting in an average tilt of the hydrophilic group toward the surface.

The kinetics at a hydrophobic surface differs from that at a hydrophilic surface. At

a hydrophobic surface, the surface excess and the adsorbed layer thickness evolves in similar manner with time, whereas at a hydrophilic surface they exhibit different time dependencies. The adsorption at a particularly bulk concentration is much faster to the hydrophobic than the hydrophilic surface. However, desorption goes much faster from a hydrophilic surface than a hydrophobic surface. At the hydrophobic surface, the emulsifi- ers become increasingly tilted toward the surface with adsorption time and surface cover- age. The concentration of the monomer emulsifiers in the immediate vicinity of the ad-

sorbed layer, C mon, s ( Γ) at a particular Γ value is equal to the inverse isotherm C b ( Γ), and

C mic, s ( Γ) ⫽ 0, except when Γ ⬇ Γ p . The adsorption rate when C b is much less than the CMC is given by the following equation:

δ 冢 Γ p ⫺Γ 冣

C b k ⫺ dt

D mon

When the bulk surfactant concentration quickly adjusts to C b ⬇ 0, the desorption rate is given by the following equation:

D mon k Γ ⫽ dt

δ Γ p ⫺Γ In general, nonionic emulsifiers are adsorbed as submonolayers or monolayers at

hydrophobic surfaces, whereas surface micelles or bilayer type aggregates are formed above a critical concentration on hydrophilic surfaces. At hydrophobic surfaces, adsorbed emulsifiers are tilted toward the surface at low coverages. The degree of hydration of the headgroups is relatively independent of the surface coverage. An increasing surface excess at a hydrophilic surface is accompanied by an increase in the number of surface micelles and in some cases a substantial in-plane growth of the surface micelles, resulting in the formation of bilayer type structures. The kinetics of adsorption and desorption are strongly dependent on the surface properties. These processes are diffusion limited at both hy- drophobic and hydrophilic surfaces. Thus, the rates are determined by the stagnant layer thickness, the diffusion coefficients of monomers and micelles, and the concentration gra- dients over the stagnant layer.

3. Ionic Emulsifiers The similarity of the physical structure (i.e., short hydrocarbon tail and small headgroup)

of a typical ionic emulsifier to that of a nonionic emulsifier that exhibits fast local exchange between solution and interface suggests that ionic emulsifiers should exchange at similar of a typical ionic emulsifier to that of a nonionic emulsifier that exhibits fast local exchange between solution and interface suggests that ionic emulsifiers should exchange at similar

4. Micellar Lifetime Micelles are in a dynamic equilibrium state with monomers in solution. The micelles are

dissociated into monomers and monomers are associated into micelles continuously. The multiequilibrium, stepwise association process of micelle formation can be described as:

A 1 ⫹A n⫺1 ⫽A n where A 1 denotes the emulsifier monomer and A n represents the micellar aggregate with

an aggregation number of n. These micellar kinetics have been studied by stopped-flow, temperature-jump, pressure-jump, and ultrasonic absorption methods since Aniansson de- veloped a theoretical model to describe kinetic micellization (Aniansson et al., 1976). There are two relaxation processes: The first one is the fast relaxation process with relax-

ation time ( τ 1 , in the microsecond range), which is associated with the fast exchange of monomers between micelles and bulk aqueous phase. This process is considered as the collision between emulsifier monomers and micelles. The second relaxation time ( τ 2 , in the millisecond range) is related to the micelle dissociation kinetics (Oh and Shah, 1994). The average micellar lifetime is given by the following expression:

1 ⫺1 n 2 σ 2

⫽ 1⫹ τ a

2 na (1) (m) ⫽ σ 2 1⫹ 2 na (1) (m) ⫽ σ 2 1⫹

and a ⫽ (A tot ⫺A 1 )/A 1 .A tot and A 1 are the total emulsifier concentration and mean mono- mer concentration, respectively. When the concentration of an emulsifier is much greater than the critical micelle concentration, the micellar lifetime is approximately equal to n τ 2 .

During dynamic processes such as emulsification, the interface between two phases ex- pands rapidly. The dynamic surface tension of the micellar solutions determines the char- acteristics of these dynamic processes. However, the dynamic surface tension is influenced by the rate of diffusion of emulsifier monomers from the bulk liquid to the interface (Tiberg, 1996). Thus, relatively unstable micelles can increase significantly the diffusion rate of monomers upon a rapid breakdown of micelles into emulsifier monomers.

Generally, emulsion systems have minimal thermodynamic stability and tend to phase separate. Conventional emulsions or macroemulsions are thermodynamically unsta- ble and turbid systems. This phenomena is even more pronounced in double emulsions, which are termed ‘‘emulsions of emulsions.’’ Double emulsions are thermodynamically unstable systems with a strong tendency for coalescence, flocculation, and creaming. Most double emulsions consist of relatively large droplets, cannot withstand storage regimes, and have a strong tendency to release entrapped matter in an uncontrolled manner (Garti, 1997). However, it is possible to have emulsion systems with ultralow interfacial tensions which are thermodynamically stable. These emulsions are called microemulsions, which are thermodynamically stable dispersions produced by using emulsifiers able to reduce the interfacial energy to values close to zero (De Buruaga et al., 1998; Perrin and Lafuma, 1998). Both oil-in-water (direct) and water-in-oil (inverse) microemulsions can be pro- duced and are transparent. Their structural entities are much smaller than the wavelength of light, which is the reason for their transparency (Perrin and Lafuma, 1998).

The primary driving force for phase separation is droplet interfacial free energy (Opawale and Burgess, 1998). During emulsification, a lot of energy is required to disperse one liquid into another as small droplets. The interfacial area is greatly increased during this dispersion process. The work done to expand the interfacial area is given by the following expression:

W ⫽ γ(∆A) where γ is the dynamic interfacial tension and ∆A is the increase in the interfacial area.

For a constant W, a greater value of γ yields a smaller ∆A. Thus, the emulsion droplet size increases as the interfacial tension increases. The dynamic interfacial tension between oil and water during the emulsification process is determined by how effectively the emul- sifier monomers adsorb into the interface from the bulk solution (from the water phase for O/W emulsions, from the oil phase for W/O emulsions). Most of the emulsifier mole- cules exist as the micellar form in solution. Therefore, the effectiveness of emulsifier monomer adsorption depends on the rate of micellar breakdown because micelles them- selves cannot adsorb at the interface.