Physical emulsion systems

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

This paper presents the physical mechanism and the structure of a comprehensive dynamic Emulsion Polymerization Model (EPM). EPM combines the theory of coagulative nucleation of homogeneously nucleated precursors with detailed species material and energy balances to calculate the time evolution of the concentration, size, and colloidal characteristics of latex particles, the monomer conversions, the copolymer composition, and molecular weight in an emulsion system. The capabilities of EPM are demonstrated by comparisons of its predictions with experimental data from the literature covering styrene and styrene/methyl methacrylate polymerizations. EPM can successfully simulate continuous and batch reactors over a wide range of initiator and added surfactant concentrations. 

Creams are semisolid emulsion systems having a creamy appearance as the result of reflection of light from their emulsified phases. This contrasts them with simple ointments, which are translucent. Little agreement exists among professionals as to what constitutes a cream, and thus the term has been applied both to absorption bases containing emulsified water (w/o emulsions) and to semisolid o/w systems, which are physicochemically totally different, strictly because of their similar creamy appearances. Logically, classification of these systems should be based on their physical natures, in which case absorption bases would be ointments and the term cream could be reserved exclusively for semisolid o/w systems, which in all instances derive their structures from their emulsifiers and internal phases. 

The range of monomers which can be employed is largely dictated by the physical chemistry of the emulsion system. For instance, monomers must be sufficiently hydrophobic to allow the formation of stable w/o HIPEs. In addition, most systems which have been studied have used polymerisation methods which require either an initiation step, or addition of a catalyst. This is due to the fact that the first step in the preparation of the polymer is the preparation of HIPE this can only proceed satisfactorily in the absence of any significant degree of polymerisation. Thus, it can be seen that radical addition polymerisation is suitable for the synthesis of PolyHIPE polymers, whereas condensation polymerisation can be more problematical. Also, the latter reactions often generate water as the by-product, hence the aqueous component of the HIPE is inhibiting to the polycondensation. 

Viscosity is an important physical property of emulsions in terms of emulsion formation and stability (1, 4). Lissant (1 ) has described several stages of geometrical droplet rearrangement and viscosity changes as emulsions form. As the amount of internal phase introduced into an emulsion system increases, the more closely crowded the droplets become. This crowding of droplets reduces their motion and tendency to settle while imparting a "creamed" appearance to the system. The apparent viscosity continues to increase, and non-Newtonian behavior becomes more marked. Emulsions of high internal-phase ratio are actually in a "super-creamed" state. 

The concentrated milk is homogenized at 140 to 210 kg/cm2 (2000 to 3000 lb/in2) at about 48°C (Hall and Hedrick 1966). This process is essential to provide adequate physical stability to the milk fat emulsion system to withstand prolonged storage at room temperature (Brunner 1974). However, homogenization lowers the heat stability of concentrated milk products (Parry 1974), which may be due to increased adsorption of casein micelles onto the newly created milk fat globule surfaces, thus making them more sensitive to heat-induced aggregation. 

In the food industry a range of practical or descriptive tests are used to evaluate product quality and the stability of whippable emulsions. Using such methods a number of reliable and commercially valuable whippable emulsions have been developed over the years. To develop new whippable emulsion systems which are more difficult to stabilize, i.e. primarily low-fat products, more advanced physical methods have been used to elucidate the fundamental mechanisms behind the behaviour of whippable emulsions. 

Self-Emulsifying Systems Emulsion systems have the disadvantage of being physically unstable, and over time a separation between the oil and water phases of the emulsion will occur. The use of conventional emulsions is also less attractive due to poor precision of the taken dose and the relatively large volume that has to be administered. To overcome these limitations, self-emulsifying drug delivery systems (SEDDS) have been developed. The... 

The physical or chemical crosslinking of polymers can be also realized in water-in-oil (W/O) emulsion systems. In this case, aqueous droplets of prepolymers are stabilized by oil-soluble surfactants in a continuous oil phase. Hyaluronan-based microgels were prepared by crosslinking of carboxylic units of hyaluronan with adipic dihydrazide in aqueous droplets [19]. Chitosan-based microgels were prepared by crosslinking of chitosan chains with glutaraldehyde in aqueous droplets [20-25],... 

Multiphase Systems Antioxidant activity depends very much on the lipid substrate used for evaluation and the hydrophilic/lipophihc nature of the anti-oxidative compound. Solubility and partition properties of the compound in the medium affect the activity of antioxidants in the bulk hpid systems. As most foods cannot be related to bulk oil systems (e.g., meat, fish, eggs, mayonnaise, salad dressings, etc.), evaluation of antioxidants in multiphase systems is more relevant to their physical and chemical nature. Because of the very same reasons, several studies have found that compounds exhibiting strong activity against oxidation of lipids in bulk systems are often inefficient in colloidal and emulsion systems. 

Aside from microscopy, the techniques for determining the size distribution of the dispersed phase in emulsion systems can be broadly divided into three categories techniques that depend upon the differences in electrical properties between the dispersed and continuous phases, those that effect a physical separation of the dispersed droplet sizes, and those that depend upon scattering phenomena due to the presence of the dispersed phase. Overviews of these types of techniques are found elsewhere 1-4,13, 46-49). 

In solid particulate systems, direct observation is justifiably the last word. In emulsions where creaming, sedimentation, and coalescence can change the nature of the sample, microscopic observation has unique sample handling problems. If these special sampling problems are addressed, then microscopy can indeed provide the benchmark for the physical characterization of the dispersed phase in emulsion systems. 

As long as the possible problems are known, microscopy can be regarded as the single most important emulsion characterization tool. In the appropriate circumstances it can give information about the relative amounts of oil, water, and solids in an emulsion system their interactions or associations the size distribution of the dispersed phase and the rate of coalescence of the dispersed droplets. Various microscopic techniques can be used to define not only the physical nature of the sample, but also the chemical composition, both mineral and organic. 

Gasperlin, M. Tusar, L. Tusar, M. Kristi, J. Smid-Korbar, J. Lipophilic semisolid emulsion systems viscoelastic behaviour and prediction of physical stability by neural 82. network modelling. Int. J. Pharm. 1998,168, 243-254. 

These are emulsion systems with a size range of 20 to 200 nm. Like emulsions, they are only kineticaUy stable but, due to the very small size, they have much longer physical stability ... 

Physical Chemistry of Emulsion Systems 1165 For a perfectly spherical droplet rj = rj = r and... 

Finally, creaming is a process which is related to flocculation in that it occurs without the loss of individual drop identities (Fig. 11.2d). Creaming will occur over time with almost all emulsion systems in which there is a difference in the density of the two phases. The rate of creaming will be dependent on the physical characteristics of the system, especially the viscosity of the continuous phase. It does not necessarily represent a change in the dispersed state of the system, however, and can often be reversed with minimal energy input. If the dispersed phase happens to be the more dense of the two phases, the separation process is termed sedimentation. 

The potential importance of the temperature effect on surfactant properties has been recognized for some time and led to the concept of using the PIT as a quantitative tool for the evaluation of surfactants in emulsion systems. As a general procedure, emulsions of oil, aqueous phase, and approximately 5% surfactant were prepared by shaking at various temperatures. The temperature at which the emulsion was found to be inverted from o/w to w/o (or vice versa) was then defined as the PIT of the system. Since the effect of temperature on the solubility of nonionic surfactants is reasonably well understood, the physical principles underlying the PIT phenomenon follow directly. 

With all these factors in mind, we have attempted to carry out the emulsions aspect of the investigations at the University of Florida Improved Oil Recovery Research Program (4,5). The emulsion systems contain TRS 10-410, isobutanol, sodium chloride, dodecane and water. Extensive physical property data and micro-structural studies of the aqueous surfactant formulations have been already reported by Vijayan et al. (6). Also, the structural aspects of the emulsions containing the same species with aqueous to oil ratio of 1 1 as well as various physical property data as a function of salt concentration have been reported by Vijayan et al. (7). A detailed study of the middle phases formed by the same surfactant formulation with dodecane oil with respect to microstructural changes and microemulsion (swollen micelle) phase inversion has been reported by Ramachandran et al. (8). 

Additions of alkoxysilanes to aqueous polymer emulsions give a complex physical-chemical system that may undergo marked changes in the latex and in deposited films with aging, but allows separate deposition of silane and polymer from a single solution. 

The physical states of lipid systems affect the distribution of antioxidants and influence their activity. a-Tocopherol and Trolox exhibit complex interfacial properties between air-oil and oil-water interfaces that significantly affect their relative activities in different lipid systems (see Chapter 10). In the bulk oil system, the hydrophilic Trolox is apparently more protective by being oriented in the air-oil interface (Figure 10.8). In the emulsion system, the lipophilic a-tocopherol is more protective by being oriented in the oil-water interface. Because of its tendency to form micelles, linoleic acid is not an appropriate lipid for testing antioxidants since their behavior in this substrate would be significantly different from that in foods composed mainly of triacylglycerols. 

Lactoferrin is a glycoprotein found in mammalian milk that tightly binds two ferric ions producing an iron complex more physically and chemically stable than the uncomplexed protein. Bovine lactoferrin inhibited oxidation in com oil-in-water emulsions and lecithin liposome systems (Table 10.8). At the same molar concentration, lactoferrin was less effective than EDTA in inhibiting hydroperoxide formation in a com oil emulsion. This lower antioxidant activity of lactoferrin may be explained by its partial iron saturation and lower affinity for ferric ions. The formation constant for ferric-EDTA is 1.3 x 10 compared to 10 ° for the ferric-lactoferrin complex. Lactoferrin was a better iron chelator in the liposome than in the emulsion systems. Inhibition in liposomes with iron-lactoferrin mixtures was in the order 1 2 > 1 1 > 2 1. This order suggested that lactoferrin also chelated metal impurities as well as added iron to inhibit lipid oxidation. Lactoferrin did not inhibit the copper-catalysed... 

Due to the well-known enzymatic lability or environmental sensitivity of proteins, a common characteristic among protein delivery systems is the ability to protect the protein from the external environment. Modes of protection may be chemical or physical. A well-recognized system that provides physical protection of proteins is microcapsules, in which a solid membrane separates the solid contents of the microcapsule from the external environment. An alternative approach would be to replace a solid membrane with a liquid membrane, i.e., multiphase or multiple emulsions. Multiple emulsions may be prepared as either oil-in-water-in-oil (OAV/0) or water-in-oil-in-water (W/OAV) systems. Multiple-emulsion systems, for the purpose of delivering proteins, would be comprised of an interior aqueous phase, containing the water-soluble protein, separated from the external aqueous phase by an oil phase, i.e., W/OAV emulsions (Fig. 1). Multiple emulsions, therefore, provide an alternative technique for the encapsulation of proteins and other materials that would otherwise be metabolized, rapidly cleared, or toxic to the patient. Multiple emulsions have been utilized for parenteral and oral administration (Brodin et al, 1978). Although there is a physical resemblance to microcapsules, multiple... 

Tadros, Tharwat F. Applied Surfactants Principles and Applications. Weinheim, Germany Wiley-VCH Verlag, 2005. Author covers a wide range of topics on the preparation and stabilization of emulsion systems and highlights the importance of emulsion science in many modern-day industrial applications discusses physical chemistry of emulsion systems, adsorption of sm-fectants at liquid/liquid interferes, emulsifier selection, polymeric surfectants, and more. 

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