Thermodynamic stability emulsions

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

For a colloidal system containing a mixture of different biopolymers, in particular a protein-stabilized emulsion containing a hydrocolloid thickening agent, it is evident that the presence of thermodynamically unfavourable interactions (A u > 0) between the biopolymers, which increases their chemical potentials (thermodynamic activity) in the bulk aqueous phase, has important consequences also for colloidal structure and stability (Antipova and Semenova, 1997 Antipova et al., 1997 Dickinson and Semenova, 1992 Dickinson et al., 1998 Pavlovskaya et al., 1993 Tsap-kina et al., 1992 Semenova et al., 1999a Makri et al., 2005 Vega et al., 2005 Semenova, 2007). 

Figure 7.10 Effect of the thermodynamic incompatibility of otsi/p-casein + high-methoxy pectin (pH = 7.0, / = 0.01 M) on phase diagram of the mixed solutions and elastic modulus of corresponding casein-stabilized emulsions (40 vol% oil, 2 wt% protein), (a) (O) Binodal line for p-casein + pectin solution with critical point ( ) ( ) binodal line for asi-casein + pectin solution with critical point ( ). (b) Complex shear modulus G (1 Hz) is plotted against the pectin concentration (O) p-casein ( ) o i -casein. Dotted lines indicate the range of pectin concentration for phase separation in the mixed solutions. The pectin was added to the protein solution before emulsion preparation. Data are taken front Semenova et al. (1999a).

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

Microemulsions represent an intermediate state between micelles and ordinary emulsions, and it is a debatable issue whether or not they should be considered as swollen micelles rather than as small-droplet emulsions. Droplet size, though small, is nonetheless large enough to justify classification as emulsions. On the other hand, the observed thermodynamic stability and reproducibility is uncharacteristic of ordinary emulsions. 

Microemulsions are defined as isotropic, transparent, and thermodynamically stable (in contrast to conventional emulsions) mixtures of a hydrophobic phase (lipid), a hydrophilic phase (often water), a surfactant, and in many cases a co-surfactant. From a lipid formulation perspective, microemulsions are generally regarded as the ultimate extension of the decreased particle size/increased surface area mantra, because emulsion particle sizes are usually less than 50 nm. Microemulsions also have additional pharmaceutical advantages in terms of their solubilizing capacity [54, 55], thermodynamic stability, and capacity for stable, infinite dilution. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

In colloid science the term colloid stability means that a specified process that causes the colloid to become a macrophase, such as aggregation, does not proceed at a significant rate. Colloid stability is different from thermodynamic stability (see Ref. [978]). The term colloid stability must be used with reference to a specific and clearly defined process, for example, a colloidally metastable emulsion may signify a system in which the droplets do not participate in aggregation, coalescence, or creaming at a significant rate. See also Kinetic Stability, Thermodynamic Stability. 

A special kind of stabilized emulsion in which the dispersed droplets are extremely small (<100 nm) and the emulsion is thermodynamically stable. These emulsions are transparent and can form spontaneously. In some usage a lower size-limit of about 10 nm is implied in addition to the upper limit see also Micellar Emulsion. In some usage the term microemulsion is reserved for a Winsor type IV system (water, oil, and surfactants all in a single phase). See also Winsor Type Emulsions. 

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

The second contribution to the steric interaction arises from the loss of configurational entropy of the chains on significant overlap. This effect is referred to as entropic, volume restriction, or elastic interaction, Gei. The latter increases very sharply with a decrease in h when the latter is less than 8. A schematic representation of the variation of Gmix, Gei, G, and Gj =G X + Gei + Ga) is given in Fig. 10. The total energy-distance curve shows only one minimum, at h 25, the depth of which depends on 5, R, and A. At a given R and A, G decreases with an increase in 5. With small particles and thick adsorbed layers (5 > 5 nm), G, becomes very small (approaches thermodynamic stability. This shows the importance of steric stabilization in controlling the flocculation of emulsions and suspensions. 

The stability of an emulsion denotes its ability to resist changes in its properties over time (i.e., higher emulsion stability implies slower change in emulsion properties). When considering the stability of an emulsion, it is of major importance to distinguish between thermodynamic stability and kinetic stability. Thermodynamics predict whether or not a process will occur, whereas kinetics predict the rate of the process, if it does occur. All food emulsions are thermodynamically unstable and thus will break down if left long enough. 

The term microemulsion, which implies a close relationship to ordinary emulsions, is misleading because the microemulsion state embraces a number of different microstructures, most of which have little in common with ordinary emulsions. Although microemulsions may be composed of dispersed droplets of either oil or water, it is now accepted that they are essentially stable, single-phase swollen micellar solutions rather than unstable two-phase dispersions. Microemulsions are readily distinguished from normal emulsions by their transparency, their low viscosity, and more fundamentally their thermodynamic stability and ability to form spontaneously. The dividing line, however, between the size of a swollen micelle ( 10-140 nm) and a fine emulsion droplet ( 100-600 nm) is not well defined, although microemulsions are very labile systems and a microemulsion droplet may disappear within a fraction of a second whilst another droplet forms spontaneously elsewhere in the system. In contrast, ordinary emulsion droplets, however small, exist as individual entities until coalescence or Ostwald ripening occurs. 

The forces such as electrical double layer, forces between emulsion droplets, hydrodynamic inertial forces, entropic (Diffusional) forces and the dispersion forces which act on the droplets or between the droplets separated at tens or hundreds of nanometers. Sedimentation and flocculation processes involve the forces such as the centrifugal force, applied electrostatic force and gravitational force. Before discussing the emulsion stability in terms of these forces, we would like to explain the thermodynamics of emulsion stabilization. 

It has also been shown from thermodynamic consideration (Equation 3), that if the interfacial tension is very low, the thermodynamically stable emulsions can be formed. Previous investigators (20,45,47,48) have calculated that for a situation likely to occur in microemulsion formation, the interfacial tensions would need to be in the order of 10 to 10 5 dynes/cm for thermodynamic stabilization and for spontaneous formation of microemulsions. 

Microemulsions are macroscopically isotropic mixtures of at least a hydrophilic, a hydrophobic and an amphiphilic component. Their thermodynamic stability and their nanostructure are two important characteristics that distinguish them from ordinary emulsions which are thermodynamically unstable. Microemulsions were first observed by Schulman [ 1 ] and Winsor [2] in the 1950s. While the former observed an optically transparent and thermodynamically stable mixture by adding alcohol, the latter induced a transition from a stable oil-rich to a stable water-rich mixture by varying the salinity. In 1959, Schulman et al. [3] introduced the term micro-emulsions for these mixtures which were later found to be nano-structured. 

In summary, the use of micro emulsions offers several advantages for the combustion process. Besides the thermodynamic stability of the aqueous fuel (which automatically means that the energy input for the production is minimal), water, surfactants and freezing point-decreasing components that are homogeneously distributed on a nano-scale can be optimally used for the reduction of soot and NOx emissions. 

Friberg was certainly the one who made the most important contributions to establish the thermodynamic stability of micro emulsions, providing key phase diagrams and being very active in refuting arguments of kinetic stability in the scientific literature and at conferences. He also at an early stage realised the problem of microstructure. This was... 

March GC, Napper DH. The thermodynamic limit to the flocculation stability of sterically stabilized emulsions. J Colloid Interface Sci 1977 61 383-387. 

Microemulsions. Unlike emulsions, microemulsions are transparent and thermodynamically stable colloidal systems, formed under certain concentrations of surfactant, water, and oil (Fig. 18.8). The transparency is because the droplet size of the microemulsions is small enough (<100 nm) that they do not reflect light. Because of its thermodynamic stability, microemulsions may have long shelf lives and spontaneously form with gentle agitation. However, microemulsions are not infinitely stable upon dilution because dilution... 

Three factors distinguish a microemulsion from an emulsion (i) the transparency, as the microemulsion is an optically isotropic solution, (ii) the thermodynamic stability of a microemulsion and the (iii) heterogeneity at the molecular level with droplets of the size 60-800 A (micelles). 

The thermodynamic limit to the stability of sterically stabilized emulsions was investigated by March and Napper (1977). It would be expected that in this limit, both oil-in-water and water-in-oil emulsions would obey the same general rules as are observed for sterically stabilized latices (Phillips, 1975). 

The thermodynamic limit to the stability of sterically stabilized emulsions ... 

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