Thermodynamics emulsion formation

An illustration is given in fig. 3.16 for a spherical cavity. As with convex double layers, the potential drops more rapidly (inweird from the surface) with distance in 10 M than in 10" M solution in the latter case the decrease is so weak that in the centre a substantial overlap potential remains. Similar results have been obtained by others 2-3 emphasizing other aspects, such as the ton uptake (or exclusion), the Gibbs energy and the disjoining pressure. This information underlies the thermodynamics of vesicle and micro-emulsion formation. 

The ease of emulsion formation increases and the droplet size achievable decreases as the interfacial tension falls. Systems in which the interfacial tension falls to near zero j<10-3 mNm (dyne cm-1)] may emulsify spontaneously under the influence of thermal energy and produce droplets so small (<10 nm diameter) that they scatter little light and give rise to clear dispersions. The micro emulsions so formed occupy a place between coarse emulsions and micelles. They are usually effectively monodispersc and unlike coarse emulsions are thermodynamically stable. Microemulsion droplets have sometimes been classified as swollen micelles. In fact, there probably exists an essentially continuous sequence of states from association colloids to coarse emulsions,... 

The interfacial aspects of emulsification, including thermodynamics of emulsion formation and breakdown, have been reviewed and described recently by Tadros. The role of emulsifiers is discussed in detail and the mechanisms outlined, although complex, are related to the particle size if for no other reason than that the number density is proportional to l/(dd). 

In most cases, AAyj2 —TAS °, which means that AG ° is positive, i.e. emulsion formation is non-spontaneous and the system is thermodynamically unstable. In the absence of any stabilization mechanism, the emulsion will breakdown by flocculation, coalescence, Ostwald ripening or a combination of all these processes - This is illustrated in Figure 6.3, which shows several paths for emulsion breakdown [1]. 

There is no doubt that the reading of all these pioneering works is quite helpful in understanding concepts. Shah [9], a well-known scholar in the field, explained that the next critical step for these pioneers was to understand how these dispersions could be thermodynamically stabilized into a single phase, as this was in apparent conflict with the well-accepted fact that emulsions are two-phase systems that always end up coalescing. In effect, it is well known that an emulsion formation requires an energy input at least equivalent to AG = yAA, where y is the interfadal tension and AA is the produced interfacial area, and thus cannot occur spontaneously. 

Emulsions are dispersions of two immiscible liquids into each other. They are thermodynamically unstable, but the addition of surfactant molecules can provide significant kinetic stability. Emulsions are extensively used in food, cosmetic, and pharmaceutical industries, just to name a few. Because of the thermodynamic penalty, emulsion formation requires an energy input. In bulk systems, this can most easily be achieved by vigorous stirring or shaking of the whole oil/water/surfactant system. This approach leads to an pulsion with broad droplet size distribution. Microfluidics allows for the minimization of polydispersity and the creation of droplets that are virtually identical in size. 

Although nano-emulsions are thermodynamically imstable systems, they may possess high kinetic stability. This property together with their transparent or translucent visual aspect and a viscosity similar to that of water makes them of special interest for practical applications. Nano-emulsions are used in the pharmaceutical field as drug delivery systems [8,17, 18,25,28-33], in cosmetics as personal-care formulations [2,4,6,7,10,19-21,23,24,27], in agrochemical applications for pesticide delivery [3,34,35], in the chemical industry for the preparation of latex particles [9,22,26,36-38], etc. In addition, the formation of kinetically stable liquid/liquid dispersions of such small sizes is of great interest from a fimdamental viewpoint. 

There s another example of water-in-oil compartmentation, which can circumvent this problem water-in-oil emulsions. These can be prepared by adding to the oil a small amount of aqueous surfactant solution, with the formation of more or less spherical aggregates (water bubbles) having dimensions in the range of 20-100 p,m in diameter. These systems are generally not thermodynamically stable, and tend to de-nfix with time. However, they can be long-lived enough to permit the observation of chemical reactions and a kinetic study. 

When a biopolymer mixture is either close to phase separation or lies in the composition space of liquid-liquid coexistence (see Figure 7.6a), the effect of thermodynamically unfavourable interactions is to induce biopolymer multilayer formation at the oil-water interface, as observed for the case of legumin + dextran (Dickinson and Semenova, 1992 Tsapkina et al, 1992). Figure 7.6b shows that there are three concentration regions describing the protein adsorption onto the emulsion droplets. The first one (Cprotein< 0.6 wt%) corresponds to incomplete saturation of the protein adsorption layer. The second concentration region (0.6 wt% < 6 proiem < 6 wt%) represents protein monolayer adsorption (T 2 mg m 2). And the third region (Cprotein > 6 wt%) relates to formation of adsorbed protein multilayers on the emulsion droplets. 

It seems that there is probably greater availability of positively charged residues on the adsorbed protein for electrostatic interaction with sulfate groups of the anionic polysaccharide. This could lead to a greater extent of neutralization of dextran sulfate as a result of complex formation, and consequently to a lower thermodynamic affinity of the complexes for the aqueous medium and a lower value of the ( -potential for emulsion droplets in bilayer emulsions. 

Azad and Fitch (5) investigated the effect of low molecular weight hydrocarbon additives on the formation of colloidafr particles in suspension polymerization of methyl methacrylate and vinyl acetate. It was found that the additives n-octane, n-dodecane, n-octadecane, n-tetracosane and mineral oil exerted a thermodynamic affect depending upon water-solubility and molecular weight. Since these effects on emulsion polymerization have not been considered by the earlier investigators, we have chosen n-pentane and ethyl benzene as additives with limited water-solubility and n-octadecane, and n-tetracosane as water-insoluble ones. Seeded emulsion polymerization was chosen so that the number of particles could be kept constant throughout the experiments and only the effect of the other parameters on the rate could be determined. 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

Concerning thermodynamically unstable emulsions, the creation of new interfaces from the disruption of the disperse phase increases the free energy of the system, which tends to return to the original two separate systems. Therefore, the use of emulsifier is necessary not only to reduce the interracial tension, but also to avoid the coalescence and the formation of macroaggregates thanks to electrostatic repulsion between adsorbed emulsifier. 

Dependence of the lifetime of foam bilayers on the concentration of dissolved surfactant. The stability of foam, emulsion and membrane bilayers can be characterised by their mean lifetime r which is the time elapsing form the moment of formation of a bilayer with a given radius until the moment of its rupture. Obviously, this is a kinetic characteristic of the bilayer stability and can only be applied to thermodynamically metastable bilayers. 

The critical concentration Cc for formation of foam and emulsion bilayers of Do(EO)22 are 4-10 6 mol dm 3 and 1.6 10 5 mol dm 3, respectively, and are in good correlation with the lowest concentrations, 2-31 O 6 mol dm 3 and 10 5 mol dm 3 [421] at which maximum filling of the surfactant adsorption monolayer is attained. It should also be noted that in the case of the emulsion bilayers, CMC < Ce which implies that it is not possible to obtain infinitely stable (i.e. with r = °°) bilayers of Do(EO)22 between two droplets of nonane under the described conditions. For this reason, it may be thought that thermodynamically stable nonane-in-water emulsions stabilised with Do(EO)22 do not exist. 

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