Aggregation, droplet

Figure 3.7. State of aggregation of water and glycerol droplets in different oils (C H2 +2) as a function of n and of the absolute value refractive index mismatch Arir between the dispersed and the continuous phase. The surfactant concentration (SMO) is equal to 1 wt%. The droplet volume fraction is set at 5%. Water and glycerol droplets have a diameter close to 0.4 um. Black symbols, aggregated droplets empty symbols, dispersed droplets. (Adapted from [13].)...

This equation shows why, for example, aggregated droplets in an emulsion cream faster than individual droplets, and why coagulated solids in a suspension usually sediment faster than individual particles (in both cases the effective radius is larger). 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

Nuclear magnetic resonance relaxation is a useful experimental technique to study surfactant aggregation in liquid solutions and liquid crystals [2,50,51]. It yields information on the local dynamics and the conformational state of the surfactant hydrocarbon chain and has, for example, demonstrated the liquidlike interior of surfactant micelles. However, the aim of NMR relaxation studies of microemulsions is often to study properties such as the surfactant aggregate (droplet) size. 

For higher volume fractions, the viscosity data were more recently interpreted by a similar model of aggregating droplets, and such a model is able to account for the experimental data up to volume fractions of 0.4 [65]. 

Fig. 8.13. Changes in an emulsion. 1 The droplets are dispersed in a continuous phase. 2 The droplets form aggregates. An increase in particle diameter results in acceleration of their flotation or sedimentation. 3 Coalescence the aggregated droplets merge into larger droplets. Finally, two continuous phases are formed the emulsion is destroyed...

Here denotes moles monomer on average bound in aggregates. This equation describes coexistence between a gas of monomers and aggregate droplets. Using Eq. (3.94) yields... 

Here, r is positive and there is thus an increased vapor pressure. In the case of water, P/ is about 1.001 if r is 10" cm, 1.011 if r is 10" cm, and 1.114 if r is 10 cm or 100 A. The effect has been verified experimentally for several liquids [20], down to radii of the order of 0.1 m, and indirect measurements have verified the Kelvin equation for R values down to about 30 A [19]. The phenomenon provides a ready explanation for the ability of vapors to supersaturate. The formation of a new liquid phase begins with small clusters that may grow or aggregate into droplets. In the absence of dust or other foreign surfaces, there will be an activation energy for the formation of these small clusters corresponding to the increased free energy due to the curvature of the surface (see Section IX-2). 

The term flotoflocculation is used to describe the process of aggregating dispersed oil droplets by the aid of polymeric flocculants (flocculation) then subjecting them to conventional flotation. It is also used, genericaHy, to describe situations where particles are first aggregated then floated. 

Dispersion of a soHd or Hquid in a Hquid affects the viscosity. In many cases Newtonian flow behavior is transformed into non-Newtonian flow behavior. Shear thinning results from the abiHty of the soHd particles or Hquid droplets to come together to form network stmctures when at rest or under low shear. With increasing shear the interlinked stmcture gradually breaks down, and the resistance to flow decreases. The viscosity of a dispersed system depends on hydrodynamic interactions between particles or droplets and the Hquid, particle—particle interactions (bumping), and interparticle attractions that promote the formation of aggregates, floes, and networks. 

In the suspension methods, agglomerate formation occurs by hardening of feed droplets into soHd particles, by layering of soHds deposited from the feed onto existing nuclei, and by adhesion of small particles into aggregates as binding soHds from the dispersed feed are deposited. The product size achievable in these methods is usually limited to ca 5 mm and is often much smaller (see Drying). 

Before determining the degree of stabiUty of an emulsion and the reason for this stabiUty, the mechanisms of its destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top, and an aqueous layer appears on the bottom. This separation is the final state of the destabilization of the emulsion the initial two processes are called flocculation and coalescence (Fig. 5). In flocculation, two droplets become attached to each other but are stiU separated by a thin film of the Hquid. When more droplets are added, an aggregate is formed, ia which the iadividual droplets cluster but retain the thin Hquid films between them, as ia Figure 5a. The emulsifier molecules remain at the surface of the iadividual droplets duiing this process, as iadicated ia Figure 6. 

Fig. 5. (a) In a flocculated emulsion the droplets are aggregated but separated by a thin film. Coalescence means that the thin film between droplets bursts and the aggregates become single droplets, (b) The result is an emulsion with a wide distribution of droplet sizes but without aggregates. 

There are two essential consequences of this relation. Because larger droplets sediment or rise much faster (a 5-p.m drop rises 625 times faster than a 0.2-p.m droplet), the process is equal to shearing, leading to enhanced flocculation. The ratio between flocculation due to shear and to diffusion of droplets is proportional to the cube of the radius. Secondly, flocculation to droplet aggregates means an enhanced sedimentation rate. Sis drops ia an octahedral arrangement gives approximately four times the sedimentation rate. 

The reaction is considerably modified if the so-called emulsion polymerisation technique is used. In this process the reaction mixture contains about 5% soap and a water-soluble initiator system. The monomer, water, initiator, soap and other ingredients are stirred in the reaction vessel. The monomer forms into droplets which are emulsified by some of the soap molecules. Excess soap aggregates into micelles, of about 100 molecules, in which the polar ends of the soap molecules are turned outwards towards the water whilst the non-polar hydrocarbon ends are turned inwards (Figure 2.17). 

A low reactor temperature may not fully vaporize the feed unvaporized feed droplets will aggregate to form coke around the feed nozzles on the reactor walls and/or the transfer line. A long residence time in the reactor and transfer line also accelerate coke buildup. 

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