Barrier to coalescence

In most systems, stable bubbles can only be formed in water or an aqueous solution if a surface active agent is present at the interface. When two bubbles are brought into contact, the resistance to coalescence will be determined by the nature of the surfactant film or monolayer. The result of coalescence is always a decrease of interfacial area. However, before coalescence can occur, the surface films around the bubbles are compressed. This gives an increase in O and provides an elastic restoring force tending to oppose the compression. 

Three test tubes containing about 10 cm of pure water are provided. The first one is vigorously shaken. No foam is produced (if some bubbles are seen, this shows that the water is not pure and contains a minute amoimt of surface active compounds). A small amount of a solid 

In the study of the effects of wheat nonpolar lipids on breadmaking, deleterious effects were found to be caused by free fatty acids. Within the fatty acids, detrimental effects were related to linoleic acid but not to palmitic acid. Could the difference be due to different behavior of these fatty acids at the gas-liquid interface  

Long chain saturated alcohols with even numbers of carbon atoms give condensed monolayers whose D-A curves extrapolate to 20 0.25 at 20°C. It is foimd that 36 gL of a 1.0-mg/mL solution of an unknown alcohol spreads to give a monolayer whose n-A curve extrapolated to an area of 162.8 cm. Deduce the identity of the alcohol. 

Bloksma, A. H. 1990. Dough structure, dough rheology and baking quality. Cereal Foods World 35 237-244. 

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The charge on a droplet surface produces a repulsive barrier to coalescence into the London-van der Waals primary attractive minimum (see Section VI-4). If the droplet size is appropriate, a secondary minimum exists outside the repulsive barrier as illustrated by DLVO calculations shown in Fig. XIV-6 (see also Refs. 36-38). Here the influence of pH on the repulsive barrier between n-hexadecane drops is shown in Fig. XIV-6a, while the secondary minimum is enlarged in Fig. XIV-6b [39]. The inset to the figures contains t,. the coalescence time. Emulsion particles may flocculate into the secondary minimum without further coalescence. 

The non-aqueous HIPEs showed similar properties to their water-containing counterparts. Examination by optical microscopy revealed a polyhedral, poly-disperse microstructure. Rheological experiments indicated typical shear rate vs. shear stress behaviour for a pseudo-plastic material, with a yield stress in evidence. The yield value was seen to increase sharply with increasing dispersed phase volume fraction, above about 96%. Finally, addition of water to the continuous phase was studied. This caused a decrease in the rate of decay of the emulsion yield stress over a period of time, and an increase in stability. The added water increased the strength of the interfacial film, providing a more efficient barrier to coalescence. 

An example of the importance of the wettability of fine particles is provided by what are called Pickering emulsions, that is, emulsions stabilized by a fdm of fine particles. The particles can be quite close-packed and the stabilizing fdm between droplets can be quite rigid, providing a strong mechanical barrier to coalescence. See Section 5.4.1. 

High surface viscosity and/or mechanically strong interfacial film - this acts as a barrier to coalescence and may be enhanced by adsorption of fine solids, or of dose-packed surfactant molecules. 

Steric stabilization appears to be the dominant stabilizing force in most food colloids [78,824], Casein-coated emulsion droplets provide an example. The presence of protein in an adsorption layer can also contribute viscoelasticity and provide a barrier to coalescence. 

An emulsion stabilized by fine particles. The particles form a close-packed structure at the oil-water interface, with significant mechanical strength, which provides a barrier to coalescence. 

Galactomannans, particularly, become adsorbed and organized on an oil-drop surface as lamellar liquid crystals that perform as steric and mechanical barriers to coalescence (Reichman and Garti, 1991). The surfactancy of xanthan was found to be related to the amount adsorbed (Young and Torres, 1989). 

With cetyl alcohol, there is the complication that the polarity of the molecule may cause it to reside at the surface of the droplet, imparting additional colloidal stability. Here, the surfactant and costabilizer form an ordered structure at the monomer-water interface, which acts as a barrier to coalescence and mass transfer. Support for this theory lies in the method of preparation of the emulsion as well as experimental interfacial tension measurements [79]. It is well known that preparation of a stable emulsion with fatty alcohol costabilizers requires pre-emulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol costabilizer in the water prior to monomer addition, it is believed that an ordered structure forms from the two surfactants. Upon addition of the monomer (oil) phase, the monomer diffuses through the aqueous phase to swell these ordered structures. For long chain alkanes that are strictly oil-soluble, homogenization of the oil phase is required to produce a stable emulsion. Although both costabilizers produce re-... 

Stable emulsions often form during industrial processing. On the microscopic scale, the reasons that the droplets remain dispersed fall into two broad categories (1) physical barriers to coalescence and (2) electrical repulsion between droplets. An example of a physical barrier is the presence of finely divided solids at the oil-water interface. Of primary concern, however, is the consideration of electrical forces because their influence is significant at relatively longer distances. Electrical repulsive forces arise... 

Surface-active agents. Surface-active agents such as emulsifiers and surfactants play a very significant role in the stability of emulsions. They greatly extend the time of coalescence, and thus they stabilize the emulsions. Mechanisms by which the surface-active agents stabilize the emulsion are discussed in detail by Becher (19) and Coskuner 14). They form mechanically strong films at the oil-water interface that act as barriers to coalescence. The emulsion droplets are sterically stabilized by the asphaltene and resin fractions of the crude oil, and these can reduce interfacial tension in some systems even at very low concentrations (i7, 20). In situ emulsifiers are formed from the asphaltic and resinous materials found in crude oils combined with ions in the brine and insoluble dispersed fines that exist in the oil-brine system. Certain oil-soluble organic acids such as naphthenic, fatty, and aromatic acids contribute to emulsification 21). 

Existence of an Electrical or Steric Barrier to Coalescence on the Dispersed... 

This assumes that every collision is effective in decreasing the number of particles. In the presence of an energy barrier to coalescence E, which is present in all dispersed systems,... 

A is a constant for a particular system, called the collision factor. The effect of the surfactants used as the emulsifying agent is seen in the value of E, the energy barrier to coalescence, which includes both mechanical and electrical barriers. 

Calculations for larger drops are complicated by phenomena such as shape deformation, wake oscillations, and eddy shedding, making theoretical estimates of E difficult. The overall process of rain formation is further complicated by the fact that drops on collision trajectories may not coalesce but bounce off each other. The principal barrier to coalescence is the cushion of air between the two drops that must be drained before they can come into contact. An empirical coalescence efficiency Ec suggested by Whelpdale and List (1971) to address droplet bounce-off is... 

In 1960, Blair [59] and Dodd [68] published key studies on water-in-crude oil emulsions and their films (see [1-6] for references). Using a Cenco surface film balance to study the water-oil interface, Blair showed that the principal source of stability arises from the formation of a condensed and viscous interfaciai film by adsorption of soluble material from the petroleum phase, such film presenting a barrier to coalescence of the dispersed droplets. This primary film may be augmented by secondary adsorption of large particles or micelles originally suspended in the petroleum. The classical picture of emulsion stabilization by an adsorbed monolayer yielding low interfaciai tension values does not seem to be an accurate one in this case. It appears that a primary adsorbed layer is initially formed, almost certainly comprised of asphaltenes, and a secondary layer superimposes on this primary layer and is likely comprised of asphaltenes, wax particles, and possibly... 

The increased temperature of the system may lower the interfacial shear viscosity, which may lead to an improved rate of film drainage between adjacent drops. Menon and Wasan (62) demonstrated the decrease in interfacial viscosity of shale oil/water at increased temperatures and showed analogous behavior at increased demulsifier concentration. Thompson et al. (133) reported that, at increased temperatures, some incompressible nomelaxing films tend to relax and the rate of buildup of the resistance to film compression increases. Demulsifiers then reduce the kinetic barrier to coalescence. 

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