Coalescence surface hydrophobicity

Electrolytes influence almost every facet of cavitation from nucleation to coalescence, to the conditions of the bubble interior which determine the nature and severity of inertial collapse. Burikin et al. [34] investigated optical cavitation and found that on both hydrophobic and hydrophilic surfaces the cavitation probability... 

A well-studied example of a bioemulsifier is emulsan, a cell surface-exposed molecule that allows Acinetobacter calcoaceticus RAG-1 to attach to crude oil droplets [123]. Upon depletion of the short-chain alkanes utilised by this strain, the emulsan molecules were released from the bacterial surface, thereby allowing the cells to leave the oil droplet and to find a new substrate. Important positive side-effects of this mechanism seem to be that the remaining emulsan hydrophilises the droplet and prevents both the reattachment of A. calcoaceticus RAG-1 and the coalescence of the used oil droplet with other droplets that still contain unexploited alkanes. Bredholt et al. [124] studied the oil-emulsifying activity of Rhodococcus sp. strain 094. When exposed to inducers of crude-oil emulsification, the cells developed a strongly hydrophobic character, which was rapidly lost when crude-oil emulsification started. This indicated that the components responsible for the formation of cell-surface hydrophobi-city acted as emulsion stabilisers after release from the cells. 

The weak surfactant solution forms bubbles on agitation in the aqueous phase, which are stabilized as microscopic spheres from coalescing into large bubbles by the orientation of insoluble surfactant across the air/ liquid interface and by adhering to the hydrophobic surface created on the cement particle by the adsorbed surfactant. This is shown diagrammatically in Fig. 3.15 [16]. 

It is believed that in the presence of dampproofing admixtures, the surfaces of the concrete, and the internal surfaces of the pores become coated with either a layer of molecules in the case of stearic acid and other fatty acids (Fig. 4.5b) or a layer of coalesced or separate particles of material in the case of waxes and bitumens, etc. (Fig. 4.5c). The end result in both cases is the production of hydrophobic surfaces exhibiting high contact angles to water, as shown in Fig. 4.6. 

Another characteristic property of many biopolymers (proteins, modified starch, chitosan, etc.) which is useful for the encapsulation of bioactive molecules is their ability to adsorb at the oil-water interface and to form adsorbed layers that are capable of stabilizing oil-in-water (OAV) emulsions against coalescence (see Table 2.2). It is worthwhile to note here that the formation of an emulsion is one of the key steps in the encapsulation of hydrophobic nutraceuticals by the most common technique used nowadays in the food industry (spray-drying). The adsorption of amphiphilic biopolymers at the oil-water interface involves the attachment of their hydrophobic groups to the surface of the oil phase (or even their slight penetration into it), whilst their hydrophilic parts protrude into the aqueous phase providing a bulky interfacial layer. 

R-groups, which act much like droplets of oil that coalesce in an aqueous environment. The nonpolar R-groups thus fill up the inte rior of the folded protein and help give it its three-dimensional shape. [Note In proteins that are located in a hydrophobic envi ronment, such as a membrane, the nonpolar R-groups are found on the outside surface of the protein, interacting with the lipid environment (see Figure 1.4).] The importance of these hydrophobic interactions in stabilizing protein structure is dis cussed on p. 19. 

The chemical materials used to produce dampproofers are able to form a thin hydrophobic layer within the pores and voids and on the surfaces of the concrete in one of three ways (1) reaction with cement hydration products (2) coalescence... 

Overall there will be a concentration of the hydrophobic particles in the upper regions of the froth layer and a concentration of the hydrophilic particles in the lower regions. Figure 10.7 provides an illustration of how froth structure changes from the bottom to the top of the froth layer. At the very top of the froth bubble, coalescence and rupture will be occurring as well. The amount of ore that can be separated for any given amount of liquid is proportional to the surface area of the foam. It has been estimated that a foam with a specific surface area of 0.2 m2g-1 can separate a thousand times more ore, by mass, than the mass of its own liquid [629]. 

Emulsifying properties. One of the major functions of commercial lecithins is to emulsify fats. In an oihwater system, the phosphohpid components concentrate at the oUrwater interface. The polar, hydrophilic parts of the molecules are directed toward the aqueous phase, and the nonpolar, hydrophobic (or lipophilic) parts are directed toward the oil phase. The concentration of phospholipids at the oihwater interface lowers the surface tension and makes it possible for emulsions to form. Once the emulsion is formed, the phosphohpid molecules at the surface of the oil or water droplets act as barriers that prevent the droplets from coalescing, thus stabilizing the emulsion (159). 

Steric Hindrance. Another form of stabilization is relatively independent of ionic strength the oil droplets are prevented from making contact by simple steric hindrance. This may take two forms, either an immobilized water layer at the interface or a solid interfacial film. Emulsion stabilization by proteins, gums, and polyoxyethylene derivatives occurs by the first mechanism. Hydrophobic parts of the stabilizers adsorb at the oil surface, but adjacent large hydrophilic segments are hydrated and form an immobilized layer on the order of 10-100 nm thick (Figure 9). As mentioned, these hydrated segments frequently interact to cause flocculation, while coalescence of the oil drops themselves is prevented. Such emulsions are frequently used as carriers for oil-soluble flavors, essences, and colorants. 

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