Coalescence preventing with surfactants

The sol-gel process is used to prepare dense, spherical particles of ThOa and (Th,U)02 for sphere-pac and coated-particle fuels. The thoria is dispersed in water from nitrate solutions by slow heating and steam denitration to form a stable sol from which spherical particles are produced. The sol droplets are injected at the top of a tapered glass column containing an upward flow of 2-ethylhexarol (2-EH). The water from the sol particles is slowly extracted by suspension in the 2-EH, and the gelled spheres drop out of the column. Coalescence of the particles is prevented with surfactants in the 2-EH. The sol-gel spheres are dried in steam and Ar at 220°C and sintered in H2 at ISOO C. 

Increasing surfactant concentrations in the aeration cell has been found to decrease bubble diameter, bubble velocity, axial diffusion coefficient, but increase bubble s surface-to-volume ratio, and total bubble surface area in the system. The effect of a surface-active agent on the total surface area of the bubbles is also a function of its operating conditions. The surfactant s effect is pronounced in the case of a coarse gas diffuser where the chances of coalescence are great and the effectiveness of a surface-active solute in preventing coalescence increases with the length of its carbon chain. 

At the end of the emulsion polymerisation, polymer particles will exist in the dispersed water phase with surfactant molecules adsorbed on the particle surface. It is usually considered that it is the hydrophobic portion of the surfactant that is adsorbed onto the surface while the hydrophilic portion goes into the water phase. The role of the surfactant is now to keep the system stable by preventing coalescence of the polymer particles in the dispersion. If particle coalescence is not prevented the settling of the coagulated particles can take place, giving a non-redispersible sediment. 

The decomposition into very finely dispersed droplets is facilitated by the low interfacial tension at the transition composition. Further dilution with water does not change the droplet size at this stage of droplet formation, but can be necessary in order to go far away from the compositions with low interfacial tension, where coalescence is favored. However, in many cases the rate at which the system moves away the PIC does not seem to be very crucial in terms of avoiding coalescence, since usually surfactants adequate to be used in PIC method have long PEO chains and, as a result, a steric stabilization can prevent coalescence for some time. In fact, if liquid crystal phases are involved, a slow addition of water may be required to avoid inhomogeneities due to the relatively high viscosity of the system [58]. 

However, coalescence of the foam may occur. In aqueous systems, this may be prevented by adding surfactants to lower the surface tension. With organic solvents, this is not as facile. Hence there may be limits to applicability. For unstable gas/liquid dispersions, the micro devices described here may only be used for shortterm contacting. 

Finely divided solid particles that are wetted to some degree by both oil and water can also act as emulsifying agents. This results from the fact that they can form a particulate film around dispersed droplets, preventing coalescence. Powders that are wetted preferentially by water form O/W emulsions, whereas those more easily wetted by oil form W/O emulsions. The compounds most frequently used in pharmacy are colloidal clays, such as bentonite (aluminum silicate) and veegum (magnesium aluminum silicate). These compounds tend to be adsorbed at the interface and also increase the viscosity of the aqueous phase. They are frequently used in conjunction with a surfactant for external purposes, such as lotions or creams. 

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

Surfactants are used for stabilization of emulsions and suspensions against flocculation, Ostwald ripening, and coalescence. Flocculation of emulsions and suspensions may occur as a result of van der Waals attraction, unless a repulsive energy is created to prevent the close approach of droplets or particles. The van der Waals attraction Ga between two spherical droplets or particles with radius R and surface-to-surface separation h is given by the Hamaker equation,... 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

In the interfacial tension theory, the adsorption of a surfactant lowers the interfacial tension between two liquids. A reduction in attractive forces of dispersed liquid for its own molecules lowers the interfacial free energy of the system and prevents the coalescence of the droplets or phase separation. Therefore the surfactant facilitates the stable emulsion system of the large interfacial area by breaking up the liquid into smaller droplets. However, the emulsions prepared with sodium dodecyl (lauryl) sulfate separate into two liquids upon standing even though the interfacial tension is reduced. The lowering of the interfacial tension in the stabilization of emulsions is not the only factor we should consider. 

Steric hindrance When a nonionic surfactant adsorbs on an interface between oil and water, the hydrophobic part of the molecule wUl orient itself towards the oily phase, while the hydrophUic part will stick out into the aqueous phase. One can envisage the interface, therefore, as having a coat of hydrophihc chains sticking out of the interface. When two oil droplets approach each other, the two coats would first make contact. The only way that the two droplets can coalesce is when the nonionic surfactant molecules move away from the contact point. However, they are strongly adsorbed and therefore impede the coalescence. Hence, when two droplets with such a layer approach each other, the coats will repel each other. Thus, the droplets will move apart again, and coalescence is prevented. 

When the surfeclant concentration is high relative to the polymer concentration sufRdent sui ctant may be adsorbed on individual polymer molecules to prevent their coalescence to form latex particles or indeed to disperse prdbrmed polymer to form clear solutions in which the solute behaves as a po yelectro yte, But the influence of such effects on tbe course of emulsion polymerization reactions has not been elucidated. Sata and Saito (1952) showed that poly(vinyl acetate) preeptated from acetone solution with water could be solubibzed in sodium dodecyl sulfate solutions after removal of the acetone by dialysis. To obtain a clear solution at 20°C, a wdght of surfactant S-10 times that of pol ner was required. Althou this greatly exceeds the surfactant concentrations normally used in emulsion... 

Monitoring Emulsion Aging. The surfactants used in transport emulsions may gradually lose their ability to stabilize the oil droplets. As the oil droplets coalesce, a two-phase mixture is formed, and it remains pumpable with no significant change in effective viscosity. This process is referred to as emulsion failure. An alternative to this process is inversion of the emulsion, in which a water-in-oil emulsion is formed with a potentially very high viscosity. Proper selection of the surfactant formulation can prevent the occurrence of emulsion inversion. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

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