Surface-active agents suspensions

Suspensions of oil in water (32), such as lanolin in wool (qv) scouring effluents, are stabilized with emulsifiers to prevent the oil phase from adsorbing onto the membrane. Polymer latices and electrophoretic paint dispersions are stabilized using surface-active agents to reduce particle agglomeration in the gel-polarization layer. 

Poly(vinyl alcohol) will function as a non-ionic surface active agent and is used in suspension polymerisation as a protective colloid. In many applications it serves as a binder and thickener is addition to an emulsifying agent. The polymer is also employed in adhesives, binders, paper sizing, paper coatings, textile sizing, ceramics, cosmetics and as a steel quenchant. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

A surfactant is a surface-active agent that is used to disperse a water-insoluble drug as a colloidal dispersion. Surfactants are used for wetting and to prevent crystal growth in a suspension. Surfactants are used quite extensively in parenteral suspensions for wetting powders and to provide acceptable syringability. They are also used in emulsions and for solubilizing steroids and fat-soluble vitamins. 

Wet anti-tack agents can be soap or detergent solutions or suspensions of the dry agents in water. For example, dissolved polymers with non-ionic surface active agents which form a thin layer of polymer on unvulcanised rubber sheets magnesium stearate in water zinc stearate dispersion and aqueous dispersions of fatty acid salts. 

This is superficially similar to suspension polymerisation. But in this process a monomer dispersed in water, in presence of a surface active agent is polymerised to give a stable polymer latex. 

Apparently the acceleration of de novo purine biosynthesis by orotic acid results from a release of feedback inhibition imposed by hepatic purine nucleotides. In a related study, it was found that orotic acid feeding can prevent hyperlipaemia, which normally follows the administration of Triton WR-1339, a surface active agent [152]. The influence of orotic acid on lipid metabolism can be readily shown by the fact that depression of serum lipoproteins and milk production were observed in lactating goats when an aqueous suspension of orotic acid was administered orally [164]. 

Bubbles are formed in the sintered glass as air or other suitable gases (N2, C02, etc.) is bubbled through the solution containing the solid suspension. A suitable flotation agent (a suitable-surface active agent) is added, and the air is bubbled. 

Wettable powders are prepared by blending the toxicant in high concentration, usually from 15 ndash 95%, with a dust carrier such as attapulgite which wets and suspends properly in water. One to two percent of a surface-active agent usually is added to improve the wetting and suspensibility of the powder. Sprays of wettable powders are used widely in agriculture because of their relative safety to plants. 

Multi-walled nanotubes can also be ultrasonically dispersed in a liquid and made soluble with a surface active agent (surfactant) to inhibit coalescence. The resulting suspension is drawn through a filter to remove smaller particles and the remaining material consists of reasonably purified intact nanotubes. The surfactant is then washed off [119]. 

If a suspension is to be produced by a dispersion technique (as opposed to precipitation techniques), surfactants may be used in the formulation to aid dispersion of the solid particles in the liquid. This is particularly important if the powder is not readily wetted by the liquid vehicle. Surfactants can reduce the interfacial tension between the solid particles and the liquid vehicle. The advancing contact angle is reduced, and wetting of the solid particles promoted. Such a system is said to be deflocculated. The inclusion of a surface-active agent to improve powder wettability can often improve the bioavailability of the formulation. 

Certain compounds, because of their chemical structure, have a tendency to accumulate at the boundary between two phases. Such compounds are termed amphiphiles, surface-active agents, or surfactants. The adsorption at the various interfaces between solids, liquids and gases results in changes in the nature of the interface which are of considerable importance in pharmacy. For example, the lowering of the interfacial tension between oil and water phases facilitates emulsion formation the adsorption of surfactants on the insoluble particles enables these particles to be dispersed in the form of a suspension and the incorporation of insoluble compounds within micelles of the surfactant can lead to the production of clear solutions. 

It has been shown that the stability of colloidal suspensions can also be influenced by a pure alcohol-water mixture, without the addition of any surface active agent. In a study of the flocculation of polystyrene emulsions in ethanol-water mixtures (42), the concentration of sodium chloride required to produce rapid flocculation increases with increasing ethanol concentration up to 0.09 molar fraction, beyond this composition, the concentration of sodium chloride required for flocculation decreases rapidly. It will be very informative, therefore, to compare our coagulation rate obtained in the microemulsion media to that in pure IPA + water mixture. The results can be used to further delineate the role of inverted micellar structure on the enhancement of coagulation. 

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