Stabilizer surfactant-type

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

The common concentration of a surfactant used in a formulation varies from 0.05 to 0.5% and depends on the surfactant type and the solids content of the dispersion. In practice, very often combinations of surfactants rather than single agents are used to prepare and stabilize disperse systems. The combination of a more hydrophilic surfactant with a more hydrophobic surfactant leads to the formation of a complex film at the interface. A good example for such a surfactant pair is the Tween-Span system of Atlas-ICI [71]. 

Stability of calibrants and analytes is another frequently overlooked aspect of quality assurance, which is particularly relevant to surfactants. This aspect is discussed in Chapter 4.4. Very few intercalibration studies have been performed for the surfactant types of analytes (cf. Chapter 4.5). Currently, no certified reference material is available for surfactants. The European Commission has recently tendered for production of a reference material with certified surfactant concentrations [2]. We can conclude that quality assurance in quantitative surfactant analysis is still in its infancy when compared to analysis of PCB or chlorinated dioxins. Notwithstanding this, several important achievements have been made during recent years regarding improvement of the accuracy and reliability of qualitative analysis of surfactants, which will be the subject of the following chapters. 

On a more practical level, to use CLAs and CGAs in PDSE it is important to understand the influence of key parameters such as solvent type and polarity, and surfactant type (hydrophilic/lipophilic balance, HLB) and concentration, on the formulation and stability of CLAs and CGAs. These are discussed next. 

The choice of surfactant, which is mostly constrained by the choice of the oil and the resulting phase behaviour of the microemulsion, can have different effects on the enzyme stability and activity. In general we have to differentiate between ionic and nonionic surfactant types ... 

The epoxy-PVC plastisol type is a mixture of a plastisol-grade PVC powder, primary PVC plasticizers (e.g., dioctyl phthalate), a liquid DGEBA epoxy resin, thickeners, stabilizers, surfactants, and other additives. The epoxy serves as a secondary plasticizer, acts as a stabilizer (acid scavenger), and helps to fortify the plastisol by crosslinking during cure. 

Styrene—Ethyl Acrylate (S—EA) Copolymers. At the inception of the research on F/T stability, it was evident that more than one polymer system would require investigation before adequate conclusions could be drawn regarding the various pertinent parameters. These include, in addition to the previous considerations, surfactant type and amount as a function of polymer type. 

Modem detergent products are complex mixtures of many different ingredients. Typical formulations consist of surfactants, builders, and other additives designed to maximize performance for the consumer while maintaining reasonable raw material and manufacturing costs. Typical detergent formulations contain multiple surfactant types to optimize performance and product stability.21,22 Performance additives such as bleaches, bleach activators,... 

Musyanovych A, Rossmanith R, Tontsch C, Landfester K (2007) Effect of hydrophilic comonomer and surfactant type on the colloidal stability and size distribution of carboxyl-and amino-functionalized polystyrene particles prepared by miniemulsion polymerization. Langmuir 23(10) 5367-5376... 

The results presented below from the study of the behaviour of steady-state foams allow to estimate the role of foam films in foam stability. Two types of steady-state foam have been studied 1) wet steady-state foams from aqueous solutions of low surface active surfactants, e.g. normal alcohols [96] and 2) dry steady-state foams [121] from aqueous solutions of micellar surfactants, e.g. NaDoS, in the presence of electrolyte at different concentrations (ensuring different types of foam films). The device employed in this study represents a glass column (of inner diameter 3.4 cm) with a sintered glass filter as a bottom [94-96,121]. The gas volume passing through the column was measured by a rheometer. The total gas volume both in the foam and in the solution was measured when a steady-state was reached, i.e. when the system volume ceases changing. Usually the total gas volume V c as well as the gas rate vc were measured. 

The stability of emulsion and foam films have also been found dependent upon the micellar microstructure within the film. Electrolyte concentration, and surfactant type and concentration have been shown to directly influence this microstructure stabilizing mechanism. The effect of oil solubilization has also been discussed. The preceding stabilizing/destabilizing mechanisms for three phase foam systems have been shown to predict the effectiveness of aqueous foam systems for displacing oil in enhanced oil recovery experiments in Berea Sandstone cores. 

To stabilize emulsions, a surfactant, which increases the repulsive force between oil droplets, is used. Nonionic surfactants are the preferred type because they are effective in brines, are generally cheaper, and often form less viscous emulsions than do ionic surfactants. In addition, their emulsions are easier to break, and they do not introduce inorganic residues that might lead to refinery problems. They are chemically stable at oil reservoir temperatures and are noncorrosive and nontoxic. The surfactant type and concentration required for a particular situation can be determined by conducting laboratory tests. A typical concentration of 0.1 lb of surfactant per barrel of oil is used for emulsions containing about 50-70% oil (2). 

A wide range of surfactant types may be used to form and stabilize transport emulsions. Nonionic surfactants have the advantage of relative insensitivity to the salt content of the aqueous phase being employed (6). The group of surfactants known as ethoxylated alkylphenols, represented by the formula,... 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

Figure 10 shows the volume fraction of the splitted emulsion after treating in the electrical a.c. field for 30 seconds for various surfactant concentrations and surfactant types. It is evident that in the absence of surfactants, there is no stability and all the water droplets coagulate with coalescence. 50 vol.% water phase signifies 100% splitting efficiency. 

Figure 10. Effect of surfactant type and surfactant concentration on emulsion stability Span 20 o Span 80 V Span 85 breaking time 20 s.

FIG. 8.22 Effect of surfactant type on protease stability (AE, alcohol ethoxylate AE25-3S, alcohol ethoxy sulfate LAS, linear alkylbenzene aulfonate). (Reproduced from Kravetz, L. and Guin, K.F. J. Am. Oil Chem. Soc., 62, 943, 1985. With permission.)... 

Those factors that were previously mentioned that produce finer-textured foams also produce more stable foams. Factors such as surfactant type, concentration, increasing pressure, and higher inputs of mechanical energy generate more stable foams. For higher temperatures such as those that exist downhole, dynamic foam stability relies upon surfactant type and concentration rather than the addition of thickeners (polymer stabilizers). It is not known what rates are necessary to maintain dynamic stability in fractures, or whether those conditions typically exist. 

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