Role of Surfactants in Emulsion Formation

Surfactants lower the interfacial tension y, which in turn causes a reduction in droplet size typically, the latter will decrease with a decrease in y. For laminar flow, the droplet diameter is proportional to y, but for a TI regime the droplet diameter 

The amount of surfactant required to produce the smallest drop size will depend on its activity a (concentration) in the bulk which in turn determines the reduction in y, as given by the Gibbs adsorption equation as discussed before. 

The value of / obtained depends on the nature of the oil and surfactant used small molecules such as nonionic surfactants lower y more than polymeric surfactants such as PVA. 

Another important role of the surfactant is its effect on the interfacial dilational 

During emulsification an increase in the interfacial area A takes place and this causes a reduction in T. The equilibrium is restored by the adsorption of surfactant from the bulk, but this takes time (shorter times occur at higher surfactant activity). Thus, e is small whether a is small or large. Because of the lack or slowness of equilibrium with polymeric surfactants, e will not be the same for expansion and compression of the interface. 

In practice, surfactant mixtures are used and these have pronounced effects on y and . Some specific surfactant mixtures give lower y than either of the two individual components [9, 10]. The presence of more than one surfactant molecule at the interface tends to increase s at high surfactant concentrations. The various components vary in surface activity. Those with the lowest y tend to predominate at the interface, but, if present at low concentrations, it may take a long time to reach the lowest value. Polymer-surfactant mixtures may show some synergetic surface activity. 

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The droplet deformation increases with increases in the Weber number which means that, in order to produce small droplets, high stresses (i.e., high shear rates) are require. In other words, the production of nanoemulsions costs more energy than does the production of macroemulsions [4]. The role of surfactants in emulsion formation has been described in detail in Chapter 10, and the same principles apply to the formation of nanoemulsions. Thus, it is important to consider the effects of surfactants on the interfacial tension, interfacial elasticity, and interfacial tension gradients. 

The role of surfactant in emulsion formation is crucial and is described in detail in Chapter 6. It reduces the oil-water interfacial tension, yow adsorption at the interface. The droplet size R is directly proportional to yow It enhances deformation and break-up of the droplets by reducing the Laplace pressure p,... 

Role of Surfactants in Emulsion Polymerization Technology Formation of aggregates as micro-reactors... 

The role of surfactants on emulsion formation is detailed in Chapter 6 and the same principles apply to the formation of nano-emulsions. Thus, one must consider the effect of surfactants on the interfadal tension, interfacial elasticity, and interfacial tension gradients. 

Additives are usually amphiphilic in nature, and thus are either ionic or neutral surfactants or even polymers. The role of surfactants in solvent extraction is ambiguous. Usually, they should be avoided as they lower the interfacial tension, which may lead to emulsion formation in an agitated extractor. However, every metal-loaded ion exchanger is amphiphilic, and can adsorb at the interface or aggregate in the bulk phase. This occurrence is well known with sodium or other metals [17], and above a critical surfactant concentration (cmc, critical micelle concentration) micellar aggregates are formed. A dimensionless geometric parameter is decisive for the structure of the associates, according to Fig. 10.6 ... 

This paper reviews various aspects of macro- and micro-emulsions. The role of interfacial film of surfactants in the formation of these systems has been high-lighted. 

Emulsification—the formation of emulsions from two immiscible liquid phases—is probably the most versatile property of surface-active agents for practical applications and, as a result, has been extensively studied. Paints, polishes, pesticides, metal cutting oils, margarine, ice cream, cosmetics, metal cleaners, and textile processing oils are all emulsions or are used in emulsified form. Since there are a number of books and chapters of books devoted to emulsions and emulsification (Sjoblom, 1996 Solans and Kunieda, 1996 Becher, 2001), the discussion here covers only those aspects of emulsification that bear on the role of surfactants in this phenomenon. 

Surfactants primarily determine the size and size distribution of the particles formed during emulsion polymerization. In general, polymerizations carried out at or below the critical micelle concentration (cmc), will lead to the formation of particles with more uniform sizes. The role of surfactants in particle nucleation and in particle stability are given in more detail in Chapters 2-5. Surfactants are typically used in the range 1 -6% by weight to monomer (lower concentrations for the anionic surfactants and higher for the nonionics). 

Several references were made above to the term phase inversion temperature. With the exceptions of Eqs. (9.17) and (9.18), however, no specific reference was made to the effect of temperature on the HLB of a surfactant. From the discussions in Chapter 4, it is clear that temperature can play a role in determining the surface activity of a surfactant, especially nonionic amphiphiles in which hydration is the principal mechanism of solubilization. The importance of temperature effects on surfactant solution properties, especially the solubility or cloud point of nonionic surfactants, led to the evolution of the concept of using that property as a tool for predicting the activity of such materials in emulsions. Since the cloud point is defined as the temperature, or temperature range, at which a given amphiphile loses sufficient solubility in water to produce a normal surfactant solution, it was assumed that such a temperature-driven transition would also be reflected in the role of the surfactant in emulsion formation and stabilization. 

The role of a surfactant in emulsion polymerisation may vary during the course of the polymerisation. Initially the surfactant contributes to the rate of polymerisation and particle formation. Once polymer is present the surfactant has to solubilise the polymer preventing precipitation and when polymerisation is complete the surfactant is required to stabilise the emulsion preventing flocculation of the polymer and the formatir. . of aggregates. Where the product is utilised in latex form the surfactant plays a major part in the performance characteristics and in particular in the areas of ... 

Silica particles synthesized in nonionic w/o microemulsions (e.g., poly-oxythylene alkyl phenyl ether/alkane/water) typically have a narrow size distribution with the average value between 25 and 75 nm [54,55]. Both water and surfactant are necessary components for the formation of stable silica suspensions in microemulsions. The amounts of each phase present in the micro emulsion system has an influence on the resulting size of the silica nanoparticle. The role of residual water (that is the water that is present in the interface between the silica particle and the surfactant) is considered important in providing stability to the silica nanoparticle in the oil... 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

The freeze/thaw (F/T) stability of a polymer emulsion serves as a macroscopic probe for investigating the properties of the average particle in a polymer emulsion. A review of the factors which contribute to this stability is included. A study of styrene-ethyl acrylate-methacrylic acid polymers shows the existence of a minimum in the plot of minimum weight percent acid required for F/T stability vs. the minimum film formation temperature (MFT) of the polymer. This is considered to be a function of both the amount of associated surfactant and the minimum acid content. Thus, both the type of surfactant and the copolymer ratio—i.e., MFT—play major roles. Chain transfer between radicals and polyether surfactant resulting in covalently bonded surfactant-polymer combinations is important in interpreting the results. 

Such a relationship between Cm and the state of the adsorption layer is also found for CBFs (see Chapter 3). However, no quantitative link between Cm and the stability of these films has been found. As far as in the formation of black spots the adsorption layers at film surface plays a significant role, the clarification of the decrease in surface tension Act with the surfactant concentration is also important. Being a characteristic of surfactants Cm is in agreement with the commonly used quantity Act but is also related to the film properties. It is also important that Cm is in correlation with foam stability and, thus, is a more precise, suitable and physically better grounded characteristic. Such a correlation has been found for aqueous emulsion films [69,70]. 

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