Emulsion ionic stabilizers

Figure 2.20. (a) Disjoining pressure vs. thickness isotherm (dots, experimental data line, doublelayer fit) for an emulsion Him stabilized by 0.1% 8-casein, ionic strength of 10 mol/l NaCl, oil phase = hexadecane. (b) Comparison between the data obtained from TFB, MCT, and SFA. (Adapted from [87].)... 

Rgure 2.26. Six consecutive steps in shrinking of an emulsion film stabilized with 0.1 wt% BSA. Oil phase is hexadecane the ionic strength is 10 mol/1. The bar corresponds to 100 j,m. The local adhesion on aggregates is evident. Arrows indicate some of the points of adherence at the interfaces. (Adapted from [87].)... 

For ionic surfactants another effect often dominates and usually salt tends to stabilize emulsions. Reason without salt the distance between surfactants in the interface is large because the molecules electrostatically repel each other. This prevents a high surface excess. The addition of salt reduces this lateral repulsion and more surfactant molecules can adsorb at the interface. Then, according to the Gibbs adsorption isotherm Eq. (3.52), the surface tension is reduced and the emulsion is stabilized. 

In choosing an epoxy and polymeric latex, it is important that they have compatibility. Incompatibility usually occurs when the pH of the epoxy resin dispersion alters the pH of the latex into a range where the ionically stabilized latex is broken, causing agglomeration of the latex polymer. The pH of the epoxy resin s emulsion may need to be adjusted before blending with the polymeric latex. 

There are an enormous variety of commercial emulsifiers that are employed in emulsion polymerization. Emulsifiers are generally categorized into four major classes anionic, cationic, nonionic and zwitterionic (amphoteric). The anionic and nonionic emulsifiers are the most widely used. In addition, mixtures of emulsifiers are also often used. Since the effects of the molecular structme and chemical and physical properties of an emulsifier on particle formation are still far from being well understood, numerous experimental investigations on particle formation have been carried out to date with various nonionic emulsifiers [99-102], mixed emulsifiers (ionic and nonionic emulsifiers) [18,103-106] and reactive surfactants [33, 107-110]. Recently, polymeric surfactants have become widely used and studied in emulsion polymerizations [111-116]. A general review of polymeric surfactants was published in 1992 by Piirma [117]. Recently, emulsion polymerization stabilized by nonionic and mixed (ionic and nonionic) emulsifiers was reviewed by Capek [118]. 

For styrene nanoemulsions prepared with 10 2 M sodium lauryl sulfate and a 1 1 molar ratio of ionic surfactant fatty alcohol, the order of decreasing stability with fatty alcohols of different chain length is Ci6 > Qg > CM > Ci2 > C10. For sodium lauryl sulfate-Ci6 alcohol mixtures, the order of decreasing stability with different sodium lauryl sulfate fatty alcohol ratios is 1 3 > 1 2 > 1 1 > 1 6 > 1 0.5. The 1 3 and 1 2 ratios produce emulsions with stabilities > 1 month. The presence of rodlike liquid-crystalline structures at 1 1 to 1 3 ionic surfactant fatty alcohol ratios is believed to be essential for the preparation of a stable nanoemulsion. (El-Aaser, 1984). 

In the chapter on Pigmentation (Chapter 8), under the heading Dispersion, we considered how to make stable colloidal dispersions of solid and found that, for stability, it was necessary to keep the particles apart. This could be done by using polymer molecules, anchored strongly to the particle, but also extending out into the solvent, in which they were soluble. These polymer molecules provide a steric barrier around the particle and this method of stabilization is called steric stabilization. We also learnt that aqueous pigment dispersions could be stabilized by adsorbed surfactant molecules, which ionized in the water to produce an electrical charge barrier around the particle ionic stabilization). Exactly the same techniques are used to stabilize emulsions. 

Conduchvity measurements are more specific in their application being of particular use in the case of aqueous dispersions of particles, stabilized by some type of ionic stabilizer. The information obtained with this technique is the conductivity of the continuous phase. In the case of emulsion polymerizations stabilized with ionic surfactants, this is related to the concentration of free surfactant, which, when combined with absorption isotherms, for example, or an empirical model, can be used to follow the evolution of the surface area of a latex. This is a promising method, but given its complexity and the need to develop more robust means of linking the conductivity to properties of interest, it has not found widespread use in commercial production at the current time. 

FIGURE 15.14. The potential energy curve for a depletion flocculated ionically stabilized emulsion shows that the sum of the van der Waals attraction and the osmotic attraction cause a secondary minimum (e.g.. Fig. 15.10). 

The surfactant also prevents aggregation of the particles, once formed by producing a strong repulsive force. As with emulsions, two stabilization mechanisms can be considered, namely, electrostatic (when using ionic surfactants) and steric when using nonionic surfactants and polymers. 

It would be wrong to assume that non-ionic stabilized emulsions are immune to the effect of added electrolytes. The addition of electrolytes to non-ionic stabilized emulsions can cause pronounced effects on stability. In solutions of non-ionic surfactants, the addition of electrolytes generally causes a dehydration of the ethylene oxide chains by disruption of hydrogen bonds. Selected salts have been shown to exhibit interaction with polyethylene oxide ethers, reducing their solvation and producing more compact molecular conformations [127,128]. 

In principle, it should then be possible to predict the stability of an emulsion system from the interfacial rheology of the continuous phase. Figure 1.5 shows the relative stability to coalescence of an emulsion system stabilized by a protein (beta-lactoglobulin) with increased concentrations of non-ionic surfactant (Tween 20). In this case the presence of surfactants has entirely destabilized the protein emulsion. 

Low degrees of ethoxylation tend to render the surfactant oil soluble, whilst higher levels confer water solubility. The use of non-ionic surfactants may sometimes lead to the formation of small aggregates or grainy emulsions. This tendency is due to the weaker surface activity and the relative difficulty with which non-ionic surfactants form micelles. As a result of these deficiencies non-ionic/anionic surfactant mixtures are normally employed in emulsion polymerisation. The ionic component allows easy solubilising of the monomer whilst the non-ionic component confers emulsion polymer stability. 

Chem. Descrip. Silicone, emulsifiers, and stabilizer emulsion Ionic Nature Nonionic/anionic... 

The same approach was used by Kong et al. in their study [10] of the adsorption of non-ionic surfactant on emulsion drops stabilized with SDS. They studied the same... 

The standard NF T 65-011 distinguishes the bitumen emulsions by their ionic nature (anionic or cationic), their stability with respect to agglomerates and weight content of base binder. There are 20 grades of emulsions. 

If an ionic surfactant is present, the potentials should vary as shown in Fig. XIV-5c, or similarly to the case with nonsurfactant electrolytes. In addition, however, surfactant adsorption decreases the interfacial tension and thus contributes to the stability of the emulsion. As discussed in connection with charged monolayers (see Section XV-6), the mutual repulsion of the charged polar groups tends to make such films expanded and hence of relatively low rr value. Added electrolyte reduces such repulsion by increasing the counterion concentration the film becomes more condensed and its film pressure increases. It thus is possible to explain qualitatively the role of added electrolyte in reducing the interfacial tension and thereby stabilizing emulsions. 

The product of an emulsion polymerization is a latex ie, polymer particles on the order of 0.5—0.15 p.m stabilized by the soap. These form the basis for the popular latex paints. SoHd mbber is recovered by coagulating the latex with ionic salts and acids (see Latex technology). 

The substitution of water-borne versions of these primers is increasing as environmental restrictions on the use of organic solvents become stricter. These are generally aqueous emulsions of epoxy novolac or phenolic based resins stabilized by surfactants [34]. Non-ionic surfactants are preferred, as they are non-hygroscopic in the dried primer films. Hygroscopic ionic surfactants could result in excessive water absorption by the primer film in service. 

Lee, G.W.J. and Tadros, Th.F. (1982) Formation and stability of emulsions produced by dilution of emulsifiable concentrates. Part I. An investigation of the dispersion on dilution of emulsifiable concentrates containing cationic and non-ionic surfactants. Colloids Surf,... 

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