Emulsion stability steric protection

Emulsifier is a general term that refers to chemical species that occupy the interfacial region between the droplet and the continuous phase. Emulsifiers aid in the formation and stabilization of emulsions. Emulsifiers are amphiphilic molecules, containing both hydrophilic and lipophilic groups, which provide the molecule with some affinity for both the disperse phase and the continuous phase. Emulsion stabilizers are polymeric molecules of higher molecular weight which form a protective steric layer around the dispersed droplets and also have some affinity for both phases. The dispersed, or discontinuous, phase is also referred to as the internal phase, whereas the continuous phase is also referred to as the external phase. Emulsions with an internal phase to total volume ratio of <0.3 are called low internal phase ratio emulsions. The external phase is usually the phase having... 

The thick interfacial layer and the viscous or gelled aqueous phase might have some advantages when protection of sensitive addenda is required. Kim et al. (2003) treated the internal aqueous phase with hydroxypropyl-beta-cyclodextrin (to make the internal aqueous phase more hydrophilic) and introduced kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone) as a model antioxidant. The multiple emulsions were also stabilized sterically by addition of xanthan gum into the external aqueous phase. The authors found improved stability against coalescence and improved chemical stability. The stability of kojic acid could be maintained at 90% for 10 weeks (Figure 5.13). 

The stabilizing of aqueous latexes succeeded by using emulsifiers (anionic, nonionic) and/or their mixture, steric stabilizators (polyvinyl alcohol (PVOH), hydroxyethyl cellulose, polyethylene glycol, new protective colloids etc.), and polymerizable surfaces active agents, in general. Vinyl acetate (VAc) emulsion homopolymers and copolymers (latexes) are widely used as binders in water-based interior and exterior architectural paints, coatings, and adhesives, since they have higher mechanical and water resistance properties than the homopolymers of both monomers [2, 4, 7]. 

In this case, it is the steric stabilization mechanism that protects the interactive particles from coagulation. In addition, the use of non-ionic t)rpes improves the stability of latex product against electrolytes, freeze-thaw cycles, water and high shear rates. As a result of them, in many emulsion pol3merization recipes (particularly in industry), mixtures of anionic and non-ionic emulsifiers have been widely used together in a s)mergistic manner to control the particle size and to impart enhanced colloidal stability [33-35]. The cationic and zwitterionic emulsifiers are used infrequently in emulsion pol3metizafion applications. 

In Sects. 1.2.1 and 1.2.2 we shall first qualitatively consider double layer and Van der Waals interactions, the two contributions to the DLVO potential (Sect. 1.2.3), and then discuss (polymeric) steric stabilization by end-attached polymer in Sect. 1.2.4. While not further discussed here we mention that adsorbing polymers, proteins or particles can also be used to protect colloids against flocculation. For protein adsorption, often used for instance in food emulsions, we refer to [28]. Using particles to stabilize colloids is referred to as Ramsden-Pickering stabilization [29]. Finally, the depletion interaction will be treated in Sect. 1.2.5. 

Chains attached to colloidal surfaces provide powerful forces for stabilization. Colloidal particles that normally coagulate from a solvent dispersion can thus be stabilized by adding a small amount of polymer to the dispersion. Such polymer additives are sometimes known as protective colloids, leading to steric stabilization. Both synthetic polymers and biopolymers such as proteins and gelatin are commonly used in both nonpolar and polar solvents. Industrially they are used in paints, toners, emulsions, suspensions, cosmetics, pharmaceuticals, processed foods, and lubricants. 

Mixtures of alkyl ethoxylates are frequently employed for emulsifying purposes, and in some cases protective colloids are also added to provide electro-steric stabilization. It is of crucial importance for the rolling result that the emulsifier has an optimal eomposition. Not only the surface quality of the sheet steel but also the rolling speed are decisively influenced by the size distribution of the oil droplets. A narrow spread of droplet sizes of about 2-4 pm has been found to be optimal, and even remains stable after being fed back into the emulsion cycle. 

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