Droplet stability

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

In a suspension polymerization, monomer is suspended ia water as 0.1—5 mm droplets, stabilized by protective coUoids or suspending agents. Polymerization is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle size is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads of about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

Among the various suspension systems mentioned, the details of oil-inwater (o/w) suspension polymerizations are fully known. The criteria of droplet formation, droplet stabilization, and droplet hardening, as will be discussed for the o/w suspension system, can apply equally to the preparation of beaded polymer particles in w/o systems. 

Beaded polymeric supports are produced by a two-phase suspension polymerization in which microdrops of a monomer solution are directly converted to the corresponding microbeads. The size of a microdroplet is usually determined by a number of interrelated manufacturing parameters, which include the reactor design, the rate of stirring, the ratio of the monomer phase to water, the viscosity of both phases, and the type and concentration of the droplet stabilizer. 

Beaded acrylamide resins (28) are generally produced by w/o inverse-suspension polymerization. This involves the dispersion of an aqueous solution of the monomer and an initiator (e.g., ammonium peroxodisulfates) with a droplet stabilizer such as carboxymethylcellulose or cellulose acetate butyrate in an immiscible liquid (the oil phase), such as 1,2-dichloroethane, toluene, or a liquid paraffin. A polymerization catalyst, usually tetramethylethylenediamine, may also be added to the monomer mixture. The polymerization of beaded acrylamide resin is carried out at relatively low temperatures (20-50°C), and the polymerization is complete within a relatively short period (1-5 hr). The polymerization of most acrylamides proceeds at a substantially faster rate than that of styrene in o/w suspension polymerization. The problem with droplet coagulation during the synthesis of beaded polyacrylamide by w/o suspension polymerization is usually less critical than that with a styrene-based resin. 

After heating at 40° C, liquid anhydrous milk fat (1 v) and the different protein solutions (10 v) were premixed using a polytron (PT 3000, Kinematica) and emulsified with a homogenizer (ALMO, Legrand, France) at about 40°C in order to obtain oil-in-water emulsions. After separation from the aqueous phase by centrifugation for 5 min at lOOOg, milk fat droplets stabilized by different proteins were washed twice with a phos-... 

TABLE 1 Average Diameter of the Natural Milk Fat Globules and Emulsified Milk Fat Droplets Stabilized by Different Fat-Water Interfaces in Reconstituted Milks... 

Reconstituted milks with natural milk fat globules (CREAM) or emulsified milk fat droplets stabilized by jS-casein (BCAS), /i-lactoglobulin 5g/L (BLG5), skim milk proteins (MP). 

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

By using MCT, Leal-Calderon et al. [ 10] measured the total repulsive force between tiny colloidal droplets stabilized with sodium dodecyl sulfate (SDS) (Fig. 2.5). The measurements were performed for emulsions with three different concentrations... 

Case I (see Fig. 2.17) corresponds to the situation such that the emulsion is initially stabilized with SDS at 8 10 mold (CMC). The repulsive force as a function of distance between the ferrofluid droplets, stabilized with SDS alone is referred as 0% PVA. Then, PVA-Vac is introduced at different concentrations varying from 0.002 to 0.5 wt%. After each addition, the emulsion is incubated for 48 h to reach equilibrium. It can be seen that the force profiles remain almost the same as in the case of 0% PVA. As the surfactant concentration is equal to CMC, the expected decay length is 3.4 nm. The experimental value of the decay length obtained from the force profile, 2.9 nm (solid line), is in good agreement with the predicted value. Thus, if the emulsion is preadsorbed with surfactant molecules, the introduction of polymer does not influence the force profile significantly. 

The disjoining pressure vs. thickness isotherms of thin liquid films (TFB) were measured between hexadecane droplets stabilized by 0.1 wt% of -casein. The profiles obey classical electrostatic behavior. Figure 2.20a shows the experimentally obtained rt(/i) isotherm (dots) and the best fit using electrostatic standard equations. The Debye length was calculated from the electrolyte concentration using Eq. (2.11). The only free parameter was the surface potential, which was found to be —30 mV. It agrees fairly well with the surface potential deduced from electrophoretic measurements for jS-casein-covered particles (—30 to —36 mV). 

Observations of large contact angles in emulsions were first reported by Aronson and Princen [105,106]. The authors have studied oil-in-water droplets stabilized by anionic surfactant in the presence of various salts. Similar systems were studied by Poulin [110]. Anionic surfactants such as sulfate, sulfonate, or carboxylate surfactants [106,110] exhibit a good stability and a strong adhesion in the presence... 

Figure 2.35. Energy of adhesion between hexadecane droplets stabilized in water by SDS, at various NaCl concentrations. (Adapted from [111].)...

The energy of adhesion between hexadecane droplets stabilized in water by SDS in the presence of NaCl is shown in Fig. 2.35. It is observed that the adhesion depends strongly on the temperature and on the salt concentration. For a given salt concentration, there is a well defined temperature, T, above which there is no adhesion. As the behavior of the surface energy changes at T, this temperature can be referred to as a wetting transition temperature [109]. The dependence of T versns the salt concentration is plotted on Fig. 2.36. 

Figure 2.38. Energy of adhesion between water droplets stabilized with sorbitan monooleate (Span 80) in a silicon oil-dodecane mixture. The arrow indicates the insolubility threshold of the amphiphUe. (Adapted from [113].)...

T.D. Dimitrova and F. Leal-Calderon Forces Between Emulsion Droplets Stabilized with Tween 20 and Proteins. Langmuir 15, 8813 (1999). 

A new variation of interfacial polymerization was developed by Russell and Emrick in which functionalized nanoparticles or premade oligomers self-assemble at the interface of droplets, stabilizing them against coalescence. The functional groups are then crosslinked, forming permanent capsule shells around the droplets to make water-in-oil (Lin et al. 2003 Skaff et al. 2005) and oil-in-water (Breitenkamp and Emrick 2003 Glogowski et al. 2007) microcapsules with elastic membranes. 

Lang J, Jada A, Malliaris A (1988) Structiu-e and Dynamics of Water-in-OU Droplets Stabilized by Sodium Bis(2-Ethylhexyl) Sulfosuccinate. J Phys Chem 92 1946-1953... 

Reverse micelles are well known to be spherical water in oil droplets stabilized by a monolayer of surfactant. The phase diagram of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate, called Na(AOT), with water and isooctane shows a very large domain of water in oil droplets and often forms reverse micelles (3,23). The water pool diameter is related to the water content, w = [H20]/[ AOT], of the droplet by (23) D(nm) = 0.3w. From the existing domain of water in oil droplets in the phase diagram, the droplet diameters vary from 0.5 nm to 18 nm. Reverse micelles are dynamic (24-27) and attractive interactions between droplets take place. 

Ogawa, S., Decker, E.A., McClements, D.J. (2003). Influence of environmental conditions on the stability of oil-in-water emulsions containing droplets stabilized by lecithin-chitosan membranes. Journal of Agricultural and Food Chemistry, 51, 5522-5527. 

Table 7.2 Effect of the presence of an anionic polysaccharide on the measured zeta potential (Q of emulsion droplets stabilized by proteins under experimental conditions corresponding to protein-polysaccharide complexation. In all cases the complexes were formed in the bulk aqueous medium before emulsification.

An overview of other forms of micellar systems follows in the next three sections. Formation of reverse micelles, in nonaqueous media, is discussed briefly in Section 8.8. Sections 8.9 and 8.10 present an introduction to microemulsions (oil, or water, droplets stabilized in water or oil, respectively) and their applications. 

Interaction with water, and dispersion into large aggregates. In samples containing appreciable sodium oleate, a turbid dispersion was present. Microscopically, this phase contained isotropic oil droplets, and the phase should represent emulsified fatty acid and monoglyceride droplets stabilized by acid soap. Unfortunately, x-ray diffraction facilities were not available to characterize this phase properly. 

From the experiments it is clear that poly electrolyte is adsorbed on the surface of the black lipid film. This applies both to the experiments with gelatin and bovine serum albumin, which gave no decrease of film resistance, and to the experiments with bovine erythrocyte ghost protein and polyphosphate. The adsorption of protein on the phospholipid-water interface may be controlled independently by investigating the electrophoretic behavior of emulsion droplets, stabilized by phospholipid, in a protein solution, as a function of pH. In this way Haydon (3) established protein adsorption on the phospholipid-water interface. If the high resistance (107 ohms per sq. cm.) of black lipid films is to be ascribed to the continuous layer of hydrocarbon chains in the interior of the film, as is generally done, an increase in film conductivity is not expected from adsorption without penetration. 

Figure 12.10 Droplet stabilized by polymer (left) and by adsorbed solid particles (right). The contact angles of the solid particles with the continuous phase should be smaller than 90°.

Figure 12.14 Schematic of an oil-in-water droplet stabilized by surfactant.

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

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