Droplet stabilization, miniemulsion

Reimers and Schork [94, 95] report the use of PMMA to stabihze MM A miniemulsions enough to effect predominant droplet nucleation. Emulsions stabilized against diffusional degradation by incorporating a polymeric costabilizer were produced and polymerized. The presence of large numbers of small droplets shifted the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. Droplet diameters were in the miniemulsion range and reasonably narrowly distributed. On-hne conductance measurements were used to confirm predominant droplet nucleation. The observed reaction rates were dependent on the amount of polymeric costabilizer present. The latexes prepared with polymeric costabilizer had lower polydispersities (1.006) than either latexes prepared from macroemulsions (1.049) or from alkane-stabilized miniemulsions (1.037). 

It is certain that we do not know what the leading effect in determining droplet stability and droplet distribution in miniemulsion is at this point both colloidal and ripening effects probably play a role. Future work is therefore needed to clarify these problems. 

Batch miniemulsion polymerization of MMA using PMMA as the costabilizer was carried out with SLS as the surfactant and KPS as the initiator. Solids content was kept at -30%. A low surfactant level was used with the miniemulsions to ensure droplet nucleation. The initiator concentration of the polymer-stabilized miniemulsion polymerizations was varied from 0.0005 to 0.02 Mjq, based on the total water content. An aqueous phase retarder, (sodium nitrite) or an oil-phase inhibitor (diphenylpicrylhydrazol [DPPH]), was added to both the miniemulsions and the macro emulsions prior to initiation. Particle numbers and rates of polymerization for both systems were determined. 

The effects of the (water-soluble) initiator concentration on the polymerization of polymer-stabilized miniemulsion are shown in Table 2. An increase in the initiator concentration does not change the number of particles, but does increase the rate of polymerization. This is due to an increase in the number of radicals per particle. However, the number of radicals per particle ranged from just 0.5 to 0.8, indicating that the kinetics (after nucleation) are still essentially Smith Ewart Case II. The number of particles was found to be proportional to the initiator concentration raised to the power of 0.002 0.001. Macroemulsion polymerizations, in contrast, show a dependence of 0.2 and 0.4 for methyl methacrylate and styrene, respectively [141]. The fact that the exponent approaches zero indicates that all or nearly all of the droplets are being nucleated. 

Stabilize new particles, thereby increasing the total number of particles. Since the nucleation period is lengthened, the polydispersity increases. Figure 14 shows that the dependence of the inhibitor concentration on the number of particles is 0.176 0.010. Conversion time curves indicate that an induction period results from the presence of the inhibitor. Since polymer-stabilized miniemulsion polymerization occurs via droplet nucleation, it should be less sensitive to oil-phase inhibition. Initiator radicals will enter the droplet one after the other until all of the inhibitor is used up, and the monomer polymerizes. This does not affect the number of droplets or particles. As seen in Fig. 15, the number of particles is proportional to the DPPH concentration raised to the power of 0.0031 0.0001. Therefore, the number of particles is essentially independent of the presence of inhibitor. 

Shifting the site of nucleation to the droplets greatly enhances the robustness of the nucleation process to recipe variations, inhibition levels, and changes in operating procedure (initiation rate and/or agitation rate). As a result of droplet nucleation, polymer-stabilized miniemulsion polymerizations are far less sen-... 

From the obtained results it can be seen, that the difference in physico-chemical characteristics of the surfactants, and therefore different emulsifying properties, affect the size of the droplets. In all cases, the decrease in the amount of surfactant leads to larger droplets and a broader size distribution. Generally, the size of droplets was varied between 240 and 400 nm. Droplets with the smallest size were obtained with Span 80 regardless to its concentration indicating that the minimum droplet size for this system is reached. The addition of Tween 80 to Span 80 in a ratio 3 2 increases the stability of miniemulsions and improves the monodispersity of the droplets. In this case, both surfactants are involved in the process of the droplet stability. Hydrophobic molecules of Span 80 are oriented at the oil-droplet interface, whereas hydrophilic molecules of Tween 80 preferably stays in an aqueous phase, and... 

Whatever the mechanism of droplets stabilization is, the behavior of the miniemulsion polymerization is similar to that of microemulsion in that the nucleation process can also be quite long. For that reason, it often leads to broad particle size distribution. However it is known that the hexadecane system tends to give narrower distribution and in many cases allows retention of the size of the initial monomer droplets. 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

Miniemulsion polymerization involves the use of an effective surfactant/costabi-lizer system to produce very small (0.01-0.5 micron) monomer droplets. The droplet surface area in these systems is very large, and most of the surfactant is adsorbed at the droplet surfaces. Particle nucleation is primarily via radical (primary or oligomeric) entry into monomer droplets, since little surfactant is present in the form of micelles, or as free surfactant available to stabilize particles formed in the continuous phase. The reaction then proceeds by polymerization of the monomer in these small droplets hence there may be no true Interval II. 

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. 

Emulsions are understood as dispersed systems with liquid droplets (dispersed phase) in another, non-miscible liquid (continuous phase). Either molecular diffusion degradation (Ostwald ripening) or coalescence may lead to destabilization and breaking of emulsions. In order to create a stable emulsion of very small droplets, which is, for historical reasons, called a miniemulsion (as proposed by Chou et al. [2]), the droplets must be stabilized against molecular diffusion degradation (Ostwald ripening, a unimolecular process or r, mechanism) and... 

In 1962, Higuchi and Misra examined the quantitative aspects of the rate of growth of the large droplets and the rate of dissolution of the small droplets in emulsion for the case in which the process is diffusion controlled in the continuous phase [4]. It was proposed that unstable emulsions may be stabilized with respect to the Ostwald ripening process by the addition of small amounts of a third component, which must distribute preferentially in the dispersed phase [4]. The obtained stability in miniemulsions is said in the literature to be metastable or fully stable. The stabilization effect by adding a third component was recently theoretically described by Webster and Cates [5]. The authors considered an emulsion whose droplets contain a trapped species, which is insoluble in the continuous phase, and studied the emulsion s stability via the Lifshitz-Slyozov dynamics (Ostwald ripening). 

The droplet size and size distribution seems to be controlled by a Fokker-Planck type dynamic rate equilibrium of droplet fusion and fission processes, i.e., the primary droplets are much smaller directly after sonication, but colloidally unstable, whereas larger droplets are broken up with higher probability. This also means that miniemulsions reach the minimal droplet sizes under the applied conditions (surfactant load, volume fraction, temperature, salinity, etc.), and therefore the resulting nanodroplets are at the critical borderline between stability and instability. This is why miniemulsions directly after homogenization are called critically stabilized [19,20]. Practically speaking, miniemulsions potentially make use of the surfactant in the most efficient way possible. 

Colloidal stability is usually controlled by the type and amount of the employed surfactant. In miniemulsions, the fusion-fission rate equilibrium during sonication and therefore the size of the droplets directly after primary equilibration depends on the amount of surfactant. For sodium dodecylsulfate (SDS) and styrene at 20% dispersed phase, it spans a range from 180 nm (0.3% SDS relative to styrene) down to 32 nm (50 rel.% SDS) (Fig. 4a). Again, it is anticipated that rapidly polymerized latexes also characterize the parental miniemulsion. As... 

The stability of miniemulsion droplets against diffusional degradation results from an osmotic pressure in the droplets, which controls the solvent or monomer evaporation. The osmotic pressure is created by the addition of a substance, which has extremely low water solubility, the so-called hydrophobe. This crucial prerequisite is usually not present in microemulsions, but... 

In miniemulsion polymerization the nucleation of the particles mainly starts in the monomer droplets themselves. Therefore, the stability of droplets is a crucial factor in order to obtain droplet nucleation. The better the droplets are stabilized against Ostwald ripening, the higher is the droplet nucleation. 

It was shown that the principle of aqueous miniemulsions could be transferred to non-aqueous media [45]. Here, polar solvents, such as formamide or glycol, replace water as the continuous phase, and hydrophobic monomers are miniemulsified with a hydrophobic agent, which stabilizes the droplets against molecular diffusion processes. It turned out that steric nonionic surfactants based on poly(ethylene oxide) tails are far more efficient than ionic stabilizers,... 

---