Microemulsions, Emulsions and Latexes

Keith P. Johnston, Gunilla B. Jacobson, C. Ted Lee, Carson Meredith, Sandro R. P. Da Rocha, Matt Z. Yates, Janet DeGrazia and Theodore W. Randolph 

As compressed carbon dioxide is a nonpolar molecule with weak van der Waals forces (low polarizability per volume), it is a relatively weak solvent [1], Thus, many interesting separations and chemical reactions involving insoluble substances in CO2 can be expected to take place in heterogeneous systems, for example, microemulsions, emulsions, latexes and suspensions. Microemulsion droplets 2-10 nm in diameter are optically transparent and thermodynamically stable, whereas kinetically stable emulsions and latexes in the range from 200 nm to 10 pm are opaque and thermodynamically unstable. 

Several important criteria must be satisfied to stabilize a colloid in an SCF [5,6,8,9]. The surfactant must adsorb at the interface, and the surfactant tails must be solvated and long enough to provide steric stabilization, where the force between the droplets or particles is repulsive. Near-critical propane can solvate the tails of many hydrocarbon-based surfactants which are also utilized to form microemulsions in alkanes such as hexane, for example, bis-2- 

---

Johnston KP, Jacobson GB, Lee CT, Meredith C, Da Rocha SRP, Yates MZ, DeGrazia J, Randolph TW. Microemulsions, emulsions, and latexes. In Jessop P, Leitner W, eds. Chemical Synthesis Using Supercritical Fluids. Weinheim, Germany Wiley-VCH, 1999 127-146. 

Johnston, K.P., Jacobson, G.B., Lee, C.T., Meredith, C., Da Rocha, S.R.P., Yates, M.Z., DeGrazia, J., and Randolph, T.W. (1999) Microemulsions, emulsions and latexes. Chemical Synthesis Using Supercritical Fluids, Wiley-VCH Verlag GmbH, Weinheim, Germany, 127-146. 

With recent theoretical and experimental advances in the understanding of colloid and interface science of SCF systems, it is becoming possible to design surfactants for microemulsions, emulsions, and latexes on a rational basis. The-... 

The unique density dependence of fluid properties makes supercritical fluids attractive as solvents for colloids including microemulsions, emulsions, and latexes, as discussed in recent reviews[l-4]. The first generation of research involving colloids in supercritical fluids addressed water-in-alkane microemulsions, for fluids such as ethane and propane[2, 5]. The effect of pressure on the droplet size, interdroplet interactions[2] and partitioning of the surfactant between phases was determined experimentally[5] and with a lattice fluid self-consistent field theory[6]. The theory was also used to understand how grafted chains provide steric stabilization of emulsions and latexes. 

Surfactants have been designed to lower y in C02-based systems. The first generation of research involving surfactants in SCFs addressed water-in-oil (W/O) microemulsions and polymer latexes in ethane and propane, as reviewed elsewhere. (43-45). This work provided a foundation for studies in CO2, which has weaker van der Waals forces (a/v) than ethane. Surfactants with both C02-philic and C02-phobic segments have been used to form microemulsions, emulsions, and organic polymer latexes in CO2. 

The major structural unit of interest in emulsions, microemulsions, colloids and latexes is the particle. It is well known that the particle shape, size and distribution of a latex controls the properties and end use applications. Many latexes are manufactured with a controlled and sometimes monodisperse distribution of particle sizes. Polymer liquids, in the form of emulsions and adhesives, are wet and sticky, and therefore specimen preparation for electron microscopy is very difficxilt. As a result of the importance of the determination of particle size distribution, microscopy techniques have focused on specimen preparations which do not alter this distribution or which alter it as little as possible. Methods have included special cryotechniques (Section 4.9), staining-fixation methods (Section 4.4), microtomy (Section 4.3) and some simple methods (Section 4.1) such as dropping a solution onto a specimen holder. This section is meant to provide a brief survey of the types of microscopy applications which have been foimd useful in the evaluation of emulsions and latexes. 

A major drawback of conventional microemulsion polymerization is the high surfactant-to-monomer ratio usually needed to form the initial microemulsion. Surfactant can be used more efficiently in semi-continuous or fed polymerization processes. Several polymerization cycles can be run in a short period of time by stepwise addition of new monomer. After each cycle of monomer addition, most of the surfactant is still available to stabilize the growing hydro-phobic polymer particles, or to forms microemulsion again when a polar monomer is used. For instance, in the polymerization of vinyl acetate (VA) by a semi-continuous microemulsion process [21], latexes with a high polymer content of about 30 wt% were obtained at relatively low AOT concentrations of about 1 wt%. Moreover, their particle sizes and molecular weights were much smaller than those obtained by conventional emulsion polymerization. 

The emulsion polymerization technique usually contains a micelle-forming surfactant and a water-soluble initiator in combination with a water-insoluble monomer. Polymerization takes place in the monomer-swollen micelles and latex particles. Therefore, the term emulsion polymerization is a misnomer the starting point is an emulsion of monomer droplets in water, and the product is a dispersion of latex particles. In the case of microemulsion polymerization, the monomer droplets are made very small (typical particle radius is 10-30 nm) and they become the locus of polymerization. In order to obtain such small droplets, a co-surfactant (e.g. hexanol) is usually applied. A microemulsion is thermodynamically stable... 

We hope that we have proved with ttiis short review that acoustics and electroacoustics can be extremely helpful in characterizing particle size, zeta potential, and some other properties of concentrated emulsions, microemulsions, and latex systems. Bottimefliods are commercially available already. There are still some problems with the theoretical background for electro-acoustics, but analysis of the literature shows gradual improvement in this field. 

Molau and Keskkula [351] were among the first to study the mechanism of particle formation in rubber containing polymers. They showed that phase separation occurs between the rubber and a vinyl polymer during the polymerization of solutions of rubber in vinyl monomers which is followed by formation of an oil-in-oil emulsion. During phase inversion of the emulsion, rubber solution droplets are formed which change into solid rubber particles in the final polymer. Structural investigations by phase contrast optical microscopy, shown in this chapter (Section 5.3), reveal dispersed particle size and distribution. Ugelstad and Mork [352] reported on new diffusion methods for the preparation of emulsions and polymer dispersions where the size and distribution of the latex particles were monitored by very simple optical, SEM and TEM methods. A microemulsion polymerization has been reported for the first time [353] with... 

Emulsion and microemulsion polymerization are the most common ways to produce polymer dispersions and generate latex particles with diameters between 0.05 and 0.5 pm. In contrast to bulk radical polymerization it is possible to obtain high molecular weights at high polymerization rates. A large number of monodisperse polymer lattices with diameters between 20 and 100 nm have been synthesized in oil-in-water emulsions, most importantly for research and industrial processes, from... 

---