Surfactant-stabilized particles

Surfactants stabilize particles produced in expansion and prevent growth... 

As for PPy s, there has been an explosion of interest in the synthesis of PAn s with nanodimensions, as such materials have been shown to have enhanced electronic and electrochemical properties. Formation of PAn nanoparticles has been achieved via polymerization in micelles, using either sodium dodecyl sulfate (SDS)211 or DBSA212 214 as the surfactant stabilizer. Particle sizes in the range of 10-30 nm with conductivities as high as 24 S cm-1 have been reported. 

Since then, a number of composite particles have been developed [66-70], Lu et al. have demonstrated the effect of surfactants on the morphology of ICP-coated polystyrene (PS) or poly(styrene-co-butyl acrylate) [PST-co-BuA] particles [71]. These authors showed that when the core particles are stabilized with ionic surfactants, a raspberry-type of morphology is obtained, whereas a homogeneous coreshell morphology is achieved in the case of nonionic surfactant-stabilized particles. 

For the size growth follows equation 21, with volume u = Z> , which leads under the assumption that both K and K independent of time (ie K and K only depend on polymerization conditions) after integration to equation 21. Note that the particle size is always the imswollen size thus using equation 21 means the size dependence of the monomer concentration is neglected, which is reasonable for surfactant-stabilized particles with sizes larger than 50 nm (121). Z) and Do... 

Interaction Between Surfactant-Stabilized Particles Dynamic Aspects... 

These are the classical conditions for interaction at constant charge and at constant potential, respectively, but now adapted for the present situation of surfactant-stabilized particles. 

These rules give rise to two considerations. First, if the dynamics of surfactant-stabilized particles are elaborated on the Gouy-Stem level, one can substitute bulk mobilities for those in the Stem layer, at least if the counterions are monovalent. For bivalent ones the average ratio is about... 

By analyzing the dynamics of surfactant-stabilized particles, a link was established with electrokinetics. In this way a cross-fertilization between two different domains of interfacial science was established. The potentials of this finding deserve further elaboration. 

Nanoparticles of the semicondnctor titanium dioxide have also been spread as mono-layers [164]. Nanoparticles of TiOi were formed by the arrested hydrolysis of titanium iso-propoxide. A very small amount of water was mixed with a chloroform/isopropanol solution of titanium isopropoxide with the surfactant hexadecyltrimethylammonium bromide (CTAB) and a catalyst. The particles produced were 1.8-2.2 nm in diameter. The stabilized particles were spread as monolayers. Successive cycles of II-A isotherms exhibited smaller areas for the initial pressnre rise, attributed to dissolution of excess surfactant into the subphase. And BAM observation showed the solid state of the films at 50 mN m was featureless and bright collapse then appeared as a series of stripes across the image. The area per particle determined from the isotherms decreased when sols were subjected to a heat treatment prior to spreading. This effect was believed to arise from a modification to the particle surface that made surfactant adsorption less favorable. 

PVP, a water soluble amine-based pol5mer, was found to be an optimum protective agent because the reduction of noble metal salts by polyols in the presence of other surfactants often resulted in non-homogenous colloidal dispersions. PVP was the first material to be used for generating silver and silver-palladium stabilized particles by the polyol method [231-233]. By reducing the precur-sor/PVP ratio, it is even possible to reduce the size of the metal particles to few nanometers. These colloidal particles are isolable but surface contaminations are easily recognized because samples washed with the solvent and dried in the air are subsquently not any more pyrophoric [231,234 236]. 

Equation 3.1 and Equation 3.2) (including the mean particle sizes obtained) (Adapted from Bonnemann, H. and Brijoux, W., in Surfactant-Stabilized Nanosized Colloidal Metals and Alloys as Catalyst Precursors/Advanced Catalysts and Nanostructured Materials, Moser, W., Ed., Academic Press, San Diego, 1996, pp. 165-196, Chap. 7. With permission from Elsevier Science.)... 

In addition to surfactant-stabilized colloids, there has been work on forming metal colloids without stabilizers. Lee et al. have shown that direct reduction of mefal chloride salts in tetrahydrofuran with LiBH4 gives small nanoparticles that can be impregnated onto carbon. No further treatment is required after the removal of the solvent. This route was applied to the preparation of PfRu, PtNi, PtMo, and PtW particles. 

Surfactants increase particle number and decrease particle size as their concentration in the initial reaction charge is increased. However, one can use delayed addition of surfactant after nucleation is complete to improve particle stability, without affecting the particle number, size, and size distribution. 

Emulsions made with a fine oil droplet particle size, usually less than one micron, are more stable with the oil droplets less likely to coalesce and separate. The encapsulation of a good quality emulsion is generally more efficient with less surface oil on the spray-dried powder. We wanted to build surfactant properties into the starch backbone to improve encapsulation efficiencies. Studies of the mechanism by which surfactants stabilize emulsions were made in order to accomplish this. 

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. 

All experimental measurements were carried out with a Coulter EPICS V System. This instrument is a laser-based flow cytometer which, as one of its simpler analytical functions, utilizes light scatter measurements to accurately size cells or similar particles (e.g., artificial surfactant-stabilized microbubbles) suspended in aqueous media. The light scatter measurements are sensitive to particle sizes as small as 0.3 pm in diameter. 

Fig. 10.1. Particle size distribution determined for artificial, surfactant-stabilized microbubbles (and micelles) in distilled water.

---