Particle stability

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

Fig. 4. (a) Polymer bridging between particles and (b), particle stabilization by adsorbed polymer (32). 

In the post-dispersion process, the soHd phenoHc resin is added to a mixture of water, cosolvent, and dispersant at high shear mixing, possibly with heating. The cosolvent, frequently an alcohol or glycol ether, and heat soften the resin and permit small particles to form. On cooling, the resin particles, stabilized by dispersant and perhaps thickener, harden and resist settling and agglomeration. Both resole and novolak resins have been made by this process (25). 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

Interestingly, this behavior of the reaction mixture can be prevented by employing another principle of particle stabilization steric protection. Inclusion of pegylated comonomer (PEG-AEPD) into the reaction mixture did enable the formation of nonaggregating DNA particles. It also caused the particles to form worm -like structures (as judged by transmission electron microscopy) that have previously been observed with DNA complexes formed from block copolymers of PEL and PEG [98]. 

By performing in situ the polymerization of acrylamide in water/AOT/toluene microemulsions, clear and stable inverse latexes of water-swollen polyacrylamide particles stabilized by AOT and dispersed in toluene have been found [192-194], It was shown that the final dispersions consist of two species of particles in equilibrium, surfactant-coated polymer particles (size about 400 A) with narrow size distribution and small AOT micelles (size about 30 A). 

Organic polymers claimed to be effective swelling clay and mineral fine particle stabilizers in the patent literature can be divided into four classes. The polymers of class 1 have the quaternary nitrogen atom as part of the polymer backbone (6-10). Polymers in this class include poly(dimethylamine-co-epichlorohy-drin, abbreviated poly(DMA-co-EPl), and poly(N,N,N, N,-tetramethyl-l,4-l,4-diaminobutane-co-l,4-dichlorobutane), abbreviated poly (TMDAB-co- DCB). These low molecular weights are not surprising since these are condensation polymers. Molecular weights cited range from 800 to 800,000 daltons. 

Mineral Fine Particle Stabilization. Experiments were performed using test columns packed with a well blended mixture of 85% (by weight) 70-170 U.S. mesh sand and 15% mineral fine particles, The size distributions of the mineral fine particles are summarized below ... 

Results indicate that the effectiveness of quaternary ammonium salt polymers in stabilizing swelling clays and mineral fine particles is dependent on monomer chemical structure and polymer molecular weight. Long flexible pendant sidechains containing quaternary nitrogen atoms appear to be required for these polymers to function as mineral fine particle stabilizers. 

Nonionic copolymers of N-vinylpyrrolidinone also functioned as mineral fine particle stabilizers. 

The results of two field experiments involving a statistically significant number of wells indicated that quaternary ammonium salt polymers can function well as swelling clay and mineral fine particle stabilizers under actual field conditions. 

Sp(t), the area of polymer particles stabilized by polymer end groups rather than soap, might in the general case be important but it is very difficult to obtain an expression for it. Aj(t) on the other hand, the area of monomer droplets, is usually neglected as being quite a few orders of magnitude less than Ap(t). 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

More direct and successful methods for the preparation of non-aqueous metal sols are desirable. Especially valuable would be a method that avoids the metal salt reduction step (and thus avoids contamination by other reagents), avoids electrical discharge methods which decompose organic solvents, and avoids macromolecule stabilization. Such a method would provide pure, non-aqueous metal colloids and should make efficient use of precious metals employed. Such colloids would be valuable technologically in many ways. They would also be valuable to study so that more could be learned about particle stabilization mechanisms in non-aqueous media, of which little is known at the present time. 

The control of particle size by concentration indicates that particle growth is a kinetic phenomenon. It is unlikely that particle growth is reversible once a Au-Au bond is formed it would not break under these experimental conditions. In a dilute solution of atoms, the frequency of encounters would be lower. As the gold atom-solvent matrix warms, the atoms and subsequent metal particles become mobile. It is the number of encounters that occur before particle stabilization that is important. If metal concentration is high the frequency of encounters is higher and the particles become bigger. 

Fig. 4. Schematic of polymer colloidal particles stabilized by PFOA homopolymer [103]...

Figure 9.61 QDs containing carboxylate groups can be coupled to amine-containing proteins or other molecules using the EDC/sulfo-NHS reaction to form amide bond linkages. The intermediate sulfo-NHS ester is negatively charged and will help maintain particle stability due to like charge repulsion between particles.

Wash particles (e.g., 100 mg of 1 pm carboxylated latex beads) into coupling buffer (i.e., 50 mM MES, pH 6.0 or 50 mM sodium phosphate, pH 7.2 buffers with pH values from pH 4.5 -7.5 may be used with success however, as the pH increases the reaction rate will decrease). Suspend the particles in 5 ml coupling buffer. The addition of a dilute detergent solution may be done to increase particle stability (e.g., final concentration of 0.01 percent sodium dodecyl sulfate (SDS)). Avoid the addition of any components containing carboxylates or amines (such as acetate, glycine, Tris, imidazole, etc.). Also, avoid the presence of thiols (e.g., dithiothreitol (DTT), 2-mercaptoethanol, etc.), as these will react with EDC and effectively inactivate it. 

Dissolve SPDP in dimethylformamide (DMF) at a concentration of 6.2 mg/ml (makes a 20 mM stock solution). Add 50 pi of the SPDP solution to the 1 ml particle suspension and mix to dissolve. Note The small quantity of DMF in a polymeric particle suspension should not affect particle stability, even if the polymer type is susceptible to swelling in pure DMF. Other particle types, such as metallic or silica based, usually are not affected by organic solvent addition, unless their surfaces are non-covalently coated with a dissolvable polymer. 

At the symposium on which this book is based, the various authors presented papers on the general topic of polymer adsorption and particle stabilization/destabilization. In this volume both aqueous and nonaqueous systems are included, comprising work on both natural and synthetic polymers. Together the chapters constitute a comprehensive update of research in progress on these topics and provide broad coverage of both experimental and theoretical aspects. 

Besides temperature and addition of non-solvent, pressure can also be expected to affect the solvency of the dispersion medium for the solvated steric stabilizer. A previous analysis (3) of the effect of an applied pressure indicated that the UCFT should increase as the applied pressure increases, while the LCFT should be relatively insensitive to applied pressure. The purpose of this communication is to examine the UCFT of a nonaqueous dispersion as a function of applied pressure. For dispersions of polymer particles stabilized by polyisobutylene (PIB) and dispersed in 2-methylbutane, it was observed that the UCFT moves to higher temperatures with increasing applied pressure. These results can qualitatively be rationalized by considering the effect of pressure on the free volume dissimilarity contribution to the free energy of close approach of the interacting particles. 

The poly(vinyl acetate) particles stabilized by PIB were found, using a Coulter Nanosizer, to have a diameter of 0.3jim while the poly(methyl methacrylate) particles were 0.4pm diameter. 

The combinational contribution to AG,n for PMMA particles stabilized by PIB in 2-methylbutane is shown plotted as a function of temperature in Figure 3(a). The values of the parameters used in Equations 2 and 3 were u = 8 x 10- g cm-, a = 300 nm, >2 = 1.09 cm g- and V = 116.4 cnr mole- . The thickness of the steric barrier,L, was taken to be 25 nm and the particle separation, do, was fixed at 30 nm. It can be seen from Figure 3(a) that AGj (comb) is a positive quantity that becomes more positive as the temperature increases, indicating that in the absence of other contributions to AG, the particle would become more stable with increasing temperature. In the above calculation, we have assumed that the S function, Equation 3, remains invariant with temperature, which is incorrect. 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

The most accepted among the qualitative theories of mass spectral fragmentation are the conception of charge and unpaired electron localization and the estimation of ions and neutral particles stability. Despite their qualitative character these approaches are quite useful to work with mass spectra. Both theories use the principle of the minimal structural changes at each stage of fragmentation, while the structure of the molecular ion is considered to be the same as that of the initial molecule. Certain isomerization processes of M+ before the fragmentation are usually a matter of special study. 

Amiens, C. et al., Selective synthesis, characterization, and spectroscopic studies on a novel class of reduced platinum and palladium particles stabilized by carbonyl and phosphine ligands, J. Am. Chem. Soc., 115, 11638, 1993. 

---