Latex particle aggregation

O.D. Velev, K. Furusawa, and K. Nagayama Assembly of Latex Particles by Using Emulsion Droplets as Templates. 2. Ball-Like and Composite Aggregates. Langmuir 12, 2385 (1996). 

Polystyrene latex particles were coagulated by the addition of Ba(N03)2. The number of dispersed particles deposited onto a planar polystyrene surface was determined 15 min after the addition of salt by optical microscopy. The light microscope does not permit the aggregation of the deposited particles to be determined subsequent examination by the electron microscope gives this information. Clint et al. obtained the following results ... 

Nanoparticles consisting of noble metals have recently attracted much attention because such particles exhibit properties differing strongly from the properties of the bulk metal [1,2], Thus, such nanoparticles are interesting for their application as catalysts [3-5], sensors [6, 7], and in electronics. However, the metallic nanoparticles must be stabilized in solution to prevent aggregation. In principle, suitable carrier systems, such as microgels [8-11], dendrimers [12, 13], block copolymer micelles [14], and latex particles [15, 16], may be used as a nanoreactors in which the metal nanoparticles can be immobilized and used for the purpose at hand. 

In the conventional emulsion polymerization, monomer droplets are dispersed ip an aqueous phase containing micellar aggregates of surfactant. In this case, the dispersed phase represents a relatively small volume fraction of the system and the micellar aggregates constitute the sites of the polymerization process. In the gel(paste)-like emulsions employed here, the volume fraction of the dispersed phase can be as high as 0.99, and the cells of the concentrated emulsion lead to the polymerized latex particles. 

With 0.05 jLim latex particles in a glycerol-salt solution, large-scale shear aggregates have been shown to have the same fractal dimension as that of Brownian aggregates (i.e.. Dp = 1.8 0.1) when the shear rate is less than 1500/sec [88]. Due to shear allegation, the aggregation rate for these experiments was much faster than that of comparative Brownian aggregation rates. 

A prolific variety of composite latex particles appears in both the open and patent literatures. The subject has been reviewed (1,2) by several authors. Composite implies the presence of at least two dissimilar components either of which could, in principle, constitute the major component by volume. Some features of composite particles, which retain colloidal stability during preparation and subsequent storage, that is where the product is a dispersion in which flocculation, aggregative, and coalescence processes are largely absent so long as the continuous phase remains, will be described here. There are alternative and important processes for preparing composite particles which give flocculated particles readily separated from the liquid diluent phase and dried for use as powder. 

The two-step glutaraldehyde coupling reaction has the advantage over the one-step reaction and the carbodiimide coupling reaction in that intramolecular and intermolecular crosslinking of the antibody molecules is avoided. Furthermore, uinder these conditions, aggregation of the latex particles does not occur. 

With this size of latex particle it becomes possibfe to make direct observations on particles over a perind of time and record them with a high-speed camera. Using this technique Cornell et al (1975)) discovered that particles in an associated unit could be quite mobile. It was observed that as well as some particles leaving the aggregated unit as single particles and returning to the disperse phase there was a continued rearrangement of the particles. This was also observed with floccules at salt concentrations well above the ccc. These observations clearly support the contention that association can occur in a secondary minimum and that in this situation a liquid film is maintained between the particles. 

Under dynamic conditions of emulsion polymerization the process of formation and aggregation of primary particles is accompanied by estab-Hshment of an equilibrium adsorption of the emulsifier on the polar surface of particles. As a result of competition of these processes, secondary" panicles of complex structure are formed (Figs. 11 and 14). The rate of attainment of equilibrium adsorption, depending on monomer polarity and the nature of the emulsifier, determines the degree of limited flocculation of primary particles at which aggregate stability of the secondaiy" latex particles is reached. The formation of nuclei from micelles during the... 

Suspensions of liposomes, microspheres and microcapsules, and nanospheres and nanocapsules formed from a variety of polymers or proteins, as discussed in section 8.6.3 form a new class of pharmaceutical suspension in which physical stability is paramount. It is important that on injection these carrier systems do not aggregate, as this will change the effective size and the fate of the particles. The exception to this is the deliberate flocculation of latex particles administered to the eye, where aggregation leads to agglomerated... 

Illustrative Cases. Three cases are illustrated in Figure 9, marked by the circles labeled A, B, and C. Case A refers to classical experiments by Swift and Friedlander (27) on the coagulation of monodisperse latex particles (diameter = 0.871 pm) in shear flow and in the absence of repulsive chemical interactions. Considering a velocity gradient of 20 s 1, HA is 0.0535, log HA is — 1.27, and dfdj is 1.0 for these experimental conditions. The circle labeled A in Figure 9 marks these conditions and indicates that the hydrodynamic corrections to Smoluchowsla s model predict a reduction of about 40% in the aggregation rate by fluid shear. The experimental measurements by Swift and Friedlander showed a reduction of 64%. This observed reduction from Smoluchowski s rectilinear model was therefore primarily physical or hydro-dynamic and consistent with the curvilinear model. 

Beautiful fractal images are observed when latex particles aggregate. A fractal is a structure that replicates itself over a number of length scales and is defined by the relation... 

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