Flocculation Of Latex Particles Research Articles

Ultrasonic Trap To Monitor Morphology and Stability of Developing Microparticle Aggregates

Aggregation and flocculation of latex particles have been examined in an ultrasonic standing wave trap that draws microparticles from suspensions into a light-microscope prefocused central region of the pressure node plane. The ultrasound chamber had a single half-wavelength path length (λ/ 2 = 0.25 mm) at its resonant frequency of 3.17 MHz. The full course of aggregate growth, in water or in 10 mM CaCl 2 , of 25 μm diameter latex particles was monitored by video microscopy. The particles formed two-dimensional aggregates in the nodal plane. Particles suspended in deionized water joined the aggregate both as single particles and as clusters. Incoming particles rolled over particles at the edge of the aggregate. The aggregate then adjusted its internal order to maintain a regular array of particles in apparent close contact. In contrast, particles in salt solutions interacted strongly on contact so that the aggregates grew as dendritic structures. The perimeter fractal dimensions of 1.15 and 1.59 in water and in salt, respectively, were typical of transport-limited and reaction-limited aggregation. When the ultrasonic exposure was terminated, the compact, low fractal dimension flocs disaggregated as they sedimented while the dendritic aggregates remained intact. It is shown that at surface separations of the order of 10 nm the acoustic particle interaction forces (dependent on the separation of centers of mass) are small compared to the van der Waals and electrostatic interaction forces (dependent on surface- surface separation) that normally control microparticle aggregation. It is concluded that the ultrasound did not significantly influence the net interactions that determine aggregate morphology and stability and that the ultrasonic trap technique holds promise for elucidating patterns of aggregate growth.

Particle Nucleation and Growth Mechanisms in Styrene Microemulsion Polymerization

A water‐insoluble dye was used to study nucleation and growth of particle nuclei in styrene microemulsion polymerizations, stabilized by sodium dodecyl sulfate/1‐pentanol and initiated by sodium persulfate (SPS). Both the rate of polymerization and concentration of latex particles increase with an increase in the initial SPS concentration ([SPS]0). Least‐squares best fitting experimental data with a modified Morgan‐Nomura model leads to the conclusion that the limited flocculation of latex particles plays an important role in the reaction system. The weight percentage of dye incorporated into latex particles (Pdye) increases with the progress of polymerization. This implies that particle nucleation takes place continuously throughout the reaction. At constant monomer conversion, Pdye decreases with increasing [SPS]0, which is attributed to the increased probability of particle formation in the aqueous phase with [SPS]0. Measurements of zeta potential of latex particles and polymer molecular weight were al...