Visual microscopic studies on hetero-aggregation and selective aggregation

An optical video microscopic technique was used to study hetero-aggregation and selective aggregation phenomena among n-hexadecane oil drops (40-110 mu m in diameter) and two types of polystyrene latex particles (6.76 and 30.2 mu m, in diameter), suspended inside an aqueous medium with pH varying between 1.1 and 12.9. A single drop was produced in situ using a micropipette inside the aqueous phase-filled glass microcapillary (100-160 mu m i.d.) containing the particles. Interactions between the drop and the solid particles and among the solid particles was achieved by movement of the aqueous medium in and out of a second micropipette. Drop-particle interactions were distinctly different from particle-particle interactions. It was observed that the latex particles aggregated irreversibly with the oil drop in all cases except two, viz. for 6.76 mu m particles at around neutral pH whereas the irreversibility of aggregation in particle-particle interactions was only seen at the ends of the pH spectrum. At around neutral pH, the flocs or clusters of small particles were very weak. Visual observations at each pH are explained on the basis of the classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. Partial wetting of particles surfaces by oil appears to be a key factor in the irreversibility of drop-particle hetero-aggregation. Results indicate that the display of reversible, irreversible or weak aggregation depends on the location and depth of the secondary minimum and that the long-range, attractive, London-van der Waals force is responsible for the initial formation of an aggregate.