EXPERIMENTAL AND THEORETICAL-ANALYSIS OF THE LIMITED COALESCENCE PROCESS - STEPWISE LIMITED COALESCENCE

Experimental tools for the investigation of the mechanism of limited c oalescence (LC) processes have been developed, and a theoretical analy sis, based on Monte Carlo methods, of the particle size distributions resulting from LC, has been carried out. The method involves performin g the LC process in a stepwise fashion in one of two ways. In the firs t, a fine emulsion of an oil phase (dodecyl phthalate in our studies) in water is made using a soap (sodium laurate) as an emulsifying agent . This emulsion is then acidified in the presence of an appropriate qu antity of silica (Ludox TM) and a promoter, under which conditions a n ormal LC process occurs. In the second, a first LC process is performe d using colloidal cupric oxide as the LC stabilizer. The CuO is then d issolved in an acidic solution containing silica and a promoter, at wh ich time a second LC process occurs. The second method allows the init ial particle size distribution to be well characterized and also allow s the kinetics of the LC process to be followed. From a consideration of the results obtained from the stepwise LC studies, we conclude that practical LC procedures involve a mechanism in which reversible, shea r-dependent flocculation is followed by rate-determining coalescence. Particle collisions are driven by the stirring, and not by Brownian mo tion. The theoretical analysis, assuming either diffusional or turbule nce-driven collision, predicts particle size distributions that are in close accord with experimental results and that are much narrower tha n those resulting from the so-called ''self-preserving distribution'' resulting from simple Brownian coalescence and the even broader distri butions expected for hydrodynamically driven coalescence processes. (C ) 1995 Academic Press, Inc.