DEPOSITION OF RUTILE TITANIUM-DIOXIDE PARTICLES ONTO OPPOSITELY CHARGED SURFACES - A COMPARISON OF STAGNATION POINT FLOW AND QUIESCENT CONDITIONS AND THE FORMATION OF 2-DIMENSIONAL RAFTS

The rate of deposition of paint-grade rutile titanium dioxide particle s (similar to 0.3 mu m diameter) onto model substrates, under a variet y of conditions, has been monitored. The model substrates used were op tically-flat glass plates, whose surfaces were made cationic by reacti on with (aminopropyl)trimethoxysilane, APTMS. A flow geometry was arra nged such that the particles impinged, through a jet, normally onto th e substrate. This creates a stagnation point region near the substrate surface, opposite the mouth of the jet. In this region, at relatively low mass transfer rates, it is supposed that particles reach the surf ace by a diffusion process. The deposition (rate) of the particles was monitored visually using both a microscope and also a CCD camera link ed to an image-analysis system. This allowed the number of particles d eposited per unit area to be monitored. Deposition rates, and maximum coverages, have been established at various particle concentrations an d various background electrolyte (NaCl) concentrations. The results ob tained agreed well with experiments in which APTMS-coated glass plates were simply immersed (''vertically'') in a suspension of titania part icles, and particles were allowed to deposit under Brownian motion con ditions. In the flow experiments, provided the NaCl concentration was below a critical value, the particles remained stable in dispersion an d ''isolated'' on the substrate surface. However, over a certain salt concentration range, when the particles were precovered with an adsorb ed layer of poly(vinyl alcohol), two-dimensional aggregation (''raft f ormation'') was observed on the substrate surface.