Effect of ammonium citrate on the rheology and particle packing of alumina slurries
- Univ. of California, Santa Barbara, CA (United States). Materials Dept.
The valence of the citrate ion changes from {minus}1 to {minus}3 as the ammonium counterions of tribasic ammonium citrate dissociate with increasing pH. The citrate anion is potential determining because it strongly adsorbs to the surface of oxide particles in aqueous suspensions, changing the surface charge. At pH 8, close to the natural isoelectric point of Al{sub 2}O{sub 3}, the fully dissociated, trivalent citrate ions produce a negative surface charge. At pH 3, where the natural surface charge of Al{sub 2}O{sub 3} is positive, citrate ions can neutralize the surface. The greatest number density of citrate ions is adsorbed at pH 3 because they are only partially dissociated and the natural surface of Al{sub 2}O{sub 3} contains a high density of positive sites. Attractive particle networks are produced at both the isoelectric point and when excess salt is added to a dispersed suspension. Rheological measurements are use to compare the strength of attractive, Al{sub 2}O{sub 3} particulate networks produced with and without added citrate ions. The attractive networks produced at pH 3, the isoelectric point in the presence of citrate ions, are slightly weaker relative to those produced by pH 9, the natural isoelectric point of Al{sub 2}O{sub 3}. Much weaker networks are produced by dispersing slurries at pH 8 with small amounts of tribasic ammonium citrate and then adding excess salt (either tribasic ammonium citrate or NH{sub 4}Cl). The strength of the weakly attractive networks produced at pH 8 appears identical to those produced when an indifferent electrolyte is added to Al{sub 2}O{sub 3} slurries dispersed at pH 4, reported previously. These observations suggest that although the citrate ion is known to adsorb to the surface of Al{sub 2}O{sub 3}, as evidenced by its potential-determining effect, it does not produce a short-range repulsive potential of any significance.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 82620
- Journal Information:
- Journal of the American Ceramic Society, Journal Name: Journal of the American Ceramic Society Journal Issue: 6 Vol. 78; ISSN 0002-7820; ISSN JACTAW
- Country of Publication:
- United States
- Language:
- English
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