Aluminium phosphate adjuvants prepared by precipitation at constant pH. Part I: composition and structure

Aluminium phosphate adjuvant was precipitated under constant pH conditions in an effort to characterize materials formed at defined precipitation cond itions. A reaction vessel was designed to provide a continuous steady-state process. An aqueous solution containing aluminium chloride and sodium dihy drogen phosphate was pumped into the reaction vessel at a constant rate. A second pump infused a sodium hydroxide solution at the rate required to mai ntain the desired pH. Precipitations were performed between pH 3.0 and 7.5, at intervals of pH 0.5. The adjuvants were characterized using Al-27 NMR, FTIR, Raman and X-ray diffraction methods along with elemental analysis. Th e results of this study indicate that a continuum of amorphous aluminium hy droxyphosphates were formed having properties that changed as a continuous function of the precipitation pH. The phosphate content decreased as the pH of precipitation increased. Al-27 NMR spectra revealed that the majority o f the aluminium was octahedrally coordinated, with a small percentage of te trahedrally coordinated aluminium. The density of the adjuvants was directl y related to the pH of precipitation. The most prominent feature of the IR and Raman spectra is the P-O stretching vibration of the structural PO4 gro ups. The positions of these bands decreased linearly as the precipitation p H increased. The results of selective deuteration FTIR experiments are cons istent with high surface area materials as most of the OH groups were expos ed near the surface of the adjuvant. (C) 2000 Elsevier Science Ltd. All rig hts reserved.