THE EFFECTS OF CONCENTRATION AND HYDROLYSIS ON THE OLIGOMERIZATION AND POLYMERIZATION OF AL(III) AS EVIDENT FROM THE AL-27 NMR CHEMICAL-SHIFTS AND LINEWIDTHS

The Al concentration and forced hydrolysis influence the polymerizatio n of aqueous Al(III) and thus the Al-27 NMR spectra. An increase of th e Al concentration results in an increase of the fraction present as m onomer, in the formation of an oligomer of an OH/Al molar ratio of 2.4 , and in the disappearance of the tridecamer above an Al concentration of 0.15M. Increasing the OH/Al molar ratio at a constant Al concentra tion leads to a linear drop of the fraction present as monomer over th e entire range between 1 and 2.5 and a maximum in oligomer fraction be tween 1.5 and 2.5. At low Al concentrations, the fraction of tridecame r exhibits an optimum yield between OH/Al ratios of 2.2 and 2.4. At ra tios higher than 2.4, NMR unobservable polymers are formed. The chemic al shift and the linewidth of the monomer resonance are lowered by inc reasing spin-spin relaxation, T2, and a consequently decreasing quadru pole coupling constant upon increasing Al concentration. With increasi ng OH/Al molar ratio, [Al(H2O)6]3+ is mainly replaced by [Al(H2O)5(OH) ]2+ and [Al(H2O)4(OH)2]+ and subsequently the chemical shift and linew idth of the monomer resonance increase. The Al concentration or OH/Al molar ratio hardly affect the resonance of the central fourfold coordi nated Al of the tridecamer, due to its very strong shielding.