Measuring the surface area of aluminum hydroxide adjuvant

The traditional method of determining surface area, nitrogen gas sorption, requires complete drying of the sample prior to analysis. This technique is not suitable for aluminum hydroxide adjuvant because it is composed of submicron, fibrous particles that agglomerate irreversibly upon complete removal of water. In this study, the surface area of a commercial aluminum hydroxide adjuvant was determined by a gravimetric/FTIR method that measures the water adsorption capacity. This technique does not require complete drying of the adjuvant. Five replicate determinations gave a mean surface area of 514 m(2)/g and a 95% confidence interval of 36 m(2)/g for a commercial aluminum hydroxide adjuvant. The X-ray diffraction pattern and the Scherrer equation were used to calculate the dimensions of the primary crystallites. The average calculated dimensions were 4.5 x 2.2 x 10 nm. Based on these dimensions, the mean calculated surface area of the commercial aluminum hydroxide adjuvant was 509 m(2)/g, and the 95% confidential interval was 30 m(2)/g. The close agreement between the two surface area values indicates that either method may be used to determine the surface area of aluminum hydroxide adjuvant. The high surface area, which was determined by two methods, is an important property of aluminum hydroxide adjuvants, and is the basis for the intrinsically high protein adsorption capacity. (C) 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association.