Adsorption of Water on Nano-ball Allophane

Abstract

Allophane appears as aggregates of unit particles of hollow sphere (nano-ball) with diameter of 3.5 to 5.0nm. The wall has some pores which accommodate water molecules. Water vapor adsorption on two natural nano-ball allophane samples was examined under different relative humidities (RHs). The adsorption isotherm up to RH of 0.51 fitted to Langmuir equation, but at higher RHs the curve diverged due to capillary condensation. By using data up to RH of 0.51 and allophane unit particle model of 4.3nm in diameter and molecular weight of 46510, the Langmuir maximum adsorption was calculated as 1240-1290 water molecules per unit particle for the two samples. The allophane model has various functional groups such as AlOHAl (outer surface), SiOH (inner surface), AlOH2 (pore), Si (OH) 2 (pore) and AlOSi (pore), and total numbers of the functional groups become 1044 per allophane unit particle. These indicate that, under lower RHs, water molecules are mainly adsorbed on the functional groups through hydrogen bonding. Molecular orbital calculation (MOPAC PM3) indicated that adsorption affinity of water molecule was higher for SiOH than for AlOHAl, and that charge values on O and H atoms of the functional groups mainly governed hydrogen bond formation with the water molecules.