Humidity-induced surface modification of boric acid

Previous studies of boric acid powder suggest that the surface undergoes restructuring when exposed to changes in relative humidity of the surrounding air. This present work investigates these surface changes using atomic force microscopy (AFM) to provide quantitative topographic information, supplemented by gravimetric analysis of water sorption. The AFM images clearly show the dissolution of small rounded features and z-height growth of the larger flatter areas, indicating that Ostwald ripening was the prevailing mechanism in the restructuring. Estimation of radii of curvature of asperities on the surface enabled the approximation of supersaturation ratio in the surface film using the Kelvin equation. The value of 1.007 obtained is much less than the value of 3.2 reported for solution-grown boric acid. It is this high level of supersaturation that is thought to account for the inherent roughness of boric acid, associated with dendritic growth. Conversely, the low supersaturation ratio estimated for the adsorbed surface layer results in regular crystal growth and a decrease in surface roughness. Gravimetric analysis of boric acid revealed a steady loss in water with increase in relative humidity from 10% to 60%. This is attributed to loss of available surface area through the progressive dissolution of fine surface features. Desorption showed a monotonic decrease in water uptake, confirming that the restructuring is not reversible. Changes in surface area were estimated from the AFM images. The loss of surface area between dry and ambient conditions was greater than that inferred from the loss of mass in the gravimetric analysis. This suggests that water is retained on the surf ace in capillaries that are not apparent in the AFM surface scans. Statistical monolayer coverage of water is achieved at 10% relative humidity, which corresponds to the onset of surface restructuring indicated by the water sorption isotherm. Copyright (C) 2004 John Wiley Sons, Ltd.