Peculiar absorption of water by hydrophobized glass beads

Hydrophobization of industrial powders is frequently used to avoid caking during transport and storage. Caking is induced by the formation of liquid bridges through capillary condensation at the contact point between particles. Liquid-solid contact angles are commonly determined through macroscopic experiments but the relationship between such observations and local water interaction with the microparticle is not straightforward. Here, model powders made of glass beads with different degrees of hydrophobicity are used to establish correlations between macroscopic wetting and water absorption at a microscopic level. Glass beads were modified by reaction of surface silanol groups with trimethylchlorosilane molecules. Increasing treatment times resulted in progressively increasing macroscopic hydrophobicity analysed by capillary rise in powder beds. Surface morphology of the particles was observed by atomic force microscopy (AFM). Isotherms of water vapour sorption obtained by Dynamic Vapour Sorption (DVS) apparatus, revealed unexpected water uptake even in the case of hydrophobized beads. Analysis of the isotherms and AFM pictures revealed the presence of nanosized hygroscopic crystals, identified as sodium chloride, at the surface of the beads. Therefore, even if after silanization the beads appear as macroscopically hydrophobic, water absorption by hygroscopic salt still takes place, compromising the effect of the hydrophobic treatment.