Structure formation and interaction of silanized glass beads at water-fluid interfaces: a redispersability study

Abstract

Abstract The aggregation and spreading of silanized glass microspheres (diameter: 62–74 μm) were investigated at water (electrolyte)-fluid interfaces by evaporating hexane or chloroform from the water surface. Redispersion of aggregates was attributed to non-equilibrium interfacial tension at the water-fluid interface in the final stage of evaporation. The effect of particle wettability on this phenomenon was studied. Two different hydrophobic samples (Sample A: Θwater/air=77 ± 1°; Sample B: Θwater/air=44 ±1°) were prepared for experiments. Two-liquid wettability of particles was also characterized by contact angles (Sample A: Θ water hexane = 100 ± 4 °; Sample B: Θ water hexane = 57 ± 3 °) by means of which the colloid and capillary pair-potential energies have been calculated. In every case the aggregation yielded well-packed aggregates at the water-organic liquid interface, but spreading experiments showed that while the aggregate of the lower hydrophobic sample could disperse into little clusters or single particles, the higher hydrophobic particles remained together in the final stage of organic liquid evaporation. The speed and extent of redispersion of the lower hydrophobic sample were found to be dependent on the spreading liquids (hexane or chloroform), but the electrolyte (KCl) content of the subphase could not influence the process. The aggregates of higher hydrophobic particles could be redispersed in neither case investigated. It was proposed, on the basis of the calculated interparticle energies, that the higher hydrophobic particles could come into direct contact during the organic liquid evaporation due to the strong “immersion-type” capillary and hydrophobic attractive interactions. We characterized the dispersing ability of the spreading liquids used by a parameter (“dispersing energy”: 25.1 mN m−1 for hexane; 46.8 mN m−1 for chloroform) which could be calculated from a supposed interfacial tension difference at the water-air interface arising during the final stage of organic liquid evaporation. These values were compared with the “pull-off energy” (≈90 mN m−1) obtained earlier for silanized glass surfaces [J.L. Parker and P.M. Claesson, Langmuir, 10 (1994) 635]. The results confirmed the above suggestion that hydrophobic spheres (Sample A) can reach a situation in which the strong adhesion between them can hinder their redispersion and, furthermore, made it reasonable to assume that the aggregates of lower hydrophobic particles (Sample B) could be redispersed better if chloroform was used as spreading liquid.