Spreading of hydrophobic silica beads at water—air interfaces

Abstract

Structured monoparticle layers, formed from 3 ± 1 μm diameter silica beads silylated to a greater (sample A) and a lesser (sample B) extent, have been investigated on water and on aqueous 1.0 M NaCl subphases in a Langmuir film balance. Hydrophobicities corresponding to the different extents of silylation were estimated by determining the wettabilities of identically treated 10 ± 1 μm silica beads. The advancing water contact angles of 98 ± 4° and 76 ± 4° have been calculated for samples A and B respectively. Visual observations of spreading and redispersity, and determinations of surface pressure—surface area isotherms, hysteresis and contact cross-sectional areas led to the assessment of the structural strengths of these two sets of silica beads. Appreciably stronger attractive interactions between neighboring silica beads have been found in structured monoparticle layers prepared from sample A than those formed from sample B. The less hydrophobic silica beads (sample B) were postulated to be separated from each other by a layer of water molecules in the structured monoparticle layer. This postulate has been supported by calculating the total interparticle interaction energies by applying the DLVO theory. Introduction of 1.0 M NaCl did not appreciably affect the behavior of structured monoparticle layers prepared from sample A. Significant creasing was observed, however, in the vicinity of the moving barrier upon the compression of structured monoparticle layers prepared from sample B.