Monoparticulate Layers of Silanized Glass Spheres at the Water−Air Interface:  Particle−Particle and Particle−Subphase Interactions

Abstract

Silanization has rendered spherical (75 ± 5 μm diameter) glass particles to be weakly (sample A, ϑ = 55°), moderately (sample B, ϑ = 72°), and highly (sample C, ϑ = 90°) hydrophobic. Nonequilibrium surface pressure (Π) vs surface area ( A ) isotherms have been determined for monoparticulate layers which were prepared from samples A, B, and C at water−air interfaces in a Langmuir film balance. The effect of hydrophobicity on the particle−particle interaction and on the energy ( E r ) which is necessary for the removal of a particle from the water−air interface (particle−subphase interaction) has been elucidated. Contact cross-sectional areas (CCSA), surface coverages (SC), and collapse energies ( E c ), evaluated from Π vs A isotherms, provided semiquantitative information on the structural strength. Monoparticulate layers which were formed from the most hydrophobic glass spheres (sample C) had a structural strength which was almost 5 times greater than that of those which were formed from the least hydrophobic sample (sample A), as revealed by the E c values which were elucidated for these systems. Long-term stability, determined by time-dependent surface-pressure measurements, was only found for sample C. The energy of a particle−particle contact was calculated, for the strongly cohesive layer of sample C, to be (1.2−1.4) × 10 -10 J. The weakly cohesive layer, prepared from sample A, had a 490-nm interparticle distance at the secondary energy minimum and a total repulsive interaction energy in the range of (0.5−1.3) × 10 -13 J between two beads at an interparticle distance of 1−200 nm. Values for adhesion work ( W r ) were calculated from in situ contact-angle measurements and compared to corresponding E r values which were obtained experimentally by the isotherms. The significant discrepancies between the W r and E r values which were found for sample A or sample B were rationalized in terms of contact-angle hysteresis, dynamic wetting, and distortion of the electric double layer around the interfacial beads.