Monoparticulate layers of silanized glass spheres at the water-air interface: Particle-particle and particle-subphase interactions

Silanization has rendered spherical (75 +/- 5 mu m diameter) glass particles to be weakly (sample A, Theta = 55 degrees), moderately (sample B, Theta = 72 degrees), and highly (sample C, Theta = 90 degrees) hydrophobic. Nonequilibrium surface pressure (II) 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 II 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) x 10-(10) J. The weakly cohesive layer, prepared from sample A, had a 490-mn interparticle distance at the secondary energy minimum and a total repulsive interaction energy in the range of (0.5-1.3) x 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.