(195f) Microscale Surface Energy Measurements of SAM-Coated Particle Surfaces Using a Modified Flotation-Based Approach

Flotation methods are widely used in metallurgical engineering for measuring the surface energy (or hydrophobicity) of fine solids such as coal dusts and various types of colloidal particles. In a typical flotation experiment, particulate materials are suspended in solutions of different surface tensions while air is bubbled into the mixtures. The fraction of particles that float in each solution is determined, often by physically separating these particles from the solution and determining their contribution by mass. The surface energy of the particles is obtained by classical approaches that determine a critical surface tension from the above experiments. Typically, these methods require large amounts of the particulate sample in order to achieve reliable results. Their suitability in assessing the properties of custom synthesized particles that are often available in small quantities has been limited. To address this gap, we have developed a flotation approach for measuring particulate surface energies using a spectroscopic method that then allows measurements on small amounts of particles. In our experiments, we used porous silica particles (5 µm of diameter) that have been functionalized with mixed self-assembled monolayers (SAMs) of Cl3Si(CH2)11(EG)3OCH3 (EG3OMe) and Cl3Si(CH2)7CH3 (C8). These mixed SAMs allowed the surface energy of the particles to be varied. The SAMs were similarly formed on flat silica surfaces, where contact angle measurements could be used for characterization, thereby allowing a direct comparison of the surface energetics for these mixed SAMs on the flat and particle surfaces. The result showed that this flotation method can be used to interrogate the surface energies of small quantities of functionalized particles.