Despite the anisotropy of crystalline solids being well established and of great significance, the measurement and determination of facet specific properties is challenging.  Typical powder-based characterisation techniques may result in a non-specific surface being probed and/or a single value surface property determined, which is non-representative of a typical organic crystalline solid which is anisotropic.  In order to probe facet specific properties, one approach is to directly measure properties of individual crystalline facets, overcoming some limitations posed by powder samples.  In order to achieve this, we will employ a slow solvent evaporation methodology to grow large single macroscopic (mm to cm in size) crystals established in JH’s laboratory.  We will obtain crystalline facets of up to 1 cm² for direct measurements by liquid sessile drop contact angle and Contact Potential Difference (CPD).  Contact angle measurements with water as a probe liquid will allow for assessment of hydrophilicity/hydrophobicity of the surface.  Charge transfer occurs most readily when hydrophilic and hydrophobic surfaces come into contact.  Contact angle measurements will be extended to a range of probe liquids (diiodomethane, formamide, ethylene glycol in addition to water) to allow for the determination of the solid surface energy and its components (e.g. Owens-Wendt for dispersive and polar surface energy components or van Oss-Good-Chaudhary and Della-Volpe for the acid-base surface energy components).  Using sessile drop method on large crystals as well as acid-washed and silanised glass beads, the contact angle will be quantified to establish a correlation with KPFM measurement of WP2 and validation for WP1.  Furthermore, relating single facet and bulk powder characteristics requires measurement of Contact Potential Difference (CPD) as well as sessile drop contact angle on compacted bulk powder surfaces for establishing an averaging methodology and linking CPD to KPFM.  In addition to the above materials, the surfaces of PTFE and PDMS will also be characterised for WPs 4, 5 & 6.  This will constitute a first attempt on this line and will be highly informative for engineering applications.  The work will be based on the approach previously developed in collaboration with Pfizer [1].

References:

1. Biegaj, K. W., Rowland, M. G., Lukas, T. M., & Heng, J. Y. Y. (2017). Surface Chemistry and Humidity in Powder Electrostatics: A Comparative Study between Tribocharging and Corona Discharge. ACS Omega, 2(4), 1576–1582.