A comparison of contact angle measurement techniques applied to highly porous catalyst supports

The wettability of porous powders is typically described by the solid/liquid contact angle and finds importance in the fields of catalysis, pharmaceuticals and separation techniques. The measurement of contact angles of porous particles with liquids is notoriously difficult and different techniques produce highly variable results. A variety of catalyst supports were used to investigate several methods recommended in the literature to determine their applicability for the measurement of the wettability of highly porous particles. The catalyst supports consisted of SiO2, Al2O3, TiO2 and ZrO2.

The following methods were investigated: the capillary rise method, thin layer wicking and the sessile drop method on particles either compressed in pellets or dispersed upon glass slides. It has been found that methods based on the capillary rise cannot be used to measure contact angle of highly porous particles and this experimental observation has been confirmed by the mass balance of liquid in the bed of particles. Multiple measurements carried out with water and paraffin indicate that only the sessile drop method on particles dispersed on a glass slide gives consistent results. Using this method, no differentiation was found in terms of wettability of paraffin oil for Al2O3, SiO2, and ZrO2 particles although TiO2 was measurably different but largely wettable. In the case of water, the contact angles varied significantly and were found to be θAl2O3=91°,θSiO2=75°,θTiO2=132°,θZrO2=49°θAl2O3=91°,θSiO2=75°,θTiO2=132°,θZrO2=49°.