Controlled wettability of quartz surfaces

The wettability of flat quartz-crystal surfaces has been assessed by measurement at 25 °C of contact angle at the water-vapour/water-drop/quartz-plate three-phase line. Plates pretreated by heating in vacuum gave angles, measured through the drop 1 min after removal from vacuum, of 0–44 ° as pretreatment temperature was increased from 200 to 1000 °C. Fully hydroxylated, therefore hydrophilic, quartz surfaces are progressively rendered hydrophobic by mutual condensation of surface hydroxyls to form siloxane bridges. Hysteresis was at a maximum after heating at 700–800 °C, indicating that maximum surface chemical heterogeneity was produced by heating in this temperature range. Plates methylated subsequent to heat treatment gave angles that were constant at ca. 80° up to treatment at 600 °C and that decreased from 80 to ca. 47 ° as the pretreatment temperature was further increased to 1000 °C. This variation with temperature is consistent with a mechanism for methylation in which only non-hydrogen-bonded surface hydroxyl groups on quartz are reactive towards the methylating reagent. Contact angles on both heat-treated and methylated plates were observed to decrease following extended exposure to water vapour.