Pitch deposition at the solid‚Äö-liquid interface: Effect of surface hydrophobicity/hydrophilicity and cation specificity

The deposition rate of colloidal pitch onto hydrophobic and hydrophilic model surfaces was measured at the solid‚Äö-liquid interface by impinging jet microscopy (IJM) and the effect of cation specificity in solution on deposition was quantified. On both model surfaces, the pitch deposition was slightly faster with calcium ions than with magnesium at the same concentration (800 mg/L). This concentration is around twice the critical coagulation concentration. The rate of colloidal pitch deposition on hydrophobic surfaces was far greater (up to a 2.5 times) than on hydrophilic surfaces for both salts. Contact angle measurements inferred that in the air-surface environment, the hydrophobicity of the surface does not affect its affinity for pitch suggesting molecular mobility within the pitch colloid. IJM shows variation in the pitch shape on the model surfaces. On hydrophilic surfaces, the pitch particle size for both salts ranges from 0.33 to 0.35 m while for hydrophobic surfaces the particle size is 5 times higher for calcium salt than for magnesium salt. Film thinning or spreading of the pitch particles occurred on the hydrophobic surfaces with calcium and to a lesser extent with magnesium salt.