Fused silica as an anti-fouling material for oil sands optical instruments

Abstract

Optical instruments have application in many chemical industries for use in inline observation, measurement, and process control for product and waste streams. Typically, these instruments are fitted with sapphire windows which offer extreme hardness, optical transparency and inertness but can be susceptible to fouling with hydrocarbons in the streams. This work explored the potential of fused silica as an alternative anti-fouling material for optical instrument windows for use in aqueous streams containing hydrocarbons. The advantage of fused silica is that it offers adequate hardness and transparency while retaining chemically reactive silanol groups on its surface. Closed loop experiments conducted with oil sands mature fine tailings (MFT) containing residual bitumen, a hydrocarbon known to cause severe fouling of optical windows, demonstrated no fouling issue on fused silica. The hydrophilicity of the optical surface increased following exposure to the mildly alkaline MFT. We hypothesized that in alkaline aqueous streams the silanol groups are deprotonated to create a negatively charged surface, thereby introducing a repulsive force between the surface and the negatively charged hydrocarbon droplets and increasing the hydrophilicity of both materials. Contrarily, the relatively inert sapphire is limited in its capacity to form negative surface charges and consistently fouled with bitumen. When silanol groups on the fused silica were removed via fluoro-functionalization, it resulted in complete fouling of the optical surface. Finally, bitumen fouling was observed on both fused silica and sapphire surfaces in neutral aqueous environments compared to none on the fused silica in an alkaline environment.