Comparative studies of hydrophobic surface treatments for TiO2: n-octylphosphonic acid and n-octyltriethoxysilane
Abstract-Hydrophobic titanium dioxide powders were prepared with n-octylphosphonic acid (NOPA) and n-octyltriethoxysilane (NOS) surface treatments. In both cases, hydrophobicity and lipophilicity were optimized at surface concentrations ranging from approx. 6 ¥ 10-6 to 10 ¥ 10-6 mol/m2. Higher surface concentrations of NOS had little to no effect on hydrophobicity and lipophilicity. However, higher surface concentrations of NOPA led to an unexpected reduction in lipophilicity, and to an increase in hydrophilicity. 31P-solid-state NMR studies of the hydrophobic NOPA-treated powders revealed the presence of chemisorbed NOPA, where the chemical shift values of the surface adsorbed species were significantly different from those of the neat compound. Analogous studies of a hydrophilic NOPA-treated powder revealed the additional presence of a neat NOPA component, suggesting that the excess surface treatment forms a physically adsorbed layer that overlays the chemisorbed surface species. These findings indicate that the transition from a hydrophobic powder to a hydrophilic powder occurs when the NOPA surface concentration exceeds the available concentration of surface adsorption sites.