Deposition of Compact Hydrous Aluminum Sulfate Thin Films on Titania Particles Coated with Organic Self-Assembled Monolayers

An organic self-assembled monolayer (SAM) was deposited on submicron-sized pigment-grade TiO₂ particles to create a highly hydrophilic surface comprised of sulfonate groups. Hydrous aluminum-containing thin films were uniformly deposited on the powder from aqueous solutions at 60-80 °C. The as-deposited films were in the form of boehmite (AlO(OH)) or hydrous Al sulfate, depending on the Al salts used in the starting solution. AlCl₃ solutions gave rise to porous films of needlelike crystalline boehmite, whereas mixed Al₂(SO₄)₃/Al(NO₃)₃ solutions led to compact, uniform, adherent, dense amorphous sulfate-containing films up to ∼5 nm thick. The sulfate anion in solution is thought to promote nucleation and growth of a solid phase through its relatively strong electrostatic field. However, the sulfate anion is weakly bound, and the sulfate films can be partly leached of sulfur in neutral or weakly basic solutions at room temperature without sacrificing their compact nature.