XPS and FTIR Surface Characterization of TiO2Particles Used in Polymer Encapsulation

The surfaces of hydrophilic (P25) and hydrophobic (T805) TiO2 particles were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to gain a better understanding of the adsorption mechanism of OLOA 370 (polybutene-succinimide pentamine) on TiO2 particles dispersed in styrene monomer prior to miniemulsion encapsulation polymerizations. XPS analysis revealed that both the P25 and T805 TiO2 particles had significant amounts of hydroxyl groups on their surfaces. The XPS results showed that the surface hydroxyl concentration on the hydrophilic (P25) particles was 3.3 OH/nm2, whereas the trimethoxy octyl silane (TMOS)-surface-modified hydrophobic (T805) particles unexpectedly contained 6.6 OH/nm2. This apparent increase in the hydroxyls was attributed to hydrolysis of −OCH3 on the TMOS. The majority of these groups, however, were apparently either not acidic or not accessible to the OLOA 370 in adsorption studies, where the concentration of reactive hydroxyls on the T805 particles was estimated to be 1.8 OH/nm2. FTIR analysis showed the existence of reactive hydroxyl groups on the surfaces of both the hydrophilic and hydrophobic TiO2 particles. Exposure of the particles to ammonia indicated a large reduction in the hydroxyl groups as detected by in situ FTIR measurements. New peaks characteristic of N−H stretching bands indicated strong interactions between the ammonia and hydroxyl groups on the surface of the TiO2 particles.