Organic modification of ultrafine particles using carbon dioxide as the solvent

An organic modification of the surface of titanium dioxide particles precoated with SiO2 and Al2O3 films in a CO2 medium is performed, where CO2 of different phases is used as the solvent and a titanate coupling reagent (CH3)2CHOTi(OP(O)(OH)OP(O)(OC8H17)2)3 is the modification reagent. The results are compared with the modification carried out in a conventional organic solvent. The interaction between the functional groups of the modification reagent and the hydroxyl groups on the surface of the particles is analyzed. Experimental results indicate that the surface of particles is modified efficiently when the CO2 solvent is in a supercritical or liquid state. The particles are remarkably changed from hydrophilic to hydrophobic by the modification. A chemical bond of AlOTi is likely formed between the molecules of the organic modification reagent (CH3)2CHOTi(OP(O)(OH)OP(O)(OC8H17)2)3 and the particle surface. The actual maximum quantity order of organic modification reagent reacted on the particle surface is about 7.08×10−7 mol/m2 (1.73% by weight) from thermogravimetric analysis. The organic modification process using supercritical or liquid CO2 as the solvent has the advantages of being simple, effective, and green.