Surface-modified transition metal oxide nanoparticles, method of making the same, and curable composition and article including the same

A method of making surface-functionalized transition metal oxide nanoparticles, the method comprising:

• providing a sol comprising transition metal oxide nanoparticles dispersed in an aqueous liquid medium having a pH of less than or equal to 3.5 and stabilized with at least one mineral acid;
• combining the sol with an extractant composition comprising at least one carboxylic acid and at least one organic amine, and a water-immiscible organic solvent, wherein:
• each one of the at least one carboxylic acid is independently represented by the formula R1—CO2H, wherein R1 is a hydrocarbyl group having from 10 to 12 carbon atoms;
• and
• each one of the at least one organic amine is represented by the formula R2R3NH, wherein R2 is a hydrocarbyl group having from 6 to 30 carbon atoms, and R3 is H or an alkyl group having from 1 to 4 carbon atoms, and wherein the at least one organic amine is present in at least an amount sufficient to neutralize acid present in the sol, thereby forming discrete separable aqueous and organic phases;
• and
• separating at least a portion of the organic phase from the aqueous phase and then at least partially removing the water-immiscible organic solvent from the separated organic phase to obtain at least a portion of the surface-functionalized transition metal oxide nanoparticles.