A Monte Carlo determination of the effectiveness of nanoparticles as spacers for optimizing TiO2 opacity

Optimal TiO2 opacity is achieved when the TiO2 particles are well spaced. Nanoparticle extenders are claimed to improve TiO2 spacing by positioning themselves between the larger TiO2 particles (0.25 mu m) and preventing the TiO2 particles from approaching or touching one another. Claims have been made that this can as much as double the light-scattering efficiency of TiO2. This concept was tested by a combination of Monte Carlo simulations and image analysis techniques to determine computationally how the effectiveness of this approach is affected by TiO2 concentration, nanoparticle concentration, and nanoparticle size. Surprisingly, the results showed that nanoparticles did not, in fact, improve the spacing of TiO2 particles under any of the conditions examined (specifically, in the absence of any inter-particle interactions). Instead, TiO2 distributions and spacings were completely indifferent to the presence of smaller particles (however, large extender particles were found to cause TiO2 crowding, consistent with expectations and experiments). Explanation and implications of these findings are discussed.