A Straightforward Way To Form Close-Packed TiO2 Particle Monolayers at an Air/Water Interface

The aim of this study was to analyze if and how monolayers of TiO2 particles could be directly formed at the air/water interface and if these monolayers could be transferred to a solid surface. TiO2 particles with diameters of 300 nm, 500 nm, 1 mu m, 5 mu m, 10 mu m, and 20 mu m formed stable monolayers at pH 2. At low surface pressures, the particles formed small two-dimensional aggregates. Particles up to a radius of 5 mu m displayed close packing at increased surface pressures. Particles of 10 mu m radius formed a loose network, which is attributed to the strong adhesion caused by the weight-induced lateral capillary attraction. Every monolayer of particles could be transformed to a solid surface by the Langmuir-Blodgett deposition. At pH 6 or 11, the particles did not form stable monolayers at the air/water interface. They were instead dispersed in the aqueous phase and eventually sank to the bottom of the trough. At pH 11 the monolayer could, however, be stabilized by the addition of salt (0.5 M NaCl). The results are interpreted based on a changed wettability of the particles depending on pH and salt concentration.