- Hydrophobic titanium dioxide (TiO2) nanoparticles (NPs) are prevalent in industry and manufacturing. They are known for their white pigmentation, are found in sunscreens, paints, cosmetics, and are frequently used in wastewater treatment. Understanding the physicochemical properties of TiO2 P-25 NPs, such as hydrophobicity, are essential when determining their fate, transport, and impact in natural systems. Standard methods of testing hydrophobicity (i.e. contact angle or octanol water partitioning) are ineffective for NPs. Dye binding assays with hydrophilic and hydrophobic dyes may offer an effective alternative comparable to Kow; through the ratio of linear adsorption slopes (β), β(hydrophobic dye)/(hydrophilic dye).
Four water samples from varying locations along the Alsea River were collected and incubated with TiO2 P-25 NPs. A dye assay utilizing known hydrophilic (Nile Blue) and hydrophobic (Rose Bengal) dyes was used to characterize the hydrophobicity of the NPs after incubation. It was found that TiO2 P-25 NPs, which are known to be hydrophobic in ultrapure water, are hydrophilic in brackish wasters close to the coast. Significant differences in TiO2 P-25 NP hydrophobicity (p≤0.05) were observed after incubation in all four water samples when compared to Milli-Q water results.