Amphiphilic titania Janus nanoparticles containing ionic groups prepared in oil–water Pickering emulsion

Abstract

Titania nanoparticles with a diameter of 8 nm underwent an anisotropic modification using apolar 6-bromohexylphosphonic acid and cationic polar N,N,N-trimethyl-6-phosphonohexan-1-aminium bromide. The Janus modification was achieved through a straightforward one-step Pickering emulsion approach using toluene–water mixtures. The resulting Janus particles were compared with isotropically and statistically modified titania particles, where either a single coupling agent is attached to the surface or both coupling agents are assembled over the surface randomly, respectively. The covalent binding of the phosphonic acids to the titania surface was confirmed by FTIR and ³¹P solid-state CP-MAS NMR analyses. The grafting density was assessed using TGA, elemental analysis, and ICP-MS, revealing grafting densities of 0.1 mmol g⁻¹ to 0.5 mmol g⁻¹ for the cationic coupling agent and 1.2 mmol g⁻¹ to 1.5 mmol g⁻¹ for the apolar coupling agent, respectively. ζ-Potential titration measurements of both pristine and modified particles revealed isoelectric points at pH 4.5 to 9.3, depending on the type of modification. The ability of the particles to stabilize Pickering emulsions was tested under various conditions, with statistically and Janus-modified particles demonstrating a significant increase in stabilization compared to their isotropically modified counterparts. Furthermore, Janus particles were deposited onto glass substrates by a simple layer-by-layer approach. Through the self-assembly of these Janus particles, the glass substrate's properties could be tailored from hydrophilic to hydrophobic to hydrophilic, depending on the dipping cycle.