(474f) Design, Synthesis, and Structure-Function Relationship of Amphiphilic Organic Ligands for Stable Nanoparticle Dispersions in Various Solvents

Inorganic nanoparticles such as metals, semiconductors, and oxides have recently been recognized as promising functional materials which have physicochemical properties, including electrical, optical and mechanical properties. For example, polymer nanocomposites are one of the most attractive applications using nanoparticles. In order to take advantage of the unique property of nanoparticles, it is necessary to obtain stable dispersion of nanoparticles in several solvents, hopefully in high concentration. In most cases, the colloidal stability is achieved by modifying nanoparticles with a monolayer of organic ligands. However, the choice of suitable organic ligands for each solvent and particle generally requires trial-and-error approaches and can be problematic. In this context, we found that TiO₂ nanoparticles modified with a commercial phosphoric acid surfactant can be dispersed in both polar and less-polar organic solvents and applied this surfactant to prepare several stable colloidal dispersions. The surfactant has hydrophobic alkyl chain and hydrophilic ethylene glycol chain, which might be a key for the dispersion stabilities. However, the relationship between molecular structure and colloidal stability is unclear and the structure is not optimized for better dispersion. In this study, we designed and synthesized a series of novel amphiphilic organic ligands and used them to modify TiO₂ nanoparticles as a model. The resulting modified TiO₂nanoparticles were then dispersed in several organic solvents to study the relationship molecular structure and colloidal stability.

ã?Experimental methodã??

We designed the amphiphilic organic ligands with different lengths of alkyl and ethylene glycol chains and a phosphoric acid group as an anchoring moiety. A series of the ligands were synthesized in 3 steps from commercially available alkyl bromides. The ligands structures and purities were characterized by NMR analysis. TiO₂nanoparticle aqueous dispersion was purchased from Ishihara Sangyo Kaisha, Ltd. (STS-100, primary particle size is 5 nm) and were modified with the synthesized ligands by mechanical stirring. The modified nanoparticles were aggregated in the aqueous solution, which were then collected by centrifugation and washed with water to remove free ligands and dried in vacuum. The resulting modified nanoparticles were characterized by elemental analysis and were used for dispersion study in a wide variety of organic solvents using dynamic light scattering measurement.

ã?Result and Discussionã??

We found that the loading amount of all synthesized ligands were approximately 5 μmol/m², suggesting that the stable monolayers were formed through the phosphoric acid-TiO₂ bonding. It should be noted that the excess addition (10 μmol/m²) of the ligands caused clear aqueous solutions, while white aggregations were formed when lower amounts were used. This could be explained that the bilayers were constructed at the surface of TiO₂nanoparticles to show ionic phosphoric acid group, rendering the modified particles hydrophilic. The recovered modified particles were then dispersed into several organic solvents at 3.0 wt% using sonication. We confirmed that the dispersions were stable and the mean particle sizes were estimated to be 10-20 nm by DLS measurements in most organic solvents used. We found that the slight structural differences significantly affected the colloidal dispersion stabilities, which will be discussed in details in the presentation.

ã?Conclusionã??

The novel amphiphilic organic ligands were designed and the synthesis routes were established. We found that the slight structural differences significantly affected the colloidal dispersion stabilities, leading to the optimization of the structure. The modified TiO₂ nanoparticles were nicely dispersed in various organic solvents at useful concentrations of 3.0 wt%.