Investigation of the Degree of Functionalization and Colloidal Stability of Shell-by-Shell Functionalized TiO2 Nanoparticles as a Function of Different Phosphonic Acid Chain Lengths

In this work, a series of Shell-by-Shell (SbS)-functionalized colloidal systems consisting of 6 nm TiO2 anatase nanoparticles (NPs), one of the phosphonic acids (PAs) propylphosphonic acid (PAC3), hexylphosphonic acid (PAC6), dodecylphosphonic acid (PAC12), tetradecylphosphonic acid (PAC14), hexadecylphosphonic acid (PAC16), octadecylphosphonic acid (PAC18) and the amphiphile sodium dodecylbenzenesulfonate (SDBS) were prepared, resulting in TiO2-PACX@SDBS (X = 3, 6, 12, 14, 16, or 18) NPs dispersed in deionized water (DIW). During the whole functionalization process, the NPs were subjected to thermogravimetric analysis (TGA) to gain insights into their degree of functionalization and respective thermal stability. In addition, the colloidal stability of the NPs as a function of PA chain length was analyzed by analytical ultracentrifugation (AUC). By combining both, TGA and AUC, it was found that the resulting hierarchical NP architectures form agglomerates, with the degree of agglomeration depending on the length of the PA used for the first-shell functionalization. This allows conclusions to be drawn about the efficiency of the overlap between the PA and the SDBS carbon chain.