Quantitative TGA distinguishing physisorbed and chemisorbed water in the ligand layer of iron oxide nanoparticles.

The details of the interfacial hydration of ligand-grafted nanoparticle surfaces depend on the chemistry and grafting density of the ligand molecules. Superparamagnetic iron oxide nanoparticles (IONPs) are used in ferrofluids and biomedicine, for example, where surface functionalization with citrate is of particular interest. Despite recent progress in quantifying citrate content, identifying and enumerating interfacial water remains challenging. In this study, we examine IONPs with a diameter of 7.4 ± 1.1 nm stabilised with either citrate or diethylene glycol with a coverage ranging from a monolayer up to 8 wt% ligand content. Thermogravimetric analysis (TGA) of IONP powders equilibrated at 30 and 75% relative humidity can be fitted consistently with two models: either the activation energy requires a distribution reflecting heterogeneity in sorption sites, or a reduction of the surface area available for evaporation near the end of the drying process. In either case, two distinct water species are identified with desorption energies of 29-33 kJ mol-1 and of 40-46 kJ mol-1, characteristic of physisorbed and chemisorbed water, respectively. In this case, chemisorbed water does not involve dissociation but rather consists of water which is hydrogen-bonded to functional groups of the ligand molecules. Vapour sorption isotherms confirm the TGA data, which were collected at a rate of 5 K min-1 to resolve the interfacial water species. Thus, widely available TGA experiments can provide quantitative relationships between surface chemistry and hydration behaviour.