We present a model to calculate particle size distributions (PSDs) of colloidal ZnO nanoparticles from their absorbance spectra. Using literature values for the optical properties of bulk ZnO and correlating the measurement wavelengths in the UV-visible regime with distinct particle sizes by a tight binding model (TBM), an algorithm deconvolutes the absorbance spectra into contributions from size fractions. We find an excellent agreement between size distributions determined from TEM images and the calculated PSDs. For further validation, bimodal PSDs have been investigated and an approach to determine not only particle size but also solid concentration is introduced. We will show the applicability of our model by the determination of temperature-dependent ripening rates, which enables the calculation of solubilities, surface tensions, and the activation enthalpy of ripening. In principle, our methodology is applicable to different semiconductor nanoparticles in various solvents as long as their bulk properties are known and scattering is negligible.

Incoherent nonlinear hyper-Rayleigh scattering (HRS) has been applied to monitor the different stages in the synthesis of sub-10 nm ZnO nanoparticles. During time-resolved investigation of the ripening process, we measured an increase in the particle hyperpolarizability, βZnO, with increasing particle size. Millisecond time resolution measurements revealed regions of prevalent nucleation, growth, and ripening. Using the size-dependent hyperpolarizabilities, a new general approach is introduced for the quantitative determination of nucleation, growth, and ripening rates via in situ HRS measurements. The derived solubility also allowed the determination of reasonable values of the ZnO surface energy.

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The suspension rheology of hematite in the ionic liquid Ecoeng TM. 212 was studied in detail and compared to the pure ionic liquid. This is the first report on the rheological behavior of suspensions in ionic liquids, and it is postulated that colloidal stability and rheology must be considered to understand these results, and to overcome limitations on the production of nanosized particles in industrial applications. Concentrated suspensions of particles in the nanometer range show non-Newtonian flow behavior including shear thinning and shear thickening. These phenomena are mainly caused by particle-particle interactions in the suspension, and control of these interactions is critical. The influences of temperature and solid concentration on flow behavior were shown for the pure liquid and the suspensions. It is seen that the ionic liquid follows the Arrhenius equation for non-associating electrolytes. It is possible to shift all hematite suspension curves to a master curve according to the model of Gleißle and Baloch. Furthermore, the flow behavior of the suspensions can be modeled with the well-known Herschel-Bulkley plot. A 10 wt % suspension of Fe 2 O 3 follows Newtonian behavior over the entire range, similar to the pure ionic liquid. It is believed that the ionic liquid has an influence on the stability of the particles, leading to a decrease of attractive particle-particle forces.

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