(102d) Determination of Particle Surface Properties Using Hansen Dispersibility Parameters Demonstrated By Means of Carbon Black and Functionalized ZnO Nanoparticles

Keywords:Formulation, analytical centrifugation, dispersibility, particle functionalization, colloidal stability

The performance of small nanoparticles in various fields ranging from optoelectronic devices like solar cells or light emitting diodes (LEDs), pigments but also medical applications, largely depends on their surface chemistry and how well these particles can be embedded in an appropriate continuous phase [1-3]. Thus, formulation is absolutely decisive in terms of the later product performance [4-7]. However, to date no holistic approach to formulation of nanoparticles does exist which includes thermodynamics and kinetics of ligand binding on the molecular level, dispersibility and colloidal stability on the mesoscale, as well as product properties like optoelectronic device performance on the macroscopic level.

To overcome this limitation and to establish a novel approach for knowledge-based formulation, we introduce a new, standardized scoring method for HSP [8] determination of colloidal systems using analytical centrifugation (AC). AC quantifies optically in a direct and accelerated way the behavior of dispersed particles by concentration profiling over the entire sample height of up to 12 samples simultaneously [9, 10]. However, as for particles not dissolution but dispersion is in focus, we propose to use the term Hansen Dispersibility Parameters (HDP) instead of HSP whenever dispersibility and stability of particles against agglomeration are to be discussed.

First, we implemented a defined dispersion routine for the well-known, industrially highly relevant pigment carbon black (CB). Then, a standardized method for the evaluation of measured AC profiles and appropriate ranking of NPs in different liquids was developed. Noteworthy, in contrast to other approaches the final discrimination of “good” and “poor” liquids is not based on preset arguments but derived automatically within the HDP evaluation procedure. This is important to establish HDP as material property that can be analyzed and compared throughout different laboratories.

Then, we demonstrate outstanding reproducibility of our procedure by comparing HDP derived for the same CB material from independent experiments performed at two different affiliations by different experimenters. For further validation, we prove the predictive power of HDP and the accuracy of our approach by evaluating the dispersibility of CB in additional liquids and mixtures of so-called “good” and “poor” liquids crossing the border from stable to unstable conditions.

Finally, motivated by the enormous potential of AC to determine the HDP of colloidal systems using a standardized and non-subjective method to access particle interactions and colloidal stability, we applied the technique to surface functionalized, quantum confined ZnO semiconductor nanoparticles (quantum dots, QDs). Aim was to link mesoscopic findings on dispersibility and colloidal stability with interface properties at the molecular level. Noteworthy, for the molecular characterization, a well-validated toolbox of different methods that has been established by our group was applied [11, 12]. Thereby gained information on binding energy and in particular on surface coverage could be successfully linked to the particle level by dispersibility parameters, _δp (polar contributions), δ_d (disperse contribution) and _δh (ability to exchange electrons).

In conclusion, our hierarchical multiscale approach in combination with standardized HDP analysis allows establishing structure-property relationships between molecular features of individual ligand molecules, ligand binding to colloidal particles at the molecular level and dispersibility at the mesoscopic level. This is seen as a major step forward in the whole field of interface characterization and formulation as it paves the way from empiricism towards knowledge-based approaches.

Acknowledgements

The authors want to thank the funding of Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence “Engineering of Advanced Materials”. Moreover, we acknowledge the Federal Ministry of Economic Affairs through the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” e.V. (AiF, project no. KF 2347922UW4).

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