2024 Frieder Mugele

Frieder Mugele

Frieder Mugele (Twente)

Frieder Mugele has a long standing experience in nanoscale characterization of colloidal interaction forces using atomic force microscopy. Recent improvements of the method in combination with molecular dynamics (MD) simulations by his group have enabled deeper understanding into charge generation and charge regulation, as well as the hydration structure of aqueous electrolytes at solid surfaces.

The first paper fof the nomination is on his study on the hydration of mica in different aqueous solutions at moderate and high salt concentrations (1). In this study he described hydration force by a superposition of a monotonically decaying and an oscillatory part. His studies of adsorbing cations demonstrate the impact of the hydration energy of the ions on the hydration forces. Frieder Mugele’s team demonstrated the transition from overall repulsive to more attractive forces when reduc- ing the hydration of the cations from Li+ over Na+, Rb+ to Cs+. Later, Frieder Mugele showed that the main results are also valid for other surfaces such as silica (2).

Frieder Mugele continued these studies on hydration forces and analyzed the different facets of colloidal particles. Crystalline colloidal particles often have different facets. Even when similar facets repel each other, different facets may attract each other. To verify this effect, Mugele and his group analyzed facetted colloidal nanoparticles (3,4). Measurements on semiconducting nanoparticles of SrTiO3 and BiVO4 at variable pH demonstrated that adjacent facets with different crystallographic index usually display different local surface charge densities and a different hydration structure. This leads to a difference in isoelectric point of up to 2 pH units (sometimes) between adjacent facets, which was shown to enable facet-selective colloidal heteroaggregation.

Upon illumination of BiVO4, the different charge densities facilitate the separation of photo-generated charge carriers and are thereby expected to enhance the photocatalytic activity of these nanoparticles.

The last paper in this nomination concerns Frieder Mugele’s work on long-range electrostatic interactions at high salt concentrations (5). In the last decade there have been reports by the groups of Valtiner and Perkin about long range interactions between solid surfaces in concentrated salt solutions. These reports have attracted intense attention, because such long-range forces contradict current theory. Frieder Mugele’s measurements demonstrated that such long-range force do not exist and are more likely the results of experimental artefacts. Demonstrating that spectacular effects may not be real is not rewarding; it is usually a lot of work, and the results are difficult to publish in high-ranking journals. Still, it is very important work that prevents other scientists from wasting resources in futile studies.

With all these insights, Frieder Mugele has contributed substantially to our knowledge of colloidal interactions and of the structure of salt solutions near interfaces.

(1) S.R. van Lin, K.K. Grotz, I. Siretanu, N. Schwierz & F. Mugele, Ion-specific and pH- dependent hydration of mica-electrolyte interfaces, Langmuir 2019, 35, 5737-5745.

(2) Siretanu, S.R. van Lin & F. Mugele, Ion adsorption and hydration forces: a compari- son of crystalline mica vs. amorphous silica surfaces, Faraday Discussions 2023, 246, 274-295.

(3) S.Q. Su, I. Siretanu, D. van den Ende, B. Mei, G. Mul & F. Mugele, Facet-dependent surface charge and hydration of semiconducting nanoparticles at variable pH. Adv. Ma- terials 2021, 33, 2106229.

(4) S.Q. Su, I. Siretanu, D. van den Ende, B. Mei, G. Mul & F. Mugele, Nanometer- resolved operando photo-response of faceted BiVO4 semiconductor nanoparticles. J. Am. Chem. Soc. 2024, 146, 2248-2256

(5) S. Kumar, P. Cats, M.B. Alotaibi, S.C. Ayirala, A.A. Yousef, R. van Roij, I. Siretanu & F. Mugele, Absence of anomalous underscreening in highly concentrated aqueous elec- trolytes confined between smooth silica surfaces. J. Colloid Int. Sci 2022, 622, 819-827.