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Niels J. H. Boon: „Ionic screening and charge regulation of heterogeneous surfaces”
University of Utrecht, The Netherlands

The class of anisotropic colloidal (nano)particles is extremely versatile nowadays due to enormous advances in the colloidal synthesis. Anisotropic particles break the rotational symmetry of "classical" spherical particles by e.g. protrusions or an inhomogeneous ("patchy") distribution of chargeable molecules over the surface. The so called "Janus"-particle, with two differently charged hemispheres, is a well-known example. The electrostatic interaction between Janus particles is often totally different from what DLVO theory would predict. In this talk we extend the traditional monopole-monopole interaction towards monopole-dipole, dipole-dipole, etc, taking into account the nonlinear coupling of the electrostatic multipoles.

The electrostatic interaction of colloidal particles is not always described properly by a fixed surface charge; one often needs to account for the surface chemistry by charge regulation. This has important consequences for patchy particles. We couple charge regulation mechanisms to patchy surfaces to calculate inhomogeneous charge distributions on the surface. We find (i) that small patches do often acquire a relatively high amount of charge and (ii) a large regime of parameters for which charge regulation induces a repulsion/attraction cross-over at a distance that is governed by the patch size.