Project

surface forces; particle aggregation; atomic force microscopy; light scattering; particle deposition; multivalent ions; adsorption

Lead
Lay summary
Surfaces of minerals, colloidal particles, or microorganisms interact by various forces, mostly of electrostatic origin. These forces are essential in controlling the known behavior a wide variety of systems (e.g., rivers, soils, living organisms) and processes (e.g., water treatment, papermaking, drug delivery). While the control of these forces is essential in numerous disciplines, we are basically unable to predict their magnitude, and sometimes even cannot rationalize their origin. The presence of multivalent ions influences these forces dramatically. This is the reason why trivalent aluminium or iron ions are used for water purification, for example. But the mechanisms how these ions influence the interaction forces continue to be debated. One group of scientists asserts that the interactions are governed by ion-specific chemical forces, while others suspect that generic aspects of electrostatic interactions are responsible for the observed behavior. The present project aims at clarifying this controversy, and will elucidate adsorption and interactions of colloidal particles in the presence of multivalent ions. Numerous state-of-the-art techniques will be used, such as, light scattering, electrophoresis, and electrochemical techniques. In particular, direct force measurements between individual particles will be carried out with the atomic force microscope. The experimental results will be confronted with current theories of interaction forces with the aim to indentify the principal mechanisms. This project is expected to have impact in several applied fields, particularly, where adsorption processes and aggregation of colloidal particles are important. The clarification of the underlying mechanisms of the interaction forces will have implications in the development better strategies in numerous fields, for example, water purification, drug delivery, ro environmental remediation.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

Multivalent ions strongly modify interactions between charged interfaces, and therefore such ions represent important additives to control aggregation or deposition of colloidal particles. For example, multivalent metal ions are used in waste water treatment to accelerate particle aggregation, or they play an essential role in cement hardening. While multivalent ions were already addressed in the classical theory of colloidal aggregation by Derjaguin, Landau, Verwey, and Overbeek (DLVO), this topic is receiving renewed attention due to two recent developments. The first development relates to new theories of the electrical double layer. It is now firmly established that the classical Poisson-Boltzmann (PB) may fail for multivalent ions, and that such ions can adsorb on oppositely charged surfaces due to electrostatic interactions so strongly that the surface may reverse its charge. In the past, it was thought that such a charge reversal is only possible if the adsorption of the ions is controlled by ion-specific forces. Moreover, these theories show that in the presence of multivalent ions, additional non-DLVO attractive forces become relevant and these forces are likely to be important in colloidal aggregation and deposition.The second development concerns important advances in experimental techniques able to probe interaction forces between colloidal particles or between such particles and surfaces. One of these techniques is the colloidal probe method based on the atomic force microscope (AFM). Moreover, one can reliably measure aggregation rate constants with scattering techniques, while particle deposition can be studied by surface sensitive methods. With these techniques at hand, such processes can be investigated experimentally with confidence.The present project builds on these developments, and addresses adsorption of multivalent ions on water-solid interfaces, resulting interactions forces, and consequences for aggregation and deposition phenomena. Adsorption of multivalent ions will be studied on well-defined colloidal latex particles with electrophoresis, cross-flow filtration, and potentiometry. Forces between these particles will be directly measured with the AFM, while their aggregation rates will be probed with light scattering. Situations involving asymmetric particle systems will be studied similarly. Interaction forces between particles and planar substrates will be also assessed with AFM, while the corresponding deposition processes will be followed with optical reflectivity and quartz crystal microbalance. These processes will be modeled with classical surface complexation models and DLVO theory. Comparison with modern theories of the electrical double layer will be achieved through collaborations with groups from other universities.The main goal of the project is to elucidate whether interactions in the presence of multivalent ions are principally governed by generic electrostatic interactions or by ion-specific forces. Furthermore, the origin of additional non-DLVO attractive forces will be clarified, and it will be assessed whether these forces may explain the known discrepancies of DLVO theory in aggregation and deposition processes.