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Electric charges exist not only in electrical appliances, but also in yoghurt, paint, skin cream, blood, and wet baby diapers. These are examples of soft matter containing both mobile charges (e.g., dissolved salt ions) and charges that are fixed on larger particles, such as proteins or polymers. The mobile and fixed charges are important for physical properties, for instance the colloidal stability of paints and food emulsions or the retention of water by diapers. We will develop a sensor to measure mobile and fixed electric charges in soft matter, for science, product development, and quality control of soft matter.

The observation of phase separation in aqueous solutions of proteins and polysaccharides offers the prospect of stable water-in-water emulsions, with droplets of an aqueous phase B dispersed in a continuous aqueous phase A of different composition. On the one hand, such emulsions are of fundamental importance, because little is known about the interfacial physical chemistry of coexisting aqueous polymer solutions. On the other hand, if such emulsions could be made sufficiently stable, they would also be of great commercial interest as a way to texturize water. Possible applications include next generation fat-free health foods and water-dispersible microcapsules with an aqueous content. The ultimate aim of the project proposed here is the realization of water-water emulsions that are kinetically or even thermodynamically stable. Kinetic stability would already enable applications. Thermodynamic stability would mean furthermore that finely divided emulsions would spontaneously form in mixtures of aqueous solutions, reproducibly and with minimal energy input. Our approach is to pay special attention to the stabilizing effect of nanoparticles or larger colloids adsorbed at the water-water interface. If stable emulsions are realized in this way, they could be called "Pickering emulsions". We propose to study the effect of chemical composition on interfacial tension, interfacial capacitance, interfacial electrical potential, and emulsion stability. Our hypothesis that will guide our experiments is that an electrical potential difference exists at the interface, due to the Donnan effect, and that charged colloids could be confined at the interface using that local potential difference. Several innovating approaches will be explored and adopted. To study changes in interfacial capacitance, differences in electrical impedance will be measured using a sensitive homebuilt setup. To measure the interfacial electrical (Donnan) potential difference, we will use reference electrodes with salt bridges of variable concentrations. To measure colloidal adsorption at the water-water interface, different procedures will be attempted: (a) Gibbs adsorption equation analysis of the concentration-dependent interfacial tension from spinning drop measurements, (b) in situ dark-field optical microscopy and cryogenic electron microscopy, and (c) analytical centrifugation of colloidal transport through the liquid-liquid interface, where the colloids may remain temporarily trapped. In parallel with the interfacial analyses, the stability of emulsions will be studied by optical microscopy, light scattering, and analytical centrifugation. This is a new exciting subject for the principal investigator, who currently researches colloidal dispersions of magnetic and semiconducting nanoparticles, but who also has a PhD degree in electrochemistry, teaches physical chemistry of liquids and basic principles of colloid science, and has extensive experience with several techniques mentioned above. The co-applicant is senior researcher at NIZO Food Research. He is detached one day a week as an associate professor in the same Utrecht University group as the principal investigator. From his NIZO position he has been actively seeking new collaborations with universities to explore and to develop the properties of water-water interfaces for applications in food, health care, and waste stream valorization. The work will be carried out mainly at Utrecht University, in a well equipped laboratory and in the vicinity of researchers with longstanding experience in colloid science, where for instance thermodynamically stable oil-in-water Pickering emulsions were recently discovered. Through the co-applicant, essential collaboration will occur with NIZO Food Research. Specialized experimental setups will be available at NIZO, such as microscopy of systems under flow and diffusing wave spectroscopy for particle dynamics in turbid solutions.

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Proteins are able to assume different conformations, depending on their physical environment such as pH, ionic strength and the proximity of other molecules. The ability of proteins to switch between conformations is the basis of allostery, a collective effect that can lead to specific binding and a sensitive dependence on ligand concentration, such as in the binding of oxygen by hemoglobin. In the field of virology, there is strong evidence that the formation of virus capsids, shell-like aggregates that consist of tens to hundreds of copies of (often) the same capsid protein, coincides with a conformational change of the capsid proteins. Such a conformational switch in principle avoids long-lived intermediates (kinetic traps) and leads to strong hysteresis, which avoids that intact viruses rapidly fall apart once the coat protein concentration is below the critical aggregate concentration. However, it is very hard and perhaps even impossible to study the effect of conformational switches directly in proteins without altering other properties at the same time. In this proposal the goal is to create and study multiblock copolymers that have, by design, a well-defined and controlled conformational switch. The reason for choosing multiblock copolymers is that nowadays, in polymer chemistry, there hardly are fundamental limitations in terms of the number of the different blocks and their chemical nature that can be linked together. We address the question what the (quantitative) role is of conformational switches in: (1) the stability as well as the assembly and disassembly pathways of shell-like structures; (2) the ability to encapsulate cargo; and (3) the combined effects with so-called ‘origins of assembly’ with respect to specific encapsulation. This last effect is believed to be the basis for specific encapsulation of viral RNA in a mixture with non-specific RNA. If successful, this work will lead to a better understanding of the role of conformational switches in self-assembly in general, and opens up possible applications in material science as well as in the formulation of new types of drug and gene delivery vehicles.