On the Phase Behaviour of Soft Matter: Understanding Complex Interactions via Quantitative Imaging

Sammanfattning: A first lesson in physical chemistry invariably starts with an example of water existing in three states, or phases - gaseous (water vapour), liquid (water from the tap) and solid (ice cubes). Such phase transitions are easily observed, but their underlying cause is not so evident. Atoms and molecules are difficult to follow due to their minute size and rapid movements. So, in order to investigate phase transitions, so-called colloids are often employed. Colloids are tiny particles ranging between 1-1000nm in size: their size is large enough, and their dynamics are slow enough to be detectable with various instruments, yet at the same time, their motion and interactions still resemble molecules and atoms. Like magnets, interactions can either cause colloids to move away from each other (repulsive interactions) or draw closer to each other (attractive interactions). The microscopic interactions between colloids drive their macroscopic phase transitions, and so by investigating the phase behaviour of colloids with different interactions, we can learn more about what triggers phase transitions in atomistic and molecular matter. In recent years, the phases and phase transitions of increasingly complex colloids have been researched.My thesis follows suit, and focuses on two vastly different systems, for which we try to predict phase behaviour based on the interactions between the colloids. The first system consists of so-called microgels, which are tiny, soft polymer networks. The microgel is comparable to a microscopic sponge saturated with water, which can be squeezed (relatively) dry. Changes in sample environment, for example an increased temperature, will lead to the squeezing out of water, which can also be done via the packing of many microgels in a small space. A swollen microgel softly repulses its neighbour, while a so-called collapsed microgel, i.e. a compressed sponge, experiences attractions. As a result, the macroscopic behaviour of a microgel sample will change significantly as the temperature is increased. The aim of my thesis is to find a model which correctly predicts the interactions between microgels as a function of temperature and packing fraction.The second leg of my thesis revolves around the interactions and phase behaviour of two proteins, lysozyme and γB-crystallin, which occur naturally in the body. Proteins - consisting of many different amino acids, each with their own specific interactions - can be considered as very complex colloids. A recurring theme in physical chemistry is therefore the attempt to describe proteins with models based on simpler colloids. In the absence of salt, lysozyme is slightly attractive but mainly repulsive, and these mixed interactions leads to the formation of clusters. There is an ongoing debate whether these clusters will eventually jam and stop moving with increasing protein concentration, i.e. transitions from a liquid state into a solid one. γB-crystallin possesses attractive patches on its surface. Under physiological conditions, these patches dominate its phase behaviour and at high enough concentration, a network is formed. Any simple colloid with an overall attractiveness - as opposed to patches - would not form such a network. Predictive theories describing at what concentrations such patchy particles will arrest, and how the concentration affects the macroscopic viscosity, are not readily available. The second aim of my thesis is therefore to experimentally explore the liquid-solid transitions for these two proteins.