Workshop Sponsor & Co-organizer:

For the detailed schedule and registration, please visit the external workshop website: https://www.esi.ac.at/events/e571/

*** More information will follow in early January 2025. ***

Motivation:

Charged soft matter entails a broad variety of systems, sharing one common feature: they contain charged macromolecules or colloidal particles, typically few nanometers in size, accompanied by salt ions and other charged small molecules, and polar solvents. The relevant systems range from cellular membraneless organelles to protein solutions, bio-inspired or synthetic polymers, inverse patchy colloids or polymer-electrolytes found in modern battery systems. Although very different in nature, these systems share the same physics, predominantly controlled by interplay of electrostatic interactions and thermal fluctuations.

Unfortunately, most of the research in the field of charged soft matter is carried out in communities centered mainly around the target systems named above, allowing only limited exchange across these communities. This leads not only to differences in nomenclatures and concepts, but also to reinventions and rediscoveries of phenomena.

The aim of this workshop is to provide an interdisciplinary platform for scientific exchange between these different communities investigating charged soft matter, underlining the shared aspects of the involved physics.

Systems of interest

In this workshop, we will cover the following classes of systems:

C1: charged colloids, proteins and viruses

This class encompasses synthetic colloidal particles with charged cores [14], Janus particles [15] or heterogeneously charged patchy colloids [2], all of which can exhibit rich phase behavior, clustering or self-assembly. The possession of charges, their distribution on the colloids, but also salt conditions in the colloidal suspensions pose a versatile playground for tuning the effective interactions between the colloids [17], which in turn regulate their collective behavior. Recently, it has been proposed that charged colloids can be also used as a model for globular charged proteins [30], selected viruses [20] or monoclonal antibodies in suspensions [10], as all of these biological objects contain charged regions in geometries akin to synthetic colloids.

C2: polyelectrolytes and polypeptides

This class of materials includes polymers, both synthetic and biopolymers, such as DNA or polypeptides, carrying charged functional groups [19, 7]. While colloids and globular proteins are typically rigid objects, polyelectrolytes are flexible molecules, which can attain many different conformations. In contrast to colloids, intramolecular entropy and intramolecular interactions play a major role in polyelectrolyte systems, bringing about unique phase transitions and different modes of dynamics as compared to the colloids. The interplay of flexibility and charge can be even used as a design motif in DNA nanotechnology [24] or as a means to regulate the structure of DNA in concentrated solutions akin to viral genome [28].

C3: intrinsically disordered proteins

Intrinsically disordered proteins have entered the spotlight of biochemical research only in recent years. In the past, they have been neglected because they did not fit the established paradigm of lock and key mechanism, relating the protein structure and function. Similar to globular proteins, the disordered ones are built from unique amino acid sequences. Yet, their conformational flexibility makes them similar to polyelectrolytes and synthetic polymers. Therefore, classical polymer theories can describe the structure of individual disordered proteins [13] whereas complexation of highly charged disordered proteins can be described using the same arguments as complexation of polyelectrolytes [6]. Although interactions in disordered proteins are more diverse than in colloids or synthetic polymers, disordered proteins can be considered as a bridge between the previous two classes.

Phenomena of interest

The sessions of the workshop will be centered around the physical phenomena shared by the aforementioned classes of systems:

P1: Liquid-liquid phase separation and coacervation

Mixing of oppositely charged polyelectrolytes results in phase separation, driven by the entropy of the released counterions, yielding either solid polyelectrolyte complex, or a liquid coacervate [32]. This phenomenon has been systematically investigated using synthetic peptides with custom sequences of acidic (anionic), basic (cationic) and neutral side chains [5, 12], later described using theories previously developed for synthetic polymers [25]. Phase separation is one of the key factors determining the spontaneous compartmentalization in biological systems, leading to the formation of membraneless organelles [9] and enabling selective encapsulation of proteins or other charged solutes not only in biology, but also in artificial bio-inspired systems [3]. The main goal of this session is to stimulate the discussion of how the systematic understanding of phase separation and coacervation could be achieved in systems of increasing complexity, up to the level of biological condensates.

P2: Self-assembly, structure formation, supramolecular phenomena

Utilizing interactions between charged moieties to form supramolecular structures is a commonly used motif in self-assembly. Oppositely charged blocks of copolymer polyelectrolytes attract each other, forming complex coacervate core micelles [33] in dilute conditions and diverse mesoscopic structures from networks to lamellae or cylindrical phases in concentrated solutions [27]. On the other hand, charge-driven colloidal assembly can lead to formation of ionic solids [14] or cluster crystal phases [29]. While the qualitative ideas behind the above mechanisms are known, the control over this behavior is not quantitative. The main goals of the sessions within this topic are to discuss how to design supramolecular assemblies from charged soft matter, how to tune the emergent structures, and how to control the processes using external fields or other stimuli.

P3: Dynamics, rheology, charge transport and non-equilibrium phenomena

The theoretical understanding of the dynamics of polyelectrolytes and charged colloids in solutions is still an open problem, complicated by the presence of several length scales of hydrodynamic, electrostatic and topological screening [16]. In concentrated phases, polyelectrolytes typically exhibit only weak entanglements and their dynamics is rather fast [8] in spite of crowding, while colloids can exhibit slowdown and features of glassy dynamics. Our main goal within this topic is to shed light on the fundamental principles of dynamics of charged soft matter, and establish parallels between charge transport in natural systems of intrinsically disordered proteins [8], in ionomeric melts [11] and polyelectrolytes in redox-flow batteries [1].

P4: Structure and function of soft matter in nature and technology

The central paradigm in molecular biology assumes that the amino acid sequence in proteins determines their structure (spatial organization), and thereby their biological function. This paradigm has been challenged by recent research on disordered proteins and biological condensates. On the contrary, colloid and polymer science have traditionally worked with simple particles or chains, composed of repeating identical functional groups, achieving the desired function by tuning the connectivity and shape. Complexity has been introduced in the sequel by using multi-block copolymers, functional
side-chains or patchy colloidal particles. Only very recently, colloid theories have been extended to account for structural features [10, 22], bridging the concepts of molecular biology with those of colloid and polymer science. Simultaneously, the function of disordered proteins has been addressed using the concepts from colloid and polymer science [4, 6, 12, 13]. The goal of this session is to foster this emerging field of research by providing a discussion platform for researchers attempting to bridge these conceptually different approaches to structure and function.

P5: Interactions, charge regulation, responsive and programmable matter

The amount of charge on proteins, colloids or polyelectrolytes can be changed by the value of pH in the solution. It is further modulated by a response to changes in the local environment, termed charge regulation. In simple colloids and proteins, charge regulation has been described using the mean-field formalism, based on the Poisson-Boltzmann equation [18, 21]. The same principles apply to charge regulation in polyelectrolytes and short peptides, albeit their conformational flexibility makes the interactions more complex. In a suitably designed system, charge regulation can trigger interactions under specific conditions [23], at a desired location or in time [26]. In the context of biomaterials, a similar effect is termed nanobuffering: a simultaneous change of a charge and conformation of a macromolecule to counterbalance changes in its local environment [31]. The last example underscores the fact that different terminology is used to describe the same physics, depending on the community. Nonetheless, controlling the interactions is the key to controlling the structure, function, phase stability and also dynamical properties. The goal of this session is to establish a strong connection between the control of electrostatic interactions and the phenomena covered in the previous topics.