Tunable Phase Separation in Polyguanidinium Solutions: Unraveling the Role of π-π Interactions in Aqueous System

Liquid-liquid phase separation (LLPS) happens when a solution that initially looks uniform suddenly splits into two separate liquid phases. One part ends up rich in polymers, while the other has much fewer of them. This kind of phase separation is not just a laboratory curiosity; it plays a key role in biological systems, helping cells form structures like membrane-less organelles. It is also valuable in materials science, where it is used to create self-assembling polymers and smart materials that respond to different stimuli. Among the many ways polymers behave in solution, upper critical solution temperature (UCST) phase separation is one that attracted scientists and engineers which happen when the temperature drops below a certain point, the polymers stop mixing well with the solvent and separate into distinct phases. What makes UCST behavior especially interesting is that it depends on specific interactions between polymer chains. Unlike lower critical solution temperature (LCST) systems, where increasing the temperature makes the polymers less soluble (often due to water molecules being forced out), UCST systems do the opposite. Here, the polymer chains attract each other more strongly as the temperature drops, leading to phase separation. Figuring out how these interactions work is important because it could help in designing new materials for drug delivery, biomimetic coatings, and even artificial protein condensates. Despite its potential, there are some big challenges in using UCST behavior effectively. For one, not all polymers show this kind of phase separation under practical conditions. Many require very specific salt levels or need complex chemical modifications that make them difficult to produce. Another challenge is that the thermodynamics behind UCST transitions in water-based systems is still not fully understood. Traditional models explain phase separation based on simple interactions between molecules, but real-world polymer systems involve electrostatic forces, hydrogen bonding, and π-π stacking interactions, all of which complicate things.

To fill this important gap, Professor Soo-Hyung Choi and postdoctoral fellow Dr. Seung-Hwan Oh from Hongik University in South Korea conducted a study that was recently published in Macromolecules Journal. They focused on polyguanidinium-based polyelectrolytes and investigated how π-π stacking interactions between guanidinium groups influence UCST phase behavior. Their idea was that these interactions, when tuned by salt concentration, could act as a key trigger for phase separation. Unlike traditional polyelectrolytes, which rely mostly on electrostatic repulsion, guanidinium-rich polymers bring an extra layer of attraction, making them an exciting option for use in biomolecular condensates, salt-sensitive hydrogels, and complex coacervates. The researchers prepared polyguanidinium solutions at different concentrations and slowly lowered the temperature, keeping an eye on how much light passed through the samples. As expected for a UCST system, they noticed that as the temperature dropped, the solutions became cloudy, signaling that the polymers were separating into two liquid phases. They carefully recorded the cloud point temperature (T_cp)—the temperature where this turbidity first appeared—at different salt concentrations. They found that adding more monovalent salt (NaCl) pushed T_cp higher and confirmed that salt concentration plays a major role in controlling associative interactions, especially π-π stacking between guanidinium groups. The stronger these interactions were, the higher the temperature at which the polymers aggregated and separated from the solution.

However, simply measuring when the solutions became cloudy was not enough to explain what was happening on a molecular level. The authors performed SANS to examine how polymer chains were structuring themselves at different temperatures and salt levels. When SANS sends neutrons through the samples and analyse the scattering patterns which demonstrated as the temperature decreased, the polymer chains packed together more densely, forming a dynamic network of interactions. This was directly linked to the π-π stacking of guanidinium pairs, which became even stronger when salt reduced the usual repulsion between charged groups. A key observation was that the correlation length (ξ), which measures how far these structural changes extend, increased sharply near the phase transition, further proving that salt was actively influencing polymer interactions. To tie these structural changes to thermodynamic properties, the authors used SLS to measure the second virial coefficient (A₂) and the theta temperature (T_θ)—a point where polymer-polymer and polymer-solvent interactions balance out. Their results showed that as salt concentration rose, A₂ decreased, meaning the polymer chains experienced less repulsion and stronger attraction. More significantly, T_θ increased in a straight-line fashion with salt concentration, confirming that intermolecular attractions were becoming more dominant than solvation effects. This was strong proof that phase separation in these solutions is driven by enthalpic forces—in other words, the attraction between polymer chains is the main reason they separate, rather than entropy-related effects.

To quantify these interactions, they analyzed the effective interaction parameter (χ_eff), which describes whether polymer-polymer interactions are stronger or weaker than polymer-solvent interactions. Their random phase approximation (RPA) analysis revealed that χ_eff increased sharply with salt concentration and decreased with temperature—a trend perfectly matching UCST behavior. One of their most exciting discoveries was that the enthalpic part of χ_eff (which corresponds to polymer-polymer attraction) increased with salt, while the entropic part (linked to polymer-solvent mixing) dropped.

In conclusion, the research work of Professor Soo-Hyung Choi and Dr. Seung-Hwan Oh showed for the first time that these molecular-level interactions are just as important as other forces when it comes to determining phase behavior and precisely tune UCST behavior by adjusting salt concentration. Moreover, in biotechnology many critical biochemical processes, such as stress granule formation and RNA-protein interactions, depend on dynamic phase separation. With the understanding how guanidinium-based interactions influence these transitions, scientists now could gain new information into how cells regulate biomolecular assemblies. Indeed, it could even contribute to the development of therapies for neurodegenerative diseases, where abnormal protein aggregation such as beta amyloid is a major problem. Additionally, the new study has huge potential in materials science. The fact that salt concentration and temperature can be used to control phase transitions makes these materials perfect candidates for self-healing hydrogels, adaptive coatings, and targeted drug delivery systems. For example, in medical applications, tweaking salt conditions could control the release of bioactive molecules, ensuring they reach precise locations in the body at just the right time. In paints, adhesives, and emulsions industry that rely on complex fluid formulations, phase behavior directly affects stability and performance, which make it important factor in product design and longevity. Furthermore, by taking advantage of UCST phase transitions that are driven by molecular interactions, engineers could develop materials that withstand extreme conditions, for instance marine coatings that resist saltwater degradation or biomedical implants that remain stable in the human body.