Physicists Develop Theoretical Model of Polymer Coacervation

Coacervation is a process by which a polymer undergoes condensation in a solution, resulting in the formation of small, insoluble droplets. This simple yet effective method of working with polymers finds widespread application across various sectors, including the production of medicines, culinary sauces, and even makeup removers. HSE MIEM physicists have developed a theoretical model for coacervation in polybetaine solutions, and this model makes it possible to predict the conditions under which coacervation will occur successfully. The results will enable chemists to synthesise polymers which are specifically tailored for coacervation. The paper has been published in Soft Matter.

A polymer solution can be conceptualised as a blend of water and a substance capable of forming distinct, dense structures. When the conditions change, such as pH (the level of acidity/alkalinity of the medium) or temperature, the solution reaches a thermodynamically unfavourable state for remaining homogeneous, resulting in phase separation. The polymer within the solution then condenses into small distinct particles. As a consequence, the process yields two liquid phases: one containing particles which are rich in the substance and the other consisting of a solvent with an almost negligible concentration of the polymer. The coacervate particles remain insoluble in the medium, resembling jellyfish floating in seawater.

Coacervates have been around for a very long time: according to the Oparin-Haldane theory, the first life forms on Earth emerged within these coacervate droplets. Coacervates find widespread applications in numerous fields today, making them an omnipresent component of our daily environment. In the pharmaceutical industry, coacervation plays a crucial role in the creation of films that safeguard the active substance of a drug from environmental factors, ensuring its controlled release at the appropriate stage. Coacervates are used in various other products, such as makeup remover liquids, lotions, conditioners, and shampoos. In the food industry, coacervates are employed in the production of sauces and yogurts, where they are formed within the product and help achieve homogeneous and stable mixtures.

A team of HSE MIEM researchers has developed a theoretical model that predicts the optimal parameters for the coacervation of polymers – substances which consist of repetitive and identical structural units. The model follows the basic principles of polymer physics, considering factors such as the length of polymer chains and the strength of their mutual attraction.

The researchers built a model of the coacervation of polybetaine molecules. These compounds are characterised by a zwitterionic structure in which the polymer chain links contain both positively and negatively charged centres. Polybetaines are highly sensitive to temperature during coacervation; therefore, to enhance synthesis efficiency, droplet size and density must be controlled through fine temperature adjustments within the range of 18–20°C.

In their study, the scientists investigated solutions with varying concentrations of polybetaine and identified the optimal temperature ranges. In addition, they calculated the resulting coacervate’s surface tension, which reflects its surface stability. 

A droplet in a solution is formed by polymers that exhibit substantial dipole moments, calculated by multiplying the absolute charge values of ion groups by their respective distances. If other polymers with high polarity or charge are added to the solution, they will be drawn towards the droplet due to electrostatic forces.

Similar to a small magnet, coacervate has the ability to attract molecules which exhibit strong interactions with it and to absorb them, much like a sponge. These droplets can be used as traps to capture certain types of proteins and amino acids. Specifically, polybetaine droplets have the ability to selectively filter out biopolymers—proteins and amino acids—from solutions.

Yuri Budkov
Author of the paper, Professor at HSE MIEM

These findings can be applied to enhance the efficiency of polymer synthesis. Based on this model, chemists will be able to determine the optimal calculated values of molecular properties for polymers and to synthesise macromolecules that are most suitable for subsequent coacervation. It is noteworthy that the theoretical model proposed by the scientists has been substantiated by independent experimental data from their colleagues.

While preparing the paper for publication, we came across a recently published study that experimentally confirms our theoretical models of coacervation. Their findings align with our calculated parameters, including the distance between the charged centres and the size of the monomer link, among others. Remarkably, our theory predicts the formation of coacervates precisely within the temperature range observed in actual aqueous solutions of polybetaines. It is important not only that our theory predicts the intended effect, but also that our calculations have been experimentally verified and confirmed.

Petr Brandyshev
Research Fellow at HSE MIEM

IQ

July 12, 2023