The Molecular Picture of the Local Environment in a Stable Model Coacervate

Abstract

Polymers with electric charge, known as polyelectrolytes, are well known to form complex coacervates, which have vital implications in various biological processes and beyond. While significant advancements have been made in comprehending the molecular interactions thatdrivecomplex coacervation, the interactions thatstabilizethe coacervates against coalescence present an intricate experimental challenge and remain a subject of ongoing investigation. In a recent experimental study, polydiallyldimethylammonium chloride polycationic (PDDA) and anionic adenosine triphosphate (ATP) coacervates have been shown to stabilize upon transferring them to deionized water. Here, we perform molecular dynamics simulations of PDDA-ATP coacervates both in supernatant and in DI water, to understand the ion dynamics and structure within stable coacervates. We produced and analyzed an aggregated sum of 63μssimulation data of PDDA-ATP coacervates in explicit water when they are in supernatant and deionized (DI) water. We found that discarding the supernatant and transferring the coacervates to DI water causes an immediate ejection of a significant amount (more than 50%) of small ions (Na+andCl−) from the coacervates to the bulk solution. Subsequently, the DI water environment alters the ionic density profiles in coacervates and the surface ion dynamics. We calculated a notable slowdown for the coacervate ions when they were transferred to the DI water. These results suggest that the initial ejection of the ions from the coacervates in DI water potentially brings the outer layer of the coacervates to a physically bound state that prevents or slows down the further mobility of ions.Significance StatementComplex coacervates are promising agents for encapsulating and delivering various materials in living organisms, however, they are often prone to coalesce, limiting the range of their applications. Recently, these coacervates have been stabilized by transferring them to deionized water. However, a molecular understanding of this stability against coalescence remained elusive. This study utilizes computer simulations to model a stable coacervate system previously probed experimentally. When the coacervates were transferred to deionized water, a significant portion of the ions were immediately ejected into the solution, modifying the coacervates’ total charge and facilitating formation of possible surface crust. These molecular insights into the stable coacervates will enable their controllable design for encapsulation and delivery applications.