NSF-DFG Confine: MolPEC -- Molecular theory of weak polyelectrolytes in confined space

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Building on complementary advances in polymer density functional theory (PDFT) and molecular simulation, the US-German team will study the structure, thermodynamics, and dynamics of weak polyelectrolytes at and between surfaces. The idea is to combine and extend Ising Density Functional Theory (iDFT) and Single-Chain-in-Mean-Field (SCMF) simulation. iDFT considers single-chain configurations and ionization states of segments on equal footing and captures liquid-like packing and electrostatic correlations via the molecular density, ρ(X). This joint probability distribution of configurations and ionization states of an entire macromolecule is dictated by a single-chain potential, ω(X), which, in turn, is a functional of the molecular density. The high-dimensional molecular density will be efficiently evaluated via SCMF simulation on parallel, GPU-accelerated supercomputers, employing the iDFT single-chain potential. We will extend the simulation code, SOft coarse grained Monte-carlo Acceleration (SOMA), to incorporate electrostatic interactions and nonlocal interactions along the molecular contour. The particle-based simulation, Dynamic-iDFT (D-iDFT), accounts for long-range fluctuations and allows us to study the configuration dynamics. Additionally, we will derive a segment-based dynamic iDFT (SD-iDFT) for describing the local polymer mass and charge density in response to environmental changes.The combined computational scheme will be applied to four prototypical, scientific questions: (i) SCMF/iDFT simulations will study the adsorption of weak polyelectrolytes onto surfaces, focusing on the coupling between single-chain configurations, ionization states, and polymer-surface interactions. (ii) Additionally, iDFT will investigate surface free energies and polymer-induced interactions between surfaces, and study the role of segment sequences and surface heterogeneities. (iii) Subsequently, we will study the coacervation of polycations and polyanions in confined aqueous solution, focusing on the interplay between wetting and phase behavior in thin films and geometric confinement. (iv) Moreover, we will use D-iDFT simulations and SD-iDFT to study the collective dynamics of confined, weak polyelectrolytes in response to changes of environmental conditions, such as pH or salt concentration. These molecular simulations are able to predict the temporal evolution of segment and charge density profiles from the underlying dynamics of polymer ionization and single-chain configurations. In all cases we will study model systems to validate our computations by comparison between SCMF simulation, iDFT, and literature data, and subsequently extend our studies to more complex systems, accounting for different segment sequences and chemistry of the confinement. These theoretical and computational advances will contribute to a rational design of weak polyelectrolyte systems for a wide spectrum of technological applications.