[Paper Review] Charge separation at liquid interfaces

Arghya Majee, Christoph A. Weber+1 more|arXiv (Cornell University)|Oct 11, 2023

Polymer Surface Interaction StudiesMaterials Science3 citations

TL;DR

This paper presents a mean-field theory for phase-separated coacervates with salt, showing that interfacial charge separation leads to alternating charged layers with non-monotonic electrostatic potential profiles. Salt concentration regulates the number and amplitude of these layers, enabling selective transport of charged molecules across interfaces—suggesting biomolecular condensates may function like electrostatic filters akin to membrane compartments.

ABSTRACT

We present a theory for phase-separated liquid coacervates with salt, taking into account spatial heterogeneities and interfacial profiles. We find that charged layers of alternating sign can form around the interface while the bulk phases remain approximately charge-neutral. We show that the salt concentration regulates the number of layers and the amplitude of the layer's charge density and electrostatic potential. Such charged layers can either repel or attract single-charged molecules diffusing across the interface. Our theory could be relevant for artificial systems and biomolecular condensates in cells. Our work suggests that interfaces of biomolecular condensates could mediate charge-specific transport similar to membrane-bound compartments.

Motivation & Objective

• To understand the thermodynamic mechanisms driving interfacial charge separation in phase-separated coacervates.
• To model electrostatic potential profiles and ion distributions across coacervate interfaces in non-dilute, multi-component electrolyte mixtures.
• To investigate how salt concentration modulates interfacial charge density and electrostatic potential structure.
• To explore the implications of interfacial charge layers for the selective transport of charged molecules across coacervate boundaries.
• To establish a theoretical framework for coacervate interfaces that accounts for strong electrostatic interactions beyond the dilute limit.

Proposed method

• Develops a mean-field electrostatic model for coacervates using the Poisson-Boltzmann equation with non-dilute ion concentrations.
• Incorporates incompressibility and constant molecular volumes for all components, including macromolecules, counterions, and salt ions.
• Derives exponential concentration profiles near the interface using linearized expansions of chemical potential conditions.
• Solves for the decay constant γ via a sixth-order polynomial equation derived from electrochemical potential equality across phases.
• Uses the interface location defined by the electrostatic potential reaching ψD/2, with multiple crossings resolved by selecting the middle crossing.
• Analyzes total charge within the coacervate phase as a function of gradient cost ratios and system parameters.

Experimental results

Research questions

• RQ1 How do interfacial electrostatic potentials evolve in phase-separated coacervates with non-dilute ions and salt?
• RQ2 What determines the formation of alternating charged layers at coacervate interfaces?
• RQ3 How does salt concentration regulate the number and amplitude of interfacial charge layers?
• RQ4 Can interfacial charge profiles mediate selective transport of charged molecules across coacervate boundaries?
• RQ5 What is the role of non-ideal electrostatic interactions in shaping interfacial potential and ion distribution?

Key findings

• Interfacial charge separation generates alternating layers of positive and negative charge density, even though the bulk phases remain approximately charge-neutral.
• The electrostatic potential exhibits non-monotonic behavior with both attractive wells and repulsive barriers, enabling complex kinetic transitions for diffusing charged molecules.
• Salt concentration controls the number of interfacial charge layers and the amplitude of charge density and potential, with higher salt reducing layer formation.
• The total charge within the coacervate phase can decrease and even flip sign with increasing gradient cost ratios, particularly in complex coacervates.
• The interface location is defined by the middle crossing of the electrostatic potential at ψD/2, reflecting the non-monotonic potential profile.
• The derived sixth-order polynomial for the decay constant γ captures the interfacial structure, with the smallest root governing long-range behavior in the asymptotic limit.