Osmosis in a minimal model system

TL;DR

This study uses molecular dynamics simulations of a minimal model to explore the microscopic mechanisms of osmosis at equilibrium, deriving a virial-like relation for osmotic pressure and supporting an intuitive force-flux balance picture.

Contribution

It provides a simplified, microscopic understanding of osmosis by deriving a virial-like relation and validating it through simulations in a minimal model system.

Findings

Derived a virial-like relation for osmotic pressure.

Supported an intuitive force balance explanation for solvent concentration gradients.

Showed that complex effects are not necessary to describe basic osmosis physics in the model.

Abstract

Osmosis plays a central role in the function of living and soft matter systems. While the thermodynamics of osmosis is well understood, the underlying microscopic dynamical mechanisms remain the subject of discussion. Unraveling these mechanisms is a crucial prerequisite for eventually understanding osmosis in non-equilibrium systems. Here, we investigate the microscopic basis of osmosis, in a system at equilibrium, using molecular dynamics simulations of a minimal model in which repulsive solute and solvent particles differ only in their interactions with an external potential. For this system, we can derive a simple virial-like relation for the osmotic pressure. Our simulations support an intuitive picture in which the solvent concentration gradient, at osmotic equilibrium, arises from the balance between an outward force, caused by the increased total density in the solution, and an inward diffusive flux caused by the decreased solvent density in the solution. While more complex effects may occur in other osmotic systems, they are not required for a description of the basic physics of osmosis in this minimal model.