Phase coexistence in a polydisperse charged hard-sphere fluid: polymer mean spherical approximation

Area of Science:

• Physical Chemistry
• Thermodynamics
• Statistical Mechanics

Background:

• Charged hard sphere (CHS) mixtures exhibit complex phase behavior.
• Previous models, like the mean spherical approximation (MSA), have limitations in capturing ionic association effects.

Purpose of the Study:

• To develop a new theoretical framework for understanding the phase behavior of polydisperse CHS mixtures.
• To incorporate the concept of minimal size neutral clusters to account for ionic association.

Main Methods:

• The study employs a polymer MSA framework to model polydisperse mixtures of neutral, polar dimers.
• The model is based on truncatable free-energy principles, allowing mapping of infinite coexistence equations to a finite set.
• Generalized moments of the distribution function are used to solve the system.

Main Results:

• The developed model accurately predicts the full phase diagram, including binodals, cloud, and shadow curves.
• Fractionation effects in coexisting phases are analyzed at the distribution function level.
• Estimates for the variation of the critical point with size asymmetry and polydispersity are provided.

Conclusions:

• The concept of neutral clusters offers a more accurate description of charged systems near phase boundaries.
• The polymer MSA framework is effective for polydisperse systems and can be generalized from finite mixtures.
• This approach provides a robust method for studying phase diagrams and critical phenomena in complex ionic systems.