Electrostatics and aggregation: how charge can turn a crystal into a gel

Area of Science:

• Colloid and interface science
• Physical chemistry
• Biophysics

Background:

• Protein and colloid crystallization is often impeded by gel formation.
• Gels are aggregates with low fractal dimensions.
• Understanding factors influencing gelation versus crystallization is crucial for controlling material self-assembly.

Purpose of the Study:

• To investigate the influence of electrostatic interactions on the formation of crystalline versus gel phases.
• To develop an analytical model for hard spheres with charges and short-range attractions.
• To identify conditions that favor or suppress gel formation.

Main Methods:

• Development of an analytical model for charged hard spheres with attractive interactions.
• Analysis of electrostatic free energy contributions, focusing on counterion entropy.
• Phase diagram analysis to predict gelation and crystallization regimes.

Main Results:

• The primary electrostatic cost in assembly formation is the entropic loss of counterions.
• An open gel structure offers more accessible volume for counterions than a dense crystal, creating an entropic driving force for gelation.
• This driving force intensifies with increasing sphere charge but can be mitigated by higher counterion concentrations.

Conclusions:

• Electrostatic effects, particularly counterion entropy, play a significant role in favoring gel over crystal formation.
• Pairwise-additive interaction models commonly used in simulations may not fully capture these electrostatic phenomena.
• The study provides insights into manipulating phase diagrams to control gel formation and promote crystallization.