Phase separation of binary mixtures induced by soft centrifugal fields.

Area of Science:

• Physical Chemistry
• Thermodynamics
• Soft Matter Physics

Background:

• Binary liquid mixtures exhibit phase transitions and critical phenomena.
• Analytical Ultracentrifugation (AUC) is a technique used to determine molar masses and study phase behavior.
• Understanding concentration gradients is crucial for accurate interpretation of AUC data.

Purpose of the Study:

• To demonstrate that weak centrifugal fields can induce giant critical fluctuations in binary liquid mixtures.
• To investigate the occurrence and impact of concentration gradients in the one-phase region above the miscibility gap.
• To explain the observed phenomena using theoretical models.

Main Methods:

• Utilizing the ethanol-dodecane model system.
• Applying centrifugal fields as low as 2000g.
• Coupling the Flory-Huggins solution model with classical density functional theory (DFT).

Main Results:

• Giant critical fluctuations were observed above the miscibility gap induced by centrifugal fields.
• Strong ethanol concentration gradients formed in the one-phase region, leading to erroneous molar mass calculations in AUC.
• The interphase appeared as a thick, turbid zone, not a sharp meniscus.
• Solute distribution gradients, like those of Nile red dye, were unaffected by these composition gradients.

Conclusions:

• Weak centrifugal fields can trigger significant fluctuations and concentration gradients in binary liquid mixtures, even far from the critical point.
• Standard AUC interpretation can yield inaccurate molar masses due to these induced gradients.
• The observed phenomena are well-explained by combining solution theory with density functional theory.