Phase Diagram of the Ouzo Effect Using Temperature-Controlled UV-Vis Spectroscopy

Importance of the Topic

Phase diagrams and solubility charts are essential tools for understanding phase transitions and molecular interactions in complex mixtures. The ouzo effect, an instantaneous emulsification observed when water is added to anethole-containing liqueurs such as pastis or ouzo, provides a striking example of solubility limits and light scattering phenomena. Temperature-controlled UV-Vis spectroscopy enables rapid, quantitative mapping of these transitions, offering benefits for food science, pharmaceuticals, and industrial QA/QC.

Objectives and Overview of the Study

This application note describes the construction of an eight-point solubility phase diagram for anethole in pastis using temperature-controlled UV-Vis methods. By leveraging the multizone feature of the Agilent Cary 3500 UV-Vis spectrophotometer, four parallel temperature-ramping experiments were performed to determine precipitation onset temperatures across alcohol concentrations from 45% to 30% v/v.

Methodology

A series of eight pastis dilutions (45.0 to 30.0% v/v ethanol) were prepared in quartz cuvettes with stirring controls. Reference cells matched ethanol-water compositions. Temperature ramps from 55 °C down to –5 °C were applied in two stages: a rapid heating phase to fully dissolve anethole, and a controlled cooling phase at 1 °C/min. Absorbance at 750 nm was monitored to isolate turbidity signals, and onset temperatures were extracted via first derivative analysis. Each experiment was replicated across four independent sample zones.

Used Instrumentation

• Agilent Cary 3500 UV-Vis spectrophotometer with xenon flash lamp and double out-of-plane Littrow monochromator
• Multicell Peltier sampling module featuring four temperature-controlled zones and independent stirrers
• Compact Peltier module for two-zone comparison
• Optical fibers for simultaneous light delivery to all cuvettes
• Nitrogen purge line to prevent condensation below dewpoint
• Agilent Cary UV Workstation software for instrument control and data acquisition

Main Results and Discussion

The selected wavelength of 750 nm ensured that absorbance changes arose solely from light scattering by precipitated anethole. Cooling curves revealed a clear trend: lower ethanol content led to higher precipitation temperatures. Anethole precipitation onsets ranged from near –2 °C at high alcohol levels to approximately 25 °C at 30% v/v. A secondary phase transition near 0 °C was attributed to pure water domain effects and irreversible coalescence phenomena. The multizone approach reduced total analysis time to around 73 minutes for eight data points.

Benefits and Practical Applications

Rapid phase diagram acquisition via parallel temperature ramps significantly accelerates solubility studies of turbid systems. The technique supports formulation optimization in beverage development, pharmaceutical emulsions, and quality control processes where non-chromophoric phase changes occur.

Future Trends and Potential Uses

• Extension to other hydrophobic compound solubility mapping
• Integration with high-throughput screening platforms
• Combining UV-Vis turbidity with spectroscopic or scattering techniques
• Microfluidic implementations for single-droplet analysis

Conclusion

This study demonstrates that temperature-controlled UV-Vis spectroscopy on the Agilent Cary 3500 platform provides an efficient, reproducible method for constructing detailed solubility phase diagrams in complex emulsions. The multizone capability and precise temperature control allow rapid mapping of precipitation behavior, offering broad applicability across research and industry.

References

1. United States Pharmacopeia (2024) General Chapter {855} Nephelometry and Turbidimetry. USP-NF. Rockville MD
2. Sitnikova NL, Sprik R, Wegdam G, Eiser E (2005) Spontaneously Formed trans-Anethol/Water/Alcohol Emulsions: Mechanism of Formation and Stability. Langmuir 21(16):7083–7089