Solubility quantification and subzero-temperature phase behavior of ultra-long-chain nonionic surfactants

Area of Science:

• Surfactant Chemistry
• Materials Science
• Physical Chemistry

Background:

• Subzero solubility of ultra-long-chain (≥ C18) polyoxyethylene nonionic surfactants is poorly understood.
• Traditional cloud point (CP) models do not fully capture solubility under extreme conditions.
• Understanding molecular architecture's impact on solubility is crucial for specialized applications.

Purpose of the Study:

• To investigate the subzero solubility behavior of ultra-long-chain polyoxyethylene nonionic surfactants.
• To establish quantitative relationships between surfactant structure and solubility metrics.
• To explore how molecular architecture influences solubility beyond the CP paradigm.

Main Methods:

• Synthesis of three series of ultra-long-chain polyoxyethylene nonionic surfactants with varying headgroup molecular weights, tail lengths, and degrees of unsaturation.
• Structural characterization using 1H NMR and mass spectrometry.
• Solubility and low-temperature phase behavior evaluation using UV-vis spectrophotometry, DSC, cryo-TEM, POM, and SAXS in propylene glycol/water mixtures.

Main Results:

• Established three quantitative relationships for phase transition temperature 1 (PTT1) and cloud point (CP) based on surfactant structure.
• Identified specific surfactant structures exhibiting PTT1 below CP or at subzero temperatures.
• Determined that higher headgroup molecular weight and longer tail length decrease cryogenic solubility, while increased unsaturation enhances it.

Conclusions:

• Demonstrated that surfactant structure significantly influences subzero solubility and phase behavior.
• Provided a framework for designing ultra-long-chain surfactants for subzero applications.
• Advanced the fundamental understanding of surfactant behavior under extreme conditions.