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

Hypothesis: The subzero solubility behavior of ultra-long-chain (≥ C 18 ) polyoxyethylene nonionic surfactants remains uncharted. Correlating structural parameters with solubility metrics may reveal how molecular architecture and extreme conditions influence solubility beyond the traditional cloud point (CP) paradigm. Experiments: Three series of ultra-long-chain polyoxyethylene nonionic surfactants, U n C L - M ¯ MPEG , were synthesized, incorporating methoxy polyethylene glycol (MPEG) headgroups of varying molecular weights ( M ¯ MPEG , = 0.35, 0.50, 0.75, and 1.00 kg⋅mol −1 ), alkyl tails of different lengths ( C L , L  = 18, 20, 22, and 24), and degrees of unsaturation ( U n , n  = 0, 1, 2, and 3). Their structures were characterized using 1 H NMR and mass spectrometry, while their solubility in propylene glycol (PG)/water mixtures was evaluated via a UV–vis spectrophotometer. And low-temperatures phase behavior was investigated using DSC, cryo-TEM, polarizing optical microscope (POM), and small-angle X-ray scattering (SAXS). Findings: For the first time, three quantitative relationships were established inPG/water mixtures: PTT1 = 8.7 M ¯ MPEG – 5.6, CP = −3.6  L  + 149.3, and CP = −10.7 n  + 80.4 (where PTT1 denotes phase transition temperature 1). The surfactants U 1 C 22 - M ¯ MPEG , U 1 C 24 –0.35, and U 0 C 22 –0.35 demonstrated a PTT1 below the CP or at subzero temperatures, forming a mixed phase of liquid crystal and surfactant solid or hexagonal phase liquid crystal. Elevated molecular weight of headgroups ( M ¯ MPEG ) and extended tail length ( L ) reduced cryogenic solubility, whereas increased unsaturation ( n ) improved solubility under subzero conditions. These insights advance the fundamental understanding of how surfactant structure governs solubility and offer a strategic framework for developing ultra-long-chain surfactants tailored for subzero applications.