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

The subzero solubility behavior of ultra-long-chain (≥ C₁₈) 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.

Three series of ultra-long-chain polyoxyethylene nonionic surfactants, U_nC_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 ¹H 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).

For the first time, three quantitative relationships were established inPG/water mixtures: PTT1 = 8.7_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 U₁C₂₂-M¯_MPEG, U₁C₂₄-0.35, and U₀C₂₂-0.35 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.