Modulation mechanism of the thermo-responsive behavior of methylcellulose aqueous solutions by sulfonate surfactants with varying alkyl chain lengths.

Abstract

Methylcellulose (MC) is a typical thermo-responsive polysaccharide, whose thermo-responsive behavior arises from the balance between hydrophilic interactions and hydrophobic association. Surfactants are commonly employed as functional additives to regulate the phase transition behavior of MC aqueous solutions. In this study, we systematically investigated the thermo-responsive behavior of MC in the presence of sulfonate-based anionic surfactants with different hydrophobic chain lengths, including sodium octyl sulfonate (SOS), sodium dodecyl sulfonate (SDS), and sodium hexadecyl sulfonate (SHS). Conductivity and isothermal titration calorimetry (ITC) analyses revealed that increasing alkyl chain length enhanced surfactant-MC association, as evidenced by stronger apparent calorimetric responses. Turbidity and particle size measurements demonstrated that SDS and SHS promoted thermally induced MC aggregation and decreased the cloud point, whereas SOS exhibited a limited effect. Rheological results showed that excessive hydrophobic association impaired gel network formation, particularly in SHS-containing systems. AFM observations further revealed that longer surfactant chains induced thicker but less continuous MC fibrillar structures, providing direct morphological evidence for the deterioration of network integrity. These findings demonstrate that surfactant molecular structure, particularly alkyl chain length, provides an effective strategy for tuning MC thermo-responsive behavior. Moderate hydrophobic association facilitates MC aggregation, whereas excessively strong association leads to heterogeneous aggregation and reduced gel mechanical performance.