Synergistic thickening of low-degree substituted acetylated guar gum with a double-tailed Esterquat surfactant: A multi-scale study.

Polysaccharide-surfactant synergistic thickening systems are of great interest for applications in soft matter and consumer products, yet the role of controlled hydrophobic modification of neutral polysaccharides in enhancing such interactions remains underexplored. In this work, a series of acetylated guar gum (AGG) derivatives with low degrees of substitution (DS, 0.04-0.41) were synthesized to fine-tune the hydrophobicity of the polysaccharide backbone. Among these, AGG with a DS of 0.07 (AGG-2) exhibited improved solubility and higher intrinsic viscosity than native guar gum (GG). When combined with a bio-based double-tailed esterquat surfactant (TEQ), AGG-2 demonstrated a markedly enhanced synergistic thickening effect, with a zero-shear viscosity ca. 24 times higher than that of the AGG-2 solution alone and a synergistic index of 1.76. Using a combination of rheology, optical microscopy, quartz crystal microbalance with dissipation (QCM-D), small-angle X-ray scattering (SAXS), and molecular dynamics simulations, we reveal that the synergy originates from hydrophobic-driven adsorption of TEQ onto the mildly acetylated polysaccharide chains, forming a physically cross-linked network of polymer chains and surfactant vesicles. Notably, a moderate DS (0.07) strikes an optimal balance between hydrophobicity and chain flexibility, maximizing network extensibility and viscoelasticity. This study provides a mechanistic understanding of how controlled acetylation enhances surfactant-polysaccharide synergy and offers a sustainable strategy for designing high-performance thickening systems with reduced surfactant usage.