<p>To investigate the regulatory mechanism of hydrophobic chain length on the swelling properties of hydrogels, this study designed and synthesized a series of reactive quaternary ammonium salt surfactants (St-PKO-n) with varying hydrophobic chain segment lengths. These surfactants were created using chloromethylvinyl benzene (CSt) and tertiary amines containing amine groups (PKO) as starting materials and were employed as functional monomers to modify PAA-based hydrogels. The structures of the surfactants and hydrogels were characterized using FT-IR and ¹H-NMR techniques. The microstructure of the hydrogels was examined by SEM, and their mechanical properties, swelling behavior, and water retention capacity were evaluated. The findings indicate that as the length of the hydrophobic carbon chain in St-PKO-n increases, the crosslinking density of the hydrogel markedly rises, and the pore structure transitions from large pores to a dense lamellar honeycomb structure. Consequently, the tensile strength increases, but the elongation at break decreases. Although the swelling capacity diminishes, the water retention capacity significantly improves, particularly exhibiting enhanced salt resistance in salt solutions. This research offers a robust theoretical foundation and experimental reference for optimizing the swelling properties of hydrogels by adjusting the length of the alkyl hydrophobic carbon chain.</p>