Gellan gum-sodium carboxymethyl cellulose (GC)-based composite films with various concentrations of silicon dioxide (SiO₂) nanoparticles and octadecyldimethyl-(3-triethoxy silylpropyl)ammonium chloride (ODDMAC) were successfully prepared by the traditional solution casting method to improve the antimicrobial and water repellent properties. Fourier transform infrared (FT-IR) spectra confirm the formation of hydrogen bonds between the GC and nano-SiO₂. The microstructure and physicochemical properties were investigated by FT-IR, wide-angle X-ray diffraction, and scanning electron microscopy (SEM) analyses. The rheological properties of the GC-SiO₂ hydrogel were also characterized. The results show that the inclusion of SiO₂ nanoparticles significantly improved the viscosity and viscoelastic properties of the GC hydrogel. The GC-SiO₂ hydrogel exhibited shear-thinning behavior and its viscosity decreased at high shear rates. The storage and loss moduli of the GC composites increased as the frequency and SiO₂ concentration increased. The tensile strength and elongation at break of the GC composites increased by 75.9 and 62%, respectively, with the addition of SiO₂ and ODDMAC. In addition, nano-SiO₂ decreased the water vapor permeability and increased the hydrophobic properties of the GC-SiO₂ composites. Thermogravimetric analysis showed that the T _5% loss was in the range of 99.4-128.6 °C and the char yield was in the range of 20.1-29.9%, which was significantly enhanced by the incorporation of SiO₂ nanoparticles. The GC-SiO₂ (ODDMAC) nanocomposites effectively shielded the UV light and exhibited high antimicrobial activity against six different pathogens. The simple and cost-effective GC-SiO₂ (ODDMAC) nanocomposites gained importance in food packaging and biomedical applications.

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