Rheological response of methylcellulose toward alkanediyl-α,ω-bis(dimethylcetylammonium bromide) surfactants with varying spacer length

[Display omitted] •Interaction of 16-S-16 gemini surfactants with methylcellulose was studied.•Gemini surfactant with larger spacer length was found to be more surface active.•In pre-CAC region viscosity of MC decreased with increase in the spacer length.•Gelation temperature of MC decreased more in presence of 16-6-16 gemini surfactant. The modulation of properties of methylcellulose (MC) by cationic gemini surfactants with varying spacer lengths was studied employing tensiometry, rheometry and turbidimetry. Surface tension measurements anticipate that the gemini surfactant with longer spacer chain length saturates MC at lower concentrations owing to its greater hydrophobicity compared to shorter spacer analogues. Rheometric and turbidimetric measurements suggest that at very low concentrations of gemini surfactants, ion-dipole type of interactions between MC and gemini surfactants promote the extension of polymer chains which is manifested by an initial increase in the low shear viscosity and gelation temperature of MC–gemini surfactant systems, and lowering of turbidity. Such interactions were found to be stronger in case of 16-4-16 than 16-5-16, and almost absent in case of 16-6-16 surfactant system. However at concentrations above CAC, hydrophobic forces operative between MC and gemini surfactants were found to be more for 16-6-16 than that of 16-5-16 and 16-4-16. The final levelling of MC viscosity in presence of all the three gemini surfactants and the variation of gelation temperature suggests the solubilization of network junctions in the surfactant micelles. Moreover, the presence of gemini surfactant strongly affects the interaction of MC with a model hydrophobic drug rifampicin.The results highlight the importance of gemini surfactants and their spacer length in controlling the structural dynamics of MC and its effective use in pharmaceutical and food industry.