Effect of Methyl Red on the surface properties of DTAB in CH3OH–H2O

The purpose of this study is to investigate the effect of Methyl Red (MR) on the micellization and surface properties of a cationic surfactant, dodecyl trimethylammonium bromide (DTAB), in CH₃OH–H₂O mixed solvent systems at varying CH₃OH parts by volumes (0.1, 0.2, 0.3, and 0.4) and temperatures (298.15 K, 308.15 K and 318.15 K). Surface tension and conductivity measurements were conducted to obtain the critical micelle concentration (CMC) and various surface properties such as premicellar slope ($\frac{d\gamma }{d\log C}$), maximum surface concentration (${\Gamma }_{max}$), minimum surface area occupied (${\pi }_{CMC}$), free energy of adsorption ($\Delta G_{ads}^o$), adsorption efficiency ($p{C}_{20}$), packing parameter ($P$), and aggregation number ($N_{\mathrm{agg}}$). The results indicate that the presence of MR significantly influences the behavior of DTAB in the mixed solvent system. In the absence of MR, the CMC increased with higher CH₃OH content, while MR reduced the CMC, promoting micelle formation through dye-surfactant-ion-pair (DSIP) formation. Surface properties were enhanced in the presence of MR leading to higher surface pressure. Additionally, the free energy of adsorption ($\Delta G_{ads}^o$) became more negative with MR, indicating a more favorable adsorption process. Correlations of ($\frac{d\gamma }{d\log C}$), ${\Gamma }_{\max },{\gamma }_0/{\gamma }_{\mathrm{cmc}}$, $p{C}_{20}$, $N_{\mathrm{agg}}$, CMC/C₂₀ with the parts by volume of CH₃OH at 298.15 K, 308.15 K, and 318.15 K are discussed with and without MR. The obtained results were used to examine the effect of MR on the surface properties of DTAB in the CH₃OH–H₂O medium. The change in properties can be explained in terms of the change in polarity of the medium and dye-surfactant ion pair (DSIP) formation.