Upon dissolution in methanol-water mixed solvent, cationic and anionic surfactants dissociate into cations and anions. The critical micelle concentration occurs when these surfactants self-assemble in an aqueous solution. As the amount of solvent increases, the medium's polarity decreases. Physicochemical and thermodynamic parameters are changed due to this change in solvent polarity.

Abstract

This study investigates the impact of the interaction between SDS (anionic surfactant) and CPC (cationic surfactant) on the critical micelle concentration (CMC) and surface properties of SDS-CPC in CH₃OH-H₂O mixed solvent systems with varying methanol concentrations (0%, 10%, 20%, 30%) at 298.15, 308.15, and 318.15 K. Several surface properties, such as the premicellar slope (dγ), maximum surface concentration (Γ _max), minimum surface area occupied (π _CMC), free energy of adsorption (ΔGads∘$\mathrm{\Delta}{G}_{\mathrm{ads}}^{\circ}$), adsorption efficiency (pC₂₀), packing parameter (P), and aggregation number (N _agg) were evaluated. The results indicate that the interaction between SDS and CPC significantly influences their behavior in the mixed solvent system, promoting micelle formation through surfactant-surfactant-ion-pair (SSIP) interactions. The presence of SDS at a constant CPC concentration enhanced the surface property, leading to increased surface pressure. Additionally, the free energy of adsorption ΔGads∘$\mathrm{\Delta}{G}_{\mathrm{ads}}^{\circ}$, became more negative, indicating a more favorable adsorption process. The relationships between dγdlogC$\frac{{d\gamma }}{{d\log C}}$, Γ _max, γ₀/γ_CMC, CMC, pC₂₀, N _agg, and CMC/pC₂₀ with methanol concentration at different temperatures are discussed. The analysis reveals how temperature and methanol percentage influence the surface properties of the SDS-CPC mixed surfactant system in the CH₃OH-H₂O medium, with observed changes attributed to variations in medium polarity and the formation of surfactant-surfactant ion pairs.