Influence of Alcohol/Temperature on the Interaction of Sodium Dodecyl Sulfate with Cetyltrimethylammonium Bromide: Experimental and Theoretical Study

This study explores the mixed micellization between anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant cetyltrimethylammonium bromide (CTAB) in H2O/H2O + alcohol mixed medium over a temperature range of 298.15–318.15 K using the conductivity measurement technique. The experimentally estimated critical micelle concentration (cmc) values are lower than respective ideal cmc (cmcid), which reveals the presence of the interaction among the surfactants. The cmc values disclose that the presence of lower carbon alcohol (methanol and ethanol) favors the mixed micellization of SDS and CTAB. In the presence of alcohol, the extent of micelle ionization (g) of the system enhances. Different theoretical models (Rubingh/Motomura/Rodenas) were applied to estimate micellar mole fraction (X1), and the obtained results illustrate the significant participation of CTAB in the mixed micellization. The magnitudes of activity coefficients (f1 and f2 < 1) as well as interaction parameter (β < 0) in almost every case clarify the attractive relations between CTAB and SDS. The standard free energy of mixed micellization (ΔGm°) and excess free energy of micellization (ΔGex) illustrate the spontaneous mixed micelle formation and greater stability of the SDS + CTAB micelle than that of individual surfactants, respectively. The ΔHm° as well as ΔSm° values of SDS + CTAB micellization were also estimated and illustrated in detail. The values of ΔGm,tr° for SDS + CTAB illustrate that micellization is more spontaneous in the presence of ethanol than in the presence of methanol. The values of ΔHm,tr° and ΔSm,tr° were also calculated and discussed.