On the Thermodynamics of Micellizationof Oppositely Charged Surfactants in the Presence of Organic Additives in the Aqueous Medium

To investigate the effect of additives urea and thiourea, on the micellization behavior of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), detailed conductance measurements were carried out in aqueous media at different temperatures. The critical micelle concentration (CMC), determined from the discontinuity in the plots of molar conductance versus square root of concentration, indicated an inhibitory effect of urea and thiourea on micelle forming ability of the surfactants SDS and CTAB in the range of composition studied. The demicellizing effect of urea has been found to be more pronounced in SDS than CTAB. These observations are further augmented by the evaluation of thermodynamic parameters of micellization. A negative change in enthalpy of micellization ($\Delta{\text{H}}_{\text{m}}^{\circ}$) indicates a strong interaction between water and the additives and a positive change observed in entropy of micellization ($\Delta{\text{S}}_{\text{m}}^{\circ}$) manifest, that the micellization is an entropy-driven process. Further $\Delta{\text{H}}_{\text{m}}^{\circ}$ and $\Delta{\text{S}}_{\text{m}}^{\circ}$ change in mutually compensating manner, so that $\Delta{\text{G}}_{\text{m}}^{\circ} < 0$ is not significantly affected. Finally, the counterion binding values (β) obtained for SDS and CTAB remain practically constant from 0.6 to 0.8 between 25 °C and 45 °C indicate that the size and shape of micelle remain essentially constant. Moreover, the increase in $\Delta{\text{G}}_{{\text{II}}}^{\circ}$ values, which represent the effect of co-solvent or additive on micellization, substantiates the above observations. Many early works has investigated the micellization behavior of surfactants using a fixed additive composition. However, in this study, variable aqueous compositions of urea (0.30–1.78 wt%) and thiourea (0.24–1.41 wt%) have been considered.