Conductometric Study of the Interaction of Cetrimide with Sodium Dodecyl Sulfate in Aqueous Medium

Abstract

Synergistic interactions between cetrimide and sodium dodecyl sulfate (SDS) were studied at three different temperatures. Aqueous solutions of pure cetrimide, SDS and their mixtures were investigated conductometrically. The mixed surfactants exhibited nonideal behavior as the CMCs are less than the ideal CMC values calculated by using Clint’s equation. In addition, Rubingh’s model was used and various parameters such as the activity coefficients f ₁ and f ₂, micelllar mole fraction, X, interaction parameter, β, excess Gibbs energy and enthalpy of mixing, $\Delta G_{\text{ex}}$ and $\Delta H_{\text{ex}}$, respectively, were calculated. The values of f ₁, f ₂, and β indicate strong interaction between cetrimide and SDS and the interaction increases with increasing mole fraction of SDS in the mixture. Negative β values and |β| > |ln(CMC ₁ /CMC ₂)| clearly confirm synergism in the surfactant mixture. Negative $\Delta G_{\text{ex}}^{{}}$ and $\Delta H_{\text{ex}}^{{}}$ values of the mixed systems at different bulk mole fractions also reflect the behaviors of β and f. The values of the standard Gibbs energy and enthalpy of mixed micelle formation are negative, suggesting that micellization is thermodynamically favorable and exothermic in nature. Larger positive contributions of the entropic component, $T\Delta S_{\text{m}}^{ 0}$, than the enthalpic component, $\Delta H_{\text{m}}^{ 0}$, to $\Delta G_{\text{m}}^{ 0}$ are ascribed to the destruction of the structured water surrounding the hydrophobic groups of the surfactants when these groups are transferred from the bulk into the interior of the micelle.