Thermodynamic properties of Sodium Dodecyl Sulfate aqueous solutions with Methanol, Ethanol, n-Propanol and iso-Propanol at different temperatures

Abstract

The thermodynamic activation parameters such as, free energy (ΔG#), enthalpy (ΔH#) and entropy (ΔS#) change for the viscous flow of the ternary systems of Methanol, Ethanol, n-Propanol and iso-Propanol in 0.005 and 0.01 M aqueous Sodium Dodecyl Sulfate (SDS) were determined by using viscosity at 298.15–323.15 K with an interval of 5 K for the entire range of composition. The critical micelle concentration (CMC) of Sodium Dodecyl Sulfate (SDS) in water was found to be 0.0085 mol L−1 at 303.15 K. The concentration of SDS in pre-micellar and post-micellar regions of 0.005 and 0.01 M was used for the thermodynamic measurements. Free energy (ΔG#), increases rapidly with alcohol concentration and shows maxima in aqueous SDS rich region at 0.2–0.3 mol fraction of alcohols. The position of maxima virtually does not change remarkably with the variation of temperature. The excess free energy, (ΔG#E) values are found to be positive and large in magnitude, indicating that the aqueous SDS solutions of alcohols are highly non ideal. The heights of the maxima are in the order:iso-Propanol + aqueous SDS >n-Propanol + aqueous SDS > Ethanol + aqueous SDS > Methanol + aqueous SDS.The ΔH# values that are positive for all the studied systems indicate that positive work has to be done to overcome the energy barrier for the flow process. The variation of ΔS# is reversing the variation of the ΔH#. The excess parameters (ΔG#E, ΔH#E) data have been fitted by the least square method to the four parameter Redlich–Kister equation and the values of the parameter aj have been reported. The observed increase of thermodynamic values in the aqueous SDS region are thought to be mainly due to the combined effect of hydrophobic hydration and hydrophilic effect.