A study on solution behaviour of sodiumdodecyl sulphate and cetyltrimethylammonium bromide in water - alcohol mixed media

 The solution behaviour viz micellization of sodiumdodecyl sulphate (SDS) an anionic surfactant and cetyltrimethylammonium bromide (CTAB) a cationic surfactant in aqueous-rich mixtures of alcohols like methanol (MeOH), ethanol (EtOH), n-Propanol (n-PrOH) and iso-Propanol (i-PrOH) have been investigated between 25oC – 45oC. The critical micelle concentration (CMC) was determined by conductometric method and was further utilized to obtain thermodynamic functions of micellization. Data indicate inhibitory effect of MeOH on micellization of SDS and CTAB; whereas CMC of SDS decreased with alcohols in the order EtOH < i-PrOH < n-PrOH, it remained practically independent of the nature of these solvents in case of CTAB. Thermodynamic parameters of micellization, enthalpy (DHo m), entropy (DSo m) and free energy (DGo m) were determined from temperature dependence of CMC. Constant DHo m, DSo m values and DHo m < 0 is indicative of the stabilization of water structure by MeOH in aqueous solutions of both SDS and CTAB and near constant DGo m values is as a consequence of compensation between entropy and enthalpy of micellization. Near constant values of counterion binding (b) irrespective of nature of head group of SDS and CTAB, is indicative of the fact that size and shape of micelle remain practically constant in the range of composition studied.

Author(s): Girish Kumar1, Mohinder S Chauhan2, Akshat Kumar2, Suvercha Chauhan2 and Rajesh Kumar1

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