Study of the effect of electrolyte and temperature on the critical micelle concentration of dodecyltrimethylammonium bromide in aqueous medium

• Micellization of (DTAB + tetraalkylammonium salt) systems have been investigated by conductivity and fluorescence studies.
• CMC values of DTAB displaying a typical U-shaped behaviour with temperature.
• Hydrophobic interactions which strengthen with length of salt’s alkyl chains show the overall dominance.
• Temperature dependence of standard thermodynamic parameters of micellization has been observed.
• The entropy dominance switched to enthalpy dominance around the midpoint of the temperature range examined.

The effect of three organic electrolytes, viz. tetrapropylammonium bromide (C12H28NBr), tetrabutylammonium bromide (C16H36NBr), and tetrapentylammonium bromide (C20H44NBr) (0.01 mol · kg−1) on the aggregation behaviour of a cationic surfactant, dodecyltrimethylammonium bromide (DTAB) (1.5 to 29.8) mmol · kg−1 in aqueous medium over a wide temperature range (288.15 to 318.15) K has been studied by conductivity measurements. From conductivity versus surfactant concentration plots, the critical micelle concentration (CMC) of DTAB has been determined, which shows that all the investigated electrolytes promote the aggregate formation of DTAB. The CMC values decrease to a certain minimum value around T = 298.15 K in all cases and then increase with further rise in temperature, displaying a typical U-shaped behaviour. Further, from the temperature dependence of CMC values, various thermodynamic parameters of micellization viz.   standard free energy change (ΔGmo), standard enthalpy change (ΔHmo) and standard entropy change (ΔSmo) have been calculated. The values of ΔGmo, ΔHmo and ΔSmo indicate that the electrolytes have arbitrated DTAB aggregation and micellization process is both enthalpy as well as entropy controlled. The results reveal the major role of hydrophobic interactions in the present (surfactant + electrolyte + water) ternary system. Fluorescence probe study has also been employed to affirm the CMC values obtained from conductivity measurements.