Temperature dependence of enthalpies and heat capacities of sodium dodecyl sulfate (SDS) in water

Abstract

The enthalpies and heat capacities of solution of an anionic surfactant sodium dodecyl sulfate (SDS) in water were determined both in the monomeric as well as in the micellar state as a function of temperature from 288.15 to 323.15 K. From these results enthalpies and heat capacities of micellization (ΔH_m and ΔC_{p, m}) were evaluated as a function of temperature. With the increase in temperature, ΔH in the monomeric (ΔH_s⁰) as well as in the micellar state (ΔH_s) increases, more so in case of the latter which results in a decrease in ΔH_m as a function of temperature. We have observed a minimum in ΔC_{p, m} vs T plots around 300 K which is a consequence of a maximum in heat capacities in the monomeric state (ΔC_p,s⁰), heat capacities in the micellar state (ΔC_p,s) not showing any significant temperature dependence. These results show that to understand the thermodynamics of micellization the contribution due to polar and nonpolar groups both in the monomeric as well as micellar state should be assessed.