Studies on CMC and thermodynamic function of anionic surfactants in DMA/long-chain alcohol systems using microcalorimetric method

Abstract

The critical micelle concentration (CMC) and the thermodynamic function of the anionic surfactant, sodium laurate (SLA) and sodium dodecyl sulfate (SDS) in the N,N-dimethyl acetamide (DMA)/long-chain alcohol systems were studied using titration microcalorimetric method. The power-time curves of SLA and SDS in the presence of a long-chain alcohol (n-heptanol, n-octanol, n-nonanol, n-decanol) in the DMA medium were determined. Then, from the curves, the critical micelle concentration (CMC) and the thermodynamic standard formation functions (ΔH ϑ m, ΔG ϑ m and ΔS ϑ m) were obtained through thermodynamic theories. The relationships between temperature, alcohol’s carbon number, concentration and thermodynamic properties were discussed. For SLA or SDS in a DMA solution, under the same concentration of alcohol, the values of CMC, ΔH ϑ m and ΔS ϑ m increase, while the value of ΔG ϑ m decrease with the increase of temperature. Under the same condition of identical temperature and alcohol concentration, the values of CMC, ΔH ϑ m, ΔG ϑ m and ΔS ϑ m decrease with the increase of the alcohol’s carbon number. In the presence of the same kind of alcohol, the values of CMC and ΔG ϑ m increase, but the values of ΔH ϑ m and ΔS ϑ m decrease with the concentration increases in alcohol series at the same temperature.