Thermal Behavior of Anionic Surfactants

Abstract

Thermal behavior of anionic surfactants, sodium alkyl sulfates, has been studied by thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), infrared absorption spectrometry, X-ray powder diffraction analysis and mass spectrometry. When alkyl sulfates were heated dynamically in air, their weights decreased due to the evapora- tion of the decomposition products and oxidation products. The temperature at which the weight loss started was raised by an increase in carbon number n of the surfactant. The temperatures for the surfactants with n=8, 10, 12, 14, 16 and 18 were 125, 132, 140, 142, 165 and 168° C, respectively. The weight loss was remarkable up to 270° C, and stopped at 550° C. The residue was composed of sodium thiosulfate and sodium sulfate. The analysis by DTA indicated that a phase transition similar to the one observed in fatty acid soap (Curd-Waxy) took place, when the surfactant, whose carbon number was higher than 12, was heated. The heat of transition was about 1 kcal/mol and was ascribed to the microbrownian or rotational motion of methylene groups.