Synthesis and physicochemical properties of non-ionic and cationic surfactants derived from methionine

Amino acid based surfactants have been widely used as alternative to classical surfactants due to their interesting physicochemical properties, biological activities, biodegradability and low toxicity. The study of the physicochemical properties of surfactants such as critical micelle concentration (CMC) and Krafft temperature is an important aspect in determining their use in detergents or pharmaceutical compositions. In this study, a series of non-ionic and cationic surfactants with varying chain lengths (C8-C12), namely the O-alkyl esters of methionine and their hydrochloride derivatives respectively were synthesized and characterized by spectral data. In view of systematically studying the effect of the head group and chain length on the physicochemical properties, the critical micelle concentrations of both series were evaluated using conductivity measurements and pyrene fluorescence spectroscopy. The Krafft temperatures of selected synthesized methionine-based surfactants were also evaluated.
The methionine surfactants were synthesized in good to moderate yield (54 – 81%). For both ester and the hydrochloride series, the CMC of the surfactants were found to decrease with chain length, as commonly observed for all surfactants. The non-ionic O-alkyl esters of methionine displayed relatively lower CMC values compared to their ester hydrochloride derivatives for all the chain length studied. The ester hydrochloride derivatives showed an increase in the Krafft temperature with increasing chain length showing that the surfactants with longer chain length has a tendency to micellise at a much higher temperature.