Synthesis and physicochemical properties of star-like cationic trimeric surfactants

Efficient synthesis of four star-like triethanolamine based cationic trimeric surfactants Tn (where n is the tail chain length, n = 10, 12, 14, and 16) with ester groups in spacers has been successfully achieved, where mild reaction conditions were employed by substituting triethylamine (TEA) with potassium carbonate as the acid binding agent. The physicochemical properties of these surfactants were investigated by means of surface tension, electrical conductivity, and steady-state fluorescence measurements. The trimeric surfactants have been found to be capable of adsorbing to the air/water interface and orienting themselves through the interactions among the hydrocarbon chains. The conductivity measurement revealed the formation of premicellar aggregations in the T16 solution. It is noteworthy that the micropolarity of the surfactant aggregations decreased with the elongation of the hydrophobic chain length. Typically, dynamic light scattering and transmission electron microscopy studies showed that the aggregations of the four surfactants changed from larger ellipsoidal vesicles to smaller globule micelles with the increase of the surfactant concentration. It is assumed that this unusual aggregation behavior is related to the transformation of molecular conformation from stretched configuration to a pyramid-like shape.