Gel phase originating from molecular quasi-crystallization and nanofiber growth of sodium laurate–water system

Abstract

Adding sodium salts to a dilute sodium laurate (SL) aqueous solution, slow phase transition was observed from the micelle solution to a gel phase. The phase-transition process has great resemblance to crystal salt-out. The observation by transmission electron microscopy (TEM) proved that the hydrogel is a network structure with interweaving fibers and ribbons, which can trap an aqueous solution. The images of high-resolution TEM (HR-TEM) indicated that each of the fibers and ribbons is composed of a bundle of parallel cylindrical nanofibers. Conductivity and in situ diffuse reflectance fourier-transform infrared (DR-FT-IR) spectroscopy measurements indicated that gel formation was induced due to a crystallization of rod micelles by Na⁺ ions bonding to the oxygen groups of salts of fatty carboxylates. Differential scanning calorimetry (DSC) showed that the increase of either the chain length of sodium soaps or the concentration of Na⁺ counter-ions promoted the growth of the nanofibers. We speculated that the phase transition from the micelle solution to the gel phase is a result of a crystallization of rod micelles induced by sodium ions.