Amphiphilic second-order phase transitions determined through NMR

Abstract

In this work, nuclear magnetic resonance (NMR) spectroscopy is used to investigate surfactant phase behavior over a broad concentration region. This technique is an adaptation of a previously developed method applied to detect the critical micelle concentration (CMC) of surfactants. In this method, a surfactant concentration is correlated to the normalized intensity, or area, of the NMR signal for each surfactant. In this procedure a linear relationship develops on either side of the CMC, with a distinct change in slope where the primary phase change occurs. The research conducted herein investigates the NMR response at higher surfactant concentration, where the phase change consists of a change in micelle shape or other structural configurations. These secondary phase transitions can be expected from 10 to 90 wt.%, where the CMC measurements are conducted at concentrations <1 wt.%. The solutions were also analyzed with dynamic light scattering (DLS) and a cross-polarizer microscope to confirm suspected detected phase changes. Visible changes in the rheological response were observed and therefore carefully examined. In a large part, the results seen with all methods corresponded with visible differences in the surfactant solutions and detected changes in the NMR protocol.