A FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY OF DODECYLTRIMETHYLAMMONIUM CHLORIDE SODIUM DODECYL-SULFATE SURFACTANT MIXTURES

The utility of FT-IR in characterizing the composition-dependent changes in packing of DTAC and SDS in mixed micelles is demonstrated. As the mixed micelle composition becomes equimolar, the micelle aggregation number is known to increase, indicating a spherical to nonspherical micelle shape change. The frequency of the composite CH2 stretching bands decreases as the composition becomes equimolar, suggesting a decrease in the gauche/trans conformer ratio in the tails of the surfactants, which is a result of the increased crowding of the methylene chains accompanying the shape change. The use of SDS-d25 allows confirmation of this trend through the inspection of the shifts in the CH2 stretching bands (DTAC tails) and the CD, stretching bands (SDS-d25 tails). Electrostatic interactions between the headgroups of DTAC and SDS can also be monitored spectroscopically. The shifts in the asymmetric and symmetric S-0 stretching bands with micelle composition indicate an increase in SDS headgroup ordering in SDS-rich mixed micelles and a dominance of interactions with DTA+ ions in DTAC-rich micelles. Similarly, a large difference in the electrostatic environment of DTAC headgroups with composition can be detected by changes in CH3-N+ deformation bands. The relatively larger, nonspherical mixed micelles exhibit a synergistic increase in decane solubilization capacity. This change may be attributed directly to the decrease in Laplace pressure of the micelle core, the solubilization site for decane. The coincidence of the aggregation numbers, solubilization, and FT-IR data suggests the utility of infrared spectroscopic techniques in studying changes in micellar structure. © 1990, American Chemical Society. All rights reserved.