SANS contrast matching for the unambiguous localization of anionic dye in cationic surfactant micelles

Contrast variation in small-angle neutron scattering (SANS) was successfully applied to localize the anionic azo dye Blue in co-assemblies with the cationic surfactant dodecyltrimethylammoniumbromide (DTAB). For this purpose, the scattering contrast between DTAB and the aqueous solvent was eliminated by SANS contrast matching, leaving only the scattering signal from Blue to be detected. Results obtained by contrast matching were confirmed by NOESY NMR-spectroscopy, showing that Blue interacts with the positively charged DTAB head groups and with up to the 4th neighbouring methylene group of the DTAB C12-alkyl chain. Its localization in the outer layer of the Blue-DTAB co-assembly explains the uniaxial growth of spheroidal DTAB micelles to wormlike micelles with increasing [Blue] : [DTAB] ratio from 0 : 1 to 1 : 3. This is in line with the concept of the packing parameter for amphiphilic substances. The complementary use of SANS contrast matching and NMR-spectroscopy permits the unambiguous localization of an anionic dye in dodecyl trimethyl-ammonium bromide micelles.

Automated summary

Contrast variation in SANS was applied to localize anionic azo dye Blue in co-assemblies with cationic surfactant DTAB. Contrast matching eliminated the scattering contrast between DTAB and solvent, leaving only the signal from Blue. Results were confirmed by NOESY NMR-spectroscopy, showing interactions between Blue and DTAB and explaining the growth of DTAB micelles. The complementary use of SANS contrast matching and NMR-spectroscopy allows for the unambiguous localization of anionic dye in DTAB micelles.