An Investigation of the Micellar Packing of 10-Carbon m-s-m Dimeric Surfactants via NMR Spectroscopy

The conformation and orientation of a series of dicationic dimeric amphiphiles of the type N,N’-bis (dimethylalkyl)-α,ω-alkanediammonium dibromide (m-s-m) in aggregated form have been studied via 13C chemical shift measurements and 2D-rotating frame Overhauser enhancement spectroscopy (2D-ROESY). 13C Chemical shift changes (Δδ values) support the tendency for the intercharge arm (the spacer group) to adopt an extended (“stretched”), trans-style configuration for short spacer lengths. As the spacer length increases, the 13C chemical shifts are consistent with some of the methylene groups in the spacer bending back towards the interior of the micelle and mixing with the main carbon chain in the micelle interior. Cross-peaks between the main alkyl chains of the dimeric amphiphiles and the spacer protons are present for the three longer chain spacer dimerics in the 2D-ROESY spectra but are absent in the spectra for the shortest spacer dimeric amphiphile. These results indicate the longer spacers can loop towards the aggregate interior and interact with the protons on the carbon atoms near the end of the main surfactant chains, whereas these interactions are absent for the shortest spacer dimeric studied. These results indicate that the conformation of the spacer group along with crowding in the vicinity of the two dimeric head groups is consistent with significant changes in electrostatic interactions, hydrophobic interactions, and hydrophobic effects that help explain the complex balance of forces that leads to their self-assembly.