The use of one- and two-dimensional NMR to investigate orientational and conformational effects in micellized CTA+ surfactants having aromatic counterions

One (1D) and two dimensional (2D) nuclear magnetic resonance (NMR) techniques were utilized to gain microstructural and conformational information about cetyltrimethylammonium 3,5-dichlorobenzoate (CTA3,5) and cetyltrimethylammonium 2,6-dichlorobenzoate (CTA2,6) surfactants in their micelles. CTA3,5 forms rod-like micelles in aqueous solution which exhibits viscoelastic behavior. CTA2,6, however, forms spherical micelles in water; these solutions are not viscoelastic. Due to appreciable line-broadening effects of CTA3,5 rod-like micelles, cetryltrimethylammonium bromide (CTAB) and CTA-dichlorobenzoate mixtures were used.

^{1H chemical shift studies indicate that in their respective micelles both dichlorobenzoate counterions intercalate among the surfactants' trimethylammonium headgroups. (T, relaxation studies also support counterion intercalation.) Larger shielding effects observed on the headgroup through e methylene 1H 's in solutions of CTA3,5 rod-like micelles indicate that 3,5-dichlorobenzoate counterions insert, on average, further into the micelle than 2,6-dichlorobenzoate counterions.}

^13C chemical shift data suggest that in micelles the mid-chain CTA⁺ chain carbons experience a greater chain straightening effect (e.g. more trans conformers on average) than those at either end of the chain. Greater mid-chain effects observed in the CTA3,5 solutions suggest that the CTA⁺ chains experience more chain extension in rod-like micelles than in the CTA2,6 spherical micelles. The chemical shift effects observed for the headgroup methyl and methylene carbons near the micellar interface implies that the CTA⁺ chains experience different conformations in CTA3,5 and CTA2,6 micellar solutions. 13C chemical shifts for the carbonyl carbons of the counterions in the micellar solution suggest that the 3,5-dichlorobenzoate counterions intercalate more deeply into the micelle. Studies of 10%, 30%, and 50% CTA3,5 and CTA2,6 solutions indicate via ROESY 2D NMR cross-peak integrations that the CTA⁺ chains in the CTA3,5 micelles experience less chain reversal and are, on average, more extended than in the CTA2,6 micelles. ROESY experiments of 100% CTA2,6 solutions (10mM and 40mM) support the inference that the methylenes near the micellar interface prefer a conformation which places the ε methylene close to the headgroup. Intermolecular interactions between the CTA⁺ chain and the dichlorobenzoate counterions also appear. Relative intermolecular and intramolecular distance information from these studies has been calculated.