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NMR Investigation on the Various Aggregates Formed by a Gemini Chiral Surfactant

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journal contribution
posted on 04.01.2000, 00:00 by Luciana Luchetti, Giovanna Mancini
In this paper we report an NMR investigation on the chiral gemini surfactant (2S,3S)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1), carried out to study the aggregation in various solvents. If the aggregation equilibrium of 1 can be described by the mass action law model, by observing the variation of chemical shift with respect to [1], we can obtain the aggregation number n. In CDCl3 the values are n = 2 and 19 ± 3, at low and high [1], respectively, indicating that 1 is present in dimeric assemblies and as reversed micelles. In CD3OD, despite evidence that 1 is under aggregating conditions, the model does not hold; 1 should form a relatively flexible structure, because in the 13C NMR spectra 1J(13C,14N) coupling is observed (1J = 3.4 Hz). 1 is scarcely soluble in D2O; in the 1H NMR spectrum of 1 × 10-3 M 1, the signals relative to the tails and to one of the NCH3 groups disappear, while the other head group signals are well resolved, indicating the presence of large assemblies. These large aggregates are confirmed by preliminary experiments of dynamic laser light scattering. The signals of the two NCH3 groups are very different in all the solvents investigated. This result can be interpreted in terms of an aggregate conformation in which they undergo different environments.