Aggregation behavior of sodium deoxycholate and its interaction with cetyltrimethylammonium bromide in aqueous solution studied by NMR spectroscopy

In pure sodium deoxycholate (NaDC) dilute solution, the transverse relaxation times (T2) of nearly all proton signals of NaDC obey single-exponential decay, with exception for proton at 3-position (H3) that decays in a two-exponential manner. The two components with different mobility of H3 of NaDC indicate that the molecules are probably associated in head-to-tail pairs via hydrogen bonds in dilute solution. In the mixed NaDC/cetyltrimethylammonium bromide (CTAB) solutions, the T2 values of interested NaDC protons are far less than those in pure NaDC solution, especially when the concentrations of the two components are close to equal-molar. The results of self-diffusion coefficients and the chemical shifts confirm further that the strong interaction occurs between the two components in mixed solutions, especially for equal-molar condition. The arrangement of the mixed aggregates can be speculated for the cross-peaks of proton pairs occurring between NaDC and CTAB molecules in two-dimensional and rotating frame nuclear Overhauser enhancement spectroscopy spectra.