¹⁴N and²H NMR Study of the Mesophases of Cetyltrimethylammonium Bromide in Formamide

Two mixtures of cetyltrimethylammonium bromide (CTAB) and formamide (FA), (60/40 wt % (CTAB60) and 78/22 wt % (CTAB78)), which form lyotropic mesophases, were studied by using H-2 and N-14 NMR spectroscopy at varying temperatures. Both CTAB and FA give a N-14 resonance, whereas the H-2 resonance arises from a small amount of FA-d(3) in the samples. From low to high temperature, the following phases were observed: liquid + crystal --> hexagonal --> isotropic and liquid + crystal --> hexagonal --> cubic --> cubic + lamellar --> lamellar for CTAB60 and CTAB78, respectively. The line shapes of the H-2 NMR patterns of deuterated formamide are explained with a model in which formamide is moving fast compared to its nuclear quadrupolar coupling constants in uniaxial domains oriented in different directions. For the calculation of the orientation parameters of formamide, nuclear quadrupolar coupling constants corresponding to both gas and liquid phases were utilized. The gas phase values of electric field gradient (EFG) tensors were obtained by first principles calculations applying both coupled cluster doubles and density-functional theories. The N-14 EFG tensor was calculated for a model of CTAB to compare the orientation of formamide and CTAB. The average location and orientation of formamide among the micelles is discussed on the basis of the best suited nuclear quadrupole coupling constants and orientation parameters.