N-14 AND H-2 NMR-STUDY OF THE MESOPHASES OF CETYLTRIMETHYLAMMONIUM BROMIDE IN FORMAMIDE

Two mixtures of cetyltrimethylammonium bromide (CTAB) and formamide (F A), (60/40 wt % (CTAB60) and 78/22 wt % (CTAB78)), which form lyotropi c mesophases, were studied by using H-2 and N-14 NMR spectroscopy at v arying temperatures. Both CTAB and FA give a N-14 resonance, whereas t he 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: liq uid + crystal --> hexagonal --> isotropic and liquid + crystal --> hex agonal --> cubic --> cubic + lamellar --> lamellar for CTAB60 and CTAB 78, respectively. The line shapes of the H-2 NMR patterns of deuterate d formamide are explained with a model in which formamide is moving fa st compared to its nuclear quadrupolar coupling constants in uniaxial domains oriented in different directions. For the calculation of the o rientation parameters of formamide, nuclear quadrupolar coupling const ants corresponding to both gas and liquid phases were utilized. The ga s 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 f or a model of CTAB to compare the orientation of formamide and CTAB. T he 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.