Crystal structure of ionic and non-ionic surfactants based on polyoxyethylenated dodecanol: A13C-CP-MAS-NMR and x-ray investigation

Abstract

Crystal structure of monodisperse non-ionic surfactants having the general formula C12H25O−(CH2−CH2−O)nH (C12En),n=7,9, 10, 15, 16 and ionic derivatives, C12H25−O−(CH2−CH2−O)n−CH2COONa (C12EnC),n=3,4,5,6,7,9, 12 has been investigated by13C-CP-MAS-NMR and x-ray diffraction. A structural model, in which aliphatic and polyether segments are segregated in bilayers stacking parallel to the elongation direction of the molecules, fits the experimental data for both series. The experimental values of the repeat distance along the stacking directiond(001) are linearly dependent onn and the slope is nearly equal to twice the repeat distance of7/2 helix conformation, which is typical for crystalline polyethyleneoxide. The values ofd(001) agree very well with the values expected for the C12 segment in a planar “zig-zag” conformation, which is tilted with respect to the polyerther segment (7/2 helix conformation) in such a way that both the aliphatic and the polyether regions adopt the mass density of the corresponding crystalline compound. Two additional phases have been detected for C12EnC. One of them is probably characterized by the planar ‘zig-zag” conformation of the polyether chain. The “meader” model, previously proposed in the literature for surfactants containing polyethylene oxide segments is inconsistent with the obtained experimental data.