Compared organization of the molecules of NaDEHP and AOT: Determination of the microscopic organization of the sodium bis(2-ethylhexyl)phosphate molecule in the solid state in the reversed hexagonal liquid crystal state

Abstract A method to determine the average shape of some molecules is described and applied to sodium bis(2-ethylhexyl)phosphate (NaDEHP) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT). The volumes and lengths of the polar and apolar parts of these molecules are obtained from specific gravity measurements and from the lattice parameters of the hexagonal and lamellar liquid crystal structures. The shape of both molecules is described as a prism. The cross section of the elementary rod in the hexagonal structure intercepts respectively 2.5 and 6 molecules. The noninteger value obtained for NaDEHP is explained by a disorder inside the rod, and the mean value of 6 obtained for AOT is related neither to the symmetry of the lattice nor to the hexagonal average shape of the rod. The parameter limiting the height of the prism is the size of the phosphate coordinance tetrahedron, in the case of NaDEHP, and the length of the succinate chain, in the case of AOT.