Conformational studies of lithium phenyl stearate.
The structure and conformation of lithium phenyl stearate (and to a lesser extent, for comparative purposes, cadmium stearate) was investigated using Fourier transform infrared spectroscopy, and various modelling techniques. The infrared results for LiPS show that the aliphatic portion of the soap molecule is much more ordered at room temperature than had been expected, having only 0.62 and 0.60 gtg and gg defects per molecule respectively, where an isotropic chain would have 1.35 and 1.21 gtg and gg defects per chain respectively. As the temperature is increased the number of conformational defects increases continuously, until at <130°C the chain reaches an isotropic degree of disorder. At this point the phase transition begins, so the chain reaches liquid like disorder before the phase transition begins.Modelling of the phenyl stearic acid showed that the phenyl group was restricted to certain angle of rotation values, and that the bonds close to the phenyl group were prevented from attaining true rotational isomeric state conformations, gtg defects near the phenyl group were distorted only slightly from their usual angular position, and an additional band in the infrared spectrum of LiPS at 1363 cm-1 has been assigned to this distorted gtg/gtg' defect. The gg defects near the phenyl group have a much greater distortion (and energy) resulting in a much reduced probability of occurrence. The number of gg defects present at the phase transition (<130°C) was only 75% of that expected for an isotropic n-alkane of equivalent chain length, indicating that the four bonds nearest to the phenyl group have a reduced probability of forming a gg defect.The modelling of the ionic core of LiPS gives a reasonable estimate of between 5.6 to 7.1 A for the core radius. When this is used to calculate the hexagonal cylinder diameter, at room temperature, along with the average chain extension, it gives a value for the cylinder diameter of between 33.9 to 36.8A. The hexagonal lattice parameter determined by X-ray diffraction has a value 35.9A. Also after the LiPS sample has gone through the phase transition beginning at >130°C, the hexagonal lattice parameter is 31.4A while the cylinder diameter lies between 30.2 and 33.2A.Crystalline cadmium stearate was found to contain two crystal forms, orthorhombic which has lattice dimensions of a0=5.05A, b0=7.35A and c0=48.6A and the other eithermonoclinic or triclinic. In the reverse hexagonal phase, the cadmium stearate molecule behaves like an isotropic n-alkane of equivalent chain length. The model used to predict the core radius of divalent metal soaps gives rise to some inconsistencies: the cylinder diameter thus determined gives a result between 28.8A to 31.7A, while the lattice parameter determined by X-ray diffraction gives a value of 36.9A. The assumption that the n-carboxylate ions in a divalent metal soap behave like two independent monovalent metal ion soaps appears to be incorrect.
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Sheffield Hallam University (United Kingdom).