X-ray diffraction studies on homologous thallium soaps below the temperature range of the neat phase 2. Structure models of crystalline modifications in the temperature range below the neat phase

Abstract

In Part 1 were discussed the results of X-ray diffraction measurements on homologous thallium soaps, the lattice parameters and the unit cells of the different phases: phase C₁, phase C₂, phase I, phase II, and the neat phase.

The aim of Part 2 is the development of structural models, based on data on lattice parameters, packing densities of hydrocarbon chains and head groups. In order to supplement our experimental data we carried out infrared-spectroscopic measurements at different temperatures within the existence regions of these phases to get additional information on alterations in configurations of head groups and hydrocarbon chains of thallium soaps in these phase regions.

By comparing the structural models we can generalize that the phase C₁, phase C₂, phase I, and phase II have some properties in common: they form crystalline structures, their unit cells are monoclinic, and the angles α and β are orthogonal. The amount of the lattice parametersc is equal to the distances of two molecular bilayers and/or twice the distance between the lamellae.

Among the normal lamella, the molecules form a “herring-bone-like” packing. This indicates that the lamellar structure of the liquid-crystalline neat phase is already preformed in these crystalline modifications of phase C₁, phase C₂, phase I, and phase II. These crystalline phases differ in their lattice parameters and their packing coefficients. The transition from the crystalline phase I or phase II into the neat phase is accompanied by a temperature-dependent contraction of the distances between the lamellae, which originates in the “fluidity” of the hydrocarbon chains. Simultaneously, the lattice parametersa andb are drastically shortened and attain dimensions considerably lower than the values obtained in crystalline modifications. The X-ray scattering properties of the Tl⁺-ions suggest that the Tl⁺-ions exist in a “fluid-like” state within the head groups region of the neat phase.

We came to the conclusion that the thallium-oxygen-bond is remarkably less polar than the bonds between alkali ions and oxygen resulting from the electro-negativity of the metal-oxygen-bonds of the alkali soaps, in comparison to the thallium soaps. Hence, the absence of polymorphic liquid-crystalline semiphases of thallium soaps is due to the low polarity of the thallium-oxygen-bond. Structure formation of Tl-soaps is mainly influenced by the hydrocarbon chain packing.