Structure and Thermal Behaviour of Lanthanide(III) Soaps

During this project the structure and thermal behaviour of lanthanide(III) soaps was studied. We tried to find out the determining factor for the existence of a stable lanthanide containing mesophase and what changes could be done on the composition to form such a stable mesophase. First we studied the lanthanide(III) alkanoates (Ln(CxH2x+1CO2)3). We were able to determine the crystal structure of some lanthanide(III) butyrates. These compounds form infinite layers of lanthanide ions connected by carboxylate groups. The alkyl chains are placed perpendicular to these layers and are in the all trans conformation. Via comparison of X-ray powder diffractograms at room temperature and of IR spectra of several compounds with these of the lanthanide(III) butyrates, we extrapolated this bilayer structure to all the lanthanide(III) alkanoates. We found that the size of the lanthanide(III) ion has a critical effect on the existence of a mesophase. So only dodecanoates of the first four lanthanides with the largest ionic size (i.e. La, Ce, Pr and Nd) exhibit a mesophase and the mesophase stability range rapidly decreases traversing the lanthanide series. The fact that the other lanthanide(III) dodecanoates do not show mesomorphism can be attributed by their small ionic size and hence by unfavourable electrostatic interactions of the carboxylate groups. As the ionic radius of the lanthanide ion decreases, the distance between the carboxylate groups on either side of the plane containing lanthanide ions decreases. The amplitude of the thermal vibrations increases on increasing the temperature and thereby induces unfavourable electrostatic interactions (repulsions) between the carboxylate groups, not only between those of the two opposing layers, but also between adjacent carboxylate groups within the same layer. When the repulsive forces between the negative charges are stronger than the attractive forces between the negative (carboxylate groups) and positive (lanthanide ions) charges, the bilayer structure is no longer stable and breaks down. In this case, a rearrangement of the carboxylate groups and lanthanide ions to obtain a more stable solid structure (i.e. a crystal-crystal transition) might be expected. However, melting of the compounds is observed. The alkyl chains have sufficient thermal energy for their all trans conformation to be lost when the layer structure breaks down. This theory explains the decrease of the melting point within the lanthanide series. The smaller the lanthanide ion, the more unstable the layer structure becomes, and hence less thermal energy is required to break down the solid state structure. A mesophase is formed, when, at the melting point of the alkyl chains, the electrostatic attraction between the lanthanide ions and the carboxylate groups is still sufficiently high to maintain a layer structure. This is the case for lanthanum(III), cerium(III), praseody-mium(III) and neodymium(III) compounds. At the melting point of the non-mesogenic lanthanide(III) alkanoates and the clearing point of the mesogenic complexes, the metal soaps are converted to an isotropic less-ordered structure with a rather low viscosity. The chain length also influences the thermal behaviour of lanthanide(III) alkanoates in the sense that the melting point increases with increasing chain length, whereas the clearing point decreases. Moreover, the short chain homologues of the mesomorphic lanthanide(III) alkanoates exhibit two mesophases. This mesophase M is formed at moderate temperatures and can be considered as a layered structure with partially molten alkyl chains. Increasing the temperatures gives rise to a rearrangement of this mesophase to a smectic A phase. The fact that only the first three lanthanides (La, Ce and Pr) exhibit a mesophase across the complete homologous series (4 £ x £ 19), and that the neodymium series does not show mesomorphism for the longer chain lengths, can be explained by taking into account both the effect of the lanthanide ion and the chain length. Whereas for lanthanum(III), cerium(III) and praseodymium(III) alkanoates the ionic radius is sufficiently large to reduce unfavourable electrostatic interactions between the carboxylate groups (and thus the melting of the alkyl chains is the major factor determining the thermal properties of these compounds), for neodymium soaps there is a competition between the stabilisation of the ionic layers and melting of the alkyl chains. For the shorter alkyl chains, their melting is the determining factor, whereas for the longer soaps the thermal energy required to melt the alkyl chains is so high that the ionic layer structure breaks down before the alkyl groups are completely molten. Because the size of the lanthanide ion strongly influences the mesomorphic behaviour of lanthanide(III) alkanoates, it was interesting to find out if a mesophase could be induced by mixing a mesogenic and a non-mesogenic compound. Therefore we synthesised following complexes [LaxLn1-x(C11H23CO2)3] (x is the mole fraction La(III), Ln = Eu, Tb, Ho, Yb) and investigated their thermal behaviour. We observed the induction of a mesophase. This means that we were able to stabilise the ionic lanthanide layer by mixing a mesomorphic lanthanide(III) dodecanoate with a large ionic radius, and a non-mesogenic lanthanide(III) dodecanoate (the lanthanide ion has a small ionic radius). In the case of lanthanum-europium mixtures a very small amount of lanthanum is needed to induce mesomorphism in the mixtures. The amount lanthanum necessary increases with decreasing ionic size of the lanthanide ion. These experiments are in agreement with our theory on the effect of the size of the lanthanide ion on the thermal behaviour of lanthanide(III) alkanoates. Another possibility for the stabilisation of the ionic layer is the introduction of a neutral ligand, like 1,10-phenanthroline. We synthesised complexes with several lanthanide ions and with several chain lengths and investigated their structure and thermal behaviour. Because we were able to determine the crystal structure of three lanthanide(III) alkanoate phenanthroline complexes, we could link the thermal behaviour unambiguously to the structure. These complexes form isotropic spherical dimers without any interaction between these dimers. So it was not so surprising to find out that these complexes are not liquid crystalline. However the unfavourable ionic interactions between the carboxylate groups were stabilised, which is expressed in the high melting points. In this project we were not only interested in normal lanthanide(III) alkanoates, but also complexes with 4-alkoxybenzoic acids were topic of investigation. These ligands are well known for their mesomorphism, so we wanted to investigate the influence of complexation with lanthanide ions hereon. It was very clear that complexation strongly influences the mesomorphism of the ligand. To declare the strange behaviour of lanthanide(III) 4-alkoxybenzoates during DSC and thermo-optical microscopy measurements, we have done synchrotron radiation studies on these compounds. We found out that here also the size of the lanthanide ion determines the structure of the complex and hence the thermal behaviour. A last example of not normal lanthanide(III) alkanoates are the lanthanide(III) dodecylsulphates. Instead of the carboxylate function as coordination site, here a sulphate group is used. Lanthanide(III) dodecylsulphates are not thermotropic liquid crystals, moreover they decompose at moderate temperatures. However, we found that these complexes form lyotropic liquid crystals when brought in contact with ethylene glycol or water or mixtures hereof. It must be said that this investigation of lanthanide(III) soaps gave rise to a lot of information on the synthesis, structure and thermal behaviour of these compounds. We hope that more industrial application will be found, now their structure and thermal behaviour are understood.status: publishe

Solid state structure and lyotropic mesomorphism of rare-earth trisdodecylsulphates in the water-ethylene glycol system

The phase behaviour of lanthanide(III) dodecylsulphates, Ln (C12H25SO4)(3), by thermo-optical microscopy using Lawrence penetration technique was investigated. The lyotropic phase behaviour of lanthanide(III) dodecylsulphates in ethylene glycol water in mixtures hereof, depends on the composition of the solvent. For pure ethylene glycol and mixtures of ethylene glycol and water three different mesophases are formed, i.e. a lamellar, a cubic and a hexagonal phase, whereas when water is used as solvent no cubic phase is formed. The size of the lanthanide ion has no influence on the mesomorphism of these metallomesogens, although the smaller the lanthanide ion the lower the solubility.status: publishe

Crystal structure of lanthanum(III) butyrate monohydrate

Single crystals of lanthanum butyrate monohydrate were obtained by reaction between lanthanum hydroxide and an aqueous butyric acid solution. The crystal structure (triclinic P (1) over bar, Z=2, a=9.940(2) Angstrom, 0=12.182(2) Angstrom, c=14.652(2) Angstrom, alpha =85.98(3)degrees, beta =75.62(3)degrees, gamma =78.17(2)degrees) consists of layers parallel to (001). The alkyl chains are in an all-tr-ans conformation parallel to (110). The layers are constructed by lanthanum chains, which are connected to one another by bridging bidentate carboxylates.status: publishe

Mesomorphic rare-earth soaps

poster presented by Liesbet Jongenstatus: publishe

Thermal behaviour of lanthanide soaps

poster presented by Liesbet Jongenstatus: publishe

Solid state structure and lyomesomorphism of rare-earth dodecyl sulphates

poster presented by Koen Binnemansstatus: publishe

Thermal behaviour of lanthanide soaps

poster presented by Ben Thijsstatus: publishe

Structure and thermal behaviour of lanthanum soaps

poster presented by Liesbet Jongenstatus: publishe

Thermal behaviour of lanthanide soaps

poster presented by Liesbet Jongenstatus: publishe