Abstract
Long-chain molecules are widely used in many commercial products, including waxes, oils, fats and soaps. This study focuses on the primary n-alkylammonium chlorides that have applications as surfactants, detergents and as models for bio-membranes.
The specific topic of this investigation is the polymorphism of three series of n-alkylammonium halides. Polymorphism is the ability of a substance to exist in more than one crystal form. Due to the conformational flexibility of the long alkyl chain and the forces (hydrogen bonding and van der Waals interactions) dictating the packing in these compounds, more than one type of molecular packing is possible, resulting in the crystallization of various polymorphs for each compound.
Various investigations of the polymorphism of n-alkylammonium halides have been published in the scientific literature. This includes mainly studies on the polymorphism and structures of n-alkylammonium chlorides. Only a few reports on investigations of the polymorphism of n-alkylammonium bromides were found in the literature, but no investigation of the polymorphism of n-alkylammonium iodides could be located.
This study is limited to the medium chain length primary n-alkylammonium halides, CnH2n+1N+H3X- where n = 11 to 18 (except 17) and X = Cl, Br and I. It is expected that in this chain length range, both packing forces (hydrogen bonding and van der Waals interactions) will play a role in dictating the molecular packing. It was attempted to crystallize the maximum number of polymorphs of each compound by extensive variation of the crystallization conditions. The parameters varied include crystallization temperature, solvent and crystallization method. Information regarding the polymorphism of a compound crystallized under specific conditions were collected by the complementary techniques of X-ray diffraction and thermal analysis.
X-ray diffraction is the ideal technique to study polymorphism because the result of such an investigation is the three-dimensional packing in the crystal structure. Due to the wide scope of the investigation, only the polymorphic forms stable at room temperature were investigated. The single crystal X-ray technique allows the determination of the crystal structure of a polymorph, but due to the tendency of the compounds to crystallize in thin plates, very few single crystals of good diffraction quality were obtained. Nine crystal structures were, however, determined. Most polymorphic forms were available as polycrystalline powders. The new techniques for crystal structure determination from powder data were employed to determine two crystal structures from powder diffraction data, although at lower precision, and further refined them by the Rietveld technique.
Conventional X-ray powder diffraction is well suited to the identification of polycrystalline materials. The technique does not give direct information regarding the structural nature of the polymorph, but gives a unique fingerprint for each polymorphic form. All polymorphs that were obtained by the various crystallization techniques were characterised by X-ray powder diffraction, and the unique long
Summary iispacing of each polymorphic form was determined from the position of the low angle diffraction peaks in the diffraction pattern. Linear correlations between the chain lengths and long spacings were used to search for the presence of isostructural series amongst the phases. More than one isostructural series could be identified for each homologous series of compounds.
Thermal analysis techniques were employed to determine the phase transition temperatures and enthalpies of phase transitions occurring at temperatures above room temperature. In this investigation the thermal behaviour of polymorphs were investigated by differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and hot stage microscopy (HSM). A stepwise melting behaviour that includes various solid-solid phase transitions was observed for all compounds studied. The sequence of phase transitions that occur with an increase in temperature were found to be unique to a specific polymorphic form. Thermogravimetric analysis provided information regarding the incorporation of solvent in the crystal lattice by monitoring the change of sample weight with increase in temperature. Hot stage microscopy allowed the visual observation of changes occurring in the morphology and texture of the sample with temperature.
This investigation contributed a large amount of information to the pool of knowledge on the crystalline phases of the n-alkylammonium halides. Up to now, not much structural data on the crystal forms of these compounds were available in the literature. In this study, complex patterns of crystal packing and phase transitions were revealed. Six isostructural series of n-alkylammonium chlorides were identified, three of which have not been reported previously, and the crystal structure of one of the novel forms was determined. Four isostructural polymorphic forms of n-alkylammonium bromides were identified. Only two forms have been reported previously in the literature. Six crystal structures of compounds with a novel crystal form were determined. For the homologous series of n-alkylammonium iodides, four novel isostructural series were identified, and one structure was determined.
Relationships between chain lengths and structural parameters like long spacings, unit cell parameters and phase transition temperatures were determined and expressed as mathematical functions.
An analysis of all the known structures (structures reported in the literature and structures determined in this investigation) indicated that different molecular conformations and hydrogen bonds are responsible for differences in the packing, as expressed in the formation of polymorphs. A choice of anion for a specific compound (chloride, bromide or iodide) influenced not only the cell volume, as would be expected, but also dictated the preferential formation of pseudo-polymorphs and complex hydrogen bonding networks in the crystals themselves.
Phase transition temperatures were found to be not simply a function of chain length, but to be significantly influenced by the anion and polymorphic form present.
Prof. G.J. Kruger