Ilsa Basson

Identification of defect chain motions in the low temperature orthorhombic phase of binary mixtures of n‐alkanes by means of nuclear magnetic resonance spin‐lattice relaxation time measurements

Powder x‐ray diffraction, differential scanning calorimetry, and nuclear magnetic resonance spin‐lattice relaxation time measurements on a series of n‐alkane binary mixtures are reported. The results have been interpreted in terms of a model in which the longer chains execute thermally activated end‐gauche defect motions, while the shorter chains are involved in twofold screw motions. The results are compared with infrared and x‐ray diffraction results.

The structure and melting of paraffinic Fischer—Tropsch waxes

NMR results, which show that the mobile amorphous zone of a wax is formed only after voids in the amorphous zone have been filled by shorter chains, are presented. A model for the melting of a wax is proposed.

Identification of a defect chain motion in n‐alkanes by means of nuclear magnetic resonance spin–lattice relaxation time measurements

Proton spin–lattice relaxation time measurements in the rotating frame as a function of inverse temperature have revealed shallow minima not previously observed for a series of n‐alkanes ranging from n‐C22H46 to n‐C41H84. These minima cannot be interpreted in terms of reorientations of methyl groups or entire chains. The most likely reorientational model is one in which a thermally activated trans–gauche defect motion modulates the internuclear dipolar interactions. The results suggest that the gauche defect equilibrium orientation has a higher energy than that of the ordered all‐trans equilibrium orientation. Activation energies for the motion as well as energy differences between the two orientations have been extracted from the results for these n‐alkanes.

Implementation of the new FET Physical Sciences curriculum : teachers' perspectives

Abstract The new South African Further Education and Training (FET) curriculum is currently in the implementation phase. The paper reports on the responses and views of 59 Physical Science teachers in the northern part of South Africa about their experiences with the implementation of this curriculum. The paper is conceptualised around the belief that views inform practice. and that the teachers need to hold positive views about the new curriculum in order for it to be successfully implemented. Data from questionnaires and focus group interviews showed that even though the teachers generally held positive views about the new FET Physical Science curriculum, they had concerns about the content overload, their ability to teach it, the available resources and the level of support they were given as well as the quality of the training which they received. The perspectives of these teachers were used to evaluate the degree of implementation of the new curriculum against a theory of implementation with particular reference to developing countries.

Defect chain motions in the low temperature phase of multiple mixtures of n‐alkanes by means of nuclear magnetic resonance spin–lattice relaxation time measurements

Nuclear magnetic resonance spin–lattice relaxation time measurements in the laboratory and rotating frames, supported by differential scanning calorimetry and x‐ray diffraction measurements, are reported for a series of multiple mixtures consisting of Gaussian chain length distributions of n‐alkanes. The results revealed three types of molecular motions, viz., threefold reorientations of methyl groups, gauche defect motions of the chain ends of the long chains and twofold screw motions of the short chains in these mixtures. The main factor influencing the latter two motions is the width of the chain length distribution.

Structural framework of a medium Fischer-Tropsch wax fraction determined by electron crystallography

The structural framework of a medium hardness Fischer-Tropsch wax distillate is established quantitatively by electron crystallography and compared to model paraffin assemblies with a similar Gaussian distribution of chain lengths. The lamellar packing closely resembles the crystal structure of refined petroleum waxes with a similar distribution of defects near the lamellar interface. Nevertheless, clear differences associated with the absorption of smaller chains within the lamellar interface, detected by NMR, are not resolved by these diffraction measurements, perhaps due to artefacts induced by the high vacuum of the experiment and/or specimen preparation.

Structures and molecular dynamics of binary mixtures of n-alkanes: C46:C38 and C46:C44

Abstract The lamellar packing and molecular dynamics of n -alkane binary mixtures C 46 :C 38 and C 46 :C 44 have been investigated by employing NMR, DSC and X-ray powder diffraction techniques. Different methods of crystallization resulted in different structures for the same binary mixture. Three relaxation mechanisms were present in all samples, viz. threefold reorientation of methyl groups, defect motions of chain-ends and translational-rotational diffusion of chains prior to melting.