ray source Detector

Computer 20 (scanned)

Powdered sample in holder

Figure 2.10 Diagram representing a typical X~ray diffractometer.

Bragg showed in his equation (Equation 2.1), that X-rays appear to be scattered from a crystal only when the angle o f incidence o f the X-ray beam (0) satisfies the condition:

sin 0 = nX ... Equation 2.1 2d

Where n is a multiple o f the wavelength (A,) and d is the interplanar distance in the crystal. The diffraction angle (20) is determined from the spacing between a certain set o f planes and with the aid o f the Bragg equation the distance (d) can be calculated from the known wavelength o f the X-ray source and measured angle. Identification o f any crystalline solid is established from the position o f the lines (in terms of 20) and their relative intensities. It is an efficient technique for both qualitative and quantitative determination o f crystalline compounds provided that grinding o f the crystalline sample into a homogenous fine powder is carried out prior to testing. This way, a huge number o f small crystallites are oriented in every possible direction; thus, when the X-ray beam passes through the sample, a significant number o f particles can be expected to be oriented in a way that fulfils the Bragg condition for reflection from every possible interplanar spacing (Skoog et al., 1998). The X-ray powder diffraction technique is not considered as efficient for characterization o f amorphous solids as it is for crystalline ones since amorphous solids lack a three-dimensional long-range order. Saleki- Gerhardt et al., (1994b) showed that lower values o f standard deviation o f the measured amorphous content are noticed for samples with higher percentage of amorphous

material. On the other hand, they stated that differences in measured amorphous content in a sample that is 90% crystalline and that which is 100% crystalline could hot be distinguished using X-ray powder diffraction. This relatively low detection limit o f amorphous content is due to the fact that this technique measures bulk properties and thus the entire sample is analysed as a whole. This makes the amorphous content, comprising a small part o f the total signal, difficult to detect with sufficient confidence (Buckton and Darcy, 1999).

The instrument used in this thesis was a Philips PW 1840 using copper alpha X-ray tube with a compact diffractometer. Scan range of 2- 60° at 20 step size. Goniometer speed is 0.02 2°0/sec. Chart speed is 10 mm/ 2°0. Receiving slit width is 0.2 mm. The experiments were carried out at ambient conditions (temperature and relative humidity).

2.1.8 SCANNING ELECTRON M ICROSCOPY (SEM):

This is a microscopic method, which can be used for imaging, defining morphology and shape o f solid surfaces. Detailed knowledge o f the physical nature o f the surfaces o f solids is o f great importance in many fields including materials science as the surface o f a solid usually differs substantially from its interior in many aspects including its physical properties. The classical method for surface monitoring o f solids was by optical microscopy, but its use has been superseded by Scanning Electron Microscopy (SEM) as the latter provides considerably higher resolution and depth o f focus than the optical microscope (Skoog et al., 1998). In a scanning electron microscope, the surface o f a solid sample is swept in a raster pattern with a beam o f energetic electrons. A raster is a scanning pattern in which an electron beam is swept across a surface in a straight line in the direction of x-axis and is then returned to its starting point followed by a downward shift o f the beam in the direction o f y-axis (Skoog et al., 1998). It can thus be said that using the raster pattern, a sample is scanned in a series o f parallel tracks. This process is repeated until a desired area o f the surface has been scanned. While the scanning process is carried out, a signal is received above the surface and stored in a computer where it can be converted to the produced image. Several types o f signals are produced from the surface as a result o f interaction with the electrons. These signals most commonly include, backscattered electrons, secondary electrons and X-ray emission (Skoog et al., 1998). All o f these signals can be captured and digitally displayed. In this study, samples were adhered to an SEM stub using double sided

carbon impregnated tape. Samples were gold coated prior to viewing in an Emitech K550 Sputter Coater for 3 minutes at 40 mA. Images were captured using a Philips XL20 Scanning Electron Microscope.

2.1.9 ELEMENTAL ANALYSIS (EA):

This analytical method is used for the determination o f CHN and is based on the quantitative “dynamic flash combustion” method, which converts all organic and inorganic substances into combustion products. The procedure is carried out by holding a sample in a tin container and dropping it into a vertical quartz tube maintained at 1020 °C (combustion reactor), the sample is purged with a continuous flow o f helium gas, but when it is dropped inside the furnace, the helium flow is temporarily mixed with pure oxygen gas to trigger combustion o f the sample. The sample and tin container will then flash combust. The mixture o f evolved gases is then passed over a catalyst to promote quantitative combustion. The mixture o f gases is then passed over copper, which will get rid o f excess oxygen and will reduce nitrogen oxides into elemental nitrogen (Nz). The resulting mixture is then passed to a chromatographic column where individual components such as nitrogen (Nz), carbon dioxide (COz), water (HzO) and sulphur dioxide (SOz) are separated and eluted from the gas. These eluted components are detected by thermal conductivity detector (TCD), which feeds a potentiometric recorder giving an output signal proportional to the concentration o f each component in the gaseous mixture. Figure 2.11 shows a schematic diagram o f an Elemental Analyser.

0x1 catalyst Copper CO, H,0 S O ,* SO 3 O * He 2 To GQ N 2 , COj . HgO 8 0 ^ . He

Figure 2,11 Schematic diagram showing the principle o f operation o f an Elemental Analyser (reproducedfrom Carlo-ErbaEA 1108 Users Marmal).

Since determination of the Carbon, Hydrogen and Nitrogen percentage (CHN) depends on the original weight of the sample, it is essential that the sample be dry and free from any foreign substance (such as dust, hair, parafilm or fibres of filter paper).

The instrument used in our studies was a Carlo-Erba Elemental Analyser EA 1108 with PC based data system (Eager 200 for windows). A Sartorious ultra microbalance 4504 MP8 was used for weighing the sample. A Gas Chromatographic colunm (Porapak QS 50-80 mesh) was employed for GC separation. The combustion reactor was made of translucent silica and was filled with silvered cobalt oxide, which was deposited on top of 40mm o f silica wool. Another layer o f silica wool separates the chromium oxide from the silvered cobalt oxide. The reduction reactor was made o f alternating layers of silica wool and copper oxide wires.

2.1.10 NUCLEAR MAGNETIC RESONANCE (NMR):

This is a powerful method used widely by chemists and biochemists to elucidate the structure o f chemical species. NMR spectroscopy is based on the measurement o f electromagnetic radiation absorbed by a sample in the radio frequency range of roughly 4-900 MHz. In this case, nuclei o f atoms instead o f outer electrons (as is the case in infrared, visible and ultraviolet spectroscopy) are involved in the absorption process. It is essential however in order for the absorption o f electromagnetic radiation to occur, to place the analyte in an intensive magnetic field which supplies the nuclei with sufficient energy state that is required for absorption o f radiation. The importance o f NMR in elucidating chemical structures was made clear when chemists became more aware that molecular environment affects the absorption o f radio frequency (RF) radiation by a nucleus in a magnetic field and that this type o f effect can be linked to the molecular structure o f the chemical species in question. Two types o f NMR spectrometers are available, continuous and pulsed or (Fourier transform) spectrometers (FTNMR). Nearly all-modem NMR instruments are o f the FT type, which gives a much better resolution (high resolution spectra). The four nuclei that have been o f greatest use to organic chemists and biochemists are ^H, ^^F and Proton (^H) NMR analysis is the most frequently used and in this case, exact positions of different protons in a molecule can be determined and shown in the spectra and the molecular structure can thus be elucidated. Second to proton NMR in popularity is the carbon (^^C) NMR

though this method is about 6000 times less sensitive than proton NMR due to a weaker NMR signal from the nucleus. Proton NMR will be discussed here, as it was the one used in this study.

In proton NMR, the frequency o f RF radiation, which is absorbed by a certain nucleus, is strongly affected by its chemical environment that is the presence o f electrons or nuclei in the nearby environment. Different types o f effects can be encountered:

1- Chemical shift: Taking pure ethanol as an example {Figure 2.12), three regions o f peaks are seen (3 peaks to the far left, 5 peaks in the middle and 3 peaks to the far right). The 3 peaks to the far left are attributed to the proton o f the hydroxyl group (OH). If the hydrogen atom o f the hydroxyl group is replaced by deuterium, the three peaks to the far left will disappear and 4 peaks will replace the 5 peaks in the middle. The displacement o f the hydroxyl proton by deuterium will lead to some changes in the absorption frequency o f the proton; such changes depend on the group to which the hydrogen atom is bonded. This type o f effect is called a chemical shift.

a

I

I — C H a —

k

J(K

Frequency-

Figure 2.12 A typical proton NMR spectrum o f highly purified ethanol showing splitting

o f OH and CH2 peaks (adaptedfrom Skoog et a l, 1998).

2- Spin-spin splitting: The magnetic field produced by a spinning nucleus causes changes in electron distribution in its bonds to other nuclei. This will cause changes in the magnetic field o f adjacent nuclei and leads to splitting o f energy levels that will therefore lead to multiple transitions. In the example on pure ethanol (Figure 2.12), it is noted that the methylene (CH2) group appears in the

NMR spectra as 5 peaks in the middle, whereas the methyl (CH3) group appears

as 3 peaks in the far right side o f the spectra. A general rule is suggested for the