Dept. of Chemistry - Master's degree
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Item Open Access Sorption behaviour of Ba++, Co++ and Zn++ ions on alumina, kaolinite and magnesite(Bilkent University, 1991) Gökmenoğlu, ZelihaThe need for ultimate disposal of nuclear wastes has stimulated a renewal of interest in the adsorption behaviour of various nuclides on minerals of the type found in and around the various types of repositories that have been proposed. These adsorption studies are needed in order to estimate rates of transport of the nuclides in the event of water penetration into and through the repository. In this work the sorption-desorption behaviour of Ra·*”*·, and Zn'^'^ on minerals from different regions of Turkey has been investigated by means of a batch technique. The mineral samples used are mainly alumina, kaolinite and magnesite types. and were used as isotopic tracers. The distribution coefficients for the sorption of the three cations studied on different minerals were calculated. Alumina mineral was used for Co·*"*· and Zn'^'^ sorption experiments whereas kaolinite and magnesite were used to study the sorption behaviour of Cc»·*·*· and Zn++. The samples were seperated into different particle size ranges by AndrecLsen Pipette method. The particle size range used throughout the experiments was 20-38//m .The groundwaters used for different sorbing materials were from Beyşehir, Seydişehir and Mihalhçcık. The groundwaters used in the experiments were prepared synthetically in the laboratory. In the solutions prepared with ground- waters, initied concentrations ranged from 1.04x10“® to 1.04x10"® meq/ml for Co·*····, 7.67x10"® to 7.67x10"'* meq/ml for Zn'^'^ and 7.65x10"® to 7.65x10"® meq/ml for The samples were shaken with a shaker at 190 rpm and phase separations were carried out by centrifuging. The measurements were performed by a Nal(Tl) detector. Sorption and desorption kinetics were studied during 16 days except for the adsorption of C0++ on magnesite which was studied during 24 days, and adsorption rates were calculated from first order rate equation. Rapid adsorption was observed at high concentrations. It was observed that, about 2 days of contact was enough for values on kaolinite to reach steady-state whereas for Co^'^ on magnesite at least 25 days were needed. Rd values of on alumina mineral were quite high (2857 ml/g). Rd values for on minerals ranged from 15 (on kaolinite) to 4274 ml/g (on alumina) depending on the type of minéral and the Rd values of Zn'^'^ on minerals ranged from 26 (on magnesite) to 3800 ml/g (on alumina). It was observed that, alumina minerals adsorb ions more than kaolinite and magnesite type of minerals. Adsorption-desorption process was found to be reversible for Ba'^'^ and Co^'^ sorption on alumina and Co^'^ sorption on kaolinite. However, a paxtially reversible mechanism was observed for Zn'^'^ on.alumina and kaolinite and the sorption of (70++ and Zn++ ions on magnesite. Concentration dependent ion sorption isotherms were found to fit to Fre- undlich type of isotherms. The specific sorption concentration or the amount of radionuclide sorbed per gram of soil were calculated. It was seen that the sorbed concentration, Ca decreases with increasing mass M of the adsorbing solid. The change of the system parameter, 7 and specific sorbed concentration, Ca,o with the initial ion concentration is given by appropriate equations. The results obtained from V/M ratios indicate that in batch experiments the influence of the V/M ratio should be taken into account although according to the fundamental distribution law, distribution coefficients should be independent of V/M ratio. Distribution coeiRcients for adsorption of on binary mixtures of various minerals have been determined to see whether Rd of mixtures could be predicted from those of the pure minerals. It is shown that the system corresponds to a diluted experiment and the overall Rd is limited by the high Rd of alumina.Item Open Access Radiochronology of sediments from various regions of the Mediterranean Sea by direct gamma spectroscopy using natural 210Pb, 226Ra and fallout 137Cs(Bilkent University, 1992) Tadjiki, SoheylThe and profiles are determined at two locations in southern coast of Spain, Three locations in southern coast of Turkey and two locations in coast of Cyprus. Compaction-corrected sedimentation rates are derived for all locations using data. The rates range from 1..39± 0.12 crn.y"' (0.50±0.04 g.cm“^.y“^) to 0.08±0.01 cm.y“^ (0.039±0.003 g.cm~'^.y“^) (0.50±0.04g.cm"^.y~^). Except for BC-6A, for all of the locations the ^^’^Cs profile is used to compute sedimentation rates which are in a good agreement with those of '^^°Pb method. The result of the Constant Initial Concentration and Constant Rate of Supply dating models are in a good agreement with each other whereas in BC-6A and KS-1 cores only Constant Rate of Supply model is valid to calculate sedimentation rate. The flux of unsupported '^^°Pb vary between 0.74±0.01 pCi.cm“^ and 0.11 ±0.03 pCi.cm“^. The average depositional flux is found to be considerably low in the coast of Turkey and Cyprus, whereas it is found to be high in the coast of Spain.Item Open Access Sorption behavior of radioiodine on organic rich soil, alumina and clay minerals(Bilkent University, 1992) Assemi, ShoelehSorption behavior of radioiodine on organic rich soil, alumina, chlorite- illite clay mixture, kaolinite and bentonite have bc;en studied using the batch method. was used as tracer. Characterization of samples were done by FTIR, X-ray diffraction spectrometry and particle size distribution by Andreason pipette method. The grain size of the samples used were all below 38/xm and the experiments were performed at room temperature. The aqxreos phase used in all experiments were synthetic ground water with a composition similar to the underground waters of the regions where the samples were obtained. Stock solutions containing tracer, were prepared using synthetic ground- water. Initial concentration of inactive iodide ion in the solutions ranged from 10~®to 10“^ mmole/ml. The influence of contact time, solution/solid ratio, pH of synthetic ground- water, biomass of soil and I~ ion concentration on the sorption of radioiodine on the organic rich soil were investigated. The effect of I~ ion concentration and complexing agents on the sorption behavior of radioiodine on alumina and clay minerals were also studied. A rather slow kinetics was observed for tlie adsor])tion of radioiodine on the organic rich soil, tending saturation within 14 da.ys. It was observed that distribution ratio increased with increasing ratio of the volume of solution to the mass of soil(V/m ratio) and reached a platee.u after V/m=50. Distribution ratio did not change very much by changing the pH of the solution. Comparison of Eh-pH behavior of the solutions with the standard Eh-pH graph of iodine-water system indicated that in the whole pH rcinge, iodine is mostly preseirt in I~ form. To observe the effect of soil biomass soil samples were sterilized by heat and gamma-ray source and the results were found to be much lower than the control sample. Increasing the concentration of iodide ion in the .solution, caused a decrease in Rd. values. Desorption Wcvs ol)served only for low initial iodine concentrations < 1 X I0~^7nmol/ml), for organic rich soil which suggests that at least two sites and/or mechanisms are involved in the sorption. The sorption data were fitted to Freundlich and Dubinin-Radushkevich isotherms. The slope of Freundlich isotherm was found to be less than 1 which indicates the non-linearity of the isotherm. The mean free energy of adsorption was calculated from the slope of Dubinin-Radushkevich isotherm and was found as about 11 k.J/mol. The distribution of sites on organic rich soil was calculated using the Freundlich and Dubinin-Radushkevich constants, considering Cl~ as the competing ion with I~ in the solution. The affinities of sites were found to be about three times higher for Cl~ than I~. Among the clay minerals, the highest Rd value was found for chlorite-illite cla}^ mixture but tlie Rd values found for alumina and the other sanrples were well below those found for the organic rich soil. The sorption data were fitted to Freundlich and Dubinin-Radushkevich isotherms. All the Freundlich isotherms were non-linear (slopeItem Open Access X-ray photoelectron spectroscopic and in-situ infrared investigation of a Ru/SiO2 catalyst(Bilkent University, 1997) Sayan, ŞafakA 4 wt% Ru/Si02 catalyst which was previously prepared via an incipient wetness technique using a ruthenium nitrosyl-nitrate [Ru(N0)(N03)3] solution and a commercially available precursor (ruthenium nitrosyl-nitrate) were used in this study. The activation of the catalyst was investigated by using Infrared (IR) spectroscopy together with X-ray photoelectron spectroscopy (XPS). Special emphasis has been given to the study of formation of active species during annealing of the precursor and the catalyst for comparison purposes. The in-situ IR measurements performed on the catalyst suggested a possible metal and support interaction. XPS experiments revealed mixed oxidation states in the case of annealing of the precursor whereas annealing did not cause any change in the oxidation state of Ru present in the catalyst which lead to the conclusion that the influence of support and interactions between the metal and support prevented any reduction by annealing only. Carbon monoxide adsorption on the reduced catalyst followed by IR spectroscopy was performed to investigate the nature of active adsorption sites. The complexity of the spectrum of chemisorbed CO suggested the presence of small metal particles. The presence of Ru"^ centers as well as reduced Ru centers showed that the catalyst was not fully reduced under these conditions. Using this catalyst ammonia synthesis was achieved at 350 °C in a N2/H2 gas mixture (N2/H2 =3:1) for different reaction times. In addition to observed NH3 as reaction product, the results showed that NH^ surface species might represent intermediates in the ammonia synthesis reaction on Ru/Si02. Carbon monoxide adsorption on the catalyst after NH3 synthesis was performed to investigate the change in nature of active sites after ammonia production when compared with the reduced sample. Based on the experimental observations, participation of Ru° sites in ammonia synthesis was confirmed and a partial oxidation of the reduced Ru sites during synthesis was observed.Item Open Access Sorption studies of cesium and barium on magnesite using radiotracer and x-ray photoelectron spectroscopy(Bilkent University, 1997-06) Shahwan, TalalAs the consumption of the radioactive materials is continuously increasing, the problem of disposing the resulting v^^astes safely is becoming more challenging. One way through which these radioactive wastes could be isolated from the biological environment is by disposing them in deep geological formations. Clay minerals are proposed as backfill buffering materials that can delay the migration of the radionuclides and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. Up to now, several studies were carried out to examine the effect of such parameters on the sorption behavior of different radionuclides on various kinds of minerals. This study was conducted to investigate the effects of time, concentration and temperature on the sorption behavior of cesium and barium ions on magnesite. Cesium 137 140 and barium have the radioactive isotopes Cs (t^^^ = 30.1 y) and Ba (t^^^ = 12.8 d) 1/2 produced in high yields during the fission process which are important in radioactive waste considerations. Magnesite is a mineral composed mainly of magnesium carbonate 2+ together with minor amounts of quartz and has a single exchangeable cation. Mg . The radiotracer method and x-ray photoelectron spectroscopy, which is a powerful surface sensitive tool, were used in this study. The results obtained from both methods complemented each others and were in good agreement. Kinetic studies of the sorption process show that equilibrium was approached within one day of contact for both of cesium and barium ions. The data of the sorption of both cations using different concentrations at various temperatures were most adequately described by the Freundlich type isotherms which correspond to multilayer adsorption on heterogeneous surfaces. The values of the Freundlich constants k and n imply that barium ions have slightly larger adsorption affinity and adsorption intensity than cesium ions. The adsorption data at low concentrations were also observed to obey the Dubinin-Radushkevich type isotherms which describe monolayer adsorption on heterogeneous or homogeneous surfaces. The adsorption data were very poorly described by the Langmuir type isotherms. Thermodynamic parameters such as enthalpy change, AH°, entropy change, AS° and free energy change of adsorption, AG°, were calculated from the sorption data of cesium and barium ions at different temperatures. The values obtained for AH° and AS° were -37 kJ/moL, -0.09 kJ/moL.K and -13 kJ/moL, -0.009 kJ/moL.K for cesium and barium ions respectively. The negative AH° values indicate the exothermic nature of adsorption which means that low temperatures are favored. The decrease in entropy upon adsorption implied by the negative AS° values is indicative of the stability of adsorption for both cations. The values of AG° at different temperatures were all negative indicating the spontaneity of the adsorption process for both cesium and barium ions. The magnitudes of AG° were seen to be within the 8-16 kJ/moL range which is the energy range of ion- exchange type processes.Item Open Access Spectroscopic investigation of polyvinyl chloride photodegradation in blends with basic traps(Bilkent University, 1999) Birer, ÖzgürPolyvinyl chloride degrades via loss of HCI when it is exposed to heat, energetic particles or photons. The mechanism is known as the zip mechanism and results in conjugated segments, polyenes. Degradation also leads to loss of mechanical properties of PVC. However, from another point of view, PVC is a Bronsted acid source, with controllable emission. Furthermore, the polyenes are small segments of polyacetylene, which itself is a very interesting one-dimensional system. Understanding the building blocks clearly helps to envisage larger systems. This study has two main goals. The first goal is benefiting from the radiation induced in-situ created HCI by incorporating basic traps into the polymer matrix and inducing optical or electrical conductivity changes. The second goal is to tune the wavelength of photodegradation by introducing sensitisers into the polymer matrix to affect the chain length of the polyenes. For the first part of the study, pH indicators, and basic forms of conducting polymers were blended with PVC and the films were irradiated with UV radiation. Optical changes were monitored with UV-Vis-NIR Spectroscopy. Similar to several other dyes tried, Bromcresol Green, and Methyl Violet changed their optical properties when they were exposed to UV radiation in the PVC matrix. However, Methyl Violet, being resistant to UV radiation, proved to be a suitable component for possible dosimetric and lithographic applications. Basic forms of polyaniline and poly-2-chloro aniline were blended with PVC, and upon irradiation of the blend, they were converted to conducting salt forms as a result of doping with in-situ created HCI. The structural changes were monitored with UV-Vis-NIR spectrophotometry as well as FTIR spectroscopy. PVC/2-CI PAN I blends gave better results compared to PVC/PANI blends. For the second part of the study, hydroquinone, anthraquinone, and anthracene were introduced into the PVC matrix. The samples were irradiated with monochromatic UV radiation at the absorption maxima of these sensitisers. It was established that the nature of polyene formation is dependent on the wavelength of irradiation as well as the amount of energy transferred to the PVC chains.Item Open Access Silver nitrate-oligo (ethylene oxide) surfactant mesoporous nanocomposite films and monoliths(Bilkent University, 2000-09) Samarskaya, Ol'gaThe purpose of this work is to improve and simplify the method of synthesis of metal functionalized mesoporous materials. This study has two particular goals. The first goal is to incorporate silver in its ionic form and to achieve its homogeneous distribution within the pores of meso-silicon oxide. The second goal is to establish the influence of concentration of silver present in the system on structure of the porous silica materials. Silver nitrate salt dissolved in hexagonal mesophase of polyoxyethylene 10 lauryl ether (non-ionic PEO-type surfactant) was evenly distributed within silica framework which is tailored through liquid crystalline templating-sol-gel processing. In this approach, lyotropic liquid crystalline mixture containing silver ion and amphiphilic oligo (ethylene oxide) precursor organizes in hexagonal phase in the presence of nitric acid and water at room temperature. This preformed silver containing LC mesophase is utilized as a template for subsequent condensation-polymerization reacting of Si (OCH subscript 3) subscript 4) which results in formation of silicon oxide matrix as a direct cast mesophase formed by the template. The amount of silver nitrate homogeneously mixed in LC hexagonal phase of oligo-ethylene oxide/water system alters the mesophase. The template, lyotropic hexagonal mesophase made up by silver nitrate which is dissolved in PEO-type surfactant/water system in the certain concentration range, can be used to synthesize silver containing silica-based mesoporous materials. It is determined that C subscript 12 E subscript 10:H subscript 2 0 (50 wt%):HNO subscript 3 system preserves its hexagonal LC phase in the presence of Ag ions up to 0.9 silver to surfactant molar ratios. Higher concentrations of silver nitrate in surfactant mesophase induce formation of white soft solid phase, which is assigned to the Ag ion/surfactant/ nitrate ion complex. The template mixtures of 0.1-0.7 silver nitrate to surfactant molar ratios yield silver containing 3D-hexagonal meso-silicon oxide. However, at higher silver nitrate concentration amorphous disordered materials form. Homogeneously distributed Ag ions were successfully reduced to Ag nunoclusters on both internal and external surface of mesoporous silica materials by hydrazine in the gas phase.Item Open Access A new lyotropic liquid crystalline system: Oligo (ethylene oxide) surfactants with transition metal complexes and the synthesis of mesoporous metal sulfides(Bilkent University, 2001) Çelik, ÖzgürIn this study a new templating method, which can be used to synthesise mesporous materials, has been developed. The main objective of this work is to form organic mesophase in the presence of inorganic salts. This is an organic-inorganic hybrid mesophase, which can be used to template the growth of inorganic materials. Here for the first time, a new lyotropic liquid crystalline (LLC) system has been presented from oligo (ethylene oxide) type surfactant and transition metal aqua complexes. The temperature and the metal aqua complex concentration range of the complex/surfactant mixtures have been determined, where the mixtures have a liquid crystalline (LC) phase. Here, the complex refers to Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, and CoCl2·6H2O and the surfactant is C12H25(CH2CH2O)10OH, (C12EO10). The addition of the metal aqua complexes directly to the surfactant produces a LC phase. The LC phase obtained from the mixture of these two is more stable than the LC phase obtained from a mixture of free water and surfactant. The FT-IR and UV-Vis absorption, Polarised Optical Microscopy (POM) and Powder X-ray Diffraction measurements show that the coordinated water molecules mediate the formation of the LC phase. Our observations also show that the coordinated water molecules make a stronger interaction with ethylene oxide (EO) chains than free water molecules. The LC templating approach, which is demonstrated as a new system has been used for synthesis of meso-structured metal oxides, metal sulphides and even metal mesh. From all these studies, it is well known that in order to maintain LC phase the metal ion concentration should correspond to metal ion to surfactant mole ratio below 0.8. However, this work shows that the amount of metal aqua complex concentration can be increased up to a 6.5 complex to surfactant mole ratio by maintaining the integrity of the hexagonal and/or cubic structure of the LC phase. This may open a new area for the realisation of new mesostructured materials with better qualities and much higher yields. In the first part of the thesis, the thermal and structural properties of the new LLC phase has been established by using polarized optical microscopy (POM) with an attached hot plate, PXRD, FT-IR and UV-Vis absorption methods. In the second part, the new phase has been used as a template to synthesise mesoporus metal sulfides. The second part of the thesis deals mainly with the structure and synthesis of mesostructured CdS and ZnS. It has been demonstrated that the LC phase of Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O in oligo(ethylene oxide) surfactant survive partially during the reaction with H2S to produce the corresponding metal sulfides.Item Open Access Identification, stability and reactivity of NOx adsorbed species on titania-supported manganese catalysts(Bilkent University, 2001) Küçükkal, Mustafa U.The needs of improved fuel economy, and lower emission of green-house gases such as CO2, is projected to increase the demand for diesel engines through the world. These engines operate at air/fuel ratio higher than stoichiometric (lean-burn conditions). This results in relatively low hydrocarbon/NOx ratio in the exhaust and an additional amount of reductant (typically about 2-3% of additional fuel) should be fed upstream of de-NOx catalyst. For this reason, it is important to study the interaction of NOx species produced upon adsorption of NO/O2 mixtures on the catalyst surface with long chain saturate hydrocarbons, which are typical for diesel fuel.In recent years many de-NOx lean-burn catalysts have been proposed among which supported metal oxides have been taken in consideration for their potential thermal stability and large composition variability. Subjects of this study are titania (anatase)-supported manganese catalysts, prepared by impregnation and ion-exchange from aqueous solutions of Mn2+ ions. TiO2 (anatase) is stable in SO2 containing atmosphere, typical for the exhaust gases in diesel engines. The identification of the NOx species formed during the adsorption of NO, NO/O2 mixtures and NO2 is performed by in situ FTIR Spectroscopy. the thermal stability and reactivity of the surface NOx forms towards n-decane is followed by application of the same technique. It is established that adsorption of NO on the support and manganese-containing catalysts is reactive and leads to linearly adsorbed NO and formation of anionic nitrosyl, NO− and NO3 − species. Contrary to the impregnated catalyst, the ion-exchanged catalyst does not contain NO− species coordinated to Ti4+ ions. This experimental fact is in agreement with the high dispersion of Mn3+ ions concluded from the CO adsorption experiments.The NO/O2 co-adsorption on the anatase and catalysts studied results in formation of NO3 − species differing in the mode of their coordination. Under these conditions no NO− species are detected. The surface nitrates formed on the manganesecontaining catalysts possess lower thermal stability than those on the pure support. This difference explains the higher reactivity of the former toward the n-decane. The nitrates formed upon NO/O2 co-adsorption on the manganese-containing catalysts are able to activate and oxidize the hydrocarbon at temperatures as low as 373 K. The latter process gives rise to adsorbed CO2, formic acid and isocyanate species. The NCO species is considered as an important intermediate in the formation of nitrogen. The extent of oxidation of n-decane is higher on the ion-exchanged catalyst. It is concluded that this catalyst can be promising in the selective catalytic reduction of NO by longer-chain saturated hydrocarbons.Item Open Access Electronic structure analysis and density functional study of an alternating donor/acceptor polymer(Bilkent University, 2001) Köse, Muhammet ErkanConducting polymers have attracted great attention for the last two decades because of their potential applications in many fields. One of the major goals for the scientists who are studying in this field is to synthesize conducting polymers with extremely low band gap. Such polymers would be intrinsically conducting, and thus eliminating the need for doping. Several powerful approaches are proposed to synthesize low band gap polymers. One of them is the donor/acceptor concept, which was proposed by Havinga and his co-workers. In their model, they attached regularly alternating electron withdrawing and electron donating groups to the carbon backbone. By connecting these groups, they aimed to decrease the lowest unoccupied molecular orbital (LUMO) level, and to increase the highest occupied molecular orbital (HOMO) level of the polymer respectively. By this way they claimed that one could obtain very low band gap conducting polymers by introducing the alternating donor/acceptor groups, where the electronegativity difference between these groups is highest. Huang and Pickup synthesized donor/acceptor copolymers of 3,4- ethylenedioxythiophene (EDOT) which is considered as electron donating group and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CDM) as electron withdrawing group. By changing the monomer ratio in the copolymers, they decreased the band gap to less than 0.16 eV, however, the intrinsic conductivity was about 10-3 S/cm. Although a very low band gap was achieved, the conductivities of copolymers were still low. To understand this behaviour and also to check the validity of donor/acceptor concept, we performed theoretical studies for these systems. Monomer through hexamer of EDOT, oligomers for 1:1 copolymer of EDOT and CDM were optimized using density functional theory. Ionization potentials, electron affinities, energy gaps, and band widths of the polymers were obtained by extrapolation. The lowest band gaps are calculated for copolymer poly-CDM-EDOT and poly-CDM. Band gaps of the polymers are found to agree well with the experimental values. When we combine two monomers (EDOT: electron-rich, CDM: electrondeficient) in one to one ratio and in different ratios in order to form co-oligomers, the highest occupied molecular orbitals of co-oligomers are averaged between the HOMO levels of parent homo-oligomers. Lowest unoccupied molecular orbitals of cooligomers are also averaged between the LUMO levels of parent homo-oligomers; however, averaged LUMO levels of co-oligomers are more close in energy to the LUMO level of CDM oligomer. We also found that poly-CDM and poly-CDMEDOT have very flat conduction bands. These narrow bands are attributed to the localized states, which decrease the mobility of n-type carriers in these bands consistent with the experimental findings. Overall, we concluded that donor/acceptor concept can be used to decrease the band gap while sacrificing the dispersion of valence and conduction bands. However, band broadening does not occur in alternating donor/acceptor polymers as donor/acceptor approach suggested.Item Open Access The mechanism of hydroquinone assisted photodegradation of PVC(Bilkent University, 2002) Balcı, SinanPure poly(vinyl chloride) (PVC) degrades upon exposure of UV radiation (λ <300 nm) by the loss of HCl and formation of long conjugated structures known as polyenes. The mechanism of this process is fully understood and known as the zip mechanism, firstly involving the elimination of chlorine radical from the PVC backbone and then the formation of HCl. Addition of chromophores (light absorbing units) into the matrix of the PVC increases the wavelength absorption of the matrix. One of these compounds is Hydroquinone (HQ), which sensitizes the photodehydrochlorination of PVC at 312 nm. The purpose of this thesis is to shed light into the mechanism of HQ assisted photodehydrochlorination of PVC at 312 nm. Accordingly, the effect of (i) the minimum concentration of HQ, (ii) the temperature of the medium on the rate of PVC photodegradation, (iii) the viscosity of the PVC, (iv) the flux of the irradiation, and (v) the hydroquinone-benzoquinone composition on the rate of PVC photodegradation are investigated. Various sensitizers and quenchers are also used to compare the HQ’s sensitisation ability and to investigate the effect of the triplet-state of HQ in the mechanism of the HQ sensitized PVC photodegradation. UV-Visible, IR, and XPS spectroscopic techniques are used to determine the effect of the mentioned parameters.V In the light of all the findings, a mechanism of the HQ assisted (sensitized) photodegradation is proposed to involve mostly formation of a triplet state of HQ followed by effective transfer of this energy to the PVC matrix for dehydrochlorination.Item Open Access Spectroscopic and structural properties of TTBC J-aggregates(Bilkent University, 2002) Birkan, BurakThe aim of this thesis is to investigate the spectroscopic and structural properties of the TTBC J-aggregates. Absorption, emission and excitation spectroscopy techniques are used to study the properties of the J-aggregates of 1,1’,3,3’-tetrachlorobenzimidazolocarbocyanine (TTBC). The dependence of absorption spectra on dye and ion concentration was investigated. A model is presented to explain the asymmetrical splitting. The interaction between molecules was found to be affected by the intermolecular distance, the orientation of the molecules and the size of the aggregate chain.Item Open Access N/pi and N/sigma interactions of the amide linkage with its N-substituents : a quantum chemical study(Bilkent University, 2002) Demir, ÖzlemThe overall contribution of backbone-backbone H-bonding to the stability of proteins remains an unresolved issue. However, a wealth of spectroscopic, structural, and thermodynamic evidence indicates that the strength of those interactions increases on going from turn or 310-helix to α-helix to β-sheet. This implies that the electronic properties of the amino acid side chains and their local interactions with the peptide bonds might play a role in secondary structure stability. One such interaction is suggested by the apparent dependence of thermodynamic β-sheet propensities, and of 15N NMR chemical shifts of oligopeptides, on resonance constants of the side chains: electron-density shift from the i+1 residue into the i, i+1 peptide bond which would increase basicity of the carbonyl O and thereby strengthen its H-bond. To test this counterintuitive proposition, N/π and N/σ interactions of the amide linkage with its N substituents were investigated. Effects of substitution have been characterized by computational examination of the changes in energy, molecular geometry, electron density distribution, and electronic structure, in three series of compounds: (1) formamides HC(=X)NY2 (X=O, S, Se; Y=H, CH3, F, Cl, Br); (2) 2,3,-endo,endo-disubstituted N-acyl-7-azabicyclo(2.2.1)heptanes and 2,3,-endo,endo-disubstituted 7-bicyclo(2.2.1)-heptyl cations; and (3) 3-substituted 5,6-diaza-1- bicyclo(2.1.1)hexyl cations. Both ab initio (MP2) and DFT methods were employed using Pople’s basis sets (6-31+G(2d), 6-31+G*, and 6-31G*). N-halo substitution effect on the potential energy surface of simple formamide derivatives is found to be largely related to the electronegativity of the substituents. The exception to this general trend is found in the case of the F effect on the transition structures for inversion, where F lp donation appears to assist π-bonding across the C-N bond, stabilizing the charge polarized resonance form of the amide group. This is supported by the examination of the variation in bond v distances, bond orders, energy and extension of the canonical π-symmetry orbitals, and NBO occupancies of the localized orbitals. On the other hand, the implied π-bonding across the N-F bond is not reflected in the group transfer energies obtained as heats of the isodesmic substitution reaction, the effect being apparently too small in comparison to the total bond energies. In accord with the experimental data, N-acetyl-7-azabicyclo(2.2.1)heptane is found to be highly pyramidalized on N7. However, due to the very small barrier to inversion, chalcogen substitution, as in N-thioacetyl and N-selenoacetyl- derivatives, results in virtual planarity of the amide N. The planar geometry is readily distorted by remote substitution in 2,3-endo,endodisubstituted N-thioacetyl-7-azabicyclo(2.2.1)-heptanes. The direction of pyramidalization is the same for strongly electron-donating substituents and strongly electron-withdrawing substituents. The dual parameter treatment suggests that in the first case pyramidalization depends largely on the NBO energies of the occupied orbitals of the bicycloheptane C-C bonds, while in the second case both the occupied and vacant orbitals interact with the N center. Examination of the electron density shifts associated with the change in conformation of the π-donor substituents confirms that the thioamide N acts as a resonance acceptor of the σ C-C density. Finally, 5,6-diaza-1-bicyclo(2.1.1)hexyl cation is found to be an excellent model system to probe π-donor capacity of the range of substituents, including all the coded amni acid side chains, even in their ionized forms. The first scale of substituent constants is obtained to characterize resonance interactions in the σ-bond systems, related to the scale of conventional experimental σR constants. The findings of the present study suggest that the amide linkage can indeed act as a resonance acceptor of π- and σ-density of its N substituents. These results may further our understanding of the local interactions in proteins and the origin of secondary structure propensities of the coded amino acids.Item Open Access The crystallization behaviour of isotactic polybutene-1(Bilkent University, 2002) Erdem, Hacı BayramThe crystallization behaviour of different commercial grades of isotactic polybutene-1 was investigated and an experimental set-up was designed for this research study. The experimental set-up constructed provides both fast-cooling of the sample and the recording of the growth of spherulites as a real time movie. The radial growth rate of the spherulites were observed to be higher for the grades with higher molecular weights. The results were found to fit to the Lauritzenn-Hoffman crystallization theory, and this theory is used to calculate the lateral surface free energy, σ, and fold surface free energy, σe. The lateral surface free energy was found to be 10.4 erg/cm2 for all the grades investigated. The fold surface free energy calculated to be between 15.5 to 17.4 erg/cm2 . The fold surface free energies and the crystallization rates do not exhibit any dependence on the molecular weight of the polymer. The polymer containing nucleating agents has the highest crystallization rate. It was observed that the likelihood of deviation from spherical shape is greater for the grades with lower molecular weights.Item Open Access Identification of NO subscript x species adsorbed on cobalt(II)-supported zirconia and sulfated zirconia and investigation of their reactivity toward methane(Bilkent University, 2002) Vakkasoğlu, Ahmet SelimIn the period between the eleventh and the thirteenth centuries migrations from the Frankish heartland into different parts of Europe created a situation in which we see the “Europeanization of Europe”. In the course of time, in terms of tenurial structure, landholding, ecclesiastical and military systems as well as onomastics, different regions of Europe implemented similar patterns. This thesis examines this process through a local study of eastern Suffolk in England between 1066 and 1166. In the first place, the identity, landholdings and tenants of the post-Conquest lords in eastern Suffolk are examined, looking at the origin of the lords, their relationship with the king and the date at which they acquired their lands. Secondly, the thesis deals with the administrative and landholding system and addresses the questions: how much they changed and how far this can be related to ‘feudalism’. Finally, military and ecclesiastical changes are discussed. The conclusion of the thesis is that, although “Europeanization” helps explain some of the changes, some things did not change, while others changed not so much through the spread of European practices as through the circumstances of postConquest England and eastern Suffolk.Item Open Access Analysis of UV induced dehydrochlorinated PVC (With hydroquinone) using direct pyrolysis mass spectrometry (DPMS), TGA, UV/VIS-NIR and FTIR techniques(Bilkent University, 2003) Avcı, ErcanPoly(vinyl chloride) (PVC) degrades easily upon heat and light exposure via loss of HCl. The mechanism of this process is well understood, known as the zip mechanism and the dehydrochlorination results in conjugated segments, polyenes. It is also possible to utilize PVC polymer as an in-situ acid donor since the main degradation product is HCl. Addition of hydroquinone (HQ) into PVC matrix sensitizes the photodehydrochlorination of PVC at 312 nm. In this study the effects of photodehydrochlorination on thermal and material properties of PVC were investigated using DPMS and TGA as well as UV-Vis-NIR and FTIR techniques. In addition, the photodegradation of PVC/PVAc blend, copolymer (PVC-coPVAc) and PVAc were similarly investigated. Dehydrochlorination of the polymers resulting from UV-exposure were also investigated for doping of PANI in blends. HCl evolution behavior of the UV dehydrochlorinated PVC exhibits a characteristic property which is different from the unirradiated ones. Both DPMS and TGA results confirms the sensitization of PVC photodehydrochlorination at 312 nm by hydroquinone (HQ) resulting in a temperature onset that is the lowest (140 oC). HQ assistance upon 312 nm UV exposure is not significantly observed in the copolymer. The low temperature onset of UV-induced copolymer is a promising result to produce longer polyene chains, since polymer backbone starts to decompose after ca. 220 oC, using copolymer might be an alternative to PVC.Item Open Access Synthesis and characterization of secondary amine functional microparticles(Bilkent University, 2003) Altıntaş, Evrim BanuSecondary amine functional microparticles were prepared by the suspension polymerization of styrene (STY), divinylbenzene (DVB), and 2-(tert-butylamino)ethyl methacrylate (tBAEMA). Effects of polymerization parameters (such as polymerization time, stirring speed, temperature, STY, DVB, tBAEMA, initiator and stabilzer concentrations, etc.) on experimental amine content, average particle size, and particle size distribution were determined. The suspension polymerization experiments were carried out in three different systems. In the first one, conventional suspension polymerization system was used and the organic phase including monomers and initiator was dispersed in an aqueous medium by using sodium dodecylsulfate (SDS) as the stabilizer. In the second system, a similar experimental protocol was followed except a proper amount of Al2(SO4)3 was used to replace SDS in the dispersion medium. The third method used to prepare the secondary amine functional microparticles was again similar to first polymerization protocol, except a proper amount of Al2(SO4)3 was added to the aqueous dispersion medium as a co-stabilizer. When SDS was used alone as a stabilizer, polymerization product was agglomerated; individual and spherical microparticles were not obtained. Replacement of SDS with Al2(SO4)3 resulted spherical microparticles within the size range of 50 - 200 µm. When SDS and Al2(SO4)3 was used together, again individual and spherical microparticles were obtained within the size range of 130 - 250 µm.Item Open Access Chemical characterization and provenance studies of archeological samples(Bilkent University, 2003) Türkmen, Işık RızaData that is collected by chemical analyses of the archeological samples can be used to find out the the raw materials used and the techniques practiced in the ancient pottery production. In addition, provenance studies of archeological samples that are commercially important may give an idea on the commercial relationships between the past civilizations. This study was conducted to investigate the chemical compositions and provenance of the amphora samples that are found around Sinop and Heraclea Pontica (Black Sea Coast) in Turkey, Ibn-Hani (Eastern Mediterranean Coast) in Syria, Tanais and Gorgippia on Northern Black Sea region of Russia. Some amphorae, which are morphologically similar to those of Colchian amphorae, but having apparently distinctive clay properties, are called Pseudo-Colchian, and they were also analyzed during the studies. The mineral compositions of the samples were found out by powder X-Ray Diffraction analyses, and the elemental compositions were sought by X-Ray Fluorescence spectroscopy. Principal component analysis and cluster analysis are applied to the data collected from XRF measurements for the provenance classification of the samples. The results obtained from both statistical methods complemented each others and were in good agreement. According to the results of this study, the pink clay, red clay and white clay amphorae from Sinop are all found to be composed of the minerals quartz, feldspars, pyroxenes, calcite and hematite, but varying in amounts with respect to type and color. Considering the mineralogical compositions, it is proposed that the average baking temperature of the red clay amphorae is around 800 – 850 o C whereas it is around 950 o C for the white clay ones. The red color observed for the red clay Sinopean amphorae was attributed to the presence of hematite minerals. On the other hand white color was attributed to the formation of mineral phases such as pyroxenes, throughout the chemical reactions that take place in the clay matrix at higher baking tempertures and low oxidation environments for the white clay Sinopean amphorae. In provenance classification of the samples, it was found that the separations between the samples occur mainly due to variations in the concentration of elements Ca, Fe, Ti, Ni, Rb and Sr. White clay amphorae from Antioch and Ibn Hani, and the red clay carrot type amphora from Tanais are found to be the Sinopean production. On the other hand, the white clay amphorae from Tanais was found to be more similar to the ones from Heraclea Pontica and different from those of Sinop. The colchian amphorae from Gorgippia and Pseudo-Colchian amphorae were found to represent typical differences from all other samples, but also from each others.Item Open Access X-ray photoelectron spectroscopic investigation of gold particles deposited on (formula) system(Bilkent University, 2003) Karadaş, FerdiGold particles on SiO2/Si system were investigated by X-ray Photoelectron Spectroscopy (XPS) technique. A suitable reference point was first established in order to investigate the physical/chemical factors affecting chemical shift of gold particles. Gold particles were: i) deposited directly from aqueous solution, ii) capped with citrate agent and then deposited, iii) reduced chemically by NaBH4 and deposited on SiO2/Si system. In addition, gold particles were deposited onto different substrates (quartz, glass). Similar chemical shift of Si4+ 2p and Au 0 4f peak upon the application of external bias gave a strong evidence to the assumption that SiO2 could be chosen as reference. In addition, the derived Auger Parameters have shown that chemical shifts observed during the application of external bias are solely due to charging. It was shown that reduction and nucleation processes occur at the same time during X-ray exposure when gold particles are deposited from aqueous solution. Differential charging of gold particles was investigated by measuring the changes in: i) binding energy, ii) FWHM and iii) intensity values of Au 0 and Si4+ peaks. Our findings obtained from Angle Resolved XPS method supported the assumption that gold particles deposited from aqueous solution prefer to grow three-dimensionally. Assuming the Si 2p binding energy of Si4+ peak as a reference, the binding energy of gold particles is: i) 84.30 ± 0.05 eV when gold is deposited from aqueous solution, ii) pdfMachine trial version V 84.00 ± 0.05 when citrate capped gold particles are used, iii) 84.10 ± 0.05 when gold is chemically reduced by NaBH4. Vis-absorption and electrophoresis methods have shown that capped gold particles have negative charges and they aggregate reversibly (i.e. without coagulation) when they are deposited on SiO2/Si system from their aqueous solution (and transferred back).Item Open Access Theoretical investigation of approaches for obtaining narrow band gaps in conducting polymers(Bilkent University, 2004) Durdağı, SerdarOver the last few years, there has been a great deal of research interest in developing organic conjugated polymers with narrow energy band gaps. Narrow band gap polymers would be intrinsically conducting, and thus eliminating the need for doping. There are several approaches for the construction of low band gap systems. Copolymerization of aromatic and o-quinoid heterocycles, minimization of bond length alternation, copolymerisation of donor and acceptor moieties might be most important factors for the lowering the band gap. The main aim of this work is to determine the reasons for low band gaps and to analyse the major effects, separately. Recently a number of low band gap systems were synthesized. These systems consist of aromatic donors and quinoid acceptors. To analyse the behaviour of donor/acceptor systems, we performed theoretical studies for these systems. We chose thieno [3,4-b] pyrazine (ThP), F-substituted thieno [3,4-b] pyrazine (F-ThP) and quinoxaline (Qx) as an acceptor and thiophene (Th) and pyrrole (Py) as donor units. Monomer through octamer of ThP, monomer through tetramer of F-ThP and Qx, were optimised by using density functional theory (DFT). All calculations were performed with the Gaussian 98 program. For ThP-Th and ThP-Py co-oligomers, energy levels were calculated for monomer through tetramer and, for F-ThP-Py, monomer through trimer data were used. Ionization Potentials (IPs), electron affinities (EAs), band gaps, and band widths of polymers were obtained by extrapolation. IPs and EAs are taken as negative HOMO and LUMO energies. The order of the band gaps of the homo-polymers is found: F-PThP (1.1 eV) < PThP(1.4 eV) < PQx.(2.4 eV). Band gaps of the systems are found to agree well with the experimental results. We increased the acceptor strength of PThP by adding fluorine units. The band gap is decreased by adding fluorine, however, the valence band width decreased, too. According to the donor-acceptor concept, we predict narrow band gaps and wider band widths for donor-acceptor co-polymers. However, for ThP-Th and ThP-Py copolymers, the band gaps are smaller than those of the homo polymers, but there is no increase on conduction band width. ThP-Th and ThP-Py have the same band gap (1.2 eV). The stronger donor pyrrole does not lead to a smaller band gap and wider band widths than the weaker donor thiophene. Thus, there is a contradiction with donor-acceptor concept qualitatively, and also quantitatively. The results show that, there is no certain correlation between band gaps and inner bond lengths. The bond length alternation (BLA) data are not consistent with the band gap results. Due to our results, the geometrical mismatch between aromatic and quinoid repeat units forces geometry distortions that impose smaller band gaps. Overall, we concluded that the situation is more complicated than simple concepts imply and further studies are required before a final conclusion can be reached.