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Title: A kinetic and thermodynamic study of the reduction of peroxyacids by iodide and aryl alkyl sulfides in the presence of non-ionic surfactants and α-cyclodextrin
Author: Mousa, Salem Mansour
Awarding Body: Northumbria University
Current Institution: Northumbria University
Date of Award: 2009
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Abstract:
The objectives of this study were two-fold: firstly to add to existing knowledge about the reaction of peracid with both iodide and sulfides in the presence of micelles (anionic and non-ionic) and α-cyclodextrin. The reaction between iodide and peracid had previously been studied only at 25°C in non-ionic, anionic micellar and alpha cyclodextrin; while the reaction of sulfides and peracid had only been investigated in the presence of α-cyclodextrin at one temperature. This study has investigated the previously undetermined effect of temperature on these reactions and how changes in temperature can affect the process of reactants binding to the micelle or cyclodextrin catalyst. The second objective was to obtain quantitative information about reactivity in ordered aqueous media such as micellar systems and cyclodextrins, and find out how these media can affect and control these reactions. This might have implications for fields such as cell biology, specifically for process occurring in living cells since both cyclodextrins and micelles might be considered simple models for protein and membranes in terms of their hydrophobicity. In addition, little information is known about bimolecular reactions involving two neutral reactants in non-ionic micelles where only the hydrophobic interaction is likely to influence the reaction due to the absence of charge-charge interaction. A kinetic and thermodynamic investigation of the reactions between peracids and different reductants i.e. iodide and series of aryl alkyl sulfides in presence non-ionic micelle and a- cyclodextrin is reported in this work. The kinetics were conducted by monitoring spectrophotometrically the increase or decrease in the absorbance due to formation of triioidide or disappearance of sulfides respectively, and absorbance versus time plots were fitted to nonlinear equation in order to obtain the observed pseudo first order rate constants. For reactions carried out in micellar systems the kinetic data were treated using the multiple micellar pseudophase model developed by Davies which considers the partition of reactants between water in the bulk aqueous phase and that in the micellar pseudophases. Important parameters in this model include the binding constant of reactants to Brij (non-ionic micelle) and kn. (reaction rate in micelle). For reactions in α-cyclodextrin, data was fitted to rate equations containing first and second order dependencies on cyclodextrin using non linear regression techniques. The work was carried out in the presence of 0.003 M nitric acid as reaction medium. The effect of inorganic electrolytes (sodium nitrate, sulfate, actetate, perchlorate and chloride) on the rate of oxidation of iodide in the absence and presence of non-ionic surfactant brij-35 was also studied. The critical micelle concentration (CMC) of the surfactants was determined using kinetic techniques and was found to be inversely proportional to the salt concentration and also to the temperature. The CMC was also found to decrease as the length of the hydrophobic part of the Brij surfactants was increased, possibly due to the decrease of interfacial energy on micellization, which generally increases in with increasing hydrophobic chain length. It was shown from analysis of the kinetic data that all non-ionic micelles in the (absence of salts) and a-CD studied in this work enhanced the rate of the iodide oxidation by MCPBA and that the rate showed saturation type kinetics. Sulfate ions were shown to accelerate the reaction further, whereas perchlorate caused an inhibition of the iodide oxidation (compared to the reaction only in nitric acid) in presence of Brij-35, but an increase in the presence of α-cyclodextrin. For the oxidation of sulfides by MCPBA in micelles and a-CD the observed rate increases to a maximum with increasing micelle or a-CD concentration and then subsequently declines. In the case of sulfide oxidation by the anionic peracid, peroxymonosulfate (PMS), there was only inhibition in the rate, due to separation of reactants. The effect of temperature on both rate and equilibrium processes for these systems was determined over the range 15 to 35°C. The results showed a linear decrease in the binding of metachloroperbenzoic (MCPBA) acid and aryl alkyl sulfides to both micelle and a-CD with increasing the temperature. The thermodynamic and activation parameters for the reactions were determined by using Van't Hoff and Eyring plots. Comparison of the micellar association constants of MCPBA and the apparent micellar association constant of the transition state for the reaction with iodide, suggested that orientational restriction imposed on the peracid by Brij-35 are similar to that in the transition state. For the same reactions carried out in α-cyclodextrin at different temperatures it was determined that the binding constant enthalpy and entropy of substrates, (peracid and iodide) are more negative than that obtained in the presence of brij-35 which indicates that stronger interactions are involved and more restriction imposed on the reactant in presence of a-CD compared to brij-35.A similar approach was employed for the reaction of series of aryl alkyl sulfides with peracids (PMS and MCPBA) in presence of Brij-35 and a-CD. The aryl alkyl sulfides can form both 1:1 and 2:1 host: guest complexes in cyclodextrin; the 2:1 inclusion complexes for some sulfides were larger than the 1:1 complexes, indicating cooperative binding, with the driving force for this possibly being a substrate induced dipole-dipole interaction between the two cyclodextrin molecules. Linear free energy studies indicate that the catalytic species is the bound peracid reacting with the unbound sulfides; sulfide binding results in steric inhibition of the reaction. The reaction of the non-binding PMS with sulfides results only in inhibition as cyclodextrin concentration is increased. The enthalpy and entropy for sulfide oxidation by peracids was calculated by means of a Van't Hoff plot. The reaction in a-CD associated with more negative entropy and enthalpy for the inclusion 2:1 while for 1:1 some substrates associated with positive entropy and small negative enthalpy while other show the usual behaviour observed for complex formation (negative values for both enthalpy and entropy).In all studied reactions (related reactions) there were good relationships between enthalpy and entropy (isokinetic relationships or enthalpy-entropy compensation). Whilst in some cases it is difficult to explain why enthalpy-entropy compensation might be observed, we have suggested that in the case of binding of sulfides to cyclodextrin these plots can act as probes into the orientation of the substrate within the cyclodextrin cavity. The nature of the catalytic mechanism for the reactions of peracids with sulfides and iodide in the presence of micelles and α-cyclodextrin was examined by comparing the transition state stabilisation parameters, KTsi, for the same reaction in the two catalytic systems. It was found that for three out of five sulfides the degree of transition state stabilisation was almost identical in both Brij-35 and α-cyclodextrin, perhaps suggesting the same catalytic mechanism in each system; this could be via either decreased stabilisation of the peracid ground state in the absence of a protic solvent and/or the prevention of significant charge development in the transition state as a result of an intramolecular proton transfer step involving the peracid that is facilitated by the absence of water. Other possibilities exist, such as general acid catalysis, though these would be more dependent on the nature of the catalytic system. There was a less clear relationship for iodide.
Supervisor: Deary, Michael Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.537429  DOI: Not available
Keywords: J100 Minerals Technology ; K900 Others in Architecture, Building and Planning
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