Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582800
Title: NMR and gas sorption studies of structure-transport relationships in porous media
Author: Shiko, Elenica
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2013
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Abstract:
The work in this thesis is focused on testing the accuracy of the gas sorption and NMR cryoporometry characterization techniques to estimate the key pore descriptors which affect the activity of porous materials used as catalyst supports and drug delivery systems. Both techniques, though, assume independent pores, neglecting advanced adsorption and melting phenomena that can specifically skew the pore size distribution and subsequently lead to inaccurate predictions of catalytic or therapeutic efficiency of the porous system. Firstly, the independent domain theory for both processes was studied by breaking down the pore-filling process of a mesoporous catalyst support, into steps. The system was partially saturated with water or cyclohexane at different pressures, via adsorption and desorption, followed by a cryoporometry experiment at each saturation fraction. Moreover, scanning curves and loops, together with PFG NMR and relaxometry were employed to ascertain the spatial arrangement of the liquid ganglia at each partial saturation and for certain molten fractions. It was shown that the configuration of the liquid condensates varied with position around the hysteresis loop, deviating from the single pore hysteresis mechanism for both adsorbates. Advanced melting of water was associated with a percolation-type transition in the connectivity of the ganglia, which could be curtailed to some extent by sample fragmentation. Also, some pores filled via advanced adsorption at lower pressures. On the contrary, advanced melting of cyclohexane arose from the liquid bridging the pore cross-sections of the partially filled pores. Secondly, an integrated nitrogen-water-nitrogen experiment was employed to test the source of sorption hysteresis and to compare the extent of advanced adsorption phenomena for nitrogen and water sorption, by isolating a subset of pores. It was found that the Kelvin-Cohan equations and the DFT algorithm overestimate the width of the sorption hysteresis in independent pores of the catalyst support studied in this work. Moreover, the adsorption mechanism of nitrogen differs to that of water, and advanced adsorption of nitrogen is less severe than that of water. Thirdly, cryodiffusometry and gas sorption techniques were used to estimate the pore space descriptors (surface area, pore size, tortuosity, porosity) of two different types of mesoporous silicas, candidates for drug delivery. The structure-transport relationships in these materials were investigated to interpret the drug release profiles obtained for release studies carried out in simulated gastrointestinal fluids. It was found that the release rate was mainly controlled by the size of the silica particles and the silica solubility itself in the environment present. Also, different synthesis routes were tested to optimize the drug loaded PLGA nanoparticles, for convection-enhanced drug delivery into the brain. Various model and real hydrophobic and hydrophilic drugs were tested. In-vitro and in-vivo studies showed that the dialysis method led to production of particles with the desirable characteristics, which were successfully distributed in the mice brain. The sensitivity of the cryoporometry melting, gas sorption and imaging techniques was found inadequate to resolve the inner structure of the polymer matrix. Last, the experimental time for the cryodiffusometry experiments in this work was long due to the high recycle delay times required to maximise the signal to noise ratio. It is though found that high delay times are unnecessary when BBP-LED pulse sequence is used, even when the fluid is imbibed in a mesoporous systems.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.582800  DOI: Not available
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