Graduate Thesis Or Dissertation
 

Influence of processing parameters on diffusion of divalent nickel in wet silica sol-gel monoliths

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  • The diffusion of divalent nickel (Ni²⁺) from wet silica gels was investigated. Silica gel is gaining interest as an encapsulation matrix for biological components. The transport of biologically relevant species within the gel is determined by the structural characteristics of the gel, which are in turn governed by synthesis parameters. Gels were synthesized by an acid-base two step process from tetraethoxysilane (TEOS) precursors. Organically modified siloxane precursors, including methyltriethoxysilane (MTES), dimethyldiethoxysilane (DMDES), trimethylethoxysilane (TMES), and ethyltriethoxysilane (ETES) were also used for some samples at a concentration of 10 molar % of silicon. PEG200 was used as an additive in some samples. Sample space covered a full factorial design of three water ratios during hydrolysis of 4:1, 10:1 and 20:1, three acid catalyst concentrations as a ratio of silicon to acid, including 1:0.005, 1:0.01, and 1:0.02, and four dilution ratios during gelation to yield gels with a final silica content of 40:1, 60:1, 80:1, and 100:1, moles of water to moles of silicon. This processing space was selected due to its relevance to applications in the encapsulation of biological components. Using Ni²⁺ as a tracer due to its strong absorbance peak at 395 nm, diffusion coefficients were calculated for all samples using both an analytical solution to Fick's Law, appropriate for one-dimensional diffusion, and an exponential empirical approximation. Estimates were calculated using Microsoft Solver and ANOVA in SAS. It was found that the diffusion coefficient in TEOS gels ranged from approximately 1.4x10⁻¹⁰ m²s⁻¹ to 6.3x10⁻¹⁰ m²s⁻¹, with a mean of approximately 2.5x10⁻¹⁰ m²s⁻¹ corresponding to approximately 14% to 63% of D for Ni²⁺ in unconfined aqueous solution, estimated to be approximately 1x10⁻⁹ m²s⁻¹. The addition of 10 mol% ORMOSILS was found to have a small effect on the predicted value of the diffusion coefficient depending on silicon content. In samples with a final silicon content of 80:1, D was slightly decreased to approximately 2.0x10⁻¹⁰ m²s⁻¹, but in samples with a silicon content of 100:1, D was slightly increased to approximately 3.5x10⁻¹⁰ m²s⁻¹. Variations in hydrolysis ratio, acid catalyst content, and dilution ratio had relatively weak effects on overall diffusion rates of Ni²⁺ with the exception of a few anomalous samples which were either unstable or displayed some syneresis. It can be concluded that over this broad processing space, gels can be tailored to best suit the particular bioencapsulation application, altering the chemical environment for optimal performance with minimal variation in the diffusion transport of small cationic ions such as Ni²⁺.
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