Dissertation
Investigation of microenvironmental heterogeneity in nanoporous silica by quantitative confocal imaging and computer simulation
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2023
DOI: 10.25820/etd.006969
Abstract
Nanoporous silica particles have been widely used in chemical and biomedical engineering and sciences such as catalysis, chemical separations, and biosensors. Although many experimental and computational studies have unraveled transport kinetics and diffusivity through the pores, there is still a room for improvement of understanding microscale interfacial area between mobile phase and stationary phase in liquid chromatography. Heterogeneous distribution of adsorption and desorption sites of silanol group on the pore surfaces has accounted for low resolving power in the chemical separation: peak tailing and band broadening.
Our methods based on fluorescence and confocal microscopy visualized the distribution of effective diffusion and kinetics at adsorption sites through the intraparticle condition in the liquid chromatography. The hypothesis of the study is that the molecular dynamics inside chromatographic silica particle pores are affected by both Brownian motion and adsorption kinetics because the diffusing molecules inside the pores will have chances to adsorb and desorb at hydrophobic layers or silanol groups on the pore surfaces. Those populations are expected to be heterogenous through the silica particle because the porous structure has been observed as highly heterogeneous by many other electron microscopy imaging studies. To test this hypothesis, the translational diffusion and desorption rates of fluorescent probe in the silica particle were imaged using fluorescence correlation spectroscopy (FCS) with a confocal imaging strategy. The FCS with a confocal microscopy can reveal the hidden heterogeneity in molecular populations through the accumulation of single molecule signals at high spatial-temporal resolution. In this contribution, quantitative imaging of diffusion and adsorption kinetics in the nanoporous silica particle were constructed using our home-built Imaging FCS and computer simulation works. The result of quantitative images showed us the heterogeneity of those kinetics over the silica particle which implies the origin of the dispersive chromatographic peak resolution. The mathematical model for fitting and interpreting the autocorrelated fluctuation data was tested by generating Monte Carlo simulation combining Brownian motion and random stopping-departing motion.
Ratiometric intensity imaging can provide chemical information of equilibrium constant which describes the attraction force of a wet nanoporous silica particle. Fast scanning can be achieved by laser scanning confocal microscopy (LSCM). Based on LSCM, we constructed quantitative Gibbs free-energy (∆G) images of the C18 silica particles in a packed column. The heterogeneity of interparticle and intraparticle particles were visualized by analyzing the ∆G images, and the result provided a potential origin of the problematic band broadening issue.
Eddy diffusion and longitudinal diffusion are crucial parameters that dominantly affect the chromatographic column efficiency. Because the eddy diffusion is caused by irregular interstitial space through a packed column, 3-D visualization of the packed bed structure is an essential source for constructing interstitial diffusion simulations or any theoretical models to unravel the eddy diffusion which is one of the most unpredictable parameters in van Deemter equation. Expansion of 3-D visualization was demonstrated by combining a lateral sequent imaging strategy and a useful function of image analysis software.
Overall, the imaging strategies of transport kinetics and diffusivity, thermodynamic free energy, structural images through the heterogeneous chromatographic environment were independently developed, and these studies will be essential towards improving the understanding of the origin of resolution limit in chemical separation science. Ultimately, these scientific approaches, knowledge and insight will be required to design a better chemical plant for more efficient separations.
Details
- Title: Subtitle
- Investigation of microenvironmental heterogeneity in nanoporous silica by quantitative confocal imaging and computer simulation
- Creators
- Hong Bok Lee
- Contributors
- M Lei Geng (Advisor)Mark A Arnold (Committee Member)Leonard R MacGillivray (Committee Member)Alexei V Tivanski (Committee Member)Johna Leddy (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Summer 2023
- DOI
- 10.25820/etd.006969
- Publisher
- University of Iowa
- Number of pages
- xviii, 309 pages
- Copyright
- Copyright 2023 Hong Bok Lee
- Language
- English
- Date submitted
- 07/12/2023
- Description illustrations
- illustrations, tables, graphs
- Description bibliographic
- Includes bibliographical references (pages 300-309).
- Public Abstract (ETD)
- This contribution visualized what happens in the chemical separation material and developed the quantitative method of optical visualization. Achieving a high resolution of chemical separation is the key, and our imaging is a powerful tool to understand the heterogeneity of the material which originates any low resolution during the chemical separation. Chemical separation is essential for everyday life. Distillation or evaporation are widely known separation techniques. For example, refining petroleum can be achieved by fractional distillation, but it requires a high energy cost. If possible, we may prefer extraction or crystallization types of separation to reduce the energy cost. Extraction types can be observed even in daily life, and it can be a coffee filter paper, for example. I can enjoy drinking a cup of coffee without chewing any bulky grinds using the filter paper. Extraction of coffee here happens because the size of pores in the filter paper is small enough to block any grind particles but large enough to allow the enjoyable chemicals as a liquid phase to penetrate the filter paper, finally, gravitational force leads them to fall into a cup. Beyond the coffee filter, a somewhat higher level of sophisticated separations for chemical mixtures can be accomplished based on their differences of chemical properties. For example, removing any impurities from water or separating two slightly different chemicals, which molecular size and structure are similar, could be very challenging projects. In order to achieve a perfect separation, designing chemical plants or devices will absolutely require knowledge: molecular level understanding during the separation process. Interestingly, there is still a huge room for the improvement of understanding and characterizing. The first step should be to understand and characterize the materials that compose the separation system. Rather than macroscopic sight of view, it is desirable to visualize those reactions or structures into molecular level to measure how long exactly the solute of interest would adsorb and desorb or diffuse through the porous silica particle. We constructed quantitative chemical imaging of the porous silica particles through this contribution. Our investigation will be essential knowledge for the better design of chemical separation and the future direction of understanding liquid chromatography.
- Academic Unit
- Chemistry
- Record Identifier
- 9984454541102771
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