Date of Award

Spring 5-18-2024

Degree Type

Dissertation

Degree Name

PhD. Chemistry

Department

Chemistry and Biochemistry

Advisor

Yuri V. Kazakevich, Ph.D.

Committee Member

Alexander Y. Fadeev, Ph.D.

Committee Member

Nicholas H. Snow, Ph.D.

Committee Member

Stephan P. Kelty, Ph.D.

Keywords

Adsorption from solutions, Excess adsorption isotherms, Liquid chromatography, Adsorbent surface area, Standard isotherms, Minor disturbance method (MDM), Stander Excess Adsorption Isotherm (SEAI), Adsorbent Geometry, Surface-specific retention factor, C18 chromatographic columns

Abstract

Excess adsorption isotherms of acetonitrile and methanol from water were measured on eight commercial columns. Columns used in this study represent the latest examples in column development and include three different poroshell columns (Kinotex C18, Ascentis C18, and Halo C18) as well as conventional columns with significantly different adsorbent geometries (Allure C18, YMC C18) and various hybrid-silica columns (Gemini C18, Xterra C18, and XBridge C18). Comparison of the excess adsorption isotherms measured on all these columns and expressed in surface-specific form demonstrated significant similarity of the adsorption properties of all columns, which allows the introduction of the Standard Excess Adsorption Isotherm (SEAI) for reverse phase C18 type columns. The methodology of the evaluation of the total amount of adsorbent in the column and the effective surface area of the C18 modified adsorbent is also discussed. These terms are critical for the successful evaluation of surface-specific parameters.

One critical but underutilized parameter affecting chromatographic separation is adsorbent surface area (S.A). We introduce a concept of direct determination of the total surface area in chromatographic columns by utilizing non-destructive measurements of eluent component excess adsorption isotherm measurement and the application of the SEAI from acetonitrile-water and methanol-water binary mixtures for direct determination of the S.A. in chromatographic columns. This nondestructive technique was used to determine the surface area for five new C18 commercial columns and one in-house made C18 column. The Brunauer-Emmett-Teller traditional (BET) method was compared to the novel chromatographic nondestructive surface area determination method for adsorbents modified with octadecylsilanes. The chromatographically determined surface area was in agreement with the conventional techniques for all columns used in this study. The difference between the traditional method and our novel chromatographic method is less than 10%. Obtained columns were characterized in terms of adsorbent, geometry ( e.g., pore volume, columns void volume, inter-particle volume), ligands bonding density, and the amount used for the packing material in a chromatographic column were also studied.

In addition, we have also shown that our novel non-destructive chromatographic determination of surface area could be used directly in obtaining surface-specific retention factor ks for direct C18 column comparison.

The investigation extended to explore the applicability of the Standard Excess Adsorption Isotherm (SEAI) in estimating surface areas for column types beyond C18. Under the influence of the mobile phase, the long-chain ligands adopt a "collapsed configuration." Consequently, the adsorption behavior for stationary phases modified with alkyl chains of varied lengths proves comparable, provided bonding densities of the ligands also remain consistent.

Comparable behavior was observed across different chain lengths, indicating the potential of SEAI for surface area determination in columns beyond C18.

Stationary phases with varying ligand lengths (C1, C4, C8, and C18) were synthesized in-house and thoroughly characterized. The findings suggest that our innovative, non-destructive approach to surface area determination holds promise for C4, C8, and C18 ligands.

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