EOF in monolithic poly(styrene-co-divinylbenzene) capillary columns.

Area of Science:

• Polymer Chemistry
• Separation Science
• Analytical Chemistry

Background:

• Monolithic capillary columns offer advantages in chromatography.
• Understanding electroosmotic flow (EOF) is crucial for optimizing separation efficiency.
• Neutral polymers are desirable for specific separation modes, but their EOF behavior can be complex.

Purpose of the Study:

• To investigate the generation of EOF on uncharged poly(styrene-co-divinylbenzene)-based monolithic capillary columns.
• To compare the electroosmotic mobilities (mu(EOF)) versus pH profiles of unmodified and modified monoliths.
• To elucidate the mechanisms behind observed EOF behavior using molecular modeling.

Main Methods:

• Preparation of three types of neutral monoliths: unmodified poly(styrene-co-divinylbenzene) and two modified with octadecyl chains (grafting and Friedel-Crafts reactions).
• Measurement of electroosmotic mobilities (mu(EOF)) across a pH range (2.5-9.5).
• Molecular modeling using HyperChem software to simulate and explain surface interactions.

Main Results:

• The mu(EOF) remained relatively constant as pH increased from 2.5 to 9.5.
• A significant decrease in mu(EOF) was observed when pH was subsequently lowered.
• Molecular modeling indicated that buffer molecules, in the presence of water, can form a negatively charged layer on the neutral polystyrene surface.

Conclusions:

• The pH-dependent EOF in these neutral monoliths is primarily governed by the interaction of buffer molecules with the polymer surface.
• The observed hysteresis in mu(EOF) suggests a reversible adsorption/desorption mechanism influenced by pH changes.
• Understanding these surface interactions is key to controlling EOF in polymer-based monolithic columns for enhanced separations.