Abstract:

Submicron nonporous silica (NPS) materials feature small particle sizes, smooth surfaces, and regular shapes. They also exhibit excellent performance as a stationary phase; however, their use is limited by their low specific surface area and low phase ratio. Therefore, a novel surface modification strategy tailored for NPS microspheres was designed, involving a multi-step reaction. 3-Glycidyloxypropyltrimethoxysilane (GPTS) was first grafted onto NPS particles as a silane coupling agent. Polyethyleneimine (PEI), a high-molecular-weight polymer, was then coated onto the particles, providing numerous amino reaction sites. In the final step, an acylation reaction was initiated between stearoyl chloride and the amino groups to obtain the final product, designated as C₁₈-NH₂-GPTS-SiO₂. Elemental analysis, FT-IR spectroscopy, Zeta potential analysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were employed to investigate the success of the chemical modifications at each step. The carbon content increased from 0.55% to higher than 8.29%. Thus, it solved the low carbon loading capacity problem when modifying NPS microspheres with traditional C₁₈ reversed phase (e. g., octadecyl chlorosilane modification).

Meanwhile, the reasons for the considerable differences between NPS and porous silica (PS) microspheres in terms of the reactivity to surface modification were investigated in detail. The BET method was employed to compare the pore structures. FT-IR and ²⁹Si solid-state NMR spectroscopy were employed to analyze the differences in the structure and quantity of silanol groups on the surfaces of the NPS and PS microspheres. Differences were observed not only in the pore size and surface area, but also in the types of silanol groups. FT-IR analysis indicated that the NPS and PS microspheres had different υ_Si-OH band positions, which shifted from 955 to 975 cm^-1, respectively. ²⁹Si solid-state NMR analysis further highlighted the differences in structural information for Si atom environments. Results revealed that 16% of silicon atoms in the PS microspheres had one hydroxyl group (isolated silanols, Q₃, δ 100), while 19% had two hydroxyl groups (geminal silanols, Q₂, δ 90). On the other hand, the NPS microspheres possessed no geminal silanols, and only 30% of the Si atoms were in the Q₃ state. Therefore, the NPS microspheres had a lower density of silanol groups and lacked geminal silanol groups, compared to the PS microspheres. Geminal silanol groups have already been confirmed in previous studies to offer far higher reactivity than isolated silanols. These factors together explained the low reactivity of NPS microspheres toward surface modification. Further, the low specific surface area of the microspheres arising from their nonporous nature made it challenging to obtain a high carbon content through a simple one-step reaction.

Hydrophobic substances such as hydrocarbons from the benzene series and polycyclic aromatic hydrocarbons (PAHs) were selected to study the chromatographic performance. The hydrophobic mechanism was revealed by the separation of PAHs using different ratios of acetonitrile. Separation was achieved with a C₁₈-NH₂-GPTS-SiO₂ column, following which a hydrophobic phenomenon occurred. The presence of the amino coating led to the inversion of the electroosmotic flow (EOF) of the silica microspheres on the pressurized capillary electrochromatography (pCEC) platform. It also enhanced the linear velocity in the pCEC platform when the pH was selected to be low. The effects of the applied voltage on the separation ability of the 720 nm C₁₈-NH₂-GPTS-SiO₂ column were examined to determine optimal conditions. Rapid and effective separation was achieved in a relatively short time. Therefore, the C₁₈-NH₂-GPTS-SiO₂ stationary phase is promising for practical use with a higher phase ratio, demonstrating superiority for use in reversed-phase pCEC separation, and thus, providing a new strategy and valuable reference for the future application of submicron NPS microspheres.

Key words: pressurized capillary electrochromatography (pCEC), submicron nonporous silica microsphere, high carbon content, silanol groups, electroosmotic flow (EOF)