Effect of Hydrophobic Moieties on the Assembly of Silica Particles into Colloidal Crystals.

To boost the implementation of colloidal crystals (CCs) in separation science, the effects of the most common chromatographic reversed phases, that is, butyl and octadecyl, on the assembly of silica particles into CCs and on the optical properties of CCs are investigated. Interestingly, particle surface modification can cause phase separation during sedimentation because the assembly is highly sensitive to minute changes in surface characteristics. Solvent-induced surface charge generation through acid-base interactions of acidic residual silanol groups with the solvent is enough to promote colloidal crystallization of modified silica particles. In addition, solvation forces at small interparticle distances are also involved in colloidal assembly. The characterization of CCs formed during sedimentation or via evaporative assembly revealed that C4 particles can form CCs more easily than C18 particles because of their low hydrophobicity; the latter can only form CCs in tetrahydrofuran when C18 chains with a high bonding density have extra hydroxyl side groups. These groups can only be hydrolyzed from trifunctional octadecyl silane but not from a monofunctional one. Moreover, after evaporative assembly, CCs formed from particles with different surface moieties exhibit different lattice spacings because their surface hydrophobicity and chemical heterogeneity can modulate interparticle interactions during the two main stages of assembly: the wet stage of crystal growth and the late stage of nano dewetting (evaporation of interparticle solvent bridges). Finally, short, alkyl-modified CCs were effectively assembled inside silica capillaries with a 100 µm inner diameter, laying the foundation for future chromatographic separation using capillary columns.

Preparation of open tubular capillary column covalently coated with polystyrene sulfonate with 4,4'-Azobis(4-cyanopentanoyl chloride) as polymerization initiator for electrochromatographic separation of alkaloids, sulfonamides, and peptides.

An open tubular capillary electrochromatography column covalently bonded with polystyrene sulfonate was prepared via in situ polymerization using functionalized Azo-initiator 4,4'-Azobis(4-cyanopentanoyl chloride). Scanning electron, fluorescence, and atomic force microscopy techniques showed the formation of a relatively rough layer of polymer. In addition, -CN and C = O stretching vibrations from infrared spectroscopy proved the successful immobilization of the azo-initiator through covalent bonding and X-ray photoelectron spectroscopy confirmed the elemental composition of the formed polymer layer. The prepared column was found to be appropriate for small and medium-sized molecules separation. Compared to bare fused silica capillary column higher selectivity and resolution were obtained for the separation of alkaloids, sulfonamides, and peptides as a result of the electrostatic and pi-pi stacking interactions between the small organic molecules and the coated column without compromising the electroosmotic flow mobility. Separation efficiency was also increased compared to the bare capillary for the separation of alkaloids (about 1.5 times). Moreover, intraday, inter-day, intra-batch, and inter-batch relative standard deviation values of retention time and peak area of peptides were within 2% and 10%, respectively, indicating good repeatability of the column preparation procedure. The developed method for the covalent bonding of polymers through a functionalized azo-initiator could represent a promising stable method for the preparation of an open tubular column.

[Novel submicron nonporous silica material modification with high carbon content and its application in reversed-phase pressurized capillary electrochromatography].

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 υSi-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.