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Langmuir 2003, 19, 8608-8610
Complete Methylation of Silica Surfaces: Next Generation of Reversed-Phase Liquid Chromatography Stationary Phases J. David Sunseri, Thomas E. Gedris, Albert E. Stiegman,* and John G. Dorsey* Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390 Received April 14, 2003. In Final Form: June 10, 2003
Introduction Liquid chromatography is likely the most used method for the separation and analysis of compounds in complex solutions. Among the types of liquid chromatography, it is those employing a reversed-phase stationary phase that are the most technologically useful. In reversed-phase chromatography a solid support is derivatized, usually with a hydrocarbon, to make the surface hydrophobic. In general, the performance of a reversed-phase liquid chromatography (RPLC) column depends on the type of the solid support, the chemical nature of the surface derivative, and the degree of coverage attained. Of the possible solid supports silica has been, by far, the most widely used due to the fact that it has a number of propitious properties. These include easily controlled particle size, high surface area, and mechanical stability.1,2 More importantly, silica contains several types of surface silanols that provide the necessary functionality for chemical derivatization to be attained. Notwithstanding this, there are significant problems with derivatized silica supports that compromise their performance. In particular, they function chromatographically only over a limited pH range (3-8) and can be dissolved at pH’s higher than 8. In addition, peak tailing often occurs (especially for basic solutes), which adversely affects component separation, selectivity, and resolution. The origin of the majority of these problems lies in the incomplete derivatization of the surface, which leaves unreacted silanol groups. Alkylation of the silica surface for chromatographic purposes is usually carried out using well-established silane coupling agent chemistry. Typical silane coupling agents used for silica derivatization have the general formula ClSiR1R2R3, where R represents organic groups, which can differ from each other or all be the same.3 For reversed-phase chromatography, the silane coupling agent has traditionally been ClSi(CH3)2(C18H37), where the C18 octadecyl group yields a hydrophobic surface. The basic silane coupling reaction involves the metathesis of the chlorine with a surface silanol to form a Si-O-Si linkage with the elimination of HCl (reaction 1). This reaction, when carried out on hydroxylated silica,
tSisOH + ClSiRR′R′′ f tSisOsSiRR′R′′ + HCl (1) which typically has a surface silanol concentration of approximately 8 ( 1 µmol/m2, does not go to completion * To whom correspondence should be addressed. E-mail:
[email protected] (A.E.S.);
[email protected] (J.G.D.). (1) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A. (2) Nawrocki, J. J. Chromatogr., A 1997, 779, 29. (3) Unger, K. K. Porous Silica; Elsevier Scientific Publishing Co.: Amsterdam, 1979; p 112.
due to the steric congestion imposed by the R groups on the coupling agent. The highest coverage attained in laboratory studies has been ∼4.5 µmol/m2, while the coverage available in commercial chromatography columns is much less, usually on the order 2.7-3.5 µmol/m2. Clearly, large improvements in the selectivity and stability of chromatographic stationary phases can be realized by the development of a more efficient derivatization process. The most common way to chlorinate the surface is using thionyl chloride to yield Si-Cl bonds. In past literature, it is reported that maximum chloride coverage is achieved by driving the reaction above the boiling point of SOCl2.3 Deuel and co-workers were the first to report the conversion of Si-OH to Si-Cl on the silica surface.4 In 1973, Unger and co-workers showed that porous silica yielded higher chlorination coverage than Aerosil.5 Pesek and coworkers in 1986 reported chlorination of the silica surface using a 10% thionyl chloride solution in toluene and, in subsequent studies, used it to produce allyl-bonded stationary phases.6-10 We report here the near-quantitative methylation of all the available silanol groups on a silica surface. This is realized through chlorination of the surface using thionyl chloride followed by methylation with methyllithium. To the best of our knowledge this is the first time that this has been achieved. The resulting materials are robust under basic conditions and show significant enhancement in their chromatographic properties. This process lays the groundwork for extended length alkyl chains to be covalently bound to the silica surface. Experimental Section Silica (5 µm particles, 341 m2/g surface area, 92 Å pore diameter), Waters Symmetry, was obtained from the Waters Corp. Thionyl chloride and methyllithium (1.4 M in diethyl ether) were obtained from Aldrich and used as received. C, H, N analysis was performed on a CE Instruments NC 2500 elemental analyzer, the results of which were subsequently verified by Quantative Technologies. Chlorine analysis was performed by Galbraith Laboratories. Chlorination. A 10 g sample of silica was dried under vacuum (10-3 Torr) at 150 °C overnight, at which point it was allowed to cool to room temperature. Approximately 125 mL of thionyl chloride, degassed with three consecutive freeze-pump-thaw cycles, was bulb-to-bulb distilled onto the silica. The silica/thionyl chloride slurry was sonicated for 30 min (Branson 1210) and then stirred for 23.5 h at room temperature. The remaining thionyl chloride was then removed under a vacuum, and the silica was allowed to dry under vacuum at room temperature. The flask containing the silica was then flushed with dry nitrogen, and 125 mL of 1.4 M methyllithium in diethyl ether was added using an airtight syringe.11 Care was taken to ensure that the reaction was maintained under scrupulously anaerobic and anhydrous conditions. The slurry was again sonicated for 30 min and stirred at room temperature for 23.5 h. The silica was then filtered off and washed with an excess (250 mL) of 2-propanol (4) Deuel, H.; Wartmann, J.; Hutscheneker, K.; Schobinger, U.; Gudel, C. Helv. Chim. Acta 1959, 42, 1160. (5) Unger, K.; Thomas, W.; Adrian, P. Kolloid Z. Z. Polym. 1973, 251, 45. (6) Pesek, J. J.; Swedberg, S. A. J. Chromatogr. 1986, 361, 83. (7) Pesek, J. J.; Mahabadi, P.; Chan, S. Chromatographia 1987, 23, 3. (8) Pesek, J. J.; Guiochon, G. J. Chromatogr. 1987, 395, 1. (9) Pesek, J. J.; Tarver, E. L.; Lange, A. Chromatographia 1987, 24, 815. (10) Pesek, J. J.; Wey, W. Y. Chromatographia 1988, 25, 969. (11) Schriver, D. F.; Drezdzon, M. A. The Manipulation of AirSensitive Compounds, 2nd ed.; Wiley: New York, 1986.
10.1021/la030156n CCC: $25.00 © 2003 American Chemical Society Published on Web 09/03/2003
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to remove and deactivate any remaining methyllithium. The silica was dried at 125 °C for 24 h under vacuum. Column Packing. Samples were suspended in chloroform and then packed into 15 cm × 4.6 mm stainless steel columns (Sci-Con, Winter Haven, FL) by the traditional slurry method using an air-driven pump (Haskel Engineering and Supply Co., Burbank, CA) at 7320 psi. The derivatized silica was “pushed” into the column using three different solvents or solvent mixtures. The pushing solvents used were 200 mL each of chloroform, methanol (Sigma-Aldrich, Milwaukee, WI), and 50:50 methanol/ water (Barnstead, Dubuque, IA). Characterization of the Silica Surface. Solid-state 29Si CPMAS NMR data (99.4 MHz) were obtained with an INOVA AS500 11.7 T NMR spectrometer (Varian Inc., Palo Alto, CA). The 7.5 mm rotor was spun at 5.0 kHz. The proton 90° pulse and CP contact times were 4.5 µs and 8.0 ms, respectively. Proton decoupling at 60 kHz and a recycle delay time of 3 s were used. All spectra were secondarily referenced to tetramethylsilane. All 29Si resonances for dense silica are specified using the conventional Qn notation, where n indicates the number of bridging oxygens (-OSi) around the silicon. These resonances for a Si* designated Q1, Q2, Q3, and Q4 therefore correspond to Si*(-OSi)1, Si*(-OSi)2, Si*(-OSi)3, and Si*(-OSi)4, respectively.12,13 All diffuse reflectance Fourier transform infrared spectra were obtained on a Nicolet Avatar 360 FT-IR spectrometer using a Harrick Praying Mantis diffuse reflectance accessory. Spectra were the average of 64 scans of 4 cm-1 resolution in the 4004000 cm-1 range. RPLC Studies. Chromatograms were generated using two model 510 HPLC pumps (Waters, Milford, MA) controlled by a model 680 automated gradient controller (Waters). Solutes were detected using a Spectroflow 757 absorbance detector (Kratos Analytical, Chestnut Ridge, NY). HPLC-grade eluents were used for each separation. Reagents used for RPLC studies were methanol, pyrrolodine (Sigma-Aldrich), and purified water (Barnstead). The Engelhardt test was used to test for silanol activity.14 A 49:51 (v/v) methanol/water solution was used at 2 mL/min and 40 °C, and detection was at 254 nm. The interactions of aniline and phenol with the residual silanols showed the amount of silanol activity. If aniline elutes before phenol, there is no significant activity, but if phenol elutes before aniline, there is significant activity. Also, the asymmetry of p-ethylaniline was measured at 10% peak height for another measure of silanol activity.14 A highpH mobile phase, 55:45 MeOH/0.05 M pyrrolidine, HCl buffer, pH 11.5, was used to test the stability of the stationary phase by recording teh column pressure over time, and using the silicomolybdate complex method, the amount of silica dissolved was obtained.15
Results and Discussion Methylation of the silica surface was carried out by a two-step process. In the initial step the silica surface was chlorinated using thionyl chloride. The chlorine was then metathesized with methyllithium to introduce methyl groups and eliminate LiCl (Figure 1). This approach to alkylation is fundamentally different from that of silane coupling chemistry and, in principle, creates a surface that should be superior for chromatographic applications. In particular, alkylation using silane coupling agents occurs through the formation of a single Si-O-Si linkage, which is inherently less robust to processes such as hydrolysis. In addition, the presence of the three alkyl groups on the silica imposes steric constraints that prevent complete surface coverage. By putting the leaving group (12) Enghardt, V. G.; Altenburg, W.; Hoebbel, D.; Wieker, W. Z. Z. Anorg. Allg. Chem. 1977, 418, 43. (13) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990; p 164. (14) Engelhardt, H.; Aranglo, M.; Lobert, T. LC-GC 1997, 15, 856. (15) Iler, R. K. The Chemistry of Silica: polymerization, colloid and surface properties, and biochemistry; Wiley-Interscience: New York, 1979; p 97.
Figure 1. Alkylation of a chlorinated silica surface.
on terminal sites on the condensed silica surface, it should be possible to realize a much more robust surface with higher coverage. Despite the potential advantages of this type of alkylation process, this approach has not been extensively explored, especially for chromatographic applications. This is in part due to the extreme hydrolytic sensitivity of the chlorinated silica surface, which makes handling difficult, and to the fact that recent NMR studies have indicated that the process is too harsh and actually breaks down the silica surface.16 In our procedure, the chlorination step was carried out under rigorously anhydrous conditions using vacuum and Schlenk-line techniques.11 More importantly, to minimize possible breakdown of the silica surface, the chlorination and methylation were not carried out, as they typically are, under refluxing conditions.6 Instead the thionyl chloride/silica and the methyllithium/ silica slurries were briefly sonicated to achieve thorough mixing and activation, and then stirred at room temperature overnight. Elemental analysis of the methylated silica obtained in this process contained 3.74% carbon, which corresponds to a bonding density of 9.58 µmol/m2 (the amount of carbon in the pure silica controls was e0.1%). Within experimental error, this constitutes complete replacement of all the available silanols on the silica surface. Further, chlorine analysis indicates that a negligible amount of chlorine (
