Langmuir 1996, 12, 4741-4747
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Fundamental Study of Residual Silanol Populations on Alkylsilane-Derivatized Silica Surfaces A. B. Scholten,* J. W. de Haan, H. A. Claessens, L. J. M. van de Ven, and C. A. Cramers Eindhoven University of Technology, Laboratory of Instrumental Analysis, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received February 5, 1996. In Final Form: June 10, 1996X Two commercially available reversed phases for high-performance liquid chromatography (one dimethyln-octadecylsilane phase and one di-isobutyl-n-octadecylsilane phase) and the native silica substrate were subjected to deuterium exchange in a mobile phase-like medium of acetonitrile and deuterium oxide (90/10, v/v). 29Si cross-polarization magic-angle-spinning (CP MAS) NMR was used to obtain information on the amounts and relaxation behavior of the internal silanols of the silica substrate, and the residual silanols of the alkylsilane-derivatized phases. Consequences for the correlation of silanol 29Si CP MAS NMR data and chromatography are discussed. Detection of deuterated silanol (SiOD) signals in 29Si CP MAS NMR of deuterium-exchanged phases depends on a transfer of magnetization (cross-polarization) from silane protons to silanol silicon atoms. It is concluded that a large portion of the residual silanol NMR signal of the phases stems from internal silanols and from silanols that are spatially very close to an alkylsilane attachment site. The former are considered chromatographically irrelevant while the latter are assumed to be inaccessible for analytes during the chromatographic separation process for reasons of steric constraints. One bulky di-isobutyl-n-octadecylsilane is capable of cross-polarizing proton magnetization to 0.8 residual surface silanols, whereas this is only 0.3 for the conventional octadecylsilane with dimethyl side groups. However, the lower surface concentration of bulky alkylsilanes leaves more space for free, unhindered silanols. The most important factor determining the relaxation behavior of the residual silanol NMR signal is the mobility of the octadecylsilane side chains, bulky silanes having the larger mobility quenching effect on residual silanols.
Introduction Reversed-phase systems for HPLC consisting of a porous silica substrate covered by chemically bonded alkylsilane structures are by far the most popular in modern RPHPLC practice. The practical usefulness of the RP-HPLC technique, however, seems to distract the chromatographer’s attention from the theoretical problems that are left unsolved.1 For example, stationary phase stability and the relation between molecular stationary phase structure and chromatographic retention mechanism are still topics of discussion among stationary phase researchers.1-8 Especially residual silanol functionalities, which remain at the silica surface after derivatization with alkylsilanes, have been the subject of investigation.9-15 Apart from the solvophobic interactions between solutes and the nonpolar octadecyl phase, also the silanophilic interactions with residual silanols had to be taken into * To whom correspondence should be addressed. Telephone: 31 40 2473022. Fax: 31 40 2453762. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Wirth, M. J. LC-GC Int. 1994, 7, 626. (2) Kirkland, J. J.; Dilks, C. H., Jr.; Henderson, J. E. LC-GC Int. 1993, 6, 436. (3) Bliesner, D. M.; Sentell, K. B. Anal. Chem. 1993, 65, 1819. (4) Jinno, K.; Wu, J.; Ichikawa, M.; Takata, I. Chromatographia 1993, 37, 627. (5) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A. (6) Ha¨berlein, H.; Tschiersch, K.-P. Pharmazie 1994, 49, 769. (7) Uhl, L.; Rempfer, K.; Egelhaaf, H.-J.; Lehr, B.; Oelkrug, D. Anal. Chim. Acta 1995, 303, 17. (8) Yarovski, I.; Aguilar, M.-I.; Hearn, M. T. W. Anal. Chem. 1995, 67, 2145. (9) Suprynowicz, A.; Pilorz, K.; Lodkowski, R. J. Chromatogr. 1991, 552, 463. (10) Fo´ti, G.; Kova´ts, E. sz. Langmuir 1989, 5, 232. (11) Welsch, T.; Frank, H.; Vigh, G. J. Chromatogr. 1990, 506, 97. (12) Fo´ti, G.; Martinez, C.; Kova´ts, E. sz. J. Chromatogr. 1989, 461, 243. (13) Nawrocki, J.; Buszewski, B. J. Chromatogr. 1988, 449, 1. (14) Goworek, J.; Nooitgedacht, F.; Rijkhof, M.; Poppe, H. J. Chromatogr. 1986, 352, 399. (15) Nawrocki, J. Chromatographia 1991, 31, 177.
S0743-7463(96)00105-9 CCC: $12.00
account to explain the observed chromatographic separation characteristics of silica-based alkylsilane phases. Although chromatography itself has been used to estimate the concentration and accessibility of residual silanols,10,14,16 techniques like infrared spectroscopy and solid-state NMR spectroscopy can provide important additional information on molecular surface structures. Recently, Maciel et al. have shown how 29Si solid-state NMR can be used to determine the concentration as well as the physical environment of internal silanol groups of a silica gel.17 They used deuterium oxide (D2O) to chemically exchange accessible surface silanol protons for deuterons (as was done in the chromatographic studies of Kova´ts et al.10 and Poppe et al.14) and thus they were able to study explicitly the silanols buried in the silica gel bulk structure with 1H-29Si cross-polarization magicangle-spinning (CP MAS) NMR. Deuterium exchange of residual silanols on a RP-HPLC phase, suspended in a mobile phase-like medium, should open up the possibilities of determining the analyte accessibility of the surface silanols and their physicochemical characteristics by the same 29Si CP MAS NMR technique. The deuteriumexchange reaction itself is fast, indicating that only the extent of silanol accessibility is the limiting factor in exchanging all silanol functionalities. On a native silica, all surface silanols are in principle accessible, but in the case of a RP-HPLC octadecyl phase, their accessibility is influenced by the shielding effects exerted by the C18 chains. This shielding effect in turn is determined by the alkyl chain-solvating power of the mobile phase: the better the C18 chains are solvated, the better the mobile phase components should be able to reach the silica surface and the residual silanols. NMR studies have indicated that acetonitrile, one of the most popular organic mobile phase (16) Takeuchi, T.; Miwa, T.; Nagae, N. Chromatographia 1993, 35, 375. (17) Chuang, I.-S.; Kinney, D. R.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115, 8695.
© 1996 American Chemical Society
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components, solvates alkyl chains rather well,3,18 thereby providing a kind of ‘worst case scenario’ for residual silanol accessibility. Kova´ts et al. have demonstrated that, upon contact with acetonitrile mobile phases which contain D2O, all residual surface silanols are exchangeable, given enough time.12 In this study we have used 29Si CP MAS NMR to characterize the residual silanol populations of two commercially available RP-HPLC phases. One of these phases is a conventional dimethyloctadecylsilane-derivatized silica. The other is based on the same substrate but has been derivatized with bulky di-isobutyloctadecylsilane structures, which has proven to result in an increased stability under low pH conditions.19-21 Moreover, it was argued that the bulky side groups would increase the steric shielding of the underlying residual silanols by the silane ligand. Details on the relation between the 29Si NMR signals of these ligand silanes and the observed increased low-pH stability have been published elsewhere.22 In order to obtain information on residual silanol concentration, mobility and solute accessibility, a combination of deuterium exchange in acetonitrile/D2O and 29Si CP MAS NMR experiments was performed. CP MAS NMR relaxation measurements were primarily done in order to assure quantitatively reliable results, but also valuable information on mobility of the surface structures is obtained. Experimental Section Materials. The studied silica materials were a gift from Rockland Technologies Inc. (Wilmington, DE). The native silica substrate and two RP-HPLC stationary phases were studied. The native silica substrate was a precipitated, spherical silica gel (Rx-Sil, batch 820966.901) with a specific surface area of 180 m2/g, an average particle size of 5 µm, and an average pore diameter of 80 Å. Rx-C18 was produced by silylation with dimethyloctadecylsilane (batch 920966.108, carbon content ) 12.25%), while the phase SB-C18 was produced by silylation with di-isobutyloctadecylsilane (batch 820966.922, carbon content ) 9.85%). All materials were dried in vacuum (10-2 Torr) at 110 °C before use. It can then safely be assumed there is no significant amount of adsorbed water present at the silica surface. Acetonitrile (Lichrosolv, E. Merck, Darmstadt, Germany) was dried and refluxed over calcium hydride and distilled under a nitrogen atmosphere. Deuterium oxide (99.9 atom % D) was purchased from Campro Scientific (Veenendaal, The Netherlands). Both solvents were stored under argon (99.9997%, Hoek Loos, Schiedam, The Netherlands). Methods. All sample handling was performed under a dry atmosphere to prevent air moisture from exchanging with deuterium atoms after deuterium exchange. Typically, 1 g of siliceous material was placed in an evacuated, dried and subsequently argon filled 100 mL glass tube, and 20 mL acetonitrile was added. After vigorous stirring, the suspension was placed in an ultrasonic bath (14 W, 55 kHz) for a few seconds to expel any pore-filling gases Thereafter 2.0 mL of D2O was added. This mixture was stirred for 15 min at room temperature, and then the suspension was evaporated to dryness (as indicated by freely floating powder) by reduced pressure within another 15 min. A total of three deuterium exchange cycles were performed, resulting in a 500-fold deuterium/silanol excess for the Rx-Sil substrate and more than a 700-fold excess for the phases Rx-C18 and SB-C18. After the third cycle, the materials were dried in vacuum for 3 h at room temperature, after which they were stored under argon. Transfer of approximately 250 (18) Bliesner, D. M.; Sentell, K. B. J. Chromatogr. 1993, 631, 23. (19) Kirkland, J. J.; Glajch, J. L.; Farlee, R. D. Anal. Chem. 1988, 61, 2. (20) Sagliano, N., Jr.; Floyd, T. R.; Hartwick, R. A.; Dibussolo, J. M.; Miller, N. T. J. Chromatogr. 1988, 443, 155. (21) Sagliano, N., Jr.; Hartwick, R. A.; Patterson, R. E.; Woods, B. A.; Bass, J. L.; Miller, N. T. J. Chromatogr. 1988, 458, 225. (22) Scholten, A. B.; de Haan, J. W.; Claessens, H. A.; van de Ven, L. J. M.; Cramers, C. A. J. Chromatogr., A. 1994, 688, 25.
Scholten et al. mg of sample into 7 mm zirconia rotors for solid-state NMR was performed in a glove bag under a dry air atmosphere (
