Architecture and Dynamics of C18 Bonded Interphases with Small

Anal. Chem. 2009, 81, 10136–10142

Architecture and Dynamics of C18 Bonded Interphases with Small Molecule Spacers Maximilian Ku¨hnle, Volker Friebolin, and Klaus Albert* Institute of Organic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tu¨bingen, Germany Catherine A. Rimmer, Katrice A. Lippa, and Lane C. Sander Analytical Chemistry Division, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899 The relationship between alkyl phase structure and chromatographic performance is investigated for a series of octadecyl (C18)-modified silica surfaces with defined spacing of the alkyl surface by a “pre-end-capping” technique. Stationary phases were prepared by a two step process with (1) reaction with less than stochiometric amounts of a small monofunctional silane, followed by (2) solution or surface polymerization with octadecyltrichlorosilane. The results of solid-state and suspension nuclear magnetic resonance (NMR) spectroscopy are correlated with the chromatographic behavior regarding shape selective separations. Two sets of six different stationary phases were prepared by solution and surface polymerization approaches, yielding materials with surface coverages from 2.7 to 5.6 µmol/m2. 13C cross-polarization magic angle spinning (CP/MAS) NMR spectra show a predominance of trans conformations for the set of surface polymerized phases with a C18 coverage greater than 4.5 µmol/m2. For the solution polymerized phases, no predominance for the trans conformation was observed, even for surface coverages greater than 5.1 µmol/m2. Proton spectra in suspension indicate the trend that a higher coverage for the surface polymerized materials correlates with a more rigid alkyl chain conformation. The set of solution polymerized stationary phases confirms this tendency but minor deviations are observed for high coverages. These structural abnormalities are confirmed by differences in the 29Si CP/MAS spectra. Furthermore, the 29Si CP/MAS spectra indicate a lower amount of cross-linking for the materials with the highest amount of placeholder (spacer). The use of the different spectroscopic and chromatographic methods provides a wealth of information on the surface morphology of the systematically prepared C18 materials and extends the understanding of surface morphology of alkyl modified silica and its influences of the molecular recognition process in liquid chromatography. Since the development of reversed-phase liquid chromatography (RPLC) as an analytical separation technique in the early * To whom correspondence should be addressed. Address: Institute of Organic Chemistry, University of Tuebingen, Auf der Morgenstelle 18, D-72076 Tuebingen, Germany. Fax 49-7071-29-5875. E-mail: [email protected].

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1970s, considerable research has been performed on synthesis and characterization of stationary phases based on alkyl modified silica. Alkyl modified silica is available from a wide variety of commercial sources and is the most commonly used separation media in liquid chromatography.1,2 Two types of chromatographic sorbents can be distinguished on the basis of bonding chemistry. Monomeric materials are usually prepared using monofunctional chlorosilanes which are directly bonded to the silica surface whereas polymeric materials are prepared via the polycondensation of multifunctional silanes in the presence of water. Monomeric bonded phases may exhibit slightly improved mass transfer characteristics compared with polymeric phases, and monomeric phases are often used in preference to polymeric phases for this reason.1,3,4 Compared to monomeric bonded phases, polymerictype sorbents show higher shape selectivity for molecularly rigid solutes and, therefore, are preferred for the separation of polycyclic aromatic hydrocarbons (PAH) and carotenoids.5-12 To facilitate the assessment of shape selectivity for different chromatographic materials, Sander and Wise developed SRM 869a, a test mixture which is composed of three shape-constrained PAHs (phenanthro[3,4-c]phenanthrene, PhPh; 1,2:3,4:5,6:7,8-tetrabenzonaphtalene, TBN; and benzo[a]pyrene, BaP). The selectivity factor RTBN/BaP ) k′TBN/k′BaP is based on the relative retention of a nonplanar solute (TBN) and a planar solute (BaP). The test provides a numerical assessment of shape recognition and permits classification: monomeric-like stationary phases (with (1) Scott, R. P. W. In Encyclopedia of Separation Science; Wilson, I., Ed.; Academic Press: New York, 1999, pp 711-717. (2) Halasz, I.; Sebestian, I. Angew. Chem., Int. Ed. 1969, 8, 453–454. (3) Lippa, K. A.; Rimmer, C. A.; Sander, L. C. In Advances in Chromatography; Grushka, E.; Grinberg, N., Eds.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2008, Vol. 46, pp 235-303. (4) Ohta, H.; Saito, Y.; Nagae, N.; Pesek, J. J.; Matyska, M. T.; Jinno, K. J. Chromatogr., A 2000, 883, 55–66. (5) Fetzer, J. C.; Biggs, W. R. Chromatographia 1989, 27, 118–122. (6) Wise, S. A.; Sander, L. C.; May, W. E. J. Chromatogr., A 1993, 624, 329– 349. (7) Wise, S. A.; Sander, L. C. J. Chromatogr. 1990, 514, 111–122. (8) Dachtler, M.; Kohler, K.; Albert, K. J. Chromatogr., B 1998, 720, 211– 216. (9) Meyer, C.; Skoksberg, U.; Welsch, N.; Albert, K. Anal. Bioanal. Chem. 2005, 382, 679–690. (10) Rimmer, C. A.; Sander, L. C.; Wise, S. A. Anal. Bioanal. Chem. 2005, 382, 698–707. (11) Sander, L. C.; Sharpless, K. E.; Craft, N. E.; Wise, S. A. Anal. Chem. 1994, 66, 1667–1674. (12) Strohschein, S.; Pursch, M.; Ha¨ndel, H.; Albert, K. Fresenius J. Anal. Chem. 1997, 357, 498–502. 10.1021/ac901911w CCC: $40.75  2009 American Chemical Society Published on Web 11/11/2009

reduced shape recognition properties), polymeric-like stationary phases (with enhanced shape recognition properties), and stationaryphaseswithintermediateshapeselectivityproperties.3,13 Other chromatographic tests have been established to assess shape recognition.14 Similar information is provided by the Tanaka test, which uses the selectivity factor for triphenylene (TRI) and o-terphenyl (o-TER) to characterize shape selectivity properties.3,15 The results of these tests are dependent on the physicochemical properties of the modified silica, the type of alkylsilane (its chain length), the surface morphology of the alkyl chains including density and conformation, and the column temperature.16 Additionally, the surface morphology can be controlled by different synthesis approaches and represents a significant factor for influencing shape selectivity of stationary phases. Two approaches are used for the synthesis of polymeric materials that produce significant differences in alkyl chain morphology and corresponding shape selectivity properties. The solution polymerization technique describes a procedure in which the multifunctional silane is reacted with suspended silica particles in the presence of water. The addition of water leads to silane hydrolysis and oligomerization (the degree of polymerization has been shown to be as low as 5 units). The silane oligomers are thought to react in concert with silane monomers at the silica surface. The self-assembly of the oligomers produces alkyl clusters with chain spacing constrained by the siloxane bond (i.e., (RSiO-SiR)n), and as a result, these clusters should show more ordered alkyl chains compared to monomeric phases due to the reduced distance between the alkyl ligands.17,18 Surface polymerization describes a procedure in which a trifunctional silane is reacted with silica containing a thin film of surface absorbed water. Wirth et al. reported a process for equilibrating silica with water vapor (humidified silica).19-22 Reaction of humidified silica with a trifunctional silane can produce a dense alkyl monolayer or self-assembled monolayer with highly ordered alkyl chains. Wirth and co-workers used this technique for the preparation of self-assembled monolayers (SAM) and for the preparation of C3/C18 and C1/C18 mixed stationary phases for liquid chromatography.19-22 These materials have been shown to be well suited for chromatographic separations of charged molecules because of their reduced silanol activity.23 Sander and Wise described the preparation of surface polymerized C18 phases, and a higher surface coverage was noted for this type of material compared to those obtained with the solution polymerization technique. Also, a more regular alkyl chain

ordering was reported for high density surface polymerized materials.24-26 Considerable effort has been expended toward developing an understanding of shape selective interactions between the stationary phase and the analyte. The first model of molecular recognition of rigid solutes was proposed in 1985 by Wise and Sander.27 In order to explain the retention behavior of isomeric PAHs, an empirical “Slot Model” was described which correlates the molecular shape of the analyte, the surface morphology of the stationary phase, and the process of molecular recognition. However, fundamental aspects of molecular recognition are not well-understood and further study of the role played by stationary phase is warranted. Solid-state NMR spectroscopy is one of the most powerful tools for studying the surface morphology of alkyl stationary phases. Cross-polarization (CP)28,29 combined with magic angle spinning (MAS)30 techniques are indispensable for the analysis of modified silica gels. Probes of conformational structure, mobility, and bonding chemistry of the immobilized alkyl ligands can be achieved by recording 1H, 13C, and 29Si spectra of the sorbent.24,31-35 Other further techniques are available which can be adopted for the characterization of modified silica. High resolution magic angle spinning (HR/MAS) can be carried out in suspension and enable the recording of spectra under chromatographically relevant conditions complementing the more established high-speed 1H MAS NMR spectroscopy for the investigation of stationary phases. In the current investigation, novel stationary phases are created by bonding sparsely distributed end-capping reagents to silica prior to synthesis of polymeric C18 phases, similar to the approach utilized by Marshall et. al.36 This approach is intended to produce cavities or open regions within the stationary phase with defined characteristics. Special interest is applied to the spectroscopic characterization of the silica surface and corresponding shape selectivity properties. Six “pre-end-capped” stationary phases with different surface coverages were further modified by solution and surface polymerization approaches, using octadecyltrichlorosilane. The chromatographic materials were characterized by 29Si cross-polarization magic angle spinning (CP/MAS) NMR, 13C CP/MAS, 1H HR/MAS, and liquid chromatography. For these 12 materials, the shape selective properties are correlated to the spectroscopic data, and a model of the surface morphology of selected materials is proposed.

(13) Certificate of Analysis; National Institute of Standards and Technology (NIST): Gaithersburg, MD, 1998; Available at http://www.nist.gov/SRM. (14) Sander, L. C.; Lippa, K.; Wise, S. A. Anal. Bioanal. Chem. 2005, 382, 646– 668. (15) Tanaka, N.; Tokuda, Y.; Iwaguchi, K.; Araki, M. J. Chromatogr. 1982, 239, 761–772. (16) Engelhardt, H.; Nikolov, M.; Arangio, M.; Scherer, M. Chromatographia 1998, 48, 183–189. (17) Sander, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504–510. (18) Verzele, M.; Mussche, P. J. Chromatogr. 1983, 254, 117–122. (19) Wirth, M. J.; Fatunmbi, H. O. Anal. Chem. 1992, 64, 2783–2788. (20) Wirth, M. J.; Fatunmbi, H. O. Anal. Chem. 1993, 65, 822–826. (21) Fatunmbi, H. O.; Bruch, M. D.; Wirth, M. J. Anal. Chem. 1993, 65, 2048– 2054. (22) Fairbank, R. W. P.; Xiang, Y.; Wirth, M. J. Anal. Chem. 1995, 67, 3879– 3885. (23) Wirth, M. J.; Fairbank, R. W. P.; Fatunmbi, H. O. Science 1997, 275, 44– 47.

(24) Pursch, M.; Sander, L. C.; Egelhaaf, H. J.; Raitza, M.; Wise, S. A.; Oelkrug, D.; Albert, K. J. Am. Chem. Soc. 1999, 121, 3201–3213. (25) Sander, L. C.; Wise, S. A. Anal. Chem. 1996, 68, 4107–4113. (26) Sander, L. C.; Wise, S. A. In Advances in Chromatography; Giddings, J. C.; Grushka, E.; Cazes, J.; Brown, P.R., Eds.; Marcel Dekker: New York, 1986; pp 139-218. (27) Wise, S. A.; Sander, L. C. J. High Resolut. Chromatogr. 1985, 8, 248–255. (28) Hartmann, S. R.; Hahn, E. L. Phys. Rev. 1962, 128, 2042–2053. (29) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 56, 1776–1777. (30) Andrew, E. R.; Bradbury, A.; Eades, R. G. Nature 1959, 183, 1802–1803. (31) Wegmann, J.; Bachmann, S.; Ha¨ndel, H.; Tro ¨ltzsch, M.; Albert, K. J. Chromatogr., A 2000, 883, 27–37. (32) Meyer, C.; Pascui, O.; Reichert, L.; Sander, L. C.; Wise, S. A.; Albert, K. J. Sep. Sci. 2006, 29, 820–828. (33) Maciel, G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606–7607. (34) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1848–1851. (35) Albert, K.; Bayer, E. J. Chromatogr. 1991, 544, 345–370. (36) Marshall, D. B.; Stutler, K. A.; Lochmu ¨ ller, C. H. J. Chromatogr. Sci. 1984, 22, 217–220.


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EXPERIMENTAL SECTION Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. Material Preparation. The preparation of the stationary phases was achieved in a two step synthesis. A small monofunctional silane (isopropyldimethylchlorosilane) was bonded to the silica surface, followed by silanization with surface polymerization or solution polymerization techniques. The polymeric synthesis procedures of the bonded phases are described in detail elsewhere.17,37 For the bonding of the monofunctional silane, 8 g of silica (Restek Corporation, Bellefonte, PA; pore size 20 nm, particle size 5 µm) was suspended in 100 mL of xylene. Two grams of dimethylaminopyridine (EMD Chemicals, Gibbstown, NJ) and isopropyldimethylchlorosilane (Gelest, Morrisville, PA) were added, and the suspension was refluxed for 18 h. After refluxing, the slurry was filtered hot and washed with aliquots of hot xylene, acetone, methanol, methanol/water (50/50, volume fraction), water, and pentane (all solvents were either reagent grade or HPLC grade). The resulting chromatographic sorbent was dried in a vacuum oven at 100 °C for 2 h. After drying, the batch was split into two equal parts for surface and solution polymerization. The solution polymerization procedure is briefly described as follows: 4 g of the dried monomeric modified silica was suspended in 100 mL of xylene. After adding 10 mL of octadecyltrichlorosilane (Gelest, Morrisville, PA) in a well mixed silica slurry, 0.5 mL of water was added rapidly and the mixture was refluxed for 4 h. After refluxing, the slurry was filtered hot, washed, and dried. For the surface polymerization, 4 g of the same batch was humidified for 2 h. The humidified silica was suspended in xylene and 10 mL of octadecyltrichlorosilane was added. The suspension was kept at room temperature for 24 h and was resuspended by swirling once an hour followed by one hour at reflux. After refluxing the slurry was filtered hot, washed, and dried. Solid-State/Suspended-State NMR Spectroscopy. 29Si CP/ MAS NMR measurements were carried out with a Bruker ASX 300 MHz instrument (Bruker, GmbH, Rheinstetten, Germany). Magic angle spinning was executed at 4000 Hz. 1H 90° pulses and contact times were 6.9 µs and 5 ms, respectively, with recycle delay times of 2 s. 13 C CP/MAS were performed by use of a Bruker MSL 200 spectrometer. For 13C spectra, sample spinning was executed at 10 000 Hz, the pulse length was 4.9 µs, and the contact times and delay times were 3 ms and 1 s. The 1H high resolution/magic angle spinning were recorded in the suspended state on a Bruker ARX 400 spectrometer at a spinning rate of 6000 Hz in a 4 mm double bearing ZrO2 rotor. The spectrometer was equipped with a deuterium lock setup which was set on the resonance frequency of acetonitrile-d3. For each sample, 10 mg of stationary phase was suspended in 80 µL of acetonitrile/water (85/15, v/v), (Euriso-Top, SaintAubin Cedex, France). The 90° pulse length was set to 10.5 µs, and the delay times were 1 s. (37) Sander, L. C.; Wise, S. A. Anal. Chem. 1995, 67, 3284–3292.

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Figure 1. Nomenclature and chemical shifts of various important silyl species.

Spin-lattice times were measured with the inversion recovery experiment with a minimum relaxation delay of 60 s. The time interval between 90° and 180° pulses was varied from 0.01 to 20 s. Liquid Chromatography. The separations were performed with a flow rate of 1 mL/min and a mobile phase composition of 85/15 acetonitrile/water at various temperatures. Standard Reference Material (SRM) 869 (column selectivity test mixture for liquid chromatography) was obtained from the Standard Reference Material Program (NIST, Gaithersburg, Maryland). Chromatographic columns were prepared using 4.6 mm × 125 mm hardware. HPLC grade solvents were used in all chromatographic separations and absorbance detection was performed at 254 nm. RESULTS AND DISCUSSION 29 Si CP/MAS NMR spectrometry is an established method for the characterization of modified and native silica. The signal assignment of various silyl species are well described elsewhere in the literature24 and can, for the purpose of this work, be summarized as follows (Figure 1). A higher degree of crosslinking of silicon species and/or an increase of oxygen neighbors leads to an upfield shift in NMR spectra. Trifunctional silyl species (Tn) appear in the chemical shift region from -49 to -66 ppm and signals of the native silica (Qn) show chemical shifts between -91 and -110 ppm. Silane functionality as well as bonding chemistry has been determined previously by this spectroscopic technique.24 The 29Si CP/MAS NMR spectra for both the surface polymerized (panel a) and the solution polymerized (panel b) stationary phases are shown in Figure 2a,b, respectively. The corresponding data regarding spacer and C18 coverage are given in Table 1. As expected, stationary phases that contain high spacer coverage (as indicated by carbon analysis) leads to less immobilization of alkyl chains.

Figure 2.

29

Si CP/MAS NMR spectra of different surface (a) and solution (b) polymerized C18 interphases.

Table 1. Summarized Spectroscopic and Chromatographic Data for the 12 Sorbents shape selectivity factor (RTBN/BaP)a

total C18 spacer surface surface polymerization coverage coverage coverage coverage T1 T2 2 2 2 material approach (µmol/m ) (µmol/m ) (µmol/m ) (% C18) (%) (%)

signal T3 T2/T3 half-width (%) ratio (Hz)b 15 °C 20 °C 25 °C 35 °C 45 °C

A A1 A2 A3 A4 A5 B B1 B2 B3 B4 B5

40.3 43.3 46.9 39.8 34.4 32.3 45.7 50.3 53.2 35.2 46.7 29.2

surface surface surface surface surface surface solution solution solution solution solution solution a

5.3 4.7 5.6 2.8 3.1 3.0 4.6 5.1 5.0 3.8 3.1 2.7

0.0 0.4 0.7 0.7 1.0 1.9 0.0 0.4 0.7 0.7 1.0 1.9

5.3 5.1 6.3 3.5 4.1 4.8 4.6 5.5 5.7 4.5 4.1 4.6

100 92 89 80 75 61 100 92 88 85 75 59

4.1 3.6 0.6 1.5 1.8 6.2 1.1 2.7 0.3 5.1 4.3 7.7

55.5 53.1 52.5 58.6 63.9 61.4 53.2 47.0 46.5 59.7 49.0 63.2

1.4 1.2 1.1 1.5 1.9 1.9 1.2 0.9 0.9 1.7 1.0 2.2

52.08 47.68 69.7 60.16 30.08 73.36 56.48 70.42 39.62 25.68

0.3 0.3 1.4 1.1 1.4 0.5 0.6 0.8 1.2 1.5

0.3 0.4 0.4 1.4 1.2 1.6 0.6 0.7 0.7 1.0 1.3 1.6

0.4 0.5 0.4 1.5 1.4 1.6 0.7 0.8 0.8 1.1 1.4 1.7

0.6 0.7 0.6 1.5 1.8 0.8 1.0 1.0 1.3 1.6 1.8

0.9 0.9 0.8 1.5 1.6 1.8 1.0 1.2 1.2 1.4 1.6 1.8

b

RPLC mobile phase conditions: 85/15 acetonitrile/water, 1.0 mL/min. Signal half-width terminal methyl group.

The ratio of Q3/Q4 is very similar for all stationary phases. Differences in cross-linking are observed for the solution polymerized phases with higher C18 coverages and can be better resolved through peak deconvolution of the Tn species, as shown in Table 1 and Figure S1 in the Supporting Information. The plot of the relative Tn ratios as a function of the spacer coverage reveals an influence of the spacer molecule for both polymerization techniques. Materials produced by both the surface and solution polymerization approaches show a slight increase of T1 and T2 groups at high spacer coverages as well as significant reduction of the T3 species. Additional insight can be obtained from the 13C CP/MAS NMR spectra which are shown for the surface and solution

polymerized materials in Figure 3a,b, respectively. Of particular interest are the chemical shifts that correspond to ordered and disordered alkyl chain conformations.24,38 The set of the surface polymerized phases (Figure 3a) shows a clear correlation between the C18 coverage and the alkyl chain conformation. For higher coverages (A1 and A2), a predominance of the more ordered trans conformation is observed; whereas at coverages below 3.2 µmol/m2 (A3-A5), a predominance of the disordered gauche conformations can be found. For surface polymerization syntheses, silane bonding occurs directly at the silica surface, and the local density of the alkyl (38) Cheng, J.; Fone, M.; Ellsworth, M. Solid State Nucl. Magn. Reson. 1996, 7, 135–140.


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Figure 3.

13

C CP/MAS NMR spectra of different surface (a) and solution (b) polymerized C18 interphases.

chains is responsible for the ordering of the alkyl chains and NMR signal shifts. It can also be noted that the primary signal that corresponds to ordered alkyl chains (34.5 ppm) is prominent for the higher density phases (surface coverage greater than about 3.8 µmol/m2). In contrast to the surface polymerized materials, both populations of ordered trans conformations and disordered gauche conformations are present for the solution polymerized phases with high surface coverages (Figure 3b). This suggests the existence of surface heterogeneity, which may be the result of bonded oligomers (high density and high order), and more widely spaced silane monomers (low density and greater disorder). It is informative to examine the ratio of T2 and T3 for each of the materials, as a potential probe of surface morphology. Silane-to-silane bonds are indicated by increased values for T3, and thus, the ratio T2/T3 is indicative of the degree of silane cross-linking (see Figure S2 in the Supporting Information). The degree of the cross-linking of the different silanes is affected by the coverage of the spacer molecule. Values of T2/T3 ≈ 1 would be indicative of an extended siloxane network, while values of T2/T3 > 1 would indicate smaller-sized assemblies of cross-linked siloxane units that would contain a greater percentage of peripheral Si-OH groups. For the solution polymerized material B1 with T2/T3 ) 0.9, the 29Si spectrum indicates that a minimal amount of alkylsilane chains are bonded directly to the silica surface. This is consistent with the view that polysiloxane oligomers with multiple silane-tosilane bonds (i.e., cross-linking) are present and have only limited bonds to the silica surface. Line widths of the terminal methyl group signals in the 1H HR/MAS NMR spectra with the corresponding coverages of 10140


Figure 4. Dependence of the selectivity factor RTBN/BaP on the C18 coverage for different surface polymerized materials at 25 °C.

the different stationary phases are provided in Table 1. Strong dipolar interactions that cannot be eliminated by high spinning speeds lead to decreased spin-spin relaxation times T2, which is indicative of rigid alkyl chain conformations. The relaxation time T2 is inversely proportional to the signal half-width and can be determined via peak deconvolution. Methyl group signal half-width generally increases as a function of stationary phase surface coverage for the surface polymerized materials (Figure S3 in the Supporting Information). Thus, broader NMR signals are observed for materials with higher surface coverages and are generally indicative of rigid alkyl chain conformations. This is

Figure 5. Representation of the potential influence of the spacer molecule on the conformation of the alkyl chains; reduced alkyl chain density and higher spacer coverage (top); increased alkyl chain density and lower spacer coverage (bottom).

consistent with the observed chemical shift signal for the trans conformation of the 13C solid-state NMR measurements. The solution polymerized materials show a similar tendency. For high surface coverages, a larger signal half-width is observed, which is consistent with the small downfield shift observed in the 13 C CP/MAS NMR spectra. The larger signal half-widths that are observed for high surface coverage materials are somewhat unexpected. The 13C CP/MAS NMR spectra indicate the presence of both ordered (trans) and disordered (gauche) conformational states, and an intermediate signal half-width may be anticipated. Further consideration of this data or additional experimental investigations to probe such an effect may be warranted. Each of the C18 stationary phases was characterized with SRM 869, a chromatographic shape selectivity test mixture.13 This test mixture was developed after the investigation of the retention properties of over 100 PAH solutes; three solutes were selected that provided the most sensitive indication of selectivity changes among monomeric and polymeric C18 columns. Sander and Wise demonstrated that the elution order of these three solutes is correlated with the type of surface modification chemistry used to prepare the stationary phases and the overall shape selectivity.37,39 The selectivity factor RTBN/BaP ) k′TBN/ k′BaP provides a numerical assessment of the shape selective characteristics of a column and is a useful single measure to

compare various RPLC sorbents with regards to their shape recognition capability. Polymeric C18 columns that exhibit low RTBN/BaP values in the range of 0.3 to 1.0 typically exhibit enhanced recognition of solute shape. The majority of monomeric columns have lower shape recognition capabilities and routinely have RTBN/BaP values of greater than 1.0. Columns that are considered intermediate in their shape recognition capabilities have RTBN/ BaP values of that range from 1.0 to 1.7. Selectivity factors (RTBN/ BaP) are summarized for both the surface and solution polymerized materials over a limited range of temperatures (15 to 45 °C) in Table 1 and Figure S4 in the Supporting Information. A reduction in shape selectivity, as indicated by an increase in RTBN/BaP, was observed at higher temperatures for all stationary phases.24,40 This is similar to trends reported for more conventional monomeric and polymeric C18 columns. A representative comparison of the two different preparation techniques at 25 °C is provided in Figure 4. The relationship between the selectivity factor (RTBN/BaP ) k′TBN/k′BaP) and surface coverage is comparable to previously reported data for monomeric and polymeric C18 phases.41 Two groups of surface polymerized materials can be distinguished by their 13C CP/MAS NMR spectra. One group with low shape selectivity (RTBN/BaP ) 1.4 to 1.6) is characterized by the presence of the NMR signal corresponding to disordered

(39) Sander, L. C.; Wise, S. A. J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 11, 383–387.

(40) Sander, L. C.; Wise, S. A. Anal. Chem. 1989, 61, 1749–1754. (41) Sander, L. C.; Pursch, M.; Wise, S. A. Anal. Chem. 1999, 71, 4821–4830.


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gauche conformations. Another group shows only the NMR signal corresponding to ordered trans conformations and relatively high shape selectivity (RTBN/BaP < 0.5) which can be correlated with enhanced shape selectivity properties. In addition, higher column efficiencies were observed at high spacer coverages which is in accordance to the findings of Marshall and co-workers (Figure S5 in the Supporting Information).36 Additional insight into the influence of the spacer molecule on the alkyl chain morphology can be obtained by comparing materials A5 and A3, with spacer coverages of 1.9 µmol/m2 and 0.7 µmol/m2, and C18 coverages of 3.0 µmol/m2 and 2.8 µmol/ m2, respectively. On the basis of a consideration of C18 coverage alone, material A5 might be expected to exhibit slightly higher order and shape selectivity than material A3; however, the opposite trend is observed. For material A5, a slight upfield shift of about 0.5 ppm is observed in the NMR spectrum which is indicative of increased disorder (gauche conformations). Because material A5 has significantly increased spacer presence, C18 chains are more widely distributed than without the spacers, resulting in increased chain mobility and disorder. Similar effects were observed when comparing the observed shifts for the material pairs A5/A4 and A3/A4 that have comparable C18 surface coverages but varying amounts of the spacer. A schematic representation is provided in Figure 5. The presence of the spacer molecule precludes formation of pockets with a high local alkyl ligand density and results in a higher mobility and disordering of these alkyl chains.

selectivity properties of various “pre-capped” C18 interphases. The relation between alkyl ligand density, chain ordering, and shape selectivity are shown via various NMR experiments and liquid chromatography. In general, high alkyl ligand densities and a more ordered alkyl chain conformation result in increased shape recognition. The solid-state NMR data shows that a more defined and ordered alkyl chain morphology can be created by the use of the surface polymerization approach instead of the solution polymerization, which results in higher shape selectivity properties. The introduction of spacers prior to solution or surface polymerization appears to influence shape selectivity primarily through the reduction in C18 surface coverage. This trend is attributed to the reduction in bonding sites on the silica surface, which has contrasting effects for monomer and oligomer silanes.

CONCLUSIONS Solid-state NMR spectroscopy and liquid chromatography have been used for the investigation of the architecture and shape

Received for review August 24, 2009. Accepted October 27, 2009.

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ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft (DFG, Grant No. AL 298/14-1). M.K. would like to thank the guest researcher program at the National Institute of Standards and Technology (NIST) for providing support for research visits. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

AC901911W

Architecture and Dynamics of C18 Bonded Interphases with Small

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