Structure−Function Relationships in High-Density Octadecylsilane

Instead, alkyl chain order is largely dependent on bonding density. ... transform IR (FT-IR) spectroscopy.30 These phases possessed similar bonding de...
0 downloads 0 Views 215KB Size
Download PDF
Anal. Chem. 2002, 74, 5576-5584

Structure-Function Relationships in High-Density Octadecylsilane Stationary Phases by Raman Spectroscopy. 1. Effects of Temperature, Surface Coverage, and Preparation Procedure Michael W. Ducey, Jr.,† Christopher J. Orendorff, and Jeanne E. Pemberton*

Department of Chemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721 Lane C. Sander

National Institute of Standards and Technology, Gaithersburg, Maryland 20899

Raman spectroscopy is used to examine the effects of temperature, surface coverage, nature of the alkylsilane precursor (octadecyltrichlorosilane, methyloctadecyldichlorosilane, or dimethyloctadecylchlorosilane), and surface grafting method (surface or solution polymerized) on alkyl chain conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 µmol/m2. Conformational order is assessed using the intensity ratio of the antisymmetric and symmetric ν(CH2) modes as well as the frequency at which these Raman bands are observed. Conformational order increases with surface coverage. Temperatureinduced surface phase changes are observed between 258 and 343 K for this homologous series of stationary phases that are demonstrated to adhere to the Clapeyron equation for a simple first-order transition. Phase changes are discussed in terms of variation of the molar enthalpy, molar entropy, and molar volume of the stationary phase, all of which depend on surface coverage. For the limited range of systems investigated, a correlation between stationary-phase preparation (surface versus solution polymerized and nature of the silane precursor) and extent of alkyl chain order is not clearly observed. Instead, alkyl chain order is largely dependent on bonding density. A molecular picture of temperature-induced disorder in octadecylsilane stationary phases is proposed, with disorder originating at the distal carbon and propagating toward the proximal carbon. Although the use of reversed-phase liquid chromatography (RPLC) is widespread, significant debate remains regarding the principles and mechanisms of solute retention. This is due, in part, to a general lack of understanding of the molecular processes giving rise to retention. Such molecular processes include the interactions of solute and mobile-phase solvent with the stationary * To whom correspondence should be addressed. Tel: (520) 621-8245. E-mail: [email protected]. † Current address: Department of Chemistry, Missouri Western State College, St. Joseph, MO 64507.

5576 Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

phase and intermolecular interactions of the stationary phase itself. Such knowledge is critical, since each of these parameters is known to affect the selectivity and efficiency of chromatographic separations.1-4 Numerous experimental techniques including chromatographic methods,5-9 fluorescence spectroscopy,10-16 nuclear magnetic resonance (NMR) spectroscopy,17-22 small angle neutron scattering,23-25 differential scanning calorimetry,26-29 (1) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A-867A. (2) Dorsey, J. G.; Dill, K. A. Chem. Rev. 1989, 89, 331-346. (3) Tchapla, A.; Heron, S.; Lesellier, E. J. Chromatogr., A 1993, 656, 81-112. (4) Schunk, T. C.; Burke, M. F. J. Chromatogr., A 1993, 656, 289-316. (5) Gilpin, R. K.; Gangoda, M. E.; Krishen, A. E. J. Chromatogr. Sci. 1982, 20, 345-348. (6) Schunk, T. C.; Burke, M. F. Int. J. Environ. Anal. Chem. 1986, 25, 81103. (7) Cole, L. A.; Dorsey, J. G. Anal. Chem. 1992, 64, 1317-1323. (8) Sander, L. C.; Wise, S. A. Anal. Chem. 1995, 67, 3284-3292. (9) Sander, L. C.; Pursch, M.; Wise, S. A. Anal. Chem. 1999, 71, 4821-4830. (10) Lochmuller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. J. Am. Chem. Soc. 1984, 106, 4077-4082. (11) Lochmuller, C. H.; Marshall, D. B.; Wilder, D. R. Anal. Chem. Acta 1981, 130, 31-43. (12) Carr, J. W.; Harris, J. M. Anal. Chem. 1987, 59, 2546-2550. (13) Montgomery. M. E.; Greeen, M. A.; Wirth, M. J. Anal. Chem. 1992, 64, 1170-1175. (14) Burbage, J. D.; Wirth, M. J. J. Phys. Chem. 1992, 96, 5943-5948. (15) Montogmery, M. E.; Wirth, M. J. Anal. Chem. 1994, 66, 680-684. (16) Zulli, S. L.; Kovaleski, J. M.; Zhu, X. R.; Harris, J. M.; Wirth, M. J. Anal. Chem. 1994, 66, 1708-1712. (17) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1848-1851. (18) Kelusky, E. C.; Fyfe, C. A. J. Am. Chem. Soc. 1986, 108, 1746-1749. (19) Shah, P.; Rogers, L. B.; Fetaer, J. C. J. Chromatogr. 1987, 388, 411-419. (20) Bayer, E.; Paulus, A.; Peters, B.; Laupp. G.; Reiners, J.; Albert, K. J. Chromatogr. 1986, 364, 25-37. (21) Albert, K.; Bayer, E. J. Chromatogr. 1991, 544, 345-370. (22) Albert, K.; Brindle, R.; Martin, P.; Wilson, I. D. J. Chromatogr., A 1994, 665, 253-258. (23) Beaufils, J. P.; Hennion, M. C.; Rosset, R. Anal. Chem. 1985, 57, 25932596. (24) Glinhka, C. J.; Sander, L. C.; Wise, S. A.; Berk, N. F. Mater. Res. Soc. Symp. Proc. 1990, 166, 415-420. (25) Sander, L. C.; Glinka, C. J.; Wise, S. A. Anal. Chem. 1990, 62, 1099-1101. (26) Claudy, P.; Letoffe, J. M.; Gaget, C.; Morel, D.; Serpinet, J. J. Chromatogr. 1985, 329, 331-349. (27) Gonnet, C.; Morel, D.; Ramamnojinirina, E.; Serpinet, J.; Claudy, P. J. Chromatogr. 1985, 330, 227-241. (28) Claudy, P.; Letoffe, J. M.; Gaget, C.; Morel, D.; Ramamnojinirina, E.; Serpinet, J. Bull. Soc. Chim. Fr. 1985, 1098-1102. 10.1021/ac0203488 CCC: $22.00

© 2002 American Chemical Society Published on Web 10/08/2002

infrared (IR) spectroscopy,30-33 and Raman spectroscopy34-40 have been employed in the investigation of retention on alkylsilanemodified stationary phases. While the results of these investigations have provided valuable insight into the retention process, a complete molecular picture remains elusive. Among the many variables known to affect the retention process, the structure of the alkyl chains and the effect of chromatographic variables such as temperature and alkyl chain density on the resulting conformational order of these alkyl chains may be the least understood. This information is best provided by techniques sensitive to alkyl chain conformation and chemical environment such as NMR, IR, and Raman spectroscopies. Several models have been advanced to describe the surface structure of alkylsilane stationary phases. The alkyl component has been envisioned as brush bristles,41 a folded or collapsed structure,42 condensed liquid drops,43 and a phase in which transitions between the liquid drop and bristle structures are possible with changes in temperature.5,44 In addition, the surface distribution of alkylsilanes has sometimes been described as heterogeneous with the alkylsilane existing as island-like patches.43 Sander and co-workers reported shape selectivity in chromatographic retention for a large number of octadecylsilane stationary phases prepared using several alkylsilane precursors and grafting procedures.9 In these studies, the selectivity factor of each stationary phase for 1,2:3,4:5,6:7,8-tetrabenzonaphthalene (a nonplanar aromatic) relative to benzo[a]pyrene was assessed. Results indicate a generally linear correlation between alkylsilane surface coverage and the presence of shape selectivity; improved separation of isomers and other structurally related solute classes are usually possible with increasing surface coverage. In earlier work, Sander and co-workers examined a series of dimethylalkylsilane stationary phases, hereafter referred to as monofunctional phases, of differing alkyl chain lengths using Fourier transform IR (FT-IR) spectroscopy.30 These phases possessed similar bonding densities (2-2.5 µmol/m2) and were chosen to elucidate differences in alkyl chain conformational order as a function of length and temperature. Order within the alkyl chain was assessed by examining the spectra for the presence of bands corresponding to gauche carbon-carbon bonds and kink defects in the alkyl chains. A significant fraction of gauche defects (29) Morel, D.; Taber, K.; Serpinet, J.; Claudy, P.; Letoffe, J. M. J. Chromatogr. 1987, 395, 73-84. (30) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068-1075. (31) Ohtake, T.; Mino, N.; Ogawa, K. Langmuir 1992, 8, 2081-2083. (32) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120-1126. (33) Jinno, K.; Wu, J.; Ichikawa, M.; Takata, I. Chromatographia 1993, 37, 627634. (34) Thompson, W. R.; Pemberton, J. E. Anal. Chem. 1994, 66, 3362-3370. (35) Ho, M.; Cai, M.; Pemberton, J. E. Anal. Chem. 1997, 69, 2613-2616. (36) Doyle, C. A.; Vickers, T. J.; Mann, C. K.; Dorsey, J. G. J. Chromatogr., A 1997, 779, 91-112. (37) Ho, M.; Pemberton, J. E. Anal. Chem. 1998, 70, 4915-4920. (38) Doyle, C. A.; Vickers, T. J.; Mann, C. K.; Dorsey, J. G. J. Chromatogr., A 2000, 877, 25-39. (39) Doyle, C. A.; Vickers, T. J.; Mann, C. K.; Dorsey, J. G. J. Chromatogr., A 2000, 877, 41-59. (40) Pemberton, J. E.; Ho, M.; Orendorff, C. J.; Ducey, M. W. J. Chromotgr., A. 2001, 913, 243-252. (41) Karch, K.; Sebestian, I.; Halasz, I. J. Chromatogr. 1976, 122, 3-16. (42) Hemetsberger, H.; Behrensmeyer, P.; Henning, J.; Ricken, H. Chromatographia 1979, 12, 71-76. (43) Lochmuller, C. H.; Wilder, D. R. J. Chromatogr. 1979, 17, 574-579. (44) Gilpin, R. K.; Squires, J. A. J. Chromatogr. Sci. 1981, 1, 561-586.

was observed in the absence of solvent where the degree of disorder was comparable to that observed in the corresponding n-alkane liquids. These experimental results suggest that surfaceconfined alkylsilane components in such systems exist in a state with liquid-like properties. In addition to the large degree of conformational disorder, no evidence for temperature-dependent phase transitions was observed for these phases, although large continuous changes in conformational order were found with changes in temperature. More recently, Pursch and co-workers employed 1H magic angle spinning (MAS) NMR and 13C cross-polarization magic angle spinning (CP/MAS) NMR to assess alkyl chain order in a series of high-density octadecylsilane stationary phases with varying surface coverage.45 A more rigid alkyl chain environment was indicated for these higher surface coverage stationary phases. In addition, a larger fraction of alkyl chains in trans conformations was observed on high surface coverage stationary-phase materials. In addition to experimental investigations of alkylsilanemodified silicas, properties of these materials have also been modeled using molecular dynamics simulations.46 Employing this approach to examine monofunctional octadecylsilane stationary phases of different chain densities, Klatte and Beck46 observed that, at surface coverages from 1.5 to 4 µmol/m2, the alkyl chains form layered structures collapsed onto the exposed silica surface. The alkyl chains in these model phases were predicted to possess tilt angles exceeding 55°, supporting a picture in which the chains lie on the silica surface. The authors observed increasing order and decreasing tilt angle with surface coverages exceeding 4 µmol/m2. A larger degree of conformational disorder was observed in portions of the chains more distal to the surface pendant group, with increasing order as the surface is approached. Such behavior is attributed to an alkyl chain environment with more degrees of freedom in methyl and methylene units a greater distance from the surface. It is of interest to note that, for surface coverages ranging from 1.5 to 4 µmol/m2, the alkyl chain order in these calculations exhibited no temperature dependence from 230 to 340 K. Perhaps the most powerful experimental tool for characterizing conformational order changes in alkyl chains is Raman spectroscopy, since it provides direct information about conformational order.34-40,47-60 Additionally, such measurements are relatively free (45) Pursch, M.; Sander, L. C.; Albert, K. Anal. Chem. 1996, 68, 4107-4113. (46) Klatte, S. J.; Beck, T. L. J. Phys. Chem. 1995, 99, 16024-16032. (47) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85B116. (48) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 117168. (49) Levin, W. W.; Hill, I. R. J. Chem. Phys. 1979, 70, 842-851. (50) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395406. (51) Snyder, R. G.; Scherer, J. R. J. Chem. Phys. 1979, 71, 3221-3228. (52) Synder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 51455150. (53) MacPhail, R. A.; Synder, R. G.; Strauss, H. L. J. Chem. Phys. 1982, 77, 11181137. (54) Young, C. W.; Koehler, J. S.; McKinney, D. S. J. Am. Chem. Soc. 1947, 69, 1410-1415. (55) Lin-Vien, J. G.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991. (56) Wallach, D. F. H.; Varma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153-208. (57) Gaber, B. P.; Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260-274. (58) Harrand, M. J. Chem. Phys. 1983, 79, 5639-5651.

Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

5577

Table 1. Stationary-Phase Properties stationary phasea

previous designationb

TFC18SF TFC18SL DFC18SF DFC18SL MFC18

S1 P1 S3 P2 M1

preparation methodc surface polymerization solution polymerization surface polymerization solution polymerization

% carbon

surface coverage, Γ (µmol/m2)

19.84 17.07 17.37 15.10 12.45

6.45 5.26 5.00 4.17 3.09

a TF, trifunctional alkylsilane precursor; DF, difunctional alkylsilane precursor; MF, monofunctional alkylsilane precursor. b Designation of stationary phases in refs 8 and 45. c Preparation method as described in the text.

from spectral interference from the silica substrate or adsorbed water that plagues IR studies of such systems.30 In previous reports,35,37,40 the feasibility and utility of Raman spectroscopy for elucidating information about conformational order of alkylsilane stationary phases, both in the presence and in the absence of solvent, has been demonstrated. As part of an ongoing effort to elucidate retention mechanisms in reversed-phase liquid chromatography,34,35,37,40 these initial studies are expanded in this report to include the effects of temperature, surface coverage, stationaryphase preparation (surface polymerized or solution polymerized), and nature of the silane precursor (octadecyltrichlorosilane, methyloctadecyldichlorosilane, or dimethyloctadecylchlorosilane). The synthesis and properties,8 chromatographic behavior,8,45 and NMR spectroscopy45 of the high-density ocatdecylsilane stationary phases considered here have been previously described. As noted above, these phases show selectivity toward planar polycyclic aromatic hydrocarbons that is highly dependent on surface coverage.8,45 Such results have been interpreted as being indicative of increasing alkyl chain order with increasing surface coverage. In the work reported here, the conformational order of these stationary phases is examined as a function of temperature to elucidate trends in behavior that may be attributable to differences in surface distribution or intermolecular interactions. The resulting behavior is described using the Clapeyron relationship for first-order phase transitions. EXPERIMENTAL SECTION Stationary Phases. Octadecylsilane-modified silica-based stationary phases were prepared and characterized as previously described.8,45 Briefly, trifunctional, difunctional, and monofunctional stationary phases were prepared using octadecyltrichlorosilane, methyloctadecyldichlorosilane, and dimethyloctadecylchlorosilane precursors, respectively. Surface-polymerized phases were prepared by exposing the alkylsilane precursors to previously humidified silica, while solution-polymerized phases were prepared through the addition of water to the precursor/silica slurry. Monofunctional phases were prepared through base catalysis by 2,6-lutidine in the dimethyloctadecylchlorosilane/silica slurry. These stationary phases were prepared using one lot of YMC 3-µm silica (Waters Corp.) The identity of the precursor, grafting procedure, and resulting properties of each stationary phase are summarized in Table 1. Instrumentation. Raman spectra were collected using 100 mW of 532-nm radiation from a Coherent Verdi Nd:YVO4 laser (59) Yellin, N.; Levin, I. W. Biochim. Biophys. Acta 1977, 489, 177-190. (60) Orendorff, C. J.; Ducey, M. W.; Pemberton, J. E. J. Phys. Chem. A 2002, 106, 6991-6998.

5578 Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

or of 514.5-nm radiation from a Coherent Inova 90 Ar+ laser on a Spex Triplemate spectrograph. Slit settings of the Triplemate were (0.5/7.0/0.15) mm for all experiments, corresponding to a spectral band-pass of 5 cm-1. The detector in these experiments was a Princeton Instruments charged-coupled device (CCD) system based on a thinned, back-illuminated, antireflection coated RTE110-PB CCD of pixel format 1100 × 330, which was cooled with liquid N2 to 183 K. Samples were sealed in 5-mmdiameter NMR tubes and positioned in the laser beam using a copper sample mount. The temperature of the copper mount was regulated by circulating a temperature-controlled medium (50:50 ethylene glycol/water) through it (and around the sample tube) using a Neslab NTE-110 temperature controller, allowing temperature control from 258 to 343 K. Samples were allowed to equilibrate at each temperature for a minimum of 30 min prior to analysis to ensure that any temperature-induced changes in conformational order were complete. A minimum of three measurements were made on each sample at each temperature. Integration times for each spectrum are provided in the figure captions. Low laser power and short acquisition times were used to avoid unnecessary sample heating by the laser beam. Spectral Data Analysis. Spectra were superimposed on a linear background in the ν(CH) frequency region. This background is a complex convolution of detector response, spectrometer response, and sample scattering characteristics. Thus, raw spectral data were background-subtracted using a linear fit of the background in the region between 2750 and 3050 cm-1. Peak intensities of the νa(CH2) and νs(CH2) bands were determined as peak intensities measured from the flat baseline in the resulting spectra. RESULTS AND DISCUSSION The effect of temperature on the conformational order of the series of high-density stationary phases described above was examined by determination of the magnitudes of two Raman spectral conformational indicators in the ν(CH) region between 2750 and 3050 cm-1 over a temperature range from 258 to 343 K. Within this region, bands due to symmetric and antisymmetric ν(CH2) and ν(CH3) vibrations as well as bands due to the Fermi resonance of these modes with overtones of the δ(CH) modes are observed. As a result of the large number of CH2 groups in the alkyl component of the stationary phase, this region is spectrally complex and not readily subject to reliable curve fitting.48-54 Figure 1 represents typical Raman spectra in this region for stationary phases prepared from trifunctional (TFC18SF and TFC18SL), difunctional (DFC18SL), and monofunctional (MFC18)

Figure 1. Raman spectra in the ν(C-H) region for TFC18SF, TFC18SL, DFC18SL, and MFC18 at (a) 258, (b) 263, (c) 313, and (d) 333 K. All spectra acquired with 10-min integrations. Table 2. Raman Peak Frequencies and Assignments for High-Density Stationary Phases peak frequency (cm-1)a TFC18SF

TFC18SL

2954 2932

2954 2929 2905 sh 2897 2880 2851

2895 2880 2846 a

DFC18SF 2954 2925 2898 sh 2882 2851

DFC18SL

MFC18

assignmt

2955 2926 2905sh 2896 2884 2851

2954 2920 2905 sh 2900 2884 2853

νa(CH3) νs(CH3)FR ν(CH3)Si νs(CH3) νa(CH2) νs(CH2)

sh, shoulder.

octadecylsilane precursors with surface coverages of 6.45, 5.26, 4.17, and 3.09 µmol/m2, respectively. Vibrational mode assignments for each of these stationary-phase materials are provided in Table 2. In general, the prominent bands represented in each spectrum correspond to the νs(CH2) at 2852 cm-1, the νa(CH2) at 2885 cm-1, the νs(CH3)FR at 2924 cm-1, and the νa(CH3) at 2955 cm-1; the νs(CH3) can be observed as a shoulder at 2898 cm-1 in most spectra. Assignments for these systems are well-established and are made based on those previously reported for alkyl chain systems.47-60

While these spectral features are relatively distinct, several lessobvious features can be identified within each spectrum. Of particular significance is the νs(CH3)Si mode for the methyl groups attached to the proximal silicon atom, present in both the difunctional and monofunctional stationary-phase materials. This mode is located at 2905 cm-1 54 and is superimposed on the νs(CH3)FR at 2925 cm-1 and the νs(CH3) at 2898 cm-1. As a consequence of its position, it may also contribute slightly to the intensity of the νa(CH2) band. It was previously demonstrated that Raman spectra contain a wealth of detailed information regarding alkyl chain conformation,34,35,37,40,53,55-60 especially in the ν(CH) region. While this region exhibits considerable complexity as a result of Fermi resonance modes and a plethora of both symmetric and antisymmetric ν(CH) modes, previous work has shown47-53 that conformational order information can be empirically derived from the peak intensity ratio of the antisymmetric (2885 cm-1) to symmetric (2850 cm-1) ν(CH2) bands, I[νa(CH2)]/I[νs(CH2)], and the frequencies at which these bands are observed. Changes in peak width, and hence peak height, of buried νa(CH) modes within this complex envelope change in response to various perturbations (e.g., temperature, solvent, etc.) and, therefore, change the peak intensities of the more prominent νa(CH2) and νs(CH2) modes. Previous work from this laboratory60 and others49,51,55-57 has documented the extreme sensitivity of the I[νa(CH2)]/I[νs(CH2)] spectral indicator to subtle changes in conformational order of alkanes. This ratio ranges from 0.6 to 0.9 for liquid alkanes to 1.6 to 2.0 for crystalline alkanes.55-56 Furthermore, in a recent study in which various Raman spectral indicators were quantitatively correlated,60 it was shown that, even in the crystalline state of bulk alkanes in which no gauche conformers exist, the conformational order decreases with increasing temperature, since I[νa(CH2)]/ I[νs(CH2)] decreases from 2.1 to 1.4. This change is thought to be due to methyl and methylene rotational disorder, predominantly near the ends of the alkane chains. Further into the biphasic region of alkanes where the average number of true C-C bond gauche conformers increases, the I[νa(CH2)]/I[νs(CH2)] continues to decrease, starting at a value of 1.2 and decreasing to 0.75. In the liquid phase, the extent of deviation from the all-trans state remains constant (i.e., I[νa(CH2)]/I[νs(CH2)] is constant), while the relative number of C-C bond gauche conformers increases with increasing thermal energy. In this region, the rotational disorder responsible for the variance of I[νa(CH2)]/I[νs(CH2)] in the biphasic region is converted to true gauche conformers with increasing temperature. Thus, I[νa(CH2)]/I[νs(CH2)] reflects the total fraction of alkyl chains that deviate in any way (i.e., rotational disorder and gauche disorder) from an all-trans C-C bond configuration. The fact that quantitative changes in this parameter of less than 5% can be precisely measured make it extraordinarily useful for investigating subtle changes in conformational order in alkane-based systems. Indeed, this sensitivity is essential for the studies of chromatographically relevant changes in stationaryphase order described here. Retention in a single theoretical plate with a partition coefficient, K, of less than 100 corresponds to a free energy change, ∆G, of less than 5 kcal/mol. This small change in free energy is expected to correspond to relatively minor changes in molecular structure. Thus, spectral indicators sensitive Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

5579

Figure 2. I[νa(CH2)]/I[νs(CH2)] as a function of temperature for TFC18SF (9), TFC18SL (4), DFC18SF (b), DFC18SL (]), and MFC18 (1). Each point is the average of at least three measurements. The error bars represent one standard deviation.

to such subtle changes are required for characterization of the molecular basis of chromatographic retention. The frequencies at which the νs(CH2) and νa(CH2) modes are observed also reflect conformational order; these are observed at 2856 and 2888 cm-1, respectively, for alkanes in the liquid state, and decrease by 6-8 cm-1 for crystalline alkanes.56-57 The peak frequency of the νs(CH2) band has been related to the extent of coupling between alkyl chains, where increased chain coupling results in a decrease in peak frequency.52,60 A qualitative examination of the spectra in Figure 1a reveals a wide range of conformational order between these stationary phases as reflected by the values of I[νa(CH2)]/I[νs(CH2)]. Particularly pronounced is the difference in conformational order between TFC18SF (6.45 µmol/m2) and MFC18 (3.09 µmol/m2). Furthermore, when the temperature dependence of the Raman spectral response between 288 (Figure 1a) and 333 K (Figure 1d) is considered, a significant difference in alkyl chain order is observed and is especially apparent for the highest surface coverage material (6.45 µmol/m2). When these data are considered, it is important to recall that the intensity of the νa(CH2) mode in MFC18 may be slightly elevated by the presence of the νs(CH3)Si mode resulting in a slightly higher than expected value of I[νa(CH2)]/I[νs(CH2)]. Thus, stationary-phase materials containing Si-CH3 groups may appear slightly more ordered than they truly are. When the I[νa(CH2)]/I[νs(CH2)] order indicator is systematically examined from 258 to 343 K, the dependence on temperature becomes even more apparent for each of the stationary-phase materials examined. Figure 2 shows plots of I[νa(CH2)]/I[νs(CH2)] as a function of temperature for the five high-density phases examined here. Although the magnitudes of the changes observed in this parameter are small, the data exhibit some scatter, and the temperature dependence is in some cases complex, general trends in the behavior of this parameter with coverage and temperature are unmistakably observed that allow important conclusions to be drawn regarding stationary-phase conformational order as a function of these two variables. First, at any given temperature, the magnitude of I[νa(CH2)]/I[νs(CH2)] systemati5580

Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

cally increases with surface coverage, indicating an increase in order with increasing surface coverage. Also, for any given surface coverage, I[νa(CH2)]/I[νs(CH2)] decreases with increasing temperature, indicating that disorder is introduced with increasing temperature. The three important regions of conformational order indicated by the magnitude of I[νa(CH2)]/I[νs(CH2)] are indicated by different shading in Figure 2. The two highest surface coverage stationary phases exhibit values of I[νa(CH2)]/I[νs(CH2)] that range from almost crystalline order at the lowest temperatures to a state in which the onset of gauche conformers occurs at the higher temperatures. In contrast, the stationary phases with the lowest three surface coverages exhibit values of I[νa(CH2)]/I[νs(CH2)] that start in the region in which the onset of gauche conformers occurs below room temperature. In other words, these stationary phases exhibit more liquid-like disorder even at the lowest temperatures investigated here. The I[νa(CH2)]/I[νs(CH2)] values of these lower coverage stationary phases extend down to the region where the rapid addition of gauche conformers occurs as the temperature approaches and exceeds room temperature. Thus, for each stationary phase, systematic trends in the I[νa(CH2)]/I[νs(CH2)] values as a function of temperature are indicative of significant differences in conformational order that might reflect phase changes. In assessing whether these temperature-induced changes in I[νa(CH2)]/I[νs(CH2)] are equivalent to phase changes, it is useful to review simple first-order phase transitions. For the first-order melting (fusion) of a solid to a liquid for a system in which the density of the solid is greater than that of the liquid, the phase transition line in the phase diagram is described by the following relationship

P ) PRef + (∆Hfus/∆Vfus) ln(T/TRef)

(1)

where P is pressure, T is temperature, PRef and TRef represent an arbitrary starting reference pressure and temperature, respectively, ∆Hfus represents the molar enthalpy change of fusion, and ∆Vfus represents the molar volume change of fusion. Recognizing that PRef and TRef are arbitrary and constant values on this phase line, this expression can be rearranged to

P ) PRef + (∆Hfus/∆Vfus) ln T - (∆Hfus/∆Vfus) ln TRef

(2)

At any point on the phase transition line, the slope of its tangent is given by

dP 1 ) (∆Hfus/∆Vfus) dT T

(3)

Therefore, a plot of dP/dT as a function of 1/T should be linear with a slope of (∆Hfus/∆Vfus) for a simple first-order phase transition from a solid to a liquid. The stationary-phase materials studied here are considered a homologous series of similar materials, each of which exhibits its own first-order solid-to-liquid phase transition, based on the conformational order behavior documented in Figure 2 and the fact that they are prepared from different precursor molecules. Thus, considering this series from the lowest to highest alkylsilane

Figure 3. Schematic of phase diagrams for a homologous series of materials with increasing surface coverage, Γ.

Figure 4. R as a function of temperature for TFC18SF (9), TFC18SL (4), DFC18SF (b), DFC18SL (]), and MFC18 (1). The error bars represent one standard deviation. Best fits for R ) a ln T + b shown as solid lines.

surface coverage, one expects the phase transition line to systematically shift to the right on the phase diagram, reflecting the greater temperature required to cause “melting” at a given pressure. A schematic of the expected phase transition lines is shown in Figure 3 for five components of such a homologous series. Within a given temperature range ∆T for this series, one can see that the slope of the phase transition line at any given temperature, dP/dT, systematically decreases with alkylsilane surface coverage according to eq 3. If the inverse of I[νa(CH2)]/I[νs(CH2)] is plotted as a function of temperature for each stationary phase, the shapes and relative positions of the resulting plots resemble those of the phase diagrams in Figure 3. This suggests a proportionality between pressure and I[νs(CH2)]/I[νa(CH2)]. Defining R as follows,

R ) I[νs(CH2)]/I[νa(CH2)]

(4)

then a proportionality between pressure and R is indicated:

R ∝ P ) PRef + (∆Hfus/∆Vfus) ln T - (∆Hfus/∆Vfus) ln TRef (5) or

Figure 5. (a) Clapeyron plots of dR/dT from best fit lines in Figure 4 as a function of 1/T. TFC18SF (9), TFC18SL (4), DFC18SF (b), DFC18SL (]), and MFC18 (1). (b) Slope of Clapeyron plots as a function of alkylsilane surface coverage.

R ) KPRef + K(∆Hfus/∆Vfus) ln T - K(∆Hfus/∆Vfus) ln TRef (6) where K is the unknown constant of proportionality. Thus,

dR d ) {KPRef + (K∆Hfus/∆Vfus) ln T dT dT 1 (K∆Hfus/∆Vfus) ln TRef} ) (K∆Hfus/∆Vfus) (7) T If the temperature-induced changes observed in the Raman spectral response of these stationary phases reflect phase changes, their behavior should follow eqs 6 and 7. Figure 4 shows plots of R as a function of temperature for these stationary-phase materials (symbols) along with the best (R ) a ln T + b) fits of the data (lines) with a ) (K ∆Hfus/∆Vfus) and b ) {KPRef - (K∆Hfus/∆Vfus) ln Tref} as the variable parameters. Although the data exhibit some scatter, the logarithmic fits are acceptable based on the calculated R2 values of 0.9 or greater in all cases. The slopes of the best-fit lines, dR/dT, were determined as a function of temperature and are plotted as a function of inverse Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

5581

Figure 6. I[νa(CH2)]/I[νs(CH2)] as a function of surface coverage at 258 (9), 293 (b), and 333 K ([).

temperature in Figure 5a. As predicted by eq 7, these Clapeyron plots are linear with near-zero intercepts. Figure 5b shows the slopes of these Clapeyron plots, which reflect the magnitude of (K∆Hfus/∆Vfus) for a phase change, as a function of surface coverage of alkylsilane. For a simple first-order phase transition, the slope of the Clapeyron plot represents the tangent to the solid/ liquid-phase line at a particular temperature; as shown by the phase diagrams in Figure 3, this slope is expected to decrease with increasing temperature or pressure for a first-order phase transition in a bulk material. However, for a surface-confined system with a fixed surface density such as the stationary-phase materials described here, ∆Hfus for the phase change is expected to increase and then level off with increasing surface density (surface coverage). In contrast, the magnitude of ∆Vfus is expected to decrease and then level off with increasing surface density. As a result of these trends, one would expect to see the slope of the Clapeyron plot increase and then level off with increasing surface coverage if K is assumed to be the same for all five stationary phases. If the slopes of these curves are examined according to these criteria, such a trend is indeed generally observed (Figure 5b) for the four lowest surface coverage materials. However, a deviation from this trend is observed for the highest surface coverage material that is attributed to the breakdown of the assumptions used to treat these data. Specifically, this deviation is interpreted to indicate the assumptions of TFC18SF being part of a homologous series with the other four materials (i.e.. K the same for all five systems) and a linear correlation between R and pressure breakdown for this system. The simple observation of temperature-dependent changes in alkyl chain order is critical to describing the surface structure and extent or even presence of short- and long-range order in the alkyl component. Chromatographic manifestations of phase changes are nonlinear van’t Hoff relationships. Many previous examinations of chromatographic behavior and stationary-phase structure based on van’t Hoff plots have resulted in the observation of either no distinct phase change or a very subtle transi5582 Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

Figure 7. νs(CH2) peak frequency as a function of temperature for (a) TFC18SF (9) and TFC18SL (4), (b) DFC18SF (b), DFC18SL (]), and MFC18 (1). The error bars represent one standard deviation.

tion25,29,34,44,65-75 similar to those observed here. When such nonlinearities have been observed, they were attributed to changes in the nature of the alkyl chain condensed phase at a particular temperature under specific solvent conditions.5,7,11,44,66,76-80 Phase changes are thus indirectly reflected in chromatographic reten(61) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (62) Knobler, C. M.; Desai, R. C. Annu. Rev. Phys. Chem. 1992, 43, 207-236. (63) Overbeck, G. A.; Mobius, D. J. Phys. Chem. 1993, 97, 7999-8004. (64) Lawrie, G. A.; Barnes, G. T. J. Colloid Interface Sci. 1994, 162, 36-44. (65) Wheeler, J. F.; Beck, T. L.; Klatte, S. J.; Cole, L. A.; Dorsey, J. G. J. Chromatogr., A 1993, 656, 317-333. (66) Bell, C. M.; Sander, L. C.; Wise, S. A. J. Chromatogr., A 1997, 757, 29-39. (67) Jinno, K.; Lin, Y. Chromatographia 1995, 41, 311. (68) Sander, L. C.; Wise, S. A. Anal. Chem. 1989, 61, 1749-1754. (69) Morel, D.; Serpinet, J. J. Chromatogr. 1980, 200, 95-104. (70) Morel, D.; Serpinet, J. J. Chromatogr. 1981, 214, 202-208. (71) Morel, D.; Serpinet, J. J. Chromatogr. 1982, 248, 231-240. (72) Hansen, S. J.; Callis, J. B. J. Chromatogr. Sci. 1981, 19, 195-199. (73) Pursch, M.; Strohschein, S.; Handel, H.; Albert, K. Anal. Chem. 1996, 68, 386-393. (74) Kelusky, E. C.; Fyfe, C. A. J. Am. Chem. Soc. 1986, 108, 1746-1749. (75) Gangoda, M. E.; Gilpin, R. K. J. Magn. Reson. 1983, 53, 140-143. (76) Sentell, K. B.; Henderson, A. N. Anal. Chim Acta 1991, 246, 139-149. (77) Nahum, A.; Horvath, C. J. Chromatogr. 1981, 203, 53-63. (78) Jinno, K.; Nagoshi, T.; Tanaka, N.; Okamoto, M.; Fetzer, J. C.; Briggs, W. R. J. Chromatogr. 1988, 436, 1-10. (79) Cole, L. A.; Dorsey, J. G.; Dill, K. A. Anal. Chem. 1992, 64, 1324-1327. (80) Hammers, W. E.; Verschoor, P. B. A. J. Chromatogr. 1983, 282, 41-58.

Figure 8. Proposed temperature-dependent disordering process for (a) TFC18SF, (b) MFC18, and (c) DFC18SF.

tion data. The vast majority of these studies have been carried out on stationary phases with surface coverages below 3.0 µmol/ m2 (surface coverages typically employed for analytical separations). In light of the data presented here and the observation that the temperature dependence is generally described by the Clapeyron relationship, it is not surprising that phase transitions were not observed or were very shallow in systems with low alkylsilane surface coverages. In examining Figure 2, it appears that the differences in conformational order between stationary phases decrease with increasing temperature. This effect is more evident when I[νa(CH2)]/I[(νs(CH2)] is examined as a function of surface coverage at several temperatures (Figure 6). This plot suggests that the conformational order approaches some asymptotic value of I[νa(CH2)]/I[νs(CH2)] dictated by the temperature as opposed to the surface coverage. This value is indicative of the maximum extent to which the surface-bound alkyl chains can be disordered. It is interesting to note that this asymptotic value of I[νa(CH2)]/I[νs(CH2)] is larger than that observed for liquid alkanes. In other words, the alkyl chains in these stationary phases cannot become as disordered as in a bulk liquid. This result is not surprising considering that alkyl chains used as stationary-phase materials are bound to the surface, thereby restricting the mobility of one end. Up to this point in the discussion, only I[νa(CH2)]/I[(νs(CH2)] has been used to describe changes in conformational order in these systems. As noted above, however, the frequency at which these bands are observed also reflects alkyl chain order. The temperature dependence of the νs(CH2) frequencies is shown in Figure 7. In general, for temperatures below 313 K, the trend in

peak frequency mimics the trend observed in R shown in Figure 4 in that the peak frequency generally increases with decreasing surface coverage, reflecting a greater extent of interchain coupling (or crystallinity) in the higher coverage materials. Differences in this parameter at low temperatures most likely result from increased chain spacing with decreasing surface coverage. At high temperatures (>323 K), very little difference in interchain coupling is observed in each phase as reflected by similar νs(CH2) peak frequencies. This similarity indicates some minimum value of interchain coupling that must result from either a physical separation of the alkyl chains or, for higher surface coverage materials, thermal agitation of the chains. These effects are most likely convoluted in the data due to the use of different precursor molecules and grafting protocols. The manner in which the peak frequency changes as a function of temperature differs between these phases as well. The highest surface coverage phase, TFC18SF, exhibits sigmoidal behavior in the νs(CH2) frequency with temperature, revealing a change in interchain coupling at ∼313 K. The remaining phases (except for MFC18) exhibit νs(CH2) frequencies that level off at temperatures below 273 K and then increase in a linear fashion with increasing temperature. In contrast, the frequency of the νs(CH2) mode in MFC18 is relatively temperature-independent, indicating low interchain coupling at all temperatures. This lack of coupling is a consequence of the additional interchain spacing imposed by the three methyl groups in the monofunctional precursor. It is interesting to note that the temperature ranges over which significant changes in νs(CH2) frequency are observed for the two trifunctional stationary phases (TFC18SF and TFC18SL) do not correlate with the temperature ranges over which significant Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

5583

changes in I[νa(CH2)]/I[νs(CH2)] are observed. One must recall the exact nature of conformational order indicated by each of these parameters to obtain a clear molecular interpretation of this apparent discrepancy. In contrast to the value of I[νa(CH2)]/I[νs(CH2)], which reflects any deviation from the all-trans arrangement, the frequency of the νs(CH2) depends only on interchain coupling. It is possible to envision a situation in which alkyl chains add kinks or twist around the long axis of the chain (resulting in a decrease in I[νa(CH2)]/I[νs(CH2)]) with the chains remaining in close proximity to one another or, in other words, having insufficient thermal energy to overcome interchain coupling. An inflection point on the νs(CH2) frequency-temperature curve therefore indicates the temperature at which interchain coupling begins to change. A closer examination of the data in this light does indeed reveal differences in interchain coupling between these phases. The data also indicate only slight differences between the two stationary phases with similar surface coverages (TFC18SL, 5.26 µmol/m2; DFC18SF, 5.00 µmol/m2). This response is expected if the alkyl chains are distributed relatively homogeneously over the surface. Combining the results of Figure 2 with those of Figure 7, a more complete picture of the temperature-dependent behavior is revealed. Beginning with the highest surface coverage system, the molecular picture indicated is one in which the alkyl chains become more disordered with temperature, although in the confined area within which the each chain can move (25.7 Å2 compared with 20.8 Å2 for the maximum theoretical surface coverage of 8 µmol/m2), a significant population of gauche bonds cannot develop. Thus, the alkyl chains undergo a phase change without altering significantly the extent of alkyl chain coupling. Such a phase change may result from a change in alkyl chain tilt angle relative to the surface. At temperatures in excess of 323 K, the chains possess sufficient thermal agitation to overcome interchain coupling as reflected by the increase in νs(CH2). While interchain coupling appears to be minimized at temperatures above 323 K, large changes in I[νa(CH2)]/I[νs(CH2)] are not observed, indicating that the maximum amount of deviation from all-trans behavior that can be sustained in these chains has been achieved. The resulting molecular picture would therefore be one in which the alkyl chains undergo rapid but sterically restricted motion that does not result in the development of large populations of gauche bonds (Figure 8a). As the surface coverage decreases, the alkyl component adopts a more disordered state. The monofunctional material examined here (MFC18) has larger interchain distances (58 Å2/chain) compared to the higher surface coverage materials. This conclusion is consistent with the lack of any temperature dependence of the νs(CH2) frequency. Considering the length of the octadecyl chain (23 Å), homogeneously distributed alkyl chains at this bonding density must deviate significantly from the all-trans configuration in order to interact (Figure 8b). Figure 2 reveals that the alkyl chains in this material are more disordered than in the higher surface coverage phases, but the increased distance between chains allows for such disorder through the formation of gauche defects in the chains.

5584

Analytical Chemistry, Vol. 74, No. 21, November 1, 2002

Finally, when possible differences in architecture of the alkyl chains in the two stationary phases with similar surface coverages are being considered, the situation shown in Figure 8c is proposed. Although well-behaved in terms of their phase change behavior according to Figure 5b, it is possible to have local heterogeneity in the difunctional material that results from asymmetry in the headgroup. The ramification of this asymmetry is the possibility of alkyl chains separated by two distances, one dictated by the methyl side chains on the headgroup and the second dictated by the Si-O-Si bond. Such local heterogeneity would alter the phase behavior despite the similar surface coverage through a decrease in overall interchain coupling. CONCLUSIONS A detailed examination of the temperature-dependent behavior of a series of high-density alkylsilane stationary phases with differing surface coverages is presented. Results indicate that conformational order and interchain coupling of the alkyl component of these phases are sensitive to temperature and surface coverage. Preparation procedure and nature of the alkylsilane precursor do not appear to significantly affect the chain conformational order or interchain interactions. Each of the stationary phases exhibits subtle temperature-dependent phase changes that are generally described by the Clapeyron relationship. The temperature at which the phase change occurs is postulated to be reflective of the condensed phase adopted by the alkyl component or may be reflective of the grafting procedure employed in the preparation of the phase. The results presented here complement the chromatographic observations made by Sander and co-workers in the correlation between surface coverage and selectivity of the phase for planar molecules.45 These results also complement the observations made by Pursch and co-workers based on NMR spectroscopy that the fraction of gauche defects decreases and interchain coupling increases with increasing surface coverage. With the molecular picture of the alkyl chains and the interactions between them clarified, the behavior of each phase in the presence of a variety of different solvents will be explored in a series of upcoming publications. ACKNOWLEDGMENT The authors gratefully acknowledge support of this research by the Department of Energy (DE-FG03-95ER14546) through a grant to J.E.P. 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 or the University of Arizona, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Received for review May 24, 2002. Accepted August 15, 2002. AC0203488