Anal. Chem. 1993, 65,2048-2054
2048
29Siand I3C NMR Characterization of Mixed Horizontally Polymerized Monolayers on Silica Gel Hafeez 0. Fatunmbi, Martha D. Bruch, and Mary J. Wirth' Department of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716
A horizontally polymerized monolayer of mixed CISand C3 trifunctional silanes on silica, and conventional chromatographicphases, are studied by NMR spectroscopy. For horizontal polymerization, quantitative W and 29SiNMR measurements reveal that a dense and highly polymerized monolayer of bonded groups is formed on the surface. The conventional polymeric surface, by contrast, is oligomeric in nature: the ratio of bonded silanes to reacted substrate silanols is only slightly more than 1:1, and the average oligomer size is 5 units. The data reveal insignificant vertical polymerization. 13CNMR measurements show that the CISchain dynamics for the mixed horizontally polymerized monolayer are similar to those for the monomeric CISmonolayer,whereas the CIS chain dynamics for a conventional polymeric phase are significantly slower. These results indicate that the CISchains are randomly interspersed with the short C3 chains in the horizontally polymerized mixed phase.
INTRODUCTION For more than two decades, much effort has been put into the synthesis of organic layers onto silica substrates for use in reversed-phase high-performance liquid chromatography (RP-HPLC). These studies are still being continued and include the attachmentof mono-, di-, and trifunctional ligands to the silica substrates.'-7 The performance and selectivity of chromatographic separations that can be achieved with such bonded phases in RP-HPLC depend on the microstructure of the organic layer bonded a t the silica surface. The details of the stationary phase, together with the nature of the mobile phase, determine the thermodynamic phase equilibria of a solute and its retention time and peak shape. Despite the effort put into RP-HPLC stationary phases, many questions are still unanswered. For example, reproducible preparation of these phases for controlled and predictable selectivity has not been achieved. Furthermore, the di- and trifunctional ligands can result in uncontrolled polymeric phases and the exact structures of such phases are based on speculations. The relatively poor hydrolytic stability of the conventional phases under a number of practical conditions is also a subject of great concern and has resulted (1) Wright, P. B.; Lamb, E.; Dorsey, J. G.; Kooser, R. G. Anal. Chem. 1992, 64, 785. (2) Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 1120. (3) Bailey, R.; Cassidy, R. M. Anal. Chem. 1992,64,2277. (4) Kimata, K.; Tanaka, N.; Araki, T. J. Chromatogr. 1992, 594, 87. (5) Roumeliotis, P.; Unger, K. K. J. Chromatogr. 1978, 149, 211. (6) Karch, K.; Sebestian, I.; Halasz, I. J. Chromatogr. 1976, 122, 3. (7) Locke, D. C.; Schmermund, J. T.; Banner, B. Anal. Chem. 1972. 44,90. 0003-2700/93/0365-2046$04.00/0
in a search for better phases.Cl2 Mixed horizontally polymerized monolayers have recently been introduced, where long and short alkyltrichlorosilanes (e.g., C18 and C3) are bonded to silica. These have been demonstrated to provide very high hydrolytic stabilityl3-15 and excellent reproducibility.16 A higher material density at the silica substrate,caused by close C$Cla spacing, is believed to underlie the high stability. The lower density beyond the terminal end of the C3 spacer is believed to allow the surface to behave like a conventional monomeric surface of comparable coverage. Analytical measurements have not yet been used to probe the structure of this new type of chromatographic monolayer. The purpose of this paper is to characterize a typical mixed C$Cla horizontally polymerized monolayer by solid-state nuclear magnetic resonance (NMR) techniques and to compare this mixed phase to conventional chromatographic phases. Solid-state 13C and aSi NMR methods are used to characterize the density, structure, and dynamics of three chromatographic phases: horizontally polymerized mixed C$ (218, conventionally polymerized (318, and monomeric (218.
THEORY To maximize the sensitivity and spectral resolution of the NMR measurements, cross polarization (CP) and magic angle spinning (MAS) are employed for all NMR measurements. The details of the use of CP/MAS techniques in the characterization of silica surfaces have been described in previous reports.1620 Magic angle spinning at a frequency greater than the magnitude of the chemical shift anisotropy removes its contribution to the line width, and the resolution of the resultant spectrum is dramatically improved. Cross polarization improves the sensitivity of heteronuclear solidstate NMR experiments by transferring magnetization from the abundant nucleus, 'H, to the heteronucleus (%Sior 13C). This method is critical for observation of the rare 13Cnucleus in natural abundance. In this work, surface sensitivity in the 29Si NMR measurements is increased by cross polarization, which greatly enhances the signal from the surface silicon atoms relative to that of the bulk silicon atoms. Presumably, the 13Cnuclei cross polarize from covalently attached protons, (8) Kirkland, J. J.; Glajch, J. L.; Farlee, R. D. Anal. Chem. 1989,61, 2. (9) Feher, F. J.; Newman, D. A. J. Am. Chem. SOC.1990, 112, 1931. (10) Wikstrom, P.; Mandenius, C. F.; Larsson, P. J. Chromatogr. 1988, 455, 105. (11) Golding, R. D.; Barry, A. J.; Burke, M. F. J. Chromatogr. 1987, 384, 105. (12) Little, C. J.; Whatley, J. A,; Dale,A. D. J. Chromatogr. 1979,171, 435. (13) Wirth, M. J.; Fatunmbi, H. 0. Anal. Chem. 1992,64, 2783. (14) Wirth, M. J.;Fatunmbi,H. O.U.S.Patent Application No. 900,215, .. June 17, 1992. (15) Wirth, M. J.; Fatunmbi, H. 0. Anal. Chem. 1993, 65, 822. (16) Maciel, G. E.; Sindorf, D. W. J. Am. Chem. SOC.1980,102,7606. (17) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. SOC.1981, 103,4263. (18) Shah,P.; Rogers, L. B.; Fetzer, J. C. J. Chromatogr. 1987, 388, 411. (19) Sindorf, D. W.; Maciel, G. E. J.Am. Chem. SOC.1983,105,3767. (20) Kohler, J.; Chase, D. B.; Farlee, R. D.; Vega, A. J.; Kirkland, J. J. J. Chromatogr. 1986, 352, 275.
@ 1993 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 65,
whereas the 29Sinuclei cross polarize from either hydroxyl protons or adsorbed water. Finally, heteronuclear dipolar coupling is removed by high-power proton decoupling during data acquisition. One potential problem with the use of CP/MAS techniques is that the intensity depends upon the proton relaxation time in the rotating frame, 1H TIp,and on the efficiency of cross polarization, which depends primarily on the average 'H-X distance, where X is either 13Cor W i . Therefore, the relative intensities in a spectrum obtained with any one contact time, which is the time allowed for cross polarization to occur, are not necessarily directly comparable since different chemical types of nuclei may have differences in relaxation properties or efficiency of polarization transfer. This can be particularly problematic if the goal is to compare intensities in spectra obtained on different samples. More accurate intensities can be obtained by performing variable contact time CP/MAS experiments. Under conditions of rapid spin diffusion and efficient cross polarization, the intensity of a given NMR signal in a CP/MAS experiment as a function of the contact time, t, can be described as follows.21,22 intensity = {MJ(l- kl/k2)){exp(-klt) - exp(-k2t)] (1) In eq 1, kl is the relaxation rate in the rotating frame (kl = 1/lH TIp),k2 is the rate of cross polarization, and M, is the equilibrium intensity under conditions of full cross polarization and no relaxation. The three parameters M,, kl, and k2 can be obtained by fitting the intensities obtained in a variable contact time CP/MAS experiment to eq 1. Use of the extrapolated M, values enables direct comparison of intensities both within a spectrum and between spectra obtained on different samples.
EXPERIMENTAL SECTION 1. Reagents. Silanizationreagents were purchased from Huh America, Bristol, PA, and the n-heptane, toluene, hexane, and carbon tetrachloride were purchased from Aldrich Chemical Co. The silica gel, Whatman Partisil, had an average particle diameter of 40 pm, with pore size and surface area of 90 A and 298 m2/g, respectively. All silane reagents were used without further purification. The n-heptane was made to be anhydrous by passage through a column containing a layer of alumina, followed by a layer of silica gel, in a glovebox. 2. Synthesis of Horizontally Polymerized Phase. Prior to silanization reactions, the silica sample was heated in concentrated nitric acid overnight, cleaned with copious amounts of ultrapure water, and dried at 130 OC under nitrogen gas for 24 h. The silica gel was then cooled to room temperature with nitrogen gas and equilibrated with ultrapure water, using an apparatus already described." Octadecyltrichlorosilane and propyltrichlorosilane were mixed in the ratio 4:l (v/v) in anhydrous n-heptane. This solution of mixed silanes was then stirred gently with the silicagel, as the gas bubbles of HC1evolved. No heat was applied, and a period of 24 h was allowed for the self-assembledmonolayer to form. The bonded phase was washed several times in hexane, carbon tetrachloride, toluene, and acetone. The bonded phase was dried at 100 'C for 1 h. 3. Synthesisof MonomericPhase. The silica gel was treated in acid, washed the same way as the mixed phase, and then dried under vacuum for 24 h before silanization. The monomeric phase was prepared by exhaustive silanization with dimethyloctadecylchlorosilane. Approximately 5 g of silica was added to a solution of 100 mL of carbon tetrachloride containing a 10-fold excessof the silane (based on 8 pmol/m2). The slurry was refluxed for 4 h, filtered, washed, and dried as described above. 4. Synthesis of Conventional Polymeric Phase. The silica gel was first treated as in the mixed and monomeric syntheses. ~ _ _ _ _ _ _
(21) Sullivan, M.; Maciel, G. E. Anal. Chem. 1982,54, 1606. (22) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm, K. W. J . Am. Chem. SOC. 1983,105, 2142.
NO. 15, AUGUST 1, 1993 2049
The polymeric phase was prepared by silanization with octadecyltrichlorosilane in the presence of water, according to the method used by Sander and Wise.29 After reaction, the silica gel was filtered, washed, and dried as above. 5. Microanalysis. Carbon elemental analysis of all the bonded phases was carried out by Micro-analysis, Inc., Wilmington, DE. 6. NMR Spectroscopy. All samples were weighed before NMR analysis. mSi and 1% CP/MAS experiments were performed on a Bruker MSL 300 Fourier transform spectrometer operating at frequencies of 59.6 and 75.7 MHz, respectively.The magic angle was carefullyadjusted by minimizing the anisotropy of the lSCaromatic signal of hexamethylbenzene (HMB), and all samples were spun at 3.1-3.2 kHz. No spinning side bands were observed in any of the 13C or "Si spectra. A 90' pulse width of 6 ps was used for both 13Cand "Si experiments, which corresponds to a spin-lock field of 42 kHz. lH 7'1 values were measured by direct observation of lH M A S spectra using a standard inversion-recovery pulse sequence (RD180'-~-9O'-detect). Intensity as a function of the delay time T was fit to the equation intensity = M , - ( M , - M ( 0 ) )exp(-T/T,) (2) In eq 2, M , is the equilibrium intensity and M(0) is the intensity immediately following application of the 180' pulse.a A single 21 ' was observed for all signals in the proton spectrum of each sample, indicative of rapid spin diffusion. To allowcomparison of NMR intensities obtained on different samples, all experimental conditions were kept constant for the series of samples studied. A relaxation delay of 2 s was used, which is greater than 3 times the longest proton Tl, to eliminate intensity distortions that could arise from incomplete proton relaxation. For all l3C spectra, l k data points were acquired over a spectral width of 10 kHz, and the data were zero-filledto 4k data points prior to Fourier transformation to facilitate accurate integration of signals. After 32 dummy scans, 3600scans were averaged for each contact time, and spectra corresponding to 12-16 contact times were used to fit the data to eq 1. The same normalization constant was used to process all spectra, and 10-Hz line broadening was used to improve the signal-to-noise ratio. The Hartmann-Hahn match was established using a sample of HMB, and the chemical shifts were referenced to 17.6 ppm for the methyl resonance of HMB. For all "Si spectra, 4k data points were acquired over a spectral width of 25 kHz, and 5000 scans were accumulated for each contact time (after 32 dummy scans). Spectra were processed with 20-Hz line broadening in the absolute intensity mode. A sample of 2,bdimethyl2-silapentane-5-sulfonic acid, which resonates at 0 ppm, was used to establish the Hartmann-Hahn match. To minimize intensity distortions that arise from differences in spectrometer performance for different samples, mSidata were obtained for all sampleswithout interruption, and then l3C spectra were obtained for all samples. For all NMR experiments, fits to eqs 1and 2 were done using a three-parameter simplex method.%
RESULTS A N D DISCUSSION 1. Quantitative W NMR. For the conventional monomeric and polymeric phases, the surface density can be determined from microanalysis. However, for the mixed phase, this is not the case because the stoichiometric ratios for long and short chains must be known. Determining this ratio requires quantitative 13C NMR spectroscopy. The purpose of this section is to establish that reliable quantitative analysis is accomplished and to report the surface densities of the three chromatographic phases. Quantitative solid-state NMR studies must be approached with caution, especially when measurements on different samples must be compared. To demonstrate that quantitative (23) Sander, L. S.; Wise, S. A. Anal. Chem. 1984,56, 504. (24) Sass,M.; Ziessow, D. J . Magn. Reson. 1977,25, 263. (25) Noggle, J. H. Practical Curve Fittingand Data Analysis;Prentice Hall. Englewood Cliffs, NJ, in press.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AU(3UST 1, 1993 4-15
a
2
1
3
4-15
I6
17
18
-(CH,12SiCH,CH,CH,(CH2~l~CH~CHzCH3
a 18
and B. These values show that the NMR results agree well with the microanalysis results. This comparison demonstrates that quantitative NMR measurements are reliable for these samples. The carbon content measured by microanalysis can be converted into a more meaningful measure of surface density (pmol/mZ) from the following equation.%
lo6wt 7% c (3) (1200)(chainlength) - (wt 7% C)(MW)(298) For the horizontally polymerized mixed phase, the numberaverage molecular weight and average chain length are used in eq 3, which gives an expression equivalent to that derived previously for mixed phases.16 The average molecular weight and average chain length are calculated from the Cs/Cl8mole ratio measured from the 13C NMR spectra;16 this ratio was determined to be 1.6 in this work, in agreement with the previous work.16 The 13C NMR measurements of Mdg of sample are converted to moVg of silica from the weight percent of carbon by correcting for the contribution of the silanizing reagent to the sample weight and normalized. The NMR values were normalized to a value of 8.0mol/g of silica for the C&e mixed phase. The two estimates for the density of the bonded phases, microanalysis and lW NMR, given in Table I, columns C and D, respectively, agree well. These data reaffirm that l3C NMR can be used to make quantitative comparisons among different samples. The data of Table I, column C, show that the monomeric phase has a surface density of 1.73 pmol/m2,which is on the low side of the normal range for chromatographic applications. The conventionalpolymeric phase has a density of 5.35 pmol/ m2, which is in the normal range. The horizontally polymerized phase has a surface density of 7.98 pmol/m2. Since 8 pmol/m2is believed to be the limiting coverage for horizontal polymerization, and the density agrees with the value for samples studied previously,15 the horizontally polymerized phase under study in this work is representative of a typical, good horizontally polymerized phase. 2. Quantitative BSi NMR. The bonding of the conventionally and horizontally polymerized phases can be investigated from quantitative %SiNMR spectroscopy. Use of this technique allows determination of the composition of the bonded monolayer, thus allowing evaluationof the extent of polymerization. A crucial piece of information in evaluating the extent of polymerization of the reagents is the ratio of moles of bonded reagent to the moles of reacted substrate silanols. Determining this requires quantitative comparisons of different samples. To demonstrate that the mSi NMR measurements are also quantitative from sample to sample, the surface densities of each of the chromatographicphases are calculated from the mSi NMR data to test their agreement with the 13C NMR data and the microanalysis data. In contrast to calculating the density through the number of alkyl chains attached to the surface, as was done with 1% NMR, the density is calculated from the %SiNMR data through the number of silicon atoms that have attached alkyl chains. The CP/MAS W i spectrum of underivatizedsilica gel (acid washed and cleaned with ultrapure water) is shown in Figure 3a. The three peaks at -90, -100, and -110 ppm have been assigned previously16J7Jgaand are due, respectively, to (-0)2Si(OH)2, (-O)SSi(OH), and (-0)4Si, where (-O),Si indicates n siloxane bonds. These structures are referred to, respectively, as geminal silanols, isolated silanols, and siloxanes. Upon derivatization, three additional resonances emerged in the silane region at -45, -59, and -70 ppm, for horizontally pmoVm2 =
1'
2'
3'
-(O),SiCH,CH,CH,
/I
1,183
3-16
I
1
d& io
30
2
3
4-15
18
17
18
-(0),SiCH,CH,CH,(CH,)12CH,CH,CH,
i0
1,18
PPW
io
i
Flgwe 1. CP/MAS l%NMR spectra of the conventional monomeric phase (top), the mixed horizontally polymerized phase (middle),and the conventional polymeric phase (bottom). All spectra were obtalned with a 5-ms contact time.
o ~ : : : : l : : . . l : : : : : : : : : I : : : : : : : . : I 0 10 20 30 40 50 60
Contact T i m e ( m s e c . )
Flg~~re 2. Peak due to C4-C15 in the l% spectra. The symbolsrepresent
the l%Intensities as a function of contact tlme, and the lines represent the best-fit curve to eq 1. The squares represent the horizontally pdymerlzed mlxed phase, the asterlsks represent the conventlonal polymerlc phase, and the diamonds represent the monomeric phase.
comparisons can be made among these bonded phases, the carbon content was measured by microanalysis and by 13C NMR. The 13C CP/MAS NMR spectra for the three types of bonded phases are shown in Figure 1, and the line assignments are as indiciated, based on previous studies.16J8 Each resolved peak was fit separately to eq 1, and the individual M, values were summed and then divided by the sample weight to obtain the total Mdg of sample. Typical fits of 13C intensity versus contact time for the biggest peak (C4416)are shown in Figure 2. Sinceabsolute carbon content cannot readily be measured by NMR, the totalMdg of sample values were normalized to give a value of 17.0 for the horizontally polymerized phase. This facilitates comparison with the microanalysis results. The values of Mdg of sample and the microanalysis data are listed in Table I, columns A
(26) Claessens, H. A.; De Haan, J. W.; van de Ven, L.J. M.; De Bruyn, P.C.;Cramers, C.A. J . Chromatogr. 1988,436, 345.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, lQQ3 2051
Table 1. Carbon Content and Surface Density (A) (B) w t % c, MJg of sample,o microanal. sample 13c NMR 10.7 monomeric CIS 10.6 polymeric CIS
20.7 17.0 1.6
22.6 17.0 1.8
mixed CS/Cls polymeric Cs
(C) densityb (wnol/m2)
1aC NMR
(E) mol/g of silica: 28SiNMR
1.76 4.82 8.00 1.62
1.52 4.43 8.00 not measd
(D)
moVg of silica:
1.73 5.35 7.98 1.78
*
a Values were normalized to give 17.0 for the mixed Cs/ClS sample. Estimated relative uncertainty is +5%. Density was calculated from % carbon using eq 3. c Values for both W and 29si NMR were normalized independently to a value of 8.00for the mixed CS/Clsphase. Estimated uncertainty is +5%.
a
A
b
C 0
10
20
30
40
50
60
0
Contact T i m e (msec.)
Figure 4. Sllanol resonance of underhratlzed slllca. The asterlsks represent the *OS1intensity as a functbn of contact time, and the solld llne represents the best-fit curve to eq 1.
M
d
The 2QSiNMR data relating to the composition of the phase are given in Table 11. The relative amounts of reagent geminal silanols, reagent isolated silanols, and reagent siloxanes, obtained from the Ma values of the appropriate resonances, are listed in columna A-C of Table 11. These values were normalized by the same constant, which was chosen to give a sum of 8.00 for the case of the mixed horizontally polymerized phase. The data in Table I1 show that the horizontally polymerized phase has a considerable reagent siloxane signal (column C), an even larger reagent isolated silanol signal (column B), and a small reagent geminal silanol (column A). The table shows that the proportion of these three types of reagent groups is quite similar to that of the conventionally polymerized phase. To determine how frequently the reagent is attached to the substrate, the two types of substrate silanols, but not the siloxane, are quantitated in this work. The siloxane signal is not relevant and does not cross polarize from the same proton pool, evidenced by the dramatically longer proton TI, relaxation time obtained when the siloxane signal is fit to eq 1. This agrees with the conclusions of Chuang et al.2' To illustrate the high quality of the fit to eq 1 for the isolated substrate silanols, the dependence of the intensity on the contact time is shown in Figure 5. As in the 13C NMR study, correcting the sample weight for the contribution from the silanizing agent, the density of unreacted silanols can be expressed as Mdg of silica. Since each geminal silanol has two attached hydroxyl groups, the total number of moles of unreacted hydroxyls is given by S(geminalsilanolMa) + (lone silanol Mo). The Mdg of silica for underivatized silica gel is the first entry in column D of Table 11. The Mdg values for remaining silanols of the derivatized silica are also listed in column D of Table 11, where in each case the amount of
1 , , I w - - l - - , - - - T
20
0
-20
-40
-60
ww
-80
-100
-180
-140
Figure3. CPIMAS spectra of (a)underhratkedslllca gel, (b) horizontally polymerlred mixed phase, (c) conventional polymerlc phase, and (d) conventlonal monomeric phase. All spectra were obtained wRh a contact tlme of 5 ms.
polymerized and conventionalpolymeric phases (Figure3b,c). These correspond, respectively, to the structures (-0)Si(C,)(OH)z, (-0)&3i(C,)OH, and (-0)&iC,, where C, is the alkyl chain. These structures are referred to, respectively, as reagent geminal silanoh, reagent isolated silanoh, and reagent siloxanes. For the monomeric phase, a silane resonance at 16 ppm emerges in Figure 3d due to the structure (-0)Si(C,)(CH&. The substrate silanol signals decrease upon silanization, as shown in Figure 3b-d, for all derivatized samples. For quantitation of the amount of silanizing agent bonded to the surface, the appropriate resonances, which are at -45, -59, and -70 ppm, were used for the polymeric surfaces, and the resonance at 16 ppm was used for the monomeric surface. Figure 4 displays a typical fit of intensity versus contact time to eq 1. Since there is one silicon atom per chain in each case, the total Mdg of silica gel is proportional to the number of moles of derivatized functional groups. To aid in comparison with the density of silanizing agent obtained from microanalysis and 13C NMR, the values of MJg of silica from the mSi NMR are normalized to a value of 8.0 for the horizontally polymerized mixed phase. These values of Mdg of silica are shown in column E of Table I. They are in excellent agreement with the l3C NMR values of Mdg of silica, thus establishing that the NMR data are quantitative from sample to sample.
(27) Chuang, I.& Kinney, D. R.; Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Phys. Chem. 1992,96,4027-34.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUQUST 1, 1993
Table 11.
&/g
of Silica Calculated from the Wi NMR Data for Reacted and Unreected Surfam Silicon Smcies
sample
underivatized silica gel monomeric CIS
polymeric CIS mixed ca/cl8
a Eetimated absolute
0.31 0.66
2.43 4.35
1.69 2.99
0 1.31 3.12 2.30
8.5 7.19 5.38 6.20
0 1.52 4.43 8.00
*
error is k0.5. Estimated relative uncertainty ia +5%.
C o n t a c t T i m e Lmsec.)
Figure 5. loSl intensities as a function of contact time. The lines representthe best-fit curve to eq 1. The squares represent the reagent sitend resonance of the horizontally polymerized mixed phase, the asterisks represent reagent siianol resonance of the conventional polymeric phase, and the diamonds represent the reagent silicon resonance in the monomeric phase.
remaining silanol is smaller than that for the underivatized silica, as expected. The amount of reacted silanol is then calculated as the difference between the initial silanol content for the underivatized sample and the amount remaining unreacted for the derivatized samples. These quantities are listed in column E of Table 11. In each case, the M,Jg signal was normalized by the same factor that waa used for columns
A-C. The silanol coverage for the underivatized silica sample is expectedto be invariably 8.0 pmol/m2,2asm which is somewhat less than the 8.6 pmollm2 (Table 11,column D)obtained from mSi NMR. This discrepancy might be due to the presence of silanols,which are not on the surface but are still observed in the NMR experiment. The discrepancyis not much larger than the experimental error. 3. Structural Interpretation. The mole ratio of bonded groups to reacted silanolsis revealed by comparison of columns E and F of Table 11. For the monomeric case, the ratio of bonded groups to reacted substrate silanols (column Fcolumn E) is 1:l within experimental error (1.2f 0.4),as expected. However, Table I1 reveals the surprising result that conventional polymerization provides a ratio that is close to 1:1(1.4 f 0.2)of bonded groups to reacted substrate silanols. This clashes with the notion of vertical polymerization,which the process is sometimes called. Vertical polymerization,where the reagent siloxane linkages are out of the surface plane, would result in a ratio of at least 2:l. For the horizontally polymerized phase, the ratio of bonded groups to reacted substrate silanols is much higher, approximately 3.5. Since attachment to the substrate is one contribution to the termination of a polymeric chain, less attachment suggests ~
(28) Tuel, A,; Hommel, H.; Legrand, A. P., Kovata, E. kngmuir 1990, 6, 770. (29) Zhuravlev, L. T. Lagmuir 1987, 3, 316.
more extensive polymerization for the horizontally polymerized phase. Table I1 shows that the proportions of isolated reagent silanols,geminal reagent silanols, and reagent siloxane bonds are similar for the conventionaland horizontally polymerized phases, but that the proportions of reagent attached to the substrate are quite different. These data can be combined to describe the average oligomer size. The next three paragraphs detail how this is done, and the structural interpretation follows. The composition of the bonded phase can be determined from the data listed in Table 11. Normalizing column A to the total amount of attached reagent, column F, gives the fractional amount of reagent having two OH groups. This is denoted as F[OR(OH)zI, where the terms in brackets refer to the three functional groups attached to the silicon atom. OR denotes another reagent-bearing silicon atom attached through a siloxane bond, OH denotes a reagent silanol, and OX denotes attachment to the silica substrate. The value of F[OR(OH)2]is given in column A of Table 111. The fraction of attached reagent groups that has a single attached OH group is comprised of F[(OR)20Hl + F[OROHOXl and is given in column B of Table 111. Finally, the fraction of reagent silicon atoms involved only in siloxane bonding, which corresponds to F[(OR)31 + F[(OR)20Xl, is given in column C of Table 111. The fraction of reagent bound to the substrate, F[(OR)20Xl + F[OROHOXl, is calculated by normalizing column D to column F, both in Table 11,and is listed in column D of Table 111. The formulas of Table I11 neglect double attachment to the silicon Substrate; i.e., the speciesF[OH(OX)~land F[OR(OX)2] are ignored. This assumptionisjustified because there is less than 1:l attachment of reagent to substrate in each case. The second assumption made in this analysis is that F[(OR)2OXl= F[OROHOXl. The justification is that there is no chemical preference for attachment to the surface by reagent silicon atoms that have two reagent groups attached, compared to those that have one reagent group plus a reagent silanol. The results presented here are quite insensitive to the relative values of F[(OR)20Xl and F[OROHOXl; therefore, this assumption is more of a convenience. With these two assumptions,the individual fractions of attached reagent groups are determined. For the conventional polymeric phase, F[(OR)s] = 0.03, which indicates little branching of polymerized phase. F[(OR)zOH] and F[(OR)20Xl combine to account for 0.55 of the attached material, and these species are the ones that comprise the nonterminating groups of linear segmenta of polymers. The remaining fraction, 0.42, is composed of contributions from terminating groups, F[OR(OH)zI and F[OROHOXl. These suggest structures that are short linear chains with only occasionaly branching. One can derive an expression for the average oligomer size, N, by the stoichiometry of branched chains: the number of terminating functional groups in the average oligomer is 2 (for the two ends of the chain) plus 1 for each of the branches.
no. of terminating functional groups = 2 + NF[(OR),I (4)
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Table 111. Normalized %i NMR Spectral Data. (A) (B)F[(OR)zOHI +
sample polymeric Cle mixed Ca/Cl8 a
(C)F[(OR)zOXl+
(D)F[OROHOXl t
F[OR(OH)zI
F[OROHOX]
F[(OR)sl
F[(OR)zOXI
0.07
0.55 0.55
0.38 0.37
0.70 0.29
0.08
The data of Table 11, columns A-D, are normalized by column F to give the respective columns A-D below.
The number of terminating groups is given as follows.
Table IV. NMR 'HRelaxation Times.
s no. of terminating functional g r ~ u p = NF[OROHOXl + NF[OR(OH),I (5) Substituting eq 5 into eq 4, and rearranging, gives an expression for N.
N = 2/(F[OROHOXl + F[OR(OH)J - F[(OR)J) (6) For the conventional polymeric phase, N is calculated to be 5. This number is rather insensitive to experimental error: its error is less than 1unit. The amount of branching is negligible,asF[(OR)J = 0.03. The conventional polymeric phase is thus composed of linear oligomers of 5 units, on the average. For the self-assembledC1$C13 phase, Table I11shows that there is significantly more branching, as F[(OR)3] = 0.22. The computed N from eq 5 is so large that its specific value is meaningless. In essence, the number of terminating groups equals the number of branching groups within experimental error; therefore, the average polymer is not a simple branched oligomer. Instead, the self-assembled monolayer is a significantly cross-linked polymer in the two dimensions. The linear oligomerization of the conventional polymeric phase may be responsible for its unique selectivity for planar compounds. Sander and Wise23 have shown that planar compounds are more strongly retained than nonplanar compounds, relative to their retention times for monomeric columns. It is reasonable to speculate that the spaces among the linear oligomers would more easily accommodate planar solutes than nonplanar solutes. The high density of the horizontally polymerized phase, which occurs through significant cross-linking, is presumably the origin of its high hydrolytic stability. One would expect the reduction of defect sites, particularly reagent silanols and reagent geminal silanols, to provide further enhancement of the hydrolytic stability especially at high pH, where base attacks the silica substrate. The results show that 29SiNMR would be a valuable method for quantitating these defect sites in support of new synthetic procedures. The W i NMR signal that corresponds to geminal substrate silanols, shown in Figure 3, exhibits unusual behavior. This signal is shifted significantly downfield to -86 ppm in the '%i NMR spectra of the two polymeric phases relative to the shift of -93 ppm observed for the unreacted silica gel and the monomeric phase (Figure 3). Furthermore, the cross-polarization rate of this signal is over 3 times as large for the polymericsamples as for the unreacted or monomericsamples. These observations suggest that the geometry of the substrate HO-Si-OH bond is distorted by the polymeric layer of bonded groups on the surface and that this distortion decreases the average Si-H distance (which increases the cross-polarization efficiency). This might be caused either by steric interaction with the polymer above the surface or by restructuring of the silica surface. 4. Relaxation Times. One structural aspect of mixed horizontal polymerization that is not answerable by the quantitative results discussed above is whether or not the long and short chains are randomly interspersed. In the synthesis, n-heptane is used as the solvent to avoid a thermodynamic driving force for the long alkyl chains to
sample
underivatized silica gel monomeric Cle
polymeric C18
mixed CS/Cle a
'HI'?
(8)
0.25 0.54 0.46 0.54
alkyl lH TI,(ms) 49 19 44
OH lH TI,(me) unreact. ligand silanol 64 13 24
84 56 25 35
Relative uncertainty is &5% for all measurements.
segregatefrom the short chains. Significant segregationwould cause the alkyl chain dynamics to be very slow because the chains would be packed at a high density. It has been shown that the C18chains of conventional polymeric phases have slower dynamics than those of monomeric phases, owing to the higher density for the polymeric phase.30 If segregated, the C18chains in the CdCla mixed phase would have motional properties similar to those associated with the polymeric Cls phase. By contrast, if the C3 and CU chains were interspersed, then the chain dynamics for the horizontally polymerized phase would resemble the dynamics of the monomeric phase. That is, the C18 chains would behave more like isolated chains and would not be significantly restricted by the presence of short C3 chains. Molecular mobility can be probed by relaxation time measurements. Detailed descriptions of the relationship between molecular motion and relaxation times have been given el~ewhere.3~ In brief, TI relaxation is primarily sensitive to fast motion in the megahertz range, ,1'2 relaxation is primarily sensitive to motion in the kilohertz range, and line shapes are sensitive to both the time scales of the motions and the amount of inhomogeneous broadening. The relaxation times are summarized in Table IV. The 21' values are similar for all three types of chromatographic surfaces, presumably because spin diffusion averages out any differences. However, the 2'lp values do differ from one type of phase to another. Spin diffusion, although rapid on the Tltime scale, is not sufficiently fast to average out the 21', values. This is evidenced by the fact that the alkyl lH ,1'2 values, obtained from fits of 13C intensities to eq 1,are different for carbons at the ends of the chain compared to carbons in the middle of the chain. For the protons in the middle of the chain (C4-C16), the alkyl 'H TI,values for the horizontally polymerized mixed phase are quite similar to the T I ,values measured for the monomeric phase. By contrast, the conventionally polymerized phase has much shorter alkyl TI, values, which indicates that the mobility of these CISchains is significantly reduced, presumably due to the high density of long chains. These results show that the mobilities of the alkyl chains in the very dense mixed phase are comparable to that observed in the less dense monomeric phase. This can only occur if the short C3 chains are interspersed with the longer C18 chains. Furthermore, the hydroxyl lH T I ,values, obtained from fits of 29Si intensities to eq 1,also are shorter for the conventional polymeric phase than for the horizontally polymerized phase for both isolated substrate silanols and (30)McNally, M. E.; Rogers, L. B. J. Chromatogr. 1985, 331, 23. (31)Zeigler, R. C.; Maciel, C. E. J.Phys. Chem. 1991, 95,7345.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUQUST 1, 1993
hydroxyls near bonded groups. Hence, the inotion of the substrate hydroxyl groups relative to their environment is also more restricted in the conventional polymeric phase than in the mixed phase. Finally, the I3C line widths are broadest in the conventional polymeric phase, as shown in Figure 1. The horizontally polymerized phase has a peak width of 1.1 ppm, which is much closer to that of the monomeric phase (0.84 ppm) than it is to the conventional polymeric phase (1.8 ppm). It is concluded from the relaxation data and line shapes that the C3 and CU groups are quite randomly dispersed in the horizontally polymerized phase.
CONCLUSIONS The results show that the horizontally polymerized monolayer is both dense and extensively polymerized and that the Cle functional groups are randomly interspersed. In the extreme limit of perfect horizontal crystallization, one would have only reagent siloxane bonds and no substrate silanols consumed. The actual phase has frequent reagent silanols
and occasional surface attachment. This represents an opportunity for improvement. Presumably, the closer one approaches horizontal crystallization,the better the hydrolytic stability toward base would become, because bases attack the silica substrate. The measurements made in this work show that efforts to improve the extent of horizontal polymerization can be evaluated quantitatively by lsC and 29SiCP/MAS NMR spectroscopy.
ACKNOWLEDGMENT This work was supported by the Department of Energy under Contract DE FG02-91ER14187. The silica gel was obtained from the reference collection maintained at Duke University by Professor Charles H. Lochmuller, originally provided by a gift from Whatman Chemical Separations Division. RECEIVED for review February 5, 1993. Accepted April 26, 1993.
