Anal. Chem. 1992, 64, 2783-2786
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Horizontal Polymerization of Mixed Trifunctional Silanes on Silica: A Potential Chromatographic Stationary Phase Mary J. Wirth' and Hafeez 0. Fatunmbi Department of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716
A new deslgn for surface bonding of monolayers, applicable to chromatographicstatlonary phases, Isreported. Thb design b shown to combine the superior hydrolytlc stablllty of senassembled monolayers with the selectlvlty and reproduclblllty of monomerlc stationary phases. While the new methodology Is applkabk to any type of functlonalky, it Is demonstrated here for C18on a flat dlka surface. The phases are bonded b Y U d n g W W a mbrtve af C H S ( W ) I S ~ and CHs(CHz)&ICIs In anhydrous mhexadecane Is reacted with rlllca. The reagents and dllca are dry, except for a surface monolayer of water on the slllca, to ensure that the polymerlzatlon reaction occurs only at the surface. The Cs group act as spacers to control the density of the C18groups. The origin of the higher stablllty Is the higher density near the surface. The Cr bonding dendty can, In principle, be varied from 0 to 7 pmd/mZby varying the proportbnaof the reagents. For a C18density comparableto that of monomerk Clo phases used In chromatography, it Is shown by FTIR spectroscopy that the horizontallypolymerizedphasehas remarkably better hydrolytlc stablllty than the conventional monomeric phase for both acid and base hydrolyds.
INTRODUCTION Chromatographic bonded phases undergo hydrolysis at a rate sufficiently high as to hamper chromatographic applications as well as spectroscopic studies of chromatographic surfaces. For example, the C18 surface coverage decreases even at neutral pH, as shown by both increased base retention and decreased hydrocarbon signal in FTIR spectroscopy.1 Hydrolysis is catalyzed both by acid and base.2 Improvement in hydrolyticstability at low pH is important to the separation of basic compounds such as pharmaceuticals, agriculturalchemicals, and proteins and peptides. Separation of these types of samples at pH < 4 resulta in better peak shape and retention behavior, presumably because the surface silanololsare protonated.3 Improvement in hydrolyticstability at high pH, which is important in capillary zone electrophoresis and protein chromatography, has been achieved by forming a direct silicon-carbon bond at the surface.4 Alternatively, polymeric C18 phases have improved hydrolytic stabilities at low pH;6 however, the selectivity differs from that of monomeric phases.6 Polymeric phases also have the reputation of being irreproducible. A hydrolytically stable monomeric phase of optimal surface coverage would be valuable for chromatographic applications. To achieve improved stability, a principle can be learned from advances in self-assembled alkyl monolayers. A very
* Corresponding author.
(1) Boudreau, S. P.; Cooper, W. T. Anal. Chem. 1989, 61, 41. (2) Unger, K. K. Porous Silica; Elsevier: New York, 1979; Chapter 3.
(3) Bennett, H. J. P.; Hudson, A. M.; McMartin, C.; Purdon, G. E. Biochem. J. 1977,168,9. (4) Cobb, K. A.; Dolnik, V.; Novotny, M. Anal. Chem. 1990,62,2478. (5) Kirkland, J. J.; Glajch, J. L.; Farlee, R. D. Anal. Chem. 1989,62,
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(6) Sanders, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504.
dense Cle monolayer forms a virtually solid barrier to the substrate upon reacting CH3(CH2)17SiCls in anhydrous in n-hexadecane with humidified silica.7-11 The intrinsic monolayer of water known to be adsorbed to silica12 provides the reagent water needed for the hydrolysis. The term ''horizontal polymerization" has been used to convey the notion that a monolayer film is achieved when anhydrous reagents are used because the reaction is restricted to the surface. Polymeric stationary phases used in chromatography, by contrast, are made by using deliberately wet solventa to allow for some polymerizationbefore the reagents attach to the surface. Since the alkylsiloxane polymers from the solution would likely attach to the surface at only one point as the reaction proceeds, this process is commonly referred to asverticalpolymerization. The carbon content of polymeric stationary phases is higher than that of monomeric phases, but one can expect from the synthesis that the density at the surface is much lower than that for the horizontally polymerized monolayer. For horizontal polymerization, Moaz and Sagiv718 showed by FTIR that a nearly solid layer of alkyl chains in an alltrans conformation quickly forms on silica in contact with CH3(CH2)17SiC13 in anhydrous hexadecane. The Whitesides group has shown by ellipsometry that this dense c18 layer on silica has a thickness of 20 A and, by X-ray reflection: that the density is 21 f 3 AVchain (7.9 f 1.1pmol/m2).10 Kessel and Granick confiimed this by showing that the spectra of the c18self-assembled monolayer is virtually identical to that of a cadmium arachidate Langmuir-Blodgett f i i compressed to a density of approximately 22 &/chain (7.6 pmol/m2).11 The Whitesides group also reported that the dense f i b was only slowly removed by concentrated HN03, but the decomposition was nonuniform. The horizontally polymerized c18 surface thus achieves excellent hydrolytic stability; however, it is much too dense for chromatographic applications. Typical surface coveragesin chromatographyare on the order of 55 A2/chain(3pmol/m2). A modification of the horizontal polymerization scheme is needed to achieve optimal chromatographic retention and hydrolytic stability simultaneously. The purpose of thiswork is to investigatewhether horizontal polymerization can be applied to the design of a more hydrolytically stable stationary phase. The essential idea is that a mixture of short- and long-chain silanizing reagents, having the Cl3Si-R functionality, would result in the surface structure schematically illustrated in Figure 1. The high density and multiple bonding immediately near the surface would retard hydrolysis, while the lower density of the remainder of the long chains would allow for analyte retention. Furthermore, Sentell and Dorsey have shown that the (7) Moaz, R.; Sagiv, J. J. Colloid. Interface Sei. 1984,100, 465. (8) Moaz, R.; Sagiv, J. Langmuir 1987, 3, 1034. (9) Wasserman, S. R.; Tao,Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
(10)Wasserman,S. R.;Whitesides,G. M.;Tidswell,I. M.; Ocko,B. M.; Pershan, P. S.; Axe, J. D. J.Am. Chem. SOC.1989,111,5852. (11) Kessel, C. R.; Granick, S. Langmuir 1991, 7 , 532. (12) Gee, M. L.; Healy, T.W.; White, L. R. J. Colloid. Interface Sci. 1990,140,450.
0003-2700/92/0384-2783$03.00/0 0 1992 American Chemical Society
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Mobile phase
1 ,Silica substrate 1 Illustration of mixed, horizontal polymerization scheme, where a mixture of long and short alkyttrichlorosllanesare horlzontally polymerlzed. The hlgh density near the surface provides hydrolytic stability whlb the lower density away from the surface allows for Flgure 1.
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conventlonal chromatographlc selectivity.
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partition coefficientpasses through a maximum a t this surface coverage and that additional selectivity can be obtained at higher surface coverages.13 The scheme of Figure 1potentially allows these higher coverages to be readily achieved. In this work, mixtures of CH&H2)1,SiC13 and CH&H2)2SiC13 are used to prepare the surface, and the hydrolytic stabilities of these surfaces are tested.
EXPERIMENTAL SECTION The method of horizontal polymerization differs from previous reports in three ways. First, silica plates are used, rather than the native oxide of silicon. The surface of the silica plates is believed to be the same as the native oxide and silica gel, because it is completely wetted by water. Second, special care was taken to avoid contamination of the silica surface during monolayer self-assembly, as detailed below. Third, mixed monolayers of long and very short chains are prepared. The mixed horizontal polymerization of functional group of interest alongwith spacers on silica has not previously been reported." The silica plates were obtained from Quartz Products (Georgetown, DE). Atomic force microscopy has revealed the surfaces to be flat on the molecular scale.16 The plates were cleaned in boiling, concentrated nitric acid, rinsed in ultrapure water, and dried under nitrogen. The nitrogen itself was filtered by a silica column. The plates were then allowed to equilibrate with the vapor of ultrapure water for 60 min under the filtered nitrogen atmosphere. (It has since been determined that less than 1min is required.) It was confirmed that a plate is completely wetted by water after this procedure. The plates were then put in contact with a reagent mixture of 0.59 mole fraction of CH&H~d17SiC13 and 0.41 mole fractionof CH3(CH2)2SiC13 dissolved in anhydrous n-hexadecane. The concentrations were in great excess of the expected amount of materialneeded for complete reaction. These silanizing reagents were obtained from Hds-America. The n-hexadecane (Aldrich) was made to be anhydrous by passage through a column containing a layer of dry alumina followed by a layer of dry silica, in a glovebox. A period of 24 h at room temperature was allowed for the self-assembled monolayer to form. After the self-assembly,the plates were rinsed thoroughly in hexane, acetone, and methanol and were then stored under nitrogen. Conventional monomeric CIS surfaces were prepared by cleaning the silica plates the same way, but then refluxing them for 3 h in a solution of CH3(CH2)1,Si(CH&C1 in n-hexadecane, with n-butylamine as the catalyst. These surfaces were endcapped with trimethylchlorosilane. This was done to make the surface as hydrophobic as possible to retard the hydrolysis of the conventional monomeric phase. The FTIR spectra were obtained using a BOMEM MBl00 at 4-cm-l resolution and 200 scans. The baselines in the spectra were corrected. The plates were oriented at normal incidence withrespectto the beam. The thickplatesresultedinnoapparent (13)Sentell, K.B.; Dorsey, J. G. A n d . Chem. 1989,61,930. (14)US.Pat. Appl. 900,215,June 17,1992. (15)Burbage, J. D.; Wirth, M. J. J.Phys. Chem. 1992,96,5943.
2950
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Wavenumber spectrum of the pure, horlzontally polymerlzed Cte surface on the silica plate. Flgurr 2. FTIR
interference fringes in the FTIRspectra. The plateswere cleaned with acetone and dried with nitrogen before the spectra were obtained.
RESULTS AND DISCUSSION 1. Horizontal Polymerization. The FTIR spectrum of a horizontally polymerized, pure CISsurface, shown in Figure 2,was obtained to confirm that the experimental procedure provides the same quality of horizontal polymerization as has been reported in the literature. The spectrum is virtually identical in both absorbance (0.003) and spectral width (15 cm-1) to those reported by Moa and Sagiv7S and Kessel and Granick," indicating a surface coverage on the order of 22-25 AVchain (7.6-6.7 pmollm2). The major peaks are the methylene symmetric and asymmetric stretches a t 2850and 2917 cm-l, respectively,lB and these are narrowed significantly compared to solution-phase spectra. This narrowing has been attributed to significant all-trans conformation due to the high density. The smaller peak at 2959 cm-l is assigned to the asymmetric methyl stretch.16 Finally, this type of plate was found to bead water uniformly after boiling in concentrated nitric acid for a period of 1 h. The improvement in uniformity is attributed to the clean conditions that reduce large defects. It is concluded that the self-assemblytechnique employed in this work provides the expected horizontally polymerized monolayer. Comparable horizontal polymerization for the mixed alkyltrichlorosilane reagante is thus expected from the same procedure. The FTIR spectrum for the horizontally polymerized surface of mixed CISand C3 composition is shown in Figure 3a, and the FTIR spectrum of the end-capped monomeric surface is shown in Figure 3b. A visual comparison of these spectra shows that the ratio of CH3 to CH2 groups is qualitatively similar to that of the monomeric surface. The similarity of the spectra for the monomeric and mixed surfaces corresponds to similar C18 coverage. The spectrum for the mixed, horizontally polymerized surface has broader bands than the spectrum for the pure C18, horizontally polymerized surface of Figure 2. This is expected because the density is significantly lower owing to the spacing by the C3 groups. On the basis of the areas of the asymmetric methyl stretch and under the assumption that the coverage of the pure C18 monolayer of Figure 1 is 6.7 pmoVm2, the coverages of both the conventional monomeric and the mixed horizontally polymerized monolayers are calculated from the data of Figure (16) Wiberly, S.E.; Bunce, S. C.; Bauer, W. H. Anal. Chem. 1960,32, 217.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, l Q Q 2 2785
1
Wavenumber F W n 9. Comparison of FTIR spectra for (a)the mixed, horlzontaily polymerlted surface and (b) the conventional enclapped monomeric
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Wavenumber Flgurr 5. FTIR spectrum of the mixed, horizontally polymerlredphase before (a)and after (b) boiling in concentrated nitric acM for 25 min.
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Wavenumber Flgurr 4. FTIR spectrum of the conventional monomeric endcepped phase before (a)and after (b) boiilng in concentrated nitric acM for 10 min. 2 to be 3.2 and 2.8pmol/m2,respectively. These are coverages typically used in chromatography. 2. Acid Hydrolysis Studies. The mixed, horizontally polymerized surface and the end-capped, conventional monomeric surface were both boiled in concentrated nitric acid for 10 min. When these two plates were removed from the nitric acid solution, water beaded up on the mixed, horizontally polymerized plate, while a uniform layer of water clung to the monomeric plate. The mixed phase was further boiled in the concentrated nitric acid for an additional 15 min. For the monomeric phase, the FTIR spectrum confirmed that the hydrocarbon monolayer was completely removed, as illustrated in Figure 4. For the mixed horizontally polymerized phase, the FTIR spectrum changed little, as shown in Figure 5: after the 26 min of boiling in Concentrated nitric acid, the absorbance decreased negligibly. These results demonstrate that the mixed, horizontally polymerized surface has dramatically improved stability to acid hydrolysis. 3. Base Hydrolysis Studies. An end-capped, mixed, horizontally polymerized surface and an end-capped, conventional monomeric surface were boiled for a period of 3 h in solutions of pH 12, with 5 % n-propanol added. This solution composition at 100 O C provides a very severe test of stability toward base hydrolysis. When removed from the basic solution, the conventional monomeric surface was completely wetted by water, while the mixed, horizontally polymerized surface beaded water. The FTIR spectra for
3000
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Wavenumber Flgurr 8. FTIR spectrum of the conventlonal monomeric endcapped phase before (a) and after (b) boillng for 3 h in a solution of 5 % n-propanol in water at pH 12.
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Flguro 7. FTIR spectrum of the mixed, horlzontally polymerizedphase before (a)and after (b) bolllng for 3 h In a solution of 5 % +propend in water at pH 12.
the conventionaland mixed, horizontallypolymerizedsurfaces are summarized in Figures 6 and 7,respectively. Figure 6 c o n f i i s that the conventional monomeric surface was completely removed. Figure 7 shows that mixed, horizontally polymerized surface suffered approximately 50 96 hydrolysis over this 3-h boiling period. These results indicate that there
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is also a dramatic improvement in stability to base hydrolysis using the new bonding scheme.
CONCLUSIONS A hydrolytically stable monolayer surface can be made by covalently bonding a functional group of interest, diluted with a short-chain silanizing agent, to a silica surface. Both the reagents have trifunctional silanes, and horizontal polymerization occurs when only surface water is present. The new phase is considered pseudomonomeric because its business end is essentially the same as the monomeric phase, yet it is made from polymerizing reagents. The same bonding scheme is applicable to functional groups other than C18, such as alkylamino,alkylhydroxyl,etc. The bonding scheme could be applied to the derivatization of capillary columns for capillary zone electrophoresis, where the surface reproducibility and hydrolytic stability would be improved. (17) Silbenan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647.
The very impressive hydrolytic stability has been demonstrated using silica substrates that are flat. These phases are promising for high-performance chromatographic application if the bonding scheme is effective on rough silica surfaces, Horizontal polymerization is believed to work equally well on rough surfaces,17and experimente are underway to evaluate the improved stability of mixed, horizontally polymerized surfaces on chromatographic silica gel.
ACKNOWLEDGMENT We thank Ms.Lois Weyer at Hercules, Inc. for use of the FTIR spectrometer. This work was supported by the Department of Energy (Grant No. DE-FG02-91ER14187)and the National Science Foundation (Grant No. CHE-9113544). RECEIVED for review July 1, 1992. Accepted August 20, 1992. Registry No. PrSiCla, 141-57-1;vitreous silica, 60676-86-0.
