Microscopic Structure of Chromatographic Interfaces as Studied by

Chapter 18

Microscopic Structure of Chromatographic Interfaces as Studied by Sum-Frequency Generation Spectroscopy Downloaded by COLUMBIA UNIV on September 10, 2012 | http://pubs.acs.org Publication Date: December 18, 2000 | doi: 10.1021/bk-2001-0781.ch018

Lauren K. Wolf, Yarjing J. Yang, Rebecca L. Pizzolatto, and Marie C. Messmer Department of Chemistry and Zettlemoyer Center for Surface Studies, Lehigh University, 7 Asa Drive, Bethlehem, PA 18015

Model chromatographic interfaces were examined using sum -frequency generation spectroscopy (SFG) in order to study the effect of various solvents on the structure and conformation of the stationary phase. Monolayers formed from m i x e d alkyltrichlorosilanes on fused silica were examined in contact with air, acetonitrile, isooctane, and water. Results show that monolayers of 100% C composition show little disorder in the alkyl chains for all the solvents examined, though a very distinct solvent shift is apparent in the methyl symmetric stretch. When mixed composition (C and C ) monolayers were examined, significant disorder was induced in the alkyl chains for all the solvents. However, the largest change occurred when the monolayer was in contact with water. These results support a collapsed structure model for the mixed monolayer in contact with water. 18

18

1

Introduction Chromatographic interfaces are complex systems that require further exploration before a complete description of chromatographic separation can emerge. Stationary © 2001 American Chemical Society In Interfacial Properties on the Submicrometer Scale; Frommer, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

293

phase properties are not always well understood because of contributions from various factors, such as residual surface silanols. In addition, the importance of certain aspects of stationary phase structure such as alkyl chain density, solvent penetration, and stationary phase ordering, are still a broad area of investigation. Recently, it has been shown that stationary phases composed of long (Ci ) and short (Ci) alkylsiloxane chains significantly reduce the "tailing" effect in reversephase chromatograms (1,2). This "tailing" effect is often caused by the strong acidbase interaction of an analyte with exposed silanol groups on the surface of a silica support. The mixed monolayer stationary phases reduce this interaction by covering excess silanols with methyl spacers (Ci groups). In addition, the Ci :Ci monolayers provide lower C i chain density at the outer layer of the monolayer surface, which increases the partitioning area o f the stationary phase, and improves the chromatographic separation. Recent theoretical calculations have shown the importance of solvent phase heterogeneity near the interfacial region in mixed solvent systems (3). Thermodynamic measurements have also shown that using an acetonitrile-water mixture changes the entropy of solute "transfer" with a changing acetonitrile mole fraction (4). This change is attributed to the high degree of stationary phase order that forms at high acetonitrile concentrations. It is believed that the ordered stationary phase can reduce the entropy of transfer to an extent that retention becomes enthalpically driven (5). By causing substantial order in the substrate, the entropie cost is equal for adsorption and desorption. To determine the role of stationary phase structure in the retention process, it is of primary importance to understand its structure in various solvent environments. Studies of the interphase region using deuterium N M R have shown that it is the mobile phase composition that determines the structure of the stationary phase (6). Results have shown that water does not associate strongly with the alkyl chains. However, acetonitrile can associate strongly, even at low mole fractions because of the microheterogeneous environments that exist for the binary mixture. Previous in situ studies of the alkyl stationary phases using surface enhanced Raman spectroscopy have also concluded that little conformational change occurs with the addition of a polar solvent to the interface (7). To further explore the nature of chromatographic interfaces in situ, we examine model chromatographic systems composed of 100% C i and mixed (Ci and Ci) monolayers in several solvent environments using sum-frequency generation spectroscopy. Sum-frequency generation (SFG) spectroscopy is a powerful surface analysis tool that has proven to be invaluable in the elucidation of molecular structure at many interfaces obscured by adjacent bulk phases. The forbidden nature of the process in isotropic media within the electric dipole approximation permits this technique to be used to examine the structure and properties of the minute interfacial region between two isotropic materials. Interfaces that have been successfully explored include those at gas-liquid, liquid-liquid, liquid-solid, and gas-solid interfaces (8). 8

8

Downloaded by COLUMBIA UNIV on September 10, 2012 | http://pubs.acs.org Publication Date: December 18, 2000 | doi: 10.1021/bk-2001-0781.ch018

8

8

In Interfacial Properties on the Submicrometer Scale; Frommer, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

8

Experimental Section


Reagents Octadecyltrichlorosilane (OTS, purity 95%) and methyltrichlorosilane ( M T S , purity 99%) were obtained from Aldrich and were used without further purification. Hexadecane (99+%) and carbon tetrachloride (99.9+%) were also used as received from Aldrich. Sulfuric acid and hydrogen peroxide were analytical grade and obtained from E M Science. Acetone and methanol were H P L C and optima grade, respectively, and obtained from Fisher Scientific.

Substrate Preparation Fused silica slides (50 χ 25 χ 1 mm), which were purchased from ChemGlass, Inc., were swabbed and rinsed with methanol followed by distilled water. The slides were then dried under nitrogen and immersed in "piranha solution" (7:3 concentrated H S 0 / 30% H 0 ) for 1 hour. W A R N I N G : Piranha solution should be kept out of contact with oxidizable organic material. It is a highly reactive mixture and severely exothermic during reaction. After exposure to the solution for one hour, the slides were rinsed with large amounts of distilled water and dried with nitrogen. The surface morphology of the bare fused silica slide after piranha treatment was measured by atomic force microscopy (AFM). The rms roughness was found to be -1.67 nm. 2

4

2

2

Monolayer Formation Each freshly cleaned slide was placed in a cell in order to expose only one side of the substrate to reagents. In this way, a monolayer forms on only one side of the fused silica slide, which is necessary for the optical experiments. A mixture of 20 m M Ci :C in hexadecane and carbon tetrachloride (4:1) was then added to the cell. After 30 minutes at room temperature, the slides were rinsed with acetone and methanol, removed from the cell, and rinsed again with copious amounts of the same solvents. Finally, each slide was dried under a stream of nitrogen and stored in sealed vials. Each slide was heated at 70°C for one hour to remove residual solvent prior to sum-frequency generation experiments. 8

3

X-ray Photoelectron Spectroscopic Measurements X-ray photoelectron spectroscopy (XPS) data were acquired with a Scienta E S C A 300 using monochromatic ΑΙ Κ α X-rays. The analysis area of each sample was approximately 0.8 mm , and the pass energy of the detector was 150.0 eV. A 90° take-off angle was employed to measure the spectra. The pressure of the chamber 2


was approximately 5 x 1 0 Torr. Ten scans of the data were taken for carbon, and 5 scans were taken for silicon. A l l spectra were fitted using a Voigt function with a linear background correction using the Scienta software. The integrated peak area was divided by atomic sensitivity factors, which were provided by the Scienta software. Monolayer composition was then calculated for each sample using previously described calculations (9).

Sum-Frequency Spectroscopic Measurements The sum-frequency generation setup is described in detail elsewhere (10). Briefly, a lithium niobate ( L i N b 0 ) optical parametric oscillator is pumped with a Surelite 1 (Continuum) N d : Y A G laser using relay imaging. Tunable infrared light is generated between 2600 - 3200 cm" (3.1 - 3.8 μιη) with a pulse width of 7 ns. The wavelength is calibrated with a polystyrene sample. The visible beam at 532 nm used in the sum-frequency experiment is generated with a potassium dihydrogen phosphate (KDP) crystal. The polarization of the infrared beam is rotated with a Soleil-Babinet compensator. Sum-frequency spectra are obtained by using a total internal reflection geometry as shown in Figure 1 (11-14). The infrared and visible beams are combined through a coupling prism to the fused silica substrate with an index-matching liquid. The generated sum-frequency light is collected and passed through several collimating optics, absorptive, interference and holographic filters, and a Glan-Taylor polarizer. Signal is detected with a photomultiplier tube and then passed to a preamplifier and gated electronics. Each point is the average of 200 laser shots. The sampling size on the surface is determined by the laser spot size which was approximately 200 μιη in diameter. Each spectrum represents spectra taken from several places on the sample.


3

3 +

1

Results and Discussion To compare the effects of solvent on these model chromatographic interfaces, the sum-frequency (SF) spectra of these systems were collected in situ, exposed to one of

Figure 1: Sum-Frequency Generation Experimental Setup


three solvents or air. Figures 2A and 2B show the SF spectra of complete monolayers of 100% Ci8 on the fused silica surface in contact with water, isooctane, and air (A) and acetonitrile (B). The first important feature to note in these spectra is the lack of a methylene group symmetric stretch (-2850 cm" ). This particular vibrational mode is not SF active when a long alkyl chain achieves an all-trans conformation because of the near complete cancellation of the nonlinear polarization on adjacent carbons. As a unique feature of sum-frequency generation spectroscopy, the intensity of this peak has been successfully used to characterize backbone disorder in a variety of systems (75-/7).


1

Figure 2: SF intensity spectra of 100% Cm-modifiedfused silica surfaces taken with ssp polarization (s-polarized SF, s-polarized visible light and p-polarized infrared light). A: (A) water, (Ώ) isooctane, and (·) air, B: acetonitrile. The solid lines are fits to the data using Voigt lineshapes.



Figure 2 also shows a spectral shift of the peaks of the methyl symmetric and antisymmetric stretches. This shift corresponds to an approximate 20 cm" shift going from an air environment to a water environment. Frequently, spectral shifts in infrared spectra of alkyl chains indicate a change in the conformation of the chain backbone (18). This is not the case for these spectra. The observed lack of methylene symmetric stretch in the spectra proves that an a.\\-trans conformation dominates for each of these solvent environments. The spectral shift purely indicates the change to a polar solvent environment, which may act to broaden the vibrational potential well. The methyl groups of these systems are obscured by the methylene groups in Raman and IR spectroscopy, making observation of this spectral shift difficult. This lack of solvent-induced monolayer disorder was also observed by Pemberton and coworkers (7). Pemberton and coworkers used Raman spectroscopy to examine C i alkylsiloxane monolayers on a SiCVAg substrate and found little disruption of the alkyl chains in contact with acetonitrile, as well as water. The results of Carr and Harris (19), however, showed that a C i chromatographic stationary phase assumes a collapsed structure in water. The stationary phase can evolve from this collapsed state with the addition of an organic modifier, such as acetonitrile, to the aqueous solvent. The modifier intercalates into the monolayer, causing an increase in volume, polarity, and alkyl chain order. The lack of any spectral evidence indicating a collapse in the 100% Ci -modified surfaces may result from several differences between our model stationary phases and real chromatographic interphase regions. The first difference may be the lower surface density of alkyl chains due to the irregular surface of the silica gel particles. Although flatter than silica gel surfaces, our fused silica substrates are relatively rough, with an rms roughness of about 1.67 nm. It seems that even with some degree of roughness on the substrate, the packing density may be higher than that found in real systems. This higher packing density prevents sufficient alkyl chain conformational freedom, which is necessary for a chain collapse to occur. To observe this chain collapse in a model system, it is necessaiy to compare the SF spectra of mixed Ci and C composition monolayers in the different solvent environments. These monolayers, with their higher degree of conformational freedom of the alkyl chains, serve as a more representative model of true chromatographic interfaces. The SF spectra comparing 100% C i monolayers and partial C i monolayers are shown in Figure 3 for air and water and in Figure 4 for isooctane and acetonitrile. These spectra were taken by selecting the s-polarized output, and using s-polarized visible and /^-polarized infrared light (ssp combination). Examining Figure 3A, a partially formed monolayer displays a significantly different structure than a full monolayer in air. Appearance of the methylene symmetric stretch peak at 2856 cm" indicates the introduction of gauche conformations into the monolayers with the addition of d as "spacers". In addition, the reduction of the methyl symmetric stretch peak at 2884 cm" for the partial monolayer shows that the methyl group has attained an average orientation that is directed further away from the surface normal than when the surface composition is 100% Ci . The SF spectra obtained with the same polarization combination (ssp) and of the same surface compositions in contact with water are shown in Figure 3B. Comparing Figures 3A and 3B, one can see that the water causes a much different structural response in the stationary phase. We see the appearance of the methylene symmetric stretch, as expected, but we see a much larger change in the methyl symmetric stretch intensity than in air, indicating that a larger amount of chain disorder is induced by 8

8

8

{ 8

8

8

1

1

8


contact with water. Albert and coworkers, while examining mixed C i and Ci monomeric monolayers in contact with high water content solvents, saw a similar reduction of alkyl chain mobility and order with cross-polarization magic angle spinning (CP/MAS) N M R spectroscopy (20).


8

ι

I

2800

I

I

I

2850 2900 2950 Wavenumber (cm )

3000

2850 2900 2950 Wavenumber (cm )

3000

1

2800

1

Figure 3: SF spectra for 100% Cm-modified and partial (40%) C ιπ-modifiedfused silica surfaces taken with ssp polarization (s-polarized SF, s-polarized visible light andp-polarized infrared light). A: in contact with air, B: in contact with water. (·) 100% Cm and(0) 40% C - 60% C modified. The solid lines arefits to the data using Voigt lineshapes. I8

r

A unique trend emerges as we examine acetonitrile and isooctane in contact with the same monolayers. Figures 4A and 4B show these spectra. B y determining the ratio of the intensities of the methyl symmetric stretch OCH3SS) the methylene symmetric stretch (ICH2SS) with ssp polarization, we can get a semi-quantitative comparison of the amounts of gauche defects in the alkyl chains. The values for this ratio for 100% C i and partial C i coverages are shown in the first two rows of Table t 0

8

8


I. The change in this ratio going from a 100% C -modified surface to one in which the composition is 40% d and 60% Ci is the most interesting feature. For air, we see a change of a factor of 3.9 ± 1.5 on going to a partial C i composition, for isooctane a factor of 11.2 ± 2.5, for acetonitrile a factor of 14.4 ± 1.9 change, and for water we see a factor of approximately 31 ± 7. A larger order of magnitude change for this ratio indicates a significantly higher proportion of gauche conformations in the alkyl chain backbone. The measurements for acetonitrile were taken with a 60% Cig-modified surface because of sample degradation, but the results still indicate that a significant difference exists for the solvent environments. These results support the theoretical picture of a stationary phase collapse in water. i8

8


8

2800

2850 2900 2950 Wavenumber (cm" )

3000

2850 2900 2950 Wavenumber (cm" )

3000

1

2800

1

Figure 4: SF spectra for 100% Cm-modified and partial Cm-modified fused silica surfaces taken with ssp polarization (s-polarized SF, s-polarized visible light and ppolarized infrared light). A: in contact with isooctane, Β: in contact with acetonitrile. (·) 100% Cm and(Q) 40% Cm- 60% C modifiedfor isooctane data and 60% C 40% C/-modifiedfor acetonitrile data. The solid lines are fits to the data using Voigt lineshapes. r


18

Table I. SF Intensity Ratios for C - and C / C Modified Silica Surfaces 18

% C Composition 1H

ICH3SS/ICH2SS

Isspllsps

Air

r

Isooctane

Acetonitrile

Water

100%

1 0 . 0 ± 2.1

1 0 . 6 ± 1.9

12.4 =L 1.1

40%

2.6 ± 0 . 9

0.95 ± 0 . 1 2

0.86 ± 0 . 0 8 * 0.40 ± 0 . 0 7

100% 40%

4.8 ± 0 . 6 0.91 ± 0 . 1 8

-

14.5 ± 1.4 20 ± 5 1.74±0.19* 0 . 4 4 ± . 0 8

* 60% C Downloaded by COLUMBIA UNIV on September 10, 2012 | http://pubs.acs.org Publication Date: December 18, 2000 | doi: 10.1021/bk-2001-0781.ch018

i 8

18

12.2±2.1

surface composition

Further evidence regarding the structure of the stationary phase in these model systems emerges from polarization studies. In the previous comparison of methyl to methylene symmetric stretch intensities using ssp polarization, the decrease in the numerator indicated a general trend of the methyl group to orient at a larger angle from the surface normal. Using s-polarized infrared light in a second set of experiments, we can confirm this observation by examining the component of the antisymmetric stretch that lies parallel to the surface. Figures 5 A - D show the SF spectra taken with both ssp (s-polarized SF, s-polarized visible light and /^-polarized infrared light) and sps (s-polarized SF, p-polarized visible light and s-polarized infrared light) polarization combinations. In Figure 5A, the methyl symmetric stretch is very strong in the ssp spectrum, and the methyl antisymmetric stretch, which possesses a transition dipole perpendicular to that of the symmetric stretch, shows a very strong peak with sps polarization for the 100% C is-modified surface in air. The lack of these two peaks in the opposite polarization combinations is a very clear indication that the C axis of the methyl group lies primarily along the surface normal. This observation is interesting because of the obvious roughness of our silica substrate. Although the average tilt angle for these types of silica surfaces is approximately 15° (21), it is likely that the polymerization of the siloxanes forms a much flatter surface, away from which the alkyl chains will be directed (9). In Figure 5B, when methyl groups (methylsiloxanes) are introduced onto the surface as spacers, we see a greater intensity in the methyl symmetric stretch with sps polarization and methyl antisymmetric stretch with ssp polarization than for 100% C i monolayers. This spectral result indicates that the methyl "spacers" provide additional conformational freedom to the longer C alkyl chains, allowing the methyl transition dipoles to have an average orientation further from the surface normal. Aside from the spectral shift, the spectra for full monolayers in contact with water and air are very similar (Figure 5C). For partial monolayers (Figure 5D), the change in the methyl symmetric stretch and methyl antisymmetric stretch with changing polarization combinations is much more distinct in water. Therefore, adding the methyl "spacers" to the monolayers seems to allow the alkyl chains enough freedom to change their conformation in response to the contacting solvent. 3 v

8

] 8



Ί

2800

I

I

I

2850 2900 2950 Wavenumber (cm" )

I

3000

1

Figure 5: SF spectra for 100% Cin-modified and partial (40%) Cis-modified fused silica surfaces taken with ssp polarization (s-polarized SF, s-polarized visible light and p-polarized infrared light), and sps polarization (s-polarized SF, p-polarized visible light and s-polarized infrared light). A: 100% in contact with air, B: 40% in contact with air, C: 100% in contact with water, D: 40% in contact with water, using (·) ssp polarization and (O) sps polarization. The solid lines are fits to the data using Voigt lineshapes.


To semi-quantitatively determine the relative change in orientation of the methyl groups on going from a 100% C i to a partially covered ds-monolayer, the ratio of the methyl symmetric stretch intensity with ssp polarization (Issp) to that with sps polarization (Isps) was determined. This ratio is not a direct indication of tilt angle because of the likelihood that a very broad distribution of angles exists. It is a good indicator, however, of the changes that occur with the addition of conformational freedom. This ratio is shown in Table I for two solvents and air. Because of the low refractive index of isooctane, it was not possible to obtain sps spectra for the surface in contact with isooctane. Comparing 100% C surfaces with partial C i surfaces, we see a factor of 5.2 ± 1.2 change for air, a factor of 8.4 ± 1.2 for acetonitrile and a factor of 46 ± 14 change for water. Such a large change in this ratio as caused by the water solvent indicates a very distinct change in the structure of the stationary phase. Further comparison of these ratios can be made by examining Figures 6A and 6B. These graphs show the change in the methyl symmetric stretch to methylene symmetric stretch intensity ratios with ssp light (Figure 6A) and the methyl symmetric stretch with ssp and sps polarizations (Figure 6B). Although the methyl symmetric stretch to methylene symmetric stretch intensity ratio for the water system is similar to that of the acetonitrile and isooctane, the ratio of methyl symmetric stretch with ssp and sps polarization is quite different. This ratio indicates that the methyl groups are orienting themselves away from the interface to reduce the high energy interactions with the water. 8


! 8

8

Summary and Conclusions Results presented here show that the solvent does influence the structure of the stationary phase of mixed Cig-Ci composition monolayer. Monolayers formed from 100% Cis alkylsiloxanes show no discernable conformational differences when placed in contact with different liquids, as found with Raman studies. Only a spectral shift in the methyl symmetric stretch of these monolayers was observed when the solvent was changed. Nevertheless, when methylsiloxane is added to the monolayer, an obvious change in C alkyl chain conformation occurs. By examining the ratio of the methyl to methylene symmetric stretches, the amount of gauche conformations in the alkyl chain backbones is semi-quantitatively characterized. It is found that water, acetonitrile, and isooctane induce similar amounts of gauche conformations in the alkyl chains of the stationary phase. By examining the ratio of the methyl symmetric stretch using ssp and sps polarizations, it is shown that water causes a much larger change in methyl group orientation than does acetonitrile. This reorientation, which is directed away from the surface normal, is consistent with a view of the stationary phase as a collapsed structure when in contact with water. î 8

Acknowledgements The insightful help of Brian Nelson (Continuum) is greatly appreciated. This research was supported by N S F (CHE-9875632), the Donors of the Petroleum


161412-1 10. 00

8-

(Λ


χ

6420· Air

I Isooctane

Γ Acetonitrile

Water

Solvent 30-,

GO

a.

Acetonitrile

Water

Solvent Figure 6: SF intensity ratios for A: methyl symmetric stretch to methylene symmetric stretch intensities for ssp polarization for various solvents, B: methyl symmetric stretch for ssp to the methyl symmetric stretch for sps polarization for (A) 100% CJS and (•) partial Qs coverages.


Research Fund, administered by the American Chemical Society and the Department of Chemistry at Lehigh University.

Literature Cited


1. 2. 3. 4. 5.

Fairbank, R. W. P.; Xiang, Y . ; Wirth, M . J. Anal. Chem. 1995, 67, 3879-3885. Wirth, M. J.; Fairbank, R. W. P.; Fatunmbi, H . O. Science 1997, 275, 44-47. Slusher, J. T.; Mountain, R. D. J. Phys. Chem. Β 1999, 103, 1354. Alvarez-Zepeda, Α.; Barman, Β. N.; Martire, D. E. Anal. Chem. 1992, 64, 1978. DeVido, D. R.; Dorsey, J. G.; Chan, H . S.; Dill, K . A. J. Phys. Chem. 1998, 102, 7272. 6. Bliesner, D. M.; Sentell, Κ. B . Anal. Chem. 1993, 65, 1819. 7. Thompson, W. R.; Pemberton, J. E. Anal. Chem. 1994, 66, 3362-3370. 8. Huang, J. Y . ; Shen, Y . R. Sum-Frequency Generation as a Surface Probe; Dai, H . L . and Ho, W., Ed.; World Scientific, 1995, pp 5-53. 9. Pizzolatto, R. L.; Yang, Y . J.; Wolf, L . K . ; Messmer, M. C. Anal. Chim. Acta 1999, 397, 81. 10. Yang, Y . J.; Pizzolatto, R. L . ; Messmer, M . C. J. Opt. Soc. Am. Β 1999, (accepted). 11. Hatch, S. R.; Polizzotti, R. S.; Dougal, S.; Rabinowitz, P. Chem. Phys. Lett. 1992, 196, 97-102. 12. Hatch, S. R.; Polizzotti, R. S.; Dougal, S.; Rabinowitz, P. J. Vac. Sci. Tech. 1993, 11, 2232-2238. 13. Conboy, J. C.; Messmer, M . C.; Richmond, G . L . J. Phys. Chem. 1996, 100, 7617-7622. 14. Messmer, M . C.; Conboy, J. C . ; Richmond, G . L . Resonant Sum Frequency Generation Studies of Surfactant Ordering at the Oil-Water Interface; Hicks, J. M . , Ho, W. and Dai, H.-L., Ed.; SPIE - The International Society for Optical Engineering: San Diego, 1995; Vol. 2547, pp 135-141. 15. Messmer, M . C.; Conboy, J. C.; Richmond, G. L. J. Am. Chem. Soc. 1995, 117, 8039-8040. 16. Bain, C. D. Langmuir 1994, 10, 2060. 17. Guyot-Sionnest, P.; Hunt, J. H . ; Shen, Y . R. Phys. Rev. Lett. 1987, 59, 15971600. 18. Ulman, A . Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly, Academic Press: San Diego, 1991. 19. Carr, J. W.; Harris, J. M. Anal. Chem. 1986, 58, 626. 20. Bayer, E . ; Paulus, Α.; Peters, B . ; Laupp, G . ; Reiners, J.; Albert, K . J. Chromatogr. 1986, 364, 25-37. 21. Simpson, G . J.; Rowlen, K . L. J. Phys. Chem. Β 1999, 103, 1525.


Microscopic Structure of Chromatographic Interfaces as Studied by

Recommend Documents