Structure−Function Relationships in High-Density ... - ACS Publications

Mar 13, 2008 - Zhaohui Liao andJeanne E. Pemberton*. Department of Chemistry ... Jennifer L. Gasser-Ramirez and Joel M. Harris. Analytical Chemistry 2...
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Anal. Chem. 2008, 80, 2911-2920

Structure-Function Relationships in High-Density Docosylsilane Bonded Stationary Phases by Raman Spectroscopy and Comparison to Octadecylsilane Bonded Stationary Phases: Effects of Common Solvents Zhaohui Liao† and Jeanne E. Pemberton*

Department of Chemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721

The effects of common solvents on alkyl chain conformational order in a series of high-density C22 stationary phases with surface coverage ranging from 3.61 to 6.97 µmol/m2 are investigated by Raman spectroscopy. Conformational order is evaluated using the intensity ratio of the antisymmetric and symmetric ν(CH2) modes as well as the frequencies at which these Raman bands are observed. Solvents studied include methanol-d4, acetonitrile-d3, water-d2, toluene-d8, chloroform-d. and benzened6. Alkyl chain conformational order and, hence, solvation of the stationary phase, is dependent on the Gibbs free energy change for these molecules at infinite dilution in hexadecane (∆G°HD), as well as stationary-phase properties (polymerization method and surface coverage). In general, polar solvents increase slightly the conformational order of these C22 stationary phases, while nonpolar solvents decrease conformational order. A comparison is made between C22 and C18 bonded-phase systems to further understand the role of alkyl chain length on solvent-stationary phase interactions. The change in alkyl chain conformational order induced by solvent is also compared to that induced by temperature, which provides insight into the effect of chromatographic conditions on stationary-phase shape selectivity, an important application of these materials. Despite the widespread use of reversed-phase liquid chromatography (RPLC), in which solutes carried in a polar mobile phase are separated by their differing retentions on a nonpolar stationary phase, a complete fundamental understanding of the critical intermolecular interactions between mobile and stationary phase components, or development of a molecular mechanism in general, have not yet been achieved. Common mobile phases used in RPLC are mainly composed of an organic solvent or a mixture of a solvent and water. The utility of these mobile phases is due in part to their ability to alter the elution strength through solvation of the stationary phase. Interactions including solute-solvent, * To whom correspondence should be addressed. Phone: 520) 621-8245. E-mail: [email protected]. † Current address: Department of Chemical and Environmental Engineering, University of Arizona, 1133 E. James E. Rogers Way, Tucson, AZ 85721. 10.1021/ac702270b CCC: $40.75 Published on Web 03/13/2008

© 2008 American Chemical Society

solvent-stationary phase, solute-stationary phase and intermolecular interactions within the stationary phase are known to affect solute retention and separation.1-5 Understanding such intermolecular interactions is requisite to further elucidate the molecular basis of retention. Two models have been proposed to describe retention in RPLC: the solvophobic and partitioning models.6-11 The solvophobic model postulates solute adsorption onto the nonpolar surface established by stationary-phase alkyl chains, driven by limited solubility within the mobile phase. In this model, the stationary phase does not play an active role in solute retention. In contrast, partitioning of a solute takes place when the solute is fully embedded within the stationary phase, with the stationary phase playing a more active role in retention. It is becoming increasingly clear that these models represent extremes in what is likely a continuum of possible behaviors that depend on the specific chemical characteristics of a given stationary phasemobile phase-solute system. For example, it has been shown that a solute with a polar functional group, especially one that is able to accept or donate a hydrogen bond with a mobile-phase component, exhibits adsorption-like behavior under the same conditions that a less polar solute shows partitioning behavior.12 In addition, recent Monte Carlo simulations indicate that both partitioning and adsorption play key roles in the RPLC separation process.13 Although these models primarily describe interactions between solute and stationary phase, interactions of the mobile(1) Tchapla, A.; Heron, S.; Lesellier, E.; Colin, H. J. Chromatog., A 1993, 656, 81-112. (2) Szepesy, L. J. Chromatogr., A 2002, 960, 69-83. (3) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A-867A. (4) Seibert, D. S.; Poole, C. F. Chromatographia 1995, 41, 51-60. (5) Schunk, T. C.; Burke, M. F. J. Chromatogr. 1993, 656, 289-316. (6) Horvath, Cs.; Melander, W. R.; Molnar, I. J. Chromatogr. 1976, 125, 129156. (7) Karger, B. L.; Gant, J. R. Hartkopt A. Weiner, P. H. J. Chromatogr. 1976, 128, 65-78. (8) Martire, D. E.; Boehm, R. E. J. Phys. Chem. 1983, 87, 1045-1062. (9) Dill, K. A. J. Phys. Chem. 1987, 91, 1980-1988. (10) Dorsey, J. G.; Dill, K. A. Chem. Rev.1989, 89, 331-346. (11) Jaroniec, M.; Martire, D. E. J. Chromatogr. 1986, 351, 1-16. (12) Carr, P. W.; Tan, L. C.; Park, J. H. J. Chromatogr., A 1996, 724, 1-12. (13) Rafferty, J. L.; Zhang, L.; Siepmann, J. L.; Schure, M. R. Anal. Chem. 2007, 79, 6551-6558.

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phase solvent with the stationary phase must be similarly addressed. Although considerable research has been carried out in an attempt to understand retention, substantial evidence suggests that retention is not defined by either the adsorption or the partitioning models.3,14 The difficulty in providing a descriptive model of the interactions associated with solute retention can be attributed to the complexity of the chromatographic interface. Thus, understanding stationary-phase structure in the presence of solvent is essential for elucidating the role of the stationary phase in solute retention at a molecular level. Differences in solute retention are attributed to conformational changes in the stationary phase.5,15-20 Thus, conformational order information is useful in understanding the chemical environment of the alkyl chains and provides valuable insight into the interactions between alkyl chains and their environment. The effects of solvents on alkylsilane stationary-phase structure have been investigated by various techniques including NMR,21-30 fluorescence,31,32 IR,33,34 and Raman35-39 spectroscopies. Among these techniques, infrared and Raman spectroscopy can provide direct information about alkyl chain conformation. In situ IR studies are difficult due to the large spectral background of the mobile phase; hence, almost all studies on alkyl chain conformation have been carried out in the dry state40-43 and only two reports33,34 have considered alkyl chain conformation in the presence of solvents. (14) Horvath, Cs.; Vailaya, A. J. Chromatogr., A 1998, 829, 1-27. (15) Gilpin, R. K.; Squires, J. A. J. Chromatogr. Sci. 1981, 19, 195-199. (16) Gilpin, R. K.; Gangoda, M. E.; Krishen, A. E. J. Chromatogr. Sci. 1982, 20, 345-348. (17) Scott, R. P. W.; Kucera, P. J. Chromatogr. 1977, 142, 213-232. (18) Yonker, C. R.; Zwier, T. A.; Burke, M. F. J. Chromatogr. 1982, 241, 257268. (19) Yonker, C. R.; Zwier, T. A.; Burke, M. F. J. Chromatogr. 1982, 241, 269280. (20) Sander, L. C.; Lippa, K. A.; Wise, S. A. Anal. Bioanal. Chem. 2005, 382, 646-668. (21) Marshall, D. B.; McKenna, W. P. Anal. Chem. 1984, 56, 2090-2093. (22) Bliesner, D. M.; Sentell, K. B. Anal. Chem. 1993, 65, 1819-1826. (23) Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1991, 113, 6349-6358. (24) Albert, K.; Evers, B.; Bayer, E. J. Magn. Reson. 1985, 62, 428-436. (25) Bayer, E.; Paulus, A.; Peters, B.; Laupp, G.; Reiners, J.; Albert, K. J. Chromatogr. 1986, 364, 25-37. (26) Shah, P.; Rogers, L. B.; Fetzer, J. C. J. Chromatogr. 1987, 388, 411-419. (27) Zeigler, R. C.; Maciel, G. E. J. Phys. Chem. 1991, 95, 7345-7353. (28) Strohschein, S.; Pursch, M.; Lubda, D.; Albert, K. Anal. Chem. 1998, 70, 13-18. (29) Gilpin, R. K.; Gangoda, M. E. J. Magn. Reson. 1985, 64, 408-413. (30) Gangoda, M. E.; Gilpin, R. K. Langmuir 1990, 6, 941-944. (31) Stahlberg, J.; Almgren, M. Anal. Chem. 1985, 57, 817-821. (32) Montgomery, M. E.; Green, M. A.; Wirth, M. J. Anal. Chem. 1992, 64, 1170-1175. (33) Srinivasan, G.; Mueller, K. J. Chromatogr., A 2006, 1110, 102-107. (34) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068-1075. (35) Pemberton, J. E.; Ho, M.; Orendorff, C. J.; Ducey, M. W. J. Chromatogr., A 2001, 913, 243-252. (36) Ducey, M. W.; Orendorff, C. J.; Pemberton, J. E.; Sander, L. C. Anal. Chem. 2002, 74, 5585-5592. (37) Orendorff, C. J.; Ducey, M. W.; Pemberton, J. E.; Sander, L. C. Anal. Chem. 2003, 75, 3360-3368. (38) Orendorff, C. J.; Ducey, M. W.; Pemberton, J. E.; Sander, L. C. Anal. Chem. 2003, 75, 3369-3375. (39) Doyle, C. A.; Vickers, T.; Mann, C. K.; Dorsey, J. G. J. Chromatogr., A 2000, 877, 25-39. (40) Srinivasan, G.; Kyrlidis, A.; McNeff, C.; Mueller, K. J. Chromatogr., A 2005, 1081, 132-139. (41) Srinivasan, G.; Neumann-Singh, S.; Mueller, K. J. Chromatogr., A 2005, 1074, 31-41. (42) Srinivasan, G.; Pursch, M.; Sander, L. C.; Mueller, K. Langmuir 2004, 20, 1746-1752.

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Raman spectroscopy has been shown previously to be a powerful tool in determining subtle conformational changes in alkyl chains in the absence and presence of solvents.35-38,44-48 The empirical order indicator I[νa(CH2)]/I[νs(CH2)], the peak intensity ratio of the antisymmetric νa(CH2) (2885 cm-1) to the symmetric νs(CH2) (2850 cm-1), has been found to be particularly illuminating of stationary-phase conformational order in previous studies of RPLC phases.35-38,44,45,48 This indicator is extremely sensitive to subtle changes in alkyl chain conformational order that result from small changes in rotational disorder in short segments of alkyl chain and the onset of C-C gauche conformer formation. This parameter measures the deviation of the alkyl chain conformation from an all-trans state but does not change linearly with the number of gauche conformers present;46,47 thus, interpretation of this parameter is complex. The peak frequencies of the νs(CH2) and νa(CH2) modes have been shown to contain additional insight into conformational order of alkyl chain systems through information about degree of interchain coupling. The use of Raman spectroscopy for such conformational analysis does have several limitations, however, and it is useful to make these explicit at the outset of any Raman spectral analysis of alkyl chain systems. Specifically, Raman spectra report on only the average behavior of the alkyl chain ensemble; information about the behavior of individual chains cannot be ascertained. In addition, for the surface-bound alkyl chains on particulate silica important in RPLC, analysis of Raman spectral data cannot provide insight into the tilt angle of the alkyl chain with respect to the silica surface. Despite these limitations, the multiple spectral indicators of alkyl chain order throughout the Raman spectrum contain a wealth of valuable insight into alkyl chain order. Although the majority of RPLC is done using C18 or C8 stationary phases, C22 phases offer a higher degree of conformational order compared to these shorter chain systems with similar surface coverages.48 Thus, solvent-induced changes in conformational order for C22 stationary phases are expected to be somewhat different due to the greater rigidity of the C22 chains. Here, the effects of organic solvents (i.e., methanol-d4, acetonitriled3, water-d2, toluene-d8, chloroform-d, and benzene-d6) on C22 stationary-phase structure are studied in an effort to further understand the role of solvent-stationary-phase interactions in solute retention in a reversed-phase system. Differences in solventinduced conformational changes between C18 and C22 phase are also explored in order to understand the role of alkyl chain length on solvent-stationary-phase interactions. Changes in alkyl chain conformational order induced by solvents are also compared to those induced by temperature to better understand the roles of solvation and temperature on stationary-phase shape selectivity, an important application of these stationary phase materials.49 (43) Neumann-Singh, S.; Villanueva-Garibay, J.; Mueller, K. J. Phys. Chem. B 2004, 108, 1906-1917. (44) Ho, M.; Pemberton, J. E. Anal. Chem. 1998, 70, 4915-4920. (45) Ducey, M. W.; Orendorff, C. J.; Pemberton, J. E.; Sander, L. C. Anal. Chem. 2002, 74, 5576-5584. (46) Orendorff, C. J.; Ducey, M. W.; Pemberton, J. E. J. Phys. Chem. A 2002, 106, 6991-6998. (47) Liao, Z.; Pemberton, J. E. J. Phys. Chem. A 2006, 110, 13744-13753. (48) Liao, Z.; Orendorff, C. J.; Sander, L. C.; Pemberton, J. E. Anal. Chem. 2006, 78, 5813-5822. (49) Pursch, M.; Sander, L. C.; Egelhaaf, H.-J.; Raitza, M.; Wise, S. A.; Oelkrug, D.; Albert, K. J. Am. Chem. Soc. 1999, 121, 3201-3213.

Table 1. Stationary-Phase Properties

a

stationary phase

previous stationaryphase designationa

silane functionality

polymerization method

DFC22SL DFC22SF TFC22SL TFC22SF

A B C D

dichloromethyl dichloromethyl trichloro trichloro

solution surface solution surface

Used as designation in previously published work on these phases; see ref 49.

EXPERIMENTAL SECTION Stationary Phases. C22 stationary phases were prepared and characterized as previously described49 and were a generous gift from Dr. Lane C. Sander of the National Institute of Standards and Technology. The preparation method can be summarized as follows. Difunctional and trifunctional stationary phases were prepared using dichloromethyl- and trichlorodocosylsilane, respectively. Of the four stationary-phase materials investigated, two were synthesized by the “surface polymerization” method, and two were synthesized by the “solution polymerization” method. Surface-polymerized phases were prepared by adding the di- or trifunctional silane to previously humidified silica, while the solution-polymerized phases were prepared by the addition of water to the precursor/silica slurry. These stationary phases were prepared using 5-µm YMC silica (YMC Inc., Wilmington, DE, and YMC Europe GmbH, Schermbeck, Germany; pore size 200 Å). The identity of the precursor, polymerization method, and resulting properties of each stationary phase are summarized in Table 1. Sample Preparation. Samples were prepared by placing between 10 and 50 mg of stationary-phase material into a 5-mmdiameter NMR tube; 100 µL of solvent was then added. Samples were sonicated for 10 min and equilibrated at 293 K for a minimum of 12 h prior to spectral acquisition. In the case of water-d2, since stationary phases tend to suspend in water, a piece of glass wool was used to force immersion of the stationary phase. Raman Spectral Acquisition and Data Treatment. Raman spectra were collected and analyzed using the instrumentation and procedures described previously by this laboratory.48 A minimum of three measurements was made on each stationary phase in each solvent, and the standard deviation was determined. Error bars are plotted for all data points in the figures. In cases wherein error bars are not obvious, they are smaller than the size of the symbol used to indicate the average value. RESULTS AND DISCUSSION Solvent Dependence. Figures S1 and S2 of the Supporting Information show Raman spectra in the ν(CH) region between 2750 and 3050 cm-1 acquired in the presence of methanol-d4 (Figure S1a), acetonitrile-d3 (Figure S1b), water-d2 (Figure S1c), toluene-d8 (Figure S2a), chloroform-d (Figure S2b), and benzened6 (Figure S2c) for stationary phases prepared from difunctional (DFC22SL and DFC22SF) and trifunctional (TFC22SL and TFC22SF) docosylsilanes with surface coverages of 3.61, 4.65, 4.89, and 6.97 µmol/m2, respectively. These spectra exhibit well-known characteristics of alkyl chain Raman spectra in this region.35-38,44-48

b

% carbon

surface coverage (µmol/m2)

shape selectivity parameterb R

15.01 18.33 18.17 23.38

3.61 4.65 4.89 6.97

1.25 0.70 0.47 0.12

As defined in ref 49.

The vibrational modes of interest here include the νs(CH2) at ∼2850 cm-1, the νa(CH2) near 2880 cm-1, the νs(CH3) near 2896 cm-1, and the νs(CH3)FR near 2928 cm-1. The νs(CH3)Si mode for the methyl group attached to the proximal silicon atom is present in the spectra of the difunctional silanes, DFC22SL and DFC22SF. This mode is located at 2906 cm-1 and is superimposed on the νs(CH3)FR near 2928 cm-1 and the νs(CH3) near 2896 cm-1. As a result of its position, it may also contribute slightly to the intensity of the νa(CH2). Qualitative examination of the spectra in Figures S1 and S2 reveals a range of conformational order among these stationary phases in the same solvent as reflected by the values of I[νa(CH2)]/I[νs(CH2)]. Particularly pronounced is the difference in conformational order between DFC22SL (3.61 µmol/m2) and TFC22SF (6.97 µmol/m2), consistent with previous studies that indicate that conformational order is affected by stationary-phase surface coverage in a given solvent.35-38 For each stationary phase, the conformational order is greater in polar solvents (Figure S1) than in nonpolar solvents (Figure S2). As indicated in Figure S1, differences in conformational order among the three polar solvents are also observed for a single stationary phase. Furthermore, when the effect of nonpolar solvents (Figure S2) is considered for each stationary phase, differences in alkyl chain conformational order are observed as well. Developing a more quantitative understanding of the influence of solvent on stationary-phase conformational order requires consideration of an appropriate thermodynamic model. Carr and co-workers50 have proposed a thermodynamic model for RPLC in which the overall process of solvent transfer from bulk solution to stationary phase is divided into two sequential processes: (i) solvent transfer from bulk solution to the ideal gas phase and (ii) solvent transfer from the ideal gas phase to the stationary phase. This approach allows one to investigate separately solvent-solvent interactions in the first transfer process and solvent-stationaryphase interactions in the second transfer process. Since Raman spectral order indicators, especially I[νa(CH2)]/I[νs(CH2)], have been shown to be a direct measure of stationary-phase conformational order,46,47 this order parameter is expected to correlate to the Gibbs free energy for the second transfer process (∆G°stat). Numerous solvent characteristics, including hydrogen bond donating and accepting ability (R and β, respectively), polarizability, molecular volume, and the interaction between solvent and stationary phase, affect ∆G°stat for solvent transfer from the ideal ° gas phase to the stationary phase. In order to correlate ∆G stat (50) Ranatunga, R. P. J.; Carr, P. W. Anal. Chem. 2000, 72, 5679-5692.

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with other thermodynamic and solvatochromic parameters and to determine the most critical factors in determining ∆G°stat, eq 1, derived from the Abraham general solvation model,51 has been used previously to predict solubility. This equation is used here to help understand the “solubility” of common organic solvents in C22 stationary phases.

log L ) c + rR2 + sπ2H + aΣR2H + bΣβ2H + l log L(16) (1)

In this relationship, log L is the Ostwald solubility coefficient for the solvent, R2 is the solvent excess molar refractivity, π2H is the solvent dipolarity-polarizability descriptor, ∑R2H and ∑β2H describe solvent hydrogen bond donating and accepting ability, and log L(16) is the solvent gas-phase dimensionless Ostwald partition coefficient into hexadecane at 298 K. Log L(16) is used here as a model for solvent partitioning into the C22 stationary phases of interest. It is noted that some caution should be exercised when using log L(16) as a general model for partitioning, since it has been suggested previously13,50 that the similarity in solvation thermodynamics between hexadecane and octadecylsilane stationary phases is only true for nonpolar compounds retained within the region of the methylene segments. In fact, as will be shown below, this cautionary note has considerable validity, since a correlation between conformational order and log L(16) is only observed for nonpolar solvents. In eq 1, the parameters R2, π2H, ∑R2H, and ∑β2H are dependent on solvent properties, while the coefficients c, r, s, a, b, and l depend on the stationary phase. The r coefficient indicates the tendency of the stationary phase to interact with the solvent through polarizability-type interactions, s is a measure of the dipolarity of the stationary phase, a and b represent the hydrogen bond acidity and basicity of the stationary phase, and l is a combination of the work needed to break alkyl chain interactions to create a cavity and the general dispersion interaction energy between solvent and alkyl chains. Since the docosylsilane of these phases has no hydrogen-bond donating or accepting ability, the a and b coefficients are zero. The dipolarity of the C22 chains is similarly negligible, and the s coefficient is considered to be zero.51 Therefore, for these C22 stationary phases, eq 1 reduces to

log L ) c + rR2 + l log L(16)

(2)

where log L and log L(16) can be related to ∆G°stat and ∆G°HD (the free energy change of solvent transfer from the ideal gas phase to hexadecane), respectively:

∆G°stat ) -RT log L

(3)

∆G°HD ) -RT log L(16)

(4)

Therefore, ∆G°HD is related to ∆G°stat, the free energy change for the second transfer process from the ideal gas phase to the stationary phase: (51) Acree, W. E.; Abraham, M. H. Can. J. Chem. 2001, 79, 1466-1476.

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∆G°stat ) -RT(c + rR2) + l ∆G°HD

(5)

In eq 5, the free energy change ∆G°stat is determined by the temperature, the solvent excess molar refractivity, and ∆G°HD. In other words, the interaction between solvent and stationary phase is dependent on the temperature, the excess molar refractivity of the solvent, and the interaction between the solvent molecule and hexadecane. Therefore, it should be possible to correlate ∆G°HD with the order parameter I[νa(CH2)]/I[νs(CH2)] to reveal a relationship between the free energy change for solvent partitioning into the stationary phase and the associated stationary-phase conformational change of this partitioning. The more easily the stationary phase can accommodate solvent at infinite dilution in terms of cavity formation and intermolecular interactions, the more disruption in order is predicted. Quantitative evaluation of the effect of solvent on conformational order for each C22 stationary phase is shown in Figure 1 in the form of plots of I[νa(CH2)]/I[νs(CH2)] as a function of free energy change for transfer of a solvent molecule from the ideal gas phase to hexadecane solution (∆G°HD). The relevant parameters for determination of ∆G°HD from eq 5 for each solvent are given in Table 2.52-56 The horizontal dashed lines represent I[νa(CH2)]/I[νs(CH2)] values in air and are labeled for each stationary phase. In general, the plots in Figure 1 demonstrate that the predicted correlation between I[νa(CH2)]/I[νs(CH2)] and ∆G°HD is indeed observed for the more nonpolar solvents, with I[νa(CH2)]/I[νs(CH2)] increasing approximately linearly with ∆G°HD from toluene-d8 to acetonitrile-d3. This behavior suggests a partitioning-like interaction for these solvents within the C22 stationary phases. For the polar solvents, however, I[νa(CH2)]/ I[νs(CH2)] is approximately independent of ∆G°HD from acetonitrile-d3 to water-d2 for any given stationary phase. In these cases, solvation of the stationary phase is apparently more adsorptive in nature, presumably whereby the alkyl chain terminus regions serve as “adsorption sites”. As a result, the stationary phases are generally more ordered in the presence of these polar solvents than they are in air. Significantly, I[νa(CH2)]/I[νs(CH2)] values in the three polar solvents are greater than those in air for all stationary phases except TFC22SF in acetonitrile-d3, while for nonpolar solvents, smaller I[νa(CH2)]/I[νs(CH2)] values relative to air are observed except for DFC22SL in chloroform-d in which conformational order similar to that in air is observed. Among the three nonpolar solvents studied, toluene-d8 has the most negative ∆G°HD, indicating that toluene-d8 has the strongest interaction with the alkyl chains. Such strong interaction leads to the highest concentration of solvent molecules in the stationary phase, which induces the greatest disorder for the alkyl chains as reflected by the lowest value of I[νa(CH2)]/I[νs(CH2)] compared with other solvents. The incremental values of ∆G°HD for toluened8, benzene-d6, and chloroform-d indicate that the interactions (52) Abraham, M. H.; Whiting, G. S. J. Chem. Soc., Perkin Trans. 2 1990, 291300. (53) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877-2887. (54) Kamlet, M. J.; Doherty, R.; Taft, R. W.; Abraham, M. H. J. Am. Chem. Soc. 1983, 105, 6741-6743. (55) Kamlet, M. J.; Carr, P. W.; Taft, R. W.; Abraham, M. H. J. Am. Chem. Soc. 1981, 103, 6062-6066. (56) Barton, A. F. M. Chem. Rev. 1975, 75, 731-753.

Figure 1. I[νa(CH2)]/I[νs(CH2)] as a function of Gibbs free energy change for infinite solvent dilution in hexadecane (∆G°HD) for DFC22SL (1), DFC22SF (4), TFC22SL (9), and TFC22SF (O) in polar solvents and nonpolar solvents at 293 K. Horizontal dashed lines represent values in air. Error bars represent one standard deviation. Table 2. Solvent Parameters solvent water acetonitrile methanol benzene chloroform toluene

∆G°HDa (kcal/mol) π*,b 2.26 0.49 1.36 -1.20 -0.76 -1.94

1.09 0.75 0.60 0.59 0.58 0.54

βb

Rb

δH

δSA

Vm d (Å3) L/B h

0.18 0.31 0.62 0.10 0 0.11

1.17 0.19 0.93 0 0.44 0

23.43c 11.74d 14.32c 9.2e 9.3d 8.9d

5.3c n/af 3.0c n/af n/af n/af

10.33 37.35 28.88 71.25 66.96 88.20

1.090 1.416 1.130 2.471 1.410 1.984

a Values taken from ref 52. b Values taken from ref 53. c Values taken from ref 54. d Values taken from ref 55 e Values taken from ref 56. f Not available.

between the solvent molecules and the stationary phase increase in the order of chloroform-d < benzene-d6 < toluene-d8. Since the free energy change ∆G°HD for chloroform-d is least negative among these three nonpolar solvents, the concentration of chloroform molecules in the alkyl chains is less than that for benzene-d6 or toluene-d8. As a result, the conformational order in chloroform is higher than that in benzene or toluene. It is noted parenthetically that an alternate explanation for these differences in conformational order could be related to depth of penetration of the solvent molecules into the alkyl chains of the stationary phase. This explanation would require, however, a differential interaction between the solvent molecules and methylene units in different regions of the C22 chain, which is more difficult to rationalize. The complexity of these solvation interactions can be better understood by considering solvent properties at a molecular level. The disorder induced by toluene-d8 and benzene-d6 can be attributed to their greater planarity, rendering them better able to penetrate into the relatively densely packed alkyl chain region. It is noted that all stationary phases in toluene-d8 are slightly more disordered than in benzene-d6. This can be attributed to the greater molecular volume of toluene-d8 causing greater disruption of order. Chloroform-d, however, is not as effective at penetrating the alkyl chains as planar benzene-d6 or toluene-d8 due to its nonplanarity and relatively bulky size. These observations also

suggest that the conformational order of these C22 stationary phases may be due in part to shape selectivity toward these three solvents. The trends in solvent shape selectivity observed here are generally consistent with expectations in that greater interaction with the stationary phase occurs for molecules with larger length-to-breadth ratios (L/B).57 For these solvents, the L/B for chloroform-d (Table 2) is 1.41, less than that of either benzene-d6 (2.47) or toluene-d8 (1.98); hence, its smaller interaction with the stationary phase is consistent at least in part with expectations based on consideration of shape selectivity. Since ∆G°HD values for all three polar solvents are positive, the intercalation of polar solvents into alkyl chains is not favorable. Unlike nonpolar solvents inducing disordered alkyl chains, polar solvents actually order the stationary phase to a degree that is generally inversely proportional to stationary-phase coverage. This behavior is consistent with an adsorption-like interaction of these polar solvents with the chain termini regions of these highly ordered C22 stationary phases.36,37 The increase in order induced by methanol-d4 and acetonitrile-d3 is likely due to an interaction in which solvent methyl groups are inserted into small cavities at the distal ends of the chains allowing association either by hydrogen bonding (methanol) or dipole-dipole interaction (acetonitrile) of these solvent molecules just outside the outer edge of the C22 chains. This solvent-solvent interaction motif causes loss of end-gauche conformers and other chain terminus disorder that can be accommodated in the dry state. Water-d2 does not have a nonpolar end with which to interact with stationary phase. Under conditions of forced wetting, as experienced both here and in chromatographic use of such phases, water-d2 is not likely to undergo any specific chemical interactions with the alkyl chains. Indeed, a previous theoretical study by Lum and co-workers58 predicts a decrease in water density, or so-called drying, near extended nonpolar surfaces. This prediction has been confirmed in two recent Monte Carlo simulations of C18 alkylsilane stationary phases on silica that demonstrate little interaction between water and the alkylsilane;59,60 this prediction is similarly substantiated experimentally on several different types of nonpolar surface systems. Such work includes chromatographic studies supporting spontaneous dewetting of intraparticle pores of alkylsilane stationary phases by Walter and co-workers61 and neutron scattering studies of methyl-terminated alkanethiol self-assembled monolayers in water that indicate a lower water density immediately adjacent to the monolayer surface by Schwendel and co-workers.62 Thus, the ordering induced by water, especially with the lower coverage phases, must not result from interactions within the alkyl chain terminus region of the interface. This ordering effect relative to air must be attributable to the incorporation of water at residual silica surface silanols in the proximal region of the stationary phase. Such water has been (57) Wise, S. A.; Bonnett, W. J.; May, F. R. J. Chromatogr. Sci. 1981, 19, 457465. (58) Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 45704577. (59) Zhang, L.; Sun, L.; Siepmann, J. I.; Schure, M. R. J. Chromatogr., A 2005, 1079, 127-135. (60) Zhang, L.; Rafferty, J. L.; Siepmann, J. I.; Chen, B.; Schure, M. R. J. Chromatogr., A 2006, 1126, 219-231. (61) Walter, T. H.; Iraneta, P.; Capparella, M. J. Chromatogr., A 2005, 1075, 177-183. (62) Schwendel, D.; Hayashi, T.; Dahint, R.; Pertsin, A.; Grunze, M.; Steitz, R.; Schreiber, F. Langmuir 2003, 19, 2284-2293.

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observed to accumulate on the silica surface in recent Monte Carlo simulations of these interfaces,59,60 and an effect of stationary-phase ordering by water in inverse proportion to alksylsilane surface coverage, as observed in the data in Figure 1, is consistent with such a postulate. Apparently, the presence of water at silanols on the silica surface causes the alkyl chains to order more in the vicinity of the residual silanols than in air, albeit without a substantial change in overall chain coupling as will be seen below. The slight ordering effect of methanol-d4 and acetonitrile-d3 can be understood further by considering the self-association ability of these solvents. Methanol is known to self-associate in the liquid state by hydrogen bonding. The liquid structure of acetonitrile, on the other hand, is not completely understood. It has been reported that the ν(CN) mode for acetonitrile in both the IR and Raman spectra is 13 cm-1 higher in the gas phase than in the liquid.63-66 Antiparallel alignment of pairs of acetonitriles has been proposed for the liquid state leading to a reduced dipole moment.67 Vibrational sum frequency generation studies of acetonitrile at air-aqueous solution interfaces have shown that the CN head group changes from a more upright to a more parallel orientation at higher acetonitrile density, suggesting strong CN-CN dipolar interactions.68 Several researchers have reported that the ν(CN) band of acetonitrile in the liquid phase is composed of two bands: a narrow band, attributed to monomeric acetonitrile and a broad band, attributed to an undefined self-associated form of acetonitrile.63,69,70 Dilution studies of acetonitrile in a variety of solvents indicate that free acetonitrile does not exist in solution; rather, it is organized as aggregates of loosely defined acetonitrile clusters.63 Although the exact form of liquid acetonitrile-d3 is not clearly defined, it does exist in one or more self-associated forms. Although a value of δSA for acetonitrile-d3 is not readily available, its π* value reported in Table 2 also indicates the self-associating ability of acetonitrile-d3. In general, as π* increases, so does selfassociation of the solvent.53,54 On this basis, it is proposed that the solvent-induced ordering of these stationary phases is a result of solvent self-association at the distal ends of the C22 chains. As solvent self-association increases from methanol-d4 to acetonitriled3, the interaction with alkyl chains decreases. Therefore, methanold4-induced alkyl chain ordering is greater than that induced by acetonitrile-d3. Surface Coverage Dependence. The results in Figure 1 indicate that conformational order in solvents also depends on C22 surface coverage. As has been observed in related studies,35-38,44,45,48 the magnitude of I[νa(CH2)]/I[νs(CH2)] generally increases with increasing surface coverage in each solvent. TFC22SF (6.97 µmol/m2) exhibits the greatest degree of conformational order in all six solvents. Similarly, DFC22SF (4.65 µmol/ m2) and TFC22SL (4.89 µmol/m2), with comparable surface coverage, show similar conformational order in all three nonpolar solvents, while DFC22SL (3.61 µmol/m2), with the lowest surface coverage, is the most disordered except in chloroform-d, in which (63) Rowlen, K. L.; Harris, J. M. Anal. Chem. 1991, 63, 964-969. (64) Sadlej, J. Spectrochim. Acta 1979, 35A, 681-684. (65) Venkateswarlu, P. J. Chem. Phys. 1951, 19, 293-298. (66) Schrotter, H. W.; Berstein, H. J. J. Mol. Spectrosc. 1964, 12, 1-17. (67) Thomas, B. H.; Thomas-Orville, W. J. J. Mol. Struct. 1969, 3, 191-206. (68) Wang, H.; Borguet, E.; Yan, E. C. Y.; Zhang, D.; Gutow, J.; Eisenthal, K. B. Langmuir 1998, 14, 1472-1477. (69) Griffiths, J. E. J. Chem. Phys. Lett. 1973, 59, 751-758. (70) Lowenschuss, A.; Yellin, N. Spectrochim. Acta 1975, 31A, 207-212.

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its conformational order is similar to that of DFC22SF and TFC22SL. However, of the phases with similar surface coverage (DFC22SF and TFC22SL), the lower surface coverage DFC22SF in the three polar solvents exhibits greater order than the higher coverage phase TFC22SL, consistent with their behavior in air.48 This observation implies that surface coverage is not the only factor that dictates conformational order of these phases. Polymerization method (solution vs surface polymerization), and therefore, alkylsilane surface homogeneity, is expected to play an additional role in determining conformational state.35-38,48 It has been proposed36,37,48,71 that solution-polymerized phases are attached to the silica surface in islands as a consequence of preassembly or oligomerization in solution, whereas alkylsilanes in phases prepared by surface polymerization are more uniformly distributed across the surface. As a result, the alkyl chains of solutionpolymerized materials are less ordered than the corresponding surface-polymerized materials of comparable surface coverage.36,37,48 This rationalization explains the observation that DFC22SF is more ordered in air and in polar solvents than TFC22SL. In assessing the effect of solvent on the order of these C22 phases, it is useful to define an independent scale for the full range of 0-100% order against which changes in I[νa(CH2)]/I[νs(CH2)] can be compared. This approach is complicated due to the absence of an obvious standard system that can be used to define the 0-100% order scale for surface-bound alkyl moieties. A surfacebound alkyl chain has fewer degrees of freedom than a free alkyl chain and, therefore, cannot achieve the same degree of either order or disorder. In light of this important distinction, a conservative choice of a standard system for comparison of these stationary phases seems the best strategy. Based on these considerations, low-density polyethylene has been used successfully in previous work from this laboratory46,47,72 as an appropriate standard for these surface-bound systems.47,58 Polyethylene is not as inherently ordered as n-alkanes due to the small number of gauche defects that remain in the structure even when crystalline. These defects arise as a result of the orthorhombic crystal structure of the polymer (as compared to the hexagonal structure of the n-alkanes) in which the polymer chains contain hairpin bends.73-75 These hairpin bends restrict the movement of the alkyl chains of the polymer in a manner similar to what is experienced by alkylsilanes tethered at one end to a silica surface. For low-density polyethylene, the Raman spectral indicator I[νa(CH2)]/I[νs(CH2)] has values that range from 1.57 ((0.02) in the crystalline state at -15 °C to 0.85 ((0.03) in the liquid state at 95 °C.46,47 Thus, 0% order on this scale is assigned to the I[νa(CH2)]/I[νs(CH2)] value for the fully disordered liquid state and 100% order on this scale is assigned to the value for the fully ordered crystalline state. Using this scale, effects of solvent on alkyl chain order can be quantified with positive (increase in order) or negative (decrease in order) values of solvent-induced change. Normalized changes for the effect of solvent on conformational order are determined (71) Pursch, M.; Sander, L. C.; Albert, K. Anal. Chem. 1996, 68, 4107-4113. (72) Liao, Z.; Orendorff, C. J.; Pemberton, J. E. Chromatographia 2006, 64, 139146. (73) Boenig, H. V. in Polyolefins: Structure and Properties; Elsevier: New York, 1996. (74) Keller, A. Philos. Mag. 1957, 2, 1171-1175. (75) Niegisch, W. D.; Swan, P. R. J. Appl. Phys. 1960, 31, 1906-1910.

Figure 2. 100 ∆IS/∆ISTD, Air as a function of surface coverage in (a) polar solvents acetonitrile-d3 (b), methanol-d4 (9), and water-d2 (2) and (b) nonpolar solvents chloroform-d (O), benzene-d6 (0), and toluene-d8 (4). Error bars represent one standard deviation.

as (100 ∆IS/∆ISTD, Air). Here, ∆IS is (IS - I0, Air), where IS is I[νa(CH2)]/I[νs(CH2)] for the stationary phase in solvent at 20 °C and I0, Air is I[νa(CH2)]/I[νs(CH2)] for the stationary phase in air at 20 °C. ∆ISTD, Air is similarily calculated as (ICrystalline PE, Air -15 °C - ILiquid PE, Air 95 °C), where ICrystalline PE, Air -15 °C is I[νa(CH2)]/ I[νs(CH2)] for crystalline, low molecular weight polyethylene in air at -15 °C and ILiquid PE, Air 95 °C is I[νa(CH2)]/I[νs(CH2)] for liquid, low molecular weight polyethylene in air at 95 °C.46,47 Figure 2 shows plots of this normalized change in order (100 ∆IS/∆ISTD, Air) as a function of surface coverage for polar and nonpolar solvents. In these plots, the horizontal line indicates the zero value of the normalized order parameter; all values above the line are positive, indicating greater order in the presence of solvents, and the values below the line are negative, indicating greater disorder. Solvent-induced conformational changes depend not only on the characteristics of solvent but also on the properties of the stationary phase. As shown in Figure 2a, all stationary phases in polar solvents are more ordered except TFC22SF in acetonitriled3. It is noted parenthetically that these results represent the first spectroscopic evidence that support the proposal that spontaneous dewetting61 and not so-called stationary-phase collapse16,76 are the origin of retention loss in RPLC when switching to aqueous mobile phases. Moreover, the change in normalized order induced by methanol-d4 and water-d2 decreases with increasing surface coverage. In acetonitrile-d3, the change in normalized order also decreases with increasing surface coverage, except for TFC22SL. (76) Yang, S. S.; Gilpin, R. K. J. Chromatogr. 1987, 394, 295-303.

These observations suggest that stationary phases with lower surface coverage are more sensitive to polar solvents, giving rise to greater increases in conformational order. This could be due to several effects. First, stationary phases with lower surface coverage are initially more disordered. Therefore, the lateral ordering induced by polar solvents at the distal methyl end is more prominent compared to stationary phases with higher surface coverage. For methanol-d4 and acetonitrile-d3, it should be noted, however, that a picture in which these solvents embed their methyl ends into the distal end of the C22 chains is somewhat outside the conventional view of an adsorption model. The conventional view6,7 holds that the stationary phase plays an inactive role in interaction. Based on the differences in conformational order elucidated here, this conventional view is suggested to be oversimplified with real effects being more complex, albeit subtle. A second cause for the greater ordering effect of polar solvents on lower coverage phases could be solvent accumulation at residual silanols within the silica-C22 interfacial region. Such solvent accumulation has been clearly observed in recent Monte Carlo simulations of such interfaces.59,60 Moreover, the number of residual silanols would be larger for smaller C22 coverage; hence, any effect induced by the presence of solvent in the proximal region of the C22 phase, such as conformational ordering, would be amplified for stationary phases of lower coverage as is observed here. As shown in Figure 2b, all stationary phases in nonpolar solvents are more disordered than in air except DFC22SL in chloroform-d. Chloroform-d-induced changes in conformational order are the smallest among the three nonpolar solvents studied, while toluene-d8-induced changes are the largest. The smaller change in conformational order with increasing surface coverage observed in polar solvents is absent for these materials in chloroform-d, however. Moreover, in benzene-d6 and toluene-d8, the extent of conformational change actually increases with increasing surface coverage. In both toluene-d8 and benzene-d6, the magnitude of changes induced in surface-polymerized phases is larger than those induced in the more heterogeneous solution-polymerized phases. This is especially evident upon comparison of the behavior of the two phases with comparable surface coverage, DFC22SF and TFC22SL. Islanding in solution-polymerized layers36,37,48,71 such as TFC22SL would be expected to reduce the average free volume between alkyl chains within the island, only allowing significant penetration of the chains by benzene-d6 and toluene-d8 at island edges. In contrast, surface-polymerized stationary phases have alkyl chains that are more homogeneously distributed across the surface; hence, when benzene-d6 and toluene-d8 intercalate into these phases (e.g., DFC22SF), they have a more disruptive effect on conformational order. These differences between the structures of surface- and solution-polymerized stationary-phase layers explain why the disordering induced by nonpolar solvents for surface-polymerized phases is greater than that for solutionpolymerized phases with similar surface coverage. Regardless of polymerization method, benzene-d6- and toluene-d8-induced disorder increases with increasing surface coverage. As noted above, chloroform-d does not interact with the alkyl chains as much as the more planar solvents due to its shape. Analytical Chemistry, Vol. 80, No. 8, April 15, 2008

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Figure 3. νs(CH2) peak frequency as a function of Gibbs free energy change for infinite solvent dilution in hexadecane (∆G°HD) for DFC22SL (1), DFC22SF (4), TFC22SL (9), and TFC22SF (O) in polar solvents and nonpolar solvents at 293 K. Horizontal dashed lines represent values in air. Error bars represent one standard deviation.

Accordingly, the order of DFC22SL, the material with the largest interchain spacing, changes significantly, while TFC22SF, with the smallest interchain spacing that does not allow extensive chloroform penetration, does not. Collectively, these observations suggest that nonpolar solvents induce changes in conformational order that depend on solvent molecular volume and shape as well as the surface heterogeneity and coverage of the alkylsilane stationary phase. νs(CH2) Frequency Order Indicator Behavior. In addition to I[νa(CH2)]/I[νs(CH2)] as one indicator of conformational order, the frequency of the νs(CH2) mode also provides useful insight into alkyl chain order insofar as it reports on the degree of interchain coupling in these materials. Although this indicator is not as discriminating of order as the I[νa(CH2)]/I[νs(CH2)] parameter,46,47 it can serve as an alternate independent spectral indicator containing insight into order. Figure 3 shows plots of νs(CH2) frequency as a function of ∆G°HD for the four C22 stationary phases in these six solvents. The horizontal dashed lines represent the νs(CH2) frequencies in air. In general, the nonpolar solvents have either little effect on interchain coupling or slightly decrease the coupling, suggesting that the majority of the C22 chains stay coupled to the same extent as in air. This behavior suggests that the volume of the stationary phase that interacts with solvent is small. Taking into consideration the relatively larger change in conformational order induced by the nonpolar solvents compared to the polar solvents as indicated by I[νa(CH2)]/I[νs(CH2)], the observed behavior suggests that the I[νa(CH2)]/I[νs(CH2)] and νs(CH2) frequency indicators contain a different perspective of alkyl chain conformational state. The νs(CH2) chain coupling indicator appears to report on the average state of chain coupling over the entire alkyl chain, while the I[νa(CH2)]/I[νs(CH2)] indicator is sensitive to conformational order change at the distal end of the chain, rendering it a hypersensitive indicator for study of stationary-phase behavior in chromatographic systems. Chain coupling in methanol-d4 is noticeably unique in that all phases exhibit considerably greater coupling than in air. The chain coupling of lower coverage phases is more significantly increased by methanol-d4 than that for the higher surface coverage phases. 2918 Analytical Chemistry, Vol. 80, No. 8, April 15, 2008

With the exception of DFC22SL in acetonitrile-d3, all phases are slightly less well coupled in acetonitrile-d3 and water-d2 than in air. The stronger interactions of the methyl group of methanol-d4 with the terminal methyl group of the stationary phase may increase interchain coupling or it may be easier for the chains to “heal” (i.e., couple) around methanol molecules accumulated at residual surface silanols. In contrast, acetontrile-d3, a more selfassociated solvent, interacts with the stationary phase to a smaller extent, and as a result, acetonitrile-d3 has only a minor influence on interchain coupling. This conclusion for acetonitrile-d3 is consistent with that drawn above on the basis of the I[νa(CH2)]/ I[νs(CH2)] indicator behavior. Comparison of C22 Phases to C18 Phases in Solvents. In a previous report,48 the conformational order of C22 phases in air was compared to that of C18 phases in air. To assess the effect of alkyl chain length on conformational order in the presence of the solvents studied here, values of (100 ∆IS/∆ISTD, Air) in a given solvent relative to those in air were calculated for pairs of C18 and C22 phases with comparable surface coverage. These values are plotted against solvent ∆G°HD in Figure 4. For surface-polymerized stationary phases, both C22 and C18 phases (Figure 4a and b) are more ordered in polar solvents than in nonpolar solvents. TFC22SF and TFC18SF exhibit similar changes in conformational order in polar solvents. However, polar solvents induce a greater change in order for DFC22SF than for DFC18SF. In total, chain length plays a minimal role in conformational order in polar solvents due to weak solvent-alkyl chain interactions in surface-polymerized systems. For solution-polymerized phases (Figure 4c and d), polar solvents induce ordering for C22 phases but disordering for C18 phases. Previous studies36,37 indicate that, although polar solvents solvate the solution-polymerized materials (TFC18SL and DFC18SL) primarily through adsorption-like interactions, the solvent molecules interact not only with the distal chain terminus region but with underlying terminal methylene groups as well, inducing slight conformational disorder. However, for C22 phases, the polar solvents interact mainly with the distal chain terminus region due to their more rigid alkyl chains. Thus, for solution-polymerized phases, chain length plays a significant role in dictating conformational order in polar solvents. Nonpolar solvents induce disordering for both C18 and C22 phases regardless of polymerization method. However, for surfacepolymerized phases, more disorder is generally induced for C22 phases than for C18 phases. This difference is absent for solutionpolymerized phases. These observations are consistent with a picture in which nonpolar solvents penetrate into the alkyl chains. Surface-polymerized stationary phases have more evenly distributed alkyl chains. Thus, the alkyl chain length plays an important role in determining conformational order in nonpolar solvents, especially benzene-d6 and toluene-d8, since these two solvents appear to penetrate through more methylene units into the alkyl chains than does chloroform-d. However, for solution-polymerized phases, due to their surface heterogeneity, both alkyl chain length and number of alkyl chains at the edge of islands determine the degree of disordering induced by these solvents. It should be kept in mind that the depth of penetration for each nonpolar solvent depends not only on the solvent but also on the stationary-phase conformational state. Although solvent

Figure 4. 100 ∆IS/∆ISTD, Air as a function of Gibbs free energy change for infinite solvent dilution in hexadecane (∆G°HD) for (a) DFC18SF (b) and DFC22SF (0), (b) TFC18SF (b) and TFC22SF (0), (c) DFC18SL (b) and DFC22SL (0), and (d) TFC18SL (b) vs TFC22SL (0).

relative penetration depth can be deduced based on change in alkyl chain conformation induced by each solvent, it is almost impossible to quantify absolute depth of penetration. However, it should also be pointed out that nonpolar solutes do not have to penetrate very deeply to be embeded into the alkyl chains. For example, for benzene to be fully embedded into a highly ordered all-trans alkyl chains, it needs to penetrate approximately four methylene units, while toluene needs to penetrate approximately five methylene units. For a less ordered system possessing gauche conformers, benzene and toluene are expected to penetrate through a few more methylene units. Nonetheless, these nonpolar solvents are highly unlikely to penetrate all the way through the alkyl chains to be at or even close to the silica surface due to unfavorable interactions with polar silianols or/and a surfaceadsorbed water layer. In examining Figure 4, it is interesting to note that stationary phases prepared from difunctional precursors exhibit trends similar to those prepared from trifunctional precursors. Thus, the nature of the precursor does not play a large role in determining conformational order of these systems. Solvent-induced changes of conformational order depend primarily on stationary-phase alkyl chain length, surface coverage, and surface homogeneity. Solvent-Induced Changes Compared to TemperatureInduced Changes. C22 stationary phases are usually used in separations of shape-constrained compounds such as PAHs, since they offer a greater degree of shape selectivity as a result of their greater conformational order.49 Among all parameters that affect shape selectivity, temperature is an important variable that has been used successfully in the shape-selective separation of PAHs.49

Mobile-phase composition, on the other hand, has a negligible effect on shape recognition.77 Although the solvents studied here do induce changes in alkyl chain conformational order, it is of interest to further compare changes in conformational order induced by solvent to those induced by temperature, since such comparison may provide additional insight into the relative contributions of these parameters to shape-selective behavior. Figure 5 shows plots of (100 ∆Is/∆ISTD, Air) as a function of surface coverage for the four C22 stationary phases in methanold4, acetonitrile-d3 and water-d2, and a plot of (100 ∆IT/∆ISTD, Air) for these phases in air as temperature is decreased from 50 to 10 °C. These temperatures were chosen based on a previous chromatographic study in which the effect of temperature on the stationary-phase shape selectivity was demonstrated49 and, therefore, were considered practical parameters. Values of 100 ∆IS/ ∆ISTD, Air were calculated as discussed above. 100 ∆IT/∆ISTD, Air were calculated using I10°C,Air - I50°C,Air for ∆IT, where I10°C,Air and I50°C,Air are I[νa(CH2)]/I[νs(CH2)] in air at 10 and 50 °C, respectively. ∆ISTD, Air is calculated as above. For phases with surface coverage of >4.5 µmol/m2, temperature-induced changes in conformational order are far greater than those induced by any solvent. These results are consistent with previous chromatographic studies that demonstrate that temperature is more significant in determining shape selectivity than composition of the mobile phase solvent.49,77 Interestingly, for the lowest coverage stationary phase that exhibits relatively poor shape selectivity, no (77) Sander, L. C.; Wise, S. A. J. Chromatogr., A 1993, 656, 335-351.

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Figure 5. 100 ∆IS/∆ISTD, Air as a function of surface coverage for C22 stationary phases in the presence of acetonitrile-d3 (b), methanold4 (9), and water-d2 (2); 100 ∆IT/∆ISTD, Air as a function of surface coverage for C22 stationary phases undergoing a change in temperature from 50 to 10 °C.

difference in solvent- and temperature-induced conformational order change is apparent. CONCLUSIONS Solvent-stationary-phase interactions were probed by Raman spectroscopy for a series of high-density C22 stationary phases in the polar, self-associating solvents methanol-d4, acetonitrile-d3, and water-d2 and in the nonpolar solvents toluene-d8, chloroformd, and benzene-d6. These results are compared with those published previously for similar C18 phases. The change of free energy for a solvent at infinite dilution in hexadecane (∆G°HD) dictates the solvent-induced change in stationary-phase conformational order. In general, polar solvents with positive ∆G°HD slightly increase the conformational order of these C22 stationary phases. The interaction of methanol-d4 and acetonitrile-d3 with C22 stationary phases can be described in terms of an adsorption-like interaction in which the distal methyl terminus region serves as an adsorption site. Both solvent characteristics (i.e., structure, self-association ability) and stationary-phase properties (surface coverage, homogeneity, and chain length) play roles in determining interactions between the solvent

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and the stationary phase. In general, solvents with higher selfassociation ability interact less with stationary phase. The alkyl chains are slightly more ordered in water-d2, suggesting that dewetting of the alkyl chains may dominate the behavior in aqueous environments, consistent with recent results from chromatographic and simulation studies. Nonpolar solvents with negative ∆G°HD generally intercalate into the alkyl chains and thereby decrease conformational order. The interactions of nonpolar solvents with these stationary phases depend on solvent characteristics (structure, shape, and size) as well as stationary-phase properties (surface coverage, homogeneity, and chain length). The planar benzene-d6 and toluene-d8 molecules penetrate through more methylene units into the alkyl chains than does the nonplanar and relatively bulky chloroformd, inducing more disorder. The stationary phase with highest surface coverage that has the smallest free volume exhibits shape selectivity toward these solvents. Finally, changes in conformational order induced by common mobile-phase solvents for these C22 phases are compared to those induced by temperature. Changes in conformational order induced by temperature are generally far greater than those induced by mobile-phase solvents, consistent with the contention that mobile phase plays a minor role in determining stationary-phase shape selectivity. ACKNOWLEDGMENT The authors gratefully acknowledge support of this research by the Department of Energy (DE-FG03-95ER14546). The authors are also grateful to Dr. Lane C. Sander of the National Institute of Standards and Technology for the generous gift of the C22 stationary phases. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review November 2, 2007. Accepted January 17, 2008. AC702270B