AMI.
1910
chem. 1001, 63, 1318-1322
Solvatochromic Studies of Stationary Phases on Thin-Layer Chromatographic Plates Jennifer L. Jones and Sarah C. Rutan* Department of Chemistry, Box 2006, Virginia Commonwealth University, Richmond, Virginia 23284-2006
phase ratio (volume of mobile phase:volume of stationary Tiwoo pdarlty-lmllcating dyos, Roichardt's botah, N , N d C phase) must be used to ensure that the signal observed arises othyCennroanMno, and N-methyC2-nitroallh, havo boon from species in the stationary phase. Despite this difficulty, uucl to c h a r a c t ~ otho m o b b p h ~ t b n a r y - p h a 8 e inCarr and Harris have been able to design experiments that torfaco d thh-hyor chromatographic plalos. DHhme rdkG permit octadecyl stationary phases to be characterized for tanco spoctrorcopy was used to dotoct tho shifts in tho relatively strong solvent "a(8,9).These worker's mdts wavohngth "aof then dyw whkh havo boon rolatod indicate that methanol-solvated CISstationary phases have totho x* and €430)solved polarMy acalos. For acdonMb polarities which are slightly less than bulk methanol, for and methanol/wator moblk-phaso mMuro8, tho r w l k lndC mobile phases ranging from 50% to 80% methanol/water. A cat0 that tho rolvatod octcrd.cyl statlonary phase k conddslight decrease in the surface polarity with the addition of erably more polar than tho analogous bulk alkane rolvont. water to the mobile phase was observed. An alternative apHowovor, tho exporhuntai evldonce supports tho ldoa that proach that ensures that predominantly surface-phase speciea t h o ~ ~ ~ t h o d y . ~ h a n ~ r p are o monitored r u u is based on total internal reflection fluorescence to bulk solvent. Control oxporImont8 on solvated silica surspectroscopy (11,12). While this approach can provide valfaces Indkate that " 0 of tho dyo8 are capd rp.cllic uable information, the solid surface must be optically flat, interactions with tho rliand groups on tho surfaco. precluding studies of actual chromatographic materials. The separation of solutes in liquid chromatography is based on the differential solvation of the solutea between a stationary phase and a mobile phase. Modem chromatographic methods typically employ bonded stationary phases, which are synthesized by reacting a functionalized silane with a silica substrate. While the chemistry of the mobile phase is relatively easy to characterize, the heterogeneous nature of the bonded stationary phases makes them much more difficult to study. This difficulty is exacerbated by the fact that different mobile phases will solvate the stationary phase to different extents (1-4). Therefore, the characteristics of these solvated surface phases need to be considered in order to obtain a more complete understanding of bonded-phase chromatography. The specific purpose of the work described here is to use polarity scales previously developed for asseasing bulk solvent polarity on the basis of W-visible spectral shifts for the characterization of the solvated stationary phase in liquid chromatography. Over the past several years, there has been increasing attention focused on the elucidation of the chemical and physical properties of solvated stationary phases. One of the most commonly used techniques for probing this heterogeneous environment is fluorescence spectroscopy. The fluorescence spectra of many species have been found to be solvent sensitive. One fluorophore, pyrene, has been used to establish a solvent polarity scale, based on changes in relative band intensities (5).Other molecules show shifts in the wavelength of maximum fluorescence intensity as a function of solvent polarity (6). Several investigators have used fluorescent POlarity probes to study solvated bonded phases (7-10).One advantage of using fluorescent probes is that the background signal from the silica substrate and bonded phase is small, and scattered light usually does not cause signiicant problems in the analysis. The major difficulty that must be addressed is ensuring that the measured signal arises primarily from species that have partitioned into the solvated stationary phase and not from species which remain in the mobile phase. For strong mobile-phase conditions (where the solute undergoes only weak interactions with the stationary phase), a small 0003-2700/91/0363-131S$02.50/0
UV-visible methods have been used lese frequently than fluoreeence methods for characterizing solvated stationary phases. There are two major factors that limit the utility of W-visible methods. One is due to the significant background contribution to the signal. A second problem is that scattered light may adversely affect the determination of the peak positions in the spectra, since reflectance spectroscopy must be used. Most of the studies that have been reported to date are based on the determination of spectral characteristics of "dry" surfaces rather than solvated interphase regions (13,141. There are advantages to obtaining information based on UV-visible solvent probes, however, as numerous polarity scales have been developed on the basis of W-visible spectral characteristics, and UV-visible methods yield information different from that obtained with fluorescence methods (15). A widely used scheme for examining the properties of homogeneous liquids is based on the Kamlet-Taft solvatochromic scalea of dipolarity-polarizabfity (**I, hydrogen bond acidity (a),and hydrogen bond basicity (8) (16,17).Other schemes for characterizing the polarity of bulk solvents have also been described (15). The two major scales that are of importance in thisstudy are the E&O) scale and the ?r+ scale. The ET(30)scale was developed by Christian Reichardt, where he used "solvatochromic band" shifts of a betaine indicator, 4-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide, and its penta-tert-butyl derivative to probe solvent polarities (18,19). This dye shows pronounced solvent-dependent spectral shifts. For example, this dye is pink in methanol (A, = 515 nm), green in acetone (A- = 677 nm), and blue in acetonitrile (A= 620 nm). This scale has been shown to give an indication of both the dipolarity and hydrogen bond donating (HBD) acidity of the solvent (15). The second scale, T * , is based on the solvent-induced shifta of the frequency maxima of the r r* transitions of several indicators, where the values are normalized so that the r* of cyclohexane is 0 and the r* of dimethyl sulfoxide (DMSO) is 1(20). The T* scale of solvent dipolarity-polarizability is presumed to represent solute-solvent interactions in the absence of strong forces such as hydrogen bonding or ion-dipole interactions (20). The r* values are normalized by the following equation:
-
0 1991 Amerlcen Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1991
T*
= [u,
- v,(cyclohexane)]/s
where, Y is the frequency of the absorbance maximum. The value for s is obtained from the following equation: s = v,(DMSO) - v,(cyclohexane) (2) Several different dyes have been characterized, and the v, and s values have been tabulated (20). The two polarity scales can be related through the following equation (211
ET(30) = 30.2
+ 12.99~*- 2.746 + 1 4 . 4 5 ~+~2.138
n = 100
r = 0.987
(3)
s = 1.25
This relationship shows that the ET(30)scale is related to the dipolarity-polarizability (#), a polarizability correction fador (a), the hydrogen bond acidity (a),and the hydrogen bond basicity (8)of the solvent. The CY scale of solvent HBD acidity is generated by examining the relationship between the shifts of two different dyes. The shift for one dye, a hydrogen bond acceptor (HBA) dye, which is presumed to be sensitive to HBD acidity, is plotted vs the shift for a second dye, which is presumed to be insensitive to HBD acidity of the solvent. The shifts for solvents incapable of HBD interactions with the HBA dye should fall on a straight line, while HBD solvents will deviate from this line in an amount related to their HBD acidity (16). The @ scale of HBA basicity can be generated by a similar analysis. In these studies, the method of diffuse reflectance spectroscopy is used to measure the UV-visible spectrum of the dyes present in the solvated stationary phase. If the reflectance of the sample, R, and the reflectance of a standard, such as barium sulfate, R,, are measured with an integrating sphere attachment, the measured reflectance can be transformed as follows:
f ( R = ) = k / S L = (1 - R/RB)2/(2R/R,)
(4)
The resulting function is directly proportional to k, which in turn is directly proportional to the molar absorptivity and concentration of the absorbing species, and is inversely proportional to sL, the scattering coefficient. The scattering coefficient is dependent on the wavelength and the particle size of the stationary phase (22). This equation is based on the assumption that the sample is optically thick and that the sample is composed of absorbing and lighbscatteringparticles, which are uniformly and randomly distributed and whose dimensions are much smaller than the thickness of the layer (22). Once the reflectance data are obtained, the variable background contribution can be subtracted by using an adaptive Kalman filtering approach that has been described previously (23).
EXPERIMENTAL SECTION All spectra reported here were obtained on a Shimadzu W-265 spectrophotometer. Data were transferred from the UV-visible spectrophotometer with an IBM PC AT computer through an IEEE488 interface using a program supplied by Shimadzu. The spectrophotometer was equipped with an integrating sphere attachment for reflectance measurements. This assembly was used to obtain the spectra of the solvatochromic dyes in pure solvents, on silica, and on reversed-phase (C,dthin-layer chromatographic (TLC) plates. The stationary phases studied here were commercially available TLC plates. The silica plate used were Kodak silica gel plates with a 100-pmlayer of silica and polymeric binder on a polyester backing. The reversed phase plates used were Whatman KC-18 reversed-phase plates with a 200-pm layer of Partisil silica with an octadecylsilane polymeric bonded phase. The solvatochromic dyes studied were ET-30, NJV-diethyl-4nitroaniline,and N-methyl-2-nitroanie and were obtained from Aldrich, Frinton Laboratories, and Kodak, respectively. The solvents used were methanol and acetonitrile from EM Science
131@
and were used as received. Hydroorganic mixtures containing up to 50% (v/v) water in 10% increments were prepared with house-deionized water. The experiments were performed by obtaining a spectrum of the solvatochromic dye in a 1 mm path length cuvette in the integrating sphere attachment. The and G(30values ) obtained in this manner were then compared to the x* and E~(30)values reported in the literature. Reflectance grade barium sulfate (Kodak) was used as the reference in the integrating sphere assembly in all instances. Then, spectra of the silica and Cleplates were obtained to study the interactions of the dye with the solvated stationary phase. A small amount of the solvatochromic dye solution in the appropriate solvent was applied to the stationary phase and was subsequently covered with a microscope cover slip to minimize the evaporation of the solvent. The method of analysis used for the determination of the wavelength maximum was to note the absorbance at its maximum value. A Kalman filter background subtraction approach, described previously (23), was used to subtract the appropriate contribution due to the background signal. The background was estimated by measuring the diffuse reflectance spectrum of the stationary phases in contact with the solvent with no dye present. From the wavelength of the peak maximum, the Y- can be determined and then used to calculate r* and E~(30)values from eqs 1, 2, and 5, respectively. For the calculation of the Et(30) values, the equation used was ET(30) (kcal) = 2.8591 x lO‘(l/A(nm)) (5)
+
For the A* calculation,the,v is calculated from the conversion factor le/&and substituted into eq 1, where the v,, and s valuea were taken from previous measurements for each of the dyes as reported by Kamlet, Abboud, and Taft (18). Capacity factors (k9 were measured on the basis of TLC elution on the same stationary phases used for the spectroscopic studies, with the same mobile-phase compositions.
RESULTS AND DISCUSSION There are several effects that must be considered before examination of the results for the stationary-phase environments. The possibility of a red shift due to the wavelength dependence of the scatter coefficient in eq 4 should be considered; however, for the wavelengths and particle sizes used here, there should not be a significant wavelength dependence of the scatter coefficient, sL (22). In addition, there is the possibility that a significant amount of dye is present in the mobile phase. This might be evidenced by a broadening of the dye spectrum, indicating heterogeneity of the dye environment. In order to assess the potential of this type of difficulty, the chromatographic capacity factors, k’, were measured for each of the stationary-phase-mobile-phase combinations. These results are listed in the tables along with the solvatochromic results. We assume that the phase ratios are similar for the spectroscopic and chromatographic experiments, so the capacity factors serve as an approximate measure of the ratio of moles of dye in the stationary phase to moles of dye mobile phase. A third consideration is that the C18stationary phases used in these preliminary studies are TLC phases, which are typically derivatized with Cle groups to a lesser extent as compared to analogous column chromatographic phases (24). The first dye chosen for study was ET-30, since this dye exhibits large shifts in homogeneous solution. The results for the ET(30) measurements are summarized in Table I. Reichardt and co-workers have measured ET(30) values for the hydroorganic mixtures studied here, and these literature values are also reported in Table I (25,26). In general, our results agree with Reichardt’s within f l kcal, which is reasonable considering that our solution phase measurements have been made with the integrating sphere attachment. In all but one case (90% methanol/water), our results are lower than Reichardt’s results. This could be due to differences in the residual water content of our solvents or due to the different
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ANALYTICAL CHEMISTRY, VOL.
63,NO. 13, JULY 1, 1991 58
Table I. ET-30Results
mobile phase
soln (25,26)
100% ACN 90% ACN 80% ACN 70% ACN 60% ACN 50% ACN 100% MeOH 90% MeOH 80% MeOH 70% MeOH 60% MeOH 50% MeOH
46.0 51.4 53.4 54.4 55.0 55.6 55.5 56.6 56.0 56.3 56.6 56.9
0
3 w’
59 58 57 56 55 54 53 52
47.0 54.2 55.1 55.5 56.1 56.5 55.5 55.9 56.3 56.8 57.2 57.5
57.4 54.4 53.9 53.1 52.3 58.3 55.2
53.7 54.3 55.6 55.8 57.2 54.7 54.4
55.8
54.0
55.6 55.9 56.4 54.9
53.3 52.8 52.4 52.6
9.9 0.9 0.3 0.9 0.4 0.5 2.7 2.1 1.9 1.1 0.8 0.2
13 15 17 31 51 2.5 4.2 15 55 41
.........
/./-
50
70 % Acetonitrile
,
52
!
50
70
Z
Methanol
90
Figure 2. Variation of Ed30) values wlth organlc content for methand/water mixtues: 0buk solvent; ( 0 )slUca sutace;(A)C,, sutace. I
,
I
k’ C18 silica Cls
I
, 48 47
ET (30),kcal soln (this work) silica
,
,y 90
Flgure 1. Varlatlon of Ed30) values with organlc content for acetonltrlle/water mlxtures: )(. bulk solvent; ( 0 ) silica surface; (A)CI8 surface.
experimental configuration used in our studies. Since the trends observed here are the same as ~ b s e ~ previously, ed this should not cause difficulties in the interpretation of the data for the surface phases. For both acetonitrile and methanol, the polarity/HBD acidity increases with increasing water content. This is consistent with the known chemistry of these mixtures. Note also that, except for the 100% acetonitrile solution, the Ed301 values for the solution phase are in close proximity to one another. This may indicate a solvent-sorting effect, where the ET-30 molecule tends to create a favorable solvent environment that is similar for the different water/organic percentages (27). The results for the different stationary phases are summarized for ET-30 in Table I and are shown graphically in Figures 1and 2. The data for the solvents with higher organic content are probably more reliable, since an increase in the water content results in a decrease in the intensity of the solvatochromic band. This may be attributed to the fact that the phenoxide group in the betaine can actually be protonated in neutral aqueous solvents (pK, = 8.63) (28,29). We found that the band intensity decreased regularly upon addition of hydrochloric acid, indicating that in the higher water content solvents, the decrease in spectral intensity may be due to the fact that a substantial fraction of the ET-30 molecules are protonated by the silanols on the silica surface. The behavior of this dye as a function of pH has not been completely studied, although other dyes with similar characteristics that do not accept protons at neutral pH’s have recently been described (28). The capacity factor for ET-30 on silica with a 100% acetonitrile mobile phase is 9.9, indicating that a substantial
fraction of the molecules are adsorbed onto the surface in the presence of acetonitrile. The ET(30)value indicates that this environment is significantly more polar than that of the bulk solvent. Upon addition of water to the mobile phase, the capacity factors for ET-30 drop to less than 1 in all cases, indicating that the ET-30 molecules are no longer strongly interacting with the surface in these hydroorganic solvent mixtures. Despite this lack of interaction, some of the measured ET(30)values are significantly different from the corresponding bulk solvent values. The trend is to decreasing polarity down to 60% acetonitrile/water, with an abrupt increase in polarity at 50% acetonitrile/water. The trend from 90% to 60% is difficult to interpret; however, these observations may reflect adsorption of water onto the silanols, creating a water layer on the surface. The ET-30 molecules probably reside largely in the mobile phase under our experimental conditions; this mobile phase may be enriched in acetonitrile. Note also that ET-30 is virtually insoluble in water and would be unlikely to adsorb onto a water-rich layer on the silanol surface (15). The reason for the abrupt increase in polarity at 50% acetonitrile/water is not clear, although this change is reproducible. Our values for ET(30) of silica exposed to solvent are significantly less than those observed for “dry” alumina samples with variable water content (14). The capacity factors for ET-30 on the cl8 phase for all acetonitrile/water mixtures are greater than 10, indicating that the majority of the ET-30 molecules are dissolved in the solvated stationary phase under our experimental conditions. The measured ET(30)values show that this environment is extraordinarily polar-approaching the polarity of the bulk mobile phase for solvents that contain water. The combination of the spectroscopic and chromatographic results suggests the following model: the phenoxide group of the ET-30 molecule interacts with the residual silanols on the surface, and this interaction is stabilized relative to the silica surface case described above by the presence of the C18chains, which can undergo hydrophobic interactions with the phenyl groups in the ET-30 molecules. The strong interaction of ET-30 with the silica in the absence of water in the mobile phase supporte this model. It is unlikely that a large amount of water would partition into the cl8 phase, even if a substantial amount of residual silanols are present (as is the case for TLC stationary phases). This model demonstrates the importance of multiple interactions in determining the chromatographic behavior of this solute in the acetonitrile/C18 system. The capacity factors for ET-30 in the methanol/silica system show a regular decrease with increasing water content. This behavior contrasts with the abrupt change observed for the acetonitrile/silica system from 100% to 90%, described above. This can be explained, in part, by the fact that methanol is an HBD acid, like water (and unlike acetonitrile).
ANALYTICAL CHEMISTRY, VOL. 63,NO. 13, JULY 1, 1991
Table 11.
T*
Resultr for N~-Diethyl-a-nitroaniline
** mobile phase
soln (16)
100% ACN 90% ACN 80% ACN 70% ACN 60% ACN 50% ACN 100% MeOH 90% MeOH 80% MeOH 70% MeOH 60% MeOH 50% MeOH
0.74
Table 111.
1921
0.74
soln (this work) silica 0.72 0.87 0.99 1.09 1.13 1.18 0.82 0.86 0.97 0.99 1.03 1.06
0.87 0.98 1.13 1.15 1.21 1.20 0.88 0.94 0.97 1.04 1.15 1.13
k' -
Cle
silica
CIS
0.4 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.1 0.3 0.2 0.3
0.3 0.7 1.5 3.5 8.6 62 0.2 0.5 0.8 1.4 2.5 7.5
0.74 0.84 0.85 0.83 0.85 0.84
0.82 0.88 0.84 0.80 0.86
0.82
**Results for N-Methyl-2-nitroaniline
mobile phase
soln (this work)
100% ACN 90% ACN 80% ACN 70% ACN 60% ACN 50% ACN 100% MeOH 90% MeOH 80% MeOH 70% MeOH 60% MeOH 50% MeOH
0.73 0.88 1.00 1.10 1.19 1.25 0.81 0.90 0.95 1.02 1.05 1.11
0.73 0.79 0.99 1.08 1.17 1.28 0.78 0.88 0.96 0.99 1.01 1-09
...... ...................
0.85 0.8
.............' " . " . ,+
.........
c I
I
50
70
90
'6 Methanol Flgure 4. Variation of ?r* values derived from measurements of N,NdiethyCenltroaniIine with organic content for methanol/water mixtures: (W) bulk solvent; ( 0 ) silica surface; (A)C18surface.
1
1.51
**
soln (30, 32)
7T
silica
k' -
Cl8
silica
C,s
0.5 0.6 0.4 0.5 0.4 0.2 0.0 0.1 0.1 0.3 0.1 0.2
0.3 0.7 1.1 2.5 4.9 29 0.1 0.3 0.5 0.8 1.4 3.6
1.4-
1.3-
0.76 0.71 1.04 0.80 1.19 0.95 1.28 0.99 1.38 0.98 1.43 1.07 0.89 0.86 0.94 0.92 0.96 0.93 1.09 0.96 1.27 0.98 1.17 0.93
1.2-
1.1
-
1-
%.. ...........
.... . . * . . . . . .
%..
0.9 -
0.7 50
\
\
0.8 -
70
90
'6 Acetonitrile Flguro 5. Variation of ?r* values derlved from measurements of N-methyl-2-nitroaniline with organic content for acetonitrile/water mixtures: )(. bulk solvent; ( 0 ) silica surface; (A)CI8 Surface.
1.3 NO.
1.3
1
I
P
0.7 ! 50
70 % Acetonitrile
90
T
Figwr 3. Varlatlon of ?re values derived from measurements of N,NdkthyCCnltroaniHne with organlc content for acetonltrile/water mixtures: (). bulk solvent; ( 0 ) silica surface; (A)CI8 surface.
-
I
Even though some of the K'values for the organic-rich solvents are moderately large (k' 2), the observed ET(30) values indicate an environment very similar to the bulk solvent. As the water content of the solvent increases, the discrepancy between the solution phase and silica surface results increases, thus indicating that the polarity of the silica surface is lower than for the corresponding bulk phase. This behavior is difficult to interpret, since the capacity factors for the two systems with the highest water contents are quite small. At this time, we cannot explain these results. The capacity factors for ET-30 in the methanolG18system show a regular increase with increasing water content, which is opposite from the trend observed for silica, indicating a change in the retention mechanism from normal phase to reversed phase. These values reflect significant interactions
0.7 4 50
70 X Methanol
90
I
Figure 6. Variation of ?re values derived from measurements of N-methyCPnttroanlline with organic content for methanoi/water mixtures: (). bulk solvent; ( 0 ) silica surface; (A)C18surface.
of the ET-30 molecules with the solvated stationary phase. In this case, the ET(30)values are significantly lower than the corresponding bulk solvent values, although they indicate that the environment is still quite polar. The trend is toward a slightly legs polar stationary-phase environment upon addition of water to the mobile phase. These results are in good agreement with observations of pyrene fluorescence reported by Carr and Harris (8, 9). The major conclusion reached from examination of the results described above is that solvated C18phases are significantly more polar than bulk alkanes, where the E ~ ( 3 0 ) values are approximately 31 kcal. In addition, all of the values
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ANALYTICAL CHEMISTRY, VOL.
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reported here for the solvated surfaces range from 52.3 to 58.3 kcal, which represent environments with equal or greater polarity than ethanol (15),and are relatively small relative to the range of values possible in bulk solution-31 kcal for alkanes to 63 kcal for water (15). The results that were obtained for the a* dyes are summarized in Tables I1 and I11 and Figures 3 through 6. The data for NJV-diethyl-hitroaniline and N-methyl-2-nitroaniline can be considered simultaneously, since they are both assumed to show shifts due primarily to dipolarity-polarizability effects. Note that the amino hydrogen on N-methyl2-nitroaniline is assumed to form a strong intramolecular hydrogen bond with the adjacent nitro group, making the NH group unavailable for hydrogen bond donation to the surrounding solvent. The data for acetonitrile and methanol-based solvents for these two dyes are shown in Figures 3 through 6. Both dyes show the same approximate trends with increasing water content The bulk solution polarity increases consistently with the addition of water. In the case of N-methyl-2-nitroaniline, our solution-phase values agree with values previously determined by Cheong and Carr to within A0.03 a* units (30, 31). We feel that this is excellent agreement, considering that we are using a very different experimental setup. (No previously reported values for hydroorganic mixtures for the dye NJV-diethyl-4-nitroanilinewere found in the literature). In general, the x* values for the silica surface increased with increasing water content and were somewhat higher than the analogous bulk solution values. Since the k'values for both dyes and both solvents were extremely small (see Tables I1 and 111), these T* values set the lower limit for the effective polarity experienced by the dyes interacting with the stationary phase. We we little spectral broadening with the silica surface, so we believe these values are reasonable reflections of the polarity in the vicinity of the solvated surface. Our values are in general less than those observed for hydrated silica surfaces, which were not exposed to bulk solvent (13). The fact that the silica surface appears more polar than the surrounding bulk solvent may be attributed to the fact that molecules at the interface will experience one environment on the side facing the surface and a different environment on the side that is exposed to bulk solvent. The general trends observed for the Cla bonded phases are distinctly different from those observed for the silica surface. For both dyes and both solvent systems, the R* values remain approximately constant as the water content of the bulk solvent is increased, and the values are lower than those observed for the analogous bulk solution and silica surfaces. The a* values are probably most accurate for the higher water content phases, as these systems have relatively high capacity factore. The x* values in all cases are much higher than bulk for alkanes, which have a* values on the order of 0.12 that increase to approximately 0.3 when the alkanes are saturated with the corresponding solvent (32).In general, the R* values obtained for N-methyl-2-nitroaniline were signifcmtly higher than those observed for NJV-diethyl-4nitroaniline,especially for strongly hydrogen bonding environments. This may be due to partial disruption of the intramolecular hydrogen bond in the ortho-substituted compound. The above observations seem to indicate that the Cla phases used in this study are strongly solvated with the bulk mobile phase, giving much higher d'values than expected on the basis of bulk alkane behavior. However, the fact that the a* values remain constant with increasing water content in the bulk solvent indicates that the CI8chains are effective in protecting the dye molecules from exposure to bulk solvent. The polarities of the methanol/water solvated CISphases in the pyrene studies were significantly lower than the
analogous acetonitrile/water solvated phasea (B), whereas our results show that the C18phasea solvated with either methanol or acetonitrile give environments with similar dipolarity-polarizability characteristics. These results may be explained by the fact that our phases probably have a higher silanol content, which would give a more polar surface enivornment and would allow more methanol to solvate the stationary phase. The results reported in this paper indicate that complete characterization of solvated surfaces using semiempirical solvent polarity scales will be much more difficult than the characterization of homogeneous solutions. In particular, the results for the &(30) and d'values showed distinctly different trends. Since each dye may probe a different microenvironment within the solvated stationary phase, it is critical to study a wide range of dyes that can serve as models for a wide variety of chromatographic solutes. These studies are preliminary in nature and will be extended to stationary phases that can be better characterized by a variety of other methods. Extension of this work to other dyes, solvent systems, and stationary phases should permit qualitative models to be developed, with quantitative models possible for specific classes of chromatographic analytes.
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RECEIVED for review October 19,1990. Accepted March 11, 1991. This work was supported in part by a grant from the National Science Foundation, Grant CHEM-8921315.
