ET(30) Surface Polarity Parameters of Alkyl- and Aryl-Group

Feb 10, 1999 - ET(30) surface polarity parameters are presented for alkyl-functionalized silica particles. Sixteen different chemically modified Aeros...
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Langmuir 1999, 15, 2103-2111

2103

ET(30) Surface Polarity Parameters of Alkyl- and Aryl-Group-Functionalized Silica Particles: Differentiating the Surface Environments by Means of the Application of Differently Substituted Reichardt’s Dyes† Stefan Spange,*,‡ Anett Reuter, and Dieter Lubda§ Department of Polymer Chemistry, Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, D-09111 Chemnitz, Germany and Merck KGaA, Frankfurter Strasse 250, D-64271 Darmstadt, Germany Received March 24, 1998 ET(30) surface polarity parameters are presented for alkyl-functionalized silica particles. Sixteen different chemically modified Aerosil and LiChrospher samples were used as silicas with various degrees of grafting and different chain lengths. The surface polarities of the silica samples were examined by measuring the UV/vis absorption maxima of differently substituted 4-(2,4,6-triphenyl-N-pyridinio)phenolate betaine dyes adsorbed to functionalized silica samples suspended in 1,2-dichloroethane or cyclohexane.The value of the ET(30) surface polarity parameter of the silica particle decreases with increasing degree of functionalization of the silica surface with alkyl groups. The lengths of the alkyl groups have no observable influence upon the value of the ET(30) parameter. The measured value of the ET(30) surface polarity parameter of a partially alkyl-group-modified silica sample is lowered more by using dyes with sterically demanding alkyl substituents in the 2 and 6 position of the phenolate ring of the dye than by using the standard dye. Contributions of hydrogen bonds and dipolarity/polarizability effects on the specific ET(30) values measured of individual silica particles with various residual silanol and alkyl groups, respectively, are discussed.

Introduction The empirical ET(30) solvent-polarity scale has been applied to describe a great variety of solvent-dependent chemical processes in organic chemistry during the last three decades.1,2 The original ET(30) polarity parameter is defined as the molar transition energy corresponding to the longest-wavelength UV/vis charge transfer (CT) absorption maximum of the standard betaine dye 2,6diphenyl-4-(2,4,6-triphenyl-N-pyridinio)phenolate (1) (Chart 1 and eq 1); measured in the respective environ-

Chart 1. Formula of 1 Used for the Determination of ET(30) Values.

ET(30) (kcal mol-1) ) (2.8951 × 10-3) νmax(1) (cm-1) ) 28591/λmax (nm) (1) -1 1-3

Later, the dimenment and expressed as kcal mol . sionless normalized ETN scale was recommended in order to circumvent the use of the obsolete energy unit kcal mol-1.1,3,17 It is defined according to eq 2, using water and

ETN ) [ET(30) - 30.7]/32.4

(2)

tetramethylsilane as extremely polar and nonpolar reference solvents.1,3 The betaine dye 1 exhibits one of the largest known negative solvatochromic effects. The color of this dye ranges from greenish-yellow in diphenyl ether (λmax ) 810 nm) to orange-red in 2,2,2-trifluoroethanol (λmax ) 478 † This paper is dedicated to Professor Dr. Christian Reichardt on the ocassion of his 65th birthday. ‡ Chemnitz University of Technology. § Merck KGaA.

(1) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, Germany, 1988. (2) Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Liebigs Ann. Chem. 1963, 661, 1. (3) Reichardt, C.; Harbusch-Go¨rnert, E. Liebigs Ann. Chem. 1983, 721.

nm). Because of the usefulness of 1 for empirically measuring the polarity of solvents, its application has also been extended to polymers,4,5 micelles,6,7 and various inorganic oxidic materials.8-13 The ET(30) polarity parameter of solid materials is usually determined by measuring the energy of the π-π* transition of 1 adsorbed to bare silicas,8-12 to chemically functionalized silicas,10,12 and to alumina.13 In other works, 1 was incorporated by a sol/gel process into ormosil networks14 or chemically (4) Paley, M. S.; McGill, R. A.; Howard, S. C.; Wallace, S. E.; Harris, J. M. Macromolecules 1990, 23, 4557. (5) Zaslavsky, B. Y.; Miheeva, L. M.; Masimov, E. A.; Djaforow, S. F.; Reichardt, C. J. Chem. Soc., Faraday Trans. 1990, 86, 519. (6) Zachariasse, K. A.; Nguyen, V. P.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676. (7) Grieser, F.; Drummond, C. F. J. Phys. Chem. 1988, 92, 5580. (8) Spange, S.; Hortschansky, P.; Ulbricht, A.; Heublein, G. Z. Chem. 1987, 27, 207. (9) Chronister, C. W.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793. (10) Spange, S.; Reuter, A.; Schramm, A.; Reichardt, C. Org. React. (Tartu) 1995, 29, 951. (11) Spange, S.; Reuter, A.; Vilsmeier, E. Colloid Polym. Sci. 1996, 274, 59. (12) Taverner, S. J.; Clark, J. H.; Gray, G. W.; Heath, P. A.; Macquarrir, D. J. Chem. Soc., Chem. Commun. 1997, 1147. (13) Michels, J. J.; Dorsey, J. G. Langmuir 1990, 6, 414.

10.1021/la980328u CCC: $18.00 © 1999 American Chemical Society Published on Web 02/10/1999

2104 Langmuir, Vol. 15, No. 6, 1999

bonded to the silica surface.15 For instance, 1 exhibits an orange-red color when adsorbed to silica,11 a red-violet color when adsorbed to nondried cellulose,16 and a green color when adsorbed to macroporous polystyrene.12 Unfortunately, 1 is only poorly soluble in water and practically insoluble in weak polar solvents such as n-hexane or benzene. Therefore, various betaine dyes substituted in the peripheral phenyl groups with polar or nonpolar groups have been synthesized in order to increase the solubility in water or nonpolar solvents. Excellent linear correlations have been obtained between the ET(30) values of 1 with the corresponding ET values of the substituted betaine dyes.1-3,17,18 Thus, ET(30) polarity parameters of nonpolar solvents can be obtained by measuring the UV/vis absorption maxima by means of secondary tert-butyl substituted betaine dyes in those solvents in which 1 is not soluble.3,17 In well-behaved regular solutions, the thermal motion of the solvent molecules surrounding the solvatochromic probe molecule causes an average polarity at ambient temperature.19 This is one reason for the good linear correlations obtained between the ET values of homomorphic pyridinium-N-phenolate betaine dyes measured in different solvents.3,17,18 The situation gets more complex when measuring probe dyes which are adsorbed from solutions to suspended solid materials and are then located at the solid/liquid interface. It is obvious that the thermal motion of functional groups at the surface of a solid material is generally lower than that of an adsorbed probe dye contacting the liquid phase. The probe dye should interact with the site on the surface corresponding to a minimum of the free energy. The penetration mechanism of the dye into the specific surface environment of the solid is determined by the properties of the dye itself. This effect has been discussed by investigating the polarity of binary solvent mixtures consisting of both alcohols with long alkyl chains and water20 and micellar solutions6-8,21 by means of differently substituted polarity-indicator dyes. Amphiphilic block copolymers also exhibit two environments with different polarities. However, this problem has not yet been studied by means of solvatochromic compounds. This problem is also of importance for inorganic/organic hybrid particles bearing at the surface various functional groups with different polarities. Such materials are particularly relevant as stationary phases in chromatography.22,23 Functionalized surfaces of solid materials, e.g., alkyl-substituted silica particles with different amounts of functionalized groups, can interact in many ways with various liquids.22,23 It is generally assumed that residual silanol groups exhibit higher surface polarity than those of surfaces containing alkyl groups.22 Alkyl-grafted silica particles seem therefore suitable models to investigate the polarity of chemically modified surfaces, because the thermal motion of the various differently polar groups is (14) Rottman, C.; Grader, G. S.; Hazan, Y. D.; Avnir, D. Langmuir 1996, 12, 5505. (15) Crowther, D.; Liu, X. J. Chem. Soc., Chem. Commun. 1995, 2445. (16) Spange, S.; Keutel, D. Liebigs Ann. Chem. 1993, 981. (17) Reichardt, C. Chem. Rev. 1994, 94, 2319. (18) Reichardt, C.; Harbusch-Go¨rnert, E.; Scha¨fer, G. Liebigs Ann. Chem. 1988, 839. (19) Linert, W.; Jameson, R. F. J. Chem. Soc., Perkin Trans. 2 1993, 1415. (20) Novaki, L. P.; Seoud, O. A. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 105. (21) Kessler, M. A.; Wolfbeis, O. S. Chem. Phys. Lipids 1989, 50, 51. (22) Scott, R. P. W. Silica Gel and Bonded Phases; Wiley: Chichester, 1993. (23) Park, J. H.; Carr, P. W. J. Chromatogr. 1989, 465, 137.

Spange et al.

restricted to rotation and vibration due to their covalent linkage to the silica surface.10,12,24 ET(30) surface polarity parameters for stationary reverse phases and functionalized silica particles are reported by Rutan,25,26 Spange et al.,10 and Clark et al.12 For the UV/vis measurements of the adsorbed pyridinium phenolate betaine dyes, different procedures and techniques were applied by the various authors. Rutan et al.25,26 used thin-layer chromatographic plates together with the solvent as samples, and a reflectance technique was employed. This method seems doubtful because the polarity of the solvents used was similar to the polarity of the silicas measured. Therefore, a distinction between the solvent polarity and that of the solvated surface seems unlikely. Drago9 and Spange et al.10 used the transmission technique which requires a careful choice of the suspending liquid to exclude both disturbing influences of light scattering and specific acid-base contributions. However, this method allows a clear distinction between the polarity of the particles and that of the surrounding solvent. Clark9 used carefully dried silica samples and a reflectance technique. However, independent of the UV/vis spectroscopic technique used for bare unaffected silicas, similar values of the ET(30) parameter were always obtained (see Results and Discussion). It was also generally found that the value of the ET(30) parameter decreases with functionalization of the silica surface with alkyl or aryl groups.10,12,25,26 The decrease of the value of the ET(30) surface polarity parameter of silica after alkyl substitution can be attributed to two different effects according to the origin of this specific solute (probe)/solvent interaction. Intermolecular solute/solvent interactions can be advantageously expressed by the LSE (linear solvation energy) relationship of Kamlet and Taft.27,28 The simplified Kamlet-Taft equation27 applied to solvatochromic shifts [with XYZ ) νmax(probe)]28,29 is given by eq 3 where (XYZ)o

XYZ ) (XYZ)o + aR + bβ + (s + dδ)π*

(3)

is the νmax(probe) in a reference system, e.g., a nonpolar inert solvent; R is the HBD (hydrogen-bond donating) ability, β is the HBA (hydrogen-bond accepting) ability; π* is the dipolarity/polarizability of the solvents; δ is a polarizability correction term that is 1.0 for aromatic, 0.5 for polyhalogenated, and 0 for aliphatic solvents; and a, b, s, and d are solvent-independent regression coefficients. Marcus calculated the LSE relationship for the ET(30) solvent parameters.30 This is shown in eq 4, with n )

ET(30) ) 30.2 + 12.99 (π* - 0.21δ) + 14.45R + 2.13β n ) 100, r ) 0.976, s ) 1.25

(4)

number of solvents considered, r ) correlation coefficient, and s ) standard deviation. From eq 4, it can be concluded that the ET(30) solvent parameter reflects mainly the HBD ability (R) and the dipolarity/polarizability (π*) of solvents (24) Duchet, J.; Chabert, B.; Chapel, J. P.; Grard, J. F.; Chovelou, J. M.; Jaffrezic-Renault, N. Langmuir 1997, 13, 2271. (25) Jones, J. L.; Rutan, S. C. Anal. Chem. 1991, 63, 1318. (26) Helbrun, R. S.; Rutan, S. C.; Pompano, J.; Mitchern, D.; Patterson, T. Anal. Chem. 1994, 66, 610. (27) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (28) Taft, R. W.; Kamlet, M. J. J. Chem. Soc., Perkin Trans. 2 1979, 1723. (29) Marcus, Y. Chem. Soc. Rev. 1993, 22, 409. (30) Marcus, Y. J. Solution Chem. 1991, 20, 929.

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Langmuir, Vol. 15, No. 6, 1999 2105

and only scarcely their HBA ability (β).17,29-31 A recalculation of eq 4 for a majority of n ) 166 solvents shows that the β and δ term can be disregarded without worsening the correlation (eq 6).17,29 These correlation analyses show that the ET(30) values represent mainly the dipolarity/ polarizability and HBD ability of an environment. β parameters for functionalized silica particles have been reported.32 π* parameters for bare silica were first reported by Leffler et al.33 The authors used six structurally different Kamlet-Taft polarity indicators that are substituted nitroanilines and nitroalkoxybenzene derivatives. They found that every indicator yields another value of the π* parameter for bare silica by employing the original single-parameter equations from Kamlet and Taft. Their calculated π* values ranged from 0.65 to 2.0, depending on the basicity of the indicator used.33 This result is due to the fact that these indicators reflect both contributions of the surface polarity, the HBD capacity, and the dipolarity/polarizability. Therefore, single-parameter equations are not suitable for these indicators by applying to silica surfaces to calculate the π* term. This has been discussed by us in detail.9 The fluorescence probe pyrene seems more promising for describing the dipolarity/polarizability property of surfaces because the intensity ratio of its I1 and I3 emission bands (Py) can easily be measured and is a linear function of the Kamlet-Taft π* solvent parameter (see eq 12).17,36,37 However, in the case of strong HBD solvents or surface groups, a correction is necessary because the R term must also be considered.36 Measurements of the surface polarity of reverse-phase materials and bare silica by means of fluorescence probes are reported by several authors.37-43 A possible approach to separate the amount of the R term from the unit of measurement of the ET(30) parameter of cellulose and appropriate cellulose solvents was previously reported.34 The same procedure has been successfully applied to functionalized silica particles.35 Marshall et al.43 reported on the localization of different fluorescence probes within different sites of the bondedphase layer of silicas. However, the probes used in this study43 are not suitable to parametrize the specific polarity of different surface environments. It could be demonstrated that values of the measured ET(30) parameter for functionalized silicas (eq 5)35 are more strongly influenced

ET(30) ) 14.5R + 5.3π* + 36.1 n ) 30; r ) 0.966; s ) 1.9

(5)

by the HBD term than those of well-behaved regular solvents (eq 6).29 (31) Matyushov, D. V.; Schmid, R.; Ladany, B. M. J. Phys. Chem. B 1997, 101, 1035. (32) Spange, S.; Reuter, A.; Linert, W. Langmuir 1998, 14, 3479. (33) Lindley, S. M.; Flowers, G. C.; Leffler, J. E. J. Org. Chem. 1985, 50, 607. (34) Spange, S.; Reuter, A.; Vilsmeier, E.; Heinze, T.; Keutel, D.; Linert, W. J. Polym. Sci. 1998, 36, 1945. (35) (a) Spange, S.; Reuter, A. Langmuir 1999, 15, 141. (b) Reuter, A. Synthese und Charakterisierung modifizierter Polykieselsa¨ uren: Strukturun tersuchungen und UV/vis spektroskopische Bestimmung von Oberfla¨chenpolarita¨tspara metern mittels solvatochromer Farbstoffe. Ph.D. Thesis, Shaker Verlag, Aachen, 1997. (36) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560. (37) Krasnansky, R.; Thomas, J. K. in The Colloid Chemistry of Silica; American Chemical Society: Washington, DC, 1994; 223. (38) Wong, A. L.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1991, 63, 1076. (39) Carr, J. W.; Harris, J. M. J. Chromatogr. 1989, 481, 135. (40) Carr, J. W.; Harris, J. M. Anal. Chem. 1987, 59, 2546. (41) Carr, J. W.; Harris, J. M. Anal. Chem. 1986, 58, 626. (42) Rutan, S. C.; Harris, J. M. J. Chromatogr. A 1993, 656, 197. (43) Burns, J. W.; Bialkowski, S. E.; Marshall, D. B. Anal. Chem. 1997, 69, 3861.

ET(30) ) 15.2R + 11.5π* + 31.2 n ) 166; r ) 0.979; s ) 2.1

(6)

In other words, the susceptibility (i.e., the coefficients a and s of eq 3) of the π* and R terms upon the value of the ET(30) parameter is different for liquids (a/s ) 1.32) and moderately acidic surfaces of solids (a/s ) 2.73). Nevertheless, the Kamlet-Taft scale should be handled with caution as it assumes the solvatochromic parameters to be mathematically orthogonal (independent) and neglects any higher order term as discussed in our previous paper.35 This is true even though an averaged polarity was measured for the functionalized silicas.35 The question remains: do different sites of functionalized silica exhibit different polarities? The objective of this paper is to show that differently alkyl-substituted pyridinium-N-phenolate betaine dyes of the Reichardt-type with bulky alkyl groups in the 2 and 6 position of the phenolate ring are suitable for an adsorption to various sites of different polarity of a specifically modified silica particle. The functionalized silicas used in this work have been modified gradually by means of short and long alkyl chains. Of particular interest is the influence of the number and length of the alkyl chains upon both the specific polarity of the residual silanol groups and the polarity of the particle surface as a whole. For these investigations, different kinds of alkyl-substituted silica particles were used. The correspondence of our results with those of the probe pyrene from the literature will be discussed.37,41,42 Experimental Section Materials. The alkyl-group-modified silica particles were synthesized by reaction of thermally pretreated silica particles with different amounts of alkyltrimethoxysilanes (samples 3-5, 7), alkylmethyldimethoxysilanes (samples 12-17), chlorodimethylalkylsilane (samples 18-20), and an alkyltrichlorosilane (sample 6) according to procedures given in the literature.22,24,46 A typical experiment is as follows. Aerosil 300 (4 g) is dried in a vacuum at 400 °C for 12 h in a glass flask. After it is cooled to room temperature under dry argon, the Aerosil 300 is suspended in 70 mL of dry toluene. The desired amount of the silane is then added with stirring over 15 min. After the addition of silane, the mixture is heated for 5 h to remove the alcohol formed by azeotropic distillation. Then the mixture is stirred for ca. 12 h. The crude solid product is separated by centrifugation and then extracted with toluene in a Soxhlet extractor for 5 h. After being carefully washed with three portions each of toluene, acetone, and diethyl ether, the solid product is dried in a vacuum at 70 °C. The final product is colorless and exhibits the same external morphology as the former Aerosil 300. The modified silica samples prepared in this way have been characterized by quantitative combustion analysis (determination of C and H), Brunauer-Emmett-Teller (BET) measurements, and diffuse reflectance Fourier transform (DRIFT) spectroscopy. 13C{1H} cross-polarization/magic-angle spinning (CP MAS), 29Si{1H} CP MAS, and 1H MAS solid-state NMR spectroscopy were employed in selected cases to confirm the desired structure of the products.35,44 The assignments of the 13C and 29Si NMR signals to specific functional groups on the silica surface were carried out according to previous reports.44-46 Aerosil R 972 and R 805 were commercially available products from Degussa, Frankfurt (Main). The LiChrospher particles no. 1220 were synthesized by a procedure similar procedure to that used for the commercially available products from Merck, (44) Francke, V.; Gu¨nther, H.; Reuter, A.; Spange, S. Unpublished results. (45) Zaper, A. M.; Koenig, J. L. Polym. Compos. 1985, 6, 156. (46) Bayer, E.; Albert, K.; Reiners, J.; Nieder, M.; Mu¨ller, D. J. Chromatogr. 1983, 264, 197.

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Spange et al.

Table 1. Physical Characteristics of the Silicas and Functionalized Silicas Used in This Work material Aerosil 300 Aerosil 300 (dried) methyl-|-O-Si-CH3 n-octyl-|-O-Si-(CH2)7CH3 n-octadecyl-|-O-Si-(CH2)17CH3

aerosil R 972 (methyl-) Aerosil R 805 (n-octyl-) LiChrospher Si 100-5 µm LiChrospher Si 100-5 µm (dried) LiCh. 18e-10 µm |-Si-(CH2)17CH3 endcapped LiCh. 18-5 µm LiCh. 18-10 µm LiCh. 8-5 µm |-Si-(CH2)7CH3 LiCh. 8e-5 µm endcapped LiCh. 8-10 µm LiCh. 18-5 µm LiCh. 18-5 µm LiCh. 18-5 µm

sample

BET surface area (m2 g-1)

carbon content (%)

surface coverage (µmol m-2)

1 2 3a 4a 5a 6a

240 240 210 205 217 229

1.2 4.3 2.6 9.7

4.8 2.2 0.6 5.0

7a

200

7.4

5.1

111.5 194 366 366 321 383 321 321 383 321 270 243 200

0.9 5.4

6.7 2.9

20.4 20.8 19.7 12.2 13.1 12.8 4.7 8.2 11.2

2.9 2.5 2.8 4.0 3.6 4.2 0.81 1.57 2.6

8 9 10 11 12 13 14 15 16 17 18 19 20

a The CP-MAS 29Si{1H} NMR spectra indicate small amounts of surface oligomerized silane precursor (T3 signal appears) and a complete conversion of the geminal silanol groups (the Q2 signal is absent).35b

Chart 2. Further Substituted Pyridinium-N-phenolate 2-6 Used as Secondary Standard Dyes for Solvatochromic Measurements

Darmstadt. The surface coverages of all silica samples were calculated by the method of Kova´ts.47 The physical properties and chemical constitution of the organically modified silica samples are compiled in Table 1. The organically modified silica samples have been dried carefully in a vacuum at 70-80 °C in a glass vessel. After being cooled to room temperature under dried argon, a solution of the probe in 1,2-dichloroethane (DCE) is simply added to the silica material. The novel alkyl-substituted pyridinium-N-phenolate betaine dyes were kindly provided by C. Reichardt, Marburg. The formulas of the substituted dyes used are shown in Chart 2. Calculation of the ET(30) Surface Polarity Parameters. For the UV/vis spectroscopic measurements, we employed special UV/vis equipment for measuring the silicas in suspension. The silica suspensions were measured by means of an immersion quartz cell, TSM 5 (Carl Zeiss Jena, Germany) which has been placed directly in the slurry. The cell is connected with a diode array UV/vis spectrometer MCS 400 (Carl Zeiss Jena, Germany) via a glass fiber optic. The optical path length of the glass windows of the immersion cell is d ) 0.5 cm. Vertically shifting the immersion in the slurry allows a clear distinction between the (47) Kova´ts, E. Adv. Coll. Sci. 1976, 6, 95.

UV/vis adsorption spectra of the surrounding solution and the deposited particles. A subtraction of the spectrum taken from the supernatant solution, after the particles are deposited, from the aggregate spectrum was carried out for each sample measurement to prove the quantitative adsorption of the indicator dye to the surface of the particles. A quantitative adsorption of the respective dye used was achieved for each νmax value reported in this paper. The measurements at the solid/liquid interface are advantageous because they can be performed under inert conditions and better reflect the conditions suitable for liquid chromatography and catalysis in suspension. The following solvents have been used as suspending liquids for the bare silicas and functionalized silicas: n-hexane, cyclohexane, toluene, DCE, nitromethane, acetonitrile, and tetrahydrofuran. The light scattering of the functionalized silicas samples disturbs the accurate recording of the UV/vis transmission spectra in n-hexane, toluene, and nitromethane. In the moderate HBA solvents, acetonitrile (β ) 0.38) and tetrahydrofuran (β ) 0.55), Reichardt’s dye did not adsorb well to the silica samples studied because the silica surface is shielded by these solvents. In this case, the UV/vis absorption spectra of the dissolved dye alone is measurable. This result also shows that the ET(30) polarity parameters reported earlier24,25 are questionable because HBA solvents, i.e., acetonitrile, were used as the liquid. It should be emphasized that bare silicas yield absolutely transparent suspensions in cyclohexane and DCE. For 2 h after the mixing of the components, no shift of the UV/vis absorption maximum of the adsorbed dye in DCE is observed. The reproducibility for determination of the value of the ET(30) parameter for bare silicas is very good; the error is less than (0.2 kcal mol -1. Corresponding to an earlier report,9 no effect is observed for the concentration of the dye upon the position of the UV/vis absorption maximum of the adsorbed dye. Of course, the UV/vis absorption bands of the adsorbed dyes are rather broad in some cases when using highly functionalized silica particles. Presumably, this effect is caused by a broader distribution of the surface polarity. Despite this effect, the reproducibility for the determination of the ET(30) surface polarity parameter is also rather good; the error is (0.5 kcal mol-1 for highly functionalized silicas. Through the use of 1, the ET(30) parameter was calculated according to eq 1. Sometimes, the secondary standard betaine dyes 2-6 have been used for experimental reasons (better

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Langmuir, Vol. 15, No. 6, 1999 2107

solubility, reduced basicity, etc.). The following linear correlation equations have then been used to calculate the ET(30) parameter from the ET values of 2-6.

2: 2,6-Dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate21 ET(30) ) 0.986ET(2) - 7.644; r ) 0.990, n ) 10

(7)

3: 2,6-Dichloro-4-{2,4,6-tris[4-(tert-butyl)phenyl]1-pyridinio}phenolate48 ET(3) ) 1.116ET(30) + 2.911, r ) 0.966, n ) 23, s ) 0.796 (8) 4: 2,6-Di[4-(tert-butyl)phenyl]-4-{2,4,6-tris[4-(tert-butyl)phenyl]-1-pyridinio}phenolate17 ET(4) ) 0.942ET(30) - 1.808; n ) 16, r ) 0.999, s ) 0.17 (9) 5: 4-{4-[4-(tert-butyl)phenyl]-2,6-bis[3,5-di(tert-butyl)phenyl]-1-pyridinio}-2,6-bis[4(tert-butyl)phenyl]phenolate49 ET(5) ) 0.908ET(30) + 2.670; n ) 28, r ) 0.996, s ) 0.688 (10) 6: 4-{2,6-Bis[4-(adamantan-1-yl)phenyl]-4-[4(tert-butyl)phenyl]-1-pyridinio}-2,6-bis[4(tert-butyl)phenyl]phenolate49 ET(6) ) 0.931ET(30) + 2.438; n ) 28, r ) 0.999, s ) 0.392 (11)

Results and Discussion Altogether, we have employed six differently substituted pyridinium-N-phenolate betaine dyes 1-6 in order to examine the surface polarity of bare and organically modified silicas. The first striking result is that dyes 1-6 all yield the same ET(30) value for bare, nonmodified silica samples, whether Aerosil or LiChrosphers. In addition, several authors (Spange,8,10,11 Drago,9 and Clark et al.12) reported, independently of each other, similar values of the ET(30) polarity parameter for differently produced silicas. In Table 2, ET(30) values are compiled for different silica samples measured by means of different betaine dyes taken from the literature. The value of the ET(30) polarity parameter for bare silicas (dried in a vacuum at 150 °C) always amounts to 57 ( 1 kcal mol-1, despite the well-known complexity of the surface of bare silica particles containing geminal silanol groups, isolated silanol groups, vicinal silanol groups, and siloxane bridges. Moreover, silicas produced by various chemical methods, i.e., fumed or precipitated silicas, exhibit very similar values of the ET(30) polarity parameter. The similarities in the values of the ET(30) parameters for different catalytic activities of bare silicas are attributed to variations of the acidity property R versus the dipolarity/polarizability π* of the silica surface.11 Reports on this specific problem will be given in a subsequent paper.50b The second important experimental observation is also striking: different betaine dyes yield different values of the ET(30) parameter for partially modified silica samples. (48) Reichardt, C.; Scha¨fer, G. University of Marburg, Marburg, Germany. Unpublished results. (49) Reichardt, C.; Lo¨bbecke, S.; Mehranpour, A. M.; Scha¨fer, G. Can. J. Chem. 1998, 76, 686. (50) (a) Spange, S.; Simon, F.; Heublein, G.; Jacobasch, H.-J.; Bo¨rner, M. Colloid Polym. Sci. 1991, 269, 173. (b) Spange, S.; Adolph, S.; Vilsmeier, E.; Zimmermann, Y. Unpublished results.

Table 2. ET(30) Surface Polarity Parameters of Various Bare Silicasa silica sample and conditions for the UV/vis measurement Aerosil 300 (untreated) measured in DCE by transmission technique (TM) Kodak silica gel plates measured with 50% acetonitrile/water in reflectance measured with 60% methanol/water in reflectance Fisher silica (dried at 200 °C) measured in CH2Cl2 (TM) Aerosil 300 (dried at 150 °C) measured in DCE (TM) Aerosil 300 (untreated) measured in DCE (TM) KG 60 (dried at 200 °C) measured in DCE (TM) Aerosil 300 (dried at 200 °C) measured in C6H12 (TM) KG 60 (untreated) measured in DCE (TM) KG 100 (untreated) measured as solid in reflectance KG 100 (dried at 150 °C) measured as solid in reflectance LiChrospher Si 100 (untreated) measured in C6H12 (TM) LiChrospher Si 100 (dried at 200 °C) measured in C6H12 (TM) Aerosil 300 (untreated) measured in DCE (TM) Aerosil 300 (dried at 200 oC) measured in DCE (TM)

ET(30) (kcal mol-1) 59.2

reference Spange8 Rutan23

58.3b 56.4b

Rutan23

57.5

Drago9

58.0

Spange et al.10

59.5

Spange et al.10

57.6

Spange et al.11

57.6c

Spange et al.11

59.0

Spange et al.11

60.5

Clark et al.12

56.2

Clark et al.12

57.4c

this work

56.7c

this work

59.8

this work

57.8

this work

a Measured by means of Reichardt’s dye, taken from literature and from this work, in chronological order. b Selected values from reference 23. c Measured with penta-tert-butyl substituted 4.

Figure 1. UV/vis absorption spectra of 1 and 4, both adsorbed to Aerosil R805 from their solution in DCE.

To avoid mixing arguments, we will first discuss the results obtained with Aerosil R 805, for which the ET(30) parameter was measured by means of all six betaine dyes. Figure 1 shows the characteristic vis spectrum of 1 and 4, both adsorbed to Aerosil R 805 in DCE, with their typical broad long-wavelength intramolecular CT absorption band. As can be seen from Figure 1, 1 and 4, adsorbed to Aerosil R 805, exhibit only a single long-wavelength vis absorption band that indicates the specific polarity of this silica sample. For vis absorption, bands with two maxima or shoulders are not observed using 1-6; all adsorbed to the modified silicas used. The possible influence of the structure of the betaine dye upon the measured values of the respective ET(30) parameter is demonstrated in Figure 2 for Aerosil R 805. To visualize this result, the values of the ET(30) parameter of Aerosil R 805, measured by 1-6 are plotted against the total number of carbon atoms of the indicator dyes used.

2108 Langmuir, Vol. 15, No. 6, 1999

Figure 2. ET(30) parameter of Aerosil R 805 as measured by 1-6 as a function of the whole number of carbon atoms of the respective dye.

It is seen that there are two groups of betaine dyes measuring different values of the ET(30) polarity parameter, i.e., 1-3 and 3-5, for the specifically alkyl-groupfunctionalized silica samples. The difference is evident and corresponds to a decrease of polarity compared to those of ethanol [ET(30) ) 51.9] and DMSO [ET(30) ) 45.1]. 1-3 do not contain sterically demanding tert-butyl or 1-adamantyl groups in the para position of the phenyl rings in the 2 and 6 position of the phenolate ring. These betaine dyes evidently measure a higher surface polarity than do 4-6. 4-6 contain sterically demanding alkyl groups in the para position of the two phenyl rings linked to the 2 and 6 position of the phenolate ring, as well as a higher number of total carbon atoms. Whereas 4 and 6 measure the same value for the ET(30) parameter in the range of the standard deviation of eqs 5 and 7, 5 measures a higher value for the ET(30) parameter. 5 has been alkyl-substituted in the 3 and 5 position of the two phenyl rings linked at the 2 and 6 position of the pyridinium ring, whereas 4 and 6 bear the tertiary butyl groups in the 4 position. Perhaps different conformers of 5 do contribute to the measured UV/vis spectrum. A theoretical prediction of this effect is given earlier.51 We think that two different effects are responsible for the two different values of the ET(30) polarity parameter measured for specifically modified silica samples. First, the alkyl groups linked to the silica surface cover the residual silanol groups and therefore 4-6 have no direct access to these polar groups because the phenolate oxygen is shielded by the large tert-butyl groups. This would also explain why 5 measures a slightly higher value than those of 4 and 6 because the steric access of 4 and 6 to the silanols is strongly prevented because of the tert-butyl groups in the 4 positions which more strongly disturb the penetration toward the silanol groups. The comparatively large value of the ET(30) parameter measured dyes 1-3 can be explained by taking into account the following considerations. It is well-established that HBD molecules (such as phenol) which specifically interact with the phenolate oxygen of 1 cause a strong hypsochromic shift of the longwavelength CT absorption band.52,53 Therefore, it is expected that 1 interacts more strongly with the surface HBD silanol groups than do 4-6 in the case of the partially alkyl-modified silica samples. That is, the chromophoric (51) De Alencastro, R. B.; Da Mota Neto, J. D.; Zerner, M. C. Int. J. Quantum Chem., Quantum Chem. Symp. 1994, 28, 361; Chem. Abstr. 1995, 122, 83696t. (52) Spange, S.; Lauterbach, M.; Gyra, A. K.; Reichardt, C. Liebigs Ann. Chem. 1991, 323. (53) Coleman, C. A.; Murray, C. J. J. Org. Chem. 1992, 57, 3578.

Spange et al.

π system of 1-3 is influenced more significantly by the residual silanol groups than are the systems of 4-6. Second, it is generally expected that the dispersion forces between the betaine dye and the alkyl chains of the surface should increase with increasing numbers of alkyl groups of the betaine dye. A systematic contribution of dispersion forces to the values of the ET(30) parameters with increasing alkyl substitution cannot be measured.54 In this case, a linear correlation (and no jump) between the value of the ET(30) parameters and the total number of carbon atoms of the dyes would be expected and was observed. As can be seen from Figure 2, each group of 1-3 and 4-6 measures a definite value of the ET(30) polarity parameter within a certain range of error. It should be noted that the ET(30) values determined by means of 2-6 are calculated with the correlation equations 7-11, with reference to 1 (see Experimental Section). For the calculated values of the ET(30) parameter, the standard deviation is about 1 kcal mol-1. As a consequence from Figure 2, it can be concluded that a particular dye from each group of 1-3 and 4 or 6 should be used to determine specific ET(30) values in the framework of the ET(30) polarity scale of different environments of partially alkylgroup-modified silica samples. Because the phenolate oxygens of 4-6 have no direct access to the residual silanols, the betaine dyes measure the polarity of the assessable alkyl group’s environment. This argument shows that both intermolecular interactions are important to explain the different experimental results: (i) the steric hindrance which prevents silanol/ phenolate interactions and (ii) the dipolarity/polarizability forces between the alkyl groups of the silica with 4-6. The intermolecular alkyl/alkyl group interaction, however, does not have a measurable influence upon the chromophoric π system of 4-6.31,54 Hence, a lower value of the ET(30) parameter is measured with these betaine dyes. The contribution of the HBD property of the alkyl environments is smaller than that of the silanols. This is the reason for the observation that these betaine dyes measure a lower value of the ET(30) parameters.54 For the following experiments, we have selected 1 and 4 because both dyes are commercially available.55 5 and 6 are rather expensive compounds, and their preparation is complicated.49 The νmax values of 1 and 4, both adsorbed to various silica samples, have been measured in DCE suspensions. Unfortunately, 1 does not adsorb to highly functionalized alkyl-modified (>4 C18 chains/nm2) silica samples of the LiChrospher type. Obviously, DCE [ET(30) ) 41.3] is a stronger polar medium than these highly alkylgroup-modified silica samples. Therefore, the value of the ET(30) surface polarity parameter of the six reverse-phase materials (12-17, Table 1) could only be measured by using penta-tert-butyl substituted 4 in cyclohexane suspension. 4 is readily soluble in cyclohexane [ET(30) ) 30.9] and does quantitatively adsorb to the LiChrospher sample from this solvent. This adsorption process can easily be followed visually. The cyclohexane solution of 4 is yellow, but after the addition of the silica example to the yellow solution and the adsorption of the betaine dye to the LiChrospher, the suspension develops a green color. UV/vis absorption maxima, the corresponding values of ET(30), and normalized ETN surface polarity parameters, are listed in Tables 3 and 4 for sixteen different alkyl(54) Rauhut, G.; Clark, T.; Steinke, T. J. Am. Chem. Soc. 1993, 115, 9174. (55) Aldrich-Chemie GmbH, D-89555 Steinheim, Germany, (order no. 27, 244-2) (see also Aldrichimica Acta 1987, 20 (2), 59; 1991, 24(3), 81); Fluka-Chemie AG, CH-9471 Buchs, Switzerland (order no. 43358).

Polarity Parameters of Silica Particles

Langmuir, Vol. 15, No. 6, 1999 2109

Table 3. Maxima of vis Absorption of 1a sample

νmax 1 × 10-3 cm-1

ET(30) kcal mol-1

ETN

1 2 3 4 5 6 7 8 9 10 11 18 19 20

20.92 20.21 17.62 18.09 18.72 19.03 19.21 17.99 18.28 20.08 19.84 19.96 19.51 18.24

59.8 57.8 50.4 54.5 53.5 54.4 54.9 51.4 52.3 57.4 56.7 57.1 55.8 52.1

0.90 0.84 0.61 0.60 0.70 0.73 0.75 0.64 0.67 0.74 0.73 0.81 0.77 0.66

a

Measured after adsorption of 1 to fourteen differently functionalized silica samples in a DCE suspension, as well as the corresponding ET(30) (cf. eq 1) and normalized ETN-values (cf. eq 2). Table 4. Maxima of the vis Absorption of Penta-tert-butyl Substituted 4a sample

νmax(4) (×10-3 cm-1)

ET(30) (kcal mol-1)

ETN

2 3 4 5 6 7 8 9 12 13 14 15 16 17 18 19 20

20.15 15.78 17.43 17.66 16.37 17.65 17.23 14.98 14.10 13.37 12.97 13.04 12.95 12.43 17.54 16.92 16.75

57.6 46.0 51.0 51.7 47.7 51.6 50.4 43.5 40.9 38.6 37.4 37.6 37.4 35.8 51.3 49.4 48.9

0.84 0.56 0.56 0.68 0.64 0.70 0.63 0.56 0.31 0.24 0.21 0.21 0.21 0.16 0.64 0.58 0.56

a Measured after adsorption of to seventeen differently functionalized silica samples in a suspension of cyclohexane, as well as the corresponding ET(30) (cf. eq 5 and normalized ETN Values cf. eq 2).

group-functionalized aerosols and LiChrosphers by applying 1 and 4. Pronounced differences were observed for the νmax values and for the corresponding values of the ET(30) polarity parameters as function the chemical modification of the silica surface. All of the organically modified silica samples studied exhibit ET(30) polarity parameters lower than those of the nonmodified silicas. The ET(30) polarity parameters as function of the degree of functionalization are shown in Figure 3. In Figure 3, two curves are arbitrarily drawn to guide the eyes of the reader. The upper curve shows the plot for alkyl-functionalized Aerosils (b) in DCE and the lower curve for the LiChrosphers (9) in cyclohexane. The values of the ET(30) polarity parameter decrease steadily with increasing concentration of alkyl groups at the silica particle surface. These results are consistent with the ET(30) values reported by Clark et al.12 for phenylsilanemodified silica particles. The values of the ET(30) parameter of the LiChrosphers measured in DCE fit into the curve of the functionalized Aerosils. Therefore, the influence of the contacting liquid phase (DCE or cyclohexane) upon the value of the ET(30) surface polarity is really of importance. Both solvents are weak HBA solvents. Stronger HBA solvents such as acetonitrile or methanol significantly effect the polarity

Figure 3. Influence of the degree of chemical functionalization (measured by the surface coverage) on the vis spectroscopically determined surface polarity parameter ET(30) of some silicas treated with various alkyl and aryl silanes (sample numbers are the same as those in Tables 3 and 4).

of silica surfaces via acid-base interactions.25,26,50 It is possible that the higher dipolarity/polarizability of DCE (π* ) 0.81) compared to that of cyclohexane (π* ) 0) contributes to the overall polarity of the suspension. However, this effect is not observed for the pure nonmodified silicas. Remarkable differences are not found in these cases. It can also be seen that the increasing number of alkyl groups of the silica samples have a much greater influence upon the value of the ET(30) parameters than does the length of the alkyl chains. The length of the alkyl groups has no observable influence on the course of the curves shown in Figure 3. Therefore, we conclude that the number of alkyl groups and of the silanols, respectively, of the silica samples is mainly responsible for the observed changes in the silica surface polarity as measured by the ET(30) values. This result supports the interpretation that the decrease of the value of the ET(30) parameter is mainly caused by the decrease of the value of the R term. If dispersion forces were to play a role, a systematic influence of the number and length of alkyl groups would be expected. 29Si CP MAS NMR measurements show that only geminal silanol groups are completely converted by the modification of the Aerosils and LiChrosphers with organosilanes.44 Therefore, it is expected that, above all, the residual silanol groups, siloxane bridges, and functional groups determine the higher polarity of the functionalized silicas in DCE. It has been shown by independent measurements that the unprecedented higher polarity of the aryl-substituted Aerosils is attributed to a larger contribution of the π* term.35 Pronounced differences are observed for the ET(30) surface polarity parameters by investigating the partially alkyl-functionalized silica samples by means of 1 and 4 in DCE for each specific silica sample. 1 reflects a higher surface polarity than penta-tert-butyl substituted 4 for all of the modified silicas studied. Figure 4 shows the differences of the ET(30) parameters [∆ET(30) (1-4) ) ET(30) (1)-ET(30) (4)] for Aerosils as a function of the degree of surface coverage by alkyl groups of these silicas. The plot ∆ET(30) (1-4) as a function of surface coverage goes through a maximum indicated by the arbitrarily drawn curve in Figure 4. A similar curve is pointed out for the analogous LiChrospher plots. Unfortunately, for the samples 1217, only the ET(30) value by means of 4 is measurable because 1 does not adsorb to these samples from DCE. However, partially substituted alkyl LiChrospher and

2110 Langmuir, Vol. 15, No. 6, 1999

Spange et al. Chart 3. Model for the Preferred Adsorption of 1 and 4 to Specific Surface Groups of Different Silica Samples and the Indicated Properties of the Respective Polarity Term Measured

Figure 4. Differences of the value of the polarity parameters ET(30), measured vis spectroscopically for (() functionalized Aerosil with 1 and 4, as a function of the surface coverage (given by the ligands per nm2). The sample numbers are the same as those in Table 1. The number of the carbon atoms per alkyl chain is given in the parenthesis next to the sample number.

Aerosil samples show the same effect toward the differently substituted 1 and 4. Silica particles with uniform surface coverage, either bearing mainly silanol or alkyl groups, also exhibit uniform surface polarity toward the two probe dyes 1 and 4. This result supports the conclusion that differently substituted betaine dyes are adsorbed at different surface sites with different polarity by using partially modified silica samples. This behavior cannot occur at uniformly functionalized particles. The expected interaction mechanisms of 1 and 4, adsorbed to different surface sites of differently functionalized silica surfaces, is illustrated in the simplified model shown in Chart 3.This model is supported by the fact that the quantity of the adsorbed betaine dye is approximately 1000 times smaller than that of all of the functional groups of the surface. Thus, overlapping vis absorptions of the betaine dye located in different surface areas cannot be observed because of the low absorption coefficient of these dyes (cf. the UV/vis spectra of 1 and 4 adsorbed to Aerosil R 805 in Figure 1). However, such effects cannot completely be excluded in principle. We recommend that the polarity of each specifically functionalized silica sample should be characterized by 1 and 4. The spectroscopic method used in this paper allows the parametrizing of the polarity of silica particles in the framework of the ET(30) scale. Using adsorbed pyrene as a fluorescence probe, Harris40,41 reported an intensity ratio of the pyrene emission band I1/I3 of about 0.98 for solvent-unaffected C18functionalized silica reverse-phase materials with similar surface coverage (3-4 µmol/m2). According to Dong and Winnik,36 a π* value of about 0.26 (R ) 0) and a ET(30) value of 34 are estimated for this sample via correlation equations for nonprotic solvents. Pyrene adsorbed to a bare silica gel surface exhibits a typical I1/I3 ratio of 1.72 for its emission bands.37 This corresponds to a value of the ET(30) parameter of 59.4 for bare silica according to the correlation equation ET(30) ) f(I1/I3 of pyrene emission) for protic solvents.36 These estimated ET(30) values taken from the results of independent references37,40 are in agreement with the order of magnitude of the values of the ET(30) parameter, the bare silica, and the highly functionalized silicas reported in this paper (see Table 4). However, the question remains: how strong of a contribute do the residual silanols make to the value of the ET(30) parameter as measured by 4? To answer this question, the results from the pyrene fluorescence were also used to tackle this problem. The I1/I3 ratio (Py) increases when the solvent’s dipolarity/

polarizability π* is increased, according to eq 11. Therefore, Dong and Winnik36 recommended the LSE eq 12 as a tool

Py ) 0.64 + 1.33 (π* - 0.24δ) - 0.25 R

(12)

n ) 320, r ) 0.900 to parametrize the value of the π* parameter of environments. As can be seen from eq 12 for protic solvents and accordingly also for acidic surfaces, a knowledge of the R value is necessary to separate the π* value from the unit of measurement Py. Corresponding to the theoretical plot R ) 1.137-0.113Γ (Γ is the amount of surface coverage expressed in micromoles per square meter)35 for the LiChrospher samples 13 (Γ ) 2.5 µmol/m2) to 17 (Γ ) 4.2 µmol/m2), the R value of the unreacted silanols should to be between 0.85 and 0.66. Through the use of these R

Polarity Parameters of Silica Particles

values and the Py value of 0.98 (from Harris),40,41 values of the π* parameter of 0.39-0.36 are calculated, respectively. However, this calculation shows that the effect of residual silanols does not dramatically influence the π* value. On the contrary, the HBD property of the residual silanols would have a large influence upon a measurable value of the ET(30) parameter as estimated by means of eqs 5 or 6, using the theoretically possible values of the R and π* parameter. Then, the value of the experimental ET(30) parameter should between 42 (no. 17) and 52 (no. 12), provided the probe dye is located within the alkyl layer toward the residual silanol groups. These expected ET(30) values do not agree with the experimental results from Table 4 which are obtained by means of 4 as a polarity indicator. Furthermore, by using the Py value of 0.98, a value of the π* parameter of 0.26-0.35 is expected, assuming arbitrarily R ) 0 and R ) 0.5 (weaker acidic residual silanols), respectively. This presumption would correspond to calculated values (via eq 5) of the ET(30) parameter for a solvent-unaffected highly functionalized silica between 34 and 40 kcal mol-1. These calculated data are in excellent agreement with the experimental ET(30) values from Table 4 which are measured by 4. These reflections strongly support the main interpretation of this paper, namely that dye 4 is not located toward the silanols within the alkyl layer. However, the covered silanols do slightly modify the value of the ET(30) parameter of alkyl-group-modified silicas which are measured by means of 4. As mentioned before, the assessable silanols of bare silica can be well measured by means of 1 or 4. Accordingly, crypted silanols bear a value of the R parameter much lower than that of freely assessable silanols at the same amount of surface coverage. It has been shown that the values of the ET(30) surface polarity parameters of silicas range between 59 (bare silica) and 35 (highly alkyl-functionalized silica). For comparison, we have measured the ET(30) value of polysiloxanes H[-O-Si(CH3)2-]n-OH. The model compounds contain the same functional groups as a completely alkyl-group modified silica sample (12-17 in Table 1). Also, very small contributions of the silanol end groups should be involved. That is expected to be similar to difficult-to-detect residual silanols. The ET(30) parameters

Langmuir, Vol. 15, No. 6, 1999 2111

of polysiloxanes were measured by means of 4, which is soluble in these viscous polymers, in contrast to dye 1, which is insoluble. The results show that polysiloxanes exhibit ET(30) values of an order of magnitude similar to that of highly alkyl-group-functionalized silicas.

Silicon oil M 20: ET(30) ) 38.7 kcal mol-1 Silicon oil NMI-200: ET(30) ) 40 kcal mol-1 Silicon oil NMI-2000: ET(30) ) 38.0 kcal mol-1 56 These final measurements support the reliability of the ET(30) values determined for the samples 12-17 in Table 4. This result also shows that the R term does not have an important influence on the surface polarity of the highly functionalized LiChrosphers. Therefore, the directly measurable ET(30) polarity of the alkyl-group environment of the functionalized LiChrosphers is mainly determined by the π* term, which is the dipolarity/polarizability. Conclusion The results described in this paper have shown that the two differently substituted Reichardt’s betaine dyes 1 and 4 are suitable solvatochromic indicators for the differential investigation of the surface polarity of alkylgroup-functionalized silica samples. DCE and cyclohexane are recommended as inert solvents for the adsorption of the betaine dyes from the dye solution to the silica in order to exclude disturbing influences of light scattering by employing the transmission technique. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. Our outstanding thanks are due to Prof. C. Reichardt, University of Marburg, for helpful discussions generously providing us with novel betaine dyes and unpublished results. LA980328U (56) Aksel, N.; Heymann, L.; Reuter, A.; Spange, S. Unpublished results.