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Hydrogen-Bond Donating and Dipolarity/Polarizability Properties of Chemically Functionalized Silica Particles Stefan Spange* and Anett Reuter Department of Polymer Chemistry, Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, D-09111 Chemnitz, Germany Received August 6, 1998. In Final Form: October 15, 1998 Linear solvation energy (LSE) relationships are employed to characterize the surface polarity of organically functionalized silicas. The polarity of the silica surfaces should be quantitatively described by three independent terms: the dipolarity/polarizability (π*), the hydrogen-bond donating ability (R), and the hydrogen-bond accepting ability (β). These terms can be defined by using the Kamlet-Taft solvent parameters R, β, and π* as a reference system. Kamlet-Taft R and π* values and Reichardt’s ET(30) values are presented for 30 differently functionalized silica particles and bare silicas. The surface polarity parameters R and π* were determined by means of correlation analyses of the energy of the UV-vis absorption maxima (νmax) of selected solvatochromic probe dyes which are adsorbed to the particle surfaces. The following surface polarity indicators have been used: 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate and its penta-tert-butyl substituted derivative, cis-dicyanobis(1,10-phenanthroline)iron(II), and bis-4,4′-(N,Ndimethylamino)benzophenone. The R values of organically modified LiChrosphers, which were synthesized with monofunctional silanes, linearly decrease with the amount of surface coverage of the functional group µmol m-2. Using trifunctional silanes as reagents, the R value of modified silicas is influenced in a more complex manner because the functionalization process occurs not uniform. Correlations of the values of surface polarity parameters with each other and with literature data demonstrate that the reported values are relevant empirical constants. The ET(30) parameter of chemically modified silicas can be expressed by the specific LSE equation: ET(30)(measured) ) 14.84R + 5.33π* + 36.1; n ) 30, r2 ) 0.9334. This ET(30) LSE relationship, derived for functionalized silicas, is compared to the equation derived for pure solvents as reported by Marcus (Marcus, Y. Chem. Soc. Rev. 1993, 409).
Introduction The application of polarity scales and acid-base concepts to surface phenomena has been investigated and discussed by several authors.1-5 In this sense, the term surface polarity is an argument often used in interpreting experimental results obtained from chromatography1,2 and heterogeneous catalysis.3,4 Despite the manifold use of the term surface polarity, no exact definition is given until now: What does the expression surface polarity really mean? Therefore, it is necessary that the term surface polarity must be clearly defined. This problem will be discussed by applying the wellestablished ET(30) polarity scale and its use to determine empirically surface polarity parameters. The original ET(30) polarity parameter for solvents is defined as the molar absorption energy corresponding to the longest-wavelength UV-vis CT absorption maximum of the standard dye 2,6-diphenyl-4-(2,4,6 triphenyl-1-pyridinio)phenolate betaine (1a, standard dye) (Chart 1), measured in the respective solvent and expressed in kcal mol-1 (equation 1).6,7 Later, the dimensionless normalized ETN scale was recommended in order to circumvent the use of the obsolete energy unit kcal/mol-1. It is defined according to equation (1) Park, J. H.; Carr, P. W. J. Chromatogr. 1989, 465, 137. (2) Rutan, S. C.; Harris, J. M. J. Chromatogr. 1993 A656, 197. (3) Lindley, S. M.; Flowers, T. C.; Leffler, J. E. J. Org. Chem. 1985, 50, 607. (4) Chronister, C.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793. (5) Jensen, W. B. In Acid-Base Interactions: Relevance to Adhesion Science and Technology; Mittal, K. L., Anderson, H. R., Eds.; VSP: Utrecht; 1991; p 3. (6) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry 2nd ed.; VCH: Weinheim, New York, 1988. (7) Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Justus Liebigs Ann. Chem. 1963, 661, 1.
Chart 1. Formulas of the Probe Dyes Used
2, using water and tetramethylsilane as extreme polar and nonpolar reference solvents.8
ET(30)[ kcal mol-1] ) ((2.8591 × 10-3)νmax)(1a)[cm-1] ) 2.8591/ λmax [nm] (1) ETN ) (ET(solvent) - ET(TMS))/(ET(water) - ET(TMS)) ) [ET(30) - 30.7]/ 32.4 (2)
10.1021/la980991i CCC: $18.00 © 1999 American Chemical Society Published on Web 12/04/1998
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Empirical ET(30) polarity parameters are well accepted for solvents and various solution processes.6 Their application to inorganic particle surfaces has been successfully demonstrated for alumina,9 bare silicas,11-13 modified silica particles,2,10,13 and ormosils.14 Again, we should discuss a practicable definition of the term surface polarity. The IUPAC recommendation for the definition of the solvent polarity is as follows (extract): 15 that the polarity is the sum of all possible, non-specific and specific, intermolecular interactions between the solute ions or molecules and solvent molecules, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute. According to this general definition, surface polarity should be similarly defined, namely, as the sum of all possible interactions between a surface group and an adsorbed molecule or ion. Consequently, a specific probe, that is Reichardt’s betaine dye, does only reflect this specific sum of interactions being operative between the relevant surface groups and the relevant sites of the specific probe molecule. Accordingly, the respective parts of the intermolecular interaction, acid/ base (specific), dipolar (nonspecific), and dispersion forces (nonspecific), are different when using another polarity indicator.16,17 As a consequence of this presumption, the single ETN polarity parameter is only one option to reflect the polarity of an environment, but it is the most frequently used one because betaine dye 1a exhibits one of the largest negatively solvatochromic effects known so far.8,16 During the past decade, the Kamlet-Taft linear solvation energy (LSE) relationship18-21 has been recommended by several authors in order to describe quantitatively the manifold influences of different environments on the course of several chemical processes including micelle/solution interfaces and heterogeneous media.1,2,3,5,12,22-24 For solvatochromic shifts (with XYZ ) νmax,Probe) the LSE relationship of Kamlet and Taft has been applied in the following simplified form:25-31
XYZ ) (XYZ)0 + (s + dδ)π* + aR + bβ
(3)
(8) Reichardt, C.; Harbusch-Go¨rnert, E. Justus Liebigs Ann. Chem. 1983, 721. (9) Michels, J. J.; Dorsey, J. G. Langmuir 1990, 6, 414. (10) Jones, J. L.; Rutan, S. C. Anal. Chem. 1991, 63, 1318. (11) Spange, S.; Reuter, A.; Schramm, A.; Reichardt, C. Org. React. (Tartu) 1995, 29, 92. (12) Spange, S.; Reuter, A.; Vilsmeier, E. Colloid Polym. Sci. 1996, 274, 59. (13) Taverner, S. J.; Clark, J. H.; Gray, G. W.; Heath, P. A.; Macquarrir, D. J. J. Chem. Soc., Chem. Commun. 1997, 1147. (14) Rottman, C.; Grader, G. S.; Hazan, Y. D.; Avnir, D. Langmuir 1996, 12, 5505. (15) Mu¨ller, P. Pure Appl. Chem. 1994, 66, 1077. (16) Reichardt, C. Chem. Rev. 1994, 94, 2319. (17) Novaki, L. P.; Soued, O. A. E. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 8. (18) Kamlet, M. J.; Hall, T. H.; Bodkin, J.; Taft, R. W. J. Org. Chem. 1979, 44, 2599. (19) Taft, R. W.; Kamlet, M. J. J. Chem. Soc., Perkin Trans. 1979, 2, 1723. (20) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (21) Abraham, M. H.; Andonian-Haftvan, J.; Du, C. M.; Diart, V.; Whiting, G. S.; Grate, J. W.; McGill, R. A. J. Chem. Soc., Perkin Trans. 1995, 2, 369. (22) Spange, S.; Reuter, A.; Vilsmeier, E.; Heinze, T.; Keutel, D.; Linert, W. J. Polym. Sci. A 1998, 36, 1945. (23) Helburn, R. S.; Rutan, S. C.; Pompano, J.; Mitchern, D.; Patterson, W. T. Anal. Chem. 1994, 66, 610. (24) Brune, B. J.; Payne, P. F.; Chaubal, M. V. Langmuir 1997, 13, 5766. (25) Migron, Y.; Marcus, Y. J. Phys. Org. Chem. 1991, 4, 310. (26) Effenberger, F.; Wu¨rthner, F. Angew. Chem. 1993, 105, 742; Angew. Chem., Int. Ed. Engl 1993, 32, 719.
XYZ is the result of a solvent-dependent chemical or physical process (i.e., the molar absorption energy, the molar emission energy, or the intensity ratio of two differently placed solvent-dependent emission bands). (XYZ)0 is the value for the solvents reference system, e.g., a nonpolar medium or the gas phase, R is the HBD (hydrogen-bond donating) ability, β is the HBA (hydrogenbond accepting) ability, and π* the dipolarity/polarizability of the solvents. δ is a polarizability correction term that is 1.0 for aromatics, 0.5 for polyhalogenated solvents, and zero for aliphatic solvents; a, b, s, and d are solventindependent correlation coefficients.18-20 It should be noted that various solvent polarity scales, these are Gutmanns donor (DN) and acceptor (AN) numbers,32 Reichardt’s ET(30) parameter,6-8 and other useful scales,31,33 can be attributed to the Kamlet-Taft parameters by LSE correlations. Therefore, the KamletTaft solvent parameter set seems advantageous as a reference system to define surface polarity parameters.5,12,22-24 Marcus has shown that a multiple correlation of the ET(30) solvent parameters with the Kamlet-Taft parameters gives the following two-parameter equation 4,28 with n ) set of various solvents, r ) correlation coefficient, and s ) standard deviation.
ET(30) ) 31.2 + 15.2R + 11.5π*
(4)
r ) 0.979, n ) 166, s ) 1.25 Equation 4 demonstrates that the ET(30) solvent parameters preferably reflect the HBD ability and the dipolarity/polarizability properties of the solvents. As expected, we have found that no correlation exists between the ET(30) and β values of functionalized silicas.30 Adsorption processes of various organic dyes to inorganic particles are very often accompanied by visible detectable changes of color. We have found that shifts of UV-vis absorption maxima of solvatochromic dyes adsorbed to particle surfaces rarely correlate linearly with the relevant surfaces properties.34 Attempts to measure the π* parameters of silica and zeolites by using various nitrosubstituted benzene derivatives as indicators are reported.3,35 In ref 3 it was shown that every specific π* indicator yields another value of the π* parameter of silica by employing the original Kamlet-Taft single parameter equations. The bare silica surface exhibits both HBD and dipolarity/polarizability properties which are differently reflected by the respective polar dye used. Therefore, single-parameter equations are unsuitable to examine the Kamlet-Taft π* polarity parameters of surfaces exhibiting both acidic and dipolar properties by utilizing the solvatochromic shift of polar indicator dyes.3,35 The choice of a suitable surface polarity indicator dye should be decided by the quality of the LSE correlation in describing its solvatochromism by using the Kamlet-Taft solvent parameters utilizing both nonprotic and protic solvents.27 Therefore, the search for and the choice of suitable surface polarity indicators are important topics in surface chem(27) Spange, S.; Keutel, D. Liebigs Ann. Chem. 1992, 423. (28) Marcus, Y. Chem. Soc. Rev. 1993, 409. (29) Schneider, H., Migron, Y., Marcus, Y. Z. Phys. Chem. (Munich) 1992, 175, 145. (30) Spange, S.; Reuter, A.; Linert, W. Langmuir 1998, 14, 3479. (31) Palm, N.; Palm, V. Org. React. (Tartu) 1997, 31, 141. (32) Gutmann, V. Coord. Chem. Rev. 1996, 18, 225. (33) Kriegsmann, H. Z. Phys. Chem. (Leipzig) 1988, 269, 1030. (34) Spange, S.; Vilsmeier, E.; Fischer, K.; Mu¨ller, H.; Prause, S. Unpublished results. (35) Handreck, G. P.; Smith, T. D. J. Chem. Soc., Faraday Trans. 1988, I84, 1847.
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istry. The very suitable fluorescence probe pyrene has been applied by several authors to measure the polarity of interfaces of microenvironments that are stationary chromatographic phases,2,36-39 silica polymer composites,40,41 and bare silicas.42 The intensity ratio of the I1 and I3 emission bands of pyrene at λ ) 375 and 384 nm, respectively, is strongly dependent on the solvent polarity.43 The I1/I3 ratio (Py) increases with increasing the dipolarity/dipolarizability π* of the solvent (equation 5).43 Therefore, Dong and Winnik43 recommended the LSE equation 5 as a tool to parametrize the value of the π* parameter of different environments. It is of importance that in the case of stronger HBD solvents, the knowledge of the R parameter is necessary to separate the π* value from the unit of measurement.
Py ) 0.64 + 1.33 (π* - 0.24δ) - 0.25R
Chart 2. Drawings That Reflect the Problem To Be Investigated
(5)
r ) 0.959, n ) 32 Despite the well-established LSE relationship shown in equation 5, several authors reported only the Py values of surfaces and avoided the calculation of the corresponding π* parameters for the studied materials. The reason for this may be the lack of the value of the R term which is not directly available. According to Harris,36,37 who reported a Py value of 0.98 for a highly C18 functionalized silica, a value of the π* parameter of 0.26 until 0.35 is expected for this sample, assuming arbitrarily R ) 0 and R ) 0.5 (weak acidic residual silanols), respectively. These π* values would correspond to calculated values (via equation 4) of the ET(30) parameter for a not solventaffected highly alkyl-functionalized silica between 35 and 40 kcal mol-1. Rutan et al.2,12 reported ET(30) values in the order of 50 for a highly C18 functionalized silica in the presence of HBD solvents. The discrepancy of this result as compared to Harris’s results36,37 shows that the HBD properties of the liquid phase influence the phenolate betaine dye as well as the stationary phase. However, the results of many papers2,3,23,35 show that there is a demand for a method to determine reliable values of the R parameter for functionalized silicas. The objective of this paper is to determine the KamletTaft parameters R and π* for chemically modified silica particles in non-HBD solvents. The values of the surface polarity parameters R and π* are analyzed by means of the application of the two carefully characterized solvatochromic dyes Fe(phen)2(CN)2 (2) and Michler’s-Ketone (3).27,44 The molecular structures of the two selected surface polarity indicators are also presented in Chart 1. The correlation equations are given in the experimental part for the calculation of the polarity parameters via the UV-vis absorption maxima of the adsorbed dyes. ET(30) surface polarity parameters were determined using the standard dye 1a shown in Scheme 1 and a penta-tertbutyl substituted derivative, the 2,6-di(4-tert-butylphenyl)-4-[2,4,6-tris(4-tert-butylphenyl)-1-pyridinio]phenolate 1b. (36) Carr, J. W.; Harris, J. M. Anal. Chem. 1986, 58, 626. (37) Carr, J. W.; Harris, J. M. Anal. Chem. 1987, 59, 2546. (38) Stahlberg, J.; Almgren, M. Anal. Chem. 1985, 57, 817. (39) Urns, J. W.; Bialkowski, S. E.; Marshall, D. B. Anal. Chem. 1997, 69, 3861. (40) Nakane, K.; Suzuki, F. J. Appl. Polym. Sci. 1997, 64, 763. (41) Baker, G. A.; Jeffrey, D. J.; Bright, F. V. J. Sol-Gel Sci. Techn. 1998, 11, 43. (42) Krasnansky, R.; Thomas, J. K. In The Colloid Chemistry of Silica; American Chemical Society: Washington, DC, 1994; p 223 ff. (43) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560. (44) Spange, S.; Keutel, D.; Simon, F. J. Chim. Phys. 1992, 89, 1615.
The correctness of the individual parameters R and π* should be proved by correlation analysis according to equation 4. Then it is possible to compare the measured and independently calculated ET(30) values. Silica was used as a model surface because various chemical functional groups can easily be introduced by surface reaction with commercially available reagents. Structure characterization of the functionalized silica particles is performed by solid-state cross polarization magic angle spinning (CP MAS) NMR and diffuse reflectance IR Fourier transform (DRIFT) spectroscopy as well as by quantitative elemental analysis. This is described in the experimental part. Simplified views concerning the problems to be investigated in this paper are shown as drawings in Chart 2. Further, we expect also to get an answer for the following questions: does each individual surface group (silanol or functional group) bear a specific polarity itself, or do the versatile interactions of the different immobilized groups with each other cause an average property of the surface polarity to be measured? Experimental Section Materials. The alkyl-group-modified silica particles were synthesized by reaction of thermically pretreated silica particles with different amounts of alkyltrimethoxysilanes, alkyldimethylalkoxysilanes, chlorodialkylsilanes, and alkyltrichlorosilanes according to procedures given in the literature.45,46 A typical experiment is as follows: Four grams of Aerosil 300 is dried in a vacuum at 400 °C for 12 h in a glass flask. After cooling 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 in order 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 vacuo at 70 °C. The final product is colorless and exhibits the same external morphology as the former Aerosil 300. The functionalized silica samples have been characterized by quantitative combustion analysis (C,H,N analysis), BET measurements, and DRIFT spectroscopy. 13C{1H} CP MAS, 29Si{1H} CP MAS and 1H MAS, solid-state NMR spectroscopy were (45) Hara, S. J. Chromatogr. 1979, 186, 543. (46) Horner, L.; Ziegler, H. Z. Naturforsch. B 1987, 42b, 643.
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Table 1. Physical Properties and Chemical Constitution of the Functionalized Silicas Used in This Work surface
sample no.
Aerosil 300 (untreated) 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 (octyl-) aminopropyl-|-O-Si(CH2)3NH2 aminobutyl-|-O-Si(CH2)4NH2 cyanopropyl-|-O-Si(CH2)3CN thiocyanatopropyl-|-O-Si(CH2)3SCN LiChrospher Si 100/5 µm (untreated) LiChrospher Si 100/5 µm (dried) LiChrospher 18e-10 µm |-Si-(CH2)17CH3 endcapped LiChrospher 18-5 µm LiChrospher 18-10 µm LiChrospher 8-5 µm |-Si-(CH2)7CH3 LiChrospher 8e-5 µm endcapped LiChrospher 8-10 µm LiChrospher 18-5 µm LiChrospher 18-5 µm LiChrospher 18-5 µm LiChrospher CN-5 µm |-Si-(CH2)3CN LiChrospher NH2-5 µm |-Si-(CH2)3NH2 LiChrospher diol-5 µm |-Si-(CH2)3OH a
The solid-state CP MAS
29Si{1H}NMR
surface coverage (µmol m-2)
BET surface area (m2 g-1)
1 2 3a 4a 5a 6a
4.8 2.2 0.6 5.0
240 240 210 205 217 229
7a
5.1
200
8a
5.1
207
9a
4.4
210
10a
7.7
120
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
6.7 2.9 13.3 10.9 1.3 1.5 2.9 2.5 2.8 4.0 3.6 4.2 0.81 1.57 2.6 3.7 3.5 6.3
111.5 194 156 144 224 240 366 366 321 383 321 321 183 321 270 243 200 360 360 360
spectra indicate oligomerized silane reagents on the silica surface (T3-signal).48,50
employed in selected cases to confirm the structure formation.47,48 The assignments of the 13C and 29Si NMR signals to specific surface groups were carried out according to refs 49 and 50. Aerosil R 972 and R 805 were commercially available products from Degussa, Frankfurt/Main. The particle samples no. 17-24 and 28-30 were commercially available products from Merck. They are synthesized with dimethoxy-n-octyl- and chlorodimethyl-n-octadecylsilane, respectively, as precursor. The specific samples no. 25, 26, and 27 were kindly prepared by D. Lubda, Merck Aktiengesellschaft, Darmstadt. For these samples the chlorodimethyl-n-alkylsilane was the reagent for modification. The surface coverage was determined by the method of Kova´ts.51 The physical properties and chemical constitutions of the organically modified silica samples used are compiled in Table 1. The probe dye indicators 1a and 1b were kindly presented by C. Reichardt, University of Marburg. Fe(phen)2(CN)2 was prepared according to Schilt.52 Michler’s Ketone, N,N-dimethylamino-4-nitrobenzene (DAN), and N-Methyl-2-nitroaniline (MNA) were purchased from Merck (Darmstadt) and recrystallized twice from ethanol before use. (47) Francke, V.; Gu¨nther, H.; Reuter, A.; Spange, S. Unpublished results. (48) Reuter, A. Synthese und Charakterisierung modifizierter Polykieselsa¨uren: Strukturuntersuchungen und UV/Vis-spektroskopische Bestimmung von Oberfla¨chenpolarita¨tsparametern mittels solvatochromer Farbstoffe. Ph.D. Thesis, Technical University, Chemnitz/ Germany, Shaker Verlag Aachen, 1997. (49) Zaper, A. M.; Koenig, J. L. Polym. Compos. 1985, 6, 156. (50) Maciel, C. E.; Bronnimann, R. S.; Zeigler, J.; Ssuer Chuang, D. R.; Kinney, E. A. Keiter In The Colloid Chemistry of Silica; American Chemical Society: Washington, DC, 1994; p 260 ff. (51) Kova´ts, E. sz. Adv. Colloid Sci. 1976, 6, 95. (52) Schilt, A. J. Am. Chem. Soc. 1960, 82, 3000.
Solvatochromic Measurements. The organically modified silica samples were dried carefully in vacuo at 70-80 °C in a glass vessel. Therefore, it is expected that residual water traces below 10-4 g mol-1 cannot be excluded. After cooling to room temperature under dried argon, a solution of the probe in 1,2dichloroethane (DCE) is simply added to the silica material. Care must be taken to avoid overloading the surface with the indicators used, as multilayer absorption is expected in solution at higher concentrations or disturbing absorptions from the nonadsorbed dye from the solution. The amount of the dyes added was therefore restricted to 2-3 mg of 1a and 1b per 1 g of silica, 0.5 mg of 2 per 1 g of silica, and 0.1 mg of 3 per 1 g of silica. For the UV-vis measurement, a limited amount of silica is required in order to meet a sufficient particle concentration in the continuously stirred slurry. A 0.1 g to 0.2 g of silica in 15 mL of the solvent used is very suitable, but also a lower amount is possible. The amount of the silica portion has no influence on the measurement of the UV-vis absorption maximum of the adsorbed dye. The equipment employed was a UV-vis spectrometer MCS 400 equipped with an immersion cuvette TS 5A (Zeiss). The immersion cell can be shifted vertically, which allows the comparison of both spectra: that of the suspension and that of the supernatant solution.12 The measurements of silica particle suspensions are advantageous because they can be performed under inert conditions and do reflect better the conditions suitable for liquid chromatography and catalysis in organic solvents. The reproducibility of the UVvis spectra of the adsorbed dyes is very good. With DCE as liquid, the UV-vis absorption maximum of the adsorbed dye on the silica particle remains constant during 1-2 h (see later). The solvatochromism and properties of the indicators Fe(phen)2(CN)2 and Michler’s Ketone were already reported in detail
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Langmuir, Vol. 15, No. 1, 1999 145
in refs 27 and 44. The mathematical procedure for the determination of the LSE correlations of the indicators 2 and 3 is reported in ref 44. The following correlation equations (6) and (7) are applied in this paper for the calculation of the R and π* values by the simultaneous use of the indicators 2 and 3.42
R ) (0.086νmax,3 × 10-3) + (0.486νmax,2 × 10-3) - 10.26 (6) π* ) -(0.297νmax,3 × 10-3) -(0.229νmax,2 × 10-3) + 12.80 (7) The error for calculation of the value of the R parameter is very low (0.02, whereas the error for calculation the value of the π* parameter is (0.15. The acceptor numbers (AN) of Gutmann32 for solvents and silicas can also simply be calculated by means of equation 8, using the solvatochromism of Fe(phen)2(CN)2.27
AN ) (13.698νmax,2 × 10-3) - 208.22
(8)
Results The UV-vis spectra of the probe dyes adsorbed to the silica particles were measured in suspensions of different liquids. The following liquids with different polarity have been tested: DCE (π* ) 0.81, β ) 0.1, R ) 0), cyclohexane (π* ) 0, β ) 0, R ) 0), toluene (π* ) 0.54, β ) 0.11, R ) 0), and tetrahydrofuran (π* ) 0.58, β ) 0.55, R ) 0).28 For the UV-vis measurements, the silica particles have been suspended in the respective liquid. The UV-vis spectra were measured in the supernatant solution after the particles are deposited as well as in the continuously stirred suspension. Absolutely transparent suspensions are obtained in DCE and cyclohexane as liquids. Silica suspended in these liquids allows one to take clean transmission UV-vis spectra of the adsorbed dye by means of the immersion cell. In toluene it is not possible to measure clean UV-vis transmission spectra of the dyes adsorbed to functionalized silica because the refractive indices of toluene and Aerosil are quite different. In THF, the dyes 1a, 1b, and 3 do not quantitatively adsorb to silica and only the unaffected UV-vis absorptions of the dyes in the solution are measured. Obviously, the silanol surface groups are shielded by this HBA solvent. Therefore, we have selected DCE as a standard solvent because it bears only very weak HBA properties. In DCE, clean UV-vis spectra of the adsorbed polarity indicators could be measured in the stirred suspension in each case. Disturbing UV-vis absorptions could not be detected in the supernatant solution in DCE. Representative UVvis spectra of the dyes 2 and 3 adsorbed to Aerosil silica samples are shown in Figure 1. The shape of the UV-vis spectra of dye 2 is nonsymmetric. The relevant absorption maxima are indicated by arrows. In all cases, the UV-vis spectra taken from the suspensions show definite UV-vis absorption maxima. UV-vis absorption bands with shoulders or bands with two maxima are not observed. The UV-vis absorption bands of the adsorbed dyes are broad in some cases. It should also be noted that concentration effects of the dye upon the shift of the absorption band of the adsorbed dye have not been observed in the range of concentrations used. Table 2 collects the measured long-wavelength UVvis absorption maxima of the polarity indicators 1a or 1b, Fe(phen)2(CN)2, and Michler’s Ketone adsorbed to various functionalized silica particles.
Figure 1. UV-vis spectra of the indicator dyes Fe(phen)2(CN)2 (1a) and Michler’s Ketone (1b), both in 1,2-dichloroethane and adsorbed to dried Aerosil sample in this solvent. The arrows indicate the relevant absorption maxima.
The calculated values of ET(30), ETN, R, and π* polarity parameters are presented in Table 3. Table 3 also contains the β values for the functionalized silicas from ref 30. The polarity indicators 1a and 2 do not adsorb to the highly alkyl group functionalized (>2.5 µmol of alkyl groups per m2 of LiChrospher) silica particles no. 19-24 from DCE solution. ET(30) polarity parameters of these samples are measured by using the penta-tert-butylsubstituted dye 1b that is soluble in cyclohexane. The dye 1b adsorbs well to the highly alkyl functionalized LiChrospher particles from cyclohexane solution. Then, the values of the ET(30)-parameters were calculated by equation 9.4,7
ET(30) ) 1.0577ET(1b) - 1.9185
(9)
r ) 0.999, n ) 16 As seen in Table 3 the values of the ET(30) polarity parameter of the highly alkyl functionalized silicas are placed between the values of the ET(30) parameter of cyclohexane [ET(30) ) 30.9] and DCE [ET(30) ) 41.3] as solvents. Consequently, Fe(phen)2(CN)2 does also not adsorb to these highly alkyl functionalized LiChrosphers from DCE solution due to the low surface acidity of these samples. Unfortunately, the polarity indicator Fe(phen)2(CN)2 is insoluble in cyclohexane. The introduction of methyl substituents in the 3- and 4-position of the phenanthroline rings of Fe(phen)2(CN)2 does not improve
146 Langmuir, Vol. 15, No. 1, 1999
Spange and Reuter Table 4. Reichardt’s ET(30) and Kamlet-Taft’s π* Parameters for Three n-Octyl- and Three n-Octadecyl-Modified LiChrospher Particles in Cyclohexane, Obtained by Employing Structurally Different Solvatochromic Probe Molecules, and Calculation of Theoretically Possible Values of r Parameters by Using the π* Value Derived from 1b and the νmax of 3
Table 2. UV-vis Absorption Maxima of the Dyes 1a (1b), 2, and 3, Measured after Adsorption of These Dyes to the Functionalized Silica Samples in a 1,2-Dichloroethane Suspension νmax × 10-3/cm-1 sample no.
1aa
2
3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19b 20b 21b 22b 23b 24b 25 26 27 28 29 30
20.92 20.21 17.62 17.61 18.72 19.03 19.21 19.31 19.05 18.29 17.99 18.28 18.01 19.03 19.11 18.16 20.08 19.84 14.10(1b) 13.37(1b) 12.97(1b) 13.04(1b) 12.95(1b) 12.43(1b) 19.96 19.51 18.24 19.53 18.96 18.84
19.05 18.78 18.30 18.36 18.48 18.45 18.45 18.20 18.45 18.00 17.86 18.27 17.86 17.94 18.28 18.14 18.94 18.68
25.25 25.51 25.68 26.02 25.79 25.34 25.97 25.08 25.41 27.40 25.18 25.23 26.37 26.32 25.33 25.90 25.92 25.34 28.20 27.20 27.00 27.20 27.70 27.20 25.90 25.90 25.90 24.75 25.73 25.51
18.67 18.55 18.22 18.25 18.05 18.10
a Values are taken from ref 59 in particular. b These samples are measured in cyclohexane suspension.
Table 3. Values of the Surface Polarity Parameters ET(30), r, π*, and β for Bare Silicas and 22 Functionalized Silicas in 1,2-Dichloroethanea sample no
ET(30) (kcal mol-1)
R
π*
β
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 25 26 27 28 29 30
59.8 57.8 50.4 54.5 53.5 54.4 54.9 55.2 54.5 52.3 51.4 52.3 51.5 54.4 54.6 48.4 57.4 56.7 57.1 55.8 52.1 55.8 54.2 50.4
1.17 1.06 0.84 0.90 0.94 0.89 0.94 0.74 0.89 0.84 0.59‘ 0.79 0.69 0.72 0.80 0.78 1.16 1.01 1.04 0.98 0.82 0.74 0.73 0.73
0.94 0.92 0.98 0.87 0.91 1.05 0.86 1.18 1.03 0.54 1.23 1.12 0.88 0.87 1.09 0.95 0.79 0.96 0.83 0.86 0.94 1.27 1.02 1.08
-0.23 -0.26 -0.16 -0.05 0.04 0.42
a
1.17 1.27 1.31 -0.16 -0.19
0.40 0.77
The β values are taken from ref 30.
its solubility in weak polar solvents.53 Superficially, these samples (no. 19-24) should not exhibit HBD property toward 1a and 2. If the value of the R term approaches (53) Spange, S.; Reuter, A.; Weise, S.; Steiner, M.; Eschner, M. unpublished results.
ET(30) sample (kcal‚mol-1) 19 20 21 22 23 24
40.9 38.6 37.4 37.6 37.4 35.8
π* 1ba
1bb
3c
0.84 0.64 0.54 0.56 0.54 0.40
0.72 0.55 0.47 0.49 0.47 0.36
0.83 1.26 1.34 1.26 1.4 1.26
4d
5e
af
0.14 0.30 0.39 0.17 0.13 0.28 0.47 0.49 0.14 0.26 0.29 0.19 0.17 0.24
R 3-1g 0.00 0.80 1.03 0.89 0.64 1.09
a Calculated via eq 4, using E (30) and assuming R ) 0. T Calculated by means of the single-parameter equation from ref 20, derived for aliphatic solvents: νmax(1) ) 10.8 + 4.84π*, n ) 16, r ) 0.974. c Calculated via eq 7. d Calculated via eq 8. e Calculated via eq 9. f Averaged value of the π* parameters by using the π* values calculated by means of the indicators 1b, 4, and 5. g Calculated by means of the two-parameter LSE of 3: ν max(3) × 10-3 ) 30.03 - 2.18π* - 1.79R; r ) 0.976, n ) 32, from ref 27. b
zero for these silica particles, the use of the single νmax of Michler’s Ketone or of other common π* indicators would allow the analysis of the value of the π* parameter. But as will be seen, this presumption is particularly wrong. The application of the single-parameter LSE correlation is possible because the UV-vis absorption maxima (νmax) of Michler’s Ketone correlate linearly with the π* parameters by utilizing only non-HBD solvents with R ) 0. Then, values of the π* parameter for these silicas with R ) 0 can be calculated by equations 10 and 11.
π* ) 12.925 - (0.429 10-3)νmax (MK) [cm-1]
(10)
r ) 0.964, n ) 39, s ) 0.074 π* ) 14.3 - (0.477 × 10-3)νmax (MK) [cm-1]
(11)
r ) 0.984, n ) 23, s ) 0.113 Equation 10 was calculated from the UV-vis maxima from ref 27 and equation 11 was taken from ref 18. Effenberger et al. recommended 5-N,N-dimethylamino5′-nitro-2,2′-bisthiophene as a promising π* indicator for solvent polarity.26 We have tested this dye, too. Unfortunately, this dye does not at all adsorb from cyclohexane or other solvents to all functionalized silica samples studied. N,N-Dimethylamino-4-nitrobenzene (DAN) and N-methyl-2-nitroaniline (MNA) were also utilized as π* indicators for chromatographic materials described in the literature.10,23 We have applied also these two indicators to analyze the π* value of the highly functionalized LiChrosphers. Both dyes adsorb well to the LiChrospher samples. The correlation equations (12 and 13) for the determination of the π* parameters were taken from refs 10 and 23.
π* ) 8.178 - (0.291 × 10-3)νmax (DAN) [cm-1] (12) r ) 0.988, n ) 33, s ) 0.150 π* ) 15.43 - (0.6277 × 10-3)νmax (MNA) [cm-1] (13) r ) 0.993, n ) 25, s ) 0.055 Therefore, assuming R ) 0 for samples no. 19-24, π* values are also calculated via equation 4 using the measured ET(30) parameters (see Table 4).
Chemically Functionalized Silica Particles
The spectroscopic results and the calculated π* values are compiled in Table 4 for the selected LiChrosphers by using the indicators 1b, 3, 4 (DAN), and 5 (MNA). Unprecedented high values of the π* parameter have been obtained for the samples no. 19-24 by using 3 as a polarity indicator. However, it is conceivable that the different indicators are adsorbed to different sites of different polarity within the alkyl-functionalized layer of the particle surface. It may also be possible that dye 3 interacts stronger with an amount of residual silanols or adsorbed water. According to Kova´ts,54,55 the sum of reacted and unreacted silanol concentrations for silica gave Γ ) 10.1 ( 0.2 µmol m-2. Since a monolayer functionalization is realized for the samples no. 19-24, a concentration of residual H-bonded silanols about 6-7 µmol m2 is expected. DRIFT spectra of these samples no. 19-24 show that the single Si-OH vibration of the former silica at ν ) 3740 cm-1 disappears after the surface functionalization. Despite this result, a broad absorption with weak intensity in the region between 3100 and 3700 cm-1 indicates H-bonded silanols and traces of adsorbed water. Therefore, the presumption R ) 0 for these samples seems wrong despite that result that the indicators 1a and 2 do not adsorb to these samples. However, the presumption R ) 0 seems acceptable only for the alkyl bonded phase which shields the residual silanols. The polarity of the alkyl-grafted silica surface hence cannot be uniform. But this effect is very difficult to analyze quantitatively.39 It is likely that the π* indicators 3, DAN, and MNA are adsorbed to different sites of different polarity associated with specific acid-base interactions of the indicator caused by unreacted silanols which are covered by the grafted alkyl chains. As discussed in the Introduction, equations 10, 11, 12, and 13 should not be applied when specific acid-base interactions occur between the probe and HBD groups. It is furthermore not yet clear how different amounts of dispersion forces contribute to the value of the π* parameter of the alkyl-functionalized silica by employing the differently polar π* indicators.17,56,57 Further, the mobility and orientation of the adsorbed probe in a rigid matrix are also of great importance.58 In the case of the dyes 1a and 1b, dispersion forces scarcely contribute to the measured ET(30) values.57 In another paper we will show that the differently substituted Reichardt’s dyes 1a and 1b measure distinct different values of the ET(30) polarity parameters of partially modified silicas.59 In the case of the highly functionalized silicas and neat silicas both dyes 1a and 1b yield the same value of the ET(30) polarity parameter. Furthermore, the quantity of dyes 1a or 1b used for the solvatochromic measurements is about 50 times larger than that of dyes 3, 4, or 5. Thus, band shifts caused by residual silanols should scarcely be detectable by using the dye 1b. Unfortunately, due to the low concentrations of the dyes 3, 4, and 5, which are used for the UV-vis measurements, other spectroscopic techniques have been impracticable in order to localize the site of the probe. (54) Fo¨ti, G.; Kova´ts, E., sz. Langmuir 1989, 5, 232. (55) Tuel, A.; Hommel, H.; Legrand, A. P.; Kova´ts, E. sz. Langmuir 1990, 6, 770. (56) Rauhut, G.; Clark, T.; Steinke, T. J. Am. Chem. Soc. 1993, 115, 9174. (57) Matyushov, D. V.; Schmid, R.; Ladanyi, B. M. J. Phys. Chem. B 1997, 101, 1035. (58) Bublitz, G. U.; Boxer, S. G. J. Am. Chem. Soc. 1998, 120, 3988. (59) Spange, S.; Reuter, A.; Lubda, D. Submitted for publication in Langmuir.
Langmuir, Vol. 15, No. 1, 1999 147
Figure 2. R values as function of surface coverage Γ for the functionalized LiChrospher. The solid line is the theoretically calculated curve according to equation 14. Sample numbers are the same as in Table 1.
Figure 3. R values as function of surface coverage Γ for the functionalized Aerosils. The solid line is the theoretically expected curve derived for the LiChrospher samples for comparison. Sample numbers are the same as in Table 1.
This discussion shows the general problem for the determination of reliable π* values for alkyl-functionalized silica particles. Discussion All the organically modified silicas studied exhibit lower values of the Kamlet-Taft R and of Reichardt’s ET(30) parameter57 than bare silicas. This result is expected because the value of these two parameters should be mainly caused by the amounts of residual free silanols. Similar results were found for the values of the ET(30) parameter for phenylsilane-modified silicas11 and for alkyland aryl-functionalized silicas as a function of surface coverage by using trifunctional silane reagents.59 The influence of the surface coverage (Γ) on the values of the R parameter is shown in Figure 2 and Figure 3 for functionalized LiChrosphers and Aerosils, respectively. For the LiChrospher samples no. 25-29 a linear correlation between the values of the R term and the surface coverage is available.
R ) 1.137 - 0.113Γ (µmol/m2) (Γ are the amounts of the functional group) (14) r ) 0.991, n ) 5, s ) 0.0228 The two ∼(CH2)3CN and ∼(CH2)3NH2 functionalized silica samples fit well into this line because the expected values of the R term of the respective functional group should be lower than the R value of the residual silanols. Therefore, since the R value of the residual silanols is larger than the R value of the functional groups as compared to solvent models [-(CH2)2-CN, R ) 0.18 for propionitrile] and [-(CH2)4-NH2, R ) 0.05 for 1-amino-
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butane],28 the residual silanols determine the measured R value. In the case that the functional group possesses a larger R value than the residual silanols that is [-(CH2)4-OH, R ) 0.84 for n-butanol],28 the HBD capacity of the functional group is measured as seen by the outlaying point of the sample no. 30. It should be emphasized that all the LiChrospher samples no. 25-29 which do fit into the linear correlation (equation 14) are synthesized by monofunctional silanes [Cl-Si(CH3)2C18H37]. The theoretical R value for a nonfunctionalized LiChrospher should to be 1.137. This value is placed between the R values of the nondried and dried (at 400 °C) LiChrospher sample. This result is reasonable because the functionalized samples are dried at 80 °C under vacuo. The R value for a moderately dried LiChrospher is 1.1 ( 0.02.53 Taking into account R ) 0 for a completely alkylfunctionalized silica surface, a surface concentration of Γ ) 10.06 µmol m-2 of former silanols is available by equation 14, assuming that each silanol group is monofunctionalized. This value derived from the solvatochromic data is astonishingly in excellent agreement with values determined by two independent experimental procedures and model calculation.54,55,60 This shows the correctness and reliability of the examined R values in this work. This result also shows that an average polarity is measured by the probe dyes used because a linear successive shift of the R value is observed with increasing functionalization the silanol groups. For the functionalized Aerosils there is no single dependence obtained because of various structurally different silica samples which were considered. It should be noted that sample series with structurally similar substituents yield separate dependences as indicated by the dashed line in Figure 3. Increasing degree of functionalization causes a moderate decrease of the value of the R parameter of the whole silica surface. The “theoretical” curve derived from equation 14, which is found for monolayer functionalization, is drawn as a solid line in Figure 3 for comparison. The two commercially available octyl (no. 12) and methyl (no. 11) functionalized Aerosils are also synthesized by means of monofunctional silanes even in the gas phase. The R values of these two samples approach the theoretically expected curve. Therefore, the functionalization process of the silica surfaces plays an important role for a well-defined HBD property of the final product. It seems obvious that the larger R values observed for several Aerosil samples are due to additional residual silanols, which are not involved in the modification process because the trifunctional silanes oligomerize. Therefore, an apparently larger surface coverage is calculated than is evident. This is indicated by the results from solid-state MAS 1H- and CP MAS 29Si{1H} NMR spectroscopy.47,48,61 According to the important paper by Duchet,61 for synthesizing well-defined silica surfaces with controlled HBD properties, the use of monofunctional silane reagents is required. In a previous paper, we have also considered that residual silanols and donor ligands linked to the silica surface may interact with each other.30 The HBA responsive probe Cu(tmen)(acac)+Cl- is adsorbed via the chloride counterion bridge to the residual silanols or/and functional groups. Consequently, the Cu(tmen)(acac)+ complex measures the HBA property of the adsorbed chloride as discussed in ref 30. This assumption is supported by the
correlation of the UV-vis absorption maxima of Cu(tmen)(acac)+Cl- adsorbed to the samples with the R value of the functionalized Aerosils as shown in Figure 4. With increasing R value of the residual silanols, the HBA capacity of the adsorbed chloride decreases as indicated by the hypsochromically shifted, relatively broad Cu(tmen)(acac)+ UV-vis absorption band (∆νmax (200 cm-1).30 The result shown in Figure 4 supports both the ability of the square planar copper complex to measure the HBA property of adsorbed anions and the reliability of the values of the R term. Despite the different structures of the three indicators used, 2, 3, and Cu(tmen)(acac)+, the result in Figure 4 shows the comparability between them. The two outlaying points in Figure 4 are samples which bear additionaly either a relatively large (no. 15) or low (no. 10) π* value. At first, the values of the π* parameters of the functionalized silicas seem to be not significantly depend on the structure of the ligands which are linked to the silica surface. Increasing functionalization of the silica surface with alkyl and aryl groups does not change remarkably its dipolarity/polarizability properties. The values of the π* parameters of the functionalized silicas range between 0.8 and 1.5. This result is, perhaps, not surprising for the functionalized silicas because DCE was mainly used as a suspending liquid possessing a π* value of 0.81. This value seems to be the limit for the detection differences. Closer considerations suggest, however, the following interpretation. It is remarkable that phenyl group modified silicas exhibit π* values greater than that of bare silica samples. The latter observation is reasonable and expected from solvent model compounds.20,28 This effect should be attributed to the high polarizability of the aromatic groups. Also, partial organically modified Aerosils (sample no. 11 and no. 12) exhibit unprecedented large π* values, even higher than pure Aerosils. These two samples are prepared by functionalization of Aerosil with monofunctional silanes in the gas phase. They contain a certain amount of strained siloxane bridges. We assume that dispersion forces between the dye 3 and the siloxane bridges do contribute to the value of these π* parameters. A similar explanation was also discussed for the larger heat of interaction obtained for n-octylamine adsorption to a similarly synthesized silica sample.62 It should also be noted that sample no. 11 exhibits the lowest R value reported in this paper for a partially modified silica sample. Kno¨zinger suggested also a highly polar structure for strained siloxane bridges.63 Accordingly, a contribution
(60) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1992, 86, 5209. (61) Duchet, J.; Chabert, B.; Chapel, J. P.; Gerard, J. F.; Chovelon, J. M.; Jaffrezick-Renault, N. Langmuir 1997, 13, 2271.
(62) Arnett, E. M.; Cassidy, K. F. Rev. Chem. Intermed. 1988, 27. (63) Kno¨zinger, H.; Sta¨hlin, W. Prog. Colloid Polym. Sci. 1980, 67, 33.
Figure 4. Dependence of the UV-vis absorption maxima of adsorbed Cu(tmen)(acac)+Cl- (νmax values were taken from ref 30) on the R value of the functionalized silica sample in 1,2dichloroethane. Sample numbers are the same as in Table 1.
Chemically Functionalized Silica Particles Scheme 1. Suggested Adsorption Mechanism of Michler’s Ketone on Siloxane Bridgesa
Langmuir, Vol. 15, No. 1, 1999 149
ET(30) ) 32.8 + 17.2R + 6.7π*
(15)
r ) 0.740, n ) 16, s ) 2.03 (a/s ) 2.57) This correlation for the functionalized Aerosils is of a poor quality. With the β parameter involved into the regression analysis,64 the correlation coefficient is improved considerably (equation 16).
ET(30) ) 18.0 + 28.8R + 10.2π*+ 3.5 β a A larger contribution of 3′ to the electronic ground state of 3 explains the strong bathochromic shift of the p-p* absorption band.
of the highly polar state of 3 adsorbed to siloxane bridges is suggested as shown in Scheme 1. That model is likely to explain the strong bathochromic band shift and consequently the higher π* values of silicas, which are modified by the gas-phase reaction, by using 327 as a probe. As a consequence of this assumption, potential Lewis acid sites of silica surfaces would display large values of the π* parameter. Correspondingly, the UV-vis absorption maximum of 3+ (the pure oxycarbenium) can be observed at about νmax ) 21 000 cm-1 when 3 is adsorbed to a highly Lewis acid functionalized silica that is silica-OAl(C2H5)2.53 The calculated values of the π* parameter of the highly alkyl group functionalized LiChrospher samples in cyclohexane range between 0.24 and 0.49. These values are in good agreement with the expected order of magnitude as estimated by equation 5 using results from pyrene fluorescence of a similar sample.2,36,37 Because of the correspondence between the expected and calculated values of the π* parameters obtained by 1b, 5, and 6 (this work) and pyrene (Harris) as probes, we suggest that the unexpectedly large values of the π* parameter, obtained with 3 as indicator, is attributed to an effect caused by residual silanols and dispersion forces of the alkyl chains. A similar explanation for the unexpected positive solvatochromic band shifts of 3 caused by cooperative effects is discussed in detail in ref 27. Since the discrepancy is caused only by residual silanols, unprecedented large R values would be responsible for this effect (see Table 4). However the indicator 2, which is very acid sensitive, does not indicate any kind of silanol groups. Perhaps, Fe(phen)2(CN)2 cannot penetrate toward these silanols because it is to bulky to surmount the shielding alkyl barrier on the surface. Therefore, we conclude that the small differences in the calculated π* values on using different π* indicators (Table 4) are caused by differently accessible amounts of silanols. However, the satisfied agreement of the π* values, obtained by means of the indicators pyrene and 1b, 4, and 5, shows that for the samples no. 19-24 the alkyl-bonded phase is preferably measured and not residual silanols. Systematic UV-vis spectroscopic studies with differently alkyl-substituted Michler’s Ketone derivatives as probes seem to be necessary to bring more light to this important question. For the correlation analyses we used the averaged π* values derived from the indicators 1b, 4, and 5 because of their similar order of magnitude, and they do reflect preferably the polarity of the alkyl-bonded phase. As a final conclusion, we have correlated the experimental values of the ET(30) parameters with the independently obtained R and π* values of the modified silicas to test an individual LSE relation for modified silicas similar to the Marcus equation 4 for solvents. For the functionalized Aerosils the following equation was obtained (equation 15).
(16)
r ) 0.963, n ) 11, s ) 1.08 (a/s ) 2.8) Despite the improved quality of the correlation, equation 16 seems not scientifically sound due to the relatively low value of the (XYZ)0 reference term. In an earlier publication,61 Marcus has also shown that the β parameters scarcely contribute (about 5%) to the ET(30) solvent polarity parameters. By investigating the polarity of the cellulose solvent N,N-dimethylacetamide/LiCl, we could show that the β term cannot be disregarded in describing the ET(30) parameter by means of LSE because this solvent bears an unusual high value of the β parameter.22 Therefore, the validity of equation 16 has to be tested in the future by additional experiments. However, equations 15 and 16 hint at the fact that acidbase interactions and hydrogen bonds contribute more strongly than nonspecific interactions to the value of the ET(30) parameter of functionalized silicas. The strong contributions of acid-base interactions to the values of the ET(30) parameters of modified silicas are supported by calculating the LSE correlation for the LiChrospher samples (equation 17).
ET(30) ) 36.1 + 15.1R + 5.5π*
(17)
r ) 0.981, n ) 12, s ) 1.88 (a/s ) 2.75) The much better quality of the correlation equation 17 as compared to equation 15 is also a hint at the more defined surface properties of the LiChrospher samples and the improved synthetic procedure used. By considering the values of the R and π* polarity parameters of all the silica samples studied, the correlation equation 18 is obtained.
ET(30) ) 36.1 + 14.84R + 5.33π*
(18)
r ) 0.966, n ) 30, s ) 1.90 Figure 5 shows the correlation of the measured ET(30) versus the calculated ET(30) plots. The good agreement between measured and independently calculated ET(30) parameters for functionalized silicas is a promising indication that this LSE concept may apply for other solid surfaces as well as for solid acid catalysts. It is striking that the quotient a/s, according to equation 3, for modified silicas (a/s ) 2.75 taken from equation 18) is evidently greater than that for well-behaved regular solvents (a/s ) 1.32 taken from equation 4). Therefore, we conclude that the values of the ET(30) parameters of silicate materials reflect more strongly the HBD capacity (about 74%) than the well-known ET(30) parameters of solvents (about 57%). This explanation seems scientifically sound because the thermal motion of solvent molecules surrounding the dye 1 causes an average polarity at (64) Marcus, Y. J. Solution Chem. 1991, 20, 929.
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relations can be used to empirically parametrize the surface polarity of functionalized silica particles. The statistical improvements of equations 14 to 17 will be the subject of future research. Conclusion
Figure 5. Calculated versus experimentally obtained ET(30) parameters of the functionalized silicas studied. The solid line is the theoretically curve according to equation 18.
ambient temperature whereas the surface indicators are more strongly bonded to acidic surface sites. The value of the ET(30) parameter decreases with increasing temperature in well-behaved regular solvents.65 In comparison, the thermal motion of the surface silanols and other immobilized groups is restricted to rotation and vibration. Therefore, the effects caused by thermal motion in solution cannot occur in this manner on the Aerosil/ liquid interface because the dye is adsorbed to the silanol groups. In the temperature range from -70 to + 40 °C, the value of the ET(30) parameter of dried Aerosil (ET(30) ) 58 ( 0.5) remains constant in dichloromethane and DCE as the suspending liquid.66 This final experiment supports the main result of this paper, that specific LSE (65) Linert, W.; Jameson, R. F. J. Chem. Soc., Perkin Trans. 1993, 2, 1415. (66) Spange, S.; Vilsmeier, E.; Reuter, A. Unpublished results.
The Kamlet-Taft solvent parameters R, β, and π* can be used as a reference system to parametrize the surface polarity of organically functionalized silica particles. Our results show that Fe(phen)2(CN)2 and Michler’s Ketone are suitable surface polarity indicators besides 2,6diphenyl-4-(2,4,6-triphenyl-1-pyridinio) phenolate to analyze both the R and π* term of partially modified silica particles in DCE. Unfortunately, the excellent R indicator Fe(phen)2(CN)2 cannot be applied to silica particles from cyclohexane solutions. The solvatochromic analyses indicate that the values of the ET(30) polarity parameter of functionalized silicas are more strongly influenced by the HBD term than those of regular solvents. The results of the solvatochromic measurements generally indicate that an averaged surface polarity has been measured.67 More research is necessary in this area to establish how silica preparation and surface functionalization determine the value of the π* parameter of functionalized silicas in detail. Also, the investigation of the influence of the solvent on the dipolarity/polarizability properties of the silica/solvent interface requires more systematic research and an improved UV-vis technique to exclude disturbing influences of light scattering by using other liquids than DCE. The conclusion of this study is that the mechanism of adsorption of well-suited solvatochromic probe dyes to solid surface gives valuable information on specific surface properties under well-defined conditions. LA980991I (67) Michael, D.; Benjamin, I. J. Phys. Chem. B 1998, 102, 5145.
