Langmuir 1998, 14, 3479-3483
3479
Cu(tmen)(acac)+ as an Ultraviolet-Visible Spectroscopic Probe for the Surface Hydrogen Bond Accepting Ability of Anions Adsorbed to Silica and Chemically Functionalized Silicas Stefan Spange,*,† Anett Reuter,† and Wolfgang Linert‡ Department of Polymer Chemistry, Institute of Chemistry, Technical University of Chemnitz, Strasse der Nationen, D-09107 Chemnitz, Germany, and Institute of Inorganic Chemistry, Getreidemarkt 9, A-1060 Wien, Austria Received January 28, 1998. In Final Form: May 5, 1998 A linear solvation energy relationship is employed to characterize the specific surface polarity of organically modified silica particles. The surface polarity of silicas can be quantitatively described by three independent terms, the dipolarity/polarizability (π*), the hydrogen-bond donating ability (R), and the hydrogen-bond accepting (HBA) ability (β). These terms can be defined using the Kamlet-Taft solvent parameters R, β, and π* as a reference system. The HBA property β of organically modified silica surfaces and anions adsorbed to silica has been determined by measuring the energy of the d-d* transition (νmax) of adsorbed Cu(tmen)(acac)+X- [X- ) Cl-, Br-, NO3-, CH3CO2-, and B(C6H5)4-]. νmax correlates linearly with the β parameter of the solvents. Cu(tmen)(acac)+B(C6H5)4- is used as a β indicator for the organically functionalized silicas because the nonnucleophilic anion B(C6H5)4- does not interact with residual silanols. Cu(tmen)(acac)+Cl-, Br-, NO3-, and CH3CO2- are adsorbed to residual silanols due to the higher basicity of the counteranion. The correspondence between the residual silanol acidity and the basicity of the functional groups of specific silica samples is discussed.
1. Background Surface polarity strongly affects the ability of particles and flat surfaces to interact with adsorbed compounds in a very complex manner, for example, in chromatographic processes or in their use as catalysts.1-3 In this sense the term “surface polarity” is often ambiguously used. The surface polarity is usually estimated by measuring the energy of the π-π* transition of 2,6-diphenyl-4-(2,4,6triphenyl-1-pyridinio)phenolate (Reichardt’s dye) on bare silicas,3-5 chemically functionalized silicas,4,6 and alumina.7 In other work, this betaine dye was incorporated by means of a sol-gel process into an ormosil network8 or chemically bonded to the silica surface.9 Despite the usefulness of Reichardt’s dye for measuring the surface polarity in the framework of the ET(30) scale,10 the basicity properties of surfaces cannot be registered with this solvatochromic probe molecule. This can be shown by application of the LSE (linear solvation energy) correlation analysis of the ET(30) solvent polarity parameters. Intermolecular solute/solvent interactions can be advantageously expressed by the LSE relationship of Kamlet and Taft.10-13 The simplified Kamlet-Taft equation14 applied † ‡
Technical University of Chemnitz. Institute of Inorganic Chemistry.
(1) Lindley, S. M.; Flowers, G. C.; Leffler, J. E. J. Org. Chem. 1985, 50, 607. (2) Park, J. H.; Carr, P. W. J. Chromatogr. 1989, 465, 137. (3) Chronister, C. W.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793. (4) Spange, S.; Reuter, A.; Schramm, A.; Reichardt, C. Org. React. (Tartu) 1995, 29, 951. (5) Spange, S.; Reuter, A.; Vilsmeier, E. Colloid Polym. Sci. 1996, 274, 59. (6) Taverner, S. J.; Clark, J. H.; Gray, G. W.; Heath, P. A.; Macquarrir, D. J. J. Chem. Soc., Chem. Commun. 1997, 1147. (7) Michels, J. J.; Dorsey, J. G. Langmuir 1990, 6, 414. (8) Rottman, C.; Grader, G. S.; Hazan, Y. D.; Avnir, D. Langmuir 1996, 12, 5505. (9) Crowther, D.; Liu, X. J. Chem. Soc., Chem. Commun. 1995, 2445. (10) Reichardt, C. Chem. Rev. 1994, 94, 2319. (11) Spange, S.; Keutel, D. Justus Liebigs Ann. Chem. 1992, 423.
to solvatochromic shifts (with XYZ ) νmax, probe)10-12 is given by eq 1.
XYZ ) (XYZ)o + aR + bβ + (s + dδ)π*
(1)
(XYZ)o is νmax (probe) in a reference system, for example, a nonpolar inert solvent, R is the HBD (hydrogen bond donating) ability, β is the HBA (hydrogen bond accepting) ability, and π* the dipolarity/polarizability of the solvents. δ is a polarizability correction term that is 1.0 for aromatic, 0.5 for polyhalogenated, and zero for aliphatic solvents; a, b, s, and d are solvent-independent regression coefficients. The Kamlet-Taft parameters seem to be very useful in describing the manifold polarity properties of surfaces15 because they can be easily transmitted into the ET(30) scale by eq 210,11 as well as into other useful solvent polarity scales12,13 such as the acceptor number (AN) and donor number (DN) scale of Gutmann.16,17 Equation 2 shows the correlation of the ET(30) values with the KamletTaft parameters (r ) 0.987, n ) 100) taken from ref 13. From eq 2 it can be concluded that the ET(30) solvent parameter reflects mainly the HBD ability (R) and the dipolarity/polarizability (π*) of solvents and only scarcely their HBA ability (β).11-13 A recalculation of eq 2 for a
ET(30) ) 30.2 + 12.99 (π* - 0.21δ) + 14.45R + 2.13β (2) majority of n ) 166 solvents shows that the β and δ term (12) Marcus, Y. Chem. Soc. Rev. 1993, 409. (13) Marcus, Y. J. Solution Chem. 1991, 20, 929. (14) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (15) Jensen, W. B. Acid-Base Interactions; Mittal, K. L., Anderson, H. R., Eds.; VSP: Zeist, 1991; p 3. (16) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225. (17) Taft, R. W.; Kamlet, M. J. J. Chem. Soc., Perkin Trans. 2 1979, 1723.
S0743-7463(98)00109-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/05/1998
3480 Langmuir, Vol. 14, No. 13, 1998 Chart 1. Structure of Dye 1+
can be disregarded without worsening the correlation.10,12 These correlation analyses show that the ET(30) values represent mainly the dipolarity/polarizability and HBD ability of an environment. The determination of surface basicity parameters such as β or DN is still not established by means of the solvatochromic technique. The reason for this is the lack of suitable solvatochromic indicators, which reflect selectively the basicity properties of surfaces. For instance, common Kamlet-Taft β indicators such as 4-nitrophenol or 4-nitroaniline cannot be applied to examine individual β values of highly polar silica surfaces because of the numerous adsorption mechanisms of these indicators.1,5 Therefore, the Fourier-transform infrared (FTIR) sensitive molecular probe pyrrole has been successfully applied as far as metal oxide basicity is concerned.18 Recently, iodine was used as a promising molecular probe to evaluate the donor properties of zeolites.19 Organic π acceptors such as 7,7,8,8-tetracyanoquinodimethane or tetracyanoethene have been used as indicators to determine the donor strength of metal oxide surfaces.20 CT (charge transfer) complexes form between these π acceptors and the metal oxides, and the CT absorption maximum correlates with the ionization potential of the occupied surface sites.20 However, no correlations have been investigated in these reports between the units of UV-vis measurements and solvent basicity parameters.18-20 The position of the long-wavelength d-d* absorption of the square-planar mixed-ligands transition-metal complex N,N,N′,N′-tetramethylethylenediamine copper(II)acetylacetonate, Cu(tmen)(acac)+ (1+) (Chart 1), is strongly solvent dependent.21,22 The extent of the bathochromic shift of the UV-vis absorption maximum of 1+ depends mainly on the strength of the Lewis basicity of the interacting compounds, that is, solvents.23,24 In pure donor solvents, the d-orbital splitting of 1+ is attributed to an octahedral geometry of the complex.25 Anion coordination yields a five-coordinated 1+X- complex, and the splitting of the d-orbitals occurs in a different manner. It is, however, very difficult to distinguish between the electronic spectra of the five- and the sixcoordinated forms.25 Thus, when 1+ is dissolved in a series of solvents, the νmax values remain constant until an anion with a larger donor strength interacts with the cation 1+. As long as this situation does not occur, the νmax values refer to the UV-vis spectra of solvated or complexed 1+ and reflect the donor strength of the solvent. (18) Binet, C.; Jadi, A.; Lamotte, J.; Lavaley, J. C. J. Chem. Soc., Faraday Trans. 1996, 92, 123. (19) Choi, S. Y.; Park, Y. S.; Hong, S. B.; Yoon, K. B. J. Am. Chem. Soc. 1996, 118, 9377. (20) Meguro, K.; Esumi, K. Acid-Base Interactions; Mittal, K. L., Anderson, H. R., Jr., Eds.; VSP: Zeist, 1991; p 117. (21) Linert, W.; Taha, A. J. Coord. Chem. 1993, 29, 265. (22) Fukuda, Y.; Sone, K. Bull. Chem. Soc. Jpn. 1972, 45, 465. (23) Soukup, R. W.; Schmid, W. J. Chem. Educ. 1985, 62, 459. (24) Migron, Y.; Marcus, Y. J. Phys. Org. Chem. 1991, 4, 310. (25) Linert, W.; Jameson, R. F.; Taha, A. J. Chem. Soc., Dalton Trans. 1993, 3181.
Letters
Accordingly, cation 1+ shows a blue color in the strong HBA solvent HMPA, and in the very weak HBA solvent 1,2-dichloroethane (DCE), its color is red. The shift of the vis band maximum of 1+, measured in various solvents, correlates well with the β parameter of Kamlet and Taft22 as well as with the donor number (DN)21 of the solvent, by using B(C6H5)4- as a nonnucleophilic counteranion (see eqs 3 and 4). Both terms, Lewis basicity and HBA property, have been used to represent β in the literature.10,12
β ) 6.716 - 0.358νmax[1+B(C6H5)4-]10-3 [cm-1]
(3)
n ) 17, r ) 0.960, s ) 0.045 DN ) 195.5 - 0.0102νmax[1+B(C6H5)4-][cm-1]
(4)
n ) 12, r ) 0.990, s ) 1.37 A combined influence on the d-d* transition of 1+ is observed when counterions with a larger basicity are applied.25 Linert et al. reported a procedure for calculation the apparent donor numbers of anions by correlation analyses.25 In this way, donor numbers of several anions have been determined in various solvents. It can be demonstrated that the donor number of anions, DN (X-), is also a function of the acceptor strength of the solvent used, AN(solvent).25 By knowledge of the AN of the solvent, the DN of X- can be calculated by means of eq 5, which is taken from ref 25.
DN (X-, solvent) ) 129.6 - 0.548AN(solvent) 0.00602νmax[Cu(tmen)(acac)+] X- (5) Therefore, it is expected that a silica surface causes a similar effect to adsorbed anions due to its strong acceptor strength (AN ) 49.1),5 which is comparable to that of hydrogen-bonding solvents.16 In this paper, we report that 1+ can be used as a convenient probe for the determination of the HBA property or Lewis basicity of anions adsorbed to silica and to chemically modified silica surfaces. 2. Experimental Details The functionalized silica particles have been obtained by reaction of Aerosil 300 with commercially available trialkoxysilane and trichlorosilane reagents.26-28 A typical procedure is as follows: 4 g of Aerosil 300 is dried at 400 °C for 12 h in a 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 trialkoxysilane reagent is added while the mixture is stirred over a time period of 15 min. After complete addition of the trialkoxysilane reagent, the mixture is heated for 5 h to remove the formed alcohol by azeotropic distillation. Then the mixture is stirred overnight. The crude product is separated by centrifugation and then extracted with toluene in a Soxhlet extractor for 5 h. After careful subsequent washing with toluene, acetone, and diethyl ether, the clean product is dried in vacuo at 70 °C. The final product is colorless and exhibits the same morphology as the former Aerosil 300. The modified silica samples have been characterized by combustion analysis (C, H, N analysis), BET measurements, and diffuse reflectance IR Fourier transform (26) Hara, S. J. Chromatogr. 1979, 186, 543. (27) Horner, L.; Ziegler, H. Z. Naturforsch., Part B 1987, 42, 643. (28) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693.
Letters
Langmuir, Vol. 14, No. 13, 1998 3481 Table 1. Physical Properties and Chemical Characteristics of the Silica Samples Used
no. 1 2 3 4 5 6 7 8 9 10 11
modified silica
12
carbon content (%)
nitrogen content (%)
surface covering (µmol m2)
no. of ligands (per nm2)
240 240 156 144 224 240 210 205 217 229 120
9.2 8.5 3.8 3.4 1.2 4.3 2.6 9.7 8.0
2.9 2.2 0.4 0.5 2.6
13.3 10.9 1.3 1.5 4.8 2.2 0.6 5.0 7.7
8.0 6.6 0.8 0.9 3.0 1.4 0.4 3.0 4.6
360
4.6
-a
3.5
2.1
nondried aerosil dried aerosil |-OSiC3H6NH2 |-OSiC4H8NH2 |-OSiC3H6CN |-OSiC3H6SCN |-OSiCH3 |-OSiC8H17 |-OSiC18H37 |-OSiCH3C6H5 O Si
a
specific surface area (m2 g-1)
N
(CH2)3
N
|-OSiC3H6NH2
Not determined.
Table 2. UV-vis Absorption Maxima νmax for [Cu(acac)(tmen)]+X- in 1,2-Dichloroethane and Adsorbed to Aerosil 300 in the Presence of Various Anions, Together with the β Values and the Donor Numbers (DNx) of These Anions As Calculated from Eqs 3 and 5 νmax‚103 (cm-1)
DN(X)-
β
1+(X)
DCEa
dried Aerosil
nondried Aerosil
DCEa
dried Aerosil
nondried Aerosil
DCE
dried Aerosil
nondried Aerosil
B(C6H5)4ClBrCH3CO2NO3-
20.0 14.2 14.5 14.9 16.1
19.5b 15.6 16.1 15.2 18.0
19.4b 15.8 16.8 15.1 18.0
0.0 36.2 33.7 29.5 21.1
17.2 15.4 13.0 5.8
15.0 13.2 10.8 3.6
-0.448 1.63 1.51 1.38 0.95
-0.26 1.13 0.95 1.27 0.27
-0.23 1.06 0.70 1.31 0.27
a
Taken from ref 25. b Only little adsorption.
(DRIFT) spectroscopy. 13C{1H} cross polarization magic angle spinning (CP MAS),29 Si{1H} CP MAS, and 1H MAS solid-state NMR spectroscopy were employed in selected cases to confirm the structure.29-32 The specific properties of the organically modified silica samples used are compiled in Table 1. The organically modified silica samples were dried carefully in vacuo at 70-80 °C in a special glass vessel. After the sample was cooled to room temperature under dry argon, a solution of the solvatochromic probe in DCE is simply added to the silica material. Care must be taken to avoid overloading the surface with 1+, as multilayer adsorption in solutions at higher concentrations or disturbing absorptions from nonadsorbed dye from the solution can be expected. The amount of the dye added was therefore restricted to 0.5-2 mmol g-1. The solvent used was DCE because Aerosil/DCE suspensions are absolutely transparent in the visible region and because DCE can be considered as a weak HBA solvent. The same standard solvent was used in ref 25. The technique used for the UV-vis spectroscopic measurements with transparent suspensions of silica in DCE is described in detail in ref 5. The measurements at the solid/liquid interface are advantageous because they can be performed under inert conditions and reflect better the conditions suitable for liquid chromatography and catalysis in suspension. 3. Results and Interpretations As silica we used Aerosil 300 because it can be easily handled as a suspension in DCE. The silica surface causes (29) Francke, V.; Gu¨nther, H.; Reuter, A.; Spange, S. Unpublished results. (30) Zaper, A. M.; Koenig, J. L. Polym. Compos. 1985, 6, 156. (31) Bayer, E.; Albert, K.; Reiners, J.; Nieder, M.; Mu¨ller, D. J. Chromatogr. 1983, 264, 197. (32) Maciel, C. E.; Bronnimann, R. S.; Zeigler, J.; Ssuer Chuang, D. R.; Kinney, E. A. In The Colloid Chemistry of Silica; American Chemical Society: Washington, DC, 1994; p 260.
a strong hypsochromic shift of the visible absorption band of 1+ X- as compared to the vis spectrum measured in the DCE solution when the nucleophilic anions Cl-, Br-, NO3-, and CH3CO2- are used. These salts are completely adsorbed to Aerosil 300 as indicated by the noncolored supernatant DCE solution, which has no absorption in the visible region. In the case of the nonnucleophilic anion B(C6H5)4-, only a very small bathochromic band shift of the vis absorption of 1+ is observed, which indicates a slightly larger basicity of Aerosil 300 than DCE. Only small amounts of 1+B(C6H5)4- are adsorbed to bare Aerosil 300. Therefore, overlapping absorptions with nonadsorbed 1+ are observed in this specific case. The UV-vis spectroscopic results are compiled in Table 2. The observed hypsochromic band shift compared to the vis spectrum in DCE solution for 1+Cl-, 1+Br-, 1+NO3-, and 1+CH3CO2- adsorbed to Aerosil 300 is different for each anion. It can be concluded that the adsorption of the probe cation 1+ to the silica surface is mediated by the counterion. An influence of the concentration of 1+X- on the position of the wavelength of the d-d* transition of 1+ is not observed. The stronger HBD capacity of nondried Aerosil as compared to dried Aerosil is particularly observed for the anions Cl- and Br- due to the larger hypsochromic band shift that is caused by nondried Aerosil. This is consistent with the independently calculated AN for the Aerosil samples (ANdried < ANnondried).5 Therefore, the apparent DN parameters for the surface coordinated anions can be calculated by eq 5 (see Table 2) using the AN values for dried (AN ) 49.1) and nondried (AN ) 53.1) Aerosils, respectively.5 In the case of B(C6H5)4- as counterion, 1+ is adsorbed directly to the bare silica surface,33 and eq 5 is not applicable.25 The β values for the anions adsorbed are calculated by eq 3. As a consequence drawn from the results of Table 2, (33) Farkas, J.; Hampden-Smith, M. I.; Kodas, T. T. J. Phys. Chem. 1994, 98, 6753.
3482 Langmuir, Vol. 14, No. 13, 1998
Letters
Table 3. UV-vis Absorption Maxima νmax for [Cu(acac)(tmen)]+B(C6H5)4- and [Cu(acac)(tmen)]+ClAdsorbed at Various Silica Samples, Their Donor Numbers (DN) and Basicity Values (β) As Calculated by Eqs 3 and 4, and Their ET(30) Parameters Determined with Reichardt’s Dye and Calculated According to Eq 6 1+B(C6H5)41+Clno. 103νmax (cm-1) 103νmax (cm-1)
DN
β
ET(30) (kcal mol-1)
1 2 3 4 5 6 7 8 9 10 11 12
-2.4 -3.4 40.4 41.5 -0.3 -1.4 -0.3 2.7 1.7 16.0 37.4 26.2
-0.23 -0.26 1.27 1.31 -0.16 -0.19 -0.16 -0.05 -0.09 0.42 1.17 0.77
58.8 57.0 54.3 54.7 55.6 53.4 54.7 54.4 55.4 57.6 51.8 55.6
19.4 19.5 15.2 15.1 19.2 19.3 19.2 18.9 19.0 17.6 15.5 16.6
15.8 15.6 14.4 14.5 15.7 14.6 15.3 15.1 14.9 15.2 14.2 14.4
1+B(C6H5)4- can be applied as a Lewis basicity indicator for functionalized silica particles, because disturbing effects caused by the anion are excluded. 1+B(C6H5)4- is adsorbed quantitatively from DCE solution to all chemically modified silica samples studied. The HBA ability or Lewis basicity of a range of chemically modified Aerosils with specific degrees of functionalization is given in Table 3. Table 3 contains also the νmax values of 1+Cl- adsorbed to the silica samples and the values of the ET(30) parameters. The ET(30) parameters of the modified silica particles have been calculated according to eq 6 by measuring the vis absorption maxima (νmax) of Reichardt’s dye adsorbed to the silica samples4,5 in DCE suspension.
ET(30) [kcal mol-1] ) (2.8591 × 10-3)νmax (Reichardt’s dye) [cm-1] (6) All the organic groups studied exhibit stronger HBA properties and lower ET(30) values than bare silica. The β values for chemically functionalized silicas are comparable to solvent model compounds. Functionalized silicas bearing strong nitrogen bases (primary amine, imidazolidine) show larger β values than alkyl-modified silicas. Cyanopropyl- and isothiocyanatopropyl-modified silicas with low degrees of functionalization show lower β values than expected from solvent model compounds. We assume that the high concentrations of residual silanols are responsible for this effect. Perhaps the excess of silanols interact with the functional groups which cause a lowering of the HBA ability of the original CN and NCS groups. To prove this argument, we have investigated the HBA properties of a commercially available LiChrospher silica sample (LiCh-CN-5 µm) that contains high amounts of cyanopropyl groups (3.7 µmol/m2) and very small amounts of detectable silanols. The absorption maximum of 1+B(C6H5)4- adsorbed to this specific sample (LiCh-CN-5 µm) is νmax ) 17 300 cm-1, which corresponds to a value of the β parameter of 0.51 (DN ) 18.7). The value of the ET(30) parameter was determined to be 55.8 for this sample. The β value agrees well with the β value for butyronitrile (β ) 0.4, DN ) 16.6). It is remarkable, that the donor groups of highly functionalized silicas exhibit only slightly larger β values than comparable solvent models. As expected, there is no correlation between ET(30) and the DN or β values. In a following paper we will show by zeta-potential measurements that the β-values are reliable parameters for the surface properties of the modified
Figure 1. Differences of the UV-vis absorption maxima ∆νmax of [Cu(acac)(tmen)]+B(C6H5)4- and [Cu(acac)(tmen)]+Cl- for the various silica samples versus the β values of these samples. The numbers of the samples are the same as those in Table 2. Chart 2. Suggested Adsorption Mechanisms for 1+B(C6H5)4- and 1+Cl-, Respectively, at Modified Silica Surface. F ) Functional Group
silicas.34 The νmax values of 1+B(C6H5)4- and 1+Cl-, measured at the functionalized silicas, do not correlate with each other indicating the different adsorption mechanism of the two probes. The postulated different adsorption mechanisms for 1+Cl- and 1+B(C6H5)4- to chemically functionalized silica are shown in Chart 2. According to Chart 2, 1+Cl- can principally adsorb to both the functional group or/and the residual silanols. In all cases, 1+Cl- measures a larger HBA property of each silica sample as a whole as indicated by the bathochromically shifted 1+Cl- absorption. We assume that both species, the Cl- f 1+ and the F f 1+Cl- complexes, contribute to the measured 1+Cl- absorption. Therefore, the difference of the strength of the HBA properties of the functionalized silicas, as measured by 1+B(C6H5)4- and 1+Cl-, is smaller as the HBA property of the functional group becomes larger. This result is shown in Figure 1. Due to the broad symmetric absorption bands of 1+, a determination of which kind of species dominates is not possible by the UV-vis method. Because of the multiple absorption mechanism of 1+Cl-, this probe should be used for specific applications, that is, for proving anion adsorption. It is generally expected that 1+Cl- is adsorbed via a counterion bridge to residual silanol groups. Therefore, a low degree of functionalization of the silica surface should cause a strong coordination of the chloride ion. A strong coordination of the chloride by residual silanols or other acceptor sites causes a lowering of the HBA capacity of the adsorbed chloride. This explanation is also consistent with the observation that the β value of aminopropyl-functionalized silicas (34) Spange, S.; Reuter, A.; Jacobasch, H. J.; Bellmann, K. To be submitted for publication in Colloid Polym. Sci.
Letters
Langmuir, Vol. 14, No. 13, 1998 3483
increases with the degree of functionalization (samples 3 and 12, Table 3). The coordination of the silica-linked aminopropyl group by residual silanols causes a lowering of the HBA property of the former group toward an external HBD molecule or Lewis acid, that is, the HBA indicator. According to this explanation, the corresponding R-values of the residual silanols, the HBD property of the functionalized silicas should also be decreased. These relations will be reported in a following paper.35 Conclusion 1+B(C6H5)4-
and 1+Cl-, respectively, on Adsorption of chemically modified silica samples is an easy and rapid method for a reliable determination of their HBA ability and, inversely, for the estimation the HBD ability of residual silanol surface groups. Advantageously, the HBA ability of functional surface groups can be expressed by the Kamlet-Taft β parameter (35) Reuter, A. Synthese und Charakterisierung modifizierter Polykieselsa¨ uren: Strukturuntersuchungen und UV/VIS-spektroskopische Bestimmung von Oberfla¨ chenpolarita¨ tsparametern mittels solvatochromer Farbstoffe; Shaker Verlag: Aachen, 1997.
or the DN of Gutmann. It makes no significant difference which parameter, β or DN, is used because both parameters correlate well with each other.36 As shown, the value of the parameter of the HBA ability is a function of both nature and concentration of the functional group. Therefore, we recommend that the determination of HBA parameters for novel functionalized silica samples should be carried out for each specific case independently. Because of the simple experimental procedure of UV-vis measurements in the visible region, the method described in this paper is recommended as a convenient tool for that purpose. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft, Bonn, and the Fonds der Chemischen Industrie, Frankfurt (Main), for financial support, Merck AG, Darmstadt, for providing functionalized silica particles, and Professor C. Reichardt, Marburg, for making the ET(30) dye available to us. LA9801099 (36) Gritzner, G. J. Mol. Liq. 1997, 73/74, 487.
