Influence of Chemical Solvent Properties on the External and Internal

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Influence of Chemical Solvent Properties on the External and Internal Surface Polarity of Silica Particles in Slurry Yvonne Zimmermann, Susann Anders, Katja Hofmann, and Stefan Spange* Department of Polymer Chemistry, Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, D-09111 Chemnitz, Germany Received June 21, 2002. In Final Form: August 30, 2002 Kamlet-Taft’s R (hydrogen-bond acidity) and π* (dipolarity/polarizability) parameters of three different silica samples and a siliceous MCM-41 material determined in 17 various solvents are presented. Fe(phen)2(CN)2 [cis-dicyano-bis(1,10)phenanthrolineiron(II), (1)] and Michler’s ketone [4,4′-bis(N,N-dimethylamino)benzophenone, (2)] were used as solvatochromic surface polarity indicators. The UV/vis transmission spectra of the two surface polarity indicators 1 and 2 adsorbed on the silicas from the solvents were measured, and the absorption maxima were used to calculate the R and π* values of the silica surface. R of the silanols on the freely accessible surfaces decreases with increasing HBA (hydrogen-bond accepting) capacity of the solvent whereas the π* term of the solvent only marginally modifies the polarity of the external surface silanol groups. In the mesopores of MCM-41, the UV/vis shift of the solvatochromic dyes shows a different behavior as a function of solvent polarity than that on freely accessible silica surfaces. The dipolarity/polarizability of solvent molecules inside the pores affects significantly the solvatochromic shift of the dye in the pores. The specific polarity properties of the solvent/silica interface at freely accessible surfaces and inside the MCM-41 pores are discussed.

Introduction Silica particles have been widely used for different applications,1-3 for example, in chromatography,2 for mediating gently occurring electrophilic organic reactions,4,5 as support for heterogeneously induced chemical reactions,6 and as a component for derivatization reactions,2,3,7 because the surface silanol groups can be used for a lot of functionalization reactions. However, many of these processes on silica surfaces have been carried out in contact with organic liquids which serve either as the mobile phase in chromatography or as the solvent for the components taking part in a heterogeneously induced reaction or for the derivatizing reagent, for example, a reactive silane compound. Usually, the adsorption of a HBA (hydrogen-bond accepting) solvent on silica decreases the catalytic activity of the surface for electrophilic reactions4a due to lone electron pairs of HBA solvents, evidently interacting with the surface silanol groups as outlined in Scheme 1. (1) Iler, R. K. The Chemistry of silica; John Wiley & Sons: New York, 1979. (2) Scott, R. P. W. Silica Gel and Bonded Phases; John Wiley & Sons: New York, 1993. (3) Bergna, H. E. Colloid Chemistry of silica, An Overview; American Chemical Society: Washington, DC, 1994; ISBN 0065-2393/94/0234. (4) Kropp, P. J.; Daus, K. A.; Tubergen, M. W.; Kepler, K. D.; Wilson, V. P.; Craig, S. L.; Baillargeon, M. M.; Breton, G. W. J. Am. Chem. Soc. 1993, 115, 3071. (b) Breton, W.; Daus, K. A.; Kropp, P. J. J. Org. Chem. 1992, 57, 6646. (c) Kropp, P. J.; Daus, K. A.; Crawford, S. D.; Tubergen, M. W.; Kepler, K. D.; Craig, S. L.; Wilson, V. P. J. Am. Chem. Soc. 1990, 112, 7433. (5) Spange, S.; Fandrei, D.; Simon, F.; Jacobasch, H. J. Colloid Polym. Sci. 1994, 272, 99-10. (b) Eismann, U.; Spange, S. Macromolecules 1997, 30, 3439-3446. (c) Spange, S.; Eismann, U.; Ho¨hne, S.; Langhammer, E. Macromol. Symp. 1997, 126, 223-236. (d) Spange, S. Prog. Polym. Sci. 2000, 25, 781. (6) Clark, J. H.; Macquarrie, D. J. Chem. Commun. 1998, 853. (b) Price, P.; Clark, J. H.; Macquarrie, D. J. J. Chem. Soc., Dalton Trans. 2000, 101-110. (7) Arkles, B. Chemtech. 1977, 7, 766. (b) An excellent review for silicon compounds, their chemistry, and applicability for surface modification is given in: Arkles, B. Gelest 2000; Glest, Inc.: Tullytown, PA 19007-6308, 1998; pp 16-104 and references in this catalogue.

Several authors reported on the mechanistic aspects of the adsorption of different basic solvents on silica during the past decades. The following established results are instructive: (1) the surface silanol valence vibration decreases with increasing donicity of the solvent,8,9,10 (2) the heat of adsorption increases with increasing basicity of the interacting base,11 and (3) the retention of a HBA component in the mobile phase increases with increasing its HBA strength and dipolarity/polarizability.12 To quantify a liquid in terms of its HBA property or basicity, among others, the following physicochemical parameters have been well established: Gutmann’s donor number (DN), which corresponds to the reaction enthalpy of the HBA solvent with SbCl5, which serves as the reference electron pair acceptor, measured in 1,2-dichloroethane,8,13 the pK(BH+) value, the protonation equilibrium constant in water,14 or the β value introduced by Kamlet and Taft, which results from the solvatochromism of several reference compounds.15 A critical review of the applicability of the different empirical HBA property parameters is given by Marcus16 and Gritzner.17 However, the interaction of a solvent with a solid surface is a composite of many effects.11,18 Not only acid-base but (8) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978. (9) Winde, H.; Fink, P.; Ko¨hler, A. Z. Chem. 1977, 17, 41. (10) Pohle, W. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2101. (11) Arnett, E. M.; Cassidy, K. F. Rev. Chem. Intermed. 1988, 9, 27. (12) Park, J. H.; Carr, P. W. J. Chromatogr. 1989, 465, 137. (b) Rutan, S. C.; Carr, P. W.; Taft, R. W. J. Phys. Chem. 1989, 93, 4292-4297. (c) Weckwerth, J. D.; Carr, P. W. Anal. Chem. 1998, 70, 4793-4799. (13) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225-240. (14) Kittelmann, U.; Unger, K.; Kreis, W. Prog. Colloid Polym. Sci. 1980, 67, 19. (15) Kamlet, M. J. D.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877-2887. (b) Taft, R. W.; Kamlet, M. J. J. Chem. Soc., Perkin Trans. 2 1979, 1723-1727, (c) Kamlet, M. J.; Hall, T. H.; Bodkin, J.; Taft, R. W. J. Org. Chem. 1979, 44, 2599. (16) Marcus, Y. J. Solution Chem. 1991, 20, 929-944. (b) Marcus, Y. Chem. Soc. Rev. 1993, 409-416. (17) Gritzner, G. J. Mol. Liq. 1997, 73/74, 487.

10.1021/la020574c CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002

Surface Polarity of Silica Particles in Slurry

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Scheme 1. Interaction of Lone Electron Pairs of HBA Solvents with the Surface Silanol Groupsa

a D, solvent molecule; P, HBD sensitive probe like 1; (I) in solvents like cyclohexane, DCE, and chloroform; (II) in solvents like THF, ether, acetone, and acetonitrile; (III) in solvents like pyridine and amines.

also ion-dipole, dipole-dipole, dipole-induced dipole, and dispersion interactions contribute to the overall solvation energy of an inorganic surface.12,18 Weak HBA solvents, that is, aromatics, halogenated solvents, or alkanes, can also change significantly the surface reactivity of a solid acid.19 Alcohols bear both HBA and HBD (hydrogen-bond donor) properties. HBD solvents (carbonic acids, alcohols) can have an ambiguous influence on the reactivity and polarity of surface silanols.20-23 This means that for each solvent surface sites of different polarity as well as different contributions of specific (acid-base) and nonspecific van der Waals interactions must be taken into account. Multiple intermolecular solute/solvent interactions can be described by the LSE (linear solvation energy) relationship of Kamlet and Taft.15 The simplified KamletTaft equation is given in eq 1.

XYZ ) (XYZ)0 + aR + bβ + s(π* + dδ)

(1)

(XYZ)0 is the solute property of a reference system, for example, in a nonpolar medium or gas phase, R describes the HBD acidity, β describes the HBA basicity, and π* describes the dipolarity/polarizability of the solvent. The three parameters characterize the overall solvent polarity. δ is a polarizability correction term which is 1.0 for aromatic, 0.5 for polyhalogenated, and zero for aliphatic solvents; a, b, s, and d are solvent-independent correlation coefficients.15 Several authors have shown that the Kamlet-Taft solvent parameters R, β, and π* are well suited for the quantitative description of solvent-solid interfacial interactions, in chromatographic12 and adsorption processes,24 rather than parameters derived using water as the reference liquid.11,14 Furthermore, the advantage of the Kamlet-Taft parameters is that they can be converted by means of multiple LSE relationships into other useful polarity scales, for example, the acceptor number (AN) or donor number (DN) scales of Gutmann16,17,24 and the ET(30) scale of Reichardt.25 It is expected that, with increasing acid-base interaction of the silica surface with a HBA solvent, the HBD capacity of the silanol groups decreases. The influence of the solvent on the π* term of the silica surface is difficult to interpret, because the acid-base interaction modifies (18) Jensen, W. B. In Acid-Base-Interactions; Mittal, K. L., Anderson, H. R., Eds.; VSP: Utrecht, 1991; pp 3-23. (19) Helburn, R. S.; Rutan, S. C.; Pompano, J.; Mitchern, D.; Patterson, W. T. Anal. Chem. 1994, 66, 610-618. (b) Rutan, S. C.; Harris, J. M. J. Chromatogr., A 1993, 656, 197-215. (20) Rochester, C. H. Prog. Colloid Interface Sci. 1980, 67, 7. (21) Arnett, E. M.; Ahsan, T. J. Am. Chem. Soc. 1991, 113, 6861. (22) Dawidowicz, A. L.; Patrykiejew, A.; Wianowska, D. J. Colloid Interface Sci. 1999, 214, 362-367. (23) Daniels, M. W.; Sefcik, J.; Francis, L. F.; McCormick, A. V. J. Colloid Interface Sci. 1999, 219, 351-356. (24) Brune, B. J.; Payne, G. F.; Chaubal, M. V. Langmuir 1997, 13, 5766-5769. (25) Reichardt, C. Chem. Rev. 1994, 94, 2319-2358.

the dipolarity of the silica surface in a complex manner (Scheme 1). In previous publications we have shown that both the HBD-capacity and dipolarity/polarizability parameters are influenced by chemical surface modification and by solvent effect.26 We have also shown that the catalytic activity of silicas in enabling the triphenylmethylium ion of chlorotriphenylmethane to undergo a hydride transfer reaction depends on the surface acidity.27 Determination of the surface polarity parameters R and π* by chromatography or adsorption calorimetry is not possible. Thus, for the simultaneous determination of the R and π* terms of functionalized silicas, we used a set of solvatochromic dyes.26,28 Recently, among the established polarity probes pyrene29 and Reichardt’s dye,26c,28c,30 Nile red31 has also been recommended as a probe for measuring internal polarities in sol-gel materials. In this work we employ the established UV/vis spectroscopic method suitable for simultaneous determination of the surface acidity (R) and the dipolarity/polarizability (π*) of moderately strong solid acids using two surface polarity indicators, Fe(phen)2(CN)2 (1) and Michler’s ketone (2) (see Schemes 2 and 4).26a,b The motivation of the present study was the examination of the surface acidity and polarity parameters of bare silica particles in contact with a variety of different solvents in order to obtain detailed information on their dependence on the nature of the liquid. The transition metal complex Fe(phen)2(CN)2 [cisdicyano-bis(1,10-phenanthroline)iron(II)] has been used as a surface polarity indicator for acidic surfaces because its solvatochromic band shift in the UV/vis spectrum is significantly dependent on the acidity of the environment.28a,32,33 The visible absorption band of 1 is attributed to the MLCT (metal to ligand charge transfer) transition from the d orbitals of the central FeII atom to the LUMO of the phenanthroline ligands.32,33 The position of the (26) Spange, S.; Reuter A.; Vilsmeier, E. Colloid Polym. Sci. 1996, 274, 59-69. (b) Spange, S.; Reuter, A. Langmuir 1999, 15, 141-150. (c) Spange, S.; Reuter, A.; Lubda, D. Langmuir 1999, 15, 2103-2111. (d) Spange, S.; Vilsmeier, E.; Fischer, K.; Prause, S.; Reuter, A. Macromol. Rapid. Commun. (Feature) 2000, 21, 643. (27) Adolph, S.; Spange, S.; Zimmermann, Y. J. Phys. Chem. B 2000, 104, 6429. (28) Spange, S.; Keutel, D. Justus Liebigs Ann. Chem. 1992, 423428. (b) Spange, S.; Keutel, D.; Simon, F. J. Chim. Phys. 1992, 89, 1615-1622. (c) Spange, S.; Vilsmeier, E.; Zimmermann, Y. J. Phys. Chem. B 2000, 104, 6417. (29) Krasnansky, R.; Thomas, J. K. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; American Chemical Society: Washington, DC, 1994; p 223 ff. (b) Baker, G. A.; Jordan, J. D.; Bright, F. V. J. Sol-Gel Sci. Technol. 1998, 11, 43-54. (30) Macquarrie, D. J.; Tavener, S. J.; Gray, G. W.; Heath, P. A.; Rafelt, J. S.; Saulzet, S. I.; Hardy, J. J. E.; Clark, J. H.; Sutra, P.; Brunel, D.; di Renzo, F.; Fajula, F. New J. Chem. 1999, 23, 725-731. (31) Moreno, E. M.; Levy, D. Chem. Mater. 2000, 12, 2334-2340. (32) Burgess, J. Spectrochim. Acta 1970, 26A, 1957-1962. (33) Soukup, R. W.; Schmid, W. J. Chem. Educ. 1985, 62, 459-462. (b) Migron, Y.; Marcus, Y. J. Phys. Org. Chem. 1991, 4, 310-314.

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Scheme 2. Interactions of Michler’s Ketone with Surface Sites on Silica Particles (a) Adsorption on Weakly Acidic Silanol Groups and (b) Adsorption on Strongly Acidic Silanol Groups

νmax(MLCT) absorption of 1 measured in various solvents correlates linearly with the AN (acceptor number) of the solvents according to Gutmann.33 Therefore, it is possible to compare the two different empirical concepts. Michler’s ketone is a solvatochromic compound which mainly responds to the dipolarity/polarizability of the environment,28a,34 because the coefficient ratio s/a in eq 1 is greater than one. When adsorbed on silica, alumina, or aluminosilicates, the UV/vis spectrum of Michler’s ketone shows quite different effects. Whereas on silicas a single UV/vis absorption maximum is observed, the dye shows second or third UV/vis absorption bands when adsorbed on alumina or aluminosilicates. The first UV/vis absorption maximum at about λmax(2) ≈ 370-400 nm results from interaction with weak Brønsted acid sites, like silanol groups. The second one at about λmax(2) ≈ 500 nm is attributed to an oxycarbenium ion of 2, which is formed by a selective complexation of the carbonyl oxygen with a proton or a Lewis acid35-37 (Scheme 2). Hydrophilically substituted derivatives of 2 are equally sensitive to R and π* but also slightly to β.38 Experimental Section Chemicals. The physical properties and sources of the silica batches used in this study are given in Table 1. For the investigation of the influence of solvents on the external and internal polarity of silica surfaces, we used three different silicas and a siliceous MCM-41 material (see Table 1). The choice of silica samples has been determined by their different morphologies: Aerosil 300 nanoparticles (d ≈ 10 nm) without pores and solids with different pore sizes and pore size distributions have been used. KG 60 and SG 432 silicas possess wide pores, and MCM-41 has narrow pores. The solvents used and their empirical polarity parameters R, β, and π* are summarized in Table 2. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFI) and 2,2,2-trifluoroethanol (TFE) were used without further purification. All other solvents as well as pyridine were dried and freshly distilled before use. (34) Groenen, E. J. J.; Koelman, W. N. J. Chem. Soc., Faraday Trans. 2 1979, 75, 85. (b) Suppan, P. J. Photochem. 1982, 18, 289. (35) Sekuur, T. J.; Kranenburg, P. Spectrochim. Acta 1973, 29A, 807. (36) Spange, S.; Vilsmeier, E.; Adolph, S.; Fa¨hrmann, A. J. Phys. Org. Chem. 1999, 12, 547. (37) Spange, S.; Zimmermann, Y.; Gra¨ser, A. Chem. Mater. 1999, 11, 3245. (38) Zimmermann, Y.; El-Sayed, M. M. I.; Prause, S.; Spange, S. Monatsh. Chem. 2001, 132, 1347-1361. (b) El-Sayed, M. M. I.; Mu¨ller, H.; Rheinwald, G.; Lang, H.; Spange, S. J. Phys. Org. Chem. 2001, 14, 247. (c) Spange, S.; El-Sayed, M. M. I.; Mu¨ller, H.; Rheinwald, G.; Lang, H. Eur. J. Org. Chem., submitted. (d) El-Sayed, M. M. I.; Mu¨ller, H.; Rheinwald, G.; Lang, H.; Spange, S. Chem. Mater., submitted.

Table 1. Physical Properties and Producers of the Siliceous Materials Used

solid acid

BET surface area (m2/g)

Aerosil 300 KG 60 SG 432 MCM-41

240 423 289 753

specific pore volume (cm3/g) 0.63 1.09 0.68a

pore diameter (nm)

producer

9.0 8.5 3.6

Degussa Merck Grace Mobil Oil

a Calculated by the following equation: r (m) ) [2V (cm3/g)/ p P ABET (m2/g)] × 10-6.

Table 2. Kamlet-Taft Polarity Parameters and ET(30) Values of the Solvents Used no.

solvent

R

β

π*

ET(30) (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

n-hexane cyclohexane tetrachloromethane diethyl ether toluene tetrahydrofuran chloroform anisole dichloromethane acetone 1,2-dichloroethane ethanol methanol 2,2,2-trifluoroethanol nitromethane 1,1,1,3,3,3-hexafluoro2-propanol acetonitrile

0 0 0 0 0 0 0.2 0 0.13 0.08 0 0.86 0.98 1.51 0.22 1.96

0 0 0.1 0.47 0.11 0.55 0.1 0.32 0.1 0.43 0.1 0.75 0.66 0 0.06 0

-0.04 0 0.28 0.27 0.54 0.58 0.58 0.73 0.82 0.71 0.81 0.54 0.6 0.73 0.85 0.65

31 30.9 32.4 34.5 33.9 37.4 39.1 37.1 40.7 42.2 41.3 51.9 55.4 59.8 46.3 65.3

0.19

0.40

0.75

45.6

17

Fe(phen)2(CN)2 was prepared according to Schilt.39 Michler’s ketone was purchased from Merck, recrystallized twice from ethanol, and dried over CaH2 before use. 2,6-Di-tert-butylpyridine was purchased from Merck in spectroscopic grade. UV/vis Measurements. The equipment employed was a UV/ vis spectrometer MCS 400 connected to an immersion cuevette TSM 5A (Zeiss). The measurements of the transparent slurry and of the sedimentation solution are possible in the same cell. The UV/vis absorption maxima of the dyes 1 and 2 adsorbed on the silica were recorded using a special flask which contains the immersion cuvette. The solids were heated at 400 °C for 12 h. After cooling to room temperature under dried argon, an amount was introduced into the measurement cell and immediately suspended in the solvent. Then, a solution of the probe dye in the same solvent was added to this slurry. (39) Schilt, A. A. J. Am. Chem. Soc. 1960, 82, 3000.


Langmuir, Vol. 18, No. 24, 2002 9581

The reproducibility of the UV/vis spectra of the adsorbed dyes is very good with ∆λmax < 1 nm. The position of the UV/vis absorption maximum remains constant during 1-2 h. Details of the UV/vis absorption measurements in the various solvents are given in the Results and Discussion. Determination of the Polarity Parameters. The following dual correlations have been used for the determination of the surface polarity parameters26,28 (r ) correlation coefficient; sd ) standard deviation; n ) number of solvents; F ) significance):

R ) -7.900 + 0.453νmax(1) × 10-3 + 0.021νmax(2) × 10-3 (2) r ) 0.95; sd ) 0.17; n ) 34; F ) 0.00 π* ) 13.889 - 0.251νmax(1) × 10-3 - 0.320νmax(2) × 10-3 (3) r ) 0.57; sd ) 0.15; n ) 36; F ) 0.00 The two equations have been calculated using the solvatochromism of 1 and 2 in 34 and 36 solvents, respectively, and the original Kamlet-Taft solvent parameters set,15 which serves as a reference system.28

Results and Discussion Solvatochromic Date. The indicator dye 1 is insoluble in weakly polar solvents such as n-hexane, cyclohexane, toluene, CCl4, or THF (nos. 1-6, 8, and 10 in Table 2).27a For the systems in these media, 1 was adsorbed on silica from dichloromethane solution and then filtered, washed, and dried. 1-loaded silica samples were suspended subsequently in the corresponding solvents.38,40 From moderately or strongly polar solvents (Nos. 6, 10, 12-14, 16, and 17 in Table 2), Michler’s ketone, 2, adsorbs only weakly on silica. Thus, in the resulting UV/vis spectrum of dye 2 measured in the slurry, interference from both the adsorbed and dissolved fractions occurs. Separate spectra of the two fractions were obtained by measuring the UV/vis spectra of the slurry and the supernatant solution after the particles’ sedimentation. A detailed UV/vis spectral series demonstrating this problem is shown for the adsorption of 2 on MCM-41. In the following we present the results and LSE correlation analyses for each silica sample separately. KG 60. The measured UV/vis absorption maxima of dyes 1 and 2 adsorbed on silica KG 60 are given in Table 3. The resulting surface polarity parameters R and π* of the silica/solvent interface have been calculated from eqs 2 and 3. The complete adsorption of 1 on this type of silica from several alcohols was surprising because other probe dyes such as 2 or Reichardt’s dye26c do not adsorb from these protic solvents on the other silica samples. The reason for this fact is still under study. The UV/vis absorption maximum of the surface polarity indicator 1 [νmax(1)] when adsorbed on KG 60 shows a significant bathochromic shift with increasing basicity of the solvent. This is seen in Figure 1 and eq 4.

νmax(1)KG 60 × 10-3 (cm-1) ) 18.81 - 1.21βsolvent r ) 0.68; sd ) 0.34; n ) 17; F ) 0.003

(4)

Despite the poor correlation coefficient of eq 4, the significance emphasizes the relevance of the correlation. Four solvents show anomalous behavior: ethanol (12), methanol (13), anisole (8), and acetonitrile (17). If these points are excluded, then a significantly improved LSE (40) Spange, S.; Simon, F.; Heublein, G.; Jacobasch, H. J.; Bo¨rner, M. Colloid Polym. Sci. 1991, 269, 173-178.

Table 3. UV/vis Absorption Maxima of Fe(phen)2(CN)2 [νmax(1)] and Michler’s Ketone [νmax(2)] When Adsorbed on Silica KG 60 in Solvents at T ) 293 K as Well as the Polarity Parameters rsurf and π*surf of the Resulting Silica/Solvent Interface Calculated from Eqs 2 and 3a solvent no.

νmax(1) (10-3 cm-1)

νmax(2) (10-3 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

18.9b 18.5b 18.8b 17.7b 18.7b 17.6b 18.5 19.0b 18.6 17.9b 18.7 18.2 18.4 19.2 18.6 18.9 18.7

26.4 26.3 26.5 26.0 25.5 c 25.6 25.3 25.4 c 25.3 c c c 28.3 c c

Rsurf

π*surf

1.23 1.04 1.16 0.66 1.12

0.69 0.82 0.69 1.11 1.03

1.03 1.23 1.04

1.06 1.05 1.09

1.10

1.10

1.10

0.17

a The solvent numbers correspond to Table 2. b Preadsorption of Fe(phen)2(CN)2 from dichloromethane has been used. c No adsorption from this solvent; a separation of νmax of the adsorbed dye from that of the overall dye fraction is not possible.

Figure 1. UV/vis absorption maxima of Fe(phen)2(CN)2 (1) when adsorbed on KG 60 as a function of the β value of the solvent at T ) 293 K (solvent numbers correspond to Table 2). The line corresponds to eq 5.

relationship has been obtained (eq 5). This equation will be considered for the summary discussion.

νmax(1)KG 60 × 10-3 (cm-1) ) 18.88 - 2.43 βsolvent r ) 0.95; sd ) 0.16; n ) 13; F < 0.0001

(5)

Correlation analysis of the calculated acidity parameter R of the KG 60/solvent interface (Table 3) with the basicity (β value) of the solvent evidently supports the above interpretation (eq 6).

RKG 60 ) 1.17 - 1.03βsolvent r ) 0.91; sd ) 0.08; n ) 8; F ) 0.002

(6)

The results of eqs 5 and 6 agree well with observations 1, 2, and 3 in the Introduction. Figure 2 and eq 7 indicate a significant bathochromic shift of the UV/vis absorption maximum of Michler’s ketone when adsorbed on silica

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Zimmermann et al. Table 4. UV/vis Absorption Maxima of Fe(phen)2(CN)2 [νmax(1)] and Michler’s Ketone [νmax(2)] When Adsorbed on Silica Aerosil 300 in Solvents at T ) 293 K as Well as the Polarity Parameters rsurf and π*surf of the Resulting Silica/Solvent Interface Calculated from Eqs 2 and 3a

Figure 2. UV/vis absorption maxima of Michler’s ketone (2) measured in different pure solvents (from ref 28a) and when adsorbed on silica KG 60 in the same solvent as a function of the π* value of the solvent at T ) 293 K (solvent numbers correspond to Table 2).

KG 60 with increasing dipolarity/polarizability of the solvent.

solvent no.

νmax(1) (10-3 cm-1)

νmax(2) (10-3 cm-1)

1 2 3 4 5 6 7 8 9 10 11 15 17

19.1b 18.9b 19.6b 17.5b 18.8b 18.0b 18.3 19.2b 18.4 18.7b 18.8 19.0 18.6

26.1 26.2 26.3 c 25.6 c 25.6 c 25.4 28.7 25.5 26.7 28.3

Rsurf

π*surf

1.31 1.20 1.52

0.73 0.78 0.56

1.14

0.97

0.95

1.10

0.98 1.18 1.15 1.11 1.26

1.14 0.02 1.01 0.16 0.59


νmax(2)KG 60 × 10-3 (cm-1) ) 26.43 - 1.37π*solvent (7) r ) 0.91; sd ) 0.22; n ) 9; F ) 0.0006 This result is quite different from that with probe 1. The polar surface of silica seems to become much less solvated and polarized by nonpolar than by more polar solvents. The poor adsorption of 2 on silica from more polar solvents can be due to the competing interactions of solvents with the silica surface sites. A multiple regression analysis of the combined dependence of νmax(2) on β and π* gives a similarly good correlation equation, but the significance decreases (F ) 0.005). Altogether, the UV/vis absorption maximum of 2 undergoes a bathochromic shift with increasing dipolarity/ polarizability π* of the solvents27a,31a (Figure 2). Two straight lines result, one for pure solvents and another for the adsorbed probe dye on silica. The effect of solvent polarity on the solvatochromic UV/vis shift of 2 when adsorbed on a silica surface shows the same trend as that observed in the absence of the adsorbing silica surface. The more polar the solvent, the smaller is the difference between νmax(2)silica/solvent and νmax(2)solvent. This shows that the effect of dipolarity/polarizability of the solvent upon νmax(2) is less pronounced when 2 is adsorbed on a polar surface. This is indicated by the slope νmax(2) versus π*, which is s ) -2.11 for 2 in the pure solvents and s ) -1.37 when 2 is adsorbed on KG 60 (Figure 2). It is likely that the dipolar sites of the silica surface compete with the polar solvent molecules. Aerosil 300. Table 4 summarizes the measured UV/vis absorption maxima of the dyes 1 and 2 adsorbed on Aerosil 300 and the calculated surface polarity parameters R and π* of the Aerosil/solvent interface. As for KG 60, increasing the HBA ability of the solvent decreases the HBD ability of the silanol groups on the surface of Aerosil 300 (eq 8). A poor correlation is obtained for the solvents studied.

νmax(1)Aerosil 300 × 10

-3

-1

(cm ) ) 19.02 - 1.65βsolvent (8) r ) 0.58; sd ) 0.46; n ) 13; F ) 0.04

Parallel straight lines for two solvent groups are observed showing the measured νmax(1) values of 1 when adsorbed on Aerosil 300 as a function of the basicity of the

Figure 3. UV/vis absorption maxima of Fe(phen)2(CN)2 (1) when adsorbed on Aerosil 300 as a function of the β value of the solvent used at T ) 293 K (solvent numbers correspond to Table 2).

solvent (Figure 3). Equation 9 accounts for νmax(1) ) f(β) for solvents 1, 2, 4, 5, 7, 9, 11, and 15, and eq 10, for solvents 3, 6, 8, 10, and 17. Equation 9 will be considered for the summary discussion.

νmax(1)Aerosil 300 × 10-3 (cm-1) ) 18.99 - 3.31βsolvent (9) r ) 0.94; sd ) 0.19; n ) 8; F ) 0.0005 νmax(1)Aerosil 300 × 10-3 (cm-1) ) 20.06 - 3.48βsolvent (10) r ) 0.95; sd ) 0.23; n ) 5; F ) 0.01 For Michler’s ketone when adsorbed on Aerosil 300, a similar influence (to that for adsorption on KG 60) of the π* value of the solvent on the UV/vis absorption maximum is observed (eq 11).

νmax(2)Aerosil 300 × 10-3 (cm-1) ) 26.21 - 0.93π*solvent (11) r ) 0.89; sd ) 0.19; n ) 7; F ) 0.007 SG 432. The physical properties and morphology of SG 432 are similar to those of KG 60. The UV/vis absorption maxima of 1 and 2 when adsorbed on SG 432 and the calculated surface polarity parameters of the SG 432/


Langmuir, Vol. 18, No. 24, 2002 9583

Table 5. UV/vis Absorption Maxima of Fe(phen)2(CN)2 [νmax(1)] and Michler’s Ketone [νmax(2)] When Adsorbed on Silica SG 432 in Solvents at T ) 293 K as Well as the Polarity Parameters rsurf and π*surf of the Resulting Silica/Solvent Interface Calculated from Eqs 2 and 3a solvent no.

νmax(1) (10-3 cm-1)

νmax(2) (10-3 cm-1)

1 2 3 4 5 6 7 8 9 10 11 15 17

18.1b 18.1b 18.2b 17.6b 18.2b 17.7b 18.4 18.6b 18.3 18.1b 18.5 18.3 18.9

26.6 26.6 27.0 26.5 25.7 c 26.3 24.8 26.0 c 25.5 25.8 c

Rsurf

π*surf

0.84 0.85 0.93 0.65 0.88

0.85 0.84 0.68 0.97 1.10

0.98 1.02 0.93

0.85 1.31 0.97

0.99 0.95

1.09 1.01


solvent interface are summarized in Table 5. The transmission UV/vis absorption spectra of 1 when adsorbed on SG 432 are distorted by a scattering effect, which varies depending on the β value of the solvent. In this case the UV/vis absorption maximum is less resolved and a broad UV/vis band results. As expected, the influence of the HBA property and the dipolarity/polarizability, respectively, of the solvent on the UV/vis absorption maxima of 1 and 2 when adsorbed on SG 432 (eqs 2 and 13) is similar to that on KG 60.

νmax(1)SG 432 × 10-3 (cm-1) ) 18.39 - 1.11βsolvent (12) r ) 0.65; sd ) 0.27; n ) 11; F ) 0.03 νmax(2)SG 432 × 10-3 (cm-1) ) 26.77 - 1.44π*solvent (13) r ) 0.72; sd ) 0.48; n ) 10; F ) 0.02 The related acid-base interactions between the silica surface and the solvents are indicated by eq 12. Altogether, it seems that interactions between this specific silica surface and solvent molecules occur in a very complex manner. The manifold solvent influence is indicated by the multiple correlation analysis of νmax(1) with R, β, and π* of the solvents used (Figure 4 and eq 14). An excellent significance as well as a mediocre correlation coefficient results for the set of 12 solvents. That means that all the solvent parameters have a significant role in the probe-surface-solvent interaction.

νmax(1)SG 432 × 10-3 (cm-1) ) 18.07 + 1.44Rsolvent 1.12βsolvent + 0.53π*solvent (14) r ) 0.96; sd ) 0.10; n ) 12; F < 0.0001 A deeper mechanistic interpretation of eq 14 should be handled with caution, because of the mediocre correlation. MCM-41. The measured UV/vis absorption maxima of 1 and 2 when adsorbed on MCM-41 and the calculated polarity parameters of the silica/solvent interface in the MCM-41 channels are compiled in Table 6. In contrast to the case of other silicas studied in this paper, the UV/vis absorption maximum of 1 when adsorbed on MCM-41 does not show a significant bathochromic shift with increasing β value of the solvent. We attribute this

Figure 4. Measured versus calculated values (eq 15) of the UV/vis absorption maximum of Fe(phen)2(CN)2 adsorbed on silica SG 432 from different solvents as a function of their Kamlet-Taft polarity parameters at T ) 293 K (solvent numbers correspond to Table 2). Table 6. UV/vis Absorption Maxima of Fe(phen)2(CN)2 [νmax(1)] and Michler’s Ketone [νmax(2)] When Adsorbed within MCM-41 Mesopores in Solvents at T ) 293 K as Well as the Polarity Parameters rsurf and π*surf of the Resulting Silica/Solvent Interface Calculated from Eqs 2 and 3a solvent no.

νmax(1) (10-3 cm-1)

1 2 3 4 5 6 7 8 9 10 11 15 17

18.8b 18.9b 18.7b 18.1b 19.2b 18.9b 18.5 19.5b 18.5 19.9b 18.4 18.7 18.9

νmax(2) (10-3 cm-1) 26.2 26.2 26.7

Rsurf R1

d d 21.1

1.17 1.20 1.11

20.8

R2

π*surf π*1 π*2

0.99

0.81 0.78 0.67

2.47

1.33

1.23

0.91

2.40

20.9

1.03

0.91

0.78

2.57

20.6

1.02

0.92

1.20

2.66

20.7 d d

0.97 1.16 1.24

0.88

1.17 (0.13) 0.56

2.65

c 25.5 c 26.5 c 25.1 c 25.3 (28.3)e 26.8

a The solvent numbers correspond to Table 2. b Preadsorption of Fe(phen)2(CN)2 from dichloromethane has been used. c No adsorption from this solvent; a separation of νmax of the adsorbed dye from that of the overall dye fraction is not possible. d Not observed. e Light scattering effects take place.

result to the distinctive pore system of the material. It is likely that the solvent molecules and probes in the nanopore system have different properties than those in large pores or on flat surfaces (cavity effect). In other words, the moderate HBA capacity of the solvent does not influence the νmax(1)MCM-41 when adsorbed within narrow pores. It can also be supposed that the influence of the HBA property of the solvent is compensated by the large dipolarity/polarizability of the solvent molecules and high mobility of both probes and solvent molecules in the confined space, as shown by dielectric broad band spectroscopy and solid state 13C NMR spectroscopy of adsorbed solvents.41,42 Due to the higher mobility of solvent molecules in narrow mesopore channels than on freely accessible surfaces, specific acid-base interactions are suppressed. Whereas only a single UV/vis absorption maximum of 2 is observed when adsorbed on the other three silica (41) Kremer, F.; Huwe, A.; Arndt, M.; Behrens, P.; Schwieger, W. J. Phys.: Condens. Matter. 1999, 11A, 175. (42) Stallmach, F.; Gra¨ser, A.; Ka¨rger, J.; Krause, C.; Jeschke, M.; Oberhagemann, U.; Spange, S. Microporous Mesoporous Mater. 2001, 44-45, 745-753.

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Figure 6. UV/vis absorption maxima of Michler’s ketone (2) measured when adsorbed on KG 60 (eq 7) within MCM-41 (eq 15) and of 2+ within MCM-41 (eq 16) as a function of the π* value of the solvent at T ) 293 K. Scheme 3. Suggested Model for the Adsorption of a Dipolar Probe in Solvent Filled Mesopores of MCM-41

Figure 5. UV/vis absorption spectra of 2 when adsorbed on MCM-41. (a) 0.1020 g of MCM-41, 15 mL of dichloromethane, and 1 × 10-6 mol of 2; λmax(2) ) 365, 398, and 486 nm; (1) without DTBP; (2) 5 µL of DTBP; (3) 100 µL of DTBP. (b) 0.1001 g of MCM-41, 15 mL of dichloromethane, and 1 × 10-6 mol of 2; λmax(2) ) 355, 398, and 486 nm; (1) without pyridine; (2) 5 µL of pyridine; (3) 10 µL of pyridine; (4) 50 µL of pyridine.

surfaces at λmax(2) ) 395 ( 4 nm, it shows two UV/vis absorption maxima when adsorbed on siliceous MCM-41 in dichloromethane,37 one at λmax(2)1 ) 398 nm and a second one at λmax(2)2 ) 486 nm. In ref 37, we could show that the latter UV/vis band is caused by the oxycarbenium ion of 2 (see Scheme 2), because this UV/vis band disappears if 2,6-di-tert-butylpyridine (DTBP) is coadsorbed (Figure 5a). It also indicates that this UV/vis absorption band of 2 is caused by mobile protons in the channels of MCM-41 and not by Lewis acid sites, because DTBP cannot interact with surface Lewis acid sites.28c When the strong HBA solvent pyridine (β ) 0.64)16b is coadsorbed, probe 2 desorbs from the surface, as seen from Figure 5b. With increasing pyridine concentration the UV/vis absorption of 2 in pure solvent with λmax ) 355 nm appears in the supernatant solvent phase. UV/vis spectra 5b/2 and 5b/3 show the interference of adsorbed 2 and dissolved 2 as a function of pyridine concentration, but an UV/vis shift of the adsorbed fraction is not detectable. DTBP is not suitable to displace completely the probe 2 from the surface. The presence of acidic mobile protons within MCM-41 channels is supported by the fact that cationic polymerizations of vinyl ethers can be initiated in the pores of bare MCM-41.37,43b,c Two UV/vis absorption bands [λmax(2)1 ≈ 380 nm and max(2)2 ≈ 490 nm] are observed when 2 is adsorbed from those solvents showing high (43) Spange, S.; Gra¨ser, A.; Rehak, P.; Ja¨ger, C.; Schulz, M. Macromol. Rapid. Commun. 2000, 21, 146. (b) Spange, S.; Gra¨ser, A.; Mu¨ller, H.; Zimmermann, Y.; Rehak, P.; Ja¨ger, C.; Fuess, H.; Baehtz, C. Chem. Mater. 2001, 13, 3698. (c) Spange, S.; Gra¨ser, A.; Huwe, A.; Kremer, F.; Tintemann, C.; Behrens, P. Chem. Eur. J. 2001, 7, 3722-3728.

dipolarity/polarizability and negligible HBA property. The UV/vis absorption band of the oxycarbenium of 2 is not observed using solvents with π* ≈ 0 (cyclohexane and n-hexane). Also, in weak HBA solvents such as acetonitrile or nitromethane, this UV/vis band is not detectable. We suppose that weak HBA solvents suppress the oxycarbenium formation because they trap the highly acidic mobile protons. The resulting νmax(2)1 and νmax(2)2 values are compiled in Table 6. Generally, UV/vis absorption maxima of 2 when adsorbed within MCM-41 shift bathochromically with increasing π* value of the solvent, as shown in Figure 6 and eqs 15 and 16.

νmax(2)1,MCM-41 × 10-3 (cm-1) ) 27.41 - 2.63π*solvent (15) r ) 0.84; sd ) 0.44; n ) 5; F ) 0.07 νmax(2)2,MCM-41 × 10-3 (cm-1) ) 21.30 - 0.82π*solvent (16) r ) 0.98; sd ) 0.05; n ) 5; F ) 0.005 With increasing π* value of the solvent, the intensity of the second UV/vis absorption band of 2 at about λmax(2)2


Langmuir, Vol. 18, No. 24, 2002 9585

Scheme 4. Competitive Interaction of Fe(phen)2(CN)2 with Silica Particle Surface and Solvent

) 490 nm increases. The intensity ratio of the two UV/vis maxima [νmax(2)2/νmax(2)1] decreases with increasing solvent dipolarity/polarizability. The bathochromic shift of the UV/vis band of the oxycarbenium ion of 2, denoted as 2+, with increasing the dipolarity/polarizability of the solvent is attributed to a stronger dipole-dipole interaction in the excited state of 2+ than in the ground state. Similar influences of the polarity of a cavity on νmax of the carbenium ions and anthracene in zeolites have been discussed by Garcia et al.44 However, the effect of solvent polarity on νmax(2+) is weak. Probes 1 and 2 in pores of MCM-41 probably are in rapid equilibrium between the adsorbed and dissolved forms, resulting in an average polarity. In Scheme 3 the environment of the probe molecules in the pores of MCM41 is represented. Summary Discussion The results of LSE relationships of νmax ) f(β) for external surfaces are reasonable, because acid-base interactions of the silanol groups with the solvent play an important role.9,11,38,40 A nucleophilic attack of the lone electron pair of the solvent upon silanol groups decreases the hydrogenbond donating capacity of the silica surface and thus the interaction strength with the probe molecule. As a consequence, a bathochromic shift of the UV/vis absorption maximum of 1 takes place. The influence of the solvent on the surface polarity of a silica seems to be of a complex nature and cannot be explained by a single solvent interaction. The multiple interactions of Fe(phen)2(CN)2 (1) with silanol groups and solvent molecules are demonstrated in Scheme 4. It is possible that the anomalies in Figure 1 are caused by such interfering effects which are difficult to quantify, since a higher order term is operative. Generally, π*surf is more strongly affected by external solvents than Rsurf, which is explained by the competing influence of the dipolar solvent molecule with the dipolar silica surface, because the solvents suitable for this study differ more in π* [from -0.04 (no. 1) to 0.85 (no. 15)] than in β [from 0.00 (no. 1) to 0.55 (no. 6)]. Probe dyes 1 and 2 do not adsorb on silica from solvents with β > 0.5. However, some dipolar solvents, for example, nitromethane, acetonitrile, alcohols, and anisole, show (44) Cano, M. L.; Corma, A.; Fornes, V.; Garcia, M. A.; Miranda, C.; Baerlocher, C.; Lengauer, C. J. Am. Chem. Soc. 1996, 118, 1100611113. (b) Ma´rquez, F.; Garcia, H.; Palomares, E.; Ferna`ndez, L.; Corma, A. J. Am. Chem. Soc. 2000, 122, 6520-6521.

Table 7. Comparison of the Reasonable Slope Values Derived from νmax(1)silica ) f(β) and νmax(2)silica ) f(π*) as a Function of Solvent Influence for the Silicas Studied νmax(1) ) f(β)

νmax(2) ) f(π*)

silica

slope

n

r

slope

n

r

Aerosil 300 KG 60 SG 432 MCM-41

-3.31 -2.43 -1.11 0.44a

8 13 11 13

0.94 0.95 0.65 0.18a

-0.93 -1.37 -1.44 -2.63

7 9 10 5

0.89 0.91 0.72 0.84

a The correlation is poor, but the slope indicates the negligible influence of the solvent.

anomalous behavior on the solvatochromic UV/vis band shift of the adsorbed indicators, which would require a deeper study to explain the effects in detail. The UV/vis absorption bands of the adsorbed probes on porous adsorbents are overall values consisting of two main components, that of the probe on the external and on the internal surface. Because KG 60 and SG 432 have nanopores and mesopores, depending on the nature and size of both solvent and probe, the localization of the probe can change from internal to external surface sites. A quantitative separation is not possible by the method employed. Owing to the known morphology of the silicas used, a qualitative ordering is justified. The portion of the probe adsorbed on the internal (pore) surfaces increases in the following order for the silicas used: Aerosil 300 (only external surface) < KG 60 ≈ SG 432 < MCM-41 (internal surface is mainly observed). Table 7 gives a comparison of the calculated slope values from the reasonable LSE relationships of νmax(1) ) f(β) and νmax(2) ) f(π*) functions for the silicas studied. A comparison of the slope values and the sensitivity of these parameters of silica samples to the solvent property leads to the following conclusion. Aerosil 300 nanoparticles and MCM-41 are the two materials which differ dramatically in morphology. Owing to the properties of these materials, the solvent influence upon the UV/vis shift of an adsorbed solvatochromic probe occurs differently. Considering all solvents studied, the following conclusions can be drawn: (a) The R value of silica is most strongly affected by HBA solvents on the Aerosil 300 surface (smallest particle size and absence of pores). (b) The π* value of silicas is most strongly affected by the solvent on the MCM-41 surface. The probe is in continuous contact (rapid adsorption/desorption) within a narrow pore.

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(c) Internal pore surfaces are sensitive to the dipolarity/ polarizability of the solvent and relatively insensitive to acid-base interactions. (d) External surfaces are sensitive to the acid-base properties of the solvent. (e) Since the morphology of silica is nonuniform, then poor single LSE correlations result. Conclusion The interaction of a solvatochromic probe with a surface environment as a function of solvent polarity can be investigated by measuring the shift in the UV/vis absorption band of the solvatochromic probe when adsorbed on silica in transparent slurries. Thus, the solvatochromic probe is used as an interfacial polarity indicator. Most solvents fit well in LSE relationships, which allows an

Zimmermann et al.

interpretation of the results relating to acid-base and dipol-dipole interactions as a function of solvatochromic probe. Acid-base interactions dominate between the probe and the external silica surface, as shown by the correlation analyses of νmax(probe) with the Kamlet-Taft polarity parameters of the solvent. Dipole-dipole interactions dominate in mesopores of MCM-41, because confinement effects and rapidly occurring acid-base interactions cause an average dipolarity of the MCM-41/solvent interface. Acknowledgment. Financial support for this research by the Chemnitz University of Technology, in part by the DFG, and the Fonds der Chemischen Industrie is gratefully acknowledged. LA020574C

Influence of Chemical Solvent Properties on the External and Internal

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