3008
Langmuir 1992,8, 3008-3013
Spectroscopic Determination of Refractive Index and Dielectric Constant at Interfaces, Using Photophysical Probe Moleculest Robert J. Kavanagh, Kai-Kong Iu, and J. Kerry Thomas' Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received April 30, 1992. In Final Form: July 6, 1992 In this study a UV-vis spectroscopictechnique is presented for the determinationof the refractive index and dielectric constant in restricted regions. Both the classical Bayliss model and a modified Bekarek model were employed to calculate the refractive index and dielectric constant of the media (e.g. polymer, silicagel, and zeolite). The studiesshow that the refractiveindex (n)of these media are 1.58 (polystyrene), -1.33 (silica gel), and -1.45 (zeolite XI,which are identical to the values reported in the literature. Increasingpretreatment temperatureof silicagel is shown to decrease the refractive index, which indicates that the probe molecule is exposed to a less polar or dehydrated environment. The refractive index in a zeolite is shown to increase in the presence of coadsorbed water. On colloidal clay surfaces, increasing the concentrationof a long chain alkylpyridiniumsurfactant around the adsorbed probe molecule is shown to decrease the refractive index in agreement with the picture that the environment of the probe becomes more hydrophobic as an aliphatic surfactant layer builds up on the surface. The data agree with earlier luminescenceprobing studiesin this system. The studiespresented amplifyand extend earlier luminescence probingstudies in heterogenousmedia, and present a method of measuringadditionalphysical parameters to describe complex systems.
-
Introduction Over the past 20 years, there has been a considerable interest in the photochemistry at surfacesor in aggregated assemblies.' The main reason for such interest is the unusual properties of the interfaces or assemblieswhereby certain features of photochemical reactions are emphasized. For example, in micellar systems the local high electric field at the micelle water interface is claimed to promote electron transfer reactions in preference to other free radical processes. At solid interfaces, such as alumina or clay interfaces, the Lewis acid sites, that may be generated on these surfaces, give rise to photoinduced electron transfer reactions at energies which are much lower than those which occur in the gas phase.2 In every case, a situation arises where some unique feature of the interface or assembly promotes or enhances the reaction of favor. The unusual circumstance of the interface or aggregate excludes normal and simple eventa that occur in homogeneous solution. Hence, techniques called molecular probing have been designed to effectively assess localmicroviscosity,localrigidity, static dielectricconstant, and reactant accessibility at these sites.3 Quite often the reaction of favor is photoinduced electron transfer. The polarization of the site, where the reaction occurs, is of prime importance in describingthe energetics that operate in this photochemical process. To a good
* Author to whom correspondence should be addressed.
+ This paper is submitted to the Perrin issue of Langmuir. It is
in the vein of much of Perrin's work, i.e., physical investigation of colloidal systems. Indeed, Perrin's early work stimulated and provided a sound bash for much of the chemistry at interfaces of colloids that is now so popular. (1) (a)Fendler,J. H. MembraneMimetic Chemistry;Wiley: NewYork, 1981. (b)Thomas,J. K. The Chemistry of Emitation at Interfaces;ACS Monograph 181; American Chemical Society: Washington, DC, 1984. (c) Kalyanasundaram,K. Photochemistry in Microheterogenous Systems; Academic Preas: New York, 1987. (2) (a) Iu,K.-K.;Thomas, J. K. J. Phys. Chem. 1991,95,506-509. (b) Liu, X.;Thomas, J. K. Langmuir 1991, 7, 2808-2816. (3) (a) Chu, D. Y.; Thomas, J. K. Photochemistry and Photophysics; Rabek, Jan F., Ed.;CRC Press: Boca Ratan, FL, 1991; p 409-102. (b) Thomas, J. K. J. Phys. Chem. 1987,91, 267-276.
degree of approximation the polarization P+ can be calculated by the simple equation given below, using the high frequency dielectric constant t, and the radius r of the species 2
P+ = "[ 12r The high-frequency dielectric constant e, in turn, is connected to the square of the refractive index n,of the site where the reaction occurs. The studies presented here describe a method of accessing the refractive index and the dielectric constant of the reaction site at the interface or in the assembly. The refractive index is measured by means of a probe molecule, which also might be a typical reaction partner. The spectroscopic properties of the probe are used to give the refractive index of the host site. The ionization potential of the probe molecule at the site is related to P+,energy of the e- in the medium VO,and the gas-phase ionization potential Ig by
I = Ig + P+ + v,
(2)
Here P+ derived from t and the present studies is a major parameter controlling the decrease in energy required to promote e- transfer processes. In order to present a convincing argument, studies are first presented in simple homogeneous liquids where prior studies have shown that the relationships, which will be mentioned later, hold true. Then more complexity is introduced, e.g. a micellar system, a polymer film system, and eventually solid surfaces under various conditions. The crux of the technique is to measure, with a great deal of accuracy, the absorption spectrum of the probe in the particular system of interest. Over the last 50 years there has been considerable interest"8 in interpreting, by models of various design, (4) Baylias, N. S.J. Chem. Phys. 1950,18,292-296. (5) Bekarek,V.;Bekarek,V.,Jr. ACTA. UPOL.Fac. Rer. Nat.,Chemica XXVII 1988, 91, 91-103. (6) Bekarek, V.; Bekarek, S.;Pavlat, F. Chem. Pap. lS88,42 (2), 197203, and references cited therein.
0743-746319212408-3008$03.00/0 0 1992 American Chemical Society
Molecular Robing at Interfaces
the red shift of visible and ultraviolet electronic absorption spectra in solution compared to spectra in the gas phase. In most models, the spectral shift is related to some function of refractive index alone or with some function of dielectric constant of the solvent. The shift to longer wavelengths was initially predicted to be due to transient polarization of the solvent molecules induced by the transition dipole of the solute. However, this prediction proved to be too general and other interactions may dominate causing increased red shift or even a blue shift. Later improved theories included formulations that take into accountadditional interactions such as dipole-dipole, dipole polarization, and hydrogen bonding. Experimental Section Chemicals. Sodium exchanged zeolite X (Si/Al = 1.41, silica gel (99+% , Davisil 60 A), and polystyrene (MW 280 OOO) were obtained from Aldrich, and laponite waa obtained from Laporte industries; all were used aa received. Sodium dodecyl sulfate (SDS) (99%)was obtained from Sigma. Cetyltrimethylammonium bromide (CTAB) waa obtained from BDH. Dodecylpyridinium chloride (Py+Cl-)waa obtained from Kodak chemicals. Solventa perfluorohexane, pentane, heptane, decane,cyclohexane, benzene, ethylbenzene, and toluene were all 99+ % ,anhydrous, spectrophotometric grade received from Aldrich; solvents ethylbenzene and 1,6dioxane were Fischer A.C.S. certified. The solvents were further purified and dried by using molecular sieve 3A pellets. Pyrene, fluoranthene (Aldrich product 99 % ) were purified by liquid chromatography (silicagel and cyclohexaneaa adsorbent and eluant respectively), followed by recrystallization from cyclohexane solution. Pyrenebutyric acid (PBA) (Aldrich product) was recrystallized from methanol. Anthracene (Aldrich gold label) and perylene (K&K Laboratories, Inc.) were used aa received. 44 1-Pyreny1)butyltrimethyla"onium bromide (PN+Br-) was obtained from Molecular Probes and used aa received. Sample Preparation. Zeolites. A stock pentane solution of probe (i.e. pyrene, anthracene, perylene, fluoranthene) was prepared to give a typical concentration of 2 X lO-' mol/mL. The zeolite was dehydrated at 400-500 OC overnight. The dehydrated zeolite waa first mixed with pure pentane; probe-pentanesolution waa added into the zeolite-pentane suspension while stirring. The completion of probe molecule adsorption was checked by scanning a W-vis spectrum of the supernatant in the absorption range of the probe. Pentane waa then removed under vacuum. The loading of the probe molecule into the zeolite was about 1o-B moVg. Silica. Probe loading for the Davisil60 A waa carried out in a similar fashion aa with the zeolite, except the dehydration temperature was 100-130 OC, and probes (1X lO-' mol/g) were loaded from cyclohexane. With the PBA study, samples were dehydrated for 5 h at 130 and 500 OC, and PBA was loaded from benzene. Polystyrene Films. Polystyrene was dissolved in toluene and solutions of probe in toluene of known concentration (typically 1.25 X 10-smoVg) were added; this mixture was allowed to stir overnight to ensure that a homogeneous mixture waa obtained. Films were cast by pouring the polymer mixture into glass plates; they were then dried in air (dust free environment) and finally dried in a vacuum oven. Spectra of the f i e were taken after which further solvent removal was attempted, this cycle being repeated until the spectrum recorded exhibited no change. Micelle Systems, Sodium Dodecyl Sulfate, and Cetyltrimethylammonium Bromide. Probes at levela of 2 X lO-' SDS/CTAB solution were solubilizedin freshly prepared aqueous (7) Bekarek, V.;Bekarek, S.;Pavlat, F. 2 . Phys. Chem. (Leipzig)1988, 269,6, S,1147-1152. (8) Brady, J. E.; Can,P. W. J. Phys. Chem. 1985,89, 5759-5766. (9) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977,99, 2039-2044. (10) (a) Liu, X.;Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1989, 93, 4120-4128. (b) Iu, K.-K.; Thomas, J. K. Langmuir 1990,6,471-478. (c) Iu, K.-K.; Liu, X.; Thomas, J. K. Mater. Res. SOC.Symp. Proc. 1991,233, 119-132.
Langmuir, Vol. 8, No.12, 1992 3009 (laboratory distilled water) sodium dodecyl sulfate (0.1 M) and cetyltrimethylammonium bromide (0.01MI. The critical micelle concentrations for these surfactants are 8 X M SDS and 9.2 X 1W M1. CTAB. Zeolite with Coadsorbed Water. Zeolite samples were prepared as above with probe adsorption from the liquid phase. For zeolite-water samples, a known amount of water was added tothe stirred zeolite-probe mixture, and the sampleswere allowed to further equilibrate (supernatants being checked to ensure no probe removal). After the pentane solvent was removed under vacuum, the sample cell with the partially hydrated sample was sealed under vacuum and heated to 100 O C to allow water and probe molecules to distribute homogeneously throughout the sample. Laponite (Colloidal Clay)/DodecylpyridiniumChloride. Colloidal clay samples (5 g/L) were prepared in deionized water. Surfactant (Py+)waa added to give a final concentration (0.020.3 mM). Concentration of probe (PN+)waa 1.6 X 1Oa M in all samples. Instrumentation. Absorption and reflectance spectra were measured on a Varian Cary 3 double beam double dispersion UV-vis spectrophotometer which is centrally controlled by a Zenith 286-16 computer, with wavelength accuracy f 2 nm, wavelength reproducibility f0.04 nm, and slit bandwidth 1nm. Spectra of solid samples were measured with a diffuse reflectance accessory attached to the above instrument. The accessory consists of a 73 mm diameter integrating sphere which features a red-sensitive photomultiplier (Hamamatsu R928).
-
Results and Discussion The classical model used was that developed by Bayliss which relates the frequency shift, i.e., absorption of the sample compared to the gas phase, to the refractive index of the medium surrounding the probe molecule, eq 3.l'
The frequency shiftcompared to the gas phase Au is related also to the oscillator strength of the electronic transition f and the radius of the cavity a occupied by the solute. In the original work by Bayliss4the solvent was assumed to be a continuous dielectric medium; the effective dielectric constant was identified as n2 (nbeing the refractive index at the frequency concerned). For 38 years after Bayliss' original work, the model was tested and applied to many systems, and other modified versions were proposed. In 1985 Brady and Car# examined a number of models and concluded that no model gave a satisfactory fit. In 1987,a series of papers" were published in this area, i.e. evaluating the medium effect on the spectral absorption maxima. After lengthy discussion it was concluded that the Born expressions for the dielectric constant and the refractive index described the spectral shifta better than other models such as the Onaager: Mosotti? Bayliss-M~Rae-Ooshika~ (BMO),and Bilot-Kaw~ki-Bakhshiev~ (BKB)models. The distinction of the Born model is that it does not consider the medium effect on electronic spectra as a result of the sum of all the solute-solvent interactions but considers the work done (11) Becker, R. S. Theory and Interpretation of Fluorescence and Phosphoresence; Wiley: New York, 1969; Chapter 4. (12) Bandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wdey New York, 1989. (13) Krasnanzky,R.;Thomas,J.K.J.Photochem.Photobio2.A:Chem. 1991,57, 81-96, and references cited therein. (14) CRC Handbook of Chemistry and Physics, 72nd ed.;CRC: Boca Raton, FL, 1991-1992. (15) (a) Zeolites for Indurrtry. Chem. Znd. 1984, No. 7, April 2,237260. (b) Breck, D.W. Zeolite Molecular Sieues; Wiley: New York, 1974. (16) Okumura, Oeamu; et al. Clay Sci. 1987,27, 14-20. (17) Lebrun, A.;Liebaert, R.; Fontaine, J.; Risbourg, A., C. R. Hebt Acad. Sci. 1963, 266 (25), 5334-6.
3010 Langmuir, Vol. 8, No.12,1992
Kavanagh et al.
Table I. Absorption Maxima Y (em-') for Probes in a Set of Standard Solvents v. em-' DyTene anthracene fluoranthene Dervlene perfluorohexane 30 506 28 604 27 955 28 173 23 065 29 963 pentane 27 905 22 993 28 099 29 888 heptane 22 932 27 866 28 042 29 829 decane 27 877 22 948 28 056 methylcyclo29 836 hexane 27 867 22 945 29 829 28 049 cyclohexane 27 813 22 895 27 912 dioxane 29 721 27 742 22 785 27 837 benzene 29 564 27 761 22 797 29 629 27 869 ethylbenzene toluene 29600 27861 27 753 22 788 ~~
~~
be used to solve for the unknown refractive index and dielectric constant of our studied systems. An example of the type of spectral shifts looked at are shown in Figure 1 for pyrene in pentane, cyclohexane, and benzene. For each probe two equations are obtained, one for the plot with the Bayliss function (for this plot the value for perfluorohexane was not used as this would lead to nonlinearity for the function as described by Bekarek et al.6) and another with the Bekarek function. The standard equations are described as follows: u=A+BC
(5)
Bekarek model
Bayliss model
8
(7)
u
I
u
o
u
1
.
u
4
J
#
u
I
y
o
y
I
a
u
UHWQlw (m)
Figure 1. Absorption spectrum of pyrene in pentane (A), cyclohexane (B),and benzene (C).
in solvent polarization during the excitation process. Bekarek et al.6 conclude that the Born functions have greater applicability over a wider range of solvent that includes perfluorinated hydrocarbons or even the vapor state. They also point' to the fact that the BMO and BKB methods do not work well in interacting solvents, i.e. those that form complexes, hydrogen bonds, etc. The Born dielectric theory is used as the basis of the Bekarek model and is represented as in eq 4, which relates
In real terms A is related to a gas phase position for the absorption maxima, and Y is the absorption maxima for the probe in the system of interest. The plots for the Bayliss and Bekarek functions are shown in Figure 2 for pyrene; data were plotted in a similar fashion for the other probes. The equations describing Y are as follows: Pyrene
= 30978 - 4.450B Y = 31731 - 1.055B
Bekarek Bayliss
= 29006 - 3.733B = 29666 - 0.896B
Bekarek Bayliss
= 23554 - 2.3638 = 24303 - 0.750B
Bekarek Bayliss
= 28319 - 1.7= 28892 - 0 . 5 6 5 ~
Bekarek Bayliss
Y
anthracene Y
Y
perylene Y
Y
W = k[+]
(4)
fluoranthene
that the work difference ( W) to charge an isolated species in air as compared to solvent as proportional (k is a constant) to the dielectric constant (e) of the solvent. The argument for use of the Born dielectric term is with regard to considering the distortion polarization that takes place following excitation and how it depends on distance between solute and solvent molecules, this distance should be related to the total solvent polarizability and so to the (e - l ) / c function. In the present study the Bayliss model is employed to determine the refractive index of the surrounding medium of the probe molecule while the dielectric constant is further calculated from the determined refractive index by using the Bekarek model (see eqs 5-7 below). The molecular forces that cause the spectral shift are of short range and include effects of the immediate molecular layer surroundingthe probe. Thus, the measurements lead to molecular scale values of the refractive index and fast dielectric constant at the host site. Standard Solvent Plots. The spectral shifts of any given probe can be related to functions of refractive index and dielectric constant in a set of standard solvents,Table I. This relationship gives a straight line; this is solved for with a linear least squares fit. These equations can then
Y
pyrenebutyric acid (PBA) Y = 29910 - 2.909B v = 30943 - 0.979B
Bekarek Bayliss
Results are obtained by using Y found for the studied system and solving for B, f ( n )in the Bayliss equation and f(n,e)in the Bekarek equation. By use of relationship for the Bayliss function the refractive index is obtained, and this value together with the relationship for the Bekarek function allows the dielectric constant to be calculated. Micellar Systems (Sodium Dodecyl Sulfate, CetyltrimethylammoniumBromide). Micellar systems have received much attention,' the concept being that kinetics and chemical behavior at this molecular level are reminiscent of that occurring in lipid biomembranes. The results for SDS reported in Table I1 indicate a value of 1.47-1.49 for the refractive index and 2.19-2.27 for the dielectric constant for the host site of the probe molecule at the micelle-water interface. Differences can be attributed to different samplingwavelengths,lsas refractive index and dielectric constant are non-linear optical
Langmuir, Vol. 8, No.12,1992 3011
Molecular Probing at Interfaces so"
,
I
Table 111. Absorption Maxima v (cm-l), Baylirr Functions, Refractive Indexes, Bekarek Functions, and Dielectric Constants for Polymer Films with Respective Prober pyrene anthracene fluoranthene perylene pyrene/PVA pyrene/PVB
f
Y
ffn)
n
ffn.6)
c
29 399 27 716 27 639 22 628 29 534 29 587
2.21 2.18 2.22 2.23 2.08 2.03
1.58 f 0.02 1.57 & 0.02 1.58 f 0.02 1.59 0.02 1.53 f 0.02 1.51 f 0.02
0.35 0.35 0.38 0.39 0.33 0.31
2.45 & 0.08 2.40 & 0.08 2.74 f 0.04 2.80 f 0.06 2.32 f 0.08 2.27 & 0.08
-..U W ZB.08
ZB.07
-
28.96-
b
U M
-
2n.a
, mdW&dl(nlum
I
(mu)
Figure 3. Plot of peak maxima for PN+ against concentration of dodecylpyridinium.
Chart I Figure 2. (a) Plot of shifted v (cm-l) for pyrene against the Bayliss function for the standard solvents. (b)Plot of shifted v (cm-l) for pyrene against the Bekarek function for the standard solvents. Table 11. Absorption Maxima v (cm-l), Bayliss Functions, Refractive Indexes, Bekarek Functions, and Dielectric Constants for SDS and CTAB with Respective Probes anthracene fluoranthene perylene
27 891 27 802 22826
1.98 1.93 1.97
CTAB
v
f(n)
Pyrene anthracene perylene
29 595 27 793 22732
2.03 2.09 2.09
1.49 f 0.02 1.47 f 0.02 1.48f 0.02
0.30 0.29 0.30
n 1.50 f 0.02
f(n,c)
e
0.31 0.33 0.30
2.26 & 0.08 2.31 f 0.08 2.13 f 0.06
1.53 f 0.02 1.53 f 0.02
2.20 f 0.08 2.19 0.04 2.27 f 0.06
*
proper tie^.'^ Perhaps of more importance are differences in the probes size, the size of the cavity, and location of the interface, which will differ and depend on the probe used. These differences prevent any observations of any general trends between the different probes. Also, data are presented in Table I1 for probes in CTAB, a cationic micellar system;the values obtained show good consistency and confirm that the technique can be used in different systems with good accuracy. Polymer System/Polystyrene. The spectra of probe molecules in a homogeneous polystyrene film were studied and the results are shown in Table 111. The value ranges are refractive index 1.57-1.59and dielectricconstant 2.402.80 and compare well with literature values of 1.59-1.60 and 2.49-2.61, respectively.12 Table I11also gives data for films of poly(viny1alcohol) (PVA) and Poly(viny1butyral) (PVB) with pyrene as the (18) Van Nostrands Scientific Encyclopedia, 7th ed.; Van Nostrand Reinhold New York, 1989; 2416-17. (19) CRC, Handbook of Laser Science and Technology. Volume ZZZ Optical materials: Part 1,Section 1: Nonlinear OpticalProperties; CRC Press: Boca Raton, FL, 1982.
I
LAPONITE Surfaciants0.1mM
I I
LAPONITE Surfactant~O.4mM
I
probe; the literature12values for the refractive indices are PVA 1.49-1.53and PVB 1.48-1.49,and again the present studies give accurate values for the refractive indices of these solid polymers. Organoclay System/Colloidal Clay with Coadsorped Alkylpyridinium. In order to aid adsorption of neutral molecules onto clay surfaces, cationic surfactants have often been used, thus making an organoclay system. The probedneutral molecules (e.g. pyrene) are then able to adsorb in the hydrophobic sites provided by the surfactant. A recent studyz0investigated the nature of the cationic adsorption process on clays. The study reportad one of the few known "antiquenching" systems (i.e., decreased quenching with increased coadsorbatel quencher), this is a result of an unusual geometry restriction on the clay surface. The present study further substantiates the picture as we can now determine the refractive index and dielectric constant of the probe site. The results are plotted in Figure 3. The results can be explained by considering Chart I above. At low concentrations (50.1mM) of Py+ both adsorbates lie along the surface; at higher concentrations (10.4mM) the surfactant forms a layer with the alkane chains away from the clay surface. This leads to a realignment of the probe PN+ (-1.5 X M), the chromophore of which is lifted from the surface, and now the chromophoric moiety resides in the aliphatic region (20) Nakamura, T.; Thomas, J. K. J. Phys. Chem. 1986,90,641-644.
(21) Young, G. J. J. Colloid Sei. 1958, 13, 67. (22) (a) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (b) Hair,M. L. Infrared Spectroscopy in Surface Chemistry: Marcel Dekker: New York, 1967.
3012 Langmuir, Vol. 8,No. 12, 1992
Kavanugh et al.
Table IV. Absorption Maxima Y (om-!), Bayliss Functions, Refractive Indexes, Bekarek Functions, and Dielectric Constants for Silica with Respective Probes PYene PBA anthracene fluoranthene perylene
v
f(n)
n
f(n,c)
t
30 030 29297 28 050 28 089 23 202
1.61 1.68 1.80 1.42 1.47
1.33 f 0.02 1.36f0.02 1.41 f 0.02 1.28 i 0.02 1.26 f 0.02
0.21 0.21 0.26 0.15 0.13
1.94 f 0.08 1.84 i 0.04 2.06 f 0.08 1.60 f 0.04 1.54 f 0.06
Table V. Absorption Maxima Y (om-l), Bayliss Functions, Refractive Indexes, Bekarek Functions, and Dielectric Constants for Silica with Different Pretreatment Temperaturea ~
13OOC 500°C
v
f(n)
n
f(n,c)
t
29297 29586
1.68 1.39
1.36f 0.02 1.24f 0.02
0.21 0.11
1.84 f 0 . 0 4 1.48 f 0.04
Figure 4. Reflectance spectrum for PBA on silica pretreated at 130 O
C
( 0 ) and
500 O C
(X).
Table VI. Absorption Maxima Y (om-!), Bayliss Functions, Refractive Indexes, Bekarek Functions, and Dielectric Constants for Zeolite with Respective Probes PFene anthracene perylene
29 985 27 816 22 831
1.66 2.06 1.96
1.36 i 0.02 1.52 f 0.02 1.48 f 0.02
0.22 0.32 0.30
1.96 f 0.08 2.29 f 0.08 2.26 f 0.06
Table VII. Absorption Maxima Y (om-!), Bayliss Functions, Refractive Indexes, Bekarek Functions and Dielectric Constants for Zeolite/Probe Pyrene with Coadsorbed Water % coadsorbed water v f(n) n f(n,t) t 0 29 985 1.66 1.35 i 0.02 0.22 1.97 f 0.08 2 29880 1.75 1.39f 0.02 0.25 2 . 0 4 t 0.08 4 29 821 1.81 1.42 0.02 0.26 2.08 f 0.08 5 29 820 1.81 1.42 f 0.02 0.26 2.08 i 0.08
*
10
29 821
1.81
1.42 f 0.02
0.26
b
2.08 f 0.08
of the surfactant. These effects are reported as a spectral maximum at higher frequency,i.e., alower refractive index and dielectric constant experienced by the probe. Silica/Porous Surface System. Silica surfaces are important in a broad spectrum of chemical applications such as catalysis and chromatography. Much work has been done to probe the mi~roenvironment'~ of such surfaces. Studies were initiated on the Davisil surface which is heterogeneous and porous in order to give information on the degree of homogenity of the surface sites where the probe molecules are adsorbed. Table IV illustrates data for the silica surface, and it can be noted that a wider range for the refractive indices (1.26-1.41) and the dielectric constants (1.54-2.06)is observed. The literature14values for refractive index are stated to be 1.40-1.60. The probes experience both the surfacevacuum region and the internal pore region and the probe could locate in both or one of these regions, interactions in the respective sites being different. For a silica system as a slurry in cyclohexane (silicaand cyclohexaneare index matched), the absorption maximum falls at wavelengths which correspond more closely with the refractive index of cyclohexane,and further away from the gas phase value this indicates that on the dry surface, i.e., under vacuum, the molecular probe which on the solid surface only partially experiences the surface much of the probe experiencing vacuum environment. Hence the refractive index measured is low. A further study was done with the probe molecule PBA to investigate the dependence of surface environment on pretreatment temperature. On silica surfaces the intensities of the maxima for fluoranthene and perylene were not well defined and this led to deviations which were not seen with the other probes. The
1344
0
2
4
6
8
10
'
K witir
Figure 5. (a) Plot of Bayliss function (+) and Bekarek function ( 0 ) against % coadsorbed water (probe pyrene). (b) Plot of
refractive index (X) and dielectric constant (0) against 9% coadsorbed water (probe pyrene).
maxima for the pyrene, anthracene, and PBA probes were sharp and spectral maxima could be located with good accuracy. Table V presents data for studies on silica with the probe PBA with different pretreatment temperatures for the silica. The spectral shift is shown in Figure 4. Pyrenebutyric acid is the probe of choice in this study as the other probes fail to adsorb on samples with high pretreatment temperatures whereas PBA with its carboxylic functional group attachs to the surface. The two pretreatment temperatures result in different concentrations of silanol groups at the surface, and the probe reports different environments. The results with high temperature treated silica indicate the environment to be closer to a gaseous phase (i.e. a less polar environment)which agrees with the decrease in silanol concentration at the surface.
Molecular Robing at Interfaces Previous studies21*22 have shown that at 500 OC approximately half of the surface hydroxyl groups have been eliminated, which is consistent with the present study. This study further shows the usefulness of such atechnique in investigating surfaces. Zeolite 13XIConstrained System. Zeolites possess unique structures,ls and the probessurveya seriesof cages and channels all with size exclusive entry pores. The refractive index for the chosen zeolite is obtained from the following equation:16
n = 1.419 + 0.0457 (Al/Si) for Zeolite 13X (Aldrich) AVSi = 1.4 n = 1.48 Table VI gives data for the zeolite refractive index in the range 1.36-1.52, which includes the calculated value. Table VI1 presents data for studies on the variation of coadsorbed water on the zeolite. Other studies" have noted the sensitivity of zeolite dielectrics to adsorbed atmospheric moisture. They noted that the dielectric constant increases on absorption of water which is consistent with the present study. Figure 5a, shows plots for the two functions. The initial increase as percentage of water increases from 0 to 4% can be considered a first stage where water fills the molecular layer around the probe; additional water (above4% ) is used to fill the entire pore of the zeolite. Above 4%) there is no wavelength
Langmuir, Vol. 8, No. 12, 1992 3013 shift, indicating no change in the functions as can be seen in Figure 5a. Hence no change is observed in refractive index or dielectric constant as seen in Figure 5b. Conclusion The method developed here, using models for spectral shifts experienced by chromophores in different environments, proves a means of measuring molecular refractive indices and dielectric constants for local microregions of systems. A variety of materials can be probed successfully, polymer, micellar, metal oxide surfaces, etc., and the data are of value in describing ionic reactions on or in these materials. The two studies, coadsorption of water onto zeolite and effects of pretreatment temperature on silica, illustrate how the technique developed here can extract information on the molecular scale in such systems, information that is required to qualitatively describe ionic processes in these systems. Acknowledgment. We thank the National Science Foundation and the Environmental Protection Agency for support of this work. We also thank Dr. B. H. Milosavljevic for preparing PVA and PVB films. Registry NO. CTAB, 57-09-0;SDS,151-21-3;PfCl-, 10474-5;PBA, 3443-45-6;PN+Br-, 81341-11-9;HzO, 7732-18-6; polystyrene, 9003-53-6;pyrene, 129-00-0;fluoranthene, 206-440; anthracene, 120-12-7;perylene, 198-55-0;laponite, 53320-868.
