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Langmuir 1997, 13, 1440-1445
Adsorption of Surfactants in Porous Silicon Films Robert B. Bjorklund,* Shahin Zangooie, and Hans Arwin Department of Physics, Linko¨ ping University, S-581 83 Linko¨ ping, Sweden Received July 5, 1996. In Final Form: December 9, 1996X Surfactant adsorption in porous silicon films formed by anodic oxidation of bulk p-type silicon in HF solution was studied by spectroscopic ellipsometry. The cationic surfactant cetyltrimethylammonium bromide reversibly adsorbed in the films from aqueous solutions. No adsorption of sodium dodecyl sulfate was observed. Modification of the surfaces by addition of aluminum nitrate from ethanol solutions and titanium(IV) chloride from the gas phase, followed by thermal decomposition, altered the surface properties and surfactant adsorption behavior. A simple two-layer optical model was used to obtain the effective refractive indices for the porous films in different ambient media. Spectra calculated from the model agreed well with the experimental observations.
Introduction Porous silicon layers prepared by anodic etching of bulk silicon in hydrofluoric acid solutions have been the subject of intense research activity since the discovery by Canham of their room temperature photoluminescence.1 A number of studies have been made on surface modification procedures directed toward improving the material’s optical properties2-4 or causing controlled quenching of the luminescence.5-7 The modifications often involve immersing freshly prepared films in organic solvents containing various adsorbates such as dye molecules. In addition, contacting porous silicon films with aqueous solutions has led to processes of possible technical interest such as water splitting,8 reductive deposition of noble metal salts,9 and formation of hydroxyapatite layers to improve biocompatability.10 Porous silicon is very reactive both with the atmosphere11 and with liquids, especially water, directly after preparation. The initial hydrophobic hydride surface contains water soluble species, including fluorides, which dissolve upon contact with water.12 Prolonged storage in the atmosphere leads to changes in composition and structure of the porous film and gradual formation of an oxide monolayer which stabilizes the surface and gives it a hydrophilic character.11 Structural stabilization and oxide formation have also been reported following heat treatment at 300 °C.13 An additional concern with regard to studying porous silicon films in contact with liquids is the ability of the solvent to penetrate into the pores.12 We have previously described porous silicon films which * Corresponding author: e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Canham, L. T. Appl. Phys. Lett. 1990, 7, 1046. (2) Petrova-Koch, V.; Muschik, T.; Kur, A.; Meyer, B. K.; Koch, F. Appl Phys. Lett. 1992, 61, 943. (3) Lee, E. J.; Ha, J. S.; Sailor, M. J. J. Am. Chem. Soc. 1995, 117, 8295. (4) Zhang, L.; Coffer, J. L.; Rho, Y. G.; Pinizzotto, R. F. J. Electrochem. Soc. 1996, 143, L42. (5) Lauerhaas, J. M.; Credo, G. M.; Heinrich, J. L.; Sailor, M. J. J. Am. Chem. Soc. 1992, 114, 1911. (6) Ko, M. C.; Meyer, G. J. Chem. Mater. 1995, 7, 12. (7) Rehm, J. M.; McLendon, G. L; Tsybeskov, L.; Fauchet, P. M. Appl. Phys. Lett. 1995, 66, 3669. (8) McCord, P.; Yau, S.-L.; Bard, A. J. Science 1992, 257, 68. (9) Coulthard, I.; Jiang, D.-T.; Lorimer, J. W.; Sham, T. K.; Feng, X.-H. Langmuir 1993, 9, 3441. (10) Canham, L. T. Adv. Mater. 1995, 7, 1033. (11) Canham, L. T.; Houlton, M. R.; Leong, W. Y.; Pickering, C.; Keen, J. M. J. Appl. Phys. 1991, 70, 422. (12) Canham, L. T.; Groszek, A. J. J. Appl. Phys. 1992, 72, 1558. (13) Yon, J. J.; Barla, K.; Herino, R.; Bomchil, G. J. Appl. Phys. 1987, 62, 1042.
S0743-7463(96)00659-2 CCC: $14.00
change color under normal fluorescent lighting in response to a change in refractive index when placed in contact with a vapor-saturated atmosphere or a liquid phase.14 This color change can be used as a control to ensure that a liquid phase has entered the pores. We report here adsorption studies on oxidized porous silicon films. The instrumental technique used was spectroscopic ellipsometry. Our goal has been to extend the use of ellipsometry beyond characterization of porous silicon films15 into the area of studying the porous film/ liquid interface. Silicon dioxide surfaces have a negative charge even in solutions having low pH values16 making possible the use of positively charged cetyltrimethylammonium bromide as a probe for the surface properties of the porous silicon films. Films were also modified by addition of Al and Ti compounds, and the stability and adsorption properties of these modified surfaces were investigated. Experimental Section Porous silicon films were prepared by anodically etching p-type silicon having a (111) orientation and resistivity 0.01-0.02 Ω cm from Okmetic OY. The electrolyte was a 1:1.5:3 mixture of HF/ H2O/EtOH. The anodic current density was controlled at 50 mA/cm2 to produce uniform films of about 3 cm2 area. Samples were rinsed in ethanol and blown dry with nitrogen. Etching for 6-7 s produced reddish-yellow films which became yellow after heat treatment at 300 °C. Diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) on a Bruker IFS 48 was used to follow oxide growth on some samples following heat treatments. Surface structure was investigated with atomic force microscopy (AFM) in the tapping mode from Digital Instruments. Ellipsometric measurements were made at room temperature in the 300-1000 nm region using a variable angle spectroscopic ellipsometer (VASE) from J. A. Woollam Co. Ellipsometric characterization of samples was done in air at 65, 70, and 75° angles of incidence, and surfactant adsorption was studied at 68° with the porous silicon surface placed in a water-filled cuvette. We report here the ellipsometric angles, ψ and ∆, which were obtained from the complex reflectance ratio F ) tan ψ exp(i∆).17 Analysis and modeling were done with the WVASE32 software package. Modification of some samples was done directly after preparation by placing them in a 0.5 m solution of Al(NO)3‚9H2O, analytical reagent grade from BDH Chemicals, in ethanol or in a saturated vapor of TiCl4, 99.9% from Aldrich. The time of (14) Bjorklund, R. B.; Zangooie, S.; Arwin, H. Appl. Phys. Lett. 1996, 69, 3001. (15) Pickering, C.; Canham, L. T.; Brumhead, D. Appl. Surf. Sci. 1993, 63, 22. (16) Bousse, L.; Mostarshed, S.; van der Shoot, B.; de Rooij, N. F.; Gimmel, P.; Go¨pel, W. J. Colloid Interface Sci. 1991, 147, 22. (17) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: New York, 1977.
© 1997 American Chemical Society
Adsorption of Surfactants in Porous Silicon Films
Figure 1. Spectra for a porous silicon film in air (s) and in deionized water (- - -). exposure to TiCl4 was controlled to yield red- and green-colored surfaces containing different concentrations of Ti species. All samples were heat treated at 300 °C before being immersed in water. The cationic surfactant cetyltrimethylammonium bromide (CTAB, zur Analyse) was from Merck and anionic sodium dodecyl sulfate (SDS, 99%) was from Sigma. Oxalic acid from Kebo AB was used to test the binding strength of Ti and Al species to the porous layers. Simulated spectra were calculated using a two-layer optical model. The model for a porous film in air consisted of an outer layer containing crystalline silicon,18 silicon dioxide,19 and void volume fractions and an inner layer containing only crystalline silicon and voids. The effective optical properties, EMA, for the layers were calculated using the Bruggeman effective medium approximation.20 A similar model was used to calculate spectra for a porous silicon film immersed in water. A three-media Bruggeman model was employed and the refractive index of water as well as for water containing 1 mM surfactant was assumed to be 1.333, the measured value at 589 nm and 20 °C,21 for all wavelengths. To simplify the analysis the surface modifiers, Al2O3 and TiO2, and the surfactants were added to the model as an additional EMA component in the water-filled volume. Optical properties for the oxides were from Palik19 and refractive indices for the surfactants were set equal to the values reported for films of stearic acid (CTAB), 1.66, and lauric acid (SDS), 1.43.17
Results The porous silicon films used in this study were approximately 250 nm thick with a total pore volume of about 75% as determined by analysis of spectra taken in air. Figure 1 shows the changes in ∆ and ψ as a function of wavelength which occurred when the ambient in contact with an unmodified porous silicon surface was changed from air to water. The peak in the ∆ curve at 525 nm was (18) Aspnes, D. E.; Studna, A. A. Phys. Rev. B 1983, 27, 985. (19) Handbook of Optical Constants; Palik, E. D., Ed.: Academic Press: Orlando, FL, 1985. (20) Aspnes, D. E. Thin Solid Films 1982, 89, 249. (21) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press, Inc.: Boca Raton, FL, 1995; pp 10-302.
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Figure 2. Optical models for a porous silicon film in air (a) and immersed in deionized water (b). Modifiers and surfactants added to the model replace some of the water in layer 1.
shifted to 540 nm while the band at 860 nm became somewhat broader and shifted to 870 nm. The peak in the ψ curve at 400 nm increased and the angles above 500 nm decreased over the entire spectrum upon immersion. Simulations of the spectra for a porous silicon film in air and immersed in water were done by constructing the two-layer optical models shown in Figure 2. The composition of the layers was determined by analysis of spectra obtained in air. All attempts to include a silicon dioxide component in layer 2 were rejected by the model. Comparison of the oxide content determined by ellipsometry for freshly prepared films and films treated at different temperatures with the integrated areas of the DRIFTS peak at around 1100 cm-1 corresponding to an Si-O absorption3,22 revealed a reasonably good agreement for the trends in oxide growth between the two methods. The void volume for the film in water was determined by analysis to be only partially filled with water and layer 2 contained only air as shown in Figure 2b. The ∆ and ψ spectra calculated from these models are shown in Figure 3. The simulated curves agree well with the important features observed experimentally upon immersion of porous films in water, the shifts to longer wavelengths of the ∆ peaks together with the decrease in ψ above 500 nm. Adding SDS to the water in the cuvette resulted in essentially no change in the ∆ and ψ spectra. Significant changes were observed when CTAB was introduced as shown in Figure 4. For this sample a shift in the ∆ peak at 540 nm to 555 nm and the broad band at 860 nm to 910 nm accompanied the adsorption of CTAB in the pores. The small step in ∆ at 340 nm is an instrumental artifact due to a filter change. The peak in the ψ spectra at 380 nm and the minimum at 540 nm were also shifted. Calculating spectra for porous films immersed in water, (22) Andersson, R. C.; Muller, R. S.; Tobias, C. W. J. Electrochem. Soc. 1993, 140, 1393.
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Figure 3. Spectra calculated from the models in Figure 2 for a porous silicon film in air (s) and in deionized water (- - -).
Bjorklund et al.
Figure 5. Spectra calculated from the composition described in Figure 2 and Table 1 for a porous silicon film in deionized water (s) and in 1 mM CTAB solution (- - -). Table 1. Volume Percent Water, Metal Oxide, and Surfactant in Layer 1 of the Optical Model, in Addition to 12% Si, 10% SiO2, and 12% Void, for Surfaces Immersed in Water Containing Different Modifiers and Surfactants modifier
adsorbate
none CTAB SDS Al(NO)3 CTAB SDS oxalic acid TiCl4 red CTAB SDS green CTAB SDS
Figure 4. Spectra for a porous silicon film in deionized water (s) and in 1 mM CTAB solution (- - -).
according to the model in Figure 2b, and in water containing CTAB where a 5% CTAB volume replaced some of the water in layer 1 of the model reproduced the shifts in the experimental curves as shown in Figure 5. The volume composition of layer 1 in the model which yielded the calculated spectra shown in Figure 5 are the first two
water 66 61 66 47 47 42 59 56 53 56 41 41 41
metal oxide
surfactant 5 0
19 19 19 7 10 10 10 25 25 25
figure 3,5 5 8, 9
0 5
8 9 10
3 0 10 0 0
entries in Table 1. The CTAB adsorption was rapid, as shown in Figure 6, indicating a fairly open porous structure which was verified by AFM investigation of the surfaces (Figure 7). Desorption when the cuvette was flushed with water (also Figure 6) was a slower process, and the angles returned to nearly their original values. Porous surfaces modified with aluminum nitrate were observed to reversibly adsorb SDS as indicated by the shifts in the curves shown in Figure 8. No adsorption of CTAB was observed. Analysis of samples in air yielded the result that 19% of layer 1 in the model was Al2O3 as shown in Table 1. The shifts in the spectra shown in Figure 8 were simulated by adding a 5% SDS component to layer 1 of the model. Not all of the alumina in the pores was firmly anchored to the surface and could be removed by adding oxalic acid. Removal of alumina resulted in shifts in the ∆ and ψ peaks to lower wavelengths as shown in Figure 9 and the curves could be simulated by decreasing
Adsorption of Surfactants in Porous Silicon Films
Langmuir, Vol. 13, No. 6, 1997 1443
Figure 6. Time dependence of ∆ (s) and ψ (- - -) at 546 nm during adsorption and desorption of CTAB surfactant. The surfactant was introduced to the solution at 22 min and the cuvette was flushed with deionized water beginning at 42 min.
Figure 8. Spectra for an Al-modified porous silicon film in deionized water (s) and in a 1 mM SDS solution (- - -).
Figure 7. AFM picture of a porous silicon film surface.
the alumina content in layer 1 of the model to 7% (Table 1). Al-modified surfaces which had been treated with oxalic acid exhibited CTAB adsorption in deionized water similar to that shown in Figure 4. Porous surfaces modified with titanium(IV) chloride had spectral features which were dependent on the quantity of Ti present in the pores. The ∆ peak at about 380 nm associated with titanium dioxide was much larger for the green samples having the higher Ti content than for the red, as shown in Figure 10. The corresponding peak in the ψ spectra was shifted from 385 nm to 445 nm as the titanium dioxide concentration increased. Analysis of the surfaces in air yielded the result that the red samples contained 10 vol % TiO2 in layer 1 of the model and 25% in the green. SDS was not observed to adsorb on the Timodified surfaces. CTAB adsorbed in the red samples, shifting the peaks to longer wavelengths, but not in the green (Table 1). Mineral acids and oxalic acid caused only small changes in the spectra. Discussion Anodically etched silicon is a highly microporous material as demonstrated by nitrogen adsorption/desorption isotherms.23 The hydrophobic surface initially formed has been shown to be wetted by a number of organic solvents,12 and aging in the atmosphere22 or thermal oxidation13 have been reported to produce a thin oxide coating which makes the porous layer hydrophilic and stable in water. Thus the material has interesting (23) Herino, R.; Bomchil, G.; Barla, K.; Bertrand, C.; Ginoux, J. L. J. Electrochem. Soc. 1987, 134,1994.
Figure 9. Spectra for an Al-modified porous silicon film in deionized water (s) and in 0.1 M oxalic acid (- - -).
properties for studying the interaction of a high surface area oxide with an aqueous phase which is relevant to several areas of technology such as preparation of solid catalysts9 and potentiometric biosensing.24 In addition, a number of surface modifications based on liquid phase treatment of porous silicon layers have been developed in
1444 Langmuir, Vol. 13, No. 6, 1997
Figure 10. Spectra for Ti-modified porous silicon films having a red color (s) and a green color (- - -) in deionized water.
order to alter the luminescence properties. These include treatments aimed at modifying photoluminescence2-7,25-27 and electroluminescence.28,29 Ellipsometry is an extremely sensitive surface technique which has been used extensively for studying adsorption phenomenon at the solid/liquid interface. The formation of a surface film upon adsorption results in a change in the optical response at the interface and, as a consequence, a change in the ellipsometric angles.17 For adsorption at a porous surface, optical changes arise from both the buildup of a film at the external surface and the accumulation of additional dipole moments in the porous layer when adsorbate molecules replace solvent molecules as previously reported for flotation reagent adsorption on mineral surfaces.30 Adsorption within the pores should dominate in the porous silicon surfaces shown in Figure 7. For transparent films an increase in the total polarizability in the pores such as occurs when water replaces air or surfactant replaces solvent causes a change in the period of the interference oscillations which shifts the ∆ and ψ curves to longer wavelengths. The optical models shown in Figure 2 were constructed in order to provide a quantitative frame of reference for the experimental observations. The goal was to develop a simple method for calculating spectra for surfaces in different environments based on the information available (24) Thust, M.; Scho¨ning, M. J.; Frohnhoff, S.; Arens-Fischer, R.; Kordos, P.; Lu¨th, H. Meas. Sci. Technol. 1996, 7, 26. (25) Andsager, D.; Hilliard, J.; AbuHassan, L. H.; Plisch, M.; Nayfeh, M. H. J. Appl. Phys. 1993, 74, 4783. (26) Li, K.-H.; Tsai, C.; Sarathy, J.; Campbell, J. C. Appl. Phys. Lett. 1993, 62, 3192. (27) Steiner, P.; Kozlowski, F.; Wielunski, M.; Lang, W. Jpn. J. Appl. Phys. 1994, 33, 6075. (28) Kozlowski, F.; Wagenseil, W.; Steiner, P.; Lang, W. Mater. Res. Soc. Symp. Proc. 1995, 358, 677. (29) Zhang, L.; Coffer, J. L.; Xu, D.; Pinizzotto, R. F. J. Electrochem. Soc. 1996, 143, 1390. (30) Bjorklund, R. B.; Arwin, H. Langmuir 1992, 8, 1709.
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about the solid/liquid interface. A 10 vol % silicon dioxide component was added to layer 1 of the model based on analysis of ellipsometric spectra taken in air together with the DRIFTS results. This value agreed well with the oxidized fraction reported for porous silicon treated in oxygen at 300 °C.13 Although satisfactory simulations of the experimental spectra could be obtained using oxidefree models, an oxide phase was included in order to have a more accurate description of the surfaces under study. The result that the porous volume was only partially filled with water was determined by both analysis of spectra and generation of simulated curves from models containing different water concentrations. It is possible that the simplicity of the models, together with the lack of silicon dioxide in layer 2, contributed to the incomplete void filling shown in Figure 2b. However it is not unrealistic to postulate that the porous volume was only partially filled because of the small dimensions and some hydrophobicity in the films. The filling of 2/3 of the void volume in layer 1 is a rough estimate of how much water penetrated into the pores upon immersion. Calculation of spectra from the models in Figure 2 yielded good simulations of the experimental curves, as shown in Figure 3. Surfactant adsorption as a function of concentration, chain length, pH, and ionic strength is a frequently used method for probing the surface properties of oxide materials. In order to simplify the conditions in this study, the surfactant adsorption was performed in deionized water. Some preliminary work has been done varying the ionic strength and pH of the solutions, but these results are not included here. As shown in Figure 4, the cationic surfactant CTAB adsorbed readily in the oxidized porous silicon layers in agreement with what has been reported in a number of adsorption studies on silica surfaces.31 The adsorption was reversible and the desorption step was quite slow as shown in Figure 6.The internal surface of the porous films seemed to be readily accessible from the liquid phase, as confirmed by AFM in Figure 7, since no delay was observed for adsorption in the pores when the surfactant was first added to the cuvette. As shown in Table 1 and Figure 5, addition of a 5% CTAB volume to layer 1 in the model resulted in shifts in the calculated spectra similar to what was observed experimentally. Changing the ambient from water to a 1 mM solution of CTAB in the simulations caused no shifts in the spectra because of the insignificant change in the refractive index. Anionic SDS caused no shifts in the ellipsometric spectra. Since the model required addition of a separate surfactant phase in order to simulate adsorption, we interpret the lack of shifts in the spectra as indicating no adsorption, which is as expected for interaction of SDS with a negatively charged silicon dioxide surface.16 Shane et al. have reported the adsorption of SDS on nonoxidized porous silicon.32 The adsorbed surfactant caused a decrease in photoluminescence and investigation by DRIFTS showed that the surfactant was strongly held even during rinsing with different solvents as long as the solvent did not oxidize the surface. This would seem to indicate that the SDS adsorption was on oxide-free silicon, which is a state not normally encountered on silicon surfaces covered by a native oxide. Surfactant adsorption on modified surfaces was done in order to study the effects of metal oxides in the porous films. Metal cations have been reported to deposit in (31) Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, C. D., Rochester, C. H., Eds.; AcademicPress: London/New York, 1983; Chapter 6. (32) Shane, M. J.; Heinrich, J. L.; Smith, R. C.; Sailor, M. J. Electrochem. Soc. Proc. 1996, PV95-25, 278.
Adsorption of Surfactants in Porous Silicon Films
porous silicon from aqueous solutions. A reductive deposition of noble metal cations, such as Pd2+, has been observed9 and the photoluminescence of porous layers has been altered by adsorption of Cu2+, Ag+, and indium chloride.25,27 Mild oxidation of porous silicon by immersion in toluene solutions of aluminum isopropoxide has been done in an effort to improve stability for electroluminescence.29 Infrared measurements indicated that the oxidation of the layers was somewhat different from that obtained by thermal oxidation in air, and a Si-O-Al asymmetric vibrational mode similar to that found in zeolites was observed. Modification of porous layers with aluminum nitrate yielded surfaces which adsorbed SDS as shown in Figure 8. No CTAB adsorption was observed. This is in agreement with the fact that aluminum oxides are positively charged at neutral pH.33 The aluminum oxide species formed from thermal decomposition of the nitrate thus completely coated the pore walls or blocked certain pore entrances. Since the Al-modified surfaces exhibited a yellow to red color change upon immersion in water, a significant fraction of the pores was still accessible to the aqueous phase. The quantity of SDS in layer 1 of the model necessary to simulate the spectral shifts shown in Figure 8 was about 5% according to Table 1. This is quite a large quantity considering the shifts in Figure 8 are much smaller than those in Figure 4 and may be related to the presence of 19% alumina in layer 1 of the model. Not all aluminum oxide in the pores was strongly bound to the porous film. Over half could be removed by oxalic acid, as shown in Figure 9, which led to a reduction in alumina content from 19 to 7% in the model and the surface remaining was similar to unmodified surfaces with regard to adsorption of CTAB. Freshly prepared porous layers immersed in ethanol were not observed to adsorb aluminum nitrate which would seem to indicate that the addition of Al to these surfaces was an impregnation process with the aluminum salt remaining in the pores after the solvent evaporated as has been reported for laser dye addition to porous silicon.34 In contrast to the weak interaction of aluminum oxide species with the porous surfaces, the reaction of titanium(IV) chloride yielded strongly bound titanium dioxide. Titanium dioxide grafted to high surface area silica has a number of technical applications, and procedures have been developed based on treatments with titanyl sulfate35 and titanium(IV) chloride.36-38 In agreement with the latter technique, the modification of the porous films with TiCl4 probably involved a surface reaction with OH groups (33) Yopps, J. A.; Fuerstenau, D. W. J. Colloid. Sci. 1964, 19, 61. (34) Canham.L. T. Appl. Phys. Lett. 1993, 63, 337. (35) Hsu, W. P.; Yu, R.; Matijevic´, E. J. Colloid Interface Sci. 1993, 156, 56. (36) Ellestad, O. H.; Blindheim, U. J. Mol. Catal. 1985, 33, 273.
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or adsorbed water to yield HCl and TiClx species. Porous silicon has also been modified by reaction with SnCl4 and H2O.39 For our system the color change of the surface as a function of adsorbed TiClx served as a monitor of the relative degree of coverage. The significant differences in the ∆ and ψ spectra for the two titanium dioxide concentrations, as shown in Figure 10, made it possible to estimate the concentrations in the porous layers by incorporating a TiO2 component. Analysis of spectra taken in air yielded the result that the titanium dioxide concentration in the layer 1 was about 10 vol % for the red surfaces and about 25% for the green. The red surfaces having the lower titanium dioxide content were observed to adsorb relatively large quantities of CTAB, as indicated by shifts in the peaks shown in Figure 10 for the red sample to longer wavelengths, which would mean that a considerable surface area of silica remained exposed in the pores. The 3% CTAB in layer 1 shown in Table 1 was sufficient to simulate this adsorption. However, the green surfaces adsorbed no CTAB, which can be interpreted as a complete coverage of the pore walls with titanium dioxide since this is believed to have a low surface charge at neutral conditions.40 Oxalic acid also had access to the internal surface and caused only minor changes in ∆ and ψ, and we thus conclude that the titanium dioxide was strongly bound in the pores. Conclusions Spectroscopic ellipsometry has been shown to be a useful technique for studying the porous silicon/liquid interface. Ellipsometric monitoring of the interface makes possible the use of surfactant adsorption as a probe of porous film surface properties. The effects of surface modification by impregnation or reaction with metal compounds can be studied. Optical models provide a quantitative frame of reference for interpreting the data with regard to degree of pore filling by the solvent and amount of adsorbed surfactant. Investigation of the stability of porous silicon in contact with aqueous solutions is an additional area where ellipsometry can be utilized. Acknowledgment. The work reported here was supported by the Swedish Research Council for Engineering Sciences. LA960659B (37) Kubota, L. T.; Gushikem, Y.; de Castro, S.; Moreira, J. C. Colloids Surf. 1991, 57, 11. (38) Haukka, S.; Lakomaa, E.-L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (39) Du¨cso¨, C.; Khanh, N. Q.; Horva´th, Z.; Ba´rsony, I.; Ultriainen, M.; Lehto, S.; Nieminen, M.; Niinisto¨, L. J. Electrochem. Soc. 1996, 143, 683. (40) Yates, D. E.; Healy, T. W. J. Chem. Soc., Faraday Trans. 1 1980, 76, 9.
