Langmuir 1986,2, 715-717 high surface coverage of both CO and oxygen. This is evidenced by the linear correlation between the C 0 2 formation rate and the integrated absorbance of the 2156-cm-' band assigned to CO in the mixed surface structure. There is no detectable lag or induction time when the surface is oxygen-covered. 4. IR intensities of surface species indicate that the 2156-cm-' band is only observable under transient conditions. 5. If we start with a CO-covered surface, the relative reactivities of the various CO species are linear > bridge (100) plane > bridge (111) plane. There is little or no
715
induction time for the linearly adsorbed CO. The bridge-bonded species have induction times, and the rate of reaction increases tremendously after the induction time.
Acknowledgment. Financial support of this research by the Department of Energy University Coal Research Program, Contract DE-FG22-84PC70785, and the National Science Foundation, Grant CBT-8513127, is gratefully acknowledged. Registry No. CO, 630-08-0; 02,7782-44-7; Pd, 7440-05-3.
Novel Liquid-Membrane Oscillator with Anionic Surfactant Kenichi Yoshikawa,* Satoshi Nakata, and Teruyuki Omochi College of General Education, University of Tokushima, Minami- josanjima, Tokushima 770, Japan
Giuseppe Colacicco Department of Chemistry, University of South Florida, Tampa, Florida 33620 Received February 13, 1986. I n Final Form: J u n e 23, 1986 An oil layer of nitrobenzene and alcohol containing 2,2'-bipyridine was placed between two aqueous solutions: 0.4 mM sodium dodecyl sulfate (SDS) on one side and 500 mM sodium chloride on the other. The system produced spontaneous oscillations of electrical potential. The oscillations may be explained by a mechanism of consecutive formation and destruction of monolayer structures of the dodecyl sulfate anion at the interface between the organic and the aqueous phases. This is the first report of oscillatory electric phenomena at an oil-water interface in the presence of anionic detergent.
Introduction Recently, we described spontaneous oscillations of electrical potential across a liquid membrane of nitrobenzene and picric acid between two aqueous layers, one of which contained cetyltrimethylammonium bromide (CTAB).14 The system produced rhythmic and sustained oscillations of electrical potential of 150-300 mV with time intervals of about 1 min. It was suggested that the electrical response of the liquid membrane resembles that of biological chemoreceptive systems. In an extension of those studies, we set out to investigate the behavior of a liquid membrane between two aqueous phases, one of which contained an anionic detergent in place of the cationic one. The present report shows that anionic detergents also display rhythmic oscillations. Experimental Section As in previous studies,'"l the experimentalapparatus consisted of a U-shaped glass tube (12-mm inner diameter), a pair of Ag/AgCl electrodes, and a high-impedance electrometer (Figure 1). The organic phase, a 4-mL mixture of 80 v/v % nitrobenzene and 20 v/v % alcohol, containing 5 mM 2,2'-bipyridine, was placed at the bottom of the U cell. The aqueous solutions (10 mL on each side) were added simultaneously to the arms of the U cell and floated on the organic phase. The system was allowed to stand without stirring and was thermostated at 25 OC throughout the experiment. The pH was monitored with an Ingold combination ) Yoshikawa, K.; Matsubara, Y. Biophys. Chem. 1983,27,183-185. I) Yoshikawa, K.;Matsubara, Y. J. Am. Chem. SOC. 1984, 206,
electrode (3-mmdiameter, catalog No 6030-02). The combination electrode was placed about 30 mm from the interface.
Results and Discussion Figure 2 shows the tracings of voltage oscillations across four different nitrobenzene-alcohol membranes: (a,b) 1-propanol, (c) 1-butanol, and (d) 1-pentanol. The oil layer also contained 5 mM 2,2'-bipyridine and was placed between aqueous sodium dodecyl sulfate (SDS 0.4 mM in Figure 2a,c,d, or 0.04 mM in Figure 2b) on the left and 0.5 M sodium chloride on the right (see Figure 1). The resting potential of about 100 mV with 0.4 mM SDS (Figure 2a) was nearly double that with 0.04 mM SDS (Figure 2b); however, the oscillation frequency was greater with the less concentrated SDS. As in previous experiment^,^,^ the value of the resting potential increased with the concentration of the ionic surfactant at the interface and f or in the aqueous phase, suggesting a direct relationship between electrical potential and surface concentration of the fixed charges of SDS in the interfacial film. Interestingly, the magnitude and shape of the oscillations differed markedly with the length of the alcohol's alkyl chain. Similar oscillations were obtained when SDS was replaced by other anionic surface-active agents, such as 0.4 mM sodium o-dodecylbenzenesulfonate (SBS) and 0.4 mM sodium p-ethylbenzenesulfonate. However, no oscillations were observed in the absence of either the
-4427. I) Yoshikawa, K.;Omochi, T.; Matsubara, Y. Biophys. Chem. 1986,
111-214. b) Yoshikawa, K.;Matsubara, Y. J. Am. Chem. SOC. 1983, 105, -5969.
(5)Colacicco, G.Nature (London) 1965,207,936-938. (6)Colacicco, G.Nature (London) 1965,207, 1045-1047. (7)Toko, K.;Yoshikawa, K.; Tsukiji, M.; Nosaka, M.; Yamafuji, K. Biophys. Chem. 1985,22,151-158.
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716 Langmuir, Vol. 2, No. 6, 1986
Yoshikawa et al. a)
5.8
I L
0
b,
( f1 Figure 1. Diagram of the experimental apparatus: (a) millivolt meter; (b) salt bridge (3 M KC1); (c) Ag/AgCl electrode; (d) aqueous layer with anionic detergent; (e)aqueous layer with NaC1; (0 organic layer.
6.6
/---
20
40 t/min 60
I
I 6.4a
6.2. 6.01 0
, 20
, 40 t/min
,
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Figure 3. pH variation of the (a) right and (b)left aqueous phases after the construction of the liquid membrane. Experimental conditions same as in Figure 2a.
-2d,
’ -3
20
110
60
‘180
200
t/min
Figure 2. Tracings of electrical potential across the organic phases. Organic phase: (a,b)20% 1-propanol, (c) 20% 1-butanol, and (d) 20% 1-pentanol. Left aqueous phase: (a,c,d) 0.4 mM SDS and (b) 0.04 mM SDS. Right aqueous phase: 0.5 M sodium chloride.
alcohol, the anionic detergent, or 2,2’-bipyridine; the latter could be replaced by 1,lO-phenanthroline. Figure 3 shows the time traces of pH in the right and left aqueous phases under the same experimental condition as in Figure 2a. Fluctuation of pH was observed only for the left aqueous phase containing SDS. This clearly indicates that the oscillatory change of the electrical potential is mainly attributable to concentration changes at the left interface. The sign of the potential obviously varies with the sign of the ionic detergent and with the polarity of the circuit
in relation to the ele~trometer.~ With the experimental setup in Figure 1 the interface at the left-hand side is electrically positive relative to the right-hand side. When the concentration of sodium chloride on the right-hand side was changed from 500 to 250 mM, the magnitude of the potential decreased 20-30%, consistent with a previous interpretation of the electrical potentials a t an oil-water i n t e r f a ~ e . Some ~ ? ~ portion of the SDS molecules, initially present in the left aqueous phase, may migrate to the right interface, which produces a small negative potential. This negative potential due to the adsorption of dodecyl sulfate anion a t the right interface should be greater in the case of less concentrated NaCl(250 mM) than in 500 mM NaC1. This may result in the smaller potential difference in the case of 250 mM NaC1. The mechanisms for the repetitive oscillations with SDS should be similar to the ones described for the cationic detergent.2,5,6 State I. Dodecyl sulfate anions in the aqueous phase become oriented at the interface. Simultaneously, molecules of the amine (B) 2,2‘-bipyridine migrate from the organic toward the left interface into the aqueous phase. Thus, there follows a gradual increase in the concentration of dodecyl sulfate anions and HB+ cations of the amine B at the left interface. State 11. When the concentration of dodecyl sulfate anions at the left interface reaches a critical value, dodecyl sulfate anions and HB+ cations are abruptly transferred to the organic phase with formation of inverted micelles or water in oil microemulsions. The process obviously repeats itself. State 111. As the interface becomes depleted of dodecyl sulfate anions at the end of state 11, accumulation of dodecyl sulfate anions at the interface begins again and a new dodecyl sulfate film is formed. The aqueous phase serves as a reservoir that supplies surfactant to the interface. State IV. When the concentration of interfacial dodecyl sulfate anions increases to an upper critical value, abrupt transfer of dodecyl sulfate anions to the organic phase occurs again. The role of the alcohol is particularly intriguing. The alcohol could affect either the structure of the SDS monolayer a t the interface, the rate of migration of the given molecules from the aqueous phase to the organic phase, or the kinetics of phase changes such as in the micro-
Langmuir 1986,2, 717-722 emulsion- or micelle-to-film and film-to-microemulsion or inverted-micelle conversions. In light of known effects of the length of the alkyl chains on the thermodynamics of alcohol-water interactions? future studies must consider (8) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1963.
717
the role of hydrogen bonding in the influence that the alcohols may produce on the electrostatics and electrodynamics of interfacial water.
Acknowledgment. This work was partly supported by Grants-in-Aid for Scientific Research to K.Y. from the for Education, Science and Of Japan and a grant from Nissan Science Foundation.
Structural Changes along the Sol-Gel-Xerogel Transition in Silica As Probed by Pyrene Excited-State Emission Vered R. Kaufman and David Avnir* Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received March 4, 1986. I n Final Form: July 15, 1986 Structural variations of molecular scale along the sol to gel to xerogel transition are studied for the polymerization of Si(OCH3)4and the formation of porous SiOz,utilizing, for the first time, a photophysical probe. Changes in the emission spectra of pyrene and of the excimer of pyrene reveal complex structural changes, which proceed much beyond the gelation point. The environmental polarity of the probe molecule changes along that process. Tested parameters were pH, water to silane ratio (w/s), and alkoxy group. Polymerization-gelation occur at low w/s, whereas colloidal gelation occurs at high w/s. The final silica xerogel is an efficient trap for organic molecules: the typical pyrene/silica excimer emission disappears.
Introduction Recent developments in the material sciences now enable the preparation of various inorganic oxide glasses at relatively low temperatures. The sol-gel process, as it became known,' is based on the idea of replacing the classical and ancient high-temperature melting technique by a room-temperature polymerization process. Not only are the temperatures lowered by this technique, but it also opens the possibility of preparing unconventional oxide mixture glasses2 by avoiding evaporation of oxides upon firing and phase separation and devitrification upon cooling. The idea in designing the proper monomer is to take a molecule that will form upon polymerization Si-0Si bonds, with reaction byproducts that can easily leave the system. Metal alkoxides fulfill these requirements. For instance, for the preparation of silica network, it was found that condensation-polymerization of silicon tetralkoxides is most suitable according to the net reaction nSi(OCH3)4+ HzO (Si-O-Si),j2 + 4nCH30H
-
The sol-gel process consists of two main steps, the first of which falls into the domain of surface science. During this first step, a high surface area microporous glass is formed a t room temperature by a complex sequence of polymerization, sol formation, gelation, and gel dessication. The second step, which has been studied in great detail,laeJ consists of annealing the porous glass at elevated tem(1) (a) Sakka, S.; Kamiya, K. J. Non-Cryst. Solids 1980,42, 403. (b) Mukherjee, S. P . J . Non-Cryst. Solids 1980, 42, 477. (c) Dislich, H. J. Non-Cryst. Solids 1983, 57, 371. (d) Brinker, C. J.; Scherer, G . W. J . Non-Cryst. Solids 1985, 70, 301. (e) Klein, L. C. Annu. Rev. Mat. Sci. 1985, 15, 227. (2) (a) Yamane, M.; Aso, S.; Sakaino, T . J . Mat. Sci. 1978,13,865. (b) Yamane, M.; Aso, S.;Okano, S.;Sakaino, T. J. Mat. Sci. 1979, 14, 607. (c) Mukherjee, S.P.; Zarzycki, J.; Traverse, J. P. J. Mat. Sci. 1976, 11, 341. (d) Yoldas, B. E. J. Non-Cryst. Solids 1980,38-39, 81. (e) Suzuki, H. Saito, H.; Hayashi, T. J. J . Mat. Sci. 1984, 19, 396. (3) (a) Zarzycki, J. J . Non-Cryst. Solids 1982, 48, 105. (b) Sakka, S . Bull. Inst. Res., Kyoto Uniu. 1983, 61, 376. (c) Nogami, M.; Moriya, Y. J. Non-Cr3st. Solids 1980, 37, 191.
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peratures, in order to close the pores. This results in a shrunken, nonporous glass. The porous glass obtained at the first stage is the precursor to the second stage and, as such, determines not only the properties of the final glass but also the transition conditions from one to the other. However, the importance of the porous glass is not limited to its being a precursor material; it is an interesting material by itself with many potential applications and uses as an efficient inorganic trap for organic molecules.4+ It is therefore not surprising that much effort has been invested in understanding the route from the monomer to the final xerogel (dry gel). A variety of techniques have been utilized for this purpose, including infrared,2es3,7 Raman: and NMR8,9spectroscopies, scattering of lightlo and of X-rays at small angles,ll gas chromatographyZaJlbJ2and (4) (a) Avnir, D.; Kaufman, V. R.; Reisfeld, R. J . Non-Cryst. Solid 1985, 74, 395. (b) Avnir, D.; Levy, D.; Reisfeld, R. J . Phys. Chem. 1984, 88, 5956. (5) (a) Kaufman, V. R.; Levy, D.; Avnir, D. J. Non-Cryst. Solids 1986, 82, 103. (b) Kaufman, V. R.; Avnir, D. In Proceedings of the 2nd International Conference on Unconventional Photoactive Solids, Cleveland, Sept 1985; Scher, H., Ed.; Plenum: New York, 1986. (c) Levy, D.: Avnir, D. Ibid. (6) (a) Tani, T.; Makishima, A.; Itani, A.; Itoh, U . , ref 5b. (b) Tani, T.; Namikawa, H.; Arai, K.; Makishima, A. J. Appl. Phys. 1985,58, 3559. (7) Yoldas, B. E. J . Non-Cryst. Solids 1984, 63, 145. (8) Artaki, I.; Bradley, M.; Zerda, T . W.; Jonas, J. J . Phys. Chem. 1985, 89, 4399. (9) (a) Assink, R. A.; Kay, B. D. Mater. Res. SOC. Symp. Proc. 1984, 32, 301. (b) Rosenberger, H.; Scheler, G.; Burger, H.; Jakob, M.Colloids Surf. 1984, 12, 53. (c) Artaki, I.; Sinha, S.; Irwin, A. D.; Jonas, J. J. Non-Cryst. Solids 1985, 72, 1985. (d) Yamane, M.; Inoue, S.; Yasumori, A. J. Non-Cryst. Solids 1984, 63, 13. (10) Hunt, A. J.; Berdahl, P. Muter. Res. SOC.Symp. Proc. 1984, 32, 275. (11) (a) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S . J . Non-Cryst. Solids 1982,48,47. (b) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Assink, R. A.; Kay, B. D.; Ashley, C. S. J. Non-Cryst. Solids 1984, 63, 45. (c) Shaefer, D. W.; Martin, J. E. Phys. Reu. Lett. 1984, 52, 2371. (d) Yamane, M.; Inoui, S.;Yasumori, A. J . Non-Cryst. Solids 1984, 63, 13. (e) Strawbridge, I.; Craievich, A. F.; James, B. F.J . Non-Cryst. Solids 1985, 72, 139. (12) Klein, L. C.; Garvery, G. J. J. Non-Cryst. Solids 1980, 38-39, 45.
0 1986 American Chemical Society
