Langmuir 1985,1, 230-232
230
f
0
.5
1
Figure 15. Comparison of period one (curve 1) and longer wavelength (curve 2) solutions for the three-phase line. Shaded regions are patches of low-energy material. G for curve 1is -0.059; G for curve 2 is -0.063. a. is the average height of both curves. rently. Such calculations are possible within the present scheme. In addition, the experiment discussed above should provide valuable insight. Our modeling has described several features of capillary rise and contact angle hysteresis on a vertical plate with a doubly periodic heterogeneity of surface energies. First, hysteresis does not depend only on the coverage of the different materials composing the heterogeneous surface. Thus, experiments must examine both the coverage and the distribution of, for example, a surfactant on a high energy surface, in order to understand the hysteresis phenomenon on that surface. Hysteresis on heterogeneous surfaces can disappear for larger patches of heterogeneity than previous predictions have indi~ated.~JlEven our predictions must be considered a lower limit on critical patch size since the ability of the internal energy of the system to overcome the barriers of metastable states has not been considered. Also, for periodic patterns with average wettabilities that are translationally invariant along the direction of capillary rise, some residual hysteresis will be observed. Finally, our more detailed theory identifies a dissipative mechanism-the jumps of the three-phase line which may be related to the hysteretic behavior of the system.
Appendix The following are closed-form solutions for advancing and receding contact angles according to the horizontalaverage theory for two wettability patterns. For the centered-square and centered-circular patterns shown in Figure 2, let the contact angle of the low-energy patches be c0s-l c1 and the high-energy angle be c o d c2. With the averaged wettability E given by eq 13b, the cosines of the advancing and receding angles are determined as the minimum and maximum values of E, respectively. Using simple geometrical considerations, these angles may be calculated as functions of F, the coverage fraction of material 1. For the centered-square pattern the results are COS Ba = ~2 - Ac(F/2)lI2 F C 72 = ~2 - AC F > 72 (A.la)
F < Y2
cos 8, = c2
= c2 - Ac( 1 -
(
Y ) l 2F) >
'/z
(A.lb) where Ac is defined as c2 - c l . The analogous results for the family of centered circles are COS ea = ~2 - RAc R I1/3lI2 = ~2 - (4R2- 1)l12Ac 1 / 3 l I 2 < R C 1/2lI2 1/2lI2 IR C 1 = ~2 - AC (A.2a) cos 9, = c2 R If/z ~2 - (2R - 1)'I2Ac 7 2 C R I1 (A.2b) Here R is proportional to the circle radius and is related to the coverage fraction F according to F = (7r/2)R2 0 IR I 1/2lI2 = (2R2- 1)'l' 1 - (2R2- 1)lI2 2R2 sin-' 2R 1/2lI2 < R 5 1
+
(
)
(A.2c)
Oscillation of Electrical Potential across a Liquid Membrane Induced by Amine Vapor Kenichi Yoshikawa* and Yasuhiro Matsubara College of General Education, University of Tokushima, Minami-josanjima, Tokushima 770, Japan Received November 9, 1984 Studies were made on the electrical potential across a liquid membrane consisting of an oil layer, 90% oleic acid and 10% 1-propanolcontaining tetraphenylphosphoniumchloride, between aqueous solutions of 0.5 M NaCl and KC1. When the oil phase was exposed to amine vapor, the system showed periodic changes of electrical potential of 10-20 mV with an interval on the order of a few minutes. It is suggested that this system can serve as a model of biological olfactory transduction.
Introduction The mechanisms of sensing, such olfactory and taste sensing, are among the most important problems in biological science. In spite of extensive studies on
chemoreception, its molecular mechanism is not well understood. Recently we reported's2 that rhythmic oscilla(1) Yoshikawa, K.;Mataubara, Y.Biophys. Chem. 1983, 17, 183.
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Electrical Potential across a Liquid Membrane
Langmuir, Vol. 1, No. 2, 1985 231
2o
t
b
4
(Omin
1bO
CMmm3 f 2 0 m m 3
Figure 1. Diagram of apparatus: (a) millivolt meter; (b) KC1
salt bridge; (c) Ag/AgCl electrode; (d) 3 M KCl aqueous solution; (e) 90% oleic acid plus 10% 1-propanol containing 10 mM tetraphenylphosphonium chloride; (f) defatted cotton soaked in amine solution in a glass tube (4 mm),located ca. 10 mm above the surface of (e); (g) 0.5 M NaCl aqueous solution; (h) 0.5 M KC1 aqueous solution. tions of electrical potential are generated across a liquid membrane consisting of water-oil-water phases and that the electrical response of this liquid membrane to chemical substances added to one of the water phases resembles that of a biological chemoreceptive membrane. This system can be regarded as a model of taste. As an extension of the previous study, we found that excitation, or oscillation of electrical potential, could be induced by various amines, which are typical odorants,in a simple artificial membrane composed of water-oil-water phases. This system can be used as a model for studies on the sense of smell.
401
J
(C)
40
c I
I O min
no
Experimental Section
t
The apparatus used in this study is shown schematically in Figure 1. Aqueous solutions (4.5 mL each) of 0.5 M NaCl and 0.5 M KC1were placed in compartments (20 X 10 mm)g and h, respectively. An organic phase (2 mL) of 90% oleic acid and 10% 1-propanolcontaining 10 mM tetraphenylphosphonium chloride was layered over the aqueous solutions to a thickness of about 4.5 mm; tetraphenylphosphonium chloride was added to reduce the impedance of the organic phase and so facilitatemeasurement of the potential. Defatted cotton soaked in a solution of amine was put into a glass tube (f)above the oil layer. All experiments were performed at room temperature (25 2 “C). The voltage was measured with a Hitachi-Horiba M-8s pH/mV meter connected by salt bridges to two Ag/AgCl electrodes. All reagents were commercial products.
served. The periodicity and shape of the oscillations seemed to depend on the amine used, and studies on this relationship are now in progress.
Results
Discussion
*
The fluctuation of electrical potential observed on exposer of the system to amine vapor is shown in Figure 2, where an upward deflection indicates that the KC1 solution becomes negative with respect to the NaCl solution. On exposure to amine vapor, the electrical potential usually remained nearly constant for 5-20 min and then changed abruptly, accompanied with spontaneous, temporal movement of the organic phase. After this, rhythmic changes of the electrical potential were observed. These rhythmic changes stopped when exposure to amine vapor was discontinued and could be generated repeatedly by repeated exposure to the vapor. Oscillation of the potential did not occur when the concentration of amine was below a certain critical value. For example, when the concentration of the NH3 solution in the tube (0in Figure 1was below 1 M, no periodic change of the potential was ob(2) Yoshikawa, K.; Matsubma, Y. J. Am. Chem. SOC.1984,106,4423.
-LO
1
P
I O twin
Figure 2. Typical traces of changes of electrical potential on the exposure to vapor of (a) ammonia (6M NH&, (b) methylamine (40% aqueous solution), (c) pyridine, and (d) piperidine. The initial stage (ca. 10 min), Le., induction period on exposure to the vapor, is omitted in these figures.
In 1937, Bungenberg de Jong and Saubert, during studies of coacervates, observed that the water contents, and consequently the volumes, of oleate coavervates were quite sensitive to foreign substances of various kinds.3 They suggested that this phenomenon might be relevant to the sense of smell. Along this line, Sperber studied the electric resisitivity of a lecithin coacervate and found that its resistance decreased on addition of several 0dorants.~9~ The present results are consistent with theirs, except that we induced “excitation” directly with odorants. Odorant molecules are thought to interact directly with the membrane of the olfactory receptor cells, resulting in generation of nervous impulses. In this respect our model is better than previous ones, since the fluctuation in (3) Bungenberg de Jong, H. G.; Saubert, G. G. P. Protoplasma 1937, 28, 329. (4) Sperber, G. 0. Acta
Physiol. Scand. 1973,89, 603. ( 5 ) Sperber, G. 0. Acta Physiol. Scand. 1977,99,129.
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Langmuir 1985, 1, 232-239
electrical potential was generated directly by the odorant molecules; previous models could not explain how nervous excitation was induced in the olfactory organ. It is also noteworthy that in our system no external force, such as pressure, electrical current, or voltage, was applied. A concentration difference of sodium and potassium ions between the two aqueous phases was essential to induce the observed electrical potential. This seems analogous to the fact that a concentration difference between sodium and potassium ions is essential for the excitability of biological membra ne^.^^' There are many reportssvs on interfacial instability
(Marangoni effect) between gas and liquid phases and between liquid and liquid phases in systems that are far from equilibrium. Thus similar oscillatory phenomena to those observed in this study may occur in a wide variety of systems that have an interface. The present preliminary study suggests the possibility of developing an artificial olfactory system.
(6) Hodgikin, A. L.; Huxley, A. F. J.Physiol. (London)1952,116,449, 473,497;11 7,500. (7)Ishii, T.; Kuroda, Y.; Yoshikawa, K.; Sakabe, K.; Matsubara, Y.; Iriyama, K.Biochem. Biophys. Res. Commun. 1984,123,792.
(8)Sortensen, T. S., Ed. ‘Dynamics and Instability of Fluid Interfaces”; Springer-Verlag: Berlin, 1979. (9)Zierep, J., Oertel, H., Jr., Eds. “Convective Transport and Instability Phenomena”; G.Braun: Karlsruhe, 1982.
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research to K.Y. from the Ministry of Education, Science and Culture of Japan. Registry No. Oleic acid, 112-80-1;1-propanol,71-23-8; tetraphenylphosphonium chloride, 2001-45-8.
Films Formed on Well-Defined Stainless Steel Single-Crystal Surfaces in Borate, Sulfate, Perchlorate, and Chloride Solutions: Studies of the (111) Plane by LEED, Auger Spectroscopy, and Electrochemistry David A. Harrington, Andrzej Wieckowski, Stephen D. Rosasco, Ghaleb N. Salaita,t and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 93106 Received September 18, 1984. I n Final Form: January 2, 1985 Reported here are studies by LEED, Auger spectroscopy, electrochemistry, and thermal desorption mass spectroscopy of the (111)plane of a face-centered cubic Fe-Cr-Ni alloy single crystal of composition (70 atom % Fe, 18 atom % Cr, 12 atom % Ni) resembling that of type 304 stainless steel. Surface films resulting from treatment in borate solution, HzS04, HC104,KC1, and HC1 were amorphous and hydrated. Films formed in acidic media contained up to 20% Cr, due to selective dissolution of Fe and Ni. Annealing at 800 “C led to ordered, Cr-enriched oxide films, consisting primarily of Crz03(001),a hexagonal lattice of chromium and oxide ions. A square CrO mesh was also observed for annealed films formed in acidic media. Significant amounts of C1 were incorporated into the film formed in HCl, but only traces of C1 were detected after pitting breakdown of annealed films in HCl.
Introduction There have been numerous studies of the oxygenous f i i on stainless steel, using electrochemical and surface science techniques.’ However, most of that work dealt with polycrystalline samples or involved procedures in which the surface became contaminated (with air, abrasives, electrolytes, etc.) during preparation and transfer operations. We have previously reported studies of (111)-oriented, well-characterized surfaces of the Fe-Cr-Ni alloy single crystal of composition (70 atom % Fe, 18 atom % Cr, 12 atom % Ni) following treatment with water vapor2 or immersion into liquid water,14 and in the present article we report studies of the same alloy surface after immersion and electrolysis in various electrolytes. In these studies the surface was examined by LEED, Auger spectroscopy, XPS, and thermal desorption mass spectrometry before and after immersion, without contamination during transfer, using an electrochemistry ultrahigh vacuum (uhv) apparatus specially designed for the p ~ r p o s e . ~The present studies involved several anion types (BO3-,S042-, C1042-,Cl-), a range of acidities (pH 0-8.5), and potentials Fulbright Scholar. Permanent address: University of Jordan, Amman, Jordan.
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in the prepassive, passive, and transpassive regions, in order to study the growth, structure, and stability of the (1) (a) Frankenthal, R. P., Kruger, J., Eds. ‘Passivity of Metals”; The Electrochemical Society: Manchester, NH, 1978. (b) Froment, M., Ed. “passivity of Metals and Semiconductors”;Elsevier: New York, 1983. (c) Sato, N.; Okamoto, G. In “ComprehensiveTreatise of Electrochemistry”; Bockris, J. O’M., Conway, B. E., Yeager, E., White, R. E., Eds.; Plenum: New York, 1981;Vol. 4. (d) Adams, R. 0. J. Vac. Sci. Technol.,A 1983, 1, 12. (2)Garwood, G. A., Jr.; Hubbard, A. T.; Lumsden, J. B. Surf. Sci. 1982,121,L524. (3) Harrington, D.A.; Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Salaita, G. N.; Hubbard, A. T. Corros. Sci., in press. (4)Harrington, D. A.; Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Lumsden, 3. B., Proceedings of the Pourbaix Symposium, The Electrochemical Society, Pennington, NJ, 1984. (5)(a)Felter, T. E.; Hubbard, A. T. J.Electroanal. Chem. 1979,100, 473. Ib) Katekaru. J. Y.: Hershbereer. J.: Garwood. G. A.. Jr.: Hubbard, A. T. surf. Sci. 1982, 121, 396. -(c) Hubbard, A. T.; Young, M. A.; Schoeffel, J. A. J.Electroanal. Chem. 1980,114,273. (d) Garwood, G. A., Jr.; Hubbard, A. T. Surf,Sci. 1982,112,281;118,223. (e) Stickney, J. L.; Rosasco, S. D.; Hubbard, A. T. J.Electrochem. SOC.1984,131,260. (f) Stickney, J. L.; Roaasco, S. D.; Schardt, B. C.; Hubbard, A. T. J. Phys. Chem. 1984,88,251. (9) Wieckowski, A,; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984,23,565.(h) Solomun, T.;Wieckowski, A.; Rosasco, S. D.; Hubbard, A. T. Surf.Sci. 1984,147, 241. (i) Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Bent, B.; Zaera, F.; Somorjai, G. A. J. Am. Chem. SOC.,in press. (j) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985,1, 66.
0 1985 American Chemical Society
