Langmuir 1986,2,319-321 observed for A in the u(BH) or v(BD) regions due to the terminal BHD group and no new bands were observed for C. The expected band midway between the two BD2 stretching bands of species B was also observed. Also, following the adsorption of a 1:l 1%2H6/10B2D6mixture, three SiOB stretching bands were observed at 1386,1361, and 1340 cm-' having intensities in the ratio 1:2:1, as expected, further supporting our assignment. However, the spectrum of partially deuterated B in the v(BH2)region is again not consistent with this since two new bands were observed. Although the v(BD) and v(Si0B) data clearly support our assignment for B, the spectrum in the v(BH) region is inconsistent insofar as three bands were observed. In order to resolve this, we adsorbed either "B&6 or "B&6 on a surface for which greater than 90% of the surface d i O H groups had been exchanged with oxygen-18 to give =Si180H groups. The observed spectrum, although identical with that for an l6O surface in the presence of gaseous diborane, was strikingly different when only species B and C were present (Figure 2d) in that the lowfrequency doublet had merged into a single band almost at the midpoint of the previous doublet. The same effect was observed for both llB and O ' B diborane. To account for this observation we must assume that the low-frequency symmetric BH2 stretching mode is in Fermi resonance with an overtone or combination mode near 2475 cm-' and that this resonance is removed upon substitution of lSO for l60.The most likely combination would be the SiO'OB stretching mode at 1386 (which shifts 20 cm-l to lower wavenumber upon l80substitution) and the unobserved in plane '%H2 deformation mode which would be expected to lie near 1100 cm-' and which cannot be observed due to the strong absorption due to silica. As a
319
guess, we place this mode near 2475 - 1386 = 1089 cm-'. Therefore, we conclude that species B also exhibits only two fundamentals in the v(BH) region, as expected for SiOBH2. To this point all of the spectroscopic data support our assignment for species B with the exception of the spectrum in the u(BH) region as a result of partial deuteration. We conclude that yet another perturbation exists which gives rise to the presence of two bands in this region from =SiOBHD. Further speculation is unwarranted since we believe that the sum of the other evidence is sufficient to definitively conclude that our assignment is correct. Finally, we have noted that the spectrum of species A and B was also observed when diborane was reacted at 20 "C with silica which had been degassed at high temperatures (800-lo00 "C), under circumstances where there was an insignificant reaction with surface OH groups (e.g., without heating the sample at 100 "C). However, a strong SiH band at 2283 cm-' was also generated and we conclude that diborane reads with the reactive siloxane sites which are created by the thermal treatment (the 908- and 888cm-' bands)16J7as follows: Si--Si B2H6 SiOB2H, SiH (8)
+
-
+
A similar reaction occurs with BF3 only we were not able to observe the low-frequency E S i F mode.23
Acknowledgment. We are grateful to N.S.E.R.C. for financial support and for a postgraduate scholarship (R.A.M.). Registry No. B2Hs, 19287-45-7; SOz, 7631-86-9. (23)Morrow, B. A.; Devi, A. J. Chem. SOC.,Faraday Trans. I 1972, 68, 403.
Spontaneous Pulsing in a Porous Membrane Covered with a Langmuir-Blodgett Film of Dioleoyllecithin Separating Equimolar NaCl and KCl Aqueous Solutions Toshio Ishii,t Yumiko Kuroda,?Teruyuki Omochi,t and Kenichi Yoshikawa** School of Dental Medicine, Tsurumi University, 2-1 -3 Tsurumi, Tsurumi-ku, Yokohama 230, Japan, and College of General Education, University of Tokushima, Minami- josanjima, Tokushima 770, Japan Received December 30, 1985 The electrical potential across a Langmuir-Blodgett film of dioleoyllecithin deposited onto a fine-pore membrane, imposed between equimolar aqueous solutions of NaCl and KCl, was studied. It was found that this system showed rhythmic and sustained pulsing or oscillations of electrical potential between the two aqueous solutions. These oscillations were attributed to the change of permeability of .Na+ and K+ across the membrane, which originated from the phase transition of dioleoyllecithinmolecules. The oscillatory phenomenon reported herein is interesting because in biological nervous membranes a difference between the concentrations of Na+ and K+ across the membranes is essential for excitability.
Introduction Excitability is one of the most important properties of cell membranes. Though there is much literature on electrical phenomena accompanying electrical excitation in biological membranes, the physicochemical mechanisms t Taurumi
University. *Universityof Tokuahima.
of these phenomena are not yet clearly understood. For a better understanding of the mechanism of biological excitation, various types of artificial membranes with excitability have been investigated. Most of the artificial membranes reported were excitable under an external force, such as pressure, voltage, or electrical current. On the other hand, in excitable biomembranes, it is wellknown that the difference in the compositions of electrolytes, especially potassium and sodium ions, across the 1986 American Chemical Society
Zshii et al.
320 Langmuir, Vol. 2, No. 3, 1986 20°C -’
Figure 1. Diagram of the experimental appardus: (a) millivoltmeter for the measurement of self-oscillationof the electrical potential; (b) 1.0M KCl aqueous solution (55 mL); (c) 1.0 M NaCl aqueous solution (55 mL); (d)membrane; (e) rubber gasket having a bore of 12-mm diameter.
membrane is essentially important.’ In other words, any external force is not necessary to induce the excitatory phenomena in biomembranes. Investigations on “selfexcitable” artificial membranes are thus important in understanding the mechanism of excitation in biological systems. However, there are very few reports on this phenomenon in artificial membranes.2* Recently we showed that rhythmic and sustained oscillations of electrical potential occurred spontaneously across a fine-pore membrane doped with glycerol cy-mon ~ o l e a t e ,glycerol ~ trioleate,8 or sorbitan monooleate (Span-80)9and separating bathing solutions of equimolar NaCl and KC1 in the absence of any external stimulus such as voltage, electrical current, hydrostatic pressure, or osmotic pressure. As an extension of these studies, in the present paper we report that spontaneous firing occurred in a Langmuir-Blogett (L-B) film of dioleoyllecithin deposited onto a porous membrane when it was imposed between aqueous solutions of equimolar NaCl and KC1 solutions. This result is quite important because phospholipids, including dioleoyllecithin, are the main constituent of biomembranes.
Experimental Section L-a-Dioleoyllecithinwas available from Sigma Chemical Co., St. Louis, MO. Porous triacetylcellulose filter paper (FM-22)of 0.22-pm nominal pore size and 135-pm thickness was obtained from Fuji Photo Film, Tokyo. Through the measurement of a surface-pressurecharacteristic, it was found that dioleoyllecithin forms a very stable monomolecular layer on a distilled aqueous solution (pH 6.2) and that its collapsing pressure is around 0.050 N/m. The monomolecular layer was deposited on a porous membrane (FM-22)by the usual L-B technique developed by Kuhn et al.1° A pair of porous membranes were immersed vertically in the aqueous subphase after spreading the monolayer and then lifted upward at an appropriate rate, ca. 1cm/min, and an adequate surface pressure, 0.020 N/m. For the first dipping Y-type film was deposited onto the membrane and the succeeding (1) Hodgkin, A. L.; Huxley, A. F. J. Physiol. 1952,116,449,473,497. Hodgkin, A. L.; Huxley, A. F. J. Physiol. 1952,117,500. (2)Kobatake, Y. Adu. Chem. Phys. 1975,29,319. (3)Arisawa, J.; Furukawa, T. J. Membr. Sci. 1977,2,303. Jpn. 1981,50,1343. (4)Toko, K.; Nitta, J.; Yamafuji, K. J. Phys. SOC. (5)Pant, H. C.; Rosenberg, B. Biochim. Biophys. Acta 1971,225,379. (6)von Klitzing, L.;Daber, M.; Bergeder, H. D. Biophysik (Berlin) 1973,9,166. (7)Yoshikawa, K.; Sakabe, K.; Mataubara, Y.; Ota,T. Biophys. Chem. 1984,20, 107. (8)Ishii, T.; Kuroda, Y.; Yoshikawa, K.; Sakabe, K.; Mataubara, Y.; Iriyama, K. Biochem. Biophys. Res. Commun. 1984, 123,792. (9)Yoshikawa, K.; Sakabe, K.; Mataubara, Y.; Ota,T. Biophys. Chem. 1985,21,33. (10)Kuhn, H.; Mobius, D.; Bticher, H. Physical Methods of Chemistry; Wiley: New York, 1972;Vol. I, Part 3b,pp 577-702.
-I__x
10 min
Figure 2. Oscillation of electrical potential between the aqueous phases. On the recording of the electrical potential, an upward change denotes an increase in the positive potential in the KC1 solution. Oscillation continued for more than 24 h.
lifting cycles provided Z-type films with the deposition ratio of 1.0. After this procedure, the pair of the membranes were separated and were used for the measurement. Temporal change of the electrical potential was measured with an apparatus shown schematicallyin Figure 1. The voltage across the membrane was monitored with a recorder FBR 252A (TOA Electronics Ltd.) connected by platinum electrodes. It was found that the surface of the film became “hydrophilic”immediately after immersion of the deposited filter paper into an aqueous solution. This suggests that the phospholipid molecules in the outer layers changed their orientation of the hydrophilic moiety to the opposite direction though flip-flop motion, when the surface was contacted with the aqueous solution.
Results and Discussion Figure 2 exemplifies spontaneous firing of the electrical potential with various temperatures for the same film, where an upward change indicates an increase in the positive potential in the side of KC1 solution. A layer of the bilayer Y-film in the first operation and 19 layers of the Z-films, single layers, in the successive operations of dioleoyllecithin were deposited onto the one side of the porous membrane in this experiment. Similar electrical oscillations were also observed for the deposit membranes with 3-21 layers. The oscillations started abruptly ca. 10 min after the filter paper was placed between the solutions of 1 M NaCl and 1M KC1. When the concentrations of NaCl and KC1 were decreased, no oscillations were observed. No pulsing occurred for the film of dipalmitoyllecithin under similar experimental condition. Figure 2 clearly shows that oscillations were generated between 35 and 20 “C. Below 20 “C oscillations ceased (data are not shown). The oscillatory phenomena were found to be reproducible. These oscillations could be attributed to periodic gating, or closing, of “channels” in the membrane. It should be noted that these channels were formed by dioleoyllecithin molecules deposited onto the porous membranes in the absence of any peptide or protein. The unique property of the membrane, the spontaneous pulsing, is most probably due to a kind of the phase transitions of the lipid molecules on the porous membrane. In relation to this, we recently found that lipid molecules having the oleoyl moiety exhibit unique phase transitions caused by the changes of water content, inorganic ions, and amine ~ a p o r . ~ - ~ItJ ~is also interesting to note that spontaneous firing could occur for the membranes made of various lipid molecules having the oleoyl The oleoyl moiety is characterized by its cis double bond on its carbon skeleton. This cis double bond should play an (11)Yoshikawa, K.;Matsubara, Y. Langmuir 1985,1, 230.
Langmuir 1986,2,321-329 essential role for the unique character in the phase-tran-
sit ion.'^^ In the present study we showed that an Na+/K+ concentration gradient could cause excitation in a L-B film of dioleoyllecithin. This result is quite important in relation to the mechanism of excitation of biological systems at the molecular level and may also be significant in the development of a “molecular electronic device”. Further experimental and theoretical studies are awaited to make clear the interesting properties of the deposited membrane herein reported.
Note Added in Proof. Quite recently, we have observed that fluctuation of membrane potential and electrical
321
current was generated in a pipette-clamp bilayer membrane of dioleoyllecithin in the absence of any “channel” protein. It was found that the magnitude of the change of the conductivity through the membrane was ca. 60 pS, which is the same order as the values reported for the membranes embedded with so-called “channel” proteins. Details of this experiment will be published in a separate manuscript.
Acknowledgment. This work was partly supported by Grants-in-aid for Scientific Research to K.Y. (No. 59212029 and 59219017) from the Ministry of Education, Science and Culture of Japan, and by Nissan Science Foundation. Registry No. NaCl, 7647-14-5; KC1, 7447-40-7.
Possible Mechanism for the Origin of Lamellar Liquid Crystalline Phases of Low Surfactant Content and Their Breakup To Form Isotropic Phases Clarence A. Miller* and Olina Ghosh Department of Chemical Engineering, Rice University, Houston, Texas 77251 Received July 30, 1985. In Final Form: January 6, 1986 It is shown that the observed existence of lamellar liquid crystals in many anionic surfactantalcohol-sodium chloride-water systems containing less than 10% amphiphilic material cannot be explained in terms of DLVO theory if the surfactant bilayers are assumed to have infinite lateral extent. But if the bilayers are taken to exist as plates of appreciable but finite size, a simple model shows that the additional thermal motion increases the mean equilibrium spacing between plates and permits dilute lamellar phases to occur. This model further allows the transition of such phases to dilute isotropic phases containing platelike micelles to be explained in a straightforward way as the result of a decrease in plate size. Such transitions have been observed to occur in both anionic and nonionic surfactant systems as the surfactant becomes less hydrophilic. The mechanism for the necessary decrease in plate size is discussed and a simple model presented.
Introduction The existence of lamellar liquid crystals in many surfactant-water and surfactant-alcohol-water systems is well-known.’ In most cases the liquid crystals contain at least 30% amphiphilic material by weight and are not readily deformed owing to their high viscosities and/or elastic moduli. Such materials are useful for some purposes, e.g., for stabilizing emulsions,2 but they must be avoided in other applications such as enhanced oil recovery where they would prevent virtually all flow through the oil-bearing formation. A few years ago lamellar phases of much lower surfactant content and viscosity were found in our laboratory in certain anionic surfactant-alcohol-brine systems of interest for enhanced oil r e ~ o v e r y . ~Indeed, -~ their compositions were in the range of interest for injected fluids in such processes. Apparently these liquid crystals and their dispersions in brine had been used without difficulty in numerous laboratory experiments involving flow through (1) Ekwall, P. Adu. Lig. Cryst. 1976, 1, 1. (2) Friberg, S.;Mandell, L.; Larason, M. J. Colloid Interface Sci. 1969, 29. 155. (3) Benton, W. J.; Fort, T., Jr.; Miller, C. A. Annu. Tech. Conf.-Soc. Petrol. Eng. 1978,36th, SPE 7579. (4)Miller, C.A.; Mukherjee, S.;Benton, W. J.; Natoli, J.; Qutubuddin, S.; Fort, T., Jr. In ‘Interfacial Phenomena in Enhanced Oil Recovery”; Wasan, D. T., Payatakes, A., Eds.; American Institute of Chemical Engineers: New York, 1982; AIChE Symp. Ser. No. 212, pp 28. (5) Benton, W. J.; Miller, C. A. J. Phys. Chem. 1983,87, 4981. ~
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0743-7463/86/2402-0321$01.50/0
sandstone cores and even in some field tests. Typical formulations contained only about 10% of an anionic surfactant-alcohol mixture, the remainder being an aqueous electrolyte solution. The simplest system having such dilute lamellar phases is the aerosol OT (A0T)-water-NaC1 system. Ita phase behavior was studied by Fontell: and his basic findings have been verified in our laboratory.’ As Figure 1 shows, a lamellar liquid crystalline phase exists at surfactant concentrations as low as 5 w t % when NaCl concentration is about 1.5 w t %. One question that may be raised regarding such dilute lamellar structures is why they do not separate into a more concentrated phase and excess brine. After all, even if the bilayers are as thin as 1 nm, the brine layers in a dilute phase containing about 10% amphiphilic material must be about 9 nm thick. If instead the bilayers are 1.5 nm thick, the thickness of the brine layers is about 13.5 nm. Since both 9 and 13.5 nm are more than an order of magnitude greater than the Debye length in these systems, which have sodium chloride concentrations of at least 1% by weight (0.17 M), one might initially expect from DLVO theory that separation into a concentrated lamellar phase and brine would take place with brine layers in the former (6).Fontell, K.; In “Colloidal Dispersions and Micellar Behavior”; American Chemical Society: Washington, DC, 1975; ACS Symp. Ser. 9, p 270. (7) Ghosh, 0. Ph.D. Thesis, Rice University, Houston, TX, 1985.
1986 American Chemical Society
