Langmuir 1986,2, 315-319
315
is insignificant (equal to the ratios of their cmc’s, Le., of pentyl or hexyl sulfates suggesting it to be a protein side chain. The linear relationship obtained between anesthetic the order to 1). Pressure Reversal of Anesthesia. It is well-known potency and the ability to strengthen hydrophobic bonding that hydrostatic pressure weakens hydrophobic bonding in 8-lactoglobulin aqueous solution adds support to the in surfactant solutions63and also reverses a n e ~ t h e s i a . ~ ~ ~above conclusion. Further support came from the equivalence of the effect of hydrostatic pressure on anesthetic Kaneshina et al.63measured the effect of pressure on the action and on the ability to weaken hydrophobic bonding CMC of sodium alkyl sulfates. From their results it is found that the pressure required to weaken hydrophobic in sodium pentyl or hexyl sulfates. bonding in sodium hexyl sulfate solution just enough to The basic point raised in this study is that on introcounteract the strengthening of hydrophobic bonding ducing anesthetic agents in the aqueous phase containing free, i.e., unaggregated, hydrophobic moieties of protein produced at clinical concentrations is about 240 atm comside chains at the site of anesthetic action, aggregation of pared with the observed value of about 100-150 atm to reverse ane~thesia.”~ If sodium pentyl sulfate ws assumed hydrophobic moieties occurs causing the folding of the side instead of the hexyl homologue then the pressure required chains. This action is nonspecific and depends only on the concentration of free hydrophobic moieties in the aqueous to counteract the effect produced by anesthetic agents falls phase. Thus it is unlike the “protein-unfolding” theory within the observed range (140 atm). of Ueda et al.le or the protein “specific site” theory of Summary and Conclusions Richards et al.17 According to the point of view presented in this study the lipid bilayer does not play a direct role We have used the micellization process as a model for in anesthetic action. the action of anesthetic agents a t the site in nerve cell Finally, the new hypothesis may be looked upon as an membrane and arrived a t the conclusion that the hydroextension of the aqueous phase theories of Paulingm and phobic moiety involved during anesthetic action is of Millerz1to which the concept of “strengthening of hyequivalent to the effective hydrophobic group in sodium drophobic bonding“ has been introduced. (63) Kaneshina, S.; Tanaka, M.; Tomido, T.; and Matuura, R. J. Colloid Interface Sci. 1974, 48,4550.
Acknowledgment. I thank Dr. Karol J. Mysels for his help and interest in this work.
Infrared Spectrum of Diborane Adsorbed on Silica B. A. Morrow*? and Richard A. McFarlane Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada KIN 9B4 Received December 19, 1985 Diborane at low pressures (10-20 torr) does not react to a significant extent with surface OH groups on silica at 20 O C . However, there is some reaction at slightly elevated temperatures (such as that due to the heat from the infrared source in a dispersive spectrometer), and all OH groups react rapidly if the silica/BzH, mixture is heated at 100 “C. Regardless of the extent of reaction, three dissociativelyadsorbed surface species, called A, B, and C, are formed. Species A, =SiOB2H5,is formed from reaction with isolated =SOH groups, or with siloxane bridges, and is stable in the presence of gas-phase diborane. Upon evacuation this converts to species B, =SiOBHz, via the equilibrium =SiOB2H5 =SiOBH2 + ‘/ZBzH6, and species A can be completely restored upon readdition of diborane. A third species, C, results from the reaction of diborane with adjacent H-bonded hydroxyls via the reaction 2SiOH + ’/&& (Si0)2BH+ 2Hz and is more abundant when the initial surface OH density is greater. Diborane also reacts with reactive siloxane sites which are created as a result of high-temperature degassing to yield A (or B after evacuation) plus SiH. A definitive spectroscopic assignment was only possible by using “B2H6, “BZD6, ‘OB2H3D3,and oxygen-18 exchanged silica.
-
Diborane was one of the first probe molecules to be used in order to determine the density of hydroxyl groups on silica surfaces.’-5 It was assummed that a determination of the ratio of Hz evolved to BzH6 consumed (called R hereafter) could be used to determine not only the number of surface OH groups but that one could also determine the relative proportions of isolated =SiOH groups and “Paired” hydroxyl groups [either as H bonded Pairs of adjacent S 3 i O H groups or geminal =Si(OH)2 groups]. The reaction with isolated groups was assumed to be SiOH
+ ‘/ZBZH6
-
SiOBH2 + H2& = 2
(1)
whereas with “paired” silanols the reaction would be either of Member of the Ottawa-Carleton Chemistry Institute.
0743-7463/86/2402-0315$01.50/0
=SioH ESiOH
+
’ \Si/oH
-=Si-0
9262H6
‘/282H6
\OH
=S,-O’
\E-H
\S/O\BH / \o/
+
2H,,
+
2H2, R . 4
R = 4 (2)
(3)
It further assumed that even higher values for R might arise from highly hydroxylated surfaces where three silanols might react to produce a (=Si0)3B surface species, yielding 3H2 per 1/2BzH6. (1) Weiss, H. G.; Shapiro, I. J. Am. Chem. SOC.1963,75,1221; J.Phys. them. 1953.57. . .. . -. .- , 219. (2) W e b , H. G.; Shapiro, I.; Knight, J. A. J.Am. Chem. SOC. 1969,81, 1823. 2
( 3 ) Naccache, C.; Fancoise-Rossetti,J.; Imelik, B. Bull. SOC.Chim. Fr. 1969,404. (4) Naccache, C.; Imelik, B. C. R . Hebd. Seances Acad. Sci. 1960,250, 2019; Bull. SOC.Chim. Fr. 1961, 553. (5) Fripiat, J. J.; Van Tongelen, M. J. Catal. 1966, 5, 158.
0 1986 American Chemical Society
Morrow and McFarlane
316 Langmuir, Vol. 2, No. 3, 1986 Infrared spectroscopy has been used several times in the past 23 years to study this reactionw and the results have tended to show that the reaction is not as simple as that envisaged above. Additional surface species such as the following have been postulated from time to time, on the basis of spectroscopic evidence:
-Si\ -si'
-
--.Si\ O:B,H,
=Si
=Si-0 O ,B:H, -si-0'
=S iOB,H,
\B/H B>o \H
fSiBHBH,
$1 0
0
The objective of the present work has been to provide a definitive description of the reaction of diborane with silica. We will show that the type of IR spectrometer used is an important variable because of the heat effect of the IR beam, that some unusual vibrational perturbations exist that can only be unraveled by use of the isotopic substitution (H/D, l0B/l1B, lsO/lsO), that the tendency of BH, species to form hydride bridges is important, that the degree of hydroxylation of the silica before reaction influences the types of surface species formed, and that diborane can also significantly react with non-hydroxylic sites. Our vibrational data have been obtained by using three IR spectrometers, including FTIR. Details of previous work will be discussed later. Experimental Section Aerosil silica (Cab-0-Sil,HS-5 grade) having a B.E.T. surface area (N,) of 340 m2 g-' was used. Discs, 25-mm diameter, containing from 10 to 40 mg cm-, were pressed in a stainless steel die using low pressures of about lo7 Pa and were mounted in a standard IR cell of 300-mL v ~ l u m e . ~Infrared spectra were recorded using either a presample chopped Perkin-Elmer 13G instrument, a Perkin-Elmer 283 postsample chopped instrument which was computer controlled, or a Bomem DA3-02 FTIR. Spectral differences between instruments could be related to beam temperature at the sample, the order of decreasing temperature being 283, 13G, FTIR (see Results). Subtraction of the background spectrum of the silica or of gas-phase diborane was done by computer with the FTIR and 283 instruments. This was also effected using the 13G instrument by placing an equivalent quantity of diborane or silica in the reference beam. Natural abundance diborane (designated "B2H6)was prepared by the reaction of I, or SnC14with NaBH410or by the action of LiAlH, with BF3in ether or diglyme." The boron-10-labeled compound was prepared by decomposing the CaF2I0BF3(96% O ' B) complex to yield BF, and then reducing the resultant BF,/ether solution with LiAlH, (or LiAlD, for the deuterated '"B derivative). Oxygen-18 exchanged silicas were prepared by repeated exchange with 99% H2180 at 420 "C until greater than 90% of the isolated =SilGOH groups were converted to ei'80H. Results Our experience in studying adsorption on black silicasupported metal samples by IR spectroscopy has lead us to conclude that, depending on the type of spectrometer, sample heating in vacuum due to the IR source can be severe.12 These samples in vacuum in our 283 spectrometer, nominally a t ambient temperatures, may be at 95 "C (6) Mathieu, M. V.; Imelik, B. J. Chim. Phys. 1962,59, 1189. (7) Baverez, M.; Bastick, J. Bull. SOC.Chim. Fr. 1964, 3226. (8) Maehchenko, A. I. Kinet. Katal. 1974, 15, 903. (9) Morrow, B. A.; Ramamurthy, P. J. Phys. Chem. 1973, 77, 3052. (10) Freeguard, G. F.; Long, L. H. Chem. Ind. (London) 1965, 471.
(11) Shapiro, I.; Weiss, H. G.; Schmich, M.; Skolnik, S.;Smith, G. B. L. J. Am. Chem. SOC.1952, 74,901. (12) Morrow, B. A.; Moran, L. E. J. Catal. 1980, 62, 294.
r
2700
:
:
:
:
-
;
2200
:
:
cm"
: -:
t 1700
:
:
:
:
4 1200
Figure 1. Infrared spectra of "B2& adsorbed on silica which had
previously been degassed at 500 "C. (a) After admission of 5 torr of and heating for 30 min at 100 "C (the gas-phase contribution and silica background have been subtracted);(b) after evacuation of (a) for 30 min at 100 "C. in the IR beam, but in the 13G spectrometer they are near 50 "C, and in the FTIR spectrometer the temperature is near ambient. We assume that similar relative differences apply to "white" silica samples but that the differences are smaller. Volkov et al. reported similar effects for silica ~amp1es.l~We have no means of measuring this temperature for silica discs in vacuum, and the temperature Merence is presumably smaller still when a gas is present. With this relative difference in mind we report that if diborane a t a pressure of 10-20 torr is left in contact for 30 min with a silica sample that had been degassed in vacuum at 500 "C there is no reaction with the SiOH groups in the FTIR,about 2% of them react using the 13G spectrometer, and about 10% react using the 283 spectrometer. We conclude that B,H6 at this pressure does not react appreciably with the OH groups on aerosil at ambient temperatures. However, as will be discussed below, there was a reaction with non-hydroxylic sites if the silica had been activated a t temperatures above 400 "C. Regardless of the spectrometer used or the temperature of activation, all accessible hydroxyl groups on the above silica completely reacted within a minute when in contact with diborane at 100 "C. Figure l a shows the IR spectrum between 2700 and 1300 cm-' (silica discs are opaque to lower wavenumber) recorded using nB2H6with "B,H6 (in excess) still present in the cell. In addition to bands in the 2400-2700-cm-' region due to BH stretching modes, there are bands near 2100, 1950, and 1600 cm-' that are associated with BHB bridge stretching modes and a band a t 1336 cm-' that can be assigned to a SiO-B stretching mode (see Discussion).14 The spectrum changed upon evacuation of gaseous diborane, the rate of change depending on the beam temperature of the spectrometer used. The final effect, however, was as is shown in Figure l b after evacuation at 100 "C for 30 min, i.e., complete disappearances of bands due to bridge-bonded BHR species (2100-1500 cm-') and new bands in the 2500-cm-' spectral region and a t 1356 (boron-ll) and 1386 (boron-10) cm-'. However, readdition of diborane completely restored the original spectrum in Figure l a and subsequent reevacuation again generated
-
(13) Volkov, A. V.; Kiselev, A. V.; Lygin, V. I. Russ. J. Phys. Chem. 1974, 48, 703. (14) Duncan, J. L.; McKean, D. C.; Torto, I.; Nivellini, G. D. J. M o l . Spectrosc. 1981, 85, 16 and references therein.
Langmuir, Vol. 2, No. 3, 1986 317
Infrared Spectrum of Adsorbed Diborane c *
Table I. Observed Boron-Hydrogen (Deuterium) Stretching Frequencies (cm-') species
'OBZHe
"BzH,
'OBzDB
"BzDG
l0BzH3D3
A
2612 2574 2522 2583 2484' 2459' 2602
2597 2565 2516 2565 2479" 2456" 2585
1986 1908 1860 1956 1831
1966 1890 1848 1940 1821
2565 1912
1950
1936
B
C
$1
'Midway between the two indicated frequencies for oxygen-18 exchanged silica.
a:
0
I
1
2500
2700
cm-'
Figure 2. Infrared spectra in the BH stretching region for adsorbed "%&. (a) Same conditions as for Figure la; (b)evacuation for 1 min at 20 "C; (c) evacuation at 100 O C for 5 min; (d) evacuation at 100 "C for 30 min. Bands due to species A-C are indicated (see text for bands marked B* and A*). The dashed portion in (d) shows the spectral changes which were observed when the silica OH groups had been exchanged with oxygen-18 prior to reaction.
A
C
2700
2563 2488 1890 2602 1950
,
1
2500
2600
2400
cm-'
Figure 3. (a) Difference spectrum Figure 2b minus Figure 2c. (b) A similar difference spectrum but for adsorbed %$,. that shown in Figure l b , and this operation could be repeated indefinitely. Therefore, in the presence of gaseous diborane we have one or more adsorbed species (state I) and after evacuation we have a different state (11) and there is evidently an equilibrium between them -BO&
I'
+B&
I1
(4)
and the adsorbed species in state I1 do not contain any
BHB bridges. Figure 2 shows a series of spectra in the BH stretching region for "B2H6 adsorbed on 500 "C degassed silica starting with I (Figure 2a), then with both states (Figure 2b,c), and finally after only state I1 is present (Figure 2d). Figure 3a shows the difference spectrum of Figure 2b Figure 2c where the peaks going "up" are those of a species in state I which is being depleted (labeled A in Figures 2 and 3) and those going "down" are of a species in I1 being created (labeled B). (Bands B* and A* are almost coin-
cident in Figure 2, and since B* is more intense than A* this shows up only as a net decrease in intensity in Figure 3a.) In Figure 2 there is a single band C which does not change in intensity during the I to I1 conversion and does not appear in Figure 3a. The difference spectrum was identical between any pair of spectra in Figure 2 with respect to relative intensities observed, although the absolute intensities of course varied. Therefore, we conclude that there are two adsorbed species in state I, called A and C, and two in state 11, B and C, and that the equilibrium depicted in (4) in fact involves a single species A and B. The equilibrium between A and B was established after reaction with diborane regardless of the extent of reaction with OH groups or of the temperature used to degas the silica. However, the intensity of the band due to species C was very much greater for silica which had been degassed a t 150 "C, whereas it was barely detectable for silicas degassed a t 1000 "C. Therefore, the formation of species C is relatively more favored when there are a larger number of surface SiOH groups initially.15 For silica degassed a t 150 "C, an additional band assignable to a B-0 stretching mode of species C was observed at 1380 cm-' for the boron-ll isotope and near 1400 cm-' for boron-10. The v(BH) spectra for "BzH6were more complicated due to the 4:l mixture of l1B/loB isotopes. The high-wavenumber bands showed a c!ear loB/l1B isotopic shift, whereas only a small shift was observed for the low-frequency bands. The different spectrum (A - B) for "B2H6 is shown in Figure 3b and Table I lists all of the BH stretching frequencies which have been attributed to species A-C for both boron-10 and boron-11. Some experiments were also carried out by using "B2D6 and 'OB,& with a predeuterated silica. Figure 4 shows a series of spectra in the BD stretching region recorded with the 13G spectrometer for adsorbed 'O&D, on a silica which had been degassed a t 1000 "C prior to adsorption. The band due to A-C are indicated and the frequencies for both isotopes are listed in Table I. Finally, we observed a very weak band at 2283 cm-' following reaction with silicas that had been degassed a t 150-400 "C. When the silica was degassed at greater than 400 "C so as to lead to the appearance of bands at 908 and 888 cm-l due to reactive siloxane sites,16J7then the 2283cm-I band was much more intense after reaction with diborane. This band showed no loB/l1Bisotopic shift, but with 10B2D6it shifted to 1662 cm-' and is undoubtedly due to an SiH/D mode.18 (15) Kiselev, A. V.; Lygin, V. I. "Infrared Spectra of Surface Compounds"; Wiley: New York, 1975. (16)Morrow, B.A.;Cody, 1. A. J. Phys. Chem. 1976,80, 1995,1998. (17)Morrow, B.A.; Cody, I. A,; Lee, L. S. M. J. Phys. Chem. 1976,80, 2761. -.
(18)Lucovsky, G.J . Vac. Sci. Technol. 1979, 16, 1225. Hartwig, C. M. J. Chem. Phys. 1977,66, 227.
Morrow and McFarlane
318 Langmuir, Vol. 2, No. 3, 1986
I '
A I
I
-
C
2500
2600
ZOO0
1800
cm-'
Figure 5. Bar graph representation of the O ' BH and O ' BD stretching modes for species A-C. "he dashed lines show the new band positions for mixed H/D species. loot
'
I
2000
cm-'
1
1800
Figure 4. Infrared spectra of "BZD6 adsorbed on a silica (40 mg cm-2) which had been degassed at lo00 O C . (a) In the presence of 10 torr of O ' B& at 20 "c;(b) after evacuation at 20 for 5 h; (c) after evacuation at 20 O C for 24 h. The % T scale refers to c, the others have been displaced. Bands due to species A-C are indicated.
Discussion We have shown in this work that diborane barely reacts a t a pressure of about 10 torr with silanol groups a t ambient temperatures, but with increasing IR beam temperature, the extent of reaction increases. This effect accounts for most of the discrepancies regarding previously published work.68 Our spectra closely resemble those of Baverez and Bastik' although our interpretation differs considerably. Additional unique features of the present work include the use of boron-10, and deuterium, and the recording of spectra of adsorbed species in the presence of gas-phase diborane. A schematic bar graph representation of the u('@BH) and u('OBD) spectra for species A-C is shown in Figure 5. Three species were also found by Baverez and Bastik' using "B2HG and they assigned A to diborane which was coordinatively bonded to oxygen atoms of surface siloxane groups; B was similarly attributed to coordinated BH3, whereas C was attributed to the dissociative reaction of diborane with two surface hydroxyl groups as follows:
-
2SiOH + B2H8
( d 3 i 0 ) 2 B H+ 2H2 C
(5)
The interconversion between A and B was assumed to be attributable to the equilibrium Siz0:B2H6+ SizO:BH, -t'/&H6 A B
(6)
Thus,evacuation of excess gas-phase diborane would cause A to revert to B, and species A would be reestablished upon readdition of gas-phase BzH6. This interpretation seems incorrect orl two grounds. There are few paired hydroxyls after degassing a t high temperatures'"" and it therefore would be improbable that the only mode of dissociative chemisorption would be by reaction 5. Further, the infrared bands attributed to the BH stretching modes of coordinated borane19 are found near 2300 cm-l, not
2500-2600 cm-l. On the other hand, we do agree with the assignment for species C and we note that the intensity of the infrared bands due to C were greater when the hydroxyl density was higher, as expected for a reaction involving pairs of OH groups. The BO2 stretching mode (1380-1400 cm-l) is also very close to that reported for the cyclic boroxine.20 In view of the above, we conclude that species A results from the direct dissociative chemisorption of diborane with single OH groups as follows:
=SiOH
+
BH ,,
--
H
ZsiOk
h \BH,
+
H,
\H/ A
Species A would be expected to have three terminal BH or BD stretching modes, as well as to have several modes due to BHzBbridging vibrations near 2100,1950, and 1550 cm-' as ~ b s e r v e d . ' ~ ~Species ~ ' ~ ~ ~A is only favored when gas-phase diborane is present and upon evacuation, the following equilibrium shifts to the right. H
-Si06
bH\ H B ,Hz
-
-
ESIOBHZ
+
1/,BzH,(g)
(7)
B
A
Although the spectrum of species B shows the expected symmetric and antisymmetric u(BD2)bands there are three bands in the u(BHJ region. It is the latter observation that has posed problems in previous interpretations"* and we will show that our assignment is, nonetheless, probably correct. In order to confirm the above assignments we either added 'OB& to adsorbed "B&6 (and the reverse), or we adsorbed various 'OB2H6/"B2D6 mixtures and we were clearly able to observe the new bands due to partial H/D exchange (Figure 5). As expected, only one new band was (19) Berschied, J. R.; Purcell, K. F. Znorg. Chem. 1972,11,930. Cluff, C. L.; Taylor, R. C. Nature (London) 1958, 182, 390. Lehmann, W.; Weiss, H. G.; Shapiro, J. J. Phys. Chem. 1957, 61, 1222. Taylor, R. C.
J . Phys. Chem. 1957,26, 1131. Watari, F. Inorg. Chem. 1982,21, 1442. Jones, L. H.; Taylor, R. C.; Paine, R. T. J . Chem. Phys. 1979, 70,749. (20) Watson, S.K.; Porter, R. F. J. Phys. Chem. 1964.68, 1443. (21) Nibler, J. W. J.Am. Chem. SOC.1972, 94, 3349. (22) Lehmann, W. J.; Shapiro, I. Spectrochim. Acta 1961, 17, 396.
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%2H6/10B2D6 mixture, three SiOB stretching bands were observed a t 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-18to 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 a t 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 a t 20 "C with silica which had been degassed a t high temperatures (800-lo00 "C), under circumstances where there was an insignificant reaction with surface OH groups (e.g., without heating the sample a t 100 "C). However, a strong SiH band a t 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 dioleoyllecithin molecules. 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
