Langmuir 1988,4, 210-215
210
Effect of Chemisorbed Water on the Two-Dimensional Condensation of Water and Argon on SrF2 Yasushige Kuroda,? Shigeharu Kittaka,' Kazuhisa Miura,$ and T. Morimoto*t Department of Chemistry, Faculty of Science, Okayama University, Okayama 700, Japan, Department of Chemistry, Okayama College of Science, Ridaicho, Okayama 700, Japan, and Department of General Education, Tsuyama National College of Technology, Tsuyama 708, Japan Received May 28, 1987. I n Final Form: August 20, 1987 The surface properties of SrF, that has strongly adsorbed HzO were investigated by measurement of the adsorption isotherms of Ar and HzO and of the surface HzO content and by IR spectra. The surface of the sample was found to be uniform for the adsorption of Ar and H20; that is, a two-dimensional condensation of each adsorbate occurs on it. Pretreatment of the sample at higher temperatures and successive rehydration largely affect the surface homogeneity as determined by the adsorption isotherms of Ar and HzO. When the surface is pretreated at elevated temperatures, the amount of chemisorbed HzO decreases, which results in an increase in homogeneous area for Ar adsorption and a decrease in homogeneous area for HzO adsorption. The successive rehydration of the surface through exposure to HzO vapor recovers the homogeneous area for HzO adsorption, accompanied by the reduction of the homogeneous area for Ar adsorption. The well-developed (100) face of the SrF2crystals was found to be responsible for these phenomena. Recently, it has been discovered that the two-dimensional (2D)condensation of HzO occurs on a limited number of metal oxides, Zn0,1-3 SnOz,P-aand C I - ~ O ~at, ~ - ~ a low relative pressure and that this phenomenon takes place on the hydroxylated surface of the solids. Since the occurrence of this phenomenon has been considered to be due to a special structure of the underlying surface hydroxyls, various approaches have been attempted to clarify the structure of hydroxyls.10-12 The same phenomenon appears also on another group of solids, NaF13-16 and CaF2,16-19where the latter has chemisorbed HzO but the former does not. It is well-known that the 2D condensation appears also on the adsorption of inert gases like Ar and CHI on such solids as graphite and NaC1.20s21 In a previous paper,18 it was reported that a step appears on the adsorption isotherms of both HzO and Ar on CaF, and that the homogeneous surface for H20 adsorption differs from that for Ar adsorption. The present study has been undertaken to measure the adsorption isotherms of HzO and Ar on SrF,, which has the same crystal structure as CaF2, and to investigate the effect of chemisorbed H20 upon the surface homogeneity for the adsorption of both adsorbates, in comparison with the case of CaF,.
Experimental Section Materials. The SrFzsample used was prepared by mixing a 0.5 M Sr(N03), solution with a 1 M NHIF solution at room temperature, where the latter was used in 10% excess. The precipitate formed was washed sufficiently with H20and then stored in a desiccator. Redistilled H20 was used as the adsorbate, and the gases which might be dissolved in H20were removed by "freeze-evaporate-thaw" cycles. The N2 gas was prepared by evaporation from liquid N2,and highly pure Ar was supplied from Teikoku Sanso Go. Measurement of Adsorption Isotherms of H 2 0 and Ar. The sample was first degassed in a vacuum of Torr for 4 h at 25,150,500, and 600"C, respectively, and then the adsorption isotherm of H20was measured on these samples at 10 "C. After the measurement of the fist H20adsorptionisotherm, the sample was exposed to saturated H20vapor at 25 "C for 12 h to ensure Okayama University.
* Okayama College of Science.
Tsuyama National College of Technology. 0743-7463/88/2404-0210$01.50/0
surface hydration, followed by degassing at 25 "C to remove physisorbed HzO, and then the second adsorption isotherm of H20was measured at the same temperature as before. The second adsorption measurements were also carried out at 0 and 20 "C for the calculation of the isosteric heat of adsorption of H20. The adsorption isotherm of Ar was measured at -196 "C before every measurement of the HzO adsorption isotherm. The specific surface area of the sample was determined by applying the BET equation to the Nz adsorption data measured at -196 "C. For the measurement of adsorption isotherms, a conventional volumetric adsorption apparatus was used, which was equipped with a MKS Baratron 310-BH diaphragm manometer. Water Content Measurement. The HzOcontent was measured by means of the successive ignition loss method22on the samples which were treated at 25,150,500, and 600 "C, respectively, in vacuo for 4 h, exposed to saturated H20vapor for 12 Torr. h, and then degassed at 25 "C in a vacuum of Measurement of IR Spectra. For the measurement of IR spectra, a self-supporting disk 20 mm in diameter was prepared by compressinga 0.2-g sample under a pressure of 400 kg/cm2. (1) Morimoto, T.; Nagao, M.; Tokuda,F. Bull. Chem. SOC.Jpn. 1965, 41, 1533. (2) Nagao, M. J . Phys. Chem. 1971, 75, 3822. (3) Morimoto, T.; Nagao, M. J. Phys. Chem. 1974, 78, 1116. (4) Kittaka, S.; Kanemoto, S.; Morimoto, T. J . Chem. SOC.Faraday Trans. 1 1978, 74, 676. (5) Morimoto, T.; Yokota, Y.; Kittaka, S. J . Phys. Chem. 1978, 82, 1996. _. ... (6) Kittaka, S.; Moriehige, K.; Fujimoto, T.; Morimoto, T. J . Colloid Interface Sci. 1979, 72, 191.
(7) Kittaka. S.: Nishivama, J.; Morishiae, - K.: Morimoto, T. Colloids Surf. 1981, 3, 51. (8) Morishiee. K.: Kittaka. S.: Morimoto. T. Surf. Sci. 1981. 109. 291. (9) KittakarS.;Morishige,'K.f Nishiyama, J.; Mdrimoto, T. 2. Colloid Interface Sci. 1983, 91, 117. (10) Nagao, M.; Yunoki, K.; Muraishi, H.; Morimoto, T. J . Phys. Chem. 1978,82,1032.
(11) Iwaki, T.; Morimoto, T. Langmuir 1987, 3, 282. (12) Morishige, K.: Kittaka, S.; Morimoto, T. J.Chem. SOC., Faraday Trans. 1 1980,?6, 738. (13) Yung-Fang,Yu Yao J. Colloid Interface Sci. 1968,28, 376. (14) Banaclough, P. B.; Hall, P. G. Surf. Sci. 1982, 120, 223. (15) Morishige, K.; Kittaka, S.; Morimoto, T. Surf. Sci. 1982,120,223. (16) Morimoto,T.; Kadota, T.; Kuroda, Y. J . Colloid Interface Sci. 1985,106, 104. (17) Kuroda, Y.; Sato, H.; Morimoto, T. J. Colloid Interface Sci. 1985, 108, 341. (18) Kuroda, Y. J. Chem. SOC.,Faraday Trans. 1 1985,81, 757. (19) Kuroda, Y.; Takenaka, T.; Umemura, J.; Kittaka, S.; Morishige, K.; Morimoto, T. Langmuir 1985,1, 679. (20) Thomy, A.; Duval, X.; Regnier, J. Surf. Sci. Rep. 1981, 1, 1. (21) Ross, S.; Olivier, J. P. On Physical Adsorption; Interscience: New York, 1964. (22) Morimoto, T.; Naono, H. Bull. Chem. SOC.Jpn. 1973,46, 2000.
0 1988 American Chemical Society
20 Condensation of H 2 0 and Ar on SrF,
Langmuir, Vol. 4, No. 1, 1988 211
0.4
0.2
0
0.4 N
'E ,-.. CL
5
0.2
n
E
0.5
1.0
Coverage ( 0 )
Figure 2. Isosteric heat of adsorption of HzO, qat,as a function of coverage 0 on SrF2pretreated at various temperatures: 0 , 2 5 , 0 , 150; (D,500; 0,600 "C. The broken line represents the heat of liquefaction of H20.
0.4
0.2
n
"0
2
1
Pressure/
3 Tort
Figure 1. Adsorption isotherms of H 2 0 on SrFz pretreated at various temperatures: (a) 25, (b) 150, (c) 500, (d) 600 O C ; 0 ,first adsorption at 10 "C; 0,second adsorption. The disk was degassed at 25,150,500, and 600 O C , respectively, in a vacuum of Torr for 2 h in an in situ cell, rehydrated by exposure to saturated H20 vapor at 25 O C , and then degassed at 25 O C for 2 h in vacuo. IR spectra were measured at room temperature by using a Nippon Bunko IR-810 spectrometer. X-ray Diffraction Analysis and Electron Micrograph Measurements. The oriented and the randomly oriented crystal samples were prepared to investigatethe kinds of exposed surfaces of SrF2by means of X-ray diffraction analysis. The SrF2sample was sedimented on a glass plate from an aqueous suspension to form the sample with oriented crystals, while the randomly oriented crystal sample was prepared by only pressing the SrF2 sample on a glass plate. The electron microscopic observation and the electron diffraction analysis were also carried out on the sample crystals,by using JEOL JEM-2OOOEX electron microscope.
Results Figure 1 shows the adsorption isotherms of H20 on the SrF2 samples pretreated a t 25, 150, 500, and 600 "C, respectively. The comparison between the first and second adsorption isotherms measured a t 10 "C in Figure 1 shows that the adsorbed amount of H20 in the first adsorption isotherm is the same as that in the second one on the 25 OC treated sample, while on the 150 OC treated sample the former is larger than the latter. On the 500 and 600 OC treated samples the situation is reversed. As has been found in the cases of metal oxides,l a larger adsorbed amount in the first isotherm on the 150 OC pretreated sample indicates that additional chemisorption of H 2 0 takes place in the course of the first adsorption measurement. On the other hand, in the case of the 500 OC pretreated sample a smaller adsorbed amount in the first isotherm is due to a slow chemisorption of H 2 0 in the
process.23 It thus follows that the chemisorption of H 2 0 occurs on the surface of SrF2, whenever the sample is treated a t higher temperatures, which indicates that the sample has originally chemisorbed HzO. Another important finding in Figure 1 is the appearance of a clear step in the adsorption isotherm. If we plot the adsorption isotherm against relative pressure, the steps in the second adsorption isotherms almost coincide with each other a t a relative pressure of 0.02-0.03. When the sample is pretreated a t increasingly elevated temperatures, the height of the step in the second adsorption isotherm becomes less distinct. These characteristic features are also found on CaF2,18but the height of the step is more distinct on SrF2. By applying the Clausius-Clapeyron equation to the second adsorption isotherms measured a t different temperatures in Figure 1, we can calculate the isosteric heat of adsorption qat (illustrated in Figure 2) as a function of the coverage 8 of H20. The qat curves show a feature similar to those on metal ~ x i d e s on , ~ which ~~~~ the 2D condensation of H20 occurs; when 8 increases, the qatcurve initially decreases, passes a minimum, increases, reaches a maximum, and again decreases, where a broad maximum appears over 8 = 0.2-1.0, corresponding to the step in the adsorption isotherm. Finally, qat reaches the heat of liquefaction of H 2 0 near 8 = 1. When the pretreatment temperature is raised, the maximum becomes smaller as in the case of CaF,. The adsorption isotherms of Ar on SrF2 measured a t liquid N2temperature are illustrated in Figure 3. It is seen from Figure 3 that the adsorption isotherm of Ar also exhibits a step a t the relative pressure 0.02 -0.04, the step being more distinct when the sample is pretreated a t higher temperature, in contrast to the case of the H20 adsorption isotherm. Moreover, the step is more distinct when measured before H20 adsorption rather than when measured after H20 adsorption; in other words, the existence of chemisorbed H 2 0 makes the surface more inhomogeneous for Ar adsorption. These characteristic features on SrF2are very similar to those on Gal?,, but the phenomenon occurs more conspicuously on SrF,. The amount of H 2 0 released was measured by the successive ignition loss method on the samples pretreated a t various temperatures and subsequently hydrated, and the histogram obtained is shown in Figure 4. It can be (23) Morimoto, T.; Nagao, M.; Imai, J.Bull. Chem. SOC.Jpn. 1974,47, 2994.
212 Langmuir, Vol. 4, No. 1, 1988 0.3
Kuroda et al.
1
N
'E
0.2
2
-
0.1
5 \
0
0.3
e : 0.2 v aj
aJ
-5
0.1
Ternperat ure/OC
Figure 5. Plots of surface H 2 0 content on SrF2 against temperature. Pretreatment temperatures: 0,25; a, 150; (0, 500; 0 ,
' 0
0.3
600
0.2 0.1
0
0
5
15
10
Pressure / T o t
20 t
Figure 3. Adsorption isotherms of Ar on SrF2 pretreated at various temperatures: (a) 25, (b) 150,(c) 500, (d) 600 "C; a, before the first H20 adsorption treatment; 0,after the first H 2 0 adsorption measurement.
N
'; 1
P
*~ 0
I
j
C
I i
I
OC.
temperatures, is illustrated in Figure 5, which is obtained by summing all the amounts released at temperatures higher than a given temperature in Figure 4, where the HzO content at 800 "C is assumed to be zero. Figure 5 clearly shows that the HzO content exhibits an irregular curve corresponding to the existence of three desorption peaks.
Discussion Two-DimensionalCondensation of Water. The H20 adsorption isotherms in Figure 1 reveal a distinct step as in the cases of such metal oxides as Zn0,'-3 Sn02,4-Sand crzo37-9and such salts as NaF13-16 and CaFZ.l6-l9 The pressure at which the step appears depends on the nature of the solid, and it is found that the pressure on SrF, is very similar to that on CaFz. In the previous paper,16 the step in the HzO adsorption isotherm on CaFz was ascribed to the 2D condensation of HzO. Also on the present system, we can point out the features characteristic of the 2D condensation of HzO. The qatcurve in Figure 2 exhibits a large maximum a t 6 = 0.2-1.0, corresponding to the height of the step in the adsorption isotherm, which indicates a strong lateral interaction of HzO molecules physisorbed on a uniform surface. The adsorption isotherms in Figure 1 and the qatcurves in Figure 2 demonstrate that the surface of SrFz recovers considerable homogeneity when the sample is equilibrated with saturated H20 vapor even after the pretreatment a t 500 O C . However, the surface of SrFz loses its homogeneity when the surface is treated a t 600 O C , as in the case of CaF,. The differential entropy 9, of the physisorbed H20on SrFz was calculated by2J4325
SA= - q d T + R In W P+)SG
(1)
where p o is the standard vapor pressure (1 atm), p the equilibrium pressure, and SGthe standard entropy of H,O vapor (188.74 J / K mol) a t 298.15 K and 1 atm. The S A values thus calculated are plotted against 0 in Figure 6, where SLand Ss are the entropies of liquid HzO at 283.15 K (66.07 J / K mol) and of ice a t 273.15 K (41.4 J / K mol), respectively.26,n The entropy curves in Figure 6 show that (24) Barer, R. M. J. Colloid Interface Sci. 1966, 21, 415. ( 2 5 ) Benson, J. E.;Ushiba, K.; Boudart, M. J . Catal. 1967, 9, 22.
Langmuir, Vol. 4, No. 1, 1988 213
2 0 Condensation of H 2 0 and Ar on SrF2
"@
.......
@
... e... ............
$-j 8 :::"
.................
"
e
..
-
.,;
....
....'..
.'8 .......
1.
0
I
I
0.5
1.0
I
100
Coverage ( 8 )
I
I
I
I
I
200
300
400
500
600
Temperature / " C
Figure 6. Differential entropy of adsorbed HzO,SA,as a function of coverage on SrFz pretreated at various temperatures: 0 , 2 5 ; a, 150; @, 500; 0 , 600 "C. SLand Ss are entropies of liquid at 10 "C and solid H 2 0 at 0 "C,respectively.
3, increases initially, reaches a maximum when the step initiates in the adsorption isotherm, and decreases to a minimum corresponding to a maximum in the qst curve. SAis smallest on the 25 "C treated sample, the minimum value being approximate to the mean value between SLand Ss, which suggests that a widely ordered structure of adsorbed H20 is formed. When the pretreatment temperature is raised, the minimum value in the curve increases and approaches SL,which indicates that the surface homogeneity of SrF2 decreases when the sample is treated a t higher temperatures.2s Though the data are not illustrated here, it was found that the H 2 0 adsorption data in Figure 1 fit well the Hill-de Boer equation for 2D condensation21and that the ratio a//3 of the 2D van der Waals constants can be calculated to be 7.3,7.3, 7.0, and 6.5 kJ/mol for the 25, 150, 500, and 600 "C treated samples, respectively. Taking account of the fact that the ratio a l b of the three-dimensional van der Waals constants can be related by the form 2a//3 = a / b , these calculated values can be considered to correspond to hydrogen-bondingenergies for the adsorbed H20. Furthermore, the value is found to decrease when the sample is pretreated at increasingly elevated temperatures, suggesting an increase in surface inhomogeneityby this treatment. Surface Homogeneity of SrF2. The adsorption isotherms of H 2 0 in Figure 1and of Ar in Figure 3, on SrF2, have an initial knee and a distinct step a t a relative pressure, which implies that the surface of SrF2is homotattic, i.e., partially homogeneous,29for the adsorption of H 2 0 as well as Ar. In the previous paper,ls the homogeneous and the heterogeneous areas of CaF2were evaluated, on the basis of the adsorption isotherms of H 2 0 and Ar. For this purpose, the total monolayer capacity was first obtained by applying the B-point method to the measured isotherms. The adsorption isotherm for the heterogeneous surface was drawn by extending the initial part of the measured isotherm involving the knee with the aid of the BET equation. The monolayer capacity on the homoge-
Figure 7. Homogeneous and heterogeneous surfaces of SrF%The solid and dotted lines are concerned with the adsorption of Ar and HzO,respectively: 0 and 0,total monolayer capacity of Ar before and after the first adsorption of H20,respectively;0 and @, amount of Ar adsorbed on the homogeneous surface before and after the f i t adsorption of H20, respectively; 0 and 0 ,total monolayer capacity of H20 in the first and second adsorption isotherms, respectively; 0 and 8 , amount of H 2 0 adsorbed on homogeneous surface in the f i t and second adsorption isotherms, respectively.
s,
(26) Eisenberg, D.;Kauzmann, W. The Structure and Properties of Water; Clarendon: Oxford, 1969. (27) Rushbrooke, G.S.Introduction t o StatisticalMechanics;Clarendon: Oxford, 1951. (28) Ishuikyan, A. A.; Kiselev, A. V. J. Phys. Chem. 1962, 66,205. (29) Sanford, C.;Ross, S.J. Phys. Chem. 1954,58, 288.
i I
e
A
I
I
I
I
I
I
I
I
L
Figure 8. X-ray diffraction spectra of SrF, pretreated at various temperatures: (a, b) 25, (c) 150, (d) 500, (e) 600 "C. Sample a randomly oriented; samples &e sedimented. neous surface was then obtained by subtracting the adsorbed amount on the heterogeneous surface a t the same pressure as that at which the total monolayer capacity was read from the total monolayer capacity. The same calculation was carried out on the adsorption data of H20 and Ar on SrFz,and the calculated monolayer capacities are plotted against the pretreatment temperature of the sample in Figure 7. The data in Figure 7 show some interesting features. First, the heat treatment of the sample reduces the H20-homogeneousarea, which recovers through equilibration of the sample with H 2 0 even after the treatment a t 500 "C. After the sample is pretreated a t 600 "C, the reduced H20-homogeneous area remains almost unchanged even after equilibration with HzO. Furthermore, the Ar-homogeneous area measured before the H 2 0 adsorption increases with rising pretreatment
214 hngmuir, Vol. 4, No.1, 1988
Kuroda et at.
a
Figum 9. Electron miemgraph (a) and diffractionpattern (b) of SrF,. temperature, despite the fact that the H,O-homogeneous area decreases, keeping each of the total monolayer capacities of Ar and HzO constant. On the other hand, the Ar-homogeneous area decreases when the surface is rehydrated by the first HZOadsorption proceas. The trend of this change in surface homogeneity of SrF, on heat treatment is similar to that of CaF,, though the HzO-homogeneous area on CaF,’8 decays drastically compared with that on SrF,. It is known that solids, such s NaF,’%=2110,”SnO%es and Cr,O,,” have homogeneous surfaces for H 2 0 adsorption, i.e. give the 2D condensation of H,O, and they are also homogeneous for the adsorption of Ar or Kr. On the contrary, the H,O-homogeneous surface on CaF, was found to be heterogeneous for Ar adsorption.’8 The present result on SrF, exhibits the same trend as that on CaF,. Therefore, it is reasonable to infer that the 2D Condensation of Ar will occur on a geometrically uniform surface to form the 2D close packing of the molecule, while that of H,O should take place in a style different from that of Ar,since the latter prefers the HzO-chemisorLdsurface of SrF,. E x p o d Crystal Faee on SrF* The X-ray diffraction analysis of SrF, was carried out on the randomly oriented sample, which was pretreated a t 25 ‘C,and on the sedimented s a m ~ l e swhich , ~ were pretreated at 25,150,500, and 600 ‘C, respectively; the data are shown in Figure 8. It can be seen from Figure 8 that, when the 25 ‘C treated sample is sedimented (Figure 8b), the X-ray spectrum differs considerably from the spectrum of the randomly oriented sample (Figure that is, the relative peak intensity of the (203) face of the SrF, wtal is greater than the intensties of the (111)and (220)faces, when sedimented. As the sample is treated at increasingly elevated temperatures, the peak intensity of the (200)face of the sedimented samples decreases and becomes almost extinct after the 600 OC treatment. These results demonstrate that the 25 “C treated sample consists of a large majority of crystals with the well-developed (100)face, so that the (100)face of sedimented crystals aligns preferentially on a horizontal plate. Furthermore, the heat treatment of the sample reducea the proportion of the (100)face and instead raises that of the (111) face (30)Kittaka, 9.; W i R; Morkl~ke,K.;Morimota, T.J. Colloid Interface Sei. 1984,105,453. (31) Swanson, € E.; I.Gilfrich. N. T.;Ugrinic, C. M.Standard X-my DiffmctwnP d r Pattern; Natianal Bureau of Standard Cirmlar 659, 1955; Vol. 5.
c .-0 YI
.-YI E c
m
c
40
30
20
15
10
Wavenumber /102c”i’
Figore 10. Infrared spectra of adsorbed H20on SrF, rehydrated after pretreatment at various temperatures: (a) 25, (b) 150, (e) 500, (d) 600 O C . The electron micrograph and the electron diffraction pattern of a typical SrF, crystal in the 25 OC treated and sedimented sample are shown in Figure 9. This photograph also shows that the rectangular surface of a SrF, crystal, which is the (100) face of the CaF, type and is exposed, in agreement with the X-ray diffraction data. Though the photograph is not presented here, it was found that the crystals in the 600 ‘C treated sample were not sharp but of round peripheries, suggesting that the (100) face is largely destroyed. Desorption and Recovery of Chemisorbed H20.As described above, the surface rehydration rate of the dehydrated SrF, sample is retarded by the pretreatment at elevated temperatures, as shown in Figure 1. A h ,the H,O content of SrF, decreases with increasing pretreatment temperature, as shown in Figure 5. Figure 10 shows the IR spectra of adsorbed H 2 0 on SrF,. Prior to the measurement of the IR spectra, the sample disk was subjected to the successive treatments of evacuation at high temperature, rehydration by e x p u r e to saturated HzO vapor, and evacuation at room temperature. It is seen from Figure 10 that several absorption peaks can be observed on the 25 OC treated sample: a sharp peak at 3684 cm-’ and a broad band around 3000-3400 cm-’ due to the OH stretching vibrations of free hdyroxyls and hydrogenbonded hydroxyls, respectively, and a peak a t 1656 cm-’
Langmuir 1988,4, 215-217
215
due to the H20 deformation vibration.32@ The other two large peaks at 2568 and 1970 cm-' are characteristic of the present sample and were not distinguished on CaF,. When the pretreatment temperature is raised, all these peaks are weakened, especially in the intensity of the band a t 3000-3400 cm-'; in other words, the higher the pretreatment temperature, the smaller the amount of surface rehydration, corresponding to the decrease in H20 content. In addition, it is interesting to conclude that the molecular H 2 0 is strongly adsorbed on the surface as is seen from the peak a t 1656 cm-' and that the characteristic peaks a t 2568 and 1970 cm-l prove also the presence of the molecularly and strongly adsorbed H20.34 Table I shows the relation between the amounts of physisorbed and chemisorbed H 2 0 molecules, the latter being defmed as the amount remaining on the surface after the evacuation at Torr for 4 h a t room temperature. V, is the monolayer capacity of physisorbed H20, which is calculated from the second adsorption isotherm. The amount of chemisorbed H20, v h , was obtained as follows.
The chemisorbed H 2 0 denoted as C in Figure 4 was not reproduced, suggesting the chemisorbed H20 to be in inner layers of the crystal, while that denoted as A and B was reproduced by exposing the sample to saturated H 2 0 vapor, which indicates that these two kinds of adsorbed H 2 0 exist just on the surface. Thus, v h was estimated by subtracting the H 2 0 content of the peak C from the total amount of chemisorbed H 2 0 (Figure 4). The ratio V,/ Vh is very large, ranging from 1.32 to 2.35 on the surface on which the 2D condensation of H20 occurs. On the surface of metal oxides, a similar ratio was calculated, but v h was taken as the number of surface hydroxyls that were formed by dissociative adsorption of H,O; the ratio was found to approximately 0.5 or 1.0; the latter being often concerned with the 2D condensation of H20.2J2 In the case of CaF2,1ethe existence of the molecularly adsorbed H 2 0 was not concluded, because the characteristic IR peaks near 2568 and 1970 cm-l were inexplicit. However, the surface properties of CaF2 are found to be similar to those of SrF,, as discussed above. Therefore, it is reasonable to conclude that for the most part the adsorbed H 2 0 can be chemisorbed molecularly also on CaF,. If we take this standpoint, the ratio V,/ v h will be very large also on CaF,, since the original value should be doubled.18 Such a large ratio suggests that the 2D condensation of H20 occurs on a surface on which adsorbed H 2 0 molecules exist together with other kinds of surface species. Further development of the surface model remains to be done.
(32) Hair, M. L. Infrared Spectroscopy in Surface Cherniatry;Marcel Dekker: New York, 1967. (33) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, 1960. (34) Kuroda, Y.; Morimoto, T. Langrnuir, in press.
Acknowledgment. The present work was partly supported by a Grant-in-Aid for Scientific Research, NO. 5747007, from the Ministry of Education, Science, and Culture of the Japanese Government. Registry No. SrF2,7783-48-4; Ar, 7440-37-1; HzO, 7732-18-5.
Table I. Relation between Amounts of Physisorbed and Chemisorbed HzO on SrFz
pretreatment temn
oc
25 150 500 600
VP
vh,
molecules nm-2
molecules nm-2
10.63 10.60 10.22 10.06
8.06 6.07 4.35 3.37
lvh
H,&H,O 1.32 1.75 2.35 2.99
Anesthetic Effect and a Lipid Bilayer Transition Involving Periodic Curvature K. Larsson Chemical Center, University of Lund, Box 124, S-221 00 Lund, Sweden Received February 5, 1987. In Final Form: August 18, 1987 The presence of extremely low concentrations of chloroform, halothane, or ethyl ether induces a phase to a cubic phase in membrane model systems. Phosphatransition from a planar bilayer structure (La) tidylcholine lipids in the form of aqueous dispersions of the Laphase (liposomal dispersions) were examined, and the new phases formed were characterized by X-ray diffraction. It is proposed that the effect of inhalation anesthetics is due to a corresponding phase transition in the axon membrane. It is assumed that the cubic phase consists of a lipid bilayer forming an infinite periodic minimal surface separating two water channel systems and that there also exists a two-dimensional analogue to this type of structure. The two-dimensional periodic minimal surface structure of the lipid bilayer can be expected to block the sodium channels due to conformational effects induced. Increased disorder of the hydrocarbon chains of the bilayer toward the methyl end group region by an anesthetic agent is discussed as the driving force of the phase transition. The pressure antagonism against the anesthetic effect, the effect of different hydrocarbon chain composition of the lipid, and the relative potency of chloroform and ethyl ether can be explained according to the proposed mechanism of the anesthetic effect.
Introduction The effect of inhalation anesthetics is generally considered to be due to a change in the lipid bilayer of the neuronal membrane (cf. ref 1). The mechanism behind (1)Winter,
P. M.; Miller, J. N. Sci. Amer. 1985, 252, 94. 0743-7463/88/2404-0215$01.50/0
blocking of the sodium channels induced by this change in the b i d region ofthe membrane, however, is not known. Evidence is given in the present paper indicating that the action of anesthetics is due to a phase transition from a planar to a periodically curved lipid bilayer. The general structure of cubic lipid-water phases is consistent with a lipid bilayer forming an infinite periodic 0 1988 American Chemical Society
