348
HENRI1., ROSANO AND VICTOR K. LA MER
Vol. 60
THE RATE OF EVAPORATION OF WATER THROUGH MONOLAYERS OF ESTERS, ACIDS A N D ALCOHOLS BY HENRIL. ROSANO AND VICTORK. LA MER Contribution from the Department of Chemistry, Columbia Univclaitg, New York, N . Y. Received A w l b3, 1966
In a previous communication, the rate of evaporation was investigated by employing monolayers of the C1,, Clel CIOand
C ~ members O of the saturated fatt acid series. The rate was independent of the surface pressure of these monolayers in
the range 10-24 dyne/cm., provigd the film was spread initially under pressure to prevent the entrance of impurities. This finding has been confirmed using a similar technique. The present investigation extends the study to nine pure substances, esters, acids, alcohols and one fluorinated alcohol, The surface area, the resistance to eva oration and the surface vwcosity were measured for each substance as a function of the surface ressure. Certain of these &ms are highly compressible in the liquid phase, as compared to the incom ressible saturated z t t y acids. The compressible films are poor retardants of evaporation, whereas the incom ressible Ilms retard by a large factor. The s ecific conductances (reciprocal resistances to evaporation) are additive &r a mixed film composed of substances of s i d a r compressibility, for example, a 50-50 mol mixture of stearic and arachidic acids. A film composed of 80 mol arachidic acid (incompressible) and 20 mol ethyl palmitate (compressible)reduces the rate of evaporation. Additivity of the conductances is approached at very low pressures, whereas under high pressures the resistance approaches that of the incompressible component. No direct relationship was found between surface ressure, surface viscosity and s ecific resistance to evaporation. The correlation depends upon the compressibility of the Elm except for the fluorinated aEohol which is exceedingly viscous.
I. Introduction The permeability of surface layers involves a physicochemical model and a process, whcse study should aid in understanding many problems of biology, physics (liquid-gas interfaces) and technology (lowering of the rate of evaporation of water in reservoirs). The problem of permeability has been investigated since 1921 by Devaux,I with interesting results followed by many investigators. Restricting our interest to the rate of evaporation of water through films, we find that three sets of conditions have been employed in the measurements: (a) a current of air is passed over the surface.l-l0 (b) The air above the surface is free of convection ~ u r r e n t s . ~ -(c) ~ ~A partial vacuum,is created above the surface.16-20 At first glance, the conditions under (2) would seem the most profitable under which to study evaporation since they most closely approximate the actual conditions under which water evaporates from a reservoir. However, from the viewpoint of a physical chemist, (1) H. E. Devaux, "La permeabilite dea lames minces. Etude de I'inlluence des vapeuru at des huiles sur le8 lames minces aolidea et liquides. SOC. Fr. de physique, 20 mai, 1921, et le8 lamea tres minces at leura proprietes physiques conference 5 juin 1930, 800. Fr. de physique 23-24, Delmaa Editeur, Bordeaux. (2) G. Hedeatrand, THIS JOURNAL, 28. 1244 (1924). (3) I. Langmuir and D. B. Langmuir, ibid., 91, 1719 (1927). (4) H. N. Glazov, J . Phye. Chsm. USSR, 11, 484 (1938). (5) A. C. Heinman, ibid., 14, 118 (1940). ( 6 ) C. I. Sklaiarenko and M. K. Baranaiev, ibid., 12$ 271 (1938). (7) C. I. Sklaiarenko, M. K. Baranaiev and E. I. I. Miejouia, ibid., 18, 447 (1944). (8) A. R. Docking, E. Heymann, L. F. Kerley and K. N. Mortenaen, Nature, 146, 265 (1940). (9) E. Heymann and A. Yoffe, Trans. Faraday SOC.,98, 408 (1942). (10) A. R. Gibby and E. Hyman, A u s t d i o n J . Qci. Res., Ser. A., 197-212 (1948). (11) R. J. Archer and V. K. La Mer, Ann. N . Y . Acad. Sci., 68, 807 (1954). 69, 200 (12) R. J. Archer and V. K. La Mer, THIS JOURNAL, (1955). (13) I. Langmuir and V. J.'Schaefer, J . Franklin Znat., 996, 119 (1943). (14) W. W. Mansfield, Nature, 179, 1101 (1953). (15) E. K. Rideal, THISJOURNAL, 29, 1585 (1927). (16) M. K. Baranaiev, J . Phys. Chem. USSR. 9, 69 (1937). (17) F. Sebba and E. K. Rideal, Trans. Faraday SOC.,87, 273 1941). (18) F. Sebba and N. Sutin, J . Chem. Soc., 2513 (1952). (19) J . F. Holliman, J. F. Largier and F. Sebba, ibid., 738 (1954). (20) F. Sebba and H. V. A. Brisooe. J . Chsm. Soc.. 106 (1940).
conditions (a) and (c) introduce an additional dynamic factor not present in (b) which complicates the interpretation of the data. Archer and La Merl1J2employed the method of Langmuir and SchaefeP but obtained entirely different results in respect to the behavior of the fatty acid flms as a function of the surface pressure. Whereas Langmuir and Schaefer found a marked dependence upon surface pressure and often a hysteresis type of curve on compression and expansion, Archer and La Mer controlled the technique of spreading the fatty .acid f l m so that the resistance became a reproducible constant independent of the surface pressure in the range 10-24 dynes/cm. The upper limit exceeds 30 dynes/cm. st9 with arachidic acid, but collapse of the film with other monolayers may occur below 30 and above 24 dynes/cm. The essential point was to spread the flm from a highly volatile solvent (petroleum ether) and under an initially high pressure to prevent the entrance of impurities (solvent or foreign molecules) into the f l m during the spreading and measurement. They were thus able to obtain consistent results yielding simple relations in respect t o chain length and the energy barrier to permeability. The energy barrier for the C19 acid was determiried from the resistances at several temperatures." It is possible that a surface pressure-area isotherm as ordinarily measured may not be sensitive enough to show that the states of a film of low compressibility obtained under decompression may not lead to the same equilibrium states on compression in the liquid state. On the other hand, the resistance to evaporation is exceedingly sensitive to many factors of technique and molecular architecture. Since the spreading technique of Archer and La Mer was quite different from that of any previously employed, and their conclusions are of far-reaching importance, our first task was to satisfy ourselves that the resistances of their fatty acid monolayers were actually independent of surface pressure. Their results have been COI firmed for the CUIand Czoacids (Fig. 4, curves 6 an 1 5). We investigated nine pure substances and two mixtures. In each case we measured the surface
Mar., 1956
THERATEOF EVAPORATION OF WATERTHROUGH ESTERS, ACIDSAND ALCOHOLS 349
area of the film (compression-area isotherm), the specific resistance to evaporation and the surface viscosity all as functions of surface pressure.
11. Experimental Methods A technique analogous to that of Langmuir and Schaefer was used with modifications for measuring the specific resistance of the monolayers to the evaporation of water, taking advantage of the experience of Archer and La Mer. Figure 1 shows the trough and torsion balance of the present apparatus but equipped with oscillating mica rings V for the measurement of surface viscosity, to be described later. For the measure of rate of evaporation through the film the mica rings are replaced by the Lucite desiccant box used previously for collecting and weighing the evaporated water. This part of the apparatus has been described in detail in the previous papers,nJa and need not be repeated here. The torsion balance M firmly connected with an elevator (not shown in sketch) bears a wettable plate K of depolished mica or sand-blasted platinum which allows facile measurement of the surface pressure of a monolayer spread on water contained in the trough E. The steel torsion wire bears a mirror,galvanometer, a damping plate immersed in oil, and a 20 mm. horizontal rod, at the edge of which is attached the wettable plate by a thin silk thread. A narrow beam of light is reflected from the galvanometer mirror through a 4-meter optical path to a vertical mirror and then to a translucent scale, which acts as an image index. The sensitivity of the torsion balance is 0.1 dyne/cm.; for 0.289 g., we have 32.9 cm. of light displacement. Since the plate has a perimeter of 4 cm., we estimate visually the sensitivity (within mm.) as 0’289 329 X 4
Om5 = 0.1 dyne/cm.
The water rests in a glass trough having the dimensions 66 X 16 X 2.5 om. For the measurement of surface pressure the surface is delimited by a floating mica frame thinly coated with pure paraffin, and by two movable barriers, which are made either of glass or of mica and also coated with paraffin. The mica frame is kept in place by two glass rods F ; i t is free to move vertically but not horizontally. A given temperature is maintained by passing water from a thermostat through a glass coil immersed in the trough. A t the edge of a Lucite platform, supported by the edges of the trough, is a steel pointed rod with which one can adjust the distance between the surface of the liquid and the bottom face of the desiccant box. This box, containing LiCl separated by a water permeable membrane fits into the platform through a circular hole 10 cm. in diameter. The trough is kept level by four adjusting screws. The entire system is kept under a Vaseline-coated hood which serves to catch dust particles from the air and prevent them falling on the surface of the water. Guastalla’sPl technique of blowing air and creating suction for cleaning the water surface was used. This method consists of amassing in one corner of the water surface all the impurities which can be detected by dusting calcinated talcum powder onto the surface. To avoid the rapid reappearance of impurities the water wa8 distilled in a glass apparatus, cleaned with a mixture of H804--K&r20,. The laboratory-distilled water was successively redistilled from acid permanganate (to oxidize organic products) and hydrated barium hydroxide added for neutralization. The edges of the trough and the barriers were coated with a minute amount of paraffin before each experiment. This paraffin was replaced as soon as there was any sign of surface activity. All parts of the apparatus that came into contact with the surface of the water were handled with tweezers. No other chemical research was permitted in the room. Despite all these precautions, it was not possible to maintain a clean water surface for more than approximately one minute. The reappearance of impurities increased with time. It was not possible to evaluate the amount of such impurities, but everything possible was done to keep them at a minimum. Measurement of Surface Area Isotherm.-The Wilhelmy (wettable plate) method was used.*z If y~ is the (21) Thesis. Montpellier. France, 1948. (22) H. L. Rouano, Memorid Sem. Cham. Efat., S6, 309-341 (1951).
surface tension of pure water and y that of water covered by a film, the surface pressure p is defined as yo - y . 70 - represenb for an insoluble film the work necesssry to isothermally and reversibly increase by one cm.* the free surface of a liquid covered. It can be broken into: (1) work against instantaneous surface tension of the pure liquid; (2) work gained by the spreading of surface active molecules to an equilibrium state.26 For a very dilute surface film, the instantaneous surface tension is very close to the surface tension of the pure liquid, so that Ysoln
= YO
- Wadaorptn
YO
-p
To measure this isotherm, the surface of the water is cleaned, then the wettable plate is put into the sur€ace, noting where it registers on the scale. The surface active material, dissolved in petroleum ether, is introduced onto the water surface from a calibrated pipet. The petroleum ether was distilled in a glass apparatus (b.p. 40-55O) and was tested for purity by evaporating 20 drops on a clean water surface and noting the lack of any residue. Starting with a non-discernible surface pressure, the barrier J is gradually moved, thus decreasing or increasing the area of the film. The surface pressure is measured at each area by the displacement of the light on the scale. Measurement of the Specific Resistance.-Starting with non-discernible surface pressure we obtained reproducible area isotherms on compression. It is impossible to measure an area isotherm by decompressing a monolayer containing micro-crystals; nevertheless, we have determined the specific resistance us. surface pressure by spreading the film under high pressure and decompressing, because it seems to be the practical way to prevent impurities entering the monolayer. Reproducible results can be obtained by the following technique: (1) the surface is cleaned many times with blowing air and suction techni ue. (2) We place the platform in position. (3) The surgface water level is adjusted with the movable rod. (4) We remove the platform and the wettable plate K. (5) Barrier J is at FI and is pushed toward F2. (6) The substance is spread as a film under high surface pressure. (7) We re lace the platform and plate K. (8) We decompress the t l m to a certain aurface pressure. (9) The desiccant box is opened and placed on the platform Temperature of the box is noted. A stop watch is started. After a certain time (2 minutes, for example), the box is removed, closed and weighed. The temperature given is again noted. With this technique we have checked the results of Archer and La Mer using saturated fatty acids. When we create a new water surface (operation No. 5) and immediately deposit the solution, it seems reasonable to suppose that the surface is cleaned by the spreading of the solvent. Since we have a film under high surface resaure the impurities cannot adsorb in surface. This expLation given by Archer and La Mer seems plausible. Before adopting the technique just cited, we aleo measured the specific resistance us. surface pressure starting with a non-discernible surface pressure and compressing. The resistance to evaporation under this procedure increased with surface ressure as was reported by Lan uir and Schaeferla ant! by Archer and La Mer (Fig. 3 oreference 12).
The explanation of Archer and La Mer was based primarily on the trapping of solvent molecules in the monolayer on compression. While we do not question the importance of trapping impurities, nevertheless, i t is also possible that in the compression and decompreRsion of a monolayer we may not have the same ratio of the several possible configurations in the liquid states of the film. We shall show later that the dependence of resistance upon surface pressure is governed by the compressibility (dp/dr) of the substance forming the monolayer, Le., by the tangents to the area isotherm. T o determine the compressibilitiea as a function of the time and surface pressure or area will require a more sensitive surface manometer than we have used. This means that the surface pressure-area isotherm is relatively insensitive as compared to the resistance-pressure isotherm, so that it may not be possible to detect significant differences in compressibility on compression and decompression. Measure of the Surface Viscosity of Monola e r s . - F i p 1 shows the apparatus as used to measure s u A c e viscosity. It consists of a mica frame V coated with par&. A mica
350
HENRI L. KOSANO AND VICTOR K.L A M E R
V O I . DU
0-n2.
Fig. 1.-Sketch of apparatus for measuring surface viscosity of surface films by the damping of oscillations including the torsion type of Wilhelmy surface balance for measurement of surface pressure. Description of lettered parts in text. ring floats on the surface of the water and is attached to a vertical torsion wire. Using a light beam reflected from a mirror on the wire, i t is possible to measure the decrement of oscillations of the ring for the clean surface water and for the surface monolayer. The damping of the oscillations depends upon the concentration and nature of the surface film, whereby one calculates the surface viscosity of a monolayer. We have used a narrow mica ring coated with paraffin.2' Surface viscosity can be calculated as p = -
43
R:
- R?
-
A
c7.4
27r
Aa
+]A
111
7.4 ?= AO
in which r is the torsion module of the wire I is the initial moment of the floating system R1is the radius of the moving ring R1 is the radius of the fixed ring A0 is the decrement without film A is the decrement with film The method of damped oscillations is best adapted for viscous films.
111. Theoretical Basis for Calculating the Specific Resistance of the Monolayer to Evaporation From diffusion theory, the rate of flow of water vapor through a column of air in a steady state can be expressed by M/t =
(W
- wo)/(b/D)(A)
(2)
-where M is the mass of water (in g.) taken up by the solid absorbant in the time t (seconds), A is the cross-sectional area of the diffusion tube, b is the length of the diffusion path (in cm.), and D is the diffusion coefficient of the water vapor in air (in cm.2/sec.),w is the concentration of the water vapor (in g./cm.S) at the surface of the water just above the monolayer, and w ois the concentration in similar units at the lower surface of the silk gauze. Equation 2 suggests an analogy to Ohm's law in the form Rate = driving force/specXc resistance
thus M/t =
(W
- WO)/~
(3)
where r is the sum of the resistance (by unit of cross-sectional area of the diffusion tube) of the water surface-the air column, the membrane and the desiccant surface acting in series. In these conditions two determinations are made of the rate of ahsorption, one with a monolayer on the wa(23) h l . July, KoZluiJ 2.. 116, 35 (19623.
Fig. 2 . 4 u r f a c e pressure-surface area isotherms showing compressible (no. 1, 2, 3, 9) and non-compressible films (4, 8) for substances of Table I.
ter and one without. The specific resistance of the monolayer r is equal to r =
(W
- uo)t(A/Mrilm) - (W - W o ) t ( A / M n o r i h )
(4)
The rigorous application of equation 2 is possible if we note the temperature of the water and the desiccant absorbant at the beginning and the end of each experiment.ll The validity of the method has been established.l1*l2 The value for w o of lithium chloride is taken from graphical data published by B i c h ~ w s k y . ~Following ~ Archer and La Mer, 11,12 we have adopted a correction with regard to the additional contribution from water vapor in the surrounding air. For experimental convenience, it was necessary to establish the following relationships w = f (temperature) w o = f (temperature) wt. of water absorbed by the surrounding air = f (time) TABLE Io M.p. 24" 1 Ethyl palmitate Cl,H,,COOC&H5 C17H&OOCzH5 ~ P 1.4573 D 2 Ethyl linoleate C17H&OOCzH5 na% 1.4519 3 Ethyl elaidate C1~Ha5COOC2Hs M.p. 34.4' 4 Ethyl stearate C&(CH2),,COOH M.p. 76' 5 Arachidic acid M.P. 68.7' 6 Stearic acid CHs( CHz)&OOH CH;((CHa)&HzOH M.p. 58.2" 7 1-Octadecanol CH,( CHn)i&HaOH M.p. 49' 8 Cetyl alcohol 9 1,1,13-Trihydro- H( CFZ)IZCHZOH M.p. 110" perfluorotridecyl alcohol a The products 1, 2, 3, 4, 5, 6, 7 and 8 were dissolved in redistilled Merck petroleum ether (b.p. 45-55') and the product 9 in methyl alcohol (boiling range 64.2-64.7').
I. Experimental Results A. Pure Substances. (1) Compression Area Isotherm.-Figure 2 shows the surface pressurearea isotherm for the above substances. The data fall into two categories, the compressible monolayers (substances number 1, 2, 3 and 9), and the relatively incompressible substances (4, 5, 6, 7, 8 ) . It is not surprising to fmd that substances with double bonds (nos. 2 and 3) give compressible monolayers. Neither is it surprising in the case of the fluorinated substance as shown by Klevens and Raison.26 However, it is not always possible to predict a priori if a substance will give a compressible or an incompressible film, as in the con(24) P. 11. Riohowsky, Cl~cm.Msl. Eng.. 302 (1940). (25) lf. B. lilevcns and M. Raison, J . chim. phys.. Sl, I - S (1954).
Mar., 1956
THERATEOF EVAPORATION OF WATERTHROUGH ESTERS, ACIDSAND ALCOHOLS 351
SURFACE PRESSURE
. dyneslcm
Fig. 3.-Reduction in rate of evaporation of water through a monolayer by substances listed in Table I in terms of the specific resistance t o evaporation as a function of the surface pressure. Note the very low resistance of the esters 1, 2, 3 and the high resistance of saturated fatty acids-stearic and arachidic.
trasting behavior of the two esters: ethyl palmitate and ethyl stearate. (2) Specific Resistance os. Surface Ressure.Figure 3 gives the results of specific resistance measurements. The products 1, 2 and 3 are ineffective for reducing the rate of evaporation of water. These products also give compressible films (Fig. 2). The effective substances have relatively incompressible films. Except for the fluorinated alcohol, which we shall discuss later, we are led to the general conclusion that for substances with hydrocarbon tails, the resistance to evaporation of a monolayer i s determined by the compressibility. We have also checked the results of Archer and La Mer for the CISand C20 acids in the range of 12 t o 24 dynes/cm. where the specific resistance is independent of the surface pressure. When the surface pressure is below 12 dynes/cm. the data indicate that (a) in the case of the acid CZ0the specific resistance of the monolayer remains practically independent of the surface pressure down to 3 dynes/ cm., and (b) in the case of CIS acid below 13 dynes/ cm. the specific resistance decreases with decreasing surface pressure. From the other curves it can be seen that there are regions where the resistance is independent of surface pressure (no. 9 between 18 dynes/cm. and saturation) and also regions where the specific resistance i~ an increasing function of surface pressure (curves 4,7,s). Generally the specific resistance is independent of the surface pressure only for certain substances and certain ranges of that pressure. Those substances exhibiting compressible films show little or no resistance to evaporation. (3) Surface Viscosity us. Surface Pressure.Only three curves are given in Fig. 4 since the method used was not sensitive enough to determine the viscosity of the more fluid monolayers. The fluorinated alcohol (no. 9) has a high viscosity. Even though there appears to be no relationship between specific resistance and surface viscosity, an examination of the two curves for any one product shows that both curves have approximately the same slope at any given pressure. The discontinuities in the slope of the surface viscosity vs. sur-
SLRFACE PRESSURE
~
dyneslrn
Fig. 4.-Surface viscosity (in lo* poises) as a function of surface pressure for substances of Table I.
face pressure curves have been ascribed by J01y23926 to changes of phases in the monolayers. Since these discontinuities in the slopes of the specific resistance vs. surface pressure curves occur at the same surface pressure, it seems highly plausible that a change in the specific resistance of a monolayer reflects a change in the structure of the film. On comparing curves for products 6 (stearic acid) and 9 (fluorinated alcohol) we find that 6 is non-compressible and non-viscous, and 9 is compressible and very viscous. Yet we see that both 6 and 9 have approximately the same specific resistance in their most effective regions. This suggests that resistance to evaporation through a monolayer depends on two effects: (a) cohesive forces residing in the monolayers (compressible films are not effective) ; (b) adhesive forces between the monolayer and the sub-phase due to hydrogen bonding. In product 9 even though this film is fairly compressible, the increase in cohesive forces, in the tails of the fluorinated compound, which are evident from the extremely high value of surface viscosity, lead t o an appreciable specific resistance. B. Mixtures.-Mixtures have been studied for two reasons: (a) to see the effect of impurities on the measurement of the specific resistance; (b) to see if it is possible to get mixtures that are still effective in reducing evaporation but which have properties that are more desirable than the pure substances alone (lower melting point of mixtures for practical purposes). Under (a) we chose two substances of similar compressibility and under (b) a mixture of a compressible and a non-compressible substance. (1) Mixture of Arachidic and Stearic Acids: 50/50 Molecules.-Figure 6 shows that this mixture obeys approximately the formula l / r mixture = l / r arachidic acid
+ l / r stearic acid
The conductances are additive as was suggested by Langmuir and Schaefer. (2) Mixture of Arachidic Acid and Ethyl Palmitate : 80/20 Molecules.-The specific resistance vs. surface pressure for pure ethyl palmitate monolayers shows no appreciable decrease in the rate of evaporation (curve 1 of Fig. 3) of water. If the law of the additivity of reciprocal resistances is generally applicable, then any substance mixed (26) M. Joly.
J. chim. p h u e , 44, 213 (1947).
352
HENRIr,. ROSANO AND VICTOR K. LA MER
with ethyl palmitate should exhibit no appreciable effect on the rate of evaporation of water. However, an experiment with mixtures of (20 mole %) ethyl palmitate and (80 mole yo)arachidic acid shows an appreciable effect on the rate of evaporation contrary to expectation (Fig. 7). This result can be understood in the following way. Since the Cle and Czofatty acids exhibit almost identical compression-area isotherms, a mixture of these two acids should exhibit a similar behavior (Fig. 5 ) . Actually, the mixtures show slightly
- " I : '
I
Vol. 60 I
I
I
!
I
I
1
I
WRFPCE PRESSURE. dynrilcrn.
Fig. 7.Specific surface resistance to evaporation as function of surface pressure of mixed 6lm repared from a mixture containing 80 mols of arachidic (820) acid and 20 mols of ethyl palmitate.
limit of low pressures, the resistance of the mixture approaches that predicted by the additivity of reciprocal resistances. I n any case, the limiting tangent at low pressures approaches that of the ester and not that of the Czoacid. Fig. 5 . S u r f a c e pressure-surface area isotherm of the mixed film formed from a 50/50 mixture of pure stearic' and pure arachidic's acids.
smaller areas a t given pressures, and likewise the measured resistance (Fig. 6 ) of the mixture is somewhat below the theoretical curve for the additivity of the reciprocal resistances. These minor effects
0 IT- 82
Fig. 8.-Surface pressure-area isotherm of the mixed film of 80 mols arachidic acid-20 mols ethyl palmitate corresponding to the resistance measurements of Fig. 7.
As the pressure is increased, the resistance of the film spread from the mixture rises rapidly and in the limit seems to approach the high resistance of the arachidic acid component. This behavior is explicable. At low pressures the interaction forces between the molecules in the 4 monolayer are weak with the result that every SURFICE PRESSURE - d y n i t l c m Fig. 6.-Spccific surface resistance t o evaporation through molecule more or less exhibits its own specific rea mixed film com osed of 50 mols stearic (G8)acid and 50 sistance which adds in parallel. On the other mols arachidic (&,) acid. The theoretical curve is com- hand, with increasing pressure, the interaction puted on the basis of the additivit of the individual recip forces increase rapidly with decrease in area, so that rocal resistances or conductances o f t h e pure substances. the additivity rule fails completely. Near the pressure for the collapse of the mixed On the other hand, in the case of two substances of different chemical type (acid and ester), the re- film, the ethyl palmitate molecules are squeezed ciprocal resistances are not additive for the mixed out and the resistance approaches the high value film (Fig. 7). The resistance of ethyl palmitate is for the arachidic acid component. In practice this result has importance. A solid so low that on the basis of this rule its conductance should dominate the conductance of any mixed substance which has a pronounced effect on the rate of evaporation can be mixed with another prodfYm containing any appreciable amount of the ester. Figure 8 shows the compression-area isotherms uct thereby making a liquid easier to spread withof the pure substances and their 80 acid/20 ester out reducing the rate of evaporation, provided it is mixture. This mixture exhibits a compressibility maintained under pressure. C. Miscellaneous Observations.-Before adoptvery similar to that of the pure acid. However, when we examine the evaporation re- ing the decompression technique of measuring sistance of this mixture (Fig. 7), we find that in the resistance to evaporation, a number of substances,
Mar., 1956
THEADSORFTION OF GASESON
not listed in Table I, were tested by compressing the film spread a t low pressures. Cyclohexyl myristate and also ethyl oleate gave easily compressible films producing very little reduction in the rate of evaporation. Methyl stearate on compression from 0.3 to 11 dynes/cm. reduced the rate of evaporation by a practically constant value of 12%. Lauryl alcohol gave a low resistance to evaporation which was practically constant (0.21 to 0.37) over a wide range of pressures (1.4 t o 41 dynes/cm.) .
A
353
SILICON SURFACE
With arachidic acid even at nondiscernible pressures, there is a noticeable eflect (reduction of 15%) on the rate of evaporation. On compressing this film the rate decreased rapidly as soon as discernible pressures were noted until a t about 22 dynes/cm. the rate of evaporation becomes 60% of that of pure water. Acknowledgment.-H. L. R. wishes to gratefully acknowledge the grant of a Fellowship from the International Cooperation Administration, Washington, D. C., which made this work possible.
ADSORPTION OF GASES ON A SILICON SURFACE BY J. T. LAWAND E.E.FRANCOIS Bell Telqh3n.e Laboratories, Incorporated, Murray Hill, New Jersey Rsceivsd Augtul I.?, 1866
An investigation of the adsorption of argon, nitrogen, carbon dioxide, carbon monoxide, hydrogen, oxygen and water vapor on a silicon d a c e haa been carried out using 8 maw spectrometer and a flash filament technique. All measurements were made at 300°K.and gas preasurea between lo-' and lo-' mm. The decomposition of carbon dioxide and water vapor at various temperatures hari also been studied.
Introduction The adsorption of gases on germanium has recently been studied'.* using the flash filament technique* but u p to the present, no work has been done on silicon. It has been shown by various workers' that the state of the surface (ie., extent and nature of adsorbed material) can have a marked effect on the electrical properties of the semiconductor, so that a study of the adsorption of gases is a prerequisite to understanding the physics of the surface region. The purpose of this paper is to present a survey of the adsorption properties of gases on silicon which have been investigated using a flash filament technique and a mass spectrometer as a pressure measuring device. Any initial study of this type must use a mass spectrometer to identify decomposition products of the adsorbed gas. The adsorption of argon, nitrogen, hydrogen, carbon monoxide, carbon dioxide, oxygen and water have been studied in this manner. Experimental
During a flash run the mass spectrometer waa focused on the mass ot an ion fragment of the gaa being investigated and a record of the gas desorbed made on the recording oscillograph. At the same time the chan e in pressure waa read on the ion gage. Temperatures wit%in the range of visible radiation were determined with an optical pyrometer usin an emissivity correction of 0.40 and at the same time t%e resistance of the filament waa measured. Valuea of the conductivity obtained in this way are shown in Fig. 1 compared with data obtained by Morin and Maitae at lower temperatures. All the points fall on the same straight line and in fact the agreement is so good that it may
Apparatus.-A filament having the dimensions 2 X 2 X 100 mm. was cut from a single cryatal of 5 ohm cm. n-type silicon. It was lapped with 600 mesh carborundum and etched.6 after which molybdenum leadc were welded to the ends. The 6lament waa then mounted in a glsss jacket and connected to the inlet line of a Consolidated mass apectrometer as described previously. The tube containing the silicon filament was pumped through the ionization chamber of the 111988 epectrometer, from which the inlet leak had been removed. The heating current for the filament waa supplied by means of a manually operated varisc. Because of the large change in resistance of the filament in going from room temperature to 500", very little control of temperature over this range waa possible. (1) J. T. Law and E. E. Francoia, Ann. N. Y. A d . Sei.. S8, 925 (1054). (2) J. T. Law, THISJOURNAL. 69. 543 (1955). (3) L. Apker. Ind. E w . Chum., 40, 846 (1948); J. A. Becker and C. D. Hartman, Tal8 JOUBNAL.67, 157 (1953). (4) W. H. Brathin and J. Bardeen. Bell System Tech. J., 13, 1 (1953); E. N. Clarke, Phys. Reo., 91.756 (1953); W.L. Brown, ibid.. 01..~ 518 (1953). . (5) Twenty-five cc. of concd. nitric acid and 25 cc. of 48% HF.
I/T *
Fig. 1.-Conductivity
x
~~
(6)
10-3
of intrinsic silicon as a function of temperat.ure.
F. J. Morin and J. P. Maita, Phua. Rev.,
96, 28 (1954).
