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The permeabilities of adsorbed monolayers to water
The Permeabilities of Adsorbed Monolayers to Water M. B L A N K 1 AND P. R. M U S S E L L W H I T E Department of Physiology, Columbia University, New York and Unilever Research Laboratory, Welwyn, Herts, England
Received October 17, 1967 The permeabilities of adsorbed monolayers of sodium dodecyl sulfate, bovine serum albumin, and several other proteins have been measured by a simple technique and maximal values of the resistances calculated. The resistances are quite low, but interfacial films of proteins may still affect the interfacial transport of gas in some systems by a mechanism which involves changes in interfacial tension. Finally, the results also suggest an explanation for the enhanced evaporation from protein solutions observed by Bull. I. INTRODUCTION Protein monolayers are present at the interfaces of m a n y natural systems and are believed to be an important constituent of the plasma membranes which surround all cells. Since the regulated transport of molecules (e.g., O2, COs) across these interfaces and membranes is essential for stability, inform~tion about the permeability of protein films is of considerable interest. F r o m the technological point of view, m a n y products utilize dispersions of bubbles or droplets stabilized by protein films, and a knowledge of the permeability would be helpful in dealing with problems t h a t arise in connection with the stability of these systems. Much information has been accumulated in recent years about the transport of small molecules (e.g., H20, CO~) across monolayers of long-chain f a t t y acids, alcohols, etc. (1, 2). These monolayers are relatively impermeable, and the techniques used were sufficiently sensitive to measure the permeabilities of the films under a variety of conditions. For adsorbed monolayers, which have a 1 Supported by Research Career Development Award (K3-GM-8158) and Research Grant (GM10101) of the U. S. Public Health Service and a Research Grant (14-014)001-693) from the Office of Saline Water, U.S. Dept. of the Interior. Journal of Colloid and Interface Science, Yol. 27, No. 2, June 1968
considerable amount of open space, the above techniques are not sensitive enough. Recently, Princen and Mason (3) developed a technique for investigating relatively permeable monolayers of soluble substances. This involves the formation of a soapstabilized bubble at the solution surface, which after drainage leads to the formation of a film, approximately two monolayers thick. I t is possible to measure the decrease in diameter of the bubble as gas diffuses out and to relate this to monolayer permeability. The method is quite sensitive but unfortunately it cannot be applied to protein films, where drainage is uneven and apparently incomplete and there is frequently a considerable amount of coagulated protein in the film (4). The technique that appeared to be best suited for the study of protein films was described briefly in a recent note (5) along with some data on the effects of protein monolayers on the water evaporation rate. Thin films of protein solutions were picked up on a ring suspended from a balance, and the weight was determined as a function of time. Since the water evaporated through the adsorbed protein monolayers, it is possible to use the weight loss rate to estimate the permeability of the monolayer. This technique requires neither even nor complete drainage, and it appears to be insensitive to 188
PERMEABILITIES
OF ADSORBED M ONOL AYE RS TO WAT E R
small amounts of coagulated protein on the surface. II. EXPERIMENTAL PROCEDURE AND RESULTS A fine (0.042 cm diameter) phosphor bronze wire was made into a 1 cm diameter ring and suspended from one arm of the Electric Microbalance, EMB-1, of Research and Industrial Instruments Co. T h e Microbalance output was fed into a Cambridge Recorder in such a way that part of the signal from the balance could be backed off to put the pen on scale. Full scale (7 inches) corresponded approximately to 100 ~g. The ring, while on the balance arm, was dipped into about 20 em '~of the film-forming solution and the solution container removed. The balance potentiometer was adjusted and the weight of the ring and solution were recorded as a function of time. (Figure 1 shows four records obtained with different films under different conditions. The curves go in both directions because either balance arm could be taken as the h e a v y one.) A series of about eight films was formed; the first two were not recorded but were used to adjust the conditions in the balance case to a steady value. The recordings were run for at least one and generally over two minutes and the steady-state slopes at 1 rain. time were measured. The slopes were averaged and the mean was good to about ~ 3 %. The ring and solution container were thei~ rinsed with triple distilled water at least ten times and water at the same temperature was left in the container. The same measurements were then made on thin films formed from this solution. The traces of surfactant left by the rinsing were enough to insure the stability of the thin films, but their concentrations were such t h a t the evaporation rates recorded could be ascribed to " p u r e " water. This was the simplest way to determine the evaporation rate from a "clean" surface under conditions that were similar to the experiments with surfactant films. The resistance of the film was calculated by the method first introduced by Langmuir (6). In the system under study, evaporation takes place at the interface, and the water vapor molecules diffuse from a concentration C~ through a stagnant layer of air to the bulk
189
of air, where the concentration is C2. I n a steady-state system we can assume t h a t the rate E0 of evaporation is equal to a driving force, C1 - C2 = C, divided b y a resistance R0. If we add an interfacial film, a steady state results at a different evaporation rate, E r . This rate is due to the driving force C overcoming two resistances, the air layer and the interfacial film, acting in series. In the case of series resistances, the total resistance r~ = ro + r,
[1]
where r is the resistance due to the interfacial film. Substituting for rt and r0 and solving for r leads to
Therefore, to determine r we require two measurements of the steady-state rate, and we must know the gradient for the diffusion of the water from the surface to the air. The steady-state rates can be determined from the weight/time measurements, but the values needed to calculate C must be estimated. The value of C~ varies with the temperature of the solution during a measurement and a value of 20 m m Hg corresponding to 22°C was assumed for the vapor pressure. The humidity, which determines C2, was not controlled, but it must have varied since the evaporation rate varied b y a factor of about two on different days. In the calculations the value of C2 was neglected; this, together with the assumption of a relatively high value for C~, means that the calculated resistances are maximal values. The resistance, calculated using Eq. [2] and the above assumptions, for sodium dodeeyl sulfate (SDS) films is 0.59 :t: 0.17 sec/cm and for bovine serum albumin (BSA) films is 0.54 ~ 0.08 sec/cm. Values determined for films adsorbed from 0.1% solutions are pepsin 1.00 (in p H 2.5 solution), 7 globulin 0.30, and ovalbumin 0.23 sec/cm. These last three values are the results of single determinations and therefore m a y not be significantly different from the BSA result. However, the pepsin film result, obtained for a thicker film, where errors in temperature and v a p o r pressure are miniJournal of Colloid and Interface Science, Vol. 27, No. 2, June 1968
190
BLANK AND MUSSELLWHITE
A.
B, Phosphor 8r~'Iz¢ Ring.
Glau Ring,
2£)/4
I
20
I
40 S~monds
I
60
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I 40
I 60 Seconds
l 80
FIG. i. Initial and steady state rates of evaporation from thin films having different adsorbed monolayers. Two different rings (A, glass; B, phosphor bronze) were used to support the films. mized, is probably higher than the others. The lower values for ~/ globulin and ovalbumin may be due to the presence of visible chunks of coagulated protein. In all cases, the magnitude of the errors in the evaporation rates lead to a precision of about =t=30To in the calculated value of r, which is close to the observed precision. III. DISCUSSION
A. In~rfacial Effects on Transport. Adsorption of proteins, which is much slower than that of detergents, is accompanied by changes in molecular structure. However, the last protein molecule to adsorb has to denature against a surface pressure, whereas the first has the chance to unravel almost completely. Therefore, protein films consist of a distribution of molecules having "native" as well as unraveled (to different extents) configurations, and it is unlikely that they will form a close-packed layer without free spaces. For this reason, adsorbed protein monolayers ought to be quite permeable. The low resistances are as expected and Journal of Colloid and Interface Science, Vol. 27, No. 2, June 1968
indicate how little effect these interracial films have on water transport. (The true values may be even lower than the values quoted.) Actually, the water layer thickness of equivalent resistance may be only several hundred Angstroms, in the case of proteins. From the point of view of natural membrane structure, the observed very low resistance of protein films is in line with the conclusions reached on the basis of other measurements, that the lipids (not the protein) in the plasma membrane constitute the significant permeability barrier (7). It is possible to compare the values calculated for the permeability of 0.1% SDS films to water, with values for other gases as determined by Princen and ~.iasorl (3). For SDS permeation by H20, r = 0.6 sec/cm and if allowance is made for an overestimate, the true value is probably closer to 0 4 sec/ cm. Therefore, the permeability, the inverse of r, is approximately 2.5 sec/cm. The permeability of a film formed from 4% hexadeeyl trimethylammonium bromide ( H D T A B ) and 4 % NaBr, to a molecule the size of O~, which is comparable in size to
PERMEABILITIES OF ADSORBED MONOLAYERS TO WATER H20, is on the order of 0.5 sec/cm. This constitutes better than order of magnitude agreement, since H D T A B is longer than SDS and the presence of salt should condense the H D T A B film and make it less permeable. If one compares the SDS permeability to H20 with the H D T A B permeability to the much larger molecules C02 or N20 ( ~ 1 0 sec/cm), the agreement is poor. Contrary to what is observed, the permeability ought to be much smaller to CO2, judging from the earlier results for condensed monolayers (1, 2). However, this inconsistency is also present within the results of Princen and Mason (3), where CO2 and N20 data are not in line with their results using other gases (He, Ne, A, H 2 , 0 2 , N~). This inconsistency may arise from the fact that CO2 and N20 are very soluble in aqueous phases, almost two orders of magnitude more than the other gases studied, and the excess pressure in the soap film bubble leads to a fairly rapid dissolution of gas into the bulk aqueous phase. This effect, which is neglected because the aqueous phase is assumed to be saturated, would accelerate the transport considerably and make the permeabilities to the two gases appear much larger. From the previous discussion, one would expect adsorbed monolayers of proteins to be ineffective barriers to transport. However, interfacial films may still have a considerable influence on transport, despite their low resistances. If we consider a spherical bubble of radius R, surrounded by a film of interfacial tension ~, the pressure AP of the gas inside the bubble is equal to 27/R. (This is the Laplace law, the basic equation of capillarity, and is illustrated in Fig. 2A.) The gas diffuses out of the bubble at a rate that is governed by AP, and as R decreases, AP and the rate of efflux increase (3, 8), that is, if -~ remains constant. Obviously an interfacial film having a low 7 would tend to stabilize the bubble and minimize the transport. Furthermore, if the interracial film present had a compression characteristic such that -1 decreased with R, then this would tend to stabilize the bubble even more. For example, if the surface tension of a film varies linearly with the area (as shown in
A.
B.
Laplace Law
191
Ap.~
Coml~ssion Isotherm
6pnJ R
R2
FIG. 2. An illustration of the effect of a surface film on the pressure in a spherical bubble when the interracial tension can vary with compression. Fig. 2B), then the pressure would decrease linearly witb the radius. The problem of minimizing interracial transport from bubbles would then be one of finding a material that will adsorb preferentially, that will have a compression characteristic of the type shown in Fig. 2B over the range of temperature, pH, etc., where stability is required, and that will not desorb appreciably as the surface pressure increases. There is one physiological system where the presence of an interfacial film helps to stabilize a dispersion b y the above mechanism (9). This is the mammalian lung, where the alveoli (terminal air sacs), with a distribution of sizes that ranges over a factor of 3-4, remain stable because of the presence of a surfaetant that gives a very low 7 on compression. The surfactant is a lipoprotein having a high concentration of phospholipid containing saturated fatty acids.
B. Enhanced Evaporation from Protein Solutions. In 1938, Bull (10) found that the rate of evaporation of water through freshly adsorbed films of ovalbumin, formed on a rotating drum, is greater than from a clean surface. For quiescent surfaces, there is no apparent difference between the two rates. The greater evaporation rate through protein monolayers than from pure water comes as a surprise to those trained in the Langmuir, La Mer tradition, although some have Journal of Colloid and Interface Science, Vol. 27, No. 2, June 1968
192
BLANK AND MUSSELLWHITE
argued that this is possible (11, 12). In view of the results given here, which indicate that the proteins have a small but finite resistance, it is advisable to consider the reasons for the apparently conflicting observations. When a thin film is picked up on a wire ring, the solution is distributed fairly evenly, and it is only after some time that the film will have drained away from the ~4re to the central area. Films which drain slowly will cover a wider area for a longer time. Consequently, the slower draining film can evaporate more water even though the per unit area rate is lower. This factor could have caused the enhanced evaporation rate observed by Bull. (It is not possible to comment on the more recent report (5) which claimed enhanced evaporation from quiescent surface films of proteins because of lack of experimental detail in their paper. However, they did not use the "pure" water film method to measure the rate of evaporation from a clean surface and relied on indirect methods to estimate this rate.) The experiments described here support the proposed explanation. Since the evaporation rates were measured using two rings of approximately the same area in the center, but made with rods of different sizes, the total possible area of the protein film was set approximately equal to (a) twice the area in the center plus (b) one half the surface area of the torus. If we assume that the initial film is over the total area and that the final film is over (a) only, the ratio of initial rate to steady-state rate should be 1.33 for the glass ring and 1.13 for the bronze ring. Figure 1 shows the initial and steady-state rates for examples of protein and SDS films for the two rings. The ratios for the glass ring are 1.18 =t= 0.05 (for 1% SDS), 1.34 40.16 (for 0.5 % BSA), 1.40 4- 0.19 (for 0.1%
Journal of Colloid and Interface Science, Vol.27, No. 2, June 1968
BSA), and 1.72 4- 0.37 (for 0.5 % v globulin). In the examples (Fig. 1B) chosen for the bronze ring, the SDS result is more typical and the BSA curve shows about the largest initial effect seen. In many cases there was no observable initial slope different from the steady-state slope. These data have not been calculated, but it is quite obvious that the ratios are much lower than for the glass ring and probably only slightly above 1.00. These results show that the proposed effect of drainage rates is supported in two ways: (1) Where there is a possibility of a greater drainage effect (i.e., the glass ring), the initial slope is greater than the steady-state slope by a factor that is not too different from the one predicted on the basis of geometry. (2) The effect in the case of both rings was much more pronounced for protein solutions, which are usually much slower in draining, than for SDS solutions. REFERENCES 1. ARCHER, R. J., AND LAMER, V. K., J. Phys. Chem. 59, 200 (1955). 2. BLANK, M., AND ROUGnTON,F. J. W., Trans. Faraday Soc. 56, 1832 (1969). 3. PmNCEN, H. M., AND MASON, S. G., J. Colloid Sci. 20,353 (1965). 4. MUSSELLWHITE,t ). R., AND KITCHENER,J. A., J. Colloid and Interface Sei. 24, 80 (1967). 5. JAMES, L. K., AND BERRY, D. J. 0., Science 140,312 (1963). 6. LANGMUIR,I,, AND LANGMUIR,D. B., J. Phys. Chem. 31, 1719 (1967). 7. BLANK,M., .I. Gen. Physiology, in press. 8. GRossE, A. V., Science 156, 1220 (1967). 9. CLEMENTS,J. A., Physiologist 5, 11 (1962). 10. BULL, H. B., J. Biol. Chem. 123, 17 (1938). 11. DERYAGIN,V., BAKANOV,S. P., AND KURGIN, Y . S., Colloid J. of U.S.S.R., 23,262 (1961). 12. KINGI)ON, K. H., J. Phys. Chem. 67, 2732 (1963).
Received October 17, 1967 The permeabilities of adsorbed monolayers of sodium dodecyl sulfate, bovine serum albumin, and several other proteins have been measured by a simple technique and maximal values of the resistances calculated. The resistances are quite low, but interfacial films of proteins may still affect the interfacial transport of gas in some systems by a mechanism which involves changes in interfacial tension. Finally, the results also suggest an explanation for the enhanced evaporation from protein solutions observed by Bull. I. INTRODUCTION Protein monolayers are present at the interfaces of m a n y natural systems and are believed to be an important constituent of the plasma membranes which surround all cells. Since the regulated transport of molecules (e.g., O2, COs) across these interfaces and membranes is essential for stability, inform~tion about the permeability of protein films is of considerable interest. F r o m the technological point of view, m a n y products utilize dispersions of bubbles or droplets stabilized by protein films, and a knowledge of the permeability would be helpful in dealing with problems t h a t arise in connection with the stability of these systems. Much information has been accumulated in recent years about the transport of small molecules (e.g., H20, CO~) across monolayers of long-chain f a t t y acids, alcohols, etc. (1, 2). These monolayers are relatively impermeable, and the techniques used were sufficiently sensitive to measure the permeabilities of the films under a variety of conditions. For adsorbed monolayers, which have a 1 Supported by Research Career Development Award (K3-GM-8158) and Research Grant (GM10101) of the U. S. Public Health Service and a Research Grant (14-014)001-693) from the Office of Saline Water, U.S. Dept. of the Interior. Journal of Colloid and Interface Science, Yol. 27, No. 2, June 1968
considerable amount of open space, the above techniques are not sensitive enough. Recently, Princen and Mason (3) developed a technique for investigating relatively permeable monolayers of soluble substances. This involves the formation of a soapstabilized bubble at the solution surface, which after drainage leads to the formation of a film, approximately two monolayers thick. I t is possible to measure the decrease in diameter of the bubble as gas diffuses out and to relate this to monolayer permeability. The method is quite sensitive but unfortunately it cannot be applied to protein films, where drainage is uneven and apparently incomplete and there is frequently a considerable amount of coagulated protein in the film (4). The technique that appeared to be best suited for the study of protein films was described briefly in a recent note (5) along with some data on the effects of protein monolayers on the water evaporation rate. Thin films of protein solutions were picked up on a ring suspended from a balance, and the weight was determined as a function of time. Since the water evaporated through the adsorbed protein monolayers, it is possible to use the weight loss rate to estimate the permeability of the monolayer. This technique requires neither even nor complete drainage, and it appears to be insensitive to 188
PERMEABILITIES
OF ADSORBED M ONOL AYE RS TO WAT E R
small amounts of coagulated protein on the surface. II. EXPERIMENTAL PROCEDURE AND RESULTS A fine (0.042 cm diameter) phosphor bronze wire was made into a 1 cm diameter ring and suspended from one arm of the Electric Microbalance, EMB-1, of Research and Industrial Instruments Co. T h e Microbalance output was fed into a Cambridge Recorder in such a way that part of the signal from the balance could be backed off to put the pen on scale. Full scale (7 inches) corresponded approximately to 100 ~g. The ring, while on the balance arm, was dipped into about 20 em '~of the film-forming solution and the solution container removed. The balance potentiometer was adjusted and the weight of the ring and solution were recorded as a function of time. (Figure 1 shows four records obtained with different films under different conditions. The curves go in both directions because either balance arm could be taken as the h e a v y one.) A series of about eight films was formed; the first two were not recorded but were used to adjust the conditions in the balance case to a steady value. The recordings were run for at least one and generally over two minutes and the steady-state slopes at 1 rain. time were measured. The slopes were averaged and the mean was good to about ~ 3 %. The ring and solution container were thei~ rinsed with triple distilled water at least ten times and water at the same temperature was left in the container. The same measurements were then made on thin films formed from this solution. The traces of surfactant left by the rinsing were enough to insure the stability of the thin films, but their concentrations were such t h a t the evaporation rates recorded could be ascribed to " p u r e " water. This was the simplest way to determine the evaporation rate from a "clean" surface under conditions that were similar to the experiments with surfactant films. The resistance of the film was calculated by the method first introduced by Langmuir (6). In the system under study, evaporation takes place at the interface, and the water vapor molecules diffuse from a concentration C~ through a stagnant layer of air to the bulk
189
of air, where the concentration is C2. I n a steady-state system we can assume t h a t the rate E0 of evaporation is equal to a driving force, C1 - C2 = C, divided b y a resistance R0. If we add an interfacial film, a steady state results at a different evaporation rate, E r . This rate is due to the driving force C overcoming two resistances, the air layer and the interfacial film, acting in series. In the case of series resistances, the total resistance r~ = ro + r,
[1]
where r is the resistance due to the interfacial film. Substituting for rt and r0 and solving for r leads to
Therefore, to determine r we require two measurements of the steady-state rate, and we must know the gradient for the diffusion of the water from the surface to the air. The steady-state rates can be determined from the weight/time measurements, but the values needed to calculate C must be estimated. The value of C~ varies with the temperature of the solution during a measurement and a value of 20 m m Hg corresponding to 22°C was assumed for the vapor pressure. The humidity, which determines C2, was not controlled, but it must have varied since the evaporation rate varied b y a factor of about two on different days. In the calculations the value of C2 was neglected; this, together with the assumption of a relatively high value for C~, means that the calculated resistances are maximal values. The resistance, calculated using Eq. [2] and the above assumptions, for sodium dodeeyl sulfate (SDS) films is 0.59 :t: 0.17 sec/cm and for bovine serum albumin (BSA) films is 0.54 ~ 0.08 sec/cm. Values determined for films adsorbed from 0.1% solutions are pepsin 1.00 (in p H 2.5 solution), 7 globulin 0.30, and ovalbumin 0.23 sec/cm. These last three values are the results of single determinations and therefore m a y not be significantly different from the BSA result. However, the pepsin film result, obtained for a thicker film, where errors in temperature and v a p o r pressure are miniJournal of Colloid and Interface Science, Vol. 27, No. 2, June 1968
190
BLANK AND MUSSELLWHITE
A.
B, Phosphor 8r~'Iz¢ Ring.
Glau Ring,
2£)/4
I
20
I
40 S~monds
I
60
I ~0
I 40
I 60 Seconds
l 80
FIG. i. Initial and steady state rates of evaporation from thin films having different adsorbed monolayers. Two different rings (A, glass; B, phosphor bronze) were used to support the films. mized, is probably higher than the others. The lower values for ~/ globulin and ovalbumin may be due to the presence of visible chunks of coagulated protein. In all cases, the magnitude of the errors in the evaporation rates lead to a precision of about =t=30To in the calculated value of r, which is close to the observed precision. III. DISCUSSION
A. In~rfacial Effects on Transport. Adsorption of proteins, which is much slower than that of detergents, is accompanied by changes in molecular structure. However, the last protein molecule to adsorb has to denature against a surface pressure, whereas the first has the chance to unravel almost completely. Therefore, protein films consist of a distribution of molecules having "native" as well as unraveled (to different extents) configurations, and it is unlikely that they will form a close-packed layer without free spaces. For this reason, adsorbed protein monolayers ought to be quite permeable. The low resistances are as expected and Journal of Colloid and Interface Science, Vol. 27, No. 2, June 1968
indicate how little effect these interracial films have on water transport. (The true values may be even lower than the values quoted.) Actually, the water layer thickness of equivalent resistance may be only several hundred Angstroms, in the case of proteins. From the point of view of natural membrane structure, the observed very low resistance of protein films is in line with the conclusions reached on the basis of other measurements, that the lipids (not the protein) in the plasma membrane constitute the significant permeability barrier (7). It is possible to compare the values calculated for the permeability of 0.1% SDS films to water, with values for other gases as determined by Princen and ~.iasorl (3). For SDS permeation by H20, r = 0.6 sec/cm and if allowance is made for an overestimate, the true value is probably closer to 0 4 sec/ cm. Therefore, the permeability, the inverse of r, is approximately 2.5 sec/cm. The permeability of a film formed from 4% hexadeeyl trimethylammonium bromide ( H D T A B ) and 4 % NaBr, to a molecule the size of O~, which is comparable in size to
PERMEABILITIES OF ADSORBED MONOLAYERS TO WATER H20, is on the order of 0.5 sec/cm. This constitutes better than order of magnitude agreement, since H D T A B is longer than SDS and the presence of salt should condense the H D T A B film and make it less permeable. If one compares the SDS permeability to H20 with the H D T A B permeability to the much larger molecules C02 or N20 ( ~ 1 0 sec/cm), the agreement is poor. Contrary to what is observed, the permeability ought to be much smaller to CO2, judging from the earlier results for condensed monolayers (1, 2). However, this inconsistency is also present within the results of Princen and Mason (3), where CO2 and N20 data are not in line with their results using other gases (He, Ne, A, H 2 , 0 2 , N~). This inconsistency may arise from the fact that CO2 and N20 are very soluble in aqueous phases, almost two orders of magnitude more than the other gases studied, and the excess pressure in the soap film bubble leads to a fairly rapid dissolution of gas into the bulk aqueous phase. This effect, which is neglected because the aqueous phase is assumed to be saturated, would accelerate the transport considerably and make the permeabilities to the two gases appear much larger. From the previous discussion, one would expect adsorbed monolayers of proteins to be ineffective barriers to transport. However, interfacial films may still have a considerable influence on transport, despite their low resistances. If we consider a spherical bubble of radius R, surrounded by a film of interfacial tension ~, the pressure AP of the gas inside the bubble is equal to 27/R. (This is the Laplace law, the basic equation of capillarity, and is illustrated in Fig. 2A.) The gas diffuses out of the bubble at a rate that is governed by AP, and as R decreases, AP and the rate of efflux increase (3, 8), that is, if -~ remains constant. Obviously an interfacial film having a low 7 would tend to stabilize the bubble and minimize the transport. Furthermore, if the interracial film present had a compression characteristic such that -1 decreased with R, then this would tend to stabilize the bubble even more. For example, if the surface tension of a film varies linearly with the area (as shown in
A.
B.
Laplace Law
191
Ap.~
Coml~ssion Isotherm
6pnJ R
R2
FIG. 2. An illustration of the effect of a surface film on the pressure in a spherical bubble when the interracial tension can vary with compression. Fig. 2B), then the pressure would decrease linearly witb the radius. The problem of minimizing interracial transport from bubbles would then be one of finding a material that will adsorb preferentially, that will have a compression characteristic of the type shown in Fig. 2B over the range of temperature, pH, etc., where stability is required, and that will not desorb appreciably as the surface pressure increases. There is one physiological system where the presence of an interfacial film helps to stabilize a dispersion b y the above mechanism (9). This is the mammalian lung, where the alveoli (terminal air sacs), with a distribution of sizes that ranges over a factor of 3-4, remain stable because of the presence of a surfaetant that gives a very low 7 on compression. The surfactant is a lipoprotein having a high concentration of phospholipid containing saturated fatty acids.
B. Enhanced Evaporation from Protein Solutions. In 1938, Bull (10) found that the rate of evaporation of water through freshly adsorbed films of ovalbumin, formed on a rotating drum, is greater than from a clean surface. For quiescent surfaces, there is no apparent difference between the two rates. The greater evaporation rate through protein monolayers than from pure water comes as a surprise to those trained in the Langmuir, La Mer tradition, although some have Journal of Colloid and Interface Science, Vol. 27, No. 2, June 1968
192
BLANK AND MUSSELLWHITE
argued that this is possible (11, 12). In view of the results given here, which indicate that the proteins have a small but finite resistance, it is advisable to consider the reasons for the apparently conflicting observations. When a thin film is picked up on a wire ring, the solution is distributed fairly evenly, and it is only after some time that the film will have drained away from the ~4re to the central area. Films which drain slowly will cover a wider area for a longer time. Consequently, the slower draining film can evaporate more water even though the per unit area rate is lower. This factor could have caused the enhanced evaporation rate observed by Bull. (It is not possible to comment on the more recent report (5) which claimed enhanced evaporation from quiescent surface films of proteins because of lack of experimental detail in their paper. However, they did not use the "pure" water film method to measure the rate of evaporation from a clean surface and relied on indirect methods to estimate this rate.) The experiments described here support the proposed explanation. Since the evaporation rates were measured using two rings of approximately the same area in the center, but made with rods of different sizes, the total possible area of the protein film was set approximately equal to (a) twice the area in the center plus (b) one half the surface area of the torus. If we assume that the initial film is over the total area and that the final film is over (a) only, the ratio of initial rate to steady-state rate should be 1.33 for the glass ring and 1.13 for the bronze ring. Figure 1 shows the initial and steady-state rates for examples of protein and SDS films for the two rings. The ratios for the glass ring are 1.18 =t= 0.05 (for 1% SDS), 1.34 40.16 (for 0.5 % BSA), 1.40 4- 0.19 (for 0.1%
Journal of Colloid and Interface Science, Vol.27, No. 2, June 1968
BSA), and 1.72 4- 0.37 (for 0.5 % v globulin). In the examples (Fig. 1B) chosen for the bronze ring, the SDS result is more typical and the BSA curve shows about the largest initial effect seen. In many cases there was no observable initial slope different from the steady-state slope. These data have not been calculated, but it is quite obvious that the ratios are much lower than for the glass ring and probably only slightly above 1.00. These results show that the proposed effect of drainage rates is supported in two ways: (1) Where there is a possibility of a greater drainage effect (i.e., the glass ring), the initial slope is greater than the steady-state slope by a factor that is not too different from the one predicted on the basis of geometry. (2) The effect in the case of both rings was much more pronounced for protein solutions, which are usually much slower in draining, than for SDS solutions. REFERENCES 1. ARCHER, R. J., AND LAMER, V. K., J. Phys. Chem. 59, 200 (1955). 2. BLANK, M., AND ROUGnTON,F. J. W., Trans. Faraday Soc. 56, 1832 (1969). 3. PmNCEN, H. M., AND MASON, S. G., J. Colloid Sci. 20,353 (1965). 4. MUSSELLWHITE,t ). R., AND KITCHENER,J. A., J. Colloid and Interface Sei. 24, 80 (1967). 5. JAMES, L. K., AND BERRY, D. J. 0., Science 140,312 (1963). 6. LANGMUIR,I,, AND LANGMUIR,D. B., J. Phys. Chem. 31, 1719 (1967). 7. BLANK,M., .I. Gen. Physiology, in press. 8. GRossE, A. V., Science 156, 1220 (1967). 9. CLEMENTS,J. A., Physiologist 5, 11 (1962). 10. BULL, H. B., J. Biol. Chem. 123, 17 (1938). 11. DERYAGIN,V., BAKANOV,S. P., AND KURGIN, Y . S., Colloid J. of U.S.S.R., 23,262 (1961). 12. KINGI)ON, K. H., J. Phys. Chem. 67, 2732 (1963).