SEMIPERMEABLE MEMBRANES AND NEGATIVE ADSORPTION BY WILDER D. BANCROFT
Mathieul observed negative adsorption with a number of dilute solutions when adsorbed by porous plates, membranes, or capillary tubes. With normal solutions the concentrations in the capillary tubes were often only one-tenth that. The difference in concentration increases with decreasing radius of the capillary tube and Mathieu considers it quite possible that with very fine tubes water alone would be adsorbed, a conclusion which, as Mathieu himself points out, is of distinct importance for the theory of semipermeable membranes. If this conclusion is true and general, it accounts for the results of Bigelow2 and of Bartel13 on osmotic pressure. In the 1912 paper Bartell says: “The fact that some grades of porcelain act ‘osmotically’ was clearly demonstrated by Graham. He made use of a porcelain cylinder of about 170 cc capacity, to which was attached an outlet tube calibrated in millimeters, for measuring the change of height of the solution. This apparatus was filled with the solution to be tested, then placed in a jar of distilled water, the temperature of which was held between 56” and 64” Fahr. Some of the results obtained are as follows: A solution containing I percent of Rochelle salt produced a rise in the column of 82 mm; phosphoric acid of the same strength gave 62 mm; sugar and other organic substances gave something less than 20 mm. “Notwithstanding the fact that certain grades of porcelain do produce osmotic effects when tested, it is also true that most grades of porcelain do not produce apparent osmotic effects. The difference in the behavior of the porcelain conDrude’s Ann., 9 , 340 (1902). Jour. Am. Chem. SOC.,29, 1576, 1675 (1907); 31, 1194 (1909). Jour. Phys. Chem., 15, 659 (1911); 16, 318 (1912); Jour. Am. Chem. SOC.,36, 646 (1914); 38, 1029, 1036 (1916). Phil. Trans., 144, 177 (1854).
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sequently raises the following questions : What is characteristic of that porcelain which is capable of producing osmosis? Is the cunzfmitiun of the porcelain or the internal structurg the principal factor, and, if the latter, is there any definite relation between the size of the pores in the porcelain and osmotic effects?” Microscopic examination of different samples of unglazed porcelain showed that those which produced the osmotic effects were those with the finest texture and therefore presumably those with the smallest pore diameters. It therefore seems reasonable to suppose that, if the size of the pores makes the difference, osmotic effects should be obtained from a plate, which alone will not show osmotic effects, provided its pores be sufficiently clogged with some insoluble material. I n Table I are given the pore diameters, above which no osmotic pressure effect was obtained and the pressures at which 100-125 airbubbles are forced through the diaphragm.
TABLEI Pressure in kg/cm2 necessary to force 100-125 air-bubbles through the diaphragm. Limiting pore diameter in microns a t which osmotic effects occur Diaphragm
’ I I
Porcelain Porcelain Porcelain Porcelain Porcelain Porcelain Porcelain
I
Pressure kg/cm2
Pore diameter I
and sulphur and BaS0.t and PbCrO4 and PbS04 and CuS and Cu2Fe(CN)e
“The border line between osmotic effect and no osmotic effect is not quite the same for the different membranes; however, the values are of the same order of magnitude and probably agree as closely’as could be expected, for comparatively large errors are possible in determining the pressure a t which 100-125 bubbles appear. Since these values are not the same, it cannot be stated definitely that the nature of the substance makes no difference; but they are so nearly the same that the
Senziflermeable ,Wenzbranes and Negative Adsorption 443
‘
pore diameter seems to be one of the main factors in determining the appearance of osmotic phenomena. The success of finding within rather narrow limits a pore diameter which, if exceeded, unsuits the membrane for demonstrating osmotic effects does not exclude the following possibility. Perhaps it is only through the pores of molecular dimensions that pure water passes through to the solution while the solution as a whole is leaking outward through all the large pores. When the inflow equals the outflow, no change in level is observed; when the inflow slightly exceeds the outflow, a change in level is observed. Perhaps what was measured in this investigation was the pore diameter at which the ‘leak’ was just a little less than the osmotic effect. ’’ The general conclusion drawn by Bartell is that there is no osmotic effect when there are 100-125 pores, in the observed area of about 1.5 cm, w t h diameters greater than 0.923 ,microns. “The border line between osmotic effect and no osmotic effect seems to be practically the same for membranes clogged with insoluble crystalline precipitates and for those clogged with the so-called ‘semipermeable’ precipitates.” In the chapter on Osmotic Phenomena Bigelowl says: “By some the function of themembrane in osmotic phenomena is considered to be a subordinate question, because i t can be demonstrated that the nature of the membrane, assuming perfect permeability, makes no difference. This demonstration runs as follows : Imagine a long tube divided into compartments by two diaphragms, perfectly semipermeable, but of such natures that the first is capable of ‘generating’a higher osmotic pressure than the second. Imagine the chamber made by the diaphragm filled with a solution and the rest of the tube full of water. Water will pass in through both membranes until the pressure is the maximum the second can generate. It will continue to come in through the first but will then begin to pass out through the second. Since the membranes are not altered, and all solute is retained in the chamber, water will continue to flow in through the first and Theoretical and Physical Chemistry, 218 ( 1 9 1 2 ) .
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out through the second forever. We could insert a water wheel and make the current do work, thus realizing a perpetual motion machine, one to do work a t no expense. Such a machine is impossible, therefore the two diaphragms cannot have the difference assumed for them. Therefore, the maximum osmotic pressure must be the same for all perfect membranes. This is entirely conclusive, being just as certain as the premise that a perpetual motion machine is impossible; but, like so much thermodynamic reasoning, it leaves us exactly where we started without adding one iota to the actual sum of our knowledge. “As a matter of fact we seldom have to do with conditions wherein even approximately maximum osmotic pressures become evident, but we have to do constantly, whether we will or not, with what we may call traces of osmotic pressures, for the majority of our life processes are secretions by glands, and through membranes which permit the passage of this and hinder the passage of that. Unless the countless membranes in our bodies have their appropriate permeabilities, sickness and death ensue. Hence the majority are not satisfied t o let the matter rest with a thermodynamic demonstration that the nature of the membrane makes no difference but will continue experimental investigations in the hope to find a cure for indigestion, rheumatism, and allied complaints. “The earliest publications on osmosis considered the membranes as capillary structures, and many were the efforts to account for the endosmotic and exosmotic currents on the basis of the behavior of liquids in capillary tubes. One liquid adhering closely t o the inner walls constituted a liquid tube moving in one direction while an interior core of the other moved in the opposite direction. These suggestions are not satisfactory as a statement of the cause of the observed effects. “Then it was thought these capillary structures acted like sieves, permitting the passage of molecules of solvents because they were smaller than the holes, and preventing the passage of solute because its molecules were larger than the
Semipermeable Membranes and Negative Adsorption 445
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holes. It has been brought forward against this theory that membranes are known which permit the passage of the large molecules of various alkaloids while holding back the smaller molecules of amido acids. It is here silently assumed that the size of the formula we write for a substance is a measure of the size of the individual molecule. This is rather hypothetical. The argument might be met on its own ground by the assumption that a very long carbon chain might worm its way through a hole so small a benzene ring could not enter.l “Experiments of extraordinary interest were carried out by I,hermite.2 In a test-tube he put some water, above this a thin layer of alcohol. In the course of a day or two the alcohol had passed through the oil to the water and there were but two layers in the tube, the solution of alcohol in water and the oil above it. He substituted turpentine for the oil with the same result. He separated a layer of chloroform from a layer of ethyl ether by a layer of water and the ether passed through to the chloroform. He tried eight different combinations. Of particular interest, as foreshadowing our present devices, is the following experiment of his. He put alcohol in a porous cup, immersed i t in water and observed that the water passed in to the alcohol. He next filled the pores of the cup with castor oil, filled the cup with alcohol and immersed it in water as before. With this modification the alcohol passed outward to the water. As a result of these experiments he stated that substances which pass through membranes first dissolve in them. Many investigators at present entertain the opinion that membranes with which osmotic phenomena may be produced must be able to dissolve the substance which passes through. That this is not essential is proved by the fact that small excess pressures may be obtained with a cracked test-tube for a membrane. The writer in conjunction with F. E. Bartell has estimated the size of the capillary holes in the coarsest-grained porcelain with which small, but definite osmotic pressures may be obtained, and finds the diameters of these holes in the neighborhood of one thousand times the probable diameters of molecules. For details, see Jour. Am. Chem. SOC.,31, 1194-1199(1909). Ann. Chim. Phys. (3)43,420-431 (1855).
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“It is possible to look upon a liquid, or solution, as a capillary structure, for there must be space between the molecules else it would be continuous, infinitely divisible, and we should have to abandon our molecular theory.” Since a copper ferrocyanide membrane is distinctly colloidal, i t is necessary to consider to what extent adsorption helps or hinders osmosis. This point has been discussed at length by Trouton. “The investigation of adsorption and absorption should throw light on osmosis, as in the first place the phenomenon occurs across a surface necessarily covered with an adsorption layer, and in the second place, as we shall see, the final condition is an equilibrium between the absorption of water by the solution and that by the membrane. The study of the conditions of absorption of water throughout the mass of the colloidal substance of which osmotic membranes are made is of much interest. Little work has been done on the subject as yet, but what little has been done is very promising. “It is convenient to call the material of which a semipermeable membrane is made the semipermeable medium. The ideal semipermeable medium will not absorb any salt from the solution, but only water, but such perfection is probably seldom to be met with. If a semipermeable medium such as parchment paper be immersed in a solution, say, of sugar, less water is taken up or absorbed than is the case when the immersion is in pure water. The diminution in the amount absorbed is found to increase with the strength of the solution. It is a t the same time found that the absorption or release of water by the semipermeable medium according as the solution is made weaker or stronger is accompanied by a swelling or shrinkage greater than can be accounted for by the water taken up or rejected. The amount of water absorbed by a semipermeable medium from a solution is found by experiment to depend upon the hydrostatic pressure. If the pressure be increased the amount of water absorbed by the semipermeable medium is increased. It is always thus possiBrit. Ass. Reports, 84, 288 (1914).
Semipermeable Membranes and Negative Adsorption 447 ble by the application of pressure to force the semipermeable medium to take up from a given solution as much water as it takes up from pure water at atmospheric pressure. It is not possible for a mass of such a medium to be simultaneously in contact and in equilibrium both with pure water and with a solution all a t one and the same pressure, seeing that the part of the medium in contact with the pure water would hold more water than that part in contact with the solution, and consequently diffusion would take place through the mass of the medium. If, however, the medium be arranged so as to separate the solution and the water, and provided the medium is capable of standing the necessary strain, it is possible to increase the pressure of the solution without increasing the pressure of the water on the other side. Thus the part of the medium which is in contact with the solution is a t a higher pressure than that part in contact with the pure solvent; consequently the medium can be in equilibrium with both the solution and the solvent, for if the pressures are rightly adjusted the moisture throughout the medium is everywhere the same. “The ordinary arrangement for showing osmotic pressure is a case such as we are considering, and equilibrium throughout the membrane is only obtained when the necevsary difference in pressure exists between the two sides of the membrane. The conditions would eventually be reached no matter how thick the membrane was. It is sometimes helpful to think of the membrane as being very thick. It precludes any temptation to view molecules shooting across from one liquid to the other through some kind of peepholes in the membrane. The advantage of a thin membrane in practice is.simply that the necessary moisture is rapidly applied to the active surface, thus enabling the pressure on the side of the solution to rise quickly, but it has no effect on the ultimate equilibrium. AS far as that goes, the semipermeable membrane or saturated medium might be infinitely thick, or, in other words, there need be no receptacle or place for holding the pure solvent outside the membrane a t all. In fact the function of the recep-
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tacle containing the pure solvent is only to keep the medium moist, and is no more or less important than the vessel of water supplied to the gauze of the wet-bulb thermometer. It is merely to keep up the supply of water to the medium. “The real field where the phenomenon of osmosis takes place is the surface of separation between the saturated semipermeable medium and the solution. Imagine a large mass of colloidal substance saturated with water and having a cavity containing a solution. The pressure will now tend to rise in the cavity until it reaches the osmotic pressure-that is, until there is established an equilibrium of surface transfer of molecules from the solution into the medium and back from the medium into the solution. KO doubt the phenomenon as thus described occurs often in nature. It is just possible that the high pressure liquid cavities which mineralogists find in certain rock crystals have been formed in some such manner in the midst of a mass of semipermeable medium; the pure solvent in this case being carbon dioxide and the medium colloidal silica, which has since changed into quartz crystal. “In considering equilibrium between a saturated semipermeable medium and a solution there seems to me to be a point which should be considered carefully before being neglected in any complete theory. That is, the adsorption layer over the surface of the semipermeable medium. We have seen that solutions are profoundly modified in thp surface layers adjoining certain solids, through concentration or otherwise of the salts in the surface layer, so that the actual equilibrium of surface transfer of water molecules is not between the unmodified solution and the semipermeable medium, but between the altered solution in the absorption layer and the saturated medium. Actual determinations of the adsorption by colloids are much wanted, so as to be able to be quite sure of what this correction amounts to or even if it exists. It may turn out to be zero. If there is adsorption, however, it may be possible to account for part of the unexpectedly high value of the osmotic pressure observed a t high concen-
Semipermeable Membranes and Negative Adsorption 449 trations of the solution, the equilibrium being, as we have seen, between the saturated medium and a solution of greater concentration than the bulk of the liquid, namely, that of the adsorption layer. In addition, when above the critical adsorption point, there may be a deposit in the solid state. This may produce a kind of polarized equilibrium of surface transfer in which the molecules which discharge from the saturated medium remain unaltered in amount, but those which move back from the adsorption layer are reduced owing to this deposit, thus necessitating an increase in pressure for equilibrium. If either or both of these effects really exist, it would seem to require that the pressure should be higher for equilibrium of the molecular surface transfer than if there were no adsorption layer and the unaltered solution were to touch the medium, but a t the same time it should be remembered that there is a second surface where equilibrium must also exist-that is, the surface of separation of the adsorption layer and the solution itself. It is just possible that the two together cancel each other’s action. “Quantitative determinations of adsorption by solid media from solution are hard to carry out, but with a liquid medium are not so difficult. Ether constitutes an excellent‘ semipermeable medium for use with sugar solution, because it takes up or dissolves only a small quantity of water and no sugar. A series of experiments using these for medium and solution has shown ( I ) that the absorption of water from a solution diminishes with the strength of the solution; and (2) that the absorption of water for any given strength of solution increases with the pressure. This increase with pressure is somewhat more rapid than if it were in proportion to the pressure. On the other hand, from pure water, ether absorbs in excess of normal almost in proportion to the pressure, Certainly this is so up to IOO atmospheres. This would go to confirm the suggestion made that the departure from proportionality in the osmotic pressure is attributablq to adsorption. By applying pressure ether can be thus made to take up the same quantity of water from any given solu-
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tion as it takes up from the pure water at atmospheric pressure. It is found by experiment that this pressure is the osmotic pressure proper to the solution in question.” It is quite clear that we can get osmotic phenomena in two distinct ways depending on whether we have a continuous film or a porous one. I n the case of a continuous film it is essential that the solvent shall dissolve in the membrane and the solute shall not. Since the permeability is not dependent on adsorption, there is no reason why there should be any. fundamental difference between the adsorption of a solute which does pass through the membrane and of one which does not pass through. If we have a porous film, we get osmotic phenomena only in case the pore walls adsorb the pure solvent and the diameter of the pores is so small that the adsorbed film of the pure solvent fills the pores full. Under these circumstances the dissolved substance cannot pass through the pores. On the other hand, if the dissolved substance can pass through the membrane, it must be adsorbed by the latter. There is therefore a fundamental difference between a solute which does pass through a porous membrane and one that does not in that the first is adsorbed by the membrane and the second is not. This way of looking at it makes Bartell’s results perfectly intelligible; but it also shows that his conclusions must be modified to a certain extent. Since we are dealing with selective adsorption it cannot possibly be strictly true that osmotic pressure phenomena begin at the same pore diameter for all membranes and all solutions, though the fluctuations may well be small in many cases. Incidentally, it does not follow that all semipermeable membranes are porous because one is. When castor oil is the semipermeable membrane, we are dealing unquestionably with a continuous film and adsorption plays no essential part in the question of permeability; but it is not so clear what we are dealing with when we have a rubber or a copper ferrocyanide membrane. Kahlenbergl has shown that benzene, toluene, and Jour. Phys. Chem.,
IO,
141 (1906).
I
Semipermeable Membranes and Negative Adsorption 45 I pyridine pass through a rubber membrane very readily while water does not. This is perfectly natural on the assumption that these liquids dissolve in rubber; but on the basis of pores it is difficult to understand why water should not pass through. One might hark back to the case of the oiled sieve and say that water does not wet rubber; but the difficulty is that water does wet rubber. The case of trichloracetic acid is also interesting. When dissolved in benzene it passes through a rubber membrane readily, whereas it passes through very slowly when dissolved in water. This is quite easy to explain if we are dealing with solution and very difficult if we are dealing with adsorption. Another interesting fact which may or may not have a bearing is that Kiisterl found that the distribution of ether between rubber and water could be represented by the equation C,/CW2= const., whereas in all duly accepted cases of adsorption it is the concentration in the solid that carries the factor. While nothing is proved, I am inclined to think that a rubber membrane is not a porous one in the sense that Bartell's clogged cups are, and I believe therefore that when a rubber film acts as a semipermeable membrane, the solvent dissolves in the rubber and passes it essentially in that way. There are no published data which would enable us to tell anything about the copper ferrocyanide membrane ; but my personal feeling is that it is more like the liquid diaphragm and the rubber diaphragm than like the clogged porcelain membrane. With the clogged porcelain membrane, the semipermeability depends undoubtedly on adsorption; but I believe it to be an interesting special case and not the general one. In this discussion it has been assumed that the clogged membrane was strictly semipermeable. Diffusion from the solvent to the solution and the production of some osmotic pressure may take place even though the membrane is not semipermeable. It is merely necessary that the rate of flow from the sohent to the solution shall exceed the rate in the opposite direction. We may postulate that the adsorbed Zeit. phys. Chern.,
13, 445 (1890).
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film in the pores is not pure water but is a very dilute solution. In that case water will pass in one direction and the solute in the other; but the more dilute the solution the more the osmotic flow will counterbalance the diffusion of the solution. If the adsorbed film is practically pure water, but the pore diameters are just large. enough so that there is a thin core of uncharged solution, there will be a flow in both directions. In the later series of papers Bartelll is evidently working under conditions under which the dissolved substance passes through the membrane because he gets negative osmosis under certain circumstances. To what extent this negative osmose is a case of electrical endosmose cannot be discussed a t this point. Bigelow has urged that we have a capillary structure in the case of liquids and that a liquid may therefore be considered as a porous diaphragm. This does not seems to be a satisfactory point of view. Solution is certainly not a question of relative molecular sizes. The solubility of sugar in water and its insolubility in alcohol or benzene are certainly not due to the fact that liquid water is more porous than liquid alcohol or liquid benzene. If one is going to insist on a mechanical conception, we get solution when the dissolved substance is able to push aside and displace the molecules of the solvent. A fish does not swim through the holes in the ocean; he displaces the water. There is obviously no relation between the alleged porosity of a liquid and the formation of a colloidal solution and also none between the porosity 'of the liquid and the formation of a true solution. The general results of this paper are as follows: I . We may have osmotic phenomena with a porous diaphragm provided we have very marked negative adsorption and provided the diameter of the pores is so small that the adsorbed films fill practically the whole of the pores. 2 . A porous diaphragm will act as a semipermeable membrane in case there is no measurable adsorption of the solute and in case the adsorbed films fill the pores completely. Jour. Am. Chem. SOC.,26, 646 (1914);38, 1029,1036 (1916).
Semipermeable Membranes and Negative Adsorption 453 3. In the usual case of a semipermeable diaphragm, we do not have a porous diaphragm and the semipermeability is due to the fact that the solTrent dissolves in the diaphragm while the solute does not to any appreciable extent under the conditions of the experiment. 4. A liquid is not to be considered as a porous substance and solubility does not depend on porosity. Cornel1 University
