QUARTERLY REVIEWS MOLECULAR-SIEVE ACTION OF SOLIDS By R. M. BARRER D.Sc. Sc.D. (PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF ABERDEEN) MANY important phenomena in physical and biological chemistry result from molecular-sieve action including osmosis dialysis Donnan membrane equilibria (unequal ionic distributions hydrolysis potential differences) isotope separation and gaseous and solution chromatography. These secondary phenomena will not be examined in detail but instead the causes of molecular-sieve action will be considered together with some of the information which studies of permeability have contributed to the fundamental chemistry and physics of solids. Molecular-sieve action may be total or partial in its effect upon the permeability. I n total molecular-sieve separations the flow of one species into or through the medium is wholly prevented while the diffusion of a second species occurs at a finite rate.In partial separations both species diffuse but a t different velocities. Molecular-sieve action can moreover be demonstrated in two ways the diffusion medium can be in the form of foils or of beds of powdered sorbent through which different trans- mission velocities occur ; alternatively a sorbent may be exposed to a stationary molecular mixture and different sorption velocities may be found. Elastomers and other polymers inorganic glasses certain metals and also natural membranes forming the walls of plant and animal cells provide good examples of media effecting sometimes partial and sometimes total separations. Sorbent media showing ultra-porosity include porous glass charcoals and other xerogels but by far the most spectacular mole- cular-sieve effects are shown by some dehydrated zeolite crystals especially chabazite [ (Ca,Na2)A1,Si,012,6H20] gmelinite [ ( Na2,Ca)A1,Si401,,6H,0] mordenite [ (Ca,K,Na,)Al,Sil,0,4,6~H20] levynite (CaA12Si,0,0,5H20) and a synthetic zeolite ( BaAl2Si,0,,,~H2O) together with their cation-exchange modifications; and to a lesser extent by analcite (NaAlSi,O,,H,O) and harmotome [ ( Ba,K,)A1,Si,Ol,,5H2O].Other zeolites may show the pro- perty but not all have yet been investigated. First we may consider the ultra-porosity of sorbents and then that of compact membranes. Molecular-sieve Action in Porous Sorbents Some General Considerations.-When a gas is circulated through a porous (1) Because greater resistance is offered to diffusion of one species than medium different flow rates arise in two important ways 293 294 QUARTERLY REVIEWS of another ; e.g.if a large and a small molecule both pass along a capillary which is of greater diameter than that of the small molecule but constricts the large molecule then the small one diffuses more rapidly. ( 2 ) Because of differing sorption potentials within the porous medium through which both species are flowing. If the medium has a large internal surface considerable amounts of the diffusing species may be sorbed and so become largely immobilised. If the amount of sorbed material and thus the lifetimes of each sorbate in the sorption layer differ then chromato- graphic bands may develop or a major difference in the rates of transmission through a column of the porous medium may occur.For example in powdered dehydrated chabazite crystals exposed to a mixture of ethane and propane the ethane is very speedily occluded a t room tem- peratures and the propane is only slowly occluded because the very narrow intracrystalline diffusion paths greatly restrict the mobility of the larger propane molecule while allowing the ethane to diffuse rapidly. Therefore the crystals first become charged with ethane * which is thus quickly removed from the gas stream. However the affinity of propane for the crystals is greater than that of ethane and therefore for comparable partial pressures of the gases a t or near equilibrium there would be a larger interstitial con- centration of propane. By suitably adjusting the time of transit of the ethane-propane mixture through a column of chabazite so that factor (1) was dominant the effluent gas was wholly freed from ethane; for very slow rates of transmission however the effluent gas should be enriched in ethane.In the absence of appreciable sorption transmission through a porous bed a t high pressure follows Poiseuille’s law and there is no separation of the constituents of a gas mixture. At low pressure when the capillary radii are small compared with the mean free path of the gas the flow fol- lows Knudsen’s law and the separation of the constituent gases is inversely proportional to the square roots of their molecular weights. As the capil- lary radii become still smaller and the surface to volume ratio increases adsorption effects begin to become important.The adsorbed molecules may be immobilised but also may contribute to flow by surface diffusion. Mole- cular streaming in a capillary may be described in terms of the diffusion equation &/at = D ( a 2 c / W ) where c denotes the concentration of gas in the gas phase. The values which the diffusion coefficient D may then have in a given cylindrical capillary are These two effects can act in opposition a t one and the same time. D = (4r/3)dZkT/nm (molecular streaming only) . 4r2 (molecular streaming with adsorption to give D,= 3rdnm,/2k~ + an immobile film) . * (2) R. 31. Barrer and L. Belchetz J . SOC. Chem. I n d . 1945 64 131. R. M. Barrer 1’mn.s. Paraclay SOC. 194-8 No. 3 61. giving a zeolitic or interstitial solid solution. * Both ethane and propane are taken up interstitially throughout each crystallite, BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 295 (molecular streaming with adsorption to give a mobile film) .(3) D - 3- I n these equations z denotes the lifetime of a molecule in the adsorbed state (z = where 7 = l / v Y is the vibration frequency in the adsorbed state and AH the heat of adsorption) ; rn is the molecular mass r the radius of the capillary k the Boltzmann constant and T the tem- perature. Us = DOe-E/RT is the surface diffusion coefficient and E is the energy of activation for surface diffusion. In deriving equation (3) it was further assumed that concentration on surface c&centration in gas phase = constant = tdkT/2nm This condition is probable for many systems a t the low pressures where molecular streaming is the principal mechanism of transmission.A still further diminution in capillary diameter results in ‘‘ surface ” diffusion becoming dominant because in capillaries with diameters of mole- cular magnitudes only the diffusing species do not leave the range of surface forces. The diffusion process now occurs by a succession of jumps in a periodically varying potential energy field due to the solid. Such periodicity arises from the discontinuous atomic structure of the medium and gives rise to the energy of activation E.* The diffusion of many gases into gas-sorbing zeolites (see p. 303) takes place according to this mechanism. Capillary size distributions have been estimated 3 by means of the Kelvin equation RT lnp/p = - 2oV cos 8/r where r is the radius of a capillary in which liquid is in equilibrium with its vapour a t vapour pres- sure p ; p s is the saturation pressure of the liquid which has molecular volume V and surface tension o.The contact angle of the liquid with the wall of the capillary is 8 and if the liquid wets the capillary 8 is zero and cos 8 = 1. From the sorption isotherm one can determine V the amount sorbed. One may subtract from V the amount Vm required to form a monolayer and attribute the remainder (V - Vm) to capillary condensa- tion. Vm can be estimated by several methods,4 and is perhaps most easily derived from B E T plots of p / V ( p - p ) against p/p when the intercept on the axis of V is l/Vmc and the slope is (c - l)/Vmc where c is a con- stant. Typical structure differentiated curves of d V/dr against r derived from isotherms by the use of the Kelvin equation are given in Fig.1. Capillary condensation in the usual sense cannot occur when the capillary radius is of the order of 3 A. G. Foster Trans. Faraday SOC. 1948 No. 3 41 ; 1932 28 245; P. H. Emmett and T. de Witt J . Amer. Chem. SOC. 1943 85 1253; P. H. Emmett and M. R. Cines J . Physical Chem. 1947 51 1248 ; S. Brunauer “ The Adsorption of Gases and Vapours” O.U.P. 1944 Chap. XI where a summary of much earlier work will be found. There are limitations to the Kelvin method. “ -4dvances in Catalysis” Vol. 1 Academic Press Inc. N.Y. 194S p. 65. * Relative movements of atoms or groups in close contact whether in chemical reaction intramolecular rotations diffusion or viscous flow all tend in this way to be jumps from one stable configuration to another via a less stable transition state.296 QUARTERLY REVIEWS one or two molecular diameters and the Kelvin equation may no longer describe the phenomena occurring. Secondly a correction to r is applied 71 28 32 34 r'A* 22 26 31 ? A . I. Benzene-Fe,O,(B). I I I . H,O-SiO (B). 11. Dioxan + AlcohoZ-Fe,O,. IV. H,O-AI,O,. FIG. 1 Iv Typical structure differentiated curues for some ge2 sorbents showing range aibd distribution of pore sixes (Foster). [Reproduced by permission from Trans. Faraday SOC. Discussion on Interaction of Water and Porous materials p . 44.1 to allow for the space taken up by the monolayer film the formation of which precedes multi-layer formation and capillary condensation. 5 Also the capillaries may not be shaped as for the Kelvin equation like test- tubes. In charcoal for example many capillaries will be the space between pairs of plate-like crystallites." Finally because many capillaries are open a t both ends the condition for forming the meniscus may differ from the condition for its equilibrium once formed.L. H. Cohan considers the condition for forming liquid in a cylindrical tube open a t botlh ends to be RT In p/p8 = - oV cos O/r which may be compared with Kelvin's equa- tion. Therefore capillary sizes and size distributions are best determined on the desorption cycle of the isotherm. In spite of these limitations the distribution curve of capillary radii in terms of Kelvin's equation may have a qualitative significance. These A. G. Foster Trans. Faraday Xoc. 1948 No. 3 44. J Amer. Chem. Soc. 1938 60 433. H. L. Riley Quart. Reviews 1947 1 65.* These laminae may have diameters no more than 20-100 A. in many gas-sorbing charcoals .' BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 297 curves normally show maxima and molecular-sieve effects may become clearly defined if there is a sharp upper limit to the capillary radii or where all the capillaries are of the same dimensions (Fig. 2 curves 2 and 1). Curve 2 may be approximated to by some gel sorbents for which the isotherms have the form given in Fig. 3 curve 2. The comparatively flat upper part of the isotherms probably corresponds to condensation in a group of capillaries all of nearly the same radius followed by cessation of sorption FIG. 2 Three possible distributions of pore sizes. Curves (1) and (2) will give sharp molecuhr- sieve effects but Curve (3) will not.* Relative pressure ‘‘0 FIG. 3 Isotherms in porous solids. Isotherm (2) corresponds with pore distributions such as those in Curve 2 Pig. 2. Isotherm (3) corresponds similarly with Curve 3 Fig. 2. With pore distributions like those in Curve 1 Fig. 2 but with pore radius too small for capillary condensa- tion Langmuir-type isotherm as in Curve 1 are observed (cf. isotherms in zeolites). because no larger capillaries exist in the porous gel and because all capil- laries of smaller diameters have already been filled. Curve 1 is charac- teristic of the crystalline zeolites in which however the pore diameters are only a few A. so that capillary condensation cannot occur. Evidence of molecular-sieve effects in charcoals has been presented inter alia by Miss M. Franklin,6 who reported that apparent densities of char- coals during graphitisation depended on the dimensions of the molecules of the immersion medium (helium water carbon disulphide methanol and benzene); the larger the molecule the smaller was the apparent density owing to incomplete penetration of the pore structure.At higher tem- peratures of graphitisation all the immersion media gave the same apparent density ; but this density was - 1.7 which is lower than that of graphite suggesting sealed pores inaccessible to molecules of any medium. According to L. G. Gur- Silica gels may also show analogous effects. International Colloquium on Reactions in the Solid State Paris Oct. 1948; Bull. SOC. chim. 1949 Nos. 1 and 2 D.53. 298 QUARTERLY REVIEWS witsch's the volumes (reckoned as liquid) of all sorbates taken UP in a porous gel sorbent should be the same when the sorbent is saturated.A. G. Foster,lo however found that this was not the case in some samples of silica gel; the larger the molecule the less the volume occluded a t saturation. In one case the number of moles sorbed at saturation increased linearly with the reciprocal of the molecular volume. Intracrystalline Sorption.-Some crystalline zeolites such as those noted on p. 293 can be dehydrated by heat and evacuation without appreciable lattice shrinkage to give crystals permeated by very narrow diffusion paths no more than a molecular diameter across. Following dehydration there- fore a great internal " surface " is developed and the crystals are capable of sorbing some other molecules in place of their original crystal water.The sorptive capacity in certain cases rivals that of activated charcoals and in addition the most remarkable molecular-sieve properties are developed.ll Evidence as to the dimensions of the intracrystalline channels has been obtained both by X-ray methods and by the study of sorptive behaviour of the crystals. Structures have been obtained for the zeolites analcite,l2 natrolite,13 scolecite,13 thomsonite,13 and edingtonite.l* X-Ray studies have been made of chabazite,l5 harmotome,ls phillipsite7l7 and heulandite,lZ but detailed structures are not known. However the minerals sodalite l8 and ultramarine l9 have been successfully examined and it is believed that the aluminosilicate framework in chabazite resembles the basket-like anionic structure recurring in amodified form in these minerals.Fig. 4 shows the channels in sodalite which cross other channels a t intervals throughout the lattice. In analcite four channels diverge from each crossing point; in chabazite ultramarine or sodalite eight channels diverge from each such point. These junction points occur regularly throughout the crystal and the channels themselves are continuous although their diameter may vary periodically along their length. Zeolite crystals have been subjected progressively to more and more drastic conditions of heat and evacuation and were a t the same time exam- ined by X-ray or sorption methods.20 Taking these data together with 9 J . Russ. Phys. Chem. SOC. 1915 47 805. 10 Nature 1946 157 340. 11 J. 'CV. McBain " Sorption of Gases by Solids " Routledge & Sons 1932 Chap.V and R. M. Barrer Ann. Reports 1944 41 31 give general summaries of the sorptive behaviour of some zeolites. 12 W. H. Taylor 2. Krist. 1930 74 1. 13 W. H. Taylor C. A. Meek and W. W. Jackson ibid. 1933 84 373. l4 W. H. Taylor and R. Jackson ibid. 1933 86 53. 1 5 J. Wyart Bull. SOC. franp. Min. 1933 56 81. l6 J. Sekawina and J. Wyart ibid. 1937 60 139. 17 J. Wyart and P. Chatelain ibid. 1938 61 121. 1 8 L. Pauling Proc. Nat. Acad. Sci. 1930 16 453; 2. Krist. 1930 74 213. 1 9 F. M. Jaeger Baker Lectures Cornell University McGraw-Hill N.Y. 1930 Part I11 ; F. M. Jaeger H. G. Westenbrink anti B. A. van Melle Proc. Acad. Anisterdnm 1937 30 249. 2o W. Milligan and H. Weiser J . Physical ClLe?iL. 1937 41 1089 ; R. &I. Barror Proc. Roy. Soc. 1938 A 167 392 406. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 299 I Strength of bonding in the main direction of aluminosilicate chains greater than strength of cross-linking of chains Strength of bonding in plans of aluminosilicate sheets greater than strength of cross-linking of sheets Strength of bonding comparable in all three dimensions cleavage properties one may make a classification of some zeolite crystals in terms of the direction stability of the lattice (Table I).Only in the robust Sodalite Na,Al,S i60,,C1 FIG. 4 Some of the details of the sodalite structure showing the basket-like anionic framework and the channels running diagonally from the eight corners of the unit cell (HelzdricEs). [Reproduced by permission from Ind. Eng. Chem. 1945 37 625.1 network group of crystals is there any general tendency for other molecules to replace water within the lattice because the channels in fibrous and laminar zeolites collapse at least partly during heating and evacuation.TABLE r Crystal chemical characters of some zeolites Bonding characteristics. Type of zeolite. Fibrous Laminar Robust network cry st als Examples. Natrolite Scolecite E ding tonit e Thomsonite Heulandite Stilbite Chabazite Gmelinite Mordenite Levynite Harmotome Analcite 300 QUARTERLY REVIEWS The interstitial volume can be determined by measuring the volume of water which can be driven off from the crystals and is a substantial frac- tion of the total volume of the crystals as the data of Table I1 show. The place of water can be taken by quite small molecules only demonstrating the small cross-sectional diameter of the channels but for these small molecules the robust network zeolites are quite exceptional sorbents.In harmotome and most analcites the only molecules sorbed are small polar molecules but in the other zeolites in Table I1 and in some synthetic zeolites non-polar molecules are sorbed as we11.21 TABLE I1 Interstitial volumes in Some gas-sorbing zeolites Zeolite. Chabazite . . . Gmelinite . . . Levynite. . . . Harmotome . . Mordenite . . . Anctlcite . . . . G.c. of liquid H,O dis- placed from 100 c . ~ . of crystals (approx.). 50 50 40 35 33 20 Sorbs non-polar gases. Yes Yes Yes No; but sorbs Yes As a rule no ; but sorbs H20 NH, HC1 H2°7 NH3 The kinetics of sorption can successfully be described in terms of Pick’s law of diffusion although D the diffusion coefficient sometimes depends upon the concentration of sorbate in the crystal.21 2 2 The channels are so narrow that the sorbates move wholly in the periodic potential-energy field of the crystal and as already noted (p.295) the diffusion coefficient must then obey the relation D = Doe-E/R1’ where Do is a temperature- independent factor and E is the energy of activation for a jump from one preferred sorption site to the next (cf. Table XI). The values of E depend in a complex way upon the radius and charge of the interstitial cations and more directly upon the diameter of the diffusing molecule (Fig. 5). Thus the more closely the interstitial channel surrounds the diffusing mole- cule the larger E becomes and the smaller the diffusion rate ; until finally large molecules do not enter the crystal at all.21-25 Every degree of intra- crystalline mobility has been observed and so every degree of molecular- sieve effect can be expected.I n an early classification of a number of molecular-sieve crystals certain sorbate molecules of known dimensions were used to compare the dimensions of the intra-crystalline channels.23’ 26 *1 R. M. Barrer J. 1948 127; R. M. Barrer and D. W. Riley ibid. p. 133. 2 2 R. M. Barrer and D. Ibbitson Trans. Paraday Soc. 1944 40 206. 23 R. M. Barrer J. SOC. Chem. Id. 1945 44 1 3 0 ~ . 24 I d e m Trans. Paraday Soc. 1944 40 555. 26 I d e m ibid. 1949 45 358. 26 I d e m Ann. Reports 1944 41 31. BARRER MOLECULAR-SIEVE AC!CION O F SOLIDS 301 FIG. 6 Relation between the true energy of activation E for diffusion and the cationic radius in a series of base exchange mordenites for a given sorbate molecule.Inset is shown the relation between the true energy of activation E and the cross-sectional radius of a series of sorbate molecules for a given base-exchange mordenite here K-mordenite. [Reproduced by permission from Trans. Faraday SOC. 1949 45 363.1 "2 A ~ 4.89 A. B FIG. 6 Critical dimensions of some typical molecules used to classify molecular sieves,. Three categories of molecular sieve are described (Table 111). The critical dimensions of these reference molecules and of some other molecular species are indicated in Pig. 6. Molecular dimensions cannot of course be given 302 QUARTERLY REVIEWS with absolute certainty but are sufficiently definite to predict the sorptive behaviour of the minerals towards other possible sorbates.Thus in the series /CH2\ c1 c1 /cH2\ /cH2\ CH CH CH CH Br Br Br c1 all the molecules are of similar shape and size owing to the comparable dimensions of the C1 and Br atoms and the methyl radical. Propane is slowly occluded by chabazite and therefore the other species should also TABLE I11 Three categories of molecular-sieve zeolite Class 1. Chabazite Gmelinite Synthetic zeolite ( BaA1,Si,0,,,nH20) ClU8S 2. Mordenite (rich in Na) Class 3. Ca- and Ba-rich mc denites (prepared hydro t her mally by cation interchange) Do not occlude {so-paraffins or aromatics. Occlude n-paraffins slowly. Occlude CH, C,H, and molecules of smaller cross- Diameter of narrowest cross-section of interstitial section very rapidly. channel between 4.89 and 5.58 A. Does not occlude n-paraffins iso-paraffins or aromatics.Occludes CH and C& slowly. Occludes N, 0, and molecules of smaller cross-section Diameter of narrowest cross-section of interstitial rapidly. channel between 4.0 and 4.89 A. Do not occlude hydrocarbons including CH and C,H,. Occlude A N, and molecules of smaller cross-section. Diameter of narrowest cross-section of interstitial channel between 3.84 and 4-0 A. be slowly sorbed. In the series CHCI, CH(CH,), CHBr, since chabazite cannot occlude isobutane it is unable to occlude chloroform and bromoform molecules which are once more of comparable shape and size to isobutane. Similarly in the n-paraffin series This prediction was confirmed. CH3 /CH2\ /CH2\ etc. F2\ / CH CH CH3 CH CH /CH2\ CH CH the cross-sectional diameter of all these molecules in the fully extended configuration is the same vix.4-89 A. Since propane is slowly occluded by chabazite all the other paraffins should be sorbed. This sorption was followed as far as n-heptane and although the sorption rate decreased as the number of carbon atoms increased large amounts of these simple n-paraffins were 27 It may be inferred that diffusion occurs 27 R. M. Barrer and D. Ibbitson Trans. Paraday Soc. 1944 40 195. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 303 in a stretched-out configuration and that the ability of the chabazite to sorb the paraffin depends on the shape and cross-sectional diameter of the molecules rather than on their molecular volume (isobutane has cross- sectional diameter 5.58 A. and molecular volume 96 C.C. and is not occluded ; n-heptane has cross-sectional diameter 4.89 A.and molecular volume 145 c.c. and is occluded). Many other examples were also found in which the observed behaviour of the zeolites as sorbents was predicted from the molecular dimensions of the sorbate molecules (Table IV). The grouping of some zeolites into the three classes of molecular sieve is dependent to a marked degree upon the thoroughness of removal of water and other interstitial molecules which reduce the mobility of the sorbate. By leaving known amounts of water in chabazite crystals it was possible to transform this zeolite from a Class 1 molecular sieve into one with sorptive properties towards hydrogen oxygen and nitrogen more nearly recalling a Class 3 sorbent.28 The water molecules immobilised in the interstitial channels substantially impede the diffusion of other species freely mobile in the absence of the water.The rates of sorption are also reduced if the heat treatment during outgassing is so severe that a measure of lattice collapse occurs. In this connection chabazite has been heated to 470" while still preserving its sorptive p0wers.l Analcite harmotome gmelinite and mordenite and many of their cation-interchanged forms have been heated for indefinitely long periods at 350" without observable lattice collapse.22 2 4 9 25 The velocity of sorption is also influenced by the dimen- sions of the sorbent particles in accordance with the theory of diffusion in such crystallites.22 Apart from the considerations of the previous paragraph it has recently been shown that the grouping of certain zeolites into three categories of molecular sieve as in Table I11 is capable of considerable refinement and extension.This can be done in either of two ways (i) by working at low temperature^,^^ or (ii) by altering the intracrystalline channel dimen- sions by cation interchange or other chemical p r o c e ~ s e s . ~ ~ ~ 3O We may first consider (i) and assign to each diffusing species a charac- teristic E and Do in the relationship D = D,e-E/RT. Temperature influences the diffusion velocity differently in each individual case and other things being equal the relative sorption velocities should diverge exponentially as the temperature is lowered. This position is however modified when the varying affinities of the sorbates for the sorbents are taken into account. It has been demonstrated that in constant-volume sorption systems the interstitial concentration and concentration gradient may sometimes increase with falling temperature more rapidly than the mobilities of individual molecules decrease giving negative instead of the more usual positive tem- perature coefficients to the rates of sorption (Fig.7).25 However in those systems with positive temperature coefficients the relative sorption rates 28 P. Emmett and T. De Witt J . Amer. Chem. SOC. 1943 65 1253. Zg R. M. Barrer Nature 1947 159 508. 30 Idem ibid. 1949 104 12. X 304 CH, C,H CH,*OH CH,*NH CH3.CN CH,Cl CH,F HCN C1 QUARTERLY REVIEWS All classes of molecules in cols. 2 and 3 section (i). TABLE IV Molecules occtuded or excluded by three classes of molecular sieve Typical molecules rapidly occluded at room tempera- ture or below.Typical molecules moderately rapidly or slowly occluded at room temperature or above. Typical molecules which are not appre- ciably occluded at room temperature or above. (i) C h s 1 minerals. He Ne A H2 N2 0 c o GO2 cos cs 32 HBr NO NH3 CH,*NH2 CH,*OH CH&N HCN C1 CH,Cl CH,Br CH,F CH,C12 CH,F CH, CJ3 CBH CH20 Has CH,*SH C,H and simple higher C2H,*OH C,H,*NH C,H,F C,H,CI C,H,Br CH,Br CHJ n-paraffins 12 H I C,H,-CN C,H,*SH H*CO,Me H*CO,Et COMe CH,*CO,Me NHMe, NHEt Aromatic hydrocarbons cycZo- and iso-paraffis. Derivatives of these hydro- carbons. Heterocyclic compounds (e.g. thiophen pyrrole pyridine) . CHCl, CCl, CHCl:CCl, and analogous bromo- and iodo- compounds. Secondary strsight-chain alco- hols thiols nitriles and halides. Primary amines with NH group attached to a secondary carbon atom.Tertiary amines. Branched-chain ethers thio- ethers and secondary amines. CH,-CHCI, CHCl,*CCl, C2CI8 A HC1 NH3 (iii) Class 3 minerals. All molecules referred to in col. 3 section (ii) . usually diverge more and more as the temperature falls. Two species both rapidly sorbed at room temperature may then be sorbed a t low temperatures a t such different velocities that molecular-sieve separation should be easily possible. Fig. 8 illustrates this low-temperature differentiation between sorption velocities of oxygen nitrogen and argon in levynite.* Numerous examples of this method of magnifying the molecular-sieve effect have been observed.25 In the modification of the zeolites by cation interchange the exchange * The " foot '' in the argon curve is due to a small nearly instantaneous van der Intra-crystalline Waals adsorption upon the external surfaces of the crystal powder.sorption is almost absent for argon on the time scale of the experiment. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 305 reaction is normally effected hydrothermally in presence of excess of the exchanging salt a t temperatures which may conveniently range from 150" to 250". Simple stainless-steel autoclaves can be used as reaction vessels Ca-mordenite . . Levynite . . . . K-mordenite. . . Ba-mordenite . . FIG. 7 ExampJes of positive and negative temperature coeflcgents in trunsgent flow oj- gases in zeolitic sorbents. (a) Kr in synthetic Na-mordenite. ( c ) Curves 1 and 2 refer to N in NH4-?nordenzte and curves 3 and 4 to Kr in the same (b) A in Ba-rnordenite. sorbent. (d) A in NH,-mordenite.[Reproduced by permission from 'I'raiis. Faraday SOC. 1949 45 363.1 except in the case of ammonium salts for which the exchange is better done in glass vessels and with ammonium chloride vapour. A range of differing molecular sieves may be prepared within limits set by it given aluminosilicate framework. In Table V are given values of D/u2 for argon a t - 78" in a series of the crystals u being the mean particle radius.25 1.51 x 10-8 Na-mordenite. . . 7-6 x lo-' U-mordenite . . . 2.8 x 10-6 NH,-mordenits . . 5.5 x TABLE V DiSfusion of argon at - 78" in cation-exchanged mordenites I Crystal. II Crystal. 1 D/a* (min.-1). i D / a z ( m h - l ) . 3.8 x 10-5 4.0 x 10-4 1-35 x 10-3 306 QUARTERLY REVIEWS Widely divergent sorption rates in dehydrated levynite at - 184" due to molecular-sieve action.(Qt denotes C.C. sorbed at N.T.P./g. QCQ denotes C.C. sorbed at N.T.P./g. at equilibrium and is for 0, 10.02 ; for N, 9.77 ; f o r k 10.13.) [Reproduced by permission from Nature 1947 159 508.1 FIG. 9 Xome relative sorption rates in Ca-mordenite at - 185" a?' - 7;". At - 185" the less easily condensed gases helium hydrogen and neon show no in the rate curves but the more easily condensed guses show "feet " due to adsorption upon external surfaces of the powder. Intracrystalline diflusion for oxygen nitrogen and argon is however very slow. A t - 78" the "feet " have almost disappeared and intracrystalline diflusion rates ure much increased. foot [Reproduced by permission from Trans. Faraday SQC. 1949 45 362.1 No allowance is made in the above data for variations in the value of a but the range in the quotients exceeds anything to be expected on this count.Fig. 9 similarly shows major differences in sorption velocities for a series of gases in a given crystal (cf. also Fig. 8). For the mordenites of Table V the rate sequence Kr < A < N < 0, Ne < H < He tended, BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 307 with some exceptions to be preserved. On the other hand the affinities shown by the gases for the crystals were nearly in the converse order Kr > A N, 0 > H > Ne > He. The critical dimensions of some of these molecules are shown in Fig. 6. A special method for modifying zeolites consists fist in forming the ammonium zeolite and then slowly burning the ammonia away with oxygen to form the crystalline hydrogen ~eolite.~O The procedure is effective only for crystals capable of occluding oxygen.The removal in this way of some of the interstitial substance of the crystal results in a significant increase of the interstitial channel dimensions. Separations by Total Molecular-sieve Action.-The behaviour of single species some of which are copiously occluded while others are not sorbed (Table IV) suggests that quantitative separations might be effected merely by a single exposure of a mixture to it suitable amount of the appropriate zeolite. Such quantitative separation has been termed total molecular- sieve action (p. 293). Chabazite-type crystals were successfully used to resolve the hydro- carbon mixtures in Table VI. Derivatives of hydrocarbons may also be separated from other such constituents in admixture.31 Many such separa- tions occur only very slowly but are often accelerated by a rise in tem- perature within limits set by the decreasing sorption with increasing temperature and by the thermal stability of the species.TABLE VI Quantitative separations of hydrocarbons using Class I sorbents 28 Mineral. Chabazite Synthetic zeoIite (BaAl,Si,O,,,nH,O) Mixture. Temp. of removal. 150" 185 150 210 150 216 220 212 200 _ _ _ ~ ~ - 160 160 160 160 20 Component removed by sorption. C3H8 n-c4H10 C3H8 n-C4H10 n-C4H10 n-C7H16 n-C7H16 C,Hs and n-C,H, Some generalisations emerge from the separations effected (1) Monosubstituted met,ha,nes in which the substituent groups are small (Cl CH, OH CN NH, and the like) are very rapidly occluded by chabazite while monosubstituted ethanes with similar substituent groups are sorbed a1 R.M. Barrer J . SOC. Chem. Id. 1945 44 133. 308 QUARTERLY REVIEWS considerably more slowly. Both classes of solute can be quantitatively removed from molecules listed in col. 3 section (i) of Table IV. (2) In mordenite monosubstituted methanes were slowly occluded but monosubstituted ethanes were excluded. Separations were then obtained of substituted methanes with substituents C1 CH, OH CN or NH from similar ethane derivatives or carbon compounds with three or more carbon atoms. (3) Ca- and Ba-mordenites which did not even sorb simple methane derivatives appreciably separated only simple inorganic molecules (NH, H20 HCI) from organic species. Separations by Partial Molecular-sieve Action.-Table IV shows in a qualitative way the great extremes in rates of intracrystalline sorption.For example sorbates in col. 1 are occluded rapidly but those in col. 2 are only slowly occluded and with a great diversity of velocities even inter se. Thus in mixtures of such species it is possible to obtain separations by partial molecular-sieve action. Separations have been carried out both by exposing the mixture to the zeolite in a static system or by gaseous or liquid chromatography.l 3l One sorbate is occluded and finally removed in the time interval during which the other constituent is still largely unsorbed. Partial or complete separations were found in the mixtures C,H,-C,H ; CH,*CN-CH,Br ; C,H,*OH-CH,Br ; C,H,~OH-n-C,H, ; CH,*OH-C2H5Br ; the molecule first mentioned being removed first The most rapidly occluded molecules should preferably be more polar than those remaining.The more polar the sorbed molecule the greater the affinity between it and the zeolite tends to be. The affinity between sorbent and sorbate can exert IL strong influence upon the intracrystalline sorption velocity 25 as well as lessening the possibility of poisoning the sorbent by blocking of the intracrystalline diffusion paths by the bulkier and less mobile second component. Scope of the Molecular-sieve Method.-The molecular-sieve method can be one of unusual power which sometimes supplements other sepasation techniques. Mixtures may be resolved quantitatively which cannot be dealt with by distillation either because the species have practically the same boiling point (n-heptane and isooctane) or because azeotropic mix- tures are formed (H,O-C,H,*OH ; CH,*OH-CH,*CO*CH ; H,O-dioxan ; CS,-CH,*CO*CH ; C,H,-OH-n-heptane ; C,H,*OH-toluene).Many indi- vidual separations should be typical for groups of analogous compounds. For instance mordenite removes methyl from ethyl alcohol and ought therefore to remove it from all alcohols. Ca- or Ba-mordenite since they dry methyl and ethyl alcohol or acetone should dry all alcohols and ketones. Select'ivity in sorption shown by molecular-sieve zeolites is often the reverse of t)hat normally shown in more open capillary structures such as silica gel. In gels porous on this more macro-scale the affinity between sorbate and sorbent often increases with increasing molecular dimensions for such homologous series as the paraffin hydrocarbons.In the gas- sorbing zeolites on the other hand the sorbate-sorbent affnity drops BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 309 sharply to zero once a certain limit in molecular size and shape is exceeded. Possibilities already realised with mordenite of modifying the zeolites by cation interchange or in other ways and so of altering the molecular- sieve behaviour as desired raises the question of the availability of natural zeolites as raw materials. Except perhaps in the case of mor- denite large natural deposits are not known so that practical developments must be limited until synthetic crystals can be made cheaply. Crystals of m~rdenite,~~ a n a l ~ i t e ~ ~ harmotome,3* and a chabazite substitute 21 have been successfully grown on a laboratory scale. Gmelinite levynite chaba- zite gismondite and other zeolites which may be valuable have not been prepared.Molecular-sieve Properties of Membrmes Metals.-We have now to consider selective transmission of gases and vapours through compact membranes. Some metals act as total molecular sieves being permeable to one or two gases but to no others. In all such cases there is a specific interaction between the diffusing gas and the metal. Transition metals often have the power of forming interstitial solid solutions with some elements of the first short series of the Periodic Table. These elements must be small enough to enter the interstices in positions where their co-ordination number may be either 4 or 6 (so called tetrahedral or octahedral sites). Such phases occur only when the radius ratio of non- metallic to metallic elements is about 0.6 or less and when the difference in electronegativity between non-metal and metal is not too great.For instance though hydrogen carbon and nitrogen form a considerable num- ber of these phases oxygen forms only a few and fluorine does not form a n ~ . ~ b The interstitial atoms often show considerable mobility within the metal and if the metal is in the form of a membrane an atmosphere of the alloying* gas may be transmitted through it from the high- to the low- pressure side.36 The rate of permeation is not usually large but such membranes have been suggested as slow controlled leaks for hydrogen (using palladium) or for oxygen (using No other gases are trans- 32 R. M. Barrer J. 1948 2158. 33 Cf. idem Trans. Puratdag Soc. Discussion on Crystal Growth 1949 No.5 326. 34 Idem unpublished data. 35 For two discussions of properties of these phases see R. E. Rundle Actu C'ryst. 1948 1 180 and R. M. Barrer Tram. Fa.raday SOC. Discussion on Physical Chemistry of Metallurgical Processes 1948 No. 4 68. 36 A discussion of the solution and diffusion of gases in metals will be found in C. Smithells " Gases and Metals " Chapman & Hall 1937 ; and in R. M. Barrer " Diffusion in and through Solids " C.U.P. 1941. 37 E. L. Jossem Rev. S c i . Instr. 1940 11 164 * The term " alloy " is in many ways an appropriate description of tliese interstitial solid solutions because they retain metallic conductivity often show super-conductivity and have a metallic lustre. Moreover as in many alloys the composition of the phase may vary continuously within limits.The nature of the bond has received considerable attention. 3ii 310 QUARTERLY REVIEWS mitted by these metals. Similarly iron transmits both hydrogen and nitrogen under conditions where the corresponding interstitial phases form. There are two experimental procedures for studying the diffusion of gases in metals. In one the metal is used in the form of a hollow cathode. This method has hitherto been limited to the diffusion of hydrogen.38 In the second procedure one side of the metal membrane is exposed to the gas a t high temperature the other side being maintained under a near- vacuum. In the electrolysis method surprisingly rapid transmission may 70 4/1 FIG. 10 Exponential increase in permeation velocity P with temperature ( P = P,e-E/RT) [Reproduced by permission from Barrer “Diffusion i n and through Solids ” C.U.P.1941 p . 164.1 occur even a t room temperatures but in the thermal method temperatures upwards of several hundred degrees Centigrade are ne~essary.~g 36 Per- meation velocities increase exponentially with temperatures (Fig. lo) 40 and some typical permeability constants are given in Table VII.41 The non- 38 E.g. M. Bodenstein 2. EZektrochem. 1922 28 517 ; G. Borelius and S. Lindblom Ann. Physilc 1927 82 201 and see also ref. (36). 39 R. M. Barrer Trans. Paraday Soc. 1940 36 1235. 40 Idem ref. (36). 41 The data are taken from Barrer ref. (36) Table 42. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 311 metallic element always diffuses in the form of atoms so that sorption of diatomic molecules is preceded by dissociation.The essential features of the diffusion process may be illustrated by a diagram in which characteristic periodic variations of energy are shown as a function of the positional co-ordinate of the diffusing species relative to the membrane (Fig. 11). Distance - x i o FIG. 11 bounded by the planes x = 0 and x = 1. A diagrammatic representation of energy relations in diffusion through a membrane Adsorption is an exothermal process the heat AH being given by the depth of the trough in the potential-energy diagram just outside the sur- faces of the membrane. E, E, and 3 are very closely related" respec- tively to the energies of activation for penetration from the surface to a point just inside the metal for " jumping " from one interstitial site to the next and for passing from just within the surface to the surface itself.E and E are similarly related t o the energies of activation for adsorption and desorption. The heat of occlusion is AH, and may be either positive or negative. The phase-boundary reactions occurring with activation energies E and E are sensitive to modification in the nature and extent of the sur- face due to sintering or because of adsorbed films of foreign atoms.36 The metal surfaces show some of the variability in activity which is shown by the same surfaces functioning as catalysts as the data in Table VII illus- trate. *l Phase-boundary reactions may then become much slower than diffusion within the metal. Palladium shows extreme variations due to this cause. A partial separation of isotopic species hydrogen and deuterium can be effected when these are simultaneously transmitted through palladium or platinum.On account of their different masses the isotopic atoms have in their interstitial environment different zero-point energies that of the lighter element being the greater. For this reason energies of activation for diffusion heats of solution and solubilities of the isotopes will differ. * A more exact relation is energy of activation = height of energy barrier minus the zero-point energy of the diffusing atom. 312 QUARTERLY BEVIEWS System. TABLE VII Permeability constunts P for several metallic membranes P = Poe-E/R1' Po (c.c. per see. per cm.* per mm. thick per atm. pressure). H2-Pd 2.3 x lo-' 3.0 x lod2 I -I-___ P,/P,. ~ _ _ _ _ _ 1.84 1.40 1.36 1.24 H2-Ni Temp. 20" 106 131 186 1.3 x 0.85 x 10-2 1-4 x 1-05 x lop2 1.44 x 1.55 1.50 1.35 1.36 1.27 E (cals./g.-atom).System. ll _ _ _ _ ~ 550" 650 750 850 950 15,420 11 H,-Pt 13,800 13,400 _____ 13,260 I 0,-Ag I1 13,860 i Po (c.c. per sec. per per mm. thick per atm. pressure). 2.3 x 10-3 1.5 x 10-3 ________ 1-41 x 1.18 x 10-2 2.6 x 10-2 3.75 x 10-2 2.06 x B (cals./g.-atom) 16,600 18,700 19,600 18,000 19,800 22,600 22,600 - In the steady state and for a simple diffusion through a membrane into a vacuum in absence of slow phase boundary processes PH/~D = DHCH/DDCD where the P's D's and C's are respectively permeability diffusion coeffi- cients and concentrations just within the ingoing surface. Some experi- mental values of PH/PD for platinum 42 and palladium 43 are given in Table VITI. TABLE VIII Permeability ratios for hydrogen and deuterium Analytical for P / P .Analytical expression for PH/PD. Silica and Silicate Glasses.-Membranes of silica glass transmit helium neon and hydrogen and to a lesser extent oxygen nitrogen and argon. The diffusion process occurs without dissociation of the diffusing molecule and without specific interaction between gas and membrane as is the case when gases diffuse in metals.44 Selectivity is considerable but total molecular-sieve action does not occur (Table IX). The diffusing molecules 42R. Joum J. Phys. Radium 1936 7 101. 4 3 A. Farkas Tram. Farday SOC. 1936 32 1667. 4 4 For a discussion see R. M. Barrer ref. (36). BARRER MOLECULAR-SIEVE ACTION OF SOLIDS . . . . . . . . Gas. He. Hz. Ne. Na. A. Relative permeability * . 100 18 3-3 2.6 1.6 Apparent energy of activa- tion (cals./g.-atom or ~ ~ g.-mol.) .. . . . 5600 10,100 9500 I 29,900 32,100 I 313 oa. ~ - 31,200 TABLE IX Relative permeabilities of silica g h s at 900" I Glass. I I Relative Apparent energy of activation (cals./g.-atom or g.-mol.). * There are considerable differences between permeabilities obtained when using different examples of silica glass. 44 or atoms move in channels which sheathe them very closely. In silicate glasses the diffusion paths become still more restricted and possibly fewer in number as indicated by a rising energy of activation or decreasing permeability towards helium (Table X). The apparent energies of activa- tion in Tables IX and X are probably very nearly true energies of activation because the heats of solution of the gases in silica glass at least are very sma11.45 Silica .. . . . . . . Pyrex . . . . . . . Leadglass (283') . . . Jena 16111. . . . . . Thuringian glass. . . . Soda glass (283"). . . . 100 12.1 0.31 0.117 0.114 0.027 5700 8700 - - 8720 11,200 True and apparent energies of activation for intracrystalline diffusion in various zeolites are recorded in Table 25 By contrast with some silicate glasses and zeolites helium cannot under any conditions diffuse through metals and in increasing order of openness of diffusion paths we may write the series Metals < silicate glasses < silica glass < levynite mordenite < chabaeite In all these systems the diffusing species move entirely within the range of action of the interatomic forces of the solid. The condition described by equation (3) (in which diffusion occurs in part as above and in part as a molecuIar streaming) requires appreciably wider channels.46 G. A. Williams and J. B. Ferguson J . Amer. Chem. SOC. 1924 46 635 ; H. Wiistner Ann. Physik 1915 48 1095. 314 QUARTERLY REVIEWS TABLE XI Some true and apparent energies of activation for intracrystalline diffusion in zeolites Crystal. Chabazite Natural crystals Mordenite Ba-rich crystals Ca-rich crystals Na-rich crystals Li-rich crystals NH,-rich crystals K-rich crystals Levynite Natural crystals Diffusing gas. 2 P C2H6 CH,CN A Kr A Kr A Kr H2 0 2 N2 A Kr Ne A Kr True activation I A>parent activation energy energy (cals./g.-mol.). (cals./g.-mol.). I 6600 9800 8100 11,500 - - 9300 11,000 7300 7600 7000 9000 2500 4400 4800 8400 10,000 2600 9400 12,000 4500; 6700 8900; 8600 7100; 6800 11,100 ; 11,400 ; 9600 5900 7300 N 40,000 Organic Membranes.-Pronounced molecular-sieve effects arise when gases and liquids diffuse through organic foils.Some of these membranes such as cellulose and some proteins are largely crystalline ; others includ- ing natural and synthetic elastomers are normally amorphous. The elastomers provide an interesting new feature in the diffusion process because individual molecules of the elastomer possess considerable flexi- bility and (within limits) mobility which play an important part in the diffusion of solutes within the medium. Each unit diffusion process is believed to involve not only the solute molecule but also segments of polymer chains adjacent to the solute in a zone of activation in which BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 315 co-operative movements of polymer segments and solute result in a successful place change of the solute r n ~ l e c u l e .~ ~ ~ 47 SUbhUF %. FIG. 12 Permeability constants of some hydrocarbons in natural rubber vulcanisates ( P in C.C. of gas at N.T.P. passing per second through 1 cm.2 of membrane 1 mm. thick under a pressure diflerence of 1 cm. of Hg). [Reproduced by permission from J. Polymer Sci. 1948 3 554.1 TOO 80" 80 e Sulphur % . FIG. 13 Diffusion coeficients of some hydrocarbons in natural rubber vulcanisates (D in cm. 2sec.-1>. [Reproduced by permission from J. Polymer Sci. 1948 3 554.1 Number of carbon atoms FIG. 14 Solubility constants u for some hydrocarbons in a rubber vulcani- sate containing 1.7% of combined sulphur. The upper curve was obtained at 40° the middle curve at 60",and the lowest curve at 80".G i s measured in C.C. at N.T.P. dis- solved in 1 c . ~ . of rubber at 1 atm. pressure. [Reproduced by permission from J. Polymer Sci. 1948 3 566.1 - _ _ _ _ _ _ _ _ _ ~ __ . ~ _ _ 4 0 R. M. Barrer Trans. Faraday SOC. 1939 35 628. 47 R. M. Barrer and G. Skirrow J . Polymer Sci. 1948 3 549 564. 316 QUARTERLY REVIEWS It is interesting to trace the effects of systematically reducing the fiexi- bility and mobility of elastomer chains upon the permeability of a group of solute molecules which are themselves of regularly increasing molecular volume. Cross-linking by vulcanisation was used to reduce the mobility of elastomer chains and the n-paraffins from methane to n-butane were used as solutes. Several of the effects observed are illustrated in Figs.12 13 and 14. There was no marked change in the solubility of a given hydrocarbon as the amount of combined sulphur increased but the dif- fusion coefficients decreased steadily and the decrease was greatest for the solute of greatest molecular TABLE XI1 Relative permeability (P) and diffusion coeflcients (D) at 40" 100 73 27 17 17 I Natural rubber (1.7% combined S). I Natural rubber (11.3% combined S). 1 1.00 100 2-17 - 4.16 15.5 3.46 4.95 10.2 4.41 Gas. Relative P. I 1.00 2.90 7.4 13.0 31.8 Relative D. I Relative P. I Relative D. Typical relative permeability and diffusion coefficients are shown in Table XII. The relative value of P is proportional to the product of the diffusion coefficient D and the solubility constant. The solubility constant increases rapidly with increasing molecular weight of the paraffin (Fig.14) whereas D decreases (Fig. 13) so these two effects influence the permeability constant in opposite senses. The net effect is however that P becomes larger as the molecular weight of the diffusing hydrocarbon becomes greater (Fig. 12). However increasing the combined sulphur in the rubber decreases D proportionately more for higher molecular-weight paraffins while leaving the solubilities largely unaltered. Hence the molecular-sieve action towards the paraffins as measured by the relative value of P becomes smaller in the vulcanisate of larger combined sulphur content. On the other hand for the permanent gases where the solubility con- stants are all of the same order P usually decreases with increasing mole- cular weight.In this case the molecular-sieve action of the rubber towards the gases increases as combined sulphur content of the vulcanisate increases (Table XIII) .48 Clearly in ebonite a highly cross-linked three-dimensional network molecular dimensions exercise a dominant influence. As a final example of selective transmission one may give some relative permeabili- ties towards polar and non-polar gases of foils of celluloid rubber and gelatin (Table XIV).49 The greater permeability towards polar gases is due to their much larger sorption in the polar media. Natural-rubber membranes normally contain polar impurity such as protein. 50 48 R. 35. Barrer Trans. Faraday SOC. 1940 36 644. 50 C. Boggs and J. Blake Ind. Eng. Chern. 1926 18 224. Idem ref. (36). BARRER MOLECTTLAR-SIEVE ACTION OF SOLIDS Molecular Relative permeability in diameter,.A. low-sulphur rubber i at 250. 317 Relative permeability in high-sulphur rubber (ebonite) at 67’. TABLE XI11 Relative permeatbilities of gases in rubber and ebonite 100 3.0 1 161 4.08 I 25.8 He. . . . . H,. . . . . N . . . . . ~ Gas. 100 30.0 0-53 TABLE XIV Relative permeabilities towards polar and non-polar gases Gas . . . . . . . . . Celluloid . . . Gelatin . . 100 100 100 247 286 100 898 6160 906 1 2500 1 3600 1 413 3190 9520 Molecular-sieve Processes in Solution Although it is not possible here to discuss in detail data on selective transmission of ions and dissolved species through solids nevertheless some reference to the phenomena noted should be made. Glass membranes functioning as hydrogen electrodes are normally regarded as permeable to hydrogen ions; and other glasses may function similarly as sodium potassium silver or zinc electr0des.5~ Membranes composed of clay crystallites can also function as electrodes reversible with respect to ions which they may take up by cation interchange.62 Crystalline zeolites ultramarines sodalite-hauyne minerals as well as amorphous zeolite gels such as ‘‘ permutit ” or ‘‘ doucil ” also interchange cations comparatively freely.63 b49 55 Sometimes there is a high selectivity in these exchange reactions analcite (hTaA1Si206,H20) will exchange Na+ for K+ or NH,+ but not for Li+ Cs+ Ca++ or Ba+’+ save to a minor extent ; leucite (KA1Si206) similarly exchanges K+ for Na+ or NH,+ but not for the other ions.Such selective exchanges indicate the possibility of preferentially 51 See W.M. Clark “ Determination of Hydrogen Ions ” Baillisre Tindall & Cox 1928 3rd Edn. p. 430 for a summary of such observations. C. Marshall and C. Krinbill J . Amer. Chem. SOC. 1942 64 1814; C. Marshall and W. E. Bergmann ibid. 1941 63 1911 ; J . Physical Chem. 1942 46 527 326. 63 H. F. Walton J. Franklin Inst. 1941 232 305. 5 4 R. M. Barrer International Colloquium on Reactions in the Solid State Paris 66 S. B. Hendricks I f i d . Eng. Chem. 1945 57 625. Crystals and glasses are often highly selective in transmitting ions. Oct. 1948 ; Bull. SOC. chim. 1949 Nos. 1 and 2 D.71. 318 QUARTERLY REVIEWS interconverting sodium potassium or ammonium salts by hydrothermal chromat~graphy.~~? 56 Other zeolites such as chabazite or mordenite or zeolite gels interchange a much greater diversity of cations.Crystalline zeolites show cation interchange freely in the range 150-250" and less rapidly below 150"; zeolitic gels organic exchangers and some clays however interchange ions freely even at room temperature. Both organic and inorganic gel membranes may serve as ultra-filters for dissolved species whether ionic or non-ionic. Membranes may there- fore establish and maintain differences in potential pH or concentration between opposite faces. They may bring about hydrolysis effect dialysis and permit also the establishment and measurement of osmotic pressures. Reasons for the selective transmission giving rise to these effects may be considered. Membranes which selectively transmit cations include polyacrylic acid co-deposited with acetyl cellulose ; oxycellulose ; cellulose impregnated with dyes containing acid groups ; and to a lesser degree nitrocellulose and acetyl cellulose which both contain some free carboxyl groups.On the other hand membranes may be prepared which contain both anionic and cationic groups for instance by condensation of triethanolamine and phthalic acid. Here there are tertiary amino-groups and carboxyl groups. I n alkaline media where the ionisation of the amino-groups is suppressed and that of the carboxyl groups promoted the medium is cation-permeable. However if the tertiary amino-groups are converted into quaternary groups by reaction with methyl iodide the membrane is anion-permeable under all conditions. Protein membranes are also amphoteric and according to the pH of the solution they transmit anions (in acid media) or cations (in alkaline media).Basic membranes sorb acid dyes acid membranes sorb basic dyes. Clearly electrostatic forces govern in part the selective trans- mission in such systems.57 In membranes without ionisable groups and of very open character the mobility of ions is substantially the same as in aqueous solution. As the capillaries become more and more restricted the mobilities become increasingly different from their values in " free " trans- port. In membranes which also contain ionisable groups these molecular- sieve effects may be superposed upon the further specificity due to electro- static attractions or repulsions between charged groups in the organic net- work and the ions diffusing. A quantitative interpretation of the over-all transmission is due to T.Teorell 58 and to Meyer and Sie~ers.5~ The relative permeation velocity is NJN, where N and N are the numbers of cations and anions transmitted per unit time. This ratio in the steady state depends on the product of the ratios U/V and C,/C, where U and 7 are mobilities of cation and anion respectively and Cc and S. Green and C. McCarthy I d . Eng. Chem. 1944 36 412. 67 K. H. Meyer " Natural and Synthetic High Polymers " Interscience 1942 68 PTOC. Soc. Exp. Biol. Med. 1935 33 282. 69 Address to the Assoc. Chim. de GenGve 1935; Helv. China. Acta 1936 19 p. 631. 649 665 987 ; K. H. Meyer H. Hauptmann and J. F. Sievers ibid. p. 948. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 319 Ca are the concentrations of cations and anions just inside the ingoing surface of the membrane.As an example one may consider an uni- ivalent electrolyte which is fully dissociated. Let the concentration in the solution be C and let X denote the concentration of non-diffusible fixed ionic groups in the membrane expressed in g.-equivs. per litre of imbibed liquid; X is a constant characteristic of the membrane at any one liquid content. Also Y denotes the concentration of imbibed electro- lyte in g.-equivs. per litre of imbibed liquid. In a membrane where the fixed ionic groups are anionic the concentration of diffusible anions is Y and of cations X + Y . The Donnan membrane equilibrium condition yields * C2 = Y(Y + X ) which may be combined with the relation Nc/Na = U ( Y + X ) / V Y = UCc/VCu to give N u 1 / 4 m 2 + x v 1/4c2 + x2 - x _ _ _ - Na In this equation one sees separately the effect of the mobility ratio and that of electrolyte concentration and concentration of fixed anionic groups upon the permeability ratio.According to the equation as C becomes very small the network of the anionic membrane contains only diffusible cations and is permeable only to cations. At higher electrolyte concentrations both ions are transmitted. Membranes with graded pore sizes have been prepared from cellulose nitrate or acetate. These membranes can be used in the separation and characterisation of colloidal particles bacteria and viruses.60s 61* 62 The ester dissolved in solvents such as acetone was poured on to a suitable surface and the solvent allowed partly to evaporate. The foil still con- taining solvent was further precipitated with coagulating agents such as dilute acetic acid.By varying the conditions of formation (nature of solvent amount of solvent in partly dried film and amount of coagulant) the texture of the porous membrane was altered and a series of graded ultra-filters obtained which transmitted particles of up to 500 mp. in diameter. Such membranes must be stored in water to prevent the irre- versible shrinkage which would follow drying. Standard particles such as 80 H. Bechold and K. Silbereisen Biochem. Z. 1928,199 1 ; H. Bechold Kolbid-Z. F. Erbe {bid. 1932 59 32 ; 1933 63 277. 1934 66 329; 1934 6'7 66. 6 2 W. J. Elford Trans. Faraday SOC. 1937 33 1094. * This equation in terms of activities should be written c2 = Y(Y + X)YcYu/Y+Y- = Y(Y + X ) R where 'ye and ya are the ionic activity coefficients of the imbibed ions and y+ and y- me theso coefficients in the surrounding electrolyte.The correction factor R may then be introduced. Y 320 QUARTERLY REVIEWS hzmoglobins were employed to calibrate the membranes. It is considered that the particles being separated should be more or less spherical and that this molecular-sieve method is not suitable for separating chain polymers. The Reviewer acknowledges gratefully permission to reproduce illustrations as follows the Faraday Society Figs. 1 5 7 and 9 ; the American Chemical Society Fig. 4 ; Messrs. Macmillan & Co. Ltd. Fig. 8 ; the University Press Cambridge Fig. 10 ; and Interscience Publishers Inc. Figs. 12 13 and 14. He also thanks the Faraday Society and the Aberdeen University Press Ltd. for the loan of blocks for Figs. 5 6 and 9.