ELECTROCHEMISTRY OF CRYSTAL-POLYMER MEMBRAXES
April, 1960
the CalculiLted weight gain assuming no hydration of products enables one to calculate the amount of water adsorbed a t equilibrium a t that particular temperature. From this the equilibrium water concentration can be calculated. All rate constants obtained were corrected to an arbitrary constant value of water concentration which was chosen at the highest temperature employed. 'The proportionality constant y is defined as the ratio of this arbitrary value (2.0 mg. H,O/g. product) to the concentration at the given temperature and is given by the expression, y = 0.2/(18.6 Wr). Multiplication of the experimentally obtained rate constants by the appropriate y d u e gives a rate constant which is proportional to the diffusion rate of water vapor through the nearly unhydrated product. The variation of the new rate constant with temperature should now follow the normal temperature dependency expected for a diffusion process with constant concentration (Arrhenius equation). The Arrhenius plots for these new constants are presented in Fig. 2, where the expected straight-line relationships are obtained. The least squares slopes of these curves yield approximately the same activation energy, z i x . , 10.9 f 0.4 and 10.2 0.3 kcal./molr, as would be expected if the same mechanism pre1-ailed during the entire reaction. Since a parabolic mechanism was found, it is evident that the product film is protective; the reaction r:ite decreases as the thickness of the film increases. -1 protective film is expected only when there is not much change in molar volume during rea c t i o n . 8 ~ ~Hence, ~ assumption (4) in the introductory section is experimentally verified. X parabolic mechanisni indicates that the rate(15) J. hl. Dunoyer, Ann. Chzm., 1121 6 , 165 (1951).
r0
*
6.0
I
I
I
417 I
5.6 5.2
< 4.8 5
E" 4.4
'C 3
2
2
k-4.0
- 3.6 M
1
3.2 2.8 2.4 1.8
2.6 2.8 3.0 3.2 103. Fig. 2.--Arrhenius plots for parabolic kinetics; T = absolute temperature. 2.0
2.2
2.4
I/T
x
controlling step is volume diffusion through the film.16.17 The controlling step is probably the diffusion of water vapor from the surface to the reaction interface, although the desorption of gaseous reaction products also might be controlling. (16) N. Cabrera and N. F. M o t t , R e p t s . Prog. Phiis , 12, I63 (1949). (17) E. A. Gulbranson and K. F. Andrew, J. Electrochem., 99, 402 (1952).
ELECTROCHEMISTRY OF CRYSTAL-POLYJIER ISIEMBRILINES. PBRT I. RESISTASCE MEASURENESTS BY
R. At.
BA4RRERAND
s. D. JAMES
Physical Chemistry Laboratories, Chemistry Department, I m p e n a l College, S. W . 7 , England Received August 27, 1969
Attempts have been made to prepare membranes in which microcrystalline ion-exchanging zeolites are bonded by inert polymeric fillers in such a way that the electrochemical behavior is determined by the crystals and by crystal contacts. Measurements of electrical conductance of the bonded membranes have, however, shown that some crystal- resin pores arise which have an influence upon electrical transport. The effect of these pores upon transport can be limited by soaking the membranes in an electrochemically inert liquid. Electrolyte in crystal-resin pores does not possess the properties of bulk electrolyte, but reduces interfacial polarization a t points of contact between crystallites. This polarization becomes more marked if crystal-resin pores are in part filled by electrochemically inert liquid; it is, however, reduced by a rise in temperature.
1. Introduction et u Z . , ~ this work has been concerned with organic Since the pioneering work of Teorell' and Rfeyer PolYelectrolYtes.4 The present study, on the other hand, utilizes and Sievers? made possible the quantitative interpretation of membrane potentials, much work has some of the crystalline zeolites having robust, 3been devoted to the study of transport processes ( 3 ) c E Marshall, TIIIB J O U R U G , 43, 1155 (1939) c. E. hlarsliall . SOC , 63, 1911 ( I W I ) , c E \larin membranes containing ion-exchange materials. and W. E Beimnan, J ~ m Chem and C. A. Krinbill, tbtd., 64, 1814 (1942). C. E Marsliall and With the r,otable exception of the mark of Marshall, shall 4. D 4yer, abid., 7 0 , 1297 (1948); C. E. hlarhsall, Titie JOURNAL,6 3 , (1) T.Teorell, Proc. SOC. Ezptl. Bzol. Med., 33, 282 (1933). 1284 (1948). (2) K. H. Rfeyer and J. F. Sierers, Helu. Cham. Acta, 19, 649 (1936).
(4) E.Q , Faradag Soc. Dasc., no. 2 1 (1936).
R. M. BARRER AND S. D. JAMES
418
dimensional crystal lattices. The aluminosilicate framework of these crystals bears a net negative charge, compensated by cations located in the intracrystalline channels and cavities. The zeolites employed include the most open crystalline structures known. They are ionic solids, permeated by channel systems and intracrystalline cavities which constitute a large proportion of the crystal volume. These channel systems are permeable to simple cations and also to small uncharged molecules such as water. The known ion-exchange properties of the zeolite~~ indicate -~ that, as single crystals, they possess excellent potentialities for use in cation-exchange membranes. They have crystal structures which undergo little or no swelling in contact with electrolyte solution and t'he dimensions of which are unappreciably sffected by ion exchange. Their high exchange capacit~ies(4-6 meq./hydrated g.)8 and small pore dimensions render the zeolit'eshighly effective in the exclusion of Doiinan diffused salt, from the cryst,al structure. L-ptake of anions (or in general of neben-ions) from electrolyte solut,ion, which above conceiit,rat~ionsof about 0.1 m YaC1 result,s in loss in select,ivity by the organic gel eschanger membranes, occiirs only to a very limited extent in the crystalline zeolite^.^ However single, crystal plat'es of these materials cannot normally be obt'ained, and accordingly we have investigated the behavior of crystalline powders, in matrices of inert polymer, in order to see holy nearly .the ideal behavior of ion-sieve single cryst'al plat'es of zeolite can be approached by such heterogeneous membranes. The ion exchangers used'o in these membranes were S a X (synthetsicnear-faujasit'e, NazO A1203.2.(j7Si02.G.7H20) and NaA (a synthetic zeolite of which our sample had the composition S a 2 0.AI2Oa.2Si02.O. 14Xad10z.4.77H20). 2.
Experimental
hIembrtnes were, in general, prepared by the compression moulding (using a book-press) of intimate mixt,ures of a finely powdered polymer and the zeolitic crystals under examination. Seven membranes were prepared using a hydraulic moulding pressure of 12 tons/sq.in. Thermoplastic resins used were: polythene, polystyrene, and polymethyl met1i:tcrylate; the only thermosetting resin employed was a stage €3 phenol rrsin. The finished membranes wrre about 2.7 cm. in diameter and 1 mm. thick. Some of these membranes were treated with an electrochemically inert liquid (silicone fluid or dinonyl phthalate) subsequent to their fabrication bj. moulding. The liquid was introduced into the mcmhranes under vacuum after an outgassing period of 0.5-1 hour a t 10-3mm. A few membranes were prrpnred by the polymerization of methyl methacrylate in rompressed plugs of zeolitic crystals. Methyl methacrylate, containing -0.1:; benzoyl peroxide, was admitted to t,hc coniprmsed plug under vacuum, and subsequently polymerized at, 50". Membrane thickness was reduced to the required value (usually about 1 mm. or less) by n2:ichining on a lathe. -___
( 5 ) R.h1. Rnrrcr. W. Huscr and W. 1'. Grrittcr. R e i u . Chim.B c l n , 39, 318 (1956). (0) R. .\I. Brirrer and L. 31. Ileier, Trans. Fiwadau SOC..64, 1071 (1958); 66, 130 (1959). (7) R. 31. Bnrrer and D. C . h l n m o n , J . Chem. Soc., 2838 (1953); 675 (1956). (8) R . SI. Barrrr. I'IYJC. Chorn. ,S'OC.. 09 (1958). (9) R,31. Barrer and W. 31. hlcier, J . Ciiom. Sue., 299 (1958). (10) These materials were kindly provided by Linde Air Products Co. They arc seinj~les,respectirely, of Linde .\Iolecular Sieveb 13X
and 4.1.
Vol. 64
The electrical resistance of membranes was measured using an a.c. Wheatstone bridge, usually a t 1000 c./sec. The membranes were clamped, in KaCl solution, between rubber gaskets and the ground-glass flanges of "Industrial Glass Piping," 5/*" joints. Electrical contact with the salt solution was made through stout discs of platinized platinum welded to tungsten rods sealed through the glass wall of the half-cell. The resistance cell was placed under a thick, thermally insulating layer of cotton wool in a brass tank which was $amped in a water-bath thermostated to within zko.1
.
3. Electrical Resistance and Apparent Activation
Energies for Cationic Migration The electrical resistance of newly moulded membranes was substantially decreased by continued exposure to salt solution; the resistance at 25' of an unequilibrated membrane was lowered by as much as 607, after the resistance had been studied as a function of temperature up to 75". Hence, before measurements were made, the membranes were equilibrated in ambient 0.1 S S a C l solution a t 80' for 3 4 days. After this treatment, plots of resistance us. temperature normally became reproducible. The measured resistances in 0.1 S S a C l solution a t 25" of equilibrated membranes containing Sax, varied with the crystal content from about 200A for membranes containing 50% (by volume) of crystals t o 14000h for a 20% membrane. This corresponds to a variation in specific resistance from 4 X lo3 to 2.8 X 105A/cm. The apparent activation energy E , obtained from the slopes of plots of log[Resistance] against the reciprocal of the absolute temperature, shows considerable variation. For twelve K a S membranes varying in % NaX and in the nature of the inert resin, the mean value of E in the interval 25-75" varies from 4.6, kcal.!/mole for a 45% Sax and polythene membrane to 6.70 kcal./mole for a 50% NaX and polystyrene membrane (Table I). In the Arrhenius plots for the membraiies there is a distinct and smooth curvature through the temperature range (Fig. 1); in all cases, the E value measured by the slope of the curve at the lower temperature is considerably greater than that a t the higher temperature. Csually the value of E in thc interval 25-35' is higher by about 2 in 6 kcd. 'mole thnii 2;' in the interval 66-75'. The above results suggest that membrane resihtance is largely determined by the imbibing of ambient salt solution. It is highly uiilikely that the activation energy for cationic migration within crystals of YaX changes, in a 40" interml, by the proportion cited above for S a X memhrane~. The curvature of Xrrhenius plots must, therefore, reflect the preqence of imbibed salt wlution in the membranes. There is a strong rewmblance hetween the curvature of these plots a i d that of similar plots made for 0.1 S S a C l mlutioii. T h e TI idc mriation in E d u e s among the tm-cl\.e S:rX 121cinbranes stidied can he attrihuted to :I variatioii ill the state of agglomeration of zeolitic cryqtals in the resin matrix, and in particular t o a variation in the extent to which crystal-resin pores pcmmit permeating salt sollition to Jjypzss the cryqt:iis Plectrically. This conclusion is supported by meawrements made on the mater absorption oi the menihraneq. They were shown to absorb quaiititiw of nater up
ELECTROCHEMISTRY OF CRYSTAL-POLYMER MEMBRANES
April, 1960
419
TABLEI E I\ I Ein. However, the method of analysis, even if oversimplified, may have some general application to heterogeneous membranes. It would he of interest to develop it further if a more reliable means of obtaining E, can be devised. flow paths and so convergence of flow lines is probably not important. T h u s it is expected t h a t E, should be similar for K c and K,i. Equality is assumed in eq. 6. (15) For this membrane a t T = 30", K,lO = 0 00134 mho, Em'Q = 6780 cal /Avogadro no of unit conduction processes, while a t T = 70°, K,70 = 0 003731 mho and Em70 = 4640 c d
ELECTROCHEITISTRY OF CRYSTAL-POLYMER MEMBRASES. PART 11. MEMBRANE POTESTIALS BY R. M. BARRER AND S. D. JAMES Physical Chemistry Laboratories, Chemistry Department, Imperial College, London, S. W . 7, England Received August 6 7 , 1969
The selectivity of membranes consisting of crystalline zeolite powders in inert polymer matrices has been investigated by e.m.f. measurements of membrane cells in homoionic and heteroionic electrolyte solutions The zeolites used were Linde Sieve A, near-faujasite (Linde Sieve X), chabazite, analcite, and in addition an aluminosilicate gel exchanger. Selectivity was imperfect when moulded membranes were used, or large pieces of crystal sealed into plastic. Polymerization of methyl methacrylate around partly dried gel zeolite or crystal powder produced selective membranes, and also some excellent selective membranes were obtained when moulded membranes containing Linde Sieve A or X were impregnated with silicone oil.
Introduction A single crystal plate of a zeolite crystal if free of cracks should act as a membrane permeable to suitable cations and impermeable to anions. Moreover such a plate would show ion-sieve activity toward cstions of different size, so that t o sufficiently large cations it could be electrochemically inert. However, as pointed out earlier
(Part I),l such single crystal membranes are fragile and difficult to prepare and preserve. Heterogeneous membranes composed of a sufficient concentration of zeolite crystallites bonded by inert polymers fall short of the above ideal, as a study of their resistance has shown,' but considerable interest attaches to their electromotive behavior (1) R. M. Barrer and S, D. James, THISJOURNAL, 64, 417 (1960).
