MEMBRASE POTENTIALS OF CRYSTAL-POLYMER MEMBRANES
April, 1960
due to crystals and to solution, respectively. finally Km
=
K,
42 1
Thus and from eq. 7 and 4,and 8 and 5, we may rliminnte K , or Ks K,,, respectively, to obtain
+
+ K, + 2 "(3) + Ksi K K
Kci
Two limiting cases arise: K,, >> Kciand K,, >> These relations have the same form and n-e cannot differentiate between (Kc KO,)and Kc, so that K,,, which, from (3), give, respectively they are effectively identical. If K, is > (Kc K , = K, K , K,, (4) Kc,), or Kc, in the relevant expression, then E, > and E,. Since Kmrepresents the whole membrane conductivity and K, a part only, these inequalities Km = Ks Kw Kc (5) If we now assume that Arrhenius equations may be should be satisfied. However, as an example we applied to KmJ K,, Kci and Ks, a t any particular consider further one of the expressions (9). The values of E, may as a first approsirnation be temperature, we have taken as those or E , for the quartz-polystyrene K , = K,Q exp ( -Em/RT); K, = Kcoexp (-E,/RT); membrane, where crystal conductivity need not be K,, = KciC8exp (-E,/R?'); K,, = Ksioexp ( - E,/RT) considered (4.7 kcal. a t 30" and 4.0 kcal. a t 70"). (6) In the expression K , = Kcoexp( -E,/Rl') we may, Kmo, Em, KSloand E, can be temperature-depend- as noted above, take Kcoand E, to he independent of ent,ll but studies of the electrical conductivity~*.l3 temperature. Then using the relevant results for of ionic crystals indicate that this should not be K , and Emfor the Sax-polystyrene membrane true of K,", Kc: and E,. By differentiation and 34SX3 at 30 and a t 7Ool5we may eliminate K , and obtain substitution we derive from eq. 4 and 5
+
+ + + +
+ (Kc + Kci)Ec
(7)
L E r n = ( K 84- KBi)E84- KcEc
(8)
KmEm = KaEs
or (11) Bs refers to the conductivity of the electrolyte not in bulk eolution, b u t in the thin electrolyte films constituting the solution paths. It is likely therefore t o be modified relative to E for bulk electrolyte. (12) J. F. Laurent and J. BBnard. J. Phys. Chem. Solids, 3, 7 (1957); 3, 218 (1957). (13) R. %I. Barrer. "Diffusion in and through Solids," C.U.P., 1951. Chapter 6. (14) I n a chain of crystals in physical contact, and in absence of a conducting liquid forming menisci around the points of contact, current flow linea would converge through the contacts, and space charge effects might modify E,, a s compared with this energy for parallel flow lines in 8 single crystal. However, the liquid provides alternate
E,(0.0837)
+
+ 0.0673 = log ( E B- 4~7
-
G )
where E, is in kcal. Thence Ec 3.4 kcal., a value which is almost certainly considerably too small, since as noted we would expect E, > 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. Thus 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).
R. AI. BARRER A \ D S.D. J a m s
422
T'ol. 6-1
freer of bias potentials under the operative conditions. The e.m.f. of the membrane cells was determined potentiometrically. Amplification of the out-of-balance current made possible an accuracy of 1 0 . 1 m.v. even v-ith membranes of low conductivity. With some membranes tht. initial elrctricd resistance was very high resulting in instabilitv of the measured e.m.f .'s due to electrical interferences. This was obviated by surrounding the whole of the electrical circuits including the membrane cell, by earthed screens. AAccur:tte measurements rould then be made much earlier than would otherwise hst\.e been possible. After prolonged conditioning in salt solution the membrane resistance usually became low enough for the screens to be dispensed xvith. The membranes were supported between salt solutions as for the resistance measurements,' and provision was made for a rapid stream of nitrogen bubbles to be directed against the faces of the membrane in order to agitate layers of solution adjacent to the membrane. The humidity of this nitrogen stream was suitably adjusted by passage through the appropriate thermostated electrolyte solution, before it entered the cell, so that no concentration changes occurred in the cell as a result of the stirring. The membrane cells were thermostated in an air-bath a t 25" (zkO.3"). The seven NaCl solutions (-2-0.002 m) used in characterizing membrane selertivity in homoionic cells are identical with those employed by Spiegler, et a1.8 The ratio of mean (Jlectrolvte activity between consecutive pairs of these solutions is ronstant a t 3.00 (rt0.02). Thus, six pairs of solutions were available for direct comparison of membrane selectivity over the range -2-0.002 m, the maximum cell e.m.f. (corresponding to complete se1ect:vity) being 56.4 mv. (=to 3) in each case. Measuiements also were made of the e.m f . of cells in +3 4 3 7 33 which the membrane separated two solutions each containing two cations, such as sodium and cesium or sodium and tetraethylammonium chlorides. ilctivity coefficients (y) of tetraethylammonium chloride were evaluated by the method '4 of Randall9 from osmotic coefficient data given by Lange.lo Activity coefficients in binary mixtures of electrolytes were calculated by means of Glueckauf's equation." 4 5 8 COY5 Time Dependence of the Cell E.M.F.'s.-The general So t o n s 2 02,10673 m procedure in e.m.f. measurement was as follows. llfter a Fig. 1.-Time dependence of membrane cell e.m.f. in KaCl period for equilibration (usually overnight) during which solutions (MSX5:Ka;Y/polystyrene). .Ag/AgCI electrodes; the membrane cell was set up in the thermostat, the YaCl solutions were discarded and fresh solutions, always a t the Kernst e.m.f. = 56.4 mv. I, 11, etc., indicate the introduction cell temperature, were introduced. The cell e.m.f. was of fresh solutions into the cell. then measured till a steady value was recorded. The soluand selectivity as compared with organic ge12-6 tions were refreshed again until a reproducible value of the e.m.f. was observed. The variations of the e.m.f.'s or clay7 membranes. Accordingly a study has stable TTith time are shown in a typical instancc in Fig. 1 (membeen made of membrane potentials of a series of brane MSX5, Part 1,1 silver chloride electrodes). In the heterogeneous membranes in which the dispersed different runs, using one particular concentration ratio, phases were Linde Sieve A, Linde Sieve X (syn- the e.m.f. rarely followed the same initid curve, but the quasi-stable e.m.f. usually occurred a t about the same thetic near-faujasite), chabazite, analcite and a value. In a period of hours or dags following the subsiding synthetic aluminosilicate gel zeolite. of initial efferts a gradual decrease in e.m.f. was generally observed, which was often detectable only as a very long Experimental term effect (Fig. I), and was more marked in the stronger The preparation of typical membranes has been described sodium chloride solutions. This probably is not due to in Part 1.1 The selectivity of membranes to cation trans- osmotic transfer of water through the mrmbranes, since a port \\ ab examined by observing the e.m.f. of concentration direct experiment failed to detect measurxble transfer in a period of some days. In this experiment movement was cells studied of liquid levels in fine capillaries attached to sealed reference electrode NaCl membrane NaCl cell compartments completely filled by different electrolyte reversible to C1ml containing m2 solutions and separated by a moulded membrane. Very h-a-zeolite long time-scale drifts may therefore be a result of slow reference electrode widening, and increase in number of, the direct solution reversible t o CIpaths, TI. ith consequent gradual diminution in selectivity. The reference electrodes were initially silver /silver chloride, This behavior could follow breaking of polymer-crystal but ralomel electrodes were later employed, having proved contacts by the electrolyte solution. .4nv drifts observed on this time srale depended upon the character and history of the membrane, and vere for some membranes almost (2) E.Q., Faraday Soc. Dzs. No. 2 1 (1936). imperceptible. (3) K. Sollner, Suensk. Kemi. Tzdskrzft, 6-7, 267 (1958). Selectivity of Moulded Membranes.-The e.m.f. (4) S. Dray and K. Sollner, Biochzm. P h w . Acta. 21, 126 (1956).
I[
