556
ALVING . WINGER,RUTHFERGUSON AND ROBERT KUNIN
Vol. 60
THE ELECTROOSMOTIC TRANSPORT OF WATER ACROSS PERMSELECTIVE MEMBRANES BY ALVING. WINGER,RUTHFERGUSON AND ROBERT KUNIN Rohm & Haas Company, Philadelphia, Pennsylvania Received July 8% 1066
The electroosmotic transport of water across Amberplex ion exchanger membranes was measured, as a function of internal and external electrolyte Concentration, current density and nature of the electrolyte. The results were interpreted by assuming that the net water transport is the difference between the amounts of water transported in opposite directions under the influence of mobile cations and mobile anions. The average number of water molecules transported per ion was calculated for each ion type and correlated with ionic potentials of the unhydrated ions and with intramembrane ionic mobility ratios as measured potentiometrically.
Introduction Permselective ion exchange transmit water by electroosmosis along with ionic species during electrolysis in aqueous solution. This study was undertaken to obtain experimental data on the magnitude of this electroosmotic transport of water across ion-exchange membranes EMa function of the membrane properties and the properties of surrounding solution, since this effect is of considerable practical importance in any electrochemical process involving membranes.4,6 A very simple model was adopted to interpret the experimental data, in which a certain average number of water molecules were assumed to move with each kind of mobile ion in its movement under a potential gradient. For a binary ionic system the model assumed above leads to a simple equation for net water transport
ion under study, leached with deionized water, and equilibrated in the solution to be used in the electrolysis. B. Apparatus and Procedure. 1. Single Membrane, Two-chambered Cell, with Pt Electrodes.-A two-chambered, single membrane cell with platinum electrodes was used with hydroxide solutions, where the net result of the electrode reactions would be an electrolysis of water. The half-cells of this apparatus were machined out of Plexiglas and had a capacity of about 150 ml. each. An exact description is given in the literature.6 Circular screen latinum electrodes were mounted close to the mem!rane clamped between the half cells and openings were provided in each half cell for motor driven stirrers and for sampling. The two chambers were filled with two solutions of a certain concentration difference and the electrolyses were run for such a time as re uired to reverse the concentration difference between theftalf cells. Since the time average of the concentration gradient across the membrane was thus nearly zero for the entire run, the effects of diffusion and osmosis were minimized. Liquid levels in the two half cells were essentially equal and constant during an electrolysis, eliminating any hydraulic transfer acrosss the membrane. The average concentration of the two solutions remained ractically constant due to the equal volumes of solution on 0th sides of the membrane. At the end of the short, highcurrent-density (90 amp./ft.* = 0.097 amp./cm.*) runs employed, the volume changes were determined by careful pipetting and the concentration changes by titration of ahquots of acid or base. From these data and the current and time values, electrical transport numbers of the ions and the transport of solvent per faraday were calculated. Calculation of the latter involved correction of volume change data for transport of electrolyte and loss of water by electrolysis and evaporation. Because of the limitation of this method to strong alkali or non-volatile acid solutions, and the difficulty in measuring accurately the volume changes in the system, only preliminary results were sought which would give an indication of the effect of high current density on the apparent ion-solvent interaction and test the validity of equation 1. 2. Two-chambered, Single Membrane Cell with AgAnCl Electrodes.-A two-chambered single membrane cell wyth Ag-AgC1 electrodes was used for electrolysis of metal chloride solution, where net result of electrode reaction would be transfer of salt from anolyte to catholyte. The cell was machined from two blocks of Plexiglas with each chamber 3.5 inches in diameter and '/a inch deep. A membrane was inserted between the two sections, with a thin rubber gasket on each side and sealed tightly by means of four screws through the outside section. Two holes '/a inch in diameter were drilled diagonally in the top of each section for filling and emptying, and for stirring. The electrodes were three inch squares of 20 mesh silver screen, the corners of which were turned back about '/2 inch to serve as spacers keeping the electrodes close to the membrane surface. To prevent the electrodes from contacting the membrane, a circle of Saran screen was placed between each electrode and the membrane. The silver chloride cathode was prepared by placing it in the cell in a solution of concentrated HCI (10%) and passing an amount of current sufficient to form a slight excess of AgCl for subsequent electrolyses. After the f i s t electrolysis the electrodes can be reversed for another run.
g
where AW is the net moles of water transferred across the membrane per faraday, to and t, the electrical transport numbers of cation and anion in the membrane, nc and n, the average number of water molecules moving with cation and anion, respectively, and 2, and 2, the numerical value of the cationic and anionic valence, respectively. AW is considered positive when net movement of water is in the direction of the cationic movement.
Experimental A. Membranes.-The membranes used in this work were Amberplex C-1 and Amberplex A-1 membranes.8 These are heterogeneous membranes composed of sulfonated polystyrene cation-exchange resin and quaternary base anion-exchange resin, respectively, bound into tough pliable sheets by an inert plastic binder. A series of the former type of membranes also were used, with 4, 8.5 and 15% DVB as cross-linking agent. The sections of each type of membrane used in the electrode cells were taken from the same large sheet of membrane and each section was shown to be electrochemically practically identical with the others from that sheet, as judged from membrane potential and conductivity tests. Before use in an electroosmotic measurement, the membranes were placed in the proper ionic form by long equilibration in concentrated solution of the (1) M. R. J. Wyllie and H. W. Patnode, THIS JOURNAL,64, 204 (1950). (2) W. Juda and W. A. McRae, J . A m . Chem. Soc., 71,1044 (1950). (3) A. G. Winger, G. W. Bodamer and R . Kunin, J . Electrochem. Soc., 100, 178 (1953). (4) A. G. Winger, G . W. Bodamer, R . Kunin, C. J. Prizer and G. W. Harmon, I n d . Eng. Chem., 47, 50 (1955). (5) W. R . Walters, D. W. Weiser and L. J. Marek, ibid., 47, 61 (1955).
(6) H. C . Bramer and J. coull, ibid., 47, 67 (1955).
May, 1956
ELECTRO~SMOTIC TRANSPORT OF WATERACROSS PERMSELECTIVE MEMBRANES
In order to weigh the contents of each chamber accurately, a vacuum line trap with a special ipet type side arm for reaching into the chamber was use:. since it was impossible to remove the contents of a chamber completely even with this trap, each chamber was rinsed with the solution to be used in it and emptied with the above trap before pipetting the final solution into a chamber. In this manner error due to residual solution was practically eliminated. The entire trap, side arm included, was weighed before and after filling with the contents of each chamber. The approximate water transport per faraday of current was obtained from preliminary runs and used to calculate the coulombs necessary to transport about 2 g. of water across the membrane. The initial concentration difference between the two solutions on either side of the membrane was chosen so that passage of the calculated number of coulombs would reverse this difference, minimizing effects of osmosis and diffusion. After rinsing the cell as described above, 75 ml. of the two solutions of known density were pipetted into the chambers, the stirrers inserted, holes stoppered, and the carefully timed electrolysis begun. The ammeter was a Weston Model 280 and a 250-500 ohm variable resistor was connected in series with the cell to smooth out any current fluctuations in the low resistance cell. Constant current was maintained until the voltage drop across the cell terminals reached 1.7 volts, a t which time the run was terminated or continued for another time interval at a new, lower current density. When the calculated amount of electricity had been passed through the cell, the chamber contents were emptied into reviously weighed traps as described and weighed to t t e nearest 0.01 g. The initial and final solutions were titrated for chloride ion with standard silver nitrate using dichlorofluorescein with dextrin as indicator. The increase in chloride content (moles per faraday) in the catholyte or the decrease in the anolyte is a direct measure of the electrical transport number of the cation across the membrane, assuming the membrane to be in required ionic state. At very low concentration of external solution some electrical transport is undoubtedly due to H80+ion from the water, but the error due to this effect was considered negligible for the concentrations used. The changes in weight in catholyte and anolyte, corrected for electrolyte transfer, were generally not numerically equal. Since unavoidable loss by evaporation during the necessarily lengthy runs was shown to be the chief cause of this, the numerical average of the gain and loss in catholyte and anolyte, respectively, was taken to be the most nearly correct measure of the water transport. The individual runs were considered reproducible under identical conditions, to within =!~30/~. For chloride solutions of nickel and cobalt, a platinum cathode was used and the metal plated out on this electrode. Otherwise the procedure is similar to that described above and a propriately modified calculations give the values for no. the concentrations employed in this work, the cobalt and nickel ions are considered to be in hydrated divalent form, free of complex formation.
0
.I
.2
.3 4 5 .6 .7 .E MEMBRANE CATION TRANSPORT NUMBER.M,
.9
557
1.0
Fig. 1.-Electroosmotic transport of water across cationexchange membrane in hydroxide solutions: A, KOH; B, NaOH; C, LiOH. ’
At high concentrations of external solution, the transport numbers of the ions in the membrane approach those in the solution around it7 and the net water transport approaches zero. Extrapolation of the plots in Fig. 1, to conditions of zero net water transport, gives the following values for the cationic transport numbers in the membrane t,(LiOH) = 0.24, t,(NaOH) = 0.38, and t,(KOH) = 0.45
These can be considered the approximate cationic transport numbers in highly concentrated alkali solutions, about 6 molar. The detailed data for electrolyses of univalent chlorides in the cell with Ag-AgC1 electrodes are presented in Table I. The lithium chloride system was studied most extensively, because of its high water transport, to obtain information on the effect of such variables as current density, temperature, external concentration and membrane resin. The results in Table I indicate no definite dependence of water transport on current density or external concentration, in the ranges studied, for alkali cations. The values of n, for all above cations were calculated from the experimental data using equation 1 and assuming n, for the chloride Data and Discussion ion to be 9. This value was based on electrolyses In Fig. 1 the results of the electrolysis of strong of quaternary ammonium chloride salts using alkali solutions in a two-chambered single mem- Amberplex A-1 (anion permselective) membranes. brane cell using Pt electrodes and Amberplex C-1 is The transport number of the chloride ion in Amgiven. The cation transport number was varied berplex C-1 for the runs given in Table I was by changing the average concentration of the two always 0.1 or less, so that a considerable uncerexternal solutions on either side of the membrane. tainty in the na value used to calculate n, for those If equation 1 applies and the magnitude noand na runs would have little effect on the final value of in the membrane is constant and independent of n,. Table I1 gives the results of water transport external solution concentrations, the water transport per faraday should be a linear function of measurements on membranes with varying content cation transport number t,. Within the accuracy of the resin cross-linking agent, divinylbenzene. of the method used in these runs, this seems to be It is apparent that values of no are dependent on the case, as the plots are essentially linear. Extrapo- the ion concentration, Cf,within the resin, and/or lation of the plots to t, = 1 gives values of n, for on the “pore size” of the membrane resin. Howlithium, sodium and potassium of 14.1 f 1, ever, the n, value does not vary directly as the 8.6 f 1 and 7.3 f 1, respectively. The extrapo- water content of the resin or the membrane. As lations to t, = 0 are too lengthy for serious con- the decrease in degree of cross-linking permits more sideration but indicate a value of nOH- approx- water to enter the resin, most of the additional imately equal to 5. (7) K. Sollner, J . Eleclrochem. Soc., 97, 139C (1950).
It
ALVING. WINGER,RUTHFERGUSON AND ROBERT KUNIN
558
Vol. 60
TABLEI despite the conclusion in previous work that the TRANSPORT OF WATER ACROSSAMBERPLEXfixed ions in resins are udydrated. ELECTROOSMOTIC Bi-ionic potential measurements were made in C-1MEMBRANES A s DETERMINED FOR CHLORIDE SOLUTIONS this Laboratory on Amberplex C-1 membranes by IN CELLWITH Ag-AgC1 ELECTRODES Salt
NaCl NaCl NaCl KCl KC1 KCI’ LiCl LiCl LiCl LiCl LiCl LiCl LiCl LiC1“ RbCl RbCl CSCl CSCl ( CHahN C1 (CHa)aNCl (CH&HC1 OCH~(CH~)~NCI
Average concn., N
Current density (ma./om.B)
no
0.125 0.225 1.12 0.235 .235 ,991 .19 .50 .50 .50 .50 .50 1.10 0.50 0.20 1.00 0.20 1.08 0.145 .lo8 .IO9
1.93 1.95 2.25 2.44 1.4G 1.59 2.44 0.49 0.82 1.64 3.29 4.93 1.80 1.64 2.65 1.64 1.94 2.90 2.22 1.39 1.38
11.2 11.5 9.2 6.4 7.4 7.1 12.9 12.7 13.4 13.9 13.6 13.3 13.3 12.7 7.0 7.3 6.8 6.7 20.6 21.4 7.7b
,232
1.64
30
9.2b (II)CHs(CH&NCl .116 1.64 .24 1.78 5.5 HCl HC1 .50 1.92 5.6 HC1 .61 1.62 2.7 HCl .74 1.73 3.2 HC1 1.10 1.94 2.9 “Temperature of run = 1°C. All other runs at room temperature, 24-26”. b Amberplex A-1 used for this run, hence value given = mi-.
TABLEI1 ELECTROOSMOTIC WATER TRANSPORT ACROSS HETEROGENEOUS SULFONATED POLYSTYRENE MEMBRANES I N 0.5 N NaCl SOLUTION IS Membrane and binder
Water content of membrane % ’ D V B of resin, Wr membrane (g. HlO/meq. resin fixed groups)
A (Geon) 4 B (Geon) 8.5 C (Geon) 15 D (Polyeth- 8 . 5 ylene)
0.350 .195 .119 .195
Id
!I *Id
methods described in the literaturelO.ll to obtain the apparent mobility ratios of various ions in the membrane. The linear plots obtained when graphing bi-ionic potentials uersus mean molal activities agreed closely with the work of Wyllie and Kanaan for the monovalent ion pairs tried. Column 4 of Table 111 gives the results of-thg determination of apparent mobility ratios U i / U r . in Amberplex C-1 obtained for various cation pairs with lithium ion as reference ion. Column 5 gives the corresponding ratios of the number of water molecules apparently tending to move with each ion in the membrane n, (reference ion) n, (ion)
The gperal agreement between values for 0 (ion)/U(ref. ion) and n,(ref. ion)/n,(ion) is good and indicates that the mobility of the mobile ions in the membrane is inversely proportional to the magnitude of ion-water interaction for simple cations. If the magnitude of the interaction between polar water molecules and the charged unhydrated ions depends on the electrostatic potential near the ion, as might be expected, the observed direct proportionality between n, and ionic potential shown in Table I11 is not surprising. This correlation and the previously cited one suggest that the electroosmotic water transport across membranes of ion exchange resins gives an .indication of the relative state of solvation of ions within the ion exchange resins. TABLE I11 OF WATER TRANSPORT PERIONWITH IONIC CORRELAT~ON AND WATERTRANSPORT RATIOSWITH IONIC POTENTIAL MOBILITY RATIOS Ion
no
Ionic potentia** charge/ radius b.
Li + Na + K+ Rb CS Co++ Ni++
13.5 f:1 9.5 7.0 f1 7.2 0.5 6.9f 0 . 5 28 f 2 26 f 2
1.3 1.0 0.75 .67 .61 2.5 2.6
+
no
10.5 9.0 7.5 8.5
+
1.40 1.20‘ 1.00
2.94 1.64 1.00
* *
V (ion) (ion) 0 (ref. ion) (ref. ion) no
no
1.00 1.40 1.83 1.78 1.82 1.11 1.09
1.00 1.50 1.93 1.87 1.95 0.97 1.04
Ions with the largest “true” hydration numbers, when moving through the micro-pores of the membrane, will tend to transfer momentum to the water imbibed is apparently unaffected by mJbile most unbound water molecules giving highest counter ions or, presumably, by the fixed ions.8 values of n,. Recent determinations of “true” ion A recent publicatione of electroosmotic trans- hydration numbers by Glueckauf l2 show that their port of water across membranes similar to those values increase in the order Cs-K-Na-Li-Ca-Mg. Acknowledgment.-The authors wish to acknowlused here by Japanese workers suggests that the difference in water transport per ion for mem- edge the assistance of Ruth Cowell and Edward branes of differing cross-linking can be explained in Parkes in the experimental work reported in this terms of “fixed” and “mobile” water molecules, paper. (8) D. E. Boyd and B. A. Soldano, 2.Eleklrochem., 57, 162 (1953). (9) Y. Oda and T. Yawataya, Bull. Chem. SOC.Japan, 28, 263 (1955).
(10) M. R. J. Wyllie, THIS JOURNAL, 58, 67 (1954). (11) M . R. J. Wyllie and 8. L. Kanaan, ibid., 68, 73 (1954). (12) E. Glueckauf, Trans. Paradag Soc., 51, 1235 (1955).
