A. S. TOMBALAKIAN, I€.J. BARTON, AND W. F. GRAYDON
1006
dissolved species3 a t low pH values, is progressively reduced as GC(OI-I)~ +HGe(OH)3
+H?Ge(OEI)?+ H3GeOH +GeHl
Grecii and Robinson's failure to detect thcsc intermediates in the cathodic gas suggests that the germanols may have acidic properties as havc their carbon analogs and thc alkyl germanium hydroxyls. Thc mcchariism of rcductioii postulated does
Vol. 66
not involve the participation of clementary germanium among the intermediate products. Acknowledgments.-The author is indebted to J. I. Carasso for much helpful discussion, to Miss C. M. Lovett who carried out the mass spectrometric analysis, and to J. A I . McPherson for experimental assistance. Acknowlcdgment also is due to the Engineer-in-Chief of the British Yost Office and to the Controller of Her Britannic Majesty's Stationery Office for permission t o publish.
ELECTROOSMOTIC WATER TRANSPORT ACHOSS ION-EXCHANGE MEMBRANES B Y 11. s. ' ~ O M B A I A ~ ~ K I A H. N , ' J. BARTON, A N D \v. li'. GIZAYDON Department of Chemical Engineering and Applied Chemistry, University of Yoronto, l'oronto, Canada Receiaed September 1 1 1961 ~
A study of clectroosmotic transport of water across polystyrrnesulfonic acid ion-exchange membranes, having various exchange capacities and degrees of crosslinking, has been made. Thc results indicatc that water transport by electroiismosis is a function of the internal ion concentration of tho mcmbrane pore solution and the membrane ionic form. A marked dependency of clectroosmotic watcr transport on current density also is noted only a t high current densities.
Introduction Thc results of previous measurements of electroosmotic transport of water across polystyrenesulfonic acid ion-exchange membranes have been reported.2 Direct; measurcrnents of water transport accompanying various cations by clectroosmosis across similar membranes over a wide range of cxternal solution concentration arid current density are given in this report. This study was undcrtakcn to obtain further cxpcrimciital data on thc magnitude of electroosmotic water t'ransport, across these ion-exchange membranes. Thc dcpeiidence of electroosmotic water transport on membranc properties such as exchange capacity, moisturc coiitcnt, intenid ion concentration of t,he membrane pore solution, aiid mcmbranc ionic form was dctcrmiiicd. Experimental A. Membranes.-The membranes used in this work were preparcd by the bulk copolymerization of the propyl ester of p-styrenesulfonic acid with styrene and divinylberizene and subsequcnt hydrolysis to produce polystyrcncsulfonic acid as described previously.3 Ilowcver, instead of rising a petri dish and floating it, ovcr a mercury bath in the oven, a stairilcss stecl dish with a glass plate cover was used. This cwablcd the prcpai-ation of a mernbrune *5.25 in. long and 3 in. aide, which perrnittoci thc cutting out of various sizes of mcinbr:incs :is thcy wcre necdcd during thc course of this investigation. In tho oven the stninloss stccl dish was watcd ovcr a tablc which was supportod by threc adjustable screws. The membranc formed did not show variation in thickness more than 0.003 in. Thc mcmbranes arc designated by two digits. The first digit reprkents the nominal exchange capwity of the membrane, while the second digit represcrits the mole pcr cent. of divinylbenxcnc used in t,hc preparat,ion of the membrano. .
. .
( 1 ) Chemistry Department, Laurentian University of
S u d b ~ r yCanada. , ( 2 ) R. *J. Stewart and (1957). (3) ( a ) I .
Sudbury,
W. F. Graydun, J . Phua. Chern., 61,
184
IT. Spinnrr, .J. Civic, and M', 1:. Graydon, Can. J . Chcm., (b) W. I?, ( ; I . H Y ~ U I I and 11. .1. Stcwart, J. I'hi/s. Chcm.,
32, 1 . u ( I!).-VI);
69. 8 0 (195,j).
B. Membrane Moisture Content and Internal Solution Concentration.-In thc past it has been customary to measure the moisture content of an ion-exchange resin by the vapor sorption method and present it as onc of the main characteristics of the ion-exchange resi11.11~~ The moisture contents of the membranes w r c determined at 25' in thc lithium, hydrogcn, sodium, arid potassium forms of the resin with external solutions in the concentration range pure water to 4.0 JI. I n Table I are given the moisture contents of the membranes in the leached lithium, hydrogen, sodium, and potassium forms in contact with pure water. It can be seen from the data in Table I that the quantity of water sorbed by the membranes, in the given cationic forms of the resin, decreases in the ordcr lithium, hydrogen, sodium, and potassium. TABLE I MEMBRANE MOISTURE CONTJGNT Mmnbraiie nu.
Capacity, inoq./g. dry Moisturo content, g. IIz0/eiiiiivitlent resin N e form Li form H form Nn form K form
2-6
1.12 0.85 1.11 1.13 1.14 1.40 1.77
1.5-6 2-8 3-6
J .32 1.58 2.80
1-2 I -6B 1-8 1-10 1-6h 1-4
560 282 227 202
558 275 226 199 302 368 332
558 280 224 198 291 362 330 180
542 264 208 180
I70 27.1
Tho interiinl iou coiic:c:ntrittion of t h e n1cnit)ranc: 1)ore solution may be evaluated from ita capacity and moisturo content. For this purpose Stcn.art3'>used the membrane cxchange capacity and moisture content by the vapor sorption method. For the membranes used in this work the internal concentrations were determined both m the ratio of membrane exchange capacity to moisture content by thc vapor sorption method, and as the ratio of total cation content (exchange capacity plus sorbed electrolyte) to moisture content by direct contact with solution. The difference in the internal ion concent,ration of the membrane pore solution evaluated by the two methods was on the average 10% and
ELECTROOSMOTIC WATERTRANSPORT ACROSS ION-EXCIIAKGE MEMBRANES
,Tunc, 1902
t,he maximum deviation between the two occurred at about 1 M external solution concentration, in which solutions the vapor sorption method gave values of about 15% less than the direct contact method. I n general, the observed variations in internal solution concentration with membrane capacity and cross-linking by the two methods were very similar. C. Membrane Swelling and Density.-The thickness of a surface dried 3-6 membrane was measured with a micrometer gtsge to 0.0001 in. following the method of The mcmbrane first was allowed to come to equilibrium a t 25' in water and this procedure was repeated for solution concentrations of 0 . 1 , 0.5, 1.0, 2.0 and 4.0 M equally concentrated in sodium nitrate and hydrochloric acid. The membrane bulk volume in contact with the above solutions :it 25" also was measured using a pycnometer. The density of the membrane was evaluated by dividiiig the wet weight by the bulk volume determined. U'it,h an increase in the external solution concentration from pure water t o 4.0 M , the density of the 3-6 membrane was found to increase from 1.326 g./ml. t o 1.581 g./ml., while the membrane thickness decreased nearly 10% and the membrane bulk volume decreased by approximately 25%. This indicates t h a t within the precision of the measurement there is no marked deviation from isotropic swelling. D ElectroSsmotic Water Transport .-Measurements of elecixoosmotic water transport were made using a lucite cell following the method of Stcwart.2 The capillaries fitted into t,he cell first were calibrated by the mercury weight method. Tho electrodes were silver rods coated with silver chloride. A current densit,y of 1 mamp./em.z was used. Iluring the measurement of water transport, the cell was placed in a water-bath controlled to 25 zt 0.1 '. The heights of the solutions in both the capillary tubes were measured using a cathetometer. A measured current was passed through the cell for a time detcrmiricd by a stopwatch arid the changts in the heights of solutions were observed. This was repeated four times by reversing the direction of current every timc. All the data reported here are average values of four individual mcasurement,s. The solutions of lithium chloride, sodium ohloride, potassiuni chloride, and hydrochloric acid used were in the concentration range 0.05 to 4.0 1M. The effcct of current. density on electroiismot.ic water transport also was dctermined. The current densities varied from 1.076 to 107.6 mamp./em.*. I n Table I1 are listed the observed values of water transport at different current densities. With high current densities in the range 26.0 t o 107.6 mamp./cm.z the number of moles of water transferred per equivalent of t,ransfcrrcd sodium ions decreased as the current dcrisit,y was incrc:tsed. This decrease i n water transport, m:ty be due to the polarization of the mombrane a t high current densities. The resultant decrease of moisture content for one membrme face would reduce the water trarisport value. With a current density below 1.076 m:imp./cm.2 all ionic forms of a membrane showcd no depcndcnce of water transport on current density.
.
1'hmc,r OF
1
,I
m
$ 5
4
'br50\2 01
0 4
0,s
2b
I?
I
I
IO
LO
a.0
44
SODIUM CHLORIDE,M
lcig. 1.-l~~lcc:t,ro6smotic water transport for a 3-6 mcnibrine at 25'. Water transport values across the inembraric scparating different solutions of sodium chloride are represented by the dark ciralos. The upper and lower curves define the values of water transport for the mcmbyane iri contact with the dilute solutions and tho membrane in contact with thc coneentratcd soliitions, respectivtrly. Thc solutions of sodium chloride used werc in the conccnt,rat,iori range 0.05 to 4.0 AI.
LITHIUY
CHLORIOE,
M
Ipig. 2.-l~~lcct.roiit;motic water tr:tnsport, for various tiieiiibrarics nsirig solutions of lithium c:hloridc i n the, c:oric:ctntr:ttion rarigr 0.06 t,o 2.0 .If. 'I'he quantity AV/18 is approxiinat,ely tho molw of water transported per faraday by lithium ions.
contact with a coilcentrated solution. The values obtained are close to tho values of water transport for the membrane in contact with the conccntratod solutions. This indicates TABLE 11 that the drier face of a membrane regulates the water transCURRENT D m w w O N WATERTRAKSPORY port through the membrane. 7 -
0.010761.076
hIenihruno no.
NaCI ~oln.. inolea/l.
:I4
0.1 1. o
7.9 5 . (j
4.0
3 . :3
inamp./ cin.2
Water transport, AV/I8--26.90 inamp./ cm.'
7.7 5.5 3,2
58.82
107.6
iii:iinj)./ CIII.~
tiitmi),/ c1n.2
7.2 6.0 2.8
0.3 4.1 2.2
order l o dotctrniino thct crstctit t o which the drier fiicc o f a mombrane regul:it,cd the water trmsport, the wttcr t,r:tnsport across a 3-8 membrrino separating diBcreiit solutions of sodium chloride over a wide mrigct of solution coriccntrat,ion \vas dctermiricd using a current, densit,y of 1.070 rn:imp./ (mi.*. Thc observed valucrs of water transport arc plotted in Fig. 1 against the cstcrrial solution conccritration together with thc values of water transport representing the values for the dilute and concentrated solut,ioris. It can be seen from Fig. I th:Lt, the values of water transport across :imemt m n e scptmting differcnt solutions of sodium chlorid(: frtll b d W(WI t h v v:dut:s of a.;~i(tr 1 rrtrisport, for t h c sttinct mcrnb r m c i n c:otit:lc:t willi a dilulc solutioii a t i t l tho rricrnbr:iiic i i i Iri
r
1007
Discussion rl'hci dat'n obtaiiictd for ihc \.oluinc: cshaiqy i i i oil(:-half of t h o clcciroiismotic cell per far:Ld:iy for some mernbraim usiiig varioiis solutions of lithiiiiri chloridc, sodium c:hloridc, pohssium chloridc, aiid hydrochloric acid and B current density of 1.076 mamp./cm.2 arc prcscritcd in Fig. 2 , 3, 4, and 5, rcspcotivcly. The quantity AV,/18 has bceii shown2 t,o bc 3, good approximation to thc niolt:s of wat'cr transported per faraday by sodium ions at any of the solution concentrat,ions used. It can bc secii from Pig. 2 , 3, 4, and 5 that the water transport decrcascs with increasing external solution concentration and also in Iht: ordcr lithium, sodium, potassium, arid hydrogen. This latter is coiisistcnt with t,hc Trariat ion in the cffcct,ivc of those ions in thcir hydrutcd forms. 111 the
A.
1008
S.TOMILILAKIAN, 11. J. I ~ A I ~ T O SASD , W. F,GRAYDON
LO
05
SODIUM
CHLOniDf,
M
Fig. 3.--Electroosmotic water trans ort for various membranes using solutions of sodium chlorife in the concentration range 0.05 to 2.0 M . The quantity AV/18 is approxiniately the molcs of water transported per faraday by sodium ions. 16
1
Vol. (iti
watcr per mole of lithium, about four moles of watcr per mole of sodium arid potassium, and about one iuole of water per mole of hydrogen. The observed variations in water transport with the external solution concentration are very similar to the variations in membrane moisture content. In general, water transport for any one ion appears to follow the moisture content2*6of the membrane in that ionic form. Although the moisture content of a membrane, as shown in Table I, differs only slightly with ionic form, a large difference in water transuort between lithium, sodium, potassium, and hydrogeii ions is observed. The moving ion exerts the major control over the magnitude of water transport. I n Table 111 are given the values of the ratio of the observed water transport of one ion to the water transport of another ion for a series of membranes having nearly the same exchange capacity. I t can be seen that the ratios of water transport for any two ions are quite constant regardless of membrane network tightness within the wide range of external solution concentrations investigated. This indicates that thcrc is w r y little sieve effect resulting from membrane network iiiterferencc. TABLJG I11 RATIOOF WATERTRANSPORTS bfcinbranc
---If/i$a-0
2
1-2 1-4 1-6A 1-6B 1-8 1-10
0.306 .306
0.339 .332 ,298 .317
no.
POTASSIUM
CULQalQE.
M
Fig. 4.-Uectroosmotic water transport for various membranes using solutions of potassium chloridc in the coneentration range 0.05 to 2 0 M . Thc quantity AV/l8 is approximatrly the moles of water transported per faraday by potassium ions.
4t \--.
HIDI1OOEM
CHLORIDE,
II
M
Fig. 5.--Elcctroosinotic water transport for various membraneu using solutions of hydrochloric acid in the conccntration range 0.05 to 2.0 J f . The yuantit AV/18 is spproximately the moles of water transportedYper faraday by hydrogen ions.
( w e of the high crosslinking membranes, it appears that the limiting values for water transport have been reached. The 1-10 membrane was an cxccdingly "tight" membrane requiring many weeks t o hydrolyze and rcprcsents vrry nearly the limiting condition of membrane internal solution concentrut ion. For this membrane the valurs of \\atrr transport ;we reduced to about six molcs of
.250 .272 ,280 .288
,301 .307
-Li/K---
-Nn/li---. 0
0
2
1.79
1.65
1.08
1.17
1.82 1.78 1.81
1.72 1.70 1.76
1.17 1.09 1.10
1.10 1.08 1.10
2
The major determining parameters in water transport for a given ion appear to be the membrane moisture content and pxchangc capacity, that is, the internal concentration of the membrane pore solution. If the values of water transport for several membranes, having the same nominal exchange capacity, are plotted against the membrane equivalental moisture values (moles of water per equivalent of ion-exchange resin) over the entire range of sodium chloride solution concentration, a linear relationship between water transport and membrane equivalental moisture is observed. The slope of the line gives the ratio of water transport to membrane equivalcntal moisture (U)T/ZOD). An empirical representation of this depcndencc on membrane exchange capacity is given by '2= 0.38(Exchange Capacity)".'*
WD
where w T / w ~ is the ratio of water transport to membrane equivalental moisture. I n Table 1V are givrn the obscrvrd and calculated values of thtb ratio of watcr transport to equivalental moisture for thc membranes in the sodium form using the empirical relationship given above. It can be seen that good agreement is found between the ralculated and observed values of tht. ratio of water transport to membrane equivslcntal mois( 5 ) 4. G. \Vinger, 556 (1036).
R. I'eiguson, nntl R. Iiunin, .I. Phv? Chetn.. 60,
turc. T h e ovcr-all mean deviation of thc observed values from thc calculatcd results is 0.037. Similar ratios rcportcd by Stcwart2 also havc bcen compared to the valucs calculatcd by thc cquation, with a mcaii dcviatioii of 0.024. 1 t A T l o OF \\'hTEIt
Ifernbrsne
no.
TABLE IV ANSP SPORT TO ~ I E M B R A NEQUIVALENTAL E
-Sodium
0
Menn dev.
chloride solution-
Calcd. value of
4
U!T/WD
value
0.40 .40
-0.030 - .025 .005 .030 .025
niolsrity 1 2
and membrane pore wall may be rcprcsc.iitcd by a simple viscous forcc balance. A moving io11 under thc influence of an applied clcctric potcntial will exert a force on the water which will be balanced by the friction force bctwccii thc water and tho stationary mcmbranc porc wall. If the forces of interaction arc assumed proportional t o tho rclative velocities, then this IVould lead to the relatioilship
from the caled.
is the proportionality constant rcprcscnting the ratio of thc forces exerted by thc moving ion and the water to the velocity of the ion relative .35 to the water, and fww is the proportionality con.40 + stant representing the ratio of the forces exerted .43 + by the water and the membrane pore wall to the .39 .036 velocity of the water relative to the membrane .44 + ,050 pore wall. .46 - .050 I n the simplest case this ratio (WT/WD) might .48 ,015 be expected to be constant and indcpciidcnt of .60 - ,090 membranc paramcters. Ilowever, in a series of In Table lrarc givcn similar ratios of water trans- membranes having various cxchangc capacities port to equivalcntal moisture for the other mem- and degrees of crosslinking, the observed dccrcase brane ionic forms. The ratios listed in Table V in thc value of the ratio (WT/WD) with decreasing show a marked variation with membrane ionic mcmbrane exchange capacity indicates ail increased form. The capacity cffcct for each ionic form is water-membrane pore wall viscous intcraction. similar to that for thc sodium form as rcprcsontcd The depcndcnce givcn by the above equation is of this' sort and shows a dccrcasc in the quantity by the above equation. W T / W D with decreasing membrane exchange capacTABLE V ity. Since the exchange capacity is a measure of RATIOOF WATERTRANSPORT TO MEMBRANE EQUXVA- the amount of organic membrane material per sulfonate group, the cffect noted above is consistent LENTAL MmsruRE, VARIOUS IONIC FORMS IIIeinbrane Ext. w l n . , nith the view that thc watcr-membrane porc wall no. rnoles/l. Idithiurn Potassium Hydrogen viscous interactions incrcase with increasing amount 0.57 0 37 0.10 1-10 0 of the organic membranc material prr sulfonate .40 .ll 2 .58 group, Although some variation in the quantity .?I6 .IO 1-8 0 .57 WT-WD may be observed as a result of variation 2 .50 .37 .ll in external solution concentration and crosslinking, .10 1-613 0 .55 * 33 thcse effects arc nearly within the present limitj of 2 .56 .34 .ll experimental error, and rcquirc further study. .50 .15 1-2 0 .84 Acknowledgment.-The authors are indebted .ll 2 .46 .31 to the National Research Council, Ottawa, Canada, OIICmight consider that interactions within a the Ontario Itesearch Foundation, and to the Adrnmbranc during water t ransport, betwccn the visory Committee on Scientific Research, Univcrnio\.iiig ion and watcr, and betwecn the water sity of Toronto, for financial support. 1-10 1-8 1-6B 1-6A 1.5-6 1-2 1-4 2-8 2-6 3-6
0.36 0.38 .37 .38 .36 .35 .45 .41 .48 0 . 4 6 .44 0 . 4 3 .51 .34 .55 .43 .45 .44 .44 .41 .51 -48 -57 .53 .50 .44
+ +
+
11 here frw
