Mechanism of Neutral Carrier Mediated Ion Transport through Ion-Selective Bulk Membranes
a
A. B. Thoma, A. Viviani-Nauer, S. Arvanitis, W. E. Morf, and W. Simon” Department of Organic Chemistry, Swiss Federal Institute
of
Technology, Universitatstrasse 16, CH-8092 Zurich, Switzerland
reduces to a pure cation-response function of the Nicolsky type:
Analytically relevant neutral carrler membranes selective for K’, CaZf, Na’, LI’, and organlc cations respectively were studied. By applying an electrical potential gradlent, a catlon transference number of nearly 1 was found. To elucidate the mechanism of this catlon permselectlvlty, electrodlaiysis experlments as well as interdlffuslon studies were carried out on 0.02-cm valinomycin-based solvent polymerlc membranes, uslng ’‘C-labeled vallnomycln and millimolar aqueous solutions of 4zK+ and 36CI-. The carrler concentration proflle thus obtalned agrees with theoretical predlctions. The Ion concentratlon levels In the membrane (potassium: -0.5 mM; Chloride: -0.006 mM) lndlcate the presence of addltlonai anlonlc specles. Further experlments were performed to study the nature of these charged sites. They orlglnate from the aqueous system and offer an explanation for the catlon permselectivlty observed, the extraction behavior descrlbed by other authors, as well as for the electric characterlstlcs of neutral carrler membranes at extreme voltages.
+ CjziKipj’] In addition to the parameter t,, the potential-determining factors in Equations 1and 2 are: a’, a ” = activities of cations (index i, j ) , respectively, anions (index x) in the aqueous solutions contacting the membrane. K,, K,, k , = partition coefficients of permeating ionic forms. K , = K, f K , = selectivity factor for cations of the same charge. z,, z, = charge of permeating cations and anions respectively; in units of the proton charge. R T f F = Nernst factor. Different theories and views are called upon to explain the origin of cation permselectivity of neutral-carrier-based liquid membranes. (a) The fundamental thin membrane model by Ciani, Eisenman, and Szabo (12-14) does not stipulate electroneutrality, and cationic carrier complexes therefore may be the only charged species existing within the membrane. Hence, permselectivity for cations may be explained by a complete exclusion of hydrophilic anions from the lipid membrane. This theory was extended to thick membranes by Boles and Buck (15) who still assume large deviations from electroneutrality to occur within the membrane interior (see also ( 5 ) ) . (b) A recent suggestion made informally by Buck (5) is that slow anion interfacial kinetics permit near-Nernstian response to cations. The remaining theories (c) to (e) of thick neutral carrier membranes may be summarized under the general assumption that the anions present within the membrane are rather immobile. The reasons for such a behavior may be as follows. (c) The membrane contains permanent anions that are chemically bound to the supporting material, as was suggested by Kedem, Perry, and Bloch (16) (see also (15)). (d) The membrane contains permanent anions that are immobilized because of their poor water-solubility (11,17,18). (e) The membrane extracts anions from the sample solution but the integral mobility of these species across the membrane is low as compared to the cationic complexes (11, 19-22). We present here experimental evidence that reveals an actual mechanism for permselectivity in neutral-carrier-based solvent polymeric membranes. These results are at variance with most of the theories mentioned above.
Of the different types of ion-selective membrane electrodes known so far, liquid-membrane sensors offer a wide range of accessible ion selectivities. In these electrodes, mobile ionselective sites, e.g. ion-selective ligands, dissolved in an appropriate solvent, are interposed between the sample solution and a reference system. Extremely high selectivities can be achieved by using neutral ion-specific ligands as membrane components (1-3). These ion carriers or ionophores have the property to complex certain ions (usually cations) of the sample solution and to transport these across lipophilic membranes by carrier translocation. In spite of the wide use of carrier membrane electrodes in analytical chemistry, some facts as to their mode of function remain obscure. This point was emphasized by Buck ( 4 , 5 ) who therefore styled these systems as “apparent neutral carrier membranes” (4). There is no doubt, however, that electrically neutral complexing agents such as valinomycin, macrotetrolides, and related compounds exhibit real carrier properties for cations in membranes. This is underscored by evidence from thin lipid bilayer membranes (6, 7) as well as from thick solvent polymeric membranes (6, 8) (see also below). The predominant problem is then to explain permselectivity of carrier membranes for cations, as is observable in both potentiometric measurements and electrodialysis experiments. This means that a carrier membrane tends to be permeable for cations only and, accordingly, its electrical properties are scarcely influenced by sample anions such as chloride. For an ideal system, the integral transference number of anions, t,, comes out to be zero and the general relationship for the zero-current membrane potential (9-11):
EXPERIMENTAL
RT ZKiai’ RT Z k* a ’ E = (1- tx)In -+ txIn (1) ziF ZKiai” z, F Z k, a,
Cell Assembly. In electrodialysis experiments the cell consisted of two electrolyte compartments separated by the membrane or membrane stack studied. The cell shown in Figure 1 was used for all experiments involving radioactively labeled species. All other experiments were performed in a similar cell (electrolyte compartment volume: 10 mL) described earlier (20). Procedures. Permselective liquid membranes consisting typically of 1 w t % ligand, 33 wt % poly(vinylch1oride) and 66
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1567
0
6
5
3
1
4
5
6
7
1
I
50 mm
1
S
E D C E A
I
l
l
-c
mm
5
5
200,um
H
Flgure 1. Cell used for electrodialysis. (1)Membrane stack or compact membrane. (2) Ag/AgCI electrodes; in the presence of 36CI-platinum electrodes were used. (3)Cathode compartment (volume 230 pL) containing 200 pL electrolyte solution. (4) Anode compartment (volume 230 pL) containing -200 pL electroiyte solution. (5) Cell body (Teflon). (6) Cell support (brass). (7) 4 tightening screws. (8) Connecting-screw to wristshaker (the continuous mixing of the half-cell contents was achieved by shaking the cell with a frequency of about 5 Hz); (A, B, C, D, E) membrane segments (thickness:40 pm, diameter: 13 mm). (S)Spacer rings (Teflon, thickness: 50 pm; inner diameter: 9 mm; outer diameter: 13 mm)
-
wt % plasticizer were used throughout the experiments. For the preparation technique, see (23). For preparation and composition of silicone rubber membranes,see (24). Compact membranes were 40- to 200-pm thick. Composite membranes (stacks) consisted of 5 identical segments each 40-wm thick. In order to facilitate a swift unstacking at the end of electrodialysis,four 50-pm thick Teflon spacer rings were mounted as a skeleton between the membrane segments sticking together (see Figure 1). For the determination of transference numbers, the time-current integral was measured by means of an integrator and the quantity of the ion of interest transported to the cathode and anode compartment respectively was evaluated either by radioactivity counters (Geiger-Muller and liquid scintillation counters) or by an atomic absorption spectrophotometer. The transference number of an ion was obtained as the ratio of the charge equivalent of the transferred species to the time-current integral. For ion profile studies the membrane segments were mounted on steel sample holders and forwarded to the Geiger-Muller counting tube. For I4C-1abeled ligand profile studies, the membrane segments were dissolved in a mixture of 0.5 mL dimethylsulfoxide and 0.5 mL toluene before adding 15 mL of butyl-PBD-scintillator cocktail. For determination of the current-voltage curve of a permselective membrane, the two compartments of the electrodialysis cell were filled with identical electrolyte solutions. A stepwise decreasing voltage was applied and the respective current, constant in time, was reached after 10 to 60 min. In ion-exchange experiments, 200-pm thick membranes mounted in liquid-membrane electrode bodies (Philips IS 560; diameter of exposed membrane surface: 4 mm) were conditioned M KC1 solution for 48 h (internal filling solution: in a 2 X 2X M KC1). Then the inactive sample solution was replaced by 2 X M 42K36Cland the uptake of the labeled ions was measured as a function of time. Changes in pH of previously de-gassed electrolytes in quartz test tubes where carrier membranes had been introduced were recorded by means of a mini-pH-electrode (Philips type C71/02) dipped in the argon-flushedsample. For further details see (25). Reagents. Synthetic carriers: for preparation see (26); poly(vinylch1oride):PVC SDP hochmolekular,Lonza AG, Basle, Switzerland. Tris(2-ethylhexy1)phosphate (purity: 98%): Merck AG, Darmstadt, Germany. Dibutyl sebacate (purum), dioctyl adipate (purum), and tetrahydrofuran (puriss., p.a., 0.025%, 2,6-di-tert-butyl-4-methyl-phenol): Fluka AG, Buchs, Switzerland. Valinomycin: Calbiochem,Los Angeles, Calif. 90054; 14C-labeled valinomycin: graciously offered by Yu. A. Ovchinnikov, USSR Academy of Sciences, Shemyakin Institute for Chemistry of Natural Products, Moscow, USSR. 42Kas anhydrous K2C03: obtained by neutron activation (2 X 1013 n cm-2 s-l at the Eidgenossisches Institut fur Reaktorforschung, Wurenlingen, Switzerland)of K2C03suprapur, Merck AG, Darmstadt, Germany. 1568
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
Figure 2. Ion-selective ligands used. A racemic mixture of the enantiomers R and S of carrier 5 was used. Carrier 6 is the natural enantiomer of valinomycin
36Clas a 0.2 M HCl solution: code CIS.l, The Radiochemical Centre, Amersham, Great Britain. Butyl-PBD-scintillatorcocktail: Ciba-Geigy AG, Basle, Switzerland. Apparatus. Digital voltmeter: type LM 1440, The Solartron Electronic Group Limited, Farnborough, Hampshire, Great Britain. Microammeter: type 150 B, Keithley Instruments, Cleveland, Ohio 44139. Integrator: recorder model SRG/disc integrator, Sargent & Co., Chicago, Ill. Geiger-Muller anticoincidence counting unit: detector: Isotope Development Laboratories, Edinburgh, Great Britain; low-background anticoincidence unit: type PW 4092, Philips; high voltage power supply: type PW 4022 and PW 4023, Philips; counting unit: type ELD 5 and ELB 5, Landis & Gyr, Switzerland; printer: type ERH 1,Landis & Gyr, Switzerland. Liquid scintillation counting unit: Mark I, Nuclear Chicago. Atomic absorption spectrophotometer: type 300 (graphite cell HGA 72), Perkin-Elmer, Ueberlingen, Germany. R E S U L T S A N D DISCUSSION Evidence for Permselectivity. As a rule all the analytically relevant cation-selective neutral-carrier-based liquid-membrane electrodes studied in detail so far exhibit cation permselectivity in electrodialysis experiments (10,20,27-30). A summary of the presently available information is given in Table I. I t includes data published earlier as well as new results obtained on membranes containing the carriers 1-6 (see Figure 2). Throughout, a cation transference number of nearly 1 ( t , i= 0; Equation 1) is found which is in perfect agreement with the slope of the electrode response observed for the same membranes (see Equation 1 and last column of Table I). In one case (valinomycin in dioctyl adipate) the transference number for 42K+of 1.02 f 0.04 is corroborated by the simultaneous radiochemical determination of the transference number of the sample anion 36Cl-of 0.0004 f 0.0002. It can therefore be concluded that the integral transference number t, for anions (Equation 1)is practically zero for the systems studied. Ion a n d Ligand Profiles i n N e u t r a l C a r r i e r Membranes. In order to reveal the mechanism of permselectivity of neutral carrier membranes, the concentration profiles of cations, anions, as well as ionophores within the membrane phase were studied. T o this end radioactively labeled carriers, cations, and anions were used in electrodialysis experiments with stacked solvent polymeric membranes (see Figure 1).The
Table I. Transport Numbers and Slopes of the Electrode Response of Cation Permselective Neutral Carrier Membrane Slope of electrode response in Membrane composition Electrolytes percent of Cathode Transference number theoretical Matrix; Anode Cation Ligand; Solvent;d slope compartment for cations studied' wt 9c compartment wt % studied wt % 2;3 M CaCl, M KCl 0.99 i 0.08 ( 2 7 ) 94 ( 3 1 ) CaZ+ 0-NPOE; 65 PVC; 32 Ca2+ 2;3 DBS; 65 PVC; 32 M CaC1, M KC1 1.00 i 0.105 ( 2 7 ) ... 5 x lo', M CaC1, Caz+ 2;3 0-NPOE; 65 PVC; 32 l o T 3 M KCl 0.99 ? 0.08 ( 2 7 ) 94 (31) M MgCl, 5x x M CaCl, 5 CaZ+ 2;3 M KC1 0.99 i 0.02 ( 2 7 ) 94 ( 3 1 ) 0-NPOE; 65 PVC; 32 M NaCl 5x Ca2+ 2;3 94 ( 3 1 ) M CaC1, 1 0 - 4 M KSCN 0.995 i 0.025 ( 2 0 ) 0-NPOE; 65 PVC; 32 1; 3 95 ( 3 2 ) Ca2+ 0-NPOE; 65 PVC; 3 2 M CaC1, M KC1 1.01 i 0.08 ( 2 8 ) ... Na+ 4; 3 DBS; 65 PVC; 32 M NaCl M KCl 0.92 i 0.08 (27, 2 9 ) 4;3 DMK; 65 PVC; 32 M NaCl M KC1 0.92 i. 0.061 ( 2 7 ) Na' 4; 3 0-NPOE; 65 PVC; 32 M NaCl M KCl 0.90 i 0.075 ( 2 7 ) 96 ( 3 3 ) Na+ lo-,M KCl 0.97 i 0.11 97 ( 3 4 ) 3; 5.8 TEHP; 62.8 PVC; 31.4 lo-* M LiCl Li' Lit 97 ( 3 4 ) 3 ; 5.8 TEHP; 62.8 PVC; 31.4 M LiCl M KCl 1.02 ? 0.21 Li+ M LiCl 97 ( 3 4 ) 3;5.8 TEHP; 62.8 PVC; 31.4 M KC1 0.98 i 0.10 K' *.. 6;3 DPP; 67 PVC; 30 M KC1 lo-, M HCl 1.08i 0.07 (27, 3 0 ) K+ ... 6;5 ... Silicone lo-' M HCl 1.1 i 0.15 ( 2 7 ,3 0 ) lo-' M KC1 rubber; 95 K' ... 6; 5 ... Silicone 10.' M KC1 lo-' M HC10, 1.1 i 0.15 ( 2 7 , 30) rubber; 95 K' 6; 1 M KCl 1.02 f 0.04 100 ( 3 5 ) DOA; 66 PVC; 33 9X 9 X l o F 4M KCl (120)c (25 PEAH' DOA; 65 PVC; 34 M PEAHCl 4 x M PEAHCl 0.95 f 0.04 4x 5; 1 ' Measured on the membrane specified in the region of ohmic behavior of the current-voltage curve (at low voltage) ( 2 0 ) : 95% confidence limits. PEAH': ''C-a-phenylethylammonium cation. From electrode response in buffered lo-' M an I lo-' M solutions of the ammonium salt; activity coefficients unknown. o-NPOE: o-nitrophenyloctyl ether; DBS: dibutyl sebacate; DMK: decylmethyl ketone; TEHP: tris-(2-ethyl-hexyl)-phosphate; DPP: dipentyl phthalate; DOA: dioctyl adipate. CURRENT
rn mole 1-1
[MI
15
CARRIER VALINOMYCIN
0 0
1
2
3 TIME [HOURS]
5
CONCENTRATION BEFORE ELECTRODIALYSIS
'j 0
400! 200
Figure 3. Current-time characteristics of neutral carrier membrane. A potential gradient of 20 V was applied to a 200-1m thick valino-
mycin-based liquid membrane in contact with KCI solutions;the cell is shown in Figure 1 current-time characteristics obtained on a stack of membranes containing I4C-labeled valinomycin (0.99 wt % ) in dioctyl adipate (66.2 wt %)/PVC (32.8wt %) in contact with 4 X M KCl is given in Figure 3. One hour after reaching the steady state, the electrodialytic transport was stopped and the membranes were unstacked within 3 min. Identical experiments were carried out by bringing the membrane stack (not labeled) in contact with 9 X M 42KClin the anode and 9 X M K3%l in the cathode compartment. The measurements of the radioactivities in the different membrane sections led to the profiles displayed in Figure 4. As indicated earlier for a different carrier system (8, 36), an ionophore concentration gradient builds up which is in agreement with theoretical predictions (20). This is an unambiguous proof for carrier translocation accompanying the cation transport and it is a requirement for the back-diffusion of free carriers to end up in a steady state (20). This back-diffusion is substantiated by results presented earlier (36). There the complete decay of the ionophore concentration gradient within
MEMBRANE STACKO SEGMENT
E
I
,,I
I
I
A
Figure 4. Ionophore and ion concentration profiles within a valinomycin-based liquid membrane (stack). Resutts represent the ion profiles 3 min alter interrupting a steady-state electrodialysis. The 95% confidence iimlts are given by vertical bars
the membrane after reestablishing a zero-current state was experimentally confirmed for the valinomycin-induced transport of a-phenylethylammonium ions. These experiments led to a diffusion coefficient for valinomycin of (1.8 f 0.3) X lo-* cm2 s-l (25). In a similar experiment of the valinomycin-induced transport of a-phenylethylammonium ions (8), only the carrier in the membrane section contacting the anode compartment ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1569
(section A, Figure 1;entry for cations) was labeled. After the transport experiment, only about 20% of the labeled ligand drained off section A reached section E (8). This proves an exchange of ligands during the transport process so that a carrier-relay mechanism must be operative (25, 37). According to Figure 4, cations as well as anions enter the membrane phase. Although this was expected, it is most remarkable that the overall concentration of cations K+ exceeds by far (factor 85) the one of the sample anions C1-. This fact can be explained only when assuming either dramatic deviations from electroneutrality or the presence of anionic species other than C1- within the membrane phase. The rather high cation concentration level (see Figure 4) as well as the relatively low electrical resistance of only about lo6 ohm cm2 (25)found for the valinomycin-basedmembranes studied here are a t variance with the space-charge theory (a) mentioned above. If anions were completely excluded from the membrane phase, the limiting resistance for a thick membrane would be (10, 15, 38):
R,
=
d3 47r2e,eD
(3)
and the theoretical concentration of cationic charges in the center of the membrane would be (8, 38):
,Ol
LABELED IONS IN MEMBRANE- PHASE
[P mole1 1.11
I
>mole] Ill
42,. 20
10
0 0
1
2
3
4
5
TIME [HOURS]
Flgure 5. Timedependent uptake of labeled ions from aqueous solution
by a neutral carrier membrane. A valinomycin-based liquid membrane was equilibrated with an electrolyte solution containing unlabeled ions. Curves I and I1 refer to the left-hand scale: curve 111refers to the blown up scale on the right-hand side. Vertical bars indicate 95 % confidence limits
pothesis (b)), the initial flux J, across the phase boundary is given by (20):
(6)
J , = k,a, and therefore
(4) Inserting the experimental values d = 0.02 cm for the membrane thickness, D = 1.8 X lo-' cm2 s-l for the diffusion coefficient of carrier complexes in the membrane, e = 4 for the dielectric constant of the membrane, and the fundamental C/V cm, RTIF = 0.0257 V at 25 constants eo = 8.85 X OC, and F = 96500 C/mol, the following values are calculated:
R,
3.18 X 1013 52 em' c,(d/2) = 4.65 X lo-'' mol/cm3 =
d
m
(5)
where M , = amount of labeled ion u in membrane a t time t 2 0 (mol). A = phase boundary area (cm2). c, = concentration of unlabeled ion u in the membrane in equilibrium with the aqueous phase at t 5 0 (mol ~ m - ~D) .= diffusion coefficient of ion u in the membrane (cm2s-l). On the other hand, when the tracer transfer from the aqueous phase into the membrane is rate-determining (hy1570
(7)
where k , = rate constant (cm s-l) for the transfer of the ion u into the membrane (including a phase boundary potential contribution (20)). a, = activity of the tracer in the aqueous solution at t 2 O (mol ~ m - ~ ) . Equation 7 predicts a linear increase of M , with time (at least in the initial stage of the diffusion experiment) which clearly contradicts the curves in Figure 5. According to Equation 5 we should expect the uptakes of labeled ions u to be linear functions of having a slope proportional to c, Such a time-dependence has indeed been found in our experiment (see Figure 6) as well as in similar studies on glass membranes made earlier by Eisenman (39). The ratio of the slopes of the straight lines in Figure 6 is (CK (cc1 = 40 and compares favorably with the ratio C K / C C ~ found previously in Figure 4. Correspondingly, the diffusion coefficients DK and D C must ~ be of the same order of magnitude and we therefore have no kinetic control of the preference of K+ over C1-. We then have to conclude that hypothesis (b) is not a likely explanation for the cation permselectivity. Since the different sections of the stacked membranes were prepared under identical conditions from the same batch of raw material, the chemically bound anions suggested in (16) (hypothesis (c)) should be homogeneously distributed throughout the membrane. This is in contrast to the situation presented in Figure 4. There the difference between the K+ and C1- concentrations, which represents the apparently missing anions, varies significantly from section A to E. This, as well as the indication of cation permselectivity for liquid membranes prepared only from n-heptane and valinomycin (40), clearly rules out the fixed-site theory (c). The latter argument also contradicts hypothesis (d) because lipophilic anions permanently in the membrane phase can be introduced only through impurities or additives (17). The anion deficiency documented in Figure 4 is around 1.0 X lo-* mol L-'. Such high levels of impurities are excluded by select methods of preparation of cation permselective solvent polymeric membranes. Therefore, the apparently missing anions in the membrane phase must originate from the aqueous system.
4
Obviously, these values are far from reality and therefore a space-charge model (hypothesis (a)) appears to be untenable for thick carrier membranes. The sample anion deficiency in the membrane phase of around 99 mol % presented in Figure 4 must be due to anions different from C1-. T o differentiate between the other models suggested, diffusion experiments were carried out. A valinomycin-based solvent polymeric membrane previously conditioned with 2 X M 42K36Cl. M KC1 was brought in contact with 2 X Results of the uptake of labeled ions by the membrane are given in Figure 5. The number of moles of cations exchanged is higher by a factor of 35 to 70 than that of the exchanged anions. The ratio of the slopes of the c w e s in Figure 5, which is a measure for the ion-exchange flux ratio, varies from 25 to 80. If the interdiffusion of isotopes within the membrane is the rate-controling step, i.e. if there is no kinetic limitation at the phase boundary, the initial tracer uptake can be described by (39):
M , = 2Ac,
M , = Ak,a,t
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
a)/ 6)
LABELED IONS IN MEMBRANE-PHASE
/ 400 -
36cl300 -
15-
200 -
10 -
100-
5050
5
0
100
lo
200 300 TIME [MINUTES:
TIME [MINUTES: 0
15
10
5
15 VTIME [MINUTES] '
VTIME [MINUTES] '
Figure 6. UDtake of labeled ions by a valinomycin-based liauid membrane as a function of the square root of time. See legend to Figure 5. Vertical bars indicate the 95% confidence limits pH SAMPLE SOLUTION
CURRENT
[YA
1
1.0 tc,-
7 0.5
= 0.00014t 0.00007
tK / + = 1.03i0.03
-_
tK+ =
/-
2 0.0001 0.77t0.03
tc,- = 00004 2 00002
tK.
1.02'0.04
0 I
0
05
1
10
))
'
0
0
05
20
40
60
80 VOLTAGE
[VI
10 TIME [HOURS ]
Figure 7. Change of pH induced in sample solution by neutral carrier M KCI membrane. Changes in pH of unstlrred argon-flushed 9 X solutions after the addition of freshly prepared valinomycin-based liquid membranes to the aqueous phase. (A) 51-mg membrane in a 0.75-mL
sample solution; for the preparation of the membrane, the components were dissolved in 1 mL of stabilized tetrahydrofuran (23)(highest purity available; stabilized with 0.025% 2,6di-tert-butyl-4-methyl-phenol). (B) 86-mg membrane in a 0.75-mL sample solution; 2 mL of freshly distilled tetrahydrofuran (freed from the stabilizer and other nonvolatile substances) were used to dissolve the membrane components during preparation (23) Mechanism of Cation Permselectivity. Since in addition to theory (e) there is no clear-cut explanation for the cation permselectivity left, one has to rationalize the nature of the dominant anions in the membrane phase as well as their apparently poor mobility. In agreement with earlier observations (41), solvent polymeric membranes of the type discussed are susceptible to water uptake and water permeability. A 200-fim thick membrane prepared from valinomycin (1.0 wt %), dioctyl adipate (66.2 wt %) and PVC (32.8 wt %) equilibrated with a 9 X M aqueous KCl solution on each side shows a water concentration of about 0.15 mol L-l and a water flux of about 1.7 X mol cm-2 s?. These values were obtained by labeling the water with 3H.Even membranes prepared from n-heptane and valinomycin have a water uptake of around 0.003 mol L-' (25 "C) (42).It is therefore reasonable to assume that the dominant anions in the membrane originate from proton exchange reactions involving water. If a freshly prepared membrane is brought into contact with an aqueous
Figure 8. Current-voltage characteristics and transport behavior of
a neutral carrier membrane. Steady-state current through a 200-hm thick valinomycin-based liquid membrane (cross section: 0.28 cm2, see Figure 1)as a function of the voltage applied. The transference numbers of 42K+ and "CI- are given for 3 selected field strengths. The confidence limits are 95% KCl solution, there is indeed a flux of protons out of the membrane phase (Figure 7) according to the formal exchange reaction: K+(aqueous)+ H,O(membrane) + K+(membrane)
+
OH-(membrane) + H+(aqueous)
(8)
Because of a possible formation of water clusters involving the OH- formed by reaction 8, the mobility of these anionic sites is likely to be low. It may, however, be expected that a direct electric transference of such sites occurs under extreme conditions. In Figure 8 a current/voltage diagram including transference information covering the range from 10 to 70 V is given. Whereas up to about 30 V the theoretically predicted behavior of an ideal cation permselective carrier membrane is observed, the transference number for K+ drops well below 1.0 at extreme voltages. The contribution to the electric current by C1- with a value of 0.0007 at 70 V is still negligible (see Figure 8). The remaining 23% contribution to the current in the electrodialysis experiment a t 70 V must then be due to the transport of charged forms of water. The mechanism suggested here gives a plausible explanation for the unusual extraction behavior reported by Lev et al. (40). For a valinomycin-modified heptane membrane and KCl (as ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1571
well as other alkali metal chlorides) in the aqueous sample solutions, these authors found that the concentration CK of the extracted cations is a function of the square root of the sample concentration cKaq. This may be explained by assuming the existence of an equilibrium 8 described by CK
X
cOH
=
X
const X
cKaq
X
c H , ~
(9)
and cK = COH (10) If the concentration of water CH,O in the membrane phase as
well as the pH of the sample solution remain constant, we indeed obtain: CK
prop. @
(11)
The facts presented here lead to the conclusion that the neutral carrier membrane model developed earlier (20) (hypothesis (e)) gives the most adequate description of the observed electromotive and transport behavior. Accordingly, the origin of permselectivity is the presence of anions of low mobility in the membrane which can be considered as “fixed” charges (20). The crucial difference between this theory and the pioneering model suggested by Kedem, Perry, and Bloch (16) (hypothesis (c)) rests in the nature of the immobile sites, which we show as originating from the aqueous system.
ACKNOWLEDGMENT We thank Yu. A. Ovchinnikov for the generous supply of I4C-labeled valinomycin and V. Prelog for ligand 5. We gratefully acknowledge the stimulating discussions and the experimental support of P. Jordan and K. May (Radiochemical Laboratory, ETHZ).
LITERATURE CITED J. Koryta, “Ion-Selective Electrodes”, Cambridge University Press, Cambridge, London, New York, and Melbourne, 1975. E. Pretsch, D. Ammann, and W. Simon, Res.lDev., 25 (3), 20 (1974). D. Ammann, R. Bissig, 2. Cimerman, U. Fiedier, M. Guggi, W. E. Morf, M. Oehme, H. Osswaid, E. Pretsch, and W. Simon, in “Ion and Enzyme Electrodes in Biology and Medicine”, M. Kessier, L. C. Clark, Jr., D. W. Lubbers, I.A. Sliver, and W. Simon, Ed., Proceedings of a International Workshop at Schioss Reisensburg, Germany, September 15-18, 1974, Urban & Schwarzenberg, Munich, Berlin, Vienna, 1976, p 22. R. P. Buck, Anal. Chem., 48, 28R (1974). R. P. Buck, Anal. Chem., 48, 23R (1976). G. Eisenman, Ed., “Membranes”, Voi. 2, Marcel Dekker, New York, N.Y., 1973.
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RECEIVED for review May 6,1977. Accepted June 20,1977. Work partly supported by the Swiss National Science Foundation and by Ciba-Geigy AG, F. Hoffmann-La Roche & Co. AG, Sandoz AG, Lonza AG.
