Anal. Chem. 1986, 58,2735-2740
2735
Plasticized Poly(viny1 chloride) Properties and Characteristics of Valinomycin Electrodes. 1. High-Frequency Resistances and Dielectric Properties George Horvai, Etelka Grif, Klira Tbth, and Ern0 Pungor
Technical University of Budapest, Budapest, Hungary Richard P. Buck*
Department of Chemistry, University of North Carolina, Chapel Hill,North Carolina 27514
Impedance measurements of the high-frequency bulk semicircle provide bulk resistance and geometric capacitance values of two- and three-component membranes: plasticizer, poly(vinyl chiorlde) (PVC), and valinomycin in combinations. Intrinslc conductivity Is traced to previously postulated counterions from slightly dissoclated fixed sites in PVC. Bulk resistances of vaiinomycln-free membranes were determined for three commercial PVC types, at short soaking thnes. Bulk resistances increase with soaking times, but with different time dependences for KCI, NaCi, or pure H,O. Dielectric constants of membranes are always greater than for dry component values. Part of thls increase may depend on water content, but there is an increase from motion of plastlcized PVC chains, presumably C-Ci bonds. Responses are dominated by fixed sites and impurity counterions, modified by charge trapping in water regions (in the absence of carrier). Increased charge carrier concentration results from the presence of carriers that solubilize ions in the organic regions.
Neutral carrier ion selective electrodes (ISEs), e.g., the K+-selective electrode with valinomycin as selective extractant/carrier, have been fabricated mainly with plasticized poly(viny1 chloride) (PVC) matrices (1,2). Typical membranes consist of 33 wt % PVC, 66% plastizicer, and 1%neutral carrier. Choices of plasticizer and of low-concentration additives, like sodium tetraphenylborate, have been optimized for several neutral carriers and sample matrices ( 2 , 3 ) . PVC has been regarded as an inert matrix for the plasticizer to yield a structureless organic membrane that dissolves the neutral carrier (4). Studies of electrical properties of plasticized PVC, e.g., conductivity, dielectric constant, and dielectric loss, suggest a more heterogeneous structure (5, 6). Presence of counterion charge carriers from corresponding weakly dissociated fixed and mobile charged sites have been noted (7-10). The free, dissociated counterions contribute to conductivity depending on the preparation of PVC. Particularly important are additives used in the manufacture of PVC by suspension or by emulsion methods (11-13). In this study, we have used the impedance method to investigate ion transport in plasticized PVC membranes, plasticized PVC membranes with neutral carrier, and liquid plasticizer films with the neutral carrier valinomycin. New observations can be interpreted in terms of plasticized PVC structures containing fixed negative sites. Hypotheses for the Response Mechanism of ThreeComponent Neutral Carrier Membrane Electrodes. Various hypotheses for responses of neutral carrier-based IS& have appeared since 1969. Morf ( 4 ) has exhaustively enumerated proposed mechanisms. Based on experimental findings (through 1977), mainly electrodialysis experiments
with radiotracers by the Simon group (141, the model of selective electrode behavior depends on bulk membrane processes: complexation and transport. The cation-selective membranes from PVC are thought to be low-capacity cation exchangers of about 0.05-0.6 mM negative fixed sites, depending on polymer molecular weight and mode of polymerization. Upon exposure to aqueous solutions containing KC1 or NaC1, C1- is found in membranes, presumably immobilized in isolated water clusters. Hydroxide ions have also been suggested as isolated co-ions. The neutral carrier, which is present at, typically, 1w t % or 9/mM, is in large excess over the ion exchanger capacity. The neutral carrier selectively complexes the primary cation when the latter is available in the bathing solution and thereby induces potentiometric and transport selectivity. The rate of charge transfer between aqueous bathing solutions and the organic membrane phase has been reported using galvanostatic transients (15,16) and using the impedance method (17). Rates have been found to depend on type and concentration of the cation in the bathing solutions. In view of the extensive measurements of resistive surface film formation on plasticized membranes, reported in part 2 (181,published rates may be underestimated, except for those applying to solvent-free lipid bilayers. Equilibrium selectivities for monovalent cations are not thought to depend on surface complexation rates, although bulk complex formation and dissociation rates are crucial for the operation of the carrier mechanism. Further instructive measurements by Oesch and Simon (19, 20) give the dependences of potentiometric slopes, selectivities, and membrane resistances on carrier loading. These critical results, combined with our findings, suggest an amended ion budget and bulk-transport-controlled response mechanism, in Table I. Our model is essentially a low-capacity ion exchanger model already suggested (10)in which interfacial and bulk processes are equally involved in pure responses and in selectivity.
EXPERIMENTAL SECTION Chemicals. Valinomycin was purchased from Sigma. Bis(2ethylhexyl) sebacate (DOS), dibutyl sebacate (DBS), and 2nitrophenyl octyl ether (0-NPOE)were made by Fluka. High and low molecular weight PVC (designated HMW and LMW) came from Aldrich. GEON PVC is a product of B. F. Goodrich Co. Tetrahydrofuran(THF) was Aldrich Gold Label quality. All other chemicals were analytical grade reagents. Deionized water was distilled in Pyrex glass. Dialysis membrane from Fisher (Spectrapor 2 tubing) was pretreated to remove additives and contaminants. Apparatus. Resistances were measured from an impedance plot for each membrane using the Solartron 1250 frequency analyzer, Solartron 1186 electrochemical interface, and a Hewlett-Packard 85B computer. The amplitude of the applied sinusoidal voltage was 1 V. Lower voltages, down to 0.1 V, gave the same impedance plot but with more noise. Emfs were measured with an Orion Research Model 701 digital pH meter.
0003-2700/86/0358-2735$01.50/00 1986 American Chemical Society
2738
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table I. Predicted Response Characteristics for Low-Site Density “Dummy” and “Normal” Membranes
bathing soln
membrane materials
K+/Na+as Cl-, plasticizer F-,or OHplasticizer, valinomycin
sites X-
neutral carrier/ site ratio
Cl- in H 2 0 droplets + anionic, acidic impurities Cl- in H 2 0 droplets + anionic, acidic impurities
0
principal charge carrier M+; charge balance
H+ and Na+/K+solvate 0 5 1 transition
plasticizer, C1- in H,O droplets + anionic, acidic impurities valinomycin plasticizer, PVC fixed protonated negative sites, mainly -0.05-0.6 mM plasticizer, PVC, fixed sites, mainly -0.05-0.6 mM valinomycin
t l
0
permselectivity t+ >
less than Nernstian; poor K+/Na+selectivity transition toward Nernstian; t+ apimproving K+/Na+ proaches 1 selectivity near Nernstian; ideal Kval+; (M+) = (X-) t+ = 1 K+/Na+ selectivity less than Nernstian; poor Na+/K+-solvate; t+ = 1 (M+) = (X-) K+/Na+selectivity
0 5 1 transition
t_
t+ ap-
proaches 1
K+/Na+ oil-soluble anion Y-
plasticizer, PVC, fixed sites, mainly valinomycin -0.05-0.6 mM plasticizer, PVC, fixed sites, mainly valinomycin -0.05-0.6 mM and C1in H 2 0 droplets plasticizer, PVC, mainly extracted ion valinomycin (Y-)2 (X-)
-1
Kval+; (M+) = (X-) t+ = 1
21
Kval+; (M+) = (X-) + (Cl-)
t+ u 1
21
Kval+ and Y-; (M+)= ( X 3 + (Y-)
t+
Electrodes and Cells. Membranes were fabricated by the method suggested by Craggs et al. (1). The active membrane areas in contact with solutions were 1.99 cm2. Thicknesses of the membranes increased somewhat from center to edge. Average thickness (130 pm) has been calculated from membrane weight and density (19, 20). Because of the uncertainty arising from membrane thicknesses, dielectric constants will be reported to the nearest integer value. Reference electrodes on both sides of the membrane were chloridized silver wires, wound to a flat spiral, forming a plate with an active surface area of about 1 cm2. These planar electrodes were placed parallel, about 1 cm away from the membranes. Compositions of the free-standing membranes were varied. The compositions are designated as follows: (1)“normal”, 0.8 mg of valinomycin/66 mg of DOS or DBS/33 mg of PVC; (2) “lowloaded”, 0.08, 0.3, 0.5, and 0.7 mg of valinomycin/66 mg of DOS or DBS/33 mg of PVC; (3) “high-loaded”, 3.0 mg of valinomycin/66 mg of DOS or DBS/33 mg of PVC; (4)“dummy” or blank, no valinomycin/66 mg of DOS or DBS/33 mg of PVC; and (5) “1:2,1:4, or 41” designate PVC/plasticizer weight ratio membranes containing either no valinomycin or 0.8 wt % valinomycin. A Plexiglas cell for measurements on PVC-free, plasticizer liquid membranes consisted of two chambers with a volume of about 25 mL each and a 2.0-mm-thick Teflon spacer that could be sandwiched between the two chambers. A circular hole of 28 mm diameter cut into the Teflon spacer connected the two half-cells. The cell was assembled by bracketing the Teflon spacer between two pieces of dialysis membrane. The two half-chambers, separated by a 2-mm gap between the dialysis membranes, were filled with a solution of valinomycin in plasticizer. The two chambers were filled with aqueous solutions and chloridized silver wire electrodes (above). A conductance cell for THF solutions was conventional.
RESULTS Electrode Response Verification. Electrodes made with 1:2 (conventional PVC/plasticizer ratio) free-standing membranes containing 0.03 wt % or more valinomycin give nearly Nernstian to fully Nernstian responses. The concentration, 0.1 M NaC1, interferes only at ca. M KC1 or less. The same observations have also been made for the supported liquid “membranes” containing 0.8 wt % valinomycin solutions in plasticizers confined between dialysis membranes. The plasticizer content of the membranes must be large relative to PVC to achieve a sensitive and Nernstian response. Responses of 4:1,1:2, and 1:4 (PVC/DBS) “normal” memM KCl, and linear to branes were no response, linear to
slopes; sensitivities
“0
> < t-
05
transition toward Nernstian; improving K+/Na+ selectivity near Nernstian; ideal K+/Na+selectivity
various cases of Donnan exclusion failure
IO
15
20
25
ZR ( M a )
Figure 1. Impedance plane plot of Aldrich high molecular weight 1:2 PVCIDOS membrane: valinomycin content, 0.8 w-t %; bathing solution,
M KCI, symmetric; frequency range, 65 kHz to 0.01 Hz; left semicircle, bulk resistance on real axis; right semicircle, surface-rate, film-controlled resistance.
M KC1, respectively. Responses of “normal” 1:2 membranes at increasing valinomycin content using different types of PVC were determined. No differences in slopes could be observed on substituting HMW, LMW, and GEON types of PVC. Note that 1:2 “dummy” membranes also respond to uK+, but with sub-Nernstian, ca. 30 mV/decade, slopes, after soaking in water for about 24 h. General Impedance Characteristics. Impedance measurements were made on fresh, symmetrically bathed memM KCl solutions except as otherwise indicated) branes (in after controlled times of soaking. Impedance plane plots (-ZI vs 2,) with at least one semicircle are observed for all membranes. This high-frequency semicircle is generally joined on the low-frequency side by a second semicircle or other more complex form (Figure 1). The leftmost semicircle arises from the high-frequency resistance and the geometric capacitance, e.g., the capacitance defined by the external solution surface charge, the membrane thickness, and the membrane dielectric constant. This assignment is confirmed by observing that doubling the membrane thickness doubles the resistance and halves the capacitance calculated from the experimental time constant. The calculation is made by fitting a semicircle to the experimental points and equating its diameter with Rb,& The interpolated frequency at the semicircle maximum is (21). A discussion of the low-frequency semicircle and other low-frequency impedance curves will follow in part 2 (18). Since Cbulk depends on membrane dimensions (area A , thickness cl) and its dielectric constant (e.g., tA/cl), while Rbulk depends inversely on both membrane dimensions and
2737
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
M KCl
Table 11. Variation of Membrane Bulk Resistance Values and c Values as an Effect of Bathing in membrane type
plasticizer”
dummy normal dummy normal dummy
DBS DBS
GEON GEON
DOS DOS DBS DBS DOS DOS DBS DBS
GEON
normal
dummy normal dummy normal
PVC
5
GEON Aldrich HMW Aldrich HMW Aldrich HMW Aldrich HMW Aldrich LMW Aldrich LMW
min
bulk resistance,bkR, after l h 1 day
1200 150 1700 420 1200 200 4300 1200 2300 160
E
4
2100 250 1800 420 2100 340
1500 180 1760 460 1200 270 12 000 1100
5000 230
3200 200
a 1:2 PVC/Plasticizer. bMembrane resistances were measured in a symmetric cell containing resistance.
2.5
days
5 min
1h
14 17 12 13 17
16 18 13 13.5
16
15 17 12 13 18 16
10 10 15 25
15 16
16 17
--
4
18
M KC1; 5-min values approximate initial
P l a s t i c i z e r DBS
I II
0 . 5 ~ / 0.0
days
after
Aldrich Aldrich L.M.W H M.W PVC
ni GEON
1:2 PVCIDBS, measured in symmetric
M
KCI.
conductance (e.g., d/aA), i t is found that T = RbulkCbulk is inversely proportional to the specific membrane conductivity ( T = e/,). T is typically 1 ms for “dummy” membranes and about 0.1 ms for “normal” membranes and is known as the “electrical relaxation time”. Initial Bulk Resistances as a Function of Membrane Composition. Bulk membrane resistances depend on bathing solutions and on the soaking time. When fresh membranes are measured in M KC1, the initial 5-min value is used to compare different types of PVC. “Dummy” membranes show initial bulk resistances in the sequence RAldrich LMW
l.oL
0.00
Flgwe 2. Histogram of average bulk resistances of fresh membranes,
> RAldrich HMW >RGEON
with further dependence on dielectric constant and lipophilicity of the plasticizer selected (20,22). The resistance of GEON/DOS is greater than for GEON/DBS. Results are illustrated in Figure 2, and additional data are given in Table I1 including dielectric constant values, discussed later. The initial bulk resistances of “normal” membranes show slight dependence on the kind of PVC when the same plasticizer is used in the comparison. Absolute resistances are smaller, relative to “dummies”, by about an order of magnitude. Since Kval+ becomes the majority carrier in all of the membranes, a great reduction of resistance is expected. The magnitude of reduction is determined by Kval+ mobility, while its maximum concentration is determined mainly by the fixedand mobile-site concentration. It is these data that have led to the conclusion that cation charge carriers, counterions, in “dummy” membranes are incompletely dissociated. It is not clear that Kval’ is, itself, completely dissociated from the fixed sites, because resistances at constant valinomycin loadings vary with plasticizer type. In Table 11, when GEON/DBS data are compared with GEON/DOS, the resistances increased
2 0 PVC 4 0 (Percent) 60 80
100
Figure 3. Bulk resistance of fresh membranes as a function of PVC content: plasticizer, DBS; valinomycin content, 0.8 wt YO. Fresh M KCI. membranes are measured at 5 min in symmetric
about 2-3 times, possibly by different ion pairing with sites or different ionic mobilities. Membrane bulk resistances are a strong function of plasticizer content a t constant, normal 0.8 wt % valinomycin loading. When GEON PVC and DBS plasticizer are varied from 20 to 80 wt % DBS, the initial resistance values in M KC1 bathing solutions are an exponential symmetric function of PVC/DBS as illustrated in Figure 3. Surprisingly, log R b d k is also a linear function of PVC/DBS for “dummy” membranes using the same PVC and plasticizer. Changes in viscosity and membrane structure contribute to this dependence (20). Small changes in weight percent of plasticizer can account for large changes in apparent Rbulk values. Bulk resistances of 1:2 PVC/plasticizer membranes as a function of valinomycin loading pass through a minimum. It is to be expected that R b d k decreases with increasing valinomycin a t low loadings (0-5 wt %) because of the increase in charge carrier concentration by replacement of weakly ionized counterions, initially present, by Kval+. Using GEON PVC, with both DBS and DOS, gives the anticipated result shown in Figure 4. However, a t high valinomycin concentrations, especially in DOS,precipitation of the carrier is clear by observation of crystals in the membranes. The almost constant ratio of the corresponding values of the two curves in Figure 4 suggests the similarity of the physical processes in the two membrane systems. Bulk Resistance as a Function of Bathing Concentrations of KCl. Values of Rbulk for “normal” membranes were measured in symmetric cells with increasing levels of aqueous KC1. Resistances decreased for each increase in bathing concentration. However, the slope was not dependent
2738
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table 111. Variation of Membrane Bulk Resistance Values as a n Effect of Bathing in NaCl, H,O, and KSCN
membrane type dummy normal dummy dummy normal dummy normal (I
valinomycin content, wt % 0.8 0.8 0.8
plasticizer
PVC”
DBS DBS DBS DOS DOS DBS DBS
GEON GEON GEON GEON GEON GEON GEON
bathing soln lo-’M lo-’ M HZO HZO HZO lo-’ M lo-’ M
0
NaCl NaCl
2300 160
KSCN KSCN
8600 380 2900 250
770
bulk resistance,bkQ, after 3h 20 h 4 days 4 300 460
5 200 510 2 500
12 000
18000
640 1300 30
1000
1100
810
40
1:2 PVCiDlasticizer in all cases. bThe bulk resistance values were measured in all cases in a symmetrical cell containing
M KC1.
Table IV. Long-Term Resistance Effects of HMW Membranes no.
membrane compn
1 2
dummy dummy dummy normal normal normal low-loaded
3 4 5 6 7
bathing soln
5 min
M KC1
4300
10-1 M NaCl
8000
HzOb M KCl lo-’M NaCl HzOb M KC1
5600 1200 1500 (1500) 2800
Impedance measured briefly in a After 23 days.
l h
bulk resistance, kD, after 1 day 2 days 12000
1 week or more
14500 18500
31000” 18 100
16 600
1100
1040
3 400
3 600 3200
2 500
M KCl bathing solution.
nO
24
0 20
0 I6
07 IbIValinomycin
HOURS
DAYS SO4KING T I M E
D4YS
Flgure 5. Bulk resistance changes as a function of soaking time: M KCI for a and b, M NaCl for bathing solutions, symmetric c and d, and water for e and f; (a) 1:2 GEON (PVC)/DBS, “dummy”, (b) 1:2 GEON (PVC)/DBS, 0.8 wt % valinomycin, “normal”; (c) 1:2 E O N (PVC)/DBS, “dummy”; (d) 1:2 GEON (PVC)/DBS, 0.8 wt % valinomycin, “normal”; (e) 1 :2 GEON (PVC)/DOS, “dummy”; (f) 1:2 GEON (PVC)/DOS, 0.8 wt % valinomycin, “normal”.
lo-‘
0.0
1
I
Valinomycin ( W t , Percent)
Figure 4. Bulk resistances of fresh membranes, 1:2 GEON (PVC)/ plasticizer, with increasing valinomycin content: (A) GEON (PVC)/DOS and (B) GEON (PVC)/DBS.
on the square root of K+ activity as reported elsewhere ( 1 4 ) . In fact, resistances decreased only about 25% for 4 orders of magnitude increase in external concentrations. For lowdensity, fixed-site ion exchangers, e.g., dummy membranes, there should be, theoretically, no dependence of Rbulk on bathing activities a t low concentrations such that Donnan exclusion is not violated. However, with the variability of site concentrations and plasticizer types used in K+ sensors, it is not surprising that a variety of results can be observed. Bulk Resistance as a Function of Soaking Time for Various Bathing Solutions, The time course of bulk resistance changes for three types of PVC membranes, bathed M KCl, are summarized in Table 111. symmetrically in The calculated dielectric constants, and their changes with time, are included. The striking differences in time dependences are illustrated in Figure 5 for one PVC type (GEON) M KC1,O.l M NaCl, and water. These short-term using soaking experiments show resistance increases for all dummy
membranes regardless of bathing solution compositions. Normal membranes bathed in water or NaCl solutions show resistance increases. Only in KC1 solutions do membrane resistances show a slight increase at the beginning of bathing, then become constant or decrease with time. Additional data are given in Table 111to show minor effects of the nature of the plasticizer. However, included is the marked resistance decrease with time when a lipophilic potassium salt, KSCN, is used as a symmetric bathing electrolyte. It is already established from 13CNMR spectrometry ( 4 ) that SCN- violates Donnan exclusion and dissolves in membranes, in excess of the fixed site concentration. Typical results for dummy and normal HMW membranes are given in Table IV. The bulk values of “dummy” membranes increase by factors of 2-7 in water, KC1, and NaCl solutions, over a t least 3 weeks. These effects may be related to water content but not to water uptake rate. The latter occurs in less than 5 min for 0.1-0.2-mm membranes to yield a water concentration of 0.1-0.15 M (20). Uptake rate has been extensively noted because it is crucial for the response of so-called “dry” operating clinical analyzers that use PVCbased ISEs. Electrode 1 in Table IV was stored in air at room temperature for 2 days after its resistance had risen in the
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
course of 23 days to 31 OOO kR When it was remeasured using M KC1, the 5-min resistance value was only 11000 k!?. Water content is evidently connected with resistance increases, but long-term structural changes, perhaps involving water clusters, may be more important than uptake rate. Membranes made of DOS and HMW PVC containing valinomycin (“normal”) also show large relative increases of resistance when bathed in NaCl or in water, but not when bathed in KC1 solutions for a longer period of time. This unexpected result was traced to filterable impurities in PVC. Membranes prepared from solutions filtered through a 0.1 pm filter did not show this phenomenon. Unfiltered membranes show white or cloudy spots when soaked in water. The latter disappear after drying in air and reappear upon resoaking. Larger spots having irregular shapes appear in the same locations during repeated soak/dry cycles. The same shapes could be observed in dry membranes, as surface bumps, using low-angle illumination. It is likely that many PVC types except GEON contain filterable impurities capable of reversible water absorption. Although Oesch et al. (19,20) have also reported reversible water uptake, they observed uniform uptake as microdroplets, hardly discernible even with a microscope. Although no definite information is available on Aldrich PVC, it is apparently made by polymerizations using suspending agents such as poly(viny1alcohol) that are retained in the PVC (6) and appear in the cast membranes. Membrane Capacitance and Dielectric Constants. The capacitance of membrane electrode cells, calculated from the high-frequency semicircle, was typically between 135 and 150 pF when the membrane was 5 / 8 in. in diameter and about 130 pm thick. Capacitance does not depend on the bathing solution composition, but does increase, very slowly, with soaking time-about 10% in 10 days. Variations of capacitance among individual membranes arise from variability of membrane thickness. The average thickness was estimated by cutting out and weighing the active membrane areas after experiments were completed. The density, on average, was reported by Oesch et al. (19,20). Correction for stray p a r d e l capacitance was estimated, experimentally, to be about 10 pF. The dielectric constants of a DOS-plasticized membrane were calculated to be about 10, and of DBS about 15. This value is much higher than for pure PVC ( t = 4), pure DOS (e = 4), or pure DBS (e = 6). This apparent anomaly had been recognized earlier (23,24) with other plasticized PVC compositions. Liberation of movement of the C-Cl dipoles in PVC by loosening of its structure in the presence of plasticizer was the suggested explanation. In Table 11, values of t for DOS for DBS-plasticized membrane were given. They confirm the higher dielectric constants of DBS- relative to DOS-plasticized membranes, but the results cannot be considered precise. The membranes contain water. The relatively high dielectric constants found for plasticized PVC membranes may have an important effect on the number of dissociated charge carriers in the “dummy” membranes. Ion pair dissociation constants of -XH (or Kval+X- in “normal” membranes) can be estimated to increase about 4 orders of magnitude when the dielectric constant changes from 4 to 10. Higher ionic dissociation in plasticized PVC membranes relative to pure plasticizer is expected. On the other hand, plasticized PVC is more viscous than the plasticizer. The specific conductances of plasticized PVC and pure plasticizer membranes, both containing the same concentration of Valinomycin, were compared. The dialysis-membrane-supported liquid membrane cell, described in the Experimental Section was used. From the bulk impedance measurement, we have found that the specific conductance of the plasticized PVC membrane was about 45 times higher than that of the simple plasticizer membrane. One reason for this increase may be
2739
the high dielectric constant, but another might be the introduction of additonal charge carriers by PVC.
DISCUSSION Resistance-increase data can be interpreted by using two experimentally indicated factors: endogenous, fixed sites in PVC that are probably protonated (or in the Na+ form) and water diffusion into membranes with accumulation in hydrophilic impurity regions. Dissociated trace-level charge carriers (H+ and Na+) and anion impurities in the membrane may diffuse toward aqueous regions to be trapped with resulting resistance increase. When the hydrophilic areas have been removed by filtration, there is not so much resistance change. This effect in DOS-plasticized membranes should occur with other plasticizers too. Indeed, a “dummy” membrane made with o-NPOE or DBS plasticizer also showed a %fold increase in resistance when soaked in M KC1 for 1 day, but the resistance of the filtered membrane did not change as much. This observation suggests that the charge trapping cannot be caused exclusively by spontaneous water droplet formation, since both filtered and nonfiltered membrane should behave similarly. GEON PVC has been produced without use of a hydrophilic agent and therefore it is not expected to contain water-adsorbing inhomogeneities. GEON membranes indeed do not turn cloudy when soaked in water. But the resistances of “dummy” GEON membranes increase with soaking time (Figure 5) similar to other PVC preparations. There is, however, a marked difference in the behavior of “dummy” membranes made with GEON or other kinds of PVC when air-dried for a period of time after a period of soaking. When the dried “dummy” membranes are remeasured in M KC1 solution, immediately after resoaking, it was observed that the GEON “dummy” membranes retain their high resistance values, while the resistances of the other PVC ”dummy” membranes decrease drastically. This suggests that the charge carriers originally present in the membranes leach out in two directions: into the bathing solution and into the water-loaded hydrophilic impurities within the membranes. The latter process is reversible upon drying, whereas the charge carriers leached out into the bathing solutions are lost forever. The behavior of GEON membranes is found to be different from the filtered Aldrich HMW membranes, when both are free from hydrophilic impurities. I t may be explained by the different nature of the charge carriers introduced by the two differing PVC preparations. In addition to the mechanism suggested above, there may be other aging processes, i.e., segregations or changes of the membrane structure, that influence membrane behavior further discussed in part 2 (18). Strong complexation between hydrophobic valinomycin and potassium maintains potassium ion as Kval+ in the organic phase instead of being absorbed by the hydrophilic regions. Thus, the resistance of the “normal” membranes is increased to a smaller extent when bathed by KC1 solutions relative to the large increase when bathed in water. Lowering valinomycin concentration produces results between the “dummy” and “normal” membranes. Resistance of membranes with low valinomycin concentrations (0.03-0.07% ) pass through maxima during the first hours of soaking in M KC1 solution. Soaking ”normal” membranes in NaCl solution produces about the same resistance increases as soaking in water. To explain resistance results in “normal membranes”, we have only to make the acceptable assumption that H+ and Na+ form less stable complexes with valinomycin than K+. A probable explanation is that Na+ forms much weaker complexes with valinomycin and is not rendered immune to segregation and mobility changes. Evidence for Endogenous Charge Carriers in the Membranes. Direct evidence for negative ion exchange sites
2740
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
in PVC membranes has been obtained in parallel experiments in Hungary (25). Membranes treated with NaCl for long times, washed, and then treated with KCl in a very small cell showed by atomic absorption analysis extracted Na+ equivalent to 0.05-0.6 mM site concentrations. There was a small dependence of the result on the concentration of bathing electrolyte used. These results will be published separately. Kumins and London (8), using the Meyer-Bernfeld experiments, showed the presence of negative sites in PVC by Donnan exclusion failure analysis. Although their calculation overestimates site concentrations (26),the plasticized PVC clearly contains fixed or mobile sites. We conclude that membranes used in our experiments contain, before aqueous soaking, dissociable species, denoted by H X = H+ + X(fixed-site X-). Thoma et al. (14) also postulated the generation of additional negative ions: OH- trapped in water clusters, by a reaction with the bathing solutions, according to K+(aq)
-
+ H,O(memb) + val(memb) Kval+(memb) + OH-(memb) + H+(aq)
The generated OH- ions (or more likely C1- from incipient Donnan failure) do not contribute to conductivity since they are nearly immobile. If C1- and OH- were the sole source of negative sites, a “normal” membrane would have a much higher bulk resistance a t the moment when soaking begins rather than later. Our results show that this expectation is not observed. Also, generation of charge carriers in the “dummy” membranes without valinomycin is not feasible by this reaction. Water-trapped negative ions as the sole site source are also contradicted by the observed low-resistance values of “dummy” membranes at the beginning of soaking. Thus, we prefer the idea that the species -XH or -XNa contributes to initial conductivity and is originally present in the membranes. In addition to our site-concentration estimates, 0.05-0.6 mM, Thoma et al. (14) have found, for their membranes, equivalent to our “normal” membranes, a cation exchanger capacity on the order of 0.1 mM. These very small concentrations could be provided by the materials or by contaminations from virtually any component used in the membrane. Each component’s resistance was checked in appropriate cells. Tetrahydrofuran impurities could be excluded by casting membranes from mixtures using different volumes of THF. The other components (PVC, plasticizer, and valinomycin) were checked by systematic variations of compositions. By dissolving in 20 mL of T H F different amounts of DOS, PVC, and valinomycin, we found that 600 mg of DOS caused no resistance change, 1 mg of valinomycin increased the conductance from 1.35 X lo-, & to 2.05 X & and 170 mg pS of HMW PVC increased the conductance from 1.35 X to 5.5 x pS. It is likely that conducting species in both valinomycin and PVC are the sources of -XH or -XNa compounds in the membranes.
CONCLUSIONS Our results led to the conclusions that (1)the mobile and fixed sites with weakly dissociable charge originate from PVC
and valinomycin impurities, but not from plasticizer, (2) there are hydrophilic heterogeneities in some membranes, and (3) the higher than expected dielectric constants of plasticized PVC, suggest three structural regions: plasticized polymer, plasticizer, and water in membranes. We believe that increased ionic dissociation in plasticized regions occurs through the formation of complex species, Kval+ and Naval+, and that plasticizer gives increased ionic dissociation that can be offset by trapping of ions in very hydrophilic regions. These conclusions follow from bulk resistance measurements of two- and three-component membranes: plasticizers, PVC, and valinomycin in combinations using three commercial PVC types. Bulk resistance of “dummy” carrier-free membranes increase with soaking time, but with different dependences for KC1, NaCl, and H20. Resistances decrease by an order of magnitude when 0.8 wt% valinomycin carrier is present. Resistances of dummy and normal membranes are exponentially dependent on PVC/plasticizer ratio, but are nearly independent of external bathing KCl solutions.
ACKNOWLEDGMENT We are indebted to Thomas Berube and Miklb Gratzl for the computer programs used in this work. Registry No. PVC, 9002-86-2; KC1,7447-40-7;NaCl, 7647-14-5; DBS, 109-43-3;DOS, 122-62-3;valinomycin, 2001-95-8. LITERATURE CITED Craggs, A.; Moody, G. J.; Thomas, J. D. R. J . Chem. Ed. 1974, 5 1 , 541-544. Ammann, D.; Morf, W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Elect. Rev. 1983, 5 , 3-90. Meier, P. C.; Morf. W. E.; Laubli, M.; Simon, W. Anal. Chlm. Acta 1984, 756, 1-6. Morf, W. E. I n Studies in Analytical Chemistry; Eisevier: Amsterdam, 1981; Vol. 2. Wurstlin, F. Kol/old 2. 1957, 752, 31-36. Davidson, J. A.; Witenhafer, D. E. J. folym. Sci. folym. fhys. Ed. 1980, 78,51-69. Wurstlin, F. Kunstst-Tech. Kunstst.-Anwend. 1941, 1 7 , 269. Kumins, C. A.; London, A. J. Polym. Sci. 1960, 46, 395-408. Rance, D. G.; Zichy, E. L. Polymer 1979, 2 0 , 266-266. Perry, M.: Lobel, E.: Bloch, R. J. Membr. Scl. 1978, 1 , 223-235. Birnthaier, W. Kunststoffe 1949, 3 9 , 301-312. Leilich. K. Kunststoffe 1950, 4 0 , 36-37. Leuchs, 0. Kunststoffe 1951, 4 7 , 309-320. Thoma, A. P.; Vviani-Nauer, A.; Arvanitis, S.;Morf. W. E.; Simon, W. Anal. Chem. 1977, 4 9 , 1567-1572. Cammann, K. Anal. Chem. 1978, 5 0 , 936-940. Armstrong, R. D.; Covinoton. A. K.: Evans, G. P. J. Electroanal. Chem. 1983, 159, 33-46. Crawley. C. D.; Rechnitz, G. A. J. M m b r . Sci. 1965, 2 4 , 201-219. Toth, K.; Graf, E.; Horvai, G.; Pungor, E.; Buck, R. P. Anal. Chem.. following paper In this issue. Oesch, U. Dissertation. ETH, Zurich, 1979, Oesch, U.; Simon, W. Anal. Chem. 1980, 5 2 , 692-700. Buck, R. P. Ion-Sel. Electrcx/e Rev. 1982, 4 , 3-74. Oesch, U.; Slmon, W. Helv. Chlm. Acta 1979, 6 2 , 754-767. Fuoss, R. M. J. Am. Chem. SOC. 1939, 67, 2334-2340. Kisbenyi, M. J. folym. Sci. Pari C 1971, 33, 113-122. Graf, E.; Toth, K.; Pungor, E., unpublished results at the Technical Unlversity of Budapest, Hungary, 1985. Buck, R. P.; Stover, F. S.; Mathis, D. E. J. flectroanal. Chem. 1977, 8 2 . 345-360.
RECEIVED for review March 12,1986.
Accepted June 27,1986. Support from NSF (under Grants CHE8406976 and INST8403331) and the Hungarian Academy of Sciences is gratefully acknowledged.
