Anal. Chem. 1996,67,570-577
Cation Permselectivity at the Phase Boundary of lonophoremlncorporated Solvent Polymeric Membranes As Studied by Optical Second Harmonic Generation Koji Tohda and Yoshio Umezawa* Department of Chemistry, School of Science, The University of Tokyo, Tokyo 113, Japan Shinji Yoshiyagawa Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Satoshi Hashimoto and Masahiro Kawasaki Research Institute for Electronic Science, Hokkaido University, Sapporo 060,Japan
Optical second harmonic generation (SHG) at various plasticized poly(viny1 chloride) (PVC)-based ion-selective membranes was observed. The SHG signal from the ionophore-incorporated membranes in contact with the corresponding aqueous primary cation chloride solutions generally increased with increasing the cation concentration and then leveled off. This result can be explained by the formation of oriented and therefore SHGactive cationionophore complexes at the membrane surface. The SHG responses were analyzed on the basis of a Langmuir-type binding isotherm; a closest packed layer of the oriented complexes seems to be formed at the membrane surface at high primary cation concentrations. It was found that the membrane potential and SHG signal changed in parallel: when the lipophilic thiocyanate ion was used as counteranion, decreases in the potentiometric and SHG responses with increasing primary cation thiocyanate concentration were observed in the same concentration range. In this case, the observed membrane potentials were primarily governed by the SHGactive oriented cation complexes at the membrane surface. However, another important property of the SHG response is that the membrane potential still increased steadily at high primary cation chloride concentrations, where saturation of the SHG signals occurred. The latter result suggests that some cation complexes that contribute to the membrane potential are located behind the SHG-activelayer. F I , the observed SHG intensitywas assumed to be a measure of the charge density at the very membrane surface. Using the space charge model, the SHG intensities were used to correlate the number of SHGadive surface species with the membrane potentials. Liquid membrane-type ion-selective electrodes (ISEs) provide one of the most versatile sensing methods because it is possible to customize the sensory elements to suit the structure of the analyte. A wealth of different synthetic and natural ionophores have been developed in the past 30 years for use in liquid 570
Analytical Chemistty, Vol. 67,No. 3,February 1, 7995
membrane-type ISEs for various inorganic and organic ions.’ In extensive studies?-* the response mechanism of these ISEs has been interpreted in terms of thermodynamics and kinetics. However, there have been few achievements in the characterization of the processes occumng at the surface of ISEs at the molecular level. Pungor and co-workers first observed the complex formation between a crown ether derivative and K+ in ISE membranes by FT-IR-ATR spectrometry. They found that there was only a low concentration of the K+-crown complex at the phase boundary between the membranes and aqueous KCl; this was easily removed by rinsing with water. In contrast, membranes treated with aqueous KSCN contain a large amount of the complexes accompanied by SCN- ions; those complexes were not easily removed by rinsing, and the transport rate of the complex into the bulk of the membrane was found to be much slower than the potentiometric response time. Pungor et al. concluded that the observed potential response of this type of ISE is determined essentially at the surface of the membrane and not in the b ~ l k . ~Umezawa -~ and co-workers first evidenced by FTIR-ATR spectrometry permselective cation transport into ISE liquid membranes in terms of the quantitative stoichiometric ratio of counteranions and complex cations. When primary ion solutions with an IR-active hydrophilic counteranion, Sod2-, were used, the IR spectra of the membranes only indicated the presence of complexed cations, and no IR peak due to the counteranion was found. The intensity of the former peaks was found to be (1) Umezawa, IC; Umezawa, Y. CRC Handbook of Ion-Selective Electrodes: Selectivity Coefficients;Umezawa, Y., Ed.; CRC Press: Boca Raton, FL, 1990. (2) Buck, R P. In Ion-Selective Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, 1978 Vol. 1, pp 1-141. (3) Mod, W. E. The Principle of Ion-Selective Electrodes and of Membrane Transpott; Elsevier: Amsterdam, 1981. (4) Lindner, E.; Tbth, K.; Pungor, E. Dynamic Characteristics of IonSdective Electrodes; CRC Press: Boca Raton, FL, 1988. (5) Kellner, R; Gotzinger,G.; Pungor, E.; Tdth, K.; Polos, P.Fresenius’Z Anal. Chem. 1984,319,839-840. (6) Kellner, R; Fischbock, G.; Gotzinger, G.; Pungor, E.; Tdth, IC; Polos, P.; Lindner, E. Fresenius’Z Anal. Chem. 1985,322,151-156. (7) Tbth, K.; Lindner, E.; Pungor, E.; Zippel, E.; Kellner, R Fresenius’Z Anal. Chem. 1988,331,448-453. (8)Pungor, E. Pure Appl. Chem. 1992,64, 503-507. 0003-2700/95/0367-0570$9.00/0 Q 1995 American Chemical Society
dependent on the membrane concentration of added anionic sites, introduced either by the use of carboxylated poly(viny1chloride) or a derivative of tetraphenylborate, and also on the penetration depth of the IR beam used. In contrast, the use of a hydrophobic counteranion, SCN-, led to IR signals from both the complexed cation and the correspondingcounteranion. Their stoichiometric ratios depended on the concentrations of the primary ion and of the tetraphenylborate derivative in the membrane. It was also found that the depth of completely permselective transport of cations, that is, exclusion of the hydrophilic counteranions, was as great as 1.0pm and that the type of ionophore and the presence of added anionic sites in the membrane were significant factors governing the magnitude of this penetration depth? FT-IR-ATR has thus been demonstrated to be a very useful technique for the characterization of processes at the surface of liquid membrane ISEs. However, the depth accessible to FT-IR-ATR is of the order of 0.1-1.0 pm and is thus too large for the observation of phenomena at the very surface. Optical second harmonic generation (SHG), which is the conversion of two photons of frequency w to a single photon of frequency 2 0 , is known to be an inherently surface-sensitive technique, because it requires a noncentrosymmetric medium. At the interface between two centrosymmetricmedia, such as the interface between two liquids, only the molecules which participate in the asymmetry of the interface will contribute to the SHG.l0 SHG has been used as an in situ probe of chemisorption, molecular orientation, and adsorbate organization at a wide variety of surfaces, namely the a i r - ~ ~ l i d , liquid-~olid,~~-~33~ ~~-~~.~~ liquidair,12J6-19,21 and liquid-liquid10,20t21 interfaces, because of its ability to discriminate between surface species and species in the adjacent bulk media. Corn et al. reported the application of surface SHG to monitor the adsorption of an anionic surfactant molecule at a liquid-liquid (water/l,2-dichloroethane)interface, to which an electrical potential was applied, and determined the surface concentration of the surfactant molecule as a function of the potential.1° Shen et al. reported on the molecular orientation of surfactants at liquid-liquid (water/decane and water/carbon tetrachloride) interfaces.20 The SHG technique therefore seems to be a valuable probe for insight into the surface chemistry of liquid membrane ISEs. We report herewith the first application of surface SHG for the study of ionophore-incorporated PVC liquid membranes. (9) Umezawa, K.; Iin,X; Nishizawa, S.; Sugawara, M.; Umezawa, Y. Anal. Chim. Acta 1993, 282, 247-257 and references cited therein. (10) Higgins, D. A; Com, R M. J. Phys. Chem. 1993, 97, 489-493. (11) Com, R M. Anal. Chem. 1 9 9 1 , 63, 285-29511 (12) Higgins, D. A; Abrams, M. B.; Byerly, S. K; Corn, R M. Langmuir 1992, 8, 1994-2000. (13) Richmond, G. L. In Electroanalytical Chemistry; Bard, A J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 97-180. (14) Higgins, D. A; Abrams, M. B.; Byerly, S. K.; Com, R M. J. Phys. Chem. 1991, 95, 6984-6990. (15) Com, R M. Appl. Spectrosc. 1992, 1636, 117-124. (16) Castro, A; Bhattacharyya, IC; Eisenthal, K B. /. Chem. Phys. 1 9 9 1 , 95, 1310-1315. (17) Schoondrop, M. A; Schouten, A J.; Hulshof, J. B. E.; Feringa, B. L Langmuir 1993, 9, 1323-1326. (18) Xiao, X.; Vogel, V.; Shen, Y. R; Marowsky, G. J Chem. Phys. 1991, 94, 2315-2318. (19) Vogel, V.; Mullin, C. S.; Shen, Y. R; Kim, M. W . J Chem. Phys. 1991,95, 4620-4625. (20) Gmbb, S.G.; Kim, M. W.; Rasing, T.;Shen, Y. R Langmuir 1988,4,452454. (21) Com, R M.; Higgins, D. A Chem. Rev. 1994,94,107-125 and references cited therein.
2
1
L o
uO
3
J
4
Figure 1. Structures of the ionophores 1-4. 1, dibenzyl-14-crown4; 2, bis(benzo-15-crown-5); 3, dibenzo-18-crown-6; 4, dibenzo-24crown-8. These ionophores are selective for Li+, K+, K+, and Na+, respectively.
EXPERIMENTAL SECTION
Reagents. The chemical structures of the ionophores used in this study are shown in Figure 1. 6,6Dibenzyl-1,4,8,11tetraoxacyclotetradecane (1) and bis [ (benz~lkrown-5)-4-methyl] pimelate (2) [abbreviated to dibenzyl-14-crown-4 and bis (benzo1krown-5), respectively] were obtained from Dojindo Laboratories (Kumamoto, Japan). Dibenzo-l&rown-6 (3)and dibenzo24crown-8 (4) were purchased from Aldrich Chemical Co., Inc. (Milwaukee, Wl). Dioctyl sebacate @OS) was purchased from Wako Pure Chemical Industries (Osaka, Japan) and purified by distillation under reduced pressure. Potassium tetrakis(pch1oropheny1)borate (KTfiClPB) was purchased from Dojindo. Poly(vinyl chloride) (WC; n = 1100) was purchased from Wako. Tetrahydrofuran (THF')was freshly distilled from sodium/benzophenone under argon. All other reagents used were of analytical reagent grade. The sample solutions were prepared with deionized and charcoal-treated water obtained with a Milli-Q Type I reagent grade water system (Millipore, Bedford, MA). Preparation of Solvent Polymeric Membranes. Polymeric liquid membranes containing one of the ionophores (1-4), designated hereafter as membranes 1-4, were prepared according to a procedure described in which 180 mg of a DOS solution of the ionophore and 100 mg of W C powder were mixed, 1.2 mL of THF was added, and the suspension was stirred until all the W C dissolved. The resulting solution was carefully cast onto a slide glass and left standing for 20 h in a Petri dish to allow the THF to evaporate slowly. The thickness of the solvent polymeric membrane thus prepared was approximately 130 pm, as determined with a micrometer. SHG Measurements. A schematic diagram of the optical measurement system is shown in Figure 2. The SHG measurements were made with the s-polarized 1064 nm output of a Q-switched Nd:YAG laser system (Quanta-Ray, Model GCR 170, Spectra-Physics, Mountain view, CA) with an 8-9 ns pulse width (22) Umezawa, Y.; Kataoka, M.; Takami, W.; Kimura, E.; Koike, T.; Noda, H. Anal. Chem. 1988, 60, 2392-2396.
Analytical Chemistry, Vol. 67, No. 3, February 7, 7995
571
1
Q-Swltched Nd:YAG laser
I
I I
Y-1 recorder
the concentration of the primary cation in the aqueous solution was changed. Though SHG intensities can be expected to depend on the roughness of the membrane surface, we found that the ratio of the signal intensities for sample and blank solutions was identical for several solvent polymeric membranes with the same composition. The SHG data points in Figures 4-6 are the average from three sets of measurements. The error bars in these figures show standard deviations. Potential Measurements. Solvent polymeric membranes, prepared as described above, were mounted on an ion-selective electrode body (Denki Kagaku Keiki Co., Tokyo, Japan) with a M primary cation chloride solution as the internal reference solution. The reference electrode was of the double-junction type, with an Ag/AgCl electrode containing 3 M KCl solution in the inner compartment and 10-l M GCl solution in the outer compartment. The electrode cell assembly thus made for the potential measurements was as follows:
Ag/AgClI
Figure2 Schematic diagram of the present SHG measuring system and detailed structure of the optical cell: a, 45" right angle prism; b, glass plate reflector; c, infrared transparent filter ('690 nm); d, aqueous CuSO4 filter; e, spherical quartz lens; f, interference filter (530 nm, 10 nm of fwhm); g, beam damper; h, stainless cover; i, silicone rubber packing;j, solvent polymeric membrane; k, slide glass; I, sample cuvette.
and 10 Hz repetition rate. The SHG measuring cell was made from an acrylic resin plate by boring a circular hole (diameter, 20 mm) through one side of the cell and attaching a transparent window on the other side. A slide glass covered with the solvent polymeric membrane, prepared as described above, was mounted onto the cell, and 50 mL of deionized water was added. The membrane was conditioned with water for 90 min to obtain equilibrium between the membrane and the aqueous phase. Changes in the primary cation (analyte) concentration in the aqueous sample solution adjacent to the membrane were made by injecting a concentrated aqueous primary cation salt solution into the sample solution. No electrolytes other than the primary cation salt were added to the sample solution, which was stirred gently throughout the experiment. The laser light was irradiated onto the membrane at an incident angle of approximately 45". The cross section of the light beam was 0.62 cm2, and the typical energy density at the membrane surface was 50 mJ as measured by an optical power meter (Model 407, Scientec, Boulder, CO). An infrared transmittance filter (>690 nm) was used as the entrance window of the cell box to eliminate ambient light. The second harmonic was collected with a quartz lens (focal length, 10 cm),passed an aqueous CuSO4 filter and an interference filter (530 nm, 10 nm of fwhm), and was detected with a photomultiplier (PMT) (1P21, Hamamatsu Photonics, Hamamatsu, Japan). The output of the SHG signal was averaged with a gated integrator (SR250, Stanford Research System, Sunnyvale, CA). The measurement of other nonlinear optical phenomena such as third harmonic generation was not attempted, and the dependence of the SHG signal on polarization was not examined. All experiments were carried out at room temperature. The SHG signal from the solvent polymeric membrane became constant within 3 min after 572 Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
M primary ion chlorideImembraneIsample solution1 110-l M LiCll13 M KClIAgCl/Ag
Potential measurements were carried out at room temperature with a pH-mV meter, Model HM-GOV (TOA Electronics Ud., Tokyo, Japan). Ion activities were calculated from the concentm tions using an extended form of the Debye-Huckel equation.23 The response time t (At, AQ, defined as the time at which the differential quotient (AE/At) of the potential-time curve becomes smaller than a prechosen value (AE < 0.5 mV within At = 1.0 min in the present study), was from 1 to 3 min.a-26 RESULTS AND MSCUSSION
SHG Induced by Surface Ionophore/Metal Ion Complexation. The SHG signal intensity, 1(20),arising at the liquidliquid interface is known to be proportional to the square of the second-order nonlinear electric susceptibility, ~ ( ~ 1at, the surface,10.2021
where I(o) is the input light power, e(@) and e(20) are the electric polarization vectors describing the input and output light fields, respectively, N is the surface concentration of SHGactive species, a(2)is the molecular second-order nonlinear electric polarizability, T is the coordinate transformation connecting the laboratory and molecular axes, and the angular brackets denote an average over all molecular orientations. From eqs 1 and 2, it follows that the square root of the SHG intensity, (1(2~))l'~, is directly proportional to N, (?) and a(2), namely,
(3) The measurement of SHG intensities thus provides information (23) Meier, P. C. Anal. Chim. Acta 1982, 136,363-368. (24) Uemasu, I.; Umezawa, Y. Anal. Chem. 1982,54,1198-1200. (25) Iindner, E.: T6th. K Pungor, E.; Umezawa, Y. Anal. Chem. 1984.56.808810. (26) Lindner, E.; T6th, IC; Pungor, E. f i r e Appl. Chem. 1986.58.469-479.
c
T
T
T
20 30 40 50 Input power (mJ I pulse)
0 20
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1
40
30
50
0
Input power (mJ I pulse) Figure 3. Dependence of the square root of the SHG intensity on the power of the irradiated fundamental light, as obtained with membrane 2 with an ionophore concentration of 3.0 x lo-‘ M. The adjacent aqueous solution was 1.8 x lo-’ M KCI. Inset: dependence of the SHG intensity on the input optical power.
0 a
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t
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0.1
0.2
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0.4
0.5
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c (M) Figure 4. Dependence of the square root of the SHG intensity on Li+ ion concentrations with membrane 1 (0,ionophore concentration, 1.O x 10-2 M) or with a membrane without ionophore 1 (0).
on the concentration, N, the molecular orientation, (7J,and the polarization, a@, of SHGactive species at the interface. The dependence of the SHG intensity, Z(2w), of membrane 2 in contact with 0.18 M aqueous KC1 on the power of the irradiation light beam, Z(w), is shown in Figure 3. A linear relationship was obtained between the input power, Z(w), and the square root of the SHG intensity, (Z(2w))lI2. This relation satisfies the principle of SHG (see eq 1) and thus coniirms that the light detected in this system was indeed arising from SHG. Figure 4 shows the dependence of the square root of the SHG intensity, (Z(2~))l’~, on the concentration of Li+ ion (LiC1) in the aqueous solution, using membrane 1 with an ionophore concentration of 10 mM. The SHG intensity increased steeply at low Li+ ion concentrations but leveled off at higher concentrations. In the absence of Li+ ion, the SHG signal was found to be negligible. The SHG response for a membrane without ionophore, i.e., a WC/DOS membrane, was also negligible at otherwise identical conditions. The generation of the SHG signal can be ascribed to the formation of oriented Li+-ionophore 1 complexes at the membrane surface, facing across the interface the hydrophilic counteranions, C1-, in the adjacent aqueous phase. In fact, when a relatively hydrophobic anion, SCN-, was used instead of C1- ion as the counteranion, the observed SHG signal decreased at high salt concentrations, probably due to a decrease in the
1
2
3
4
5
6
7
1 I C (M-1) Figure 5. Plots of the reciprocal of the square root of the SHG intensities versus the reciprocal of Li+ ion concentrations in the adjacent aqueous solutions (the Langmuir isotherm) as obtained with membrane 1. The ionophore concentrations were 3.0 x (A), 1.0 x lo-‘ (0),and 3.0 x M (0).The data points present averages for three sets of measurements. Error bars show standard deviations.
number of oriented cation complexes at the surface upon extraction of SCN- ion into the membrane bulk (vide infra). The fact that the SHG signal virtually leveled off at high Li+ ion concentrations means that the number of SHGactive species located at the membrane interface became constant beyond a certain Lit ion concentration, assuming negligible changes in molecular orientation (see eq 3). The SHG responses of membranes 2 , 3 ,and 4, respectively, were similar to that of membrane 1;the SHG signal increased with increasing primary cation concentrations in the aqueous solution and leveled off at high cation concentrations (vide infra). The saturation of the SHG response at high cation concentrations suggests that the process of complex formation at the membrane surface may be treated by a Langmuir isotherm-type analysi~.’~J~ At constant temperature, the Langmuir equation is given by (4)
where N is the number of the cationic complexes located at the surface, N, is the maximum number of the cationic complexes at the interface, cb is the aqueous bulk cation concentration, and K is the binding constant (M-l) (the complexation stability constant between the cation and the ionophore at the membrane surface). It should be noted that 1/N is inversely proportional to the bulk cation concentration, Cb. A nearly linear relationship was in fact found between the reciprocal of the square root of the SHG intensity and the reciprocal of the bulk cation concentration for membranes 1,as shown in Figure 5. Also for membranes 2,3,and 4, Langmuirtype binding isotherms were found (figures not shown). It seems that a Langmuk-type “saturation” is indeed reached at the membrane surface. Figure 5 also shows the effect of the ionophore concentration on the Langmui~typebinding isotherm. ?he slope of the isotherm for a membrane with 10 mM of ionophore 1 was roughly 3 times larger than that with 30 mM of the same ionophore. The binding Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
573
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Figure 6. Dependence of the square root of the SHG intensity for membrane 2 without KTpClPB (a) and with KTpClPB (b) on K+ ion concentrations in the adjacent aqueous solution containing KCI (0)and KSCN (0).Inset: corresponding observed EMF to KCI and KSCN. The M, respectively, for both SHG and EMF measurements. The data concentrations of ionophore 2 and KTpClPB were 3.0 x low2and 1.O x points present averages for three sets of measurements. Error bars show standard deviations.
constant, K, which is inversely proportional to the slope (eq 4), was estimated to be 4.2 and 11.5M-' for the membranes with 10 and 30 mM ionophore 1,respectively. This result supports the validity of the present Langmuir analysis because the binding constant, K, should reflect the availability of the surface sites, the number of which should be proportional to the ionophore concentration, if the ionophore is not surface-active itself. In addition, the intercept of the isotherm for a membrane with 10 mM ionophore 1 was nearly equal to that of a membrane with 30 mM ionophore 1 (see Figure 5). This suggests the formation of a closest packed surface molecular layer of the SHGactive Litionophore 1 cation complex whose surface concentration is nearly equal at both ionophore concentrations. On the other hand, a totally different intercept and a very small slope of the isotherm was obtained for a membrane containing only 3 mM ionophore 1. This indicates an incomplete formation of the closest packed surface layer of the cation complexes due to a lack of free ionophores at the membrane surface, leading to a kinetic limitation. In this case, the potentiometric response of the membrane toward Li+ was also found to be very weak (vide infra). The results of the above-mentioned Langmuir analysis of the SHG responses may be interpreted in terms of a tightly packed monolayer of the SHGactive cation complexes at the membrane surface. The tight layer may, however, also be a layer thicker than a monolayer in which the potential aligns the complexes to the electric field. As a consequence of the increase of the potential near the surface, the oriented complexes would on the average be nearer to the surface than the average of all complexes. Correlation between the SHG Signals and ISE Potentials. As shown in Figure 6a, the SHG response of membrane 2 to aqueous KSCN was found to be different from that to KC1. Upon increasing the KSCN concentration, the SHG signal initially increased but reached a maximum at 0.2 M and then decreased. The potentiometric response of the same membrane also exhibited a maximum at a K+ ion activity of ca. 0.1 M (see inset in Figure 6a). Thus, the decreases in the SHG and potentiometric responses 574 Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
were found to start at roughly the same KSCN concentration. This may be attributed to a decrease in the number of oriented K+ionophore 2 complexes at the interface with the appearance of SCN- ion in the membrane. To prevent such anionic effects, so-called added anionic sites are conventionally used as additives for ISE m e m b r a n e ~ . ~In ~-~l fact, upon addition of KTPCE'B, a marked improvement of the SHG response for KSCN was observed at higher analyte concentrations (Figure 6b). Because a membrane containing only KTpClPB, i.e., a PVC/DOS/KTpClPB membrane, did not show any SHG response toward K+ ion, this result suggests that TpClPB- may assist the surface orientation of the K+-ionophore 2 complexes by inhibiting the uptake of SCN- ion from the adjacent aqueous solution. The parallel changes in membrane potentials and SHG signals allow us to conclude that the observed membrane potential changes in the above case are primarily governed by the SHGactive oriented species at the membrane surface. Anionic effects were observed by FT-IR-ATR spectrometrywith a membrane containing not ionophore 2 but a different kind of ionophore, ETH 12ge9When the lipophilic counteranion, SCN-, was used for the primary ion solutions, the spectra from both the complexed cation and the Correspondingcounteranion were seen, of which the stoichiometric ratio was nearly equal to that of ETH 129 complex-SCN salt at relatively high concentrations of the primary ion solutions. With KTPClPB in the membrane, the decrease in the uptake of SCN- ion into the ETH 129based membrane relative to the complexed cation was observed. The ATR results for the ETH 129-basedmembranes with and without KXpClPB indicate that KI)CE'B resisted and inhibited by as much (27) Mod, W. E.; Kahr, G.;Simon, W. Anal. Lett. 1974, 7, 9-22. (28) Boles, J. H.; Buck, R P. Anal. Chem. 1973, 45, 2057-2062. (29) Buck, R P.; Toth, IC; Graf, E.; Horvai, G.; Pungor, E. I. Elecfroanal. Chem. 1987,223, 51-56. (30) Eugster, R; Gehrig, P. M.; Mod, W. E.; Spichiger, U. E.; Simon, W. Anal. Chem. 1991, 63, 2285-2289. (31) Eugster, R;Spichiger, U.E.; Simon, W. Anal. Chem. 1993, 65, 689-695.
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Flgure 7. Dependence of the SHG signals and the calculated charge densities for membranes 1-4 on the primary cation concentrations in the adjacent aqueous solutions and the corresponding observed EMFs (inset). The ionophore concentrations for the membranes 1 (a), 2 (b), 3 (c), and 4 (d) are 1.0 x 3.0 x 2.5 x and 3.0 x lo-* M, and their primary ions are Li+, K+, K+, and Na+, respectively. The data points present averages for three sets of measurements. Error bars show standard deviations.
as 70%the uptake of SCN- ion into the membrane phase, which trend is consistentwith the findings relevant to the anionic effects of the potentiometric and SHG responses of membrane 2. Another important property of the SHG response of membranes 1-4 is that saturation occurs at higher cation concentrations, where Nemstian potentiometric responses are still observed (Figure 7). This shows that the generation of the membrane potential at very high primary cation concentrations is governed not only by the SHGactive cation complexes at the membrane surface but also by complexes located behind the SHGactive layer, which are SHGinactive due to a lack of molecular orientation. One should remember, however, that electroneutrality requires that almost all cation complexes be within a few Debye lengths from the membrane surface as long as their charge is not balanced by an anionic site. To evaluate the contribution of the SHGactive oriented cation complexes to the ISE potential, the SHG responses were analyzed on the basis of a space charge mode1.2~32~33 This model, which was proposed to explain the permselectivity behavior of electrical neutral ionophore-based liquid membranes, assumes that a space charge region exists at the membrane boundary: the primary function of lipophilic ionophores is to solubilize cations in the boundary region of the membrane, whereas hydrophilic counteranions are excluded from the membrane phase. Theoretical treatments of this model reported so far were essentially based on the assumption of a double-diffuselayer at the organic/aqueous solution interface and used a description of the diffuse double layer based on the classical Gouy Chapman t h e ~ r y ? ~ - ~ ~ (32)Buck, R P.Anal. Chem. 1976,48,23-39B (33)Morf, W.E.:Simon, W. Helu. Chim. Acta 1986,69,1120-1131. (34)Gavach, C.;Seta,P.; D'Epenoux, B. J. Electroanal. Chem. 1977,83,225235. (35)Reid, J. D.;Melroy, 0. R; Buck, R P. J. Electroanal. Chem. 1983,89,2936. (36)Groth, M.; Gromb, S.: Gavach, C. J. Electroanal. Chem. 1978,147,71-82.
Following this model, the charge density of the complex cations, uo,on the membrane side and that of the hydrophilic counteranions, e, on the aqueous side can be expressed as
(5) $.= - , / m s i n h [
F(40 2RT - 4lJ
]
(6)
where c b and cb are the concentrationsof the ions in the bulk of the membrane and of the aqueous solution, respectively, EO is the permittivity of vacuum, ? and E are the relative permittivities of the membrane and of water, respectively, $0 and & are the electric potentials at the outer Helmholtz planes at the interface of the membrane and of the aqueous side, respectively, &, and & are the electric potentials of the bulk of the membrane and of the aqueous solution, respectively, and R and T have their usual meanings. It is possible to describe the electric potential difference between the aqueous and membrane bulk, &, - &, as a function of the charge densities, aoand e, in the simplitied case where $o = 40,as &-&=(40-4lJ-(40-&=
When the extraction of the hydrophilic counteranion from the aqueous solution into the membrane bulk is negligible (cation permselectivity preserved), the concentration of the complex cation in the membrane bulk, c b , is equal to that of the k e d anionic sites, X-,in the membrane matrix, due to the electroneuAnalytical Chemistry, Vol. 67, No. 3, February 1, 1995
575
trality condition withii the membrane bulk:
Because the net charge at the interface should be zero, (TO - aa = 0, the electric potential difference, & - &, in eq 7 can be rewritten as
2R arcsinh(
J--)]
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8RT€,+Cb The determination of the number of the SHGactive complex cations from the corresponding SHG intensity and thus the surface charge density, uo, is not possible because the values of the molecular second-ordernonlinear electric polarizability, a(2), and molecular orientation, (7), of the SHGactive complex cation and its distribution at the membrane surface are not known (see eq 3). Although the formation of an SHGactive monolayer seems not to be the only possible explanation, we used the following method to estimate the surface charge density from the SHG results. Since the square root of the SHG intensity, (1(20))1/2, is proportional to the number of SHGactive cation complexes (see eq 3), the observed SHG intensity is a measure of the charge density (C/cmz), 8,at the membrane surface. Using the molecular area of the charged complex as estimated from a CPK molecular model and assuming a full coverage of the cation complexes, Nmax of the Langmuir equation was estimated (see eq 4 and Figure 5). Surface charge densities can then be obtained from eq 4. The surface charge density and the SHG intensity dependence on the cation concentration is shown in Figure 7 for four different membrane types. The electric potential differences, & - &, calculated from the surface charge densities, a",according to eq 9 are shown in F w e 8, together with the observed potentials. The concentration of anionic sites in the membrane, X-, which is necessary for determining &, - & according to eq 9, was varied from 0.05 to 0.6 mM. This covers the reported concentration range of anionic sites in PVC-plasticizer membranes without added sites.B Changing the X- value within this range, however, hardly affected the slope of the calculated potentials, whereas the absolute potentials shifted. For membrane 1 with 10 mM ionophore, the slope of the calculated membrane potential was 58 mV/decade in the Lit ion concentration range from 2.0 x to 3.0 x lo-' M, which is close to a Nernstian response (59.2 mV/decade, 25 "C). This is consistent with the observed EMF response. This result indicates that the potential increase in this case is essentially determined by the SHGactive cation complexes. However, for a membrane with a concentration of ionophore 1 of 30 mM, as a result of the SHG intensity saturation, the slope of the calculated potential became small at higher primary ion (Lit) concentrations. This is in clear contrast to the observed result, showing a nearly Nernstian response in the same concentration range. For the membrane with 3 mM ionophore 1, the slope of the calculated potentials was only 25 mV/decade, which is close to that of the observed value. This ionophore concentration dependence r e flects the extent of surface coverage by the SHGactive cation complexes; at high ionophore concentrations,the surface satura576 Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
tion of the SHGactive complex cation occurs even at lower primary ion concentrations. The calculated membrane potentials consequently leveled off, deviating from the observed EMF function. Similar results were observed with other membranes: the slopes of the calculated potentials for membranes 2,3,and 4 were consistent with those of the observed EMFs at relatively low cation concentrations (Figure 8b-d), and deviations between the calculated and observed potentials were found at relatively high cation concentrations. The increase in the extent of charge separation due to the increasing ionophore concentration without substantial change in the observed EMF, as can be seen in Figure 8, may be surprising. One must, however, bear in mind that the increased ionophore concentration leads to an increase in the phase boundary potentials at both the outer and inner interfaces of the ISE membrane and that these two effects should cancel out. The deviation of the calculated EMF responses from Nernstian slopes at high cation and ionophore concentrations, in contrast to the observed EMFs, may be explained as follows. In the space charge model, the surface charge is treated as the sum of point charges that are Boltzmann distributed within the diffuse double layer. Due to neglect of field-dipole forces and specific molecular interactions occurring at the phase boundary, saturation of the SHGactive layer cannot be taken into account. Although the space charge model seems to be valid for the explanation of the electric potential drop across the interface, the SHG observation is expected to provide refinements of this model in terms of the charge distribution as well as the orientation of the cation complexes. It must be concluded that the calculated potentials do not allow only the interpretation that the tight cation complex layer is a monolayer, because the slope of the membrane potential calculated from surface charge densities hardly changed when charge densities 2 or 3 times larger or smaller than the values estimated using the CPK model areas were used. CONCLUSION SHG was shown to be the first optical technique available for the selective observation of charge separation processes at the liquid-liquid interface of ion-selective electrodes. By observing the SHG signals from ionophore-incorporated liquid membranes, the existence of SHGactive, oriented cation complexes at the membrane surface was shown. At low analyte concentrations,all cation complexes contributing to the EMF seem to be SHGactive, whereas at high analyte concentrations,only a very thin layer of complexes at the very surface is SHGactive. This may be the result of the increasing thickness of the EMF-determining layer with increasing analyte concentration. The hgmuir-type saturation of the SHG signal, as well as the fact that SHG is usually believed to be monolayer sensitive, suggests the formation of a monomolecular layer of the SHGactive cation complexes. An alternative explanation may, however, be the formation of a layer of the SHGactive cation complexes in which the potential aligns the complexes to the electric field. Whereas the SHGactivelayer aligned with the electric field does not seem to contradict the space charge model, the model for the formation of a monomolecular SHGactive layer predicts a different charge distribution from the space charge model at the membrane interface because it is not based on point charges and explicitly considers intermolecular interactions. We are currently continuing to examine the
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Flgure 8. Calculated potential differences, $b - &, across the membrane boundary as a function of the primary cation activity, as calculated from eq 9. The upper data sets show the correspondingobserved EMF. (a) Membrane 1: primary ion, Li+; ionophore concentration, 3.0 x (O), 1.O x M (0).(b) Membrane 2: primary ion, K+; ionophore concentration,3.0 x (O), and 3.0 x M. (c) Membrane 3: primary M. The following ion, K+; ionophore concentration, 2.5 x lo-* M. (d) Membrane 4: primary ion, Na+; ionophore concentration, 3.0 x parameters were used: Z = 4.0, E = 78.3, T = 298 K, and X- = 1.0 x lo-' m o l ~ m - ~ .
properties of the SHGactive layer with different ionophores to further elucidate this point.
University of Tokyo, for his invaluable discussionsand comments on this work.
ACKNOWLEDGMENT
This work was hancially supported by the Ministry of Education, Science and Culture, Japan, and the Nissan Science Foundation, Tokyo, Japan. We thank Idemitsu Petrochemical Co. for part of instrumental support of this work. We also thank Dr. P. Buhlmann, Department of Chemistry, School of Science,
Received for review September 12, 1994. Accepted November 1 8 ~lgg4.@ ~ C 9 4 o gox i @
Abstract published in Advance ACS Abstracts, December 15, 1994.
Analytical Chemistty, Vol. 67, No. 3, February 1, 1995
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