Anal. Chem. 1996, 68, 1623-1631
Influence of Nonionic Surfactants on the Potentiometric Response of Hydrogen Ion-Selective Polymeric Membrane Electrodes Cecilia Espadas-Torre, Eric Bakker,† Susan Barker, and Mark E. Meyerhoff*
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-1055
The influence of poly(ethylene oxide)-based nonionic surfactants (i.e., Triton X-100 and Brij 35) in the sample phase on the response properties of hydrogen ion-selective polymeric membrane electrodes containing mobile (lipophilic amines) or covalently bound (aminated-poly(vinyl chloride)) hydrogen ion carriers is reported. In the presence of these nonionic surfactants, membrane electrode response toward interfering cation activity (e.g., Na+) in the sample phase is increased substantially and the pH measuring range shortened. The degree of cation interference for pH measurements is shown to correlate with the basicity of the hydrogen ion carrier doped within the membrane phase. The observed deterioration in selectivity arises from the partitioning of the surfactant into the membrane and concomitant extraction of metal cations by the surfactants in the organic phase. The effect of nonionic surfactants on pH electrodes prepared with aminated-PVC membranes is shown to be more complex, with additional large shifts in EMF values apparently arising from multidentate interactions between the surfactant molecules and the polymeric amine in the membrane, leading to a change in the apparent pKa values for the amine sites. The effects induced by nonionic surfactants on the EMF response function of hydrogen ionselective polymeric membrane electrodes are modeled, and experimental results are shown to correlate well with theoretical predictions. Direct potentiometry with polymer membrane-based ionselective electrodes (ISEs) is a well-established measurement technology used widely within many commercial blood gas/ electrolyte analyzers and water quality instruments. Such automated systems commonly operate with calibration and wash solutions that contain nonionic surfactants (e.g., polyether-based compounds).1 These species are added to enhance washing efficiency (i.e., decrease sample carryover) and to prevent entrapment of air bubbles in the flow channels of such analyzers. In water analysis, various surfactants may also be present in the samples themselves. Although the presence of nonionic surfactants in samples, wash solutions, and/or calibrators is not expected to influence the potentiometric response of classical glass membrane or solid-state ISEs (e.g., fluoride electrode), problems could † Present address: Department of Chemistry, Auburn University, Auburn, AL 36849. (1) Barton, E. C., et al., Eds. Advances in Automated Analysis, Technicon International Congress, 1976; Mediad: Tarrytown, NY, 1977; Vol. 2, pp 39, 43.
0003-2700/96/0368-1623$12.00/0
© 1996 American Chemical Society
potentially arise in the case of ISEs based on polymeric membranes owing to the ability of such surfactant species to enhance the extraction of lipophilic ionophores from the membrane phase to the aqueous sample, thereby decreasing ISE operational lifetimes. Surprisingly, other potential effects on the performance of polymer membrane ISEs due to the presence of nonionic surfactants (e.g., response times, potentiometric selectivity, etc.) have, to date, not been examined in detail. It is well known that surfactants of all types are capable of interacting with and partitioning into organic polymeric films. In fact, polymer membrane-based ISEs have long been employed for the determination of ionic surfactants via either direct potentiometry or, more commonly, titration methods. For example, electrodes fabricated with plasticized poly(vinyl chloride) (PVC) membranes containing ion-exchangers (often ion pairs of large cationic surfactants with anionic surfactants) have been used for the analysis of anionic or cationic surfactants.2-4 In addition, electrodes formulated with PVC membranes containing tetraphenylborate salts of barium complexes with given polyethoxylates (e.g., oxyethylated alkylphenols), originally developed as Ba2+selective electrodes, are also responsive to various acyclic polyether-based nonionic surfactants and can be used effectively for the determination of these species.5-7 Since surfactants are able to partition into/interact with the organic polymeric matrices and change their composition (and hence chemistry), it is not unexpected that interferences may occur due to the presence of surface-active substances during the determination of other ionic species with polymer membrane-type ISEs. Indeed, it is well known that cationic surfactants are able to compete favorably with analyte cations (e.g., Ca2+, K+, etc.) for extraction into polymer membranes doped with cation-selective neutral carrier ionophores and suitable lipophilic counteranionic sites, and such extraction can yield large positive errors in the measurement of cations with these devices as well as large shifts in the electrodes’ standard potential (ref 8 and references therein). A similar effect would be expected for neutral or charged carrier-based anion sensors in the presence of anionic surfactants. Although the interference of nonionic surfactants on the determination of Ba2+ with electrode membranes containing barium complexes of alkoxylates has been documented6 (owing to the unique role that the surfactant has (2) Birch, B. J.; Cockcroft, R. N. Ion-Sel. Electrode Rev. 1981, 3, 1. (3) Schulz, R.; Gerhards, R. Am. Lab. 1994, 26, 40. (4) Szczepaniak, W.; Ren, M. Electroanalysis 1994, 6, 341. (5) Jones, D. L.; Moody, G. J.; Thomas, J. D. R. Analyst 1981, 106, 974. (6) Jones, D. L.; Moody, G. J.; Thomas, J. D. R. Analyst 1981, 106, 439. (7) Chernova, R. K.; Kulapina, E. G.; Materova, E. A.; Tret’yachenko, E. V.; Novikov, A. P. Zh. Anal. Khim. 1992, 47, 1464. (8) Hulanicki, A.; Trojanowicz, M.; Pobozy, E. Analyst 1982, 107, 1356.
Analytical Chemistry, Vol. 68, No. 9, May 1, 1996 1623
a number of commercial blood gas/electrolyte instruments for rapid measurements of pH and carbon dioxide levels in undiluted blood.16,17 It will be shown that, in the presence of low concentrations of common nonionic surfactants (Triton X-100 or Brij 35) in the sample phase, the response of polymer membrane pH electrodes toward potentially interfering cations (e.g., Na+) within the sample solution can be increased substantially, shortening the pH measuring range considerably. Such effects are shown to be more pronounced for electrodes formulated with membranes containing hydrogen ion carriers of relatively low basicity. Hydrogen ion-selective electrodes prepared with aminated-PVC membranes are demonstrated to constitute a special case owing to the ability of polyether-based nonionic surfactants to interact strongly with the polymeric amine sites in the membrane phase via a multidentate interaction, altering the apparent pKa value of the amine sites immobilized on the PVC matrix. An appropriate theoretical model is developed to explain the observed interferences, which arise from the partitioning of nonionic surfactants into the membranes of pH and potentially other polymeric membrane ISEs.
Figure 1. Structures of surfactants, hydrogen ion-selective ionophores, and chromoionophores used in this work (structure shown for PIP-PVC represents only a segment of the resulting polymer; for a complete description, see ref 15).
on the electrode’s ability to exhibit Ba2+ selectivity), in most other cases, the effect of nonionic surfactants on the response of ionophore-based polymer membrane electrodes has generally been considered to be of little or no significance. Herein, we examine in detail the effect of nonionic surfactants having poly(ethylene oxide) chains on the potentiometric response properties of hydrogen ion-selective polymeric membrane ISEs employing either mobile hydrogen ion carriers (lipophilic amines) or covalently bound hydrogen ion carriers (aminated-PVC) as ionophores (see Figure 1 for ionophore and surfactant structures examined). pH electrodes of this type have been the subject of considerable research in recent years9-15 and are now used within (9) Schulthess, P.; Shijo, Y.; Pham, H. V.; Pretsch, E.; Ammann, D.; Simon, W. Anal. Chim. Acta 1981, 131, 88. (10) Oesch, U.; Brzo´zka, A.; Xu, A.; Rusterholz, B.; Suter, G.; Pham, H. V.; Welti, D. H.; Ammann, D.; Pretsch, E.; Simon, W. Anal. Chem. 1986, 58, 2285. (11) Bakker, E.; Lerchi, M.; Rosatzin, T.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211. (12) Ma, S. C.; Meyerhoff, M. E. Mikrochim. Acta 1990, 1, 197. (13) Cosofret, V. V.; Lindner, E.; Buck, R. P.; Kusy, R. P.; Whitley, J. Q. J. Electroanal. Chem. 1993, 345, 169. (14) Cosofret, V. V.; Lindner, E.; Buck, R. P.; Kusy, R. P.; Whitley, J. Q. Electroanalysis 1993, 5, 725.
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Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
EXPERIMENTAL SECTION Materials and Reagents. The following were products of Fluka (Ronkonkoma, NY): high molecular weight poly(vinyl chloride) (PVC), o-nitrophenyl octyl ether (NPOE), dioctyl sebacate (DOS), potassium tetrakis(p-chlorophenyl)borate (KTpClPB), octadecyl isonicotinate, 11-[(1-butylpentyl)oxy]-11-oxoundecyl 4{[9-(dimethylamino)-5H-benzo[a]phenoxazin-5-ylidene]amino}benzeneacetate (ETH 2439), N,N-diethyl-5-octadecanoylimino)-5Hbenzo[a]phenoxazin-9-amine (ETH 5294), and N,N-diethyl-5-[(2octyldecyl)imino]-5H-benzo[a]phenoxazin-9-amine (ETH 5350). Tridodecylamine (TDDA) was obtained from Eastman Kodak (Rochester, NY). Poly(vinyl chloride) with covalently fixed piperazine sites (PIP-PVC) was prepared according to ref 15 by reacting 1 part PVC (MW 100 000, Polysciences), 7 parts anhydrous piperazine (Aldrich), and 14 parts HPLC grade methanol (Aldrich) for 7 days at 35 °C. After extensive aqueous and methanol washing followed by drying, elemental analysis (Galbraith Laboratories, Inc., Knoxville, TN) indicated the product had 0.5 wt % nitrogen content. The surfactants Brij 35 (poly(ethylene glycol) monolauryl ether, with an average of 23 ethylene oxide units) and Triton X-100 (poly(ethylene glycol) p-isooctylphenyl ether, with an average of 10 ethylene oxide units) were obtained from Sigma and Aldrich, respectively. Tetrahydrofuran (THF) from Aldrich was freshly distilled prior to use. All other chemicals were analytical-reagent grade. Reverse osmosis/deionized water was used to prepare all solutions. Membrane Preparation. Membranes for potentiometric measurements were prepared by dissolving the components (totaling 200 mg) in 2 mL of THF and casting the solutions into 2.5-cm glass rings resting on clean glass plates. The solvent was allowed to evaporate at room temperature in a dust-free environment for 24-48 h. Aminated-PVC membranes contained 33.5 wt % PIP-PVC, 66.0 wt % NPOE, and 0.5 wt % KTpClPB. Membranes doped with free mobile carriers contained 0.004 mmol of the corresponding pH ionophore, 0.002 mmol of KTpClPB (equivalent to 0.5 wt %), 66.0 wt % NPOE, and ∼32.5 wt % PVC (balance). (15) Kusy, R. P.; Whitley, J. Q.; Buck, R. P.; Cosofret, V. V.; Lindner, E. Polymer 1994, 35, 2141. (16) Meyerhoff, M. E. Clin. Chem. 1990, 36, 1567. (17) Kost, G. J. Crit. Rev. Clin. Lab. Sci. 1993, 30, 153.
Some measurements were performed with membranes that contained no pH carrier (only KTpClPB as ion-exchanger). Cocktails for membranes used in optical measurements contained an additional ∼0.0003 mmol (per 200 mg of total components) of a suitable chromoionophore added as indicator when the primary hydrogen ion carrier was not itself a suitable chromoionophore. Potentiometric Measurements. Disks were punched from the membranes and mounted into Philips IS-560 electrode bodies (Mo¨ller Glasbla¨serei, Zu¨rich, Switzerland). Membrane electrode voltages were measured at 22.0 ( 1.0 °C vs a double-junction Ag/AgCl reference electrode from Fisher Scientific (Itasca, IL) with 1.0 M lithium acetate as salt bridge electrolyte. A Macintosh IIcx computer with an NB-MIO-16X analog/digital input/output board (National Instruments, Austin, TX) and a custom-built electrode interface module controlled by Lab-View 2 software (National Instruments) were used to obtain potentiometric response data, as described elsewhere.18 Hydrogen ion and surfactant EMF response measurements were made using a CHES/HEPES/citrate/NaOH buffer (5 mM 2-[N-cyclohexylamino]ethanesulfonic acid, 5 mM 4-[2-hydroxyethyl]-1-piperazineethanesulfonic acid, 5 mM citric acid, and 100 mM NaOH) as the sample solution. The pH of this universal buffer was adjusted by additions of small volumes of concentrated HCl. For pH calibrations, the membrane electrodes’ internal filling solution was 0.5 M HEPES buffer, pH 7.5, containing 0.1 M NaCl; for measurements of the electrodes’ dynamic response toward surfactants, the internal filling solution composition matched that of the sample solution: CHES/HEPES/citrate/NaOH buffer adjusted to pH 7.4. The pH of the test sample solutions was monitored with a calibrated glass electrode (Ingold). When surfactants were used, they were added as aliquots from 0.033 M stock solutions to achieve the desired concentration in the buffered sample. Calibration curves for pH electrodes in the presence of surfactants were obtained only after the EMF values had stabilized (approximating an equilibrium potential value at the membrane/sample interface) after the initial addition of surfactant to the solution at the highest test pH. Optical Measurements. Optical measurements of polymer membranes were used to confirm changes in the apparent pKa of aminated-PVC upon extraction of surfactants into the membranes. The following procedure was employed for such measurements: 200 µL of appropriate membrane cocktail solutions containing added lipophilic indicator dye (ETH 2439, ETH 5294, or ETH 5350 [see Figure 1 for structures], depending on the membrane type) were cast on glass plates so as to cover an area 0.8 cm wide and 1 cm long. After solvent evaporation, the plates were soaked in deionized water for 2 h and then transferred to a quartz cuvette containing the test solution, and the first absorption spectra were recorded immediately with a UV-visible spectrophotometer (Shimadzu UV160U). Subsequent spectra were recorded at different time intervals to follow changes in the ratio of protonated to deprotonated dye within the membranes as the test surfactant partitioned into the membrane phase. EXPERIMENTAL RESULTS The studies reported here were prompted by preliminary observations made when evaluating PIP-PVC membranes as a potential replacement for TDDA-based membranes for fabricating (18) Telting-Diaz, M.; Collison, M. E.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 576.
Figure 2. Dynamic response profiles of polymeric membrane electrodes upon addition (at time ) 0.2 h) of 5.5 × 10-5 M surfactant (Brij 35 or Triton X-100) in stirred CHES/HEPES/citrate/NaOH buffer (see Experimental Section), pH 7.4: (a) PIP-PVC membrane, (b) TDDA-based membrane, and (c) KTpClPB membrane (no hydrogen ion-selective carrier).
blood pH sensors. The use of amine sites covalently attached to PVC would prevent ionophore leaching and thus would be expected to increase the sensor stability and lifetime, especially within instruments with calibrant and wash solutions containing appreciable levels of surfactants. As previously reported,13-15 pH electrodes fabricated with PIP-PVC membranes function as excellent pH sensors in aqueous buffered solutions containing a physiological ionic background composition as well as in plasma samples; however, when evaluated in test solutions containing the surfactant Brij 35 or Triton X-100, large potential drifts were observed, and pH calibrations resulted in very low response slopes. Figure 2a illustrates the response profile over time of a pH electrode prepared with a PIP-PVC membrane after addition of Triton X-100 solution to the stirred buffer solution at pH 7.4. At the surfactant concentration present (final concentration ) 5.5 × 10-5 M), an immediate sharp change in the cell potential (>70 mV) is observed; after several minutes, the rate of potential change Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
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Figure 3. Typical hydrogen ion potentiometric response curves of polymer membrane pH electrodes based on (a) TDDA, (b) ETH 2439, (c) octadecyl isonicotinate, and (d) PIP-PVC in CHES/HEPES/citrate/NaOH buffer in the absence of surfactant (O) or after addition of either Brij 35 (0) or Triton X-100 (4) to a final concentration of 5.5 × 10-5 M.
becomes slower, approximating a steady state, and eventually the potential starts declining to reach a level comparable to the initial value (the time needed for this being dependent on the solution stirring rate and membrane thickness). A similar profile was observed when the surfactant added was Brij 35 at the same concentration. In this case, the maximum potential change was somewhat lower than that observed with Triton X-100 (see Figure 2a). Electrodes formulated with membranes doped with mobile TDDA and anionic sites in the form of KTpClPB, in contrast, exhibit only a very small shift in potential (a few millivolts) upon addition of the surfactants (Figure 2b). However, under the same test conditions, a similarly plasticized PVC membrane containing only lipophilic borate additives (KTpClPB) (no selective H+ carrier) produced potential shifts even larger than those observed for the electrodes prepared with the PIP-PVC membrane (Figure 2c). Based on these preliminary observations, more detailed studies were carried out to assess the effect of added surfactants on the EMF response function of pH electrodes over a wide range of pH values, using different amine structures (see Figure 1) as hydrogen ion carriers within the polymer membranes employed to construct functional electrodes. Figure 3a-c shows the effect of surfactant addition on the calibration curves of membranes based on the mobile ionophores TDDA, ETH 2439, and octadecyl isonicotinate, with corresponding pKa values of their conjugate acids (in water) of 10.6, 7.7, and 3.4, respectively (note: proportional pKa values in the organic membrane phase are expected to be a couple of units higher11). In each case, the presence of surfactant diminishes the dynamic pH measurement range by increasing the detection limit toward hydrogen ion activity. It is also evident that such a detection limit is shifted to lower pH values as the basicity of the hydrogen ion carrier decreases. 1626 Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
As shown in Figure 3d, the effect of nonionic surfactants on the hydrogen ion EMF response function of membrane electrodes formulated with PIP-PVC is somewhat different than that observed with the mobile hydrogen ion carriers (Figures 3a-c). In this case, not only is the pH measurement range shortened substantially, but there is a parallel shift in the EMF response over this shortened range of pH 5-8 (compared to response in the absence of nonionic surfactant). DISCUSSION AND MODEL Triton X-100 and Brij 35 are both nonionic surfactants with a poly(ethylene oxide) hydrophilic chain (with an average number of 10 ethylene oxide units in Triton X-100 and 23 in Brij 35; see Figure 1). It is well known19-21 that surfactants of this type are capable of binding metal ions in aqueous or organic phases by forming spiral-type structures with inwardly directed oxygen atoms that coordinate with the cations by ion-dipole interactions. In aqueous solution, such binding is considered to be weak and often is too small to be measured.22 Stability constants for the complexation of alkali and alkali-earth cations by polyether-based molecules have typically been measured in methanol. The interactions are stronger in this solvent because methanol is a much weaker solvation medium and thus competes less with the polyethers for the cations (enthalpic stabilization). The stability constants for complexes of this type in water are considered to reflect the trends observed in methanol, with a difference of 2-3 orders of magnitude between the two.22,23 Only a few values (19) Kikuchi, Y.; Takahashi, N.; Suzuki, T.; Sawada, K. Anal. Chim. Acta 1992, 256, 311. (20) Hamon, R. F.; Khan, A. S.; Chow, A. Talanta 1982, 29, 313. (21) Markuzina, N. N.; Mokron, S. B.; Stefanova, O. K.; Sementzov, S. N.; Volkov, Y. M.; Ranieva, E. A. Russ. J. Appl. Chem. 1993, 66, 1765. (22) Burger-Guerrisi, C.; Tondre, C. J. Colloid Interface Sci. 1987, 116, 100.
appear in the literature that report on the binding of cations with the specific surfactants used in the present study. The logarithm of the binding constant in methanol (at 25 °C) for the complexation of Triton X-100 with K+ was reported as 3.16.24 For Brij 35, the reported values (log Keq) are 2.07 for complexation with Na+, 2.50 with K+, and 2.48 with Ca2+.25 Therefore, at the low surfactant concentrations (in the sample phase) used in this study (5.5 × 10-5 M), there should not be any significant changes in the activities of hydrogen ions or background sodium ions in the aqueous phase due to complexation with the added surfactants. Thus, such sample phase complexation cannot account for the observed surfactant effects on the electrodes’ EMF behavior. The origin of the observed effects of such surfactants on the EMF response of polymeric pH electrodes can, however, be explained by the ability of the surfactants to partition into the membrane phase and behave as neutral carrier ionophores, thereby enhancing the extraction of metal cations from the sample phase into the membrane phase (a property that has been exploited, for example, in the development of barium-selective electrodes,26 which also display nonionic surfactant sensitivity6). An induced sensitivity toward the background cation activity in the electrolyte solution (0.1 M Na+ in experiments reported here) due to partitioning of Brij 35 or Triton X-100 into the organic membrane phase would explain the positive potential shifts initially observed (see Figure 2). That is, surfactant partitioning at the membrane/sample interface reduces the hydrogen ion selectivity of the membrane. As the neutral surfactant eventually diffuses across the membrane and reaches the internal membrane/ solution interface, a similar enhanced membrane extraction of sodium ions would take place, resulting in a gradual decrease in the net measured electrode potential (i.e., cation interference on both sides of the membrane becomes symmetrical; see Figure 2). The expected influence of nonionic surfactants on the response function of polymer membrane pH electrodes can be modeled by first examining the effect of surfactant partitioning on electrodes formulated with membranes containing only lipophilic cationexchange sites. Membranes containing ion-exchangers such as KTpClPB should respond to the metal ion activity in the aqueous sample phase with high selectivity over hydrogen ions. Provided that the composition of the internal filling solution of the membrane electrode is kept constant, the cation-exchanger electrode’s potentiometric response can be ascribed to changes in the phase boundary potential at the membrane/sample interface in accordance with the following expression:
aJ+ RT ln + E1 ) E° + F [J ]
(1)
where E1 is the electrode’s membrane/sample boundary potential in the absence of surfactants, aJ+ is the metal ion activity in the sample solution (sodium ions in studies reported here), and [J+] is the concentration of metal ion in the membrane phase boundary contacting the sample solution; R, T, and F have their usual meanings, and all other constant potential contributions are (23) Pedersen, C. J.; Frensdorff, H. K. Angew. Chem., Int. Ed. Engl. 1972, 1, 16. (24) Buschmann, H. J. Makromol. Chem. 1986, 187, 423. (25) Buschmann, H. J. Polyhedron 1985, 4, 2039. (26) Levins, R. J. Anal. Chem. 1971, 43, 1045.
included in the term E°. In this case, the concentration of J+ in the outer membrane phase boundary layer [J+] equals the concentration of anionic (borate) sites RT, so eq 1 can be rewritten as
E1 ) E° +
RT aJ+ ln F RT
(2)
However, when a nonionic surfactant S is added to the sample solution, it will rapidly partition into the organic membrane phase, where it can bind metal ions (for simplicity we assume a 1:1 stoichiometry, although nonionic surfactants with sufficiently long polyether chains could, in principle, bind more than one ion per surfactant molecule) according to the following equilibrium reaction: + J+ org + Sorg h SJorg
with the binding constant defined as
βSJ ) [SJ+]/[S][J+]
(3)
with all terms in brackets indicating concentrations in the organic phase (activity coefficients in the organic phase are considered to be constant for all ionic species; therefore, concentration values are used in place of activities for ions in the membrane phase). Under these conditions, electroneutrality in the membrane phase is given by
RT ) [SJ+] + [J+]
(4)
and the partitioning of the surfactant between the organic and aqueous phases is defined by the constant
P ) [S]/aS
(5)
where aS is the activity of the surfactant in the aqueous phase and [S] its concentration in the organic membrane phase. From eqs 3-5, it is found that
[J+] ) RT/(βSJPaS + 1)
(6)
By inserting eq 6 into eq 1, an expression for the new membrane/ sample boundary potential in the presence of nonionic surfactant can be found:
E2 ) E° +
RT aJ+(βSJPaS + 1) ln F RT
(7)
Subtraction of eq 7 from eq 2 gives
E2 - E1 )
RT ln (βSJPaS + 1) F
(8)
which describes the change in the phase boundary potential at the membrane/sample interface due to addition of surfactant to the aqueous sample solution. Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
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For experiments in which either Triton X-100 or Brij 35 is added to the sample solution, it is possible to obtain (from Figure 2c) values for E2 - E1, measured as the maximum observed shift in EMF value upon addition of the surfactant. An assumption is made that the observed E2 - E1 represents the maximum attainable value (i.e., true equilibrium extraction of the surfactant is achieved at the membrane/sample interface before the surfactant is able to reach the internal solution/membrane interface and cause a decrease in the net membrane potential). By inserting each of the maximum E2 - E1 values into eq 8, together with the corresponding surfactant concentration, an estimate of βSJP for each surfactant can be made that allows a direct comparison of a given surfactant’s ability to be extracted into the membrane phase and concomitantly bind cations, thereby inducing an initial positive EMF shift. Our experimental results indicate that the average βSJP value (with J+ ) Na+) is at least 15 times greater for Triton X-100 than that for Brij 35 (see Figure 2c). For membranes also containing a neutral pH carrier (either mobile or covalently fixed), the uncomplexed nonionic surfactant can also partition into the organic phase, but the formation of a surfactant-cation complex in the membrane is restricted by the ability of the pH ionophore to remain protonated since electroneutrality conditions in the polymeric film must be maintained (i.e., extraction of an interfering cation J+ must be counterbalanced by the release of a hydrogen ion into the sample solution). Thus, any appreciable changes in EMF due to surfactant-mediated cation extraction would only be evident in the low hydrogen ion activity range (high pH), where the interfering cation activity in the background electrolyte solution can effectively compete for sites in the membrane by exchanging with hydrogen ions in the boundary layer. To explain this more explicitly, a model developed previously for the description of the detection limits of neutral carrier-based pH electrodes27 can be adapted by incorporating the additional surfactant-dependent equilibria. The assumptions required are those made for the original model, and although all may not be valid for the current system (e.g., neglecting any possible ion pair formation within the membrane phase), the general conclusions allow useful comparisons to be made. For test solutions with low hydrogen ion activity, the partitioning of interfering cations J+ into the membrane phase will be facilitated by the presence of surfactant molecules S according to the following ion-exchange reaction: + + + J+ aq + LHorg + Sorg h SJorg + Lorg + Haq
[SJ+][L]aH+ [S][LH+]aJ+
) KaβSJKJH
(9)
where L and LH+ are the free and protonated ionophore, respectively, and βSJ is the surfactant-cation binding constant defined previously by eq 3. The acidity constant of the ionophore in the membrane phase may be expressed as (27) Bakker, E.; Xu, A.; Pretsch, E. Anal. Chim. Acta 1994, 295, 253.
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(10)
and the ion-exchange constant for the free interfering cation between the sample and the membrane phase (dependent on the relative lipophilicity of the cations) is given by
KJH ) aH+[J+]/[H+]aJ+
(11)
Given that the inner filling solution of the membrane electrode is kept constant (in terms of hydrogen ion and metal cation activities), the response of the neutral carrier pH membrane to hydrogen ion activity changes in the sample solution can be described by the expression for the phase boundary potential at the membrane/sample interface:
EH ) E° +
aH+ RT ln + F [H ]
) E° -
RT RT aH+[L] ln Ka + ln F F [LH+]
) E°′ +
RT aH+[L] ln F [LH+]
(12)
with analogous definitions for the terms as in the ion-exchanger only case described above (eq 1). By inserting the ion-exchange constant (Kexch from eq 9) and the surfactant partitioning constant (P from eq 5) into eq 12, an expression can be derived for the membrane’s EMF response as a function of the interfering ion activity, aJ+, in a sample containing nonionic surfactant:
[
]
aJ+aS RT ln KexchP + F [SJ ]
EJ ) E°′ +
(13)
Over the pH range where the electrode responds ideally to hydrogen ions (no anion or cation interference), electroneutrality in the membrane phase is given by [LH+] ) RT, and thus [L] ) LT - RT, where LT is the total concentration of ionophore; therefore, by inserting this information into eq 12, the ideal electrode response toward hydrogen ions is described by
EH ) E°′ +
with the corresponding ion-exchange constant,
Kexch )
Ka ) [L][H+]/[LH+]
[
]
aH+(LT - RT) RT ln F RT
(14)
On the other hand, in sufficiently basic samples, where the interfering metal cations completely exchanges with hydrogen ions, the electrode response will be a function of aJ+ alone, and thus by substituting [SJ+] ) RT in eq 13, it can be found for this region that
EJ ) E°′ +
[
]
aJ+aS RT ln KexchP F RT
(15)
The lower detection limit for ions is defined by the IUPAC as the ion activity corresponding to the intersection of the two extrapolated segments of the calibration curve, one segment being the
ideal pH electrode potentiometric response and the other the ideal Nernstian electrode response to the interfering cation activity. This point is found at the hydrogen ion activity for which EH ) EJ; hence, by combining eqs 14 and 15, the following expression for the cation interference-induced detection limit (aH+(DLcif)) in samples containing nonionic surfactants is found as
aJ+aS aJ+aS aH+(DLcif) ) KexchP ) KaβSJKJHP LT - RT LT - RT
(16)
Equation 16 shows that the greater the surfactant’s ability to partition into the membrane phase, the stronger its complexation of interferent metal ions, and the higher its concentration in the sample solution, the greater the change (increase) in detection limit for hydrogen ions induced by the presence of the surfactant in the test solution. Further, for those membranes doped with pH ionophores that are weaker bases (higher Ka of conjugate acids), this shortening of their useful pH measurement range can limit dramatically their applicability for pH measurements (see Figure 3a-c). Comparison of eq 16 with that obtained for the cation interference-induced detection limit of pH electrodes in surfactant-free samples,27
aJ+ aH+(DLcif) ) KaKJH LT - RT
(17)
shows that they differ by the factor βSJPaS. Thus, given a known concentration of a particular surfactant, an estimate of its corresponding βSJP could be obtained by comparing the electrode detection limits before and after addition of surfactant, provided that the shapes of the experimental curves allow an adequate determination of such detection limits. In a similar fashion, based on eq 16 alone, the relative values of βSJP for Triton X-100 and Brij 35 can be estimated by comparing the experimental detection limits obtained after addition of surfactant to the test solution. Treating the data shown in Figure 3b in this way (for which the two segments of each of the calibration curves obtained after surfactant addition are clearly defined), we again find that βSJP for Triton X-100 is ∼15 times greater than that for Brij 35. The effect of surfactant addition on the EMF response of an aminated-PVC-based membrane requires further analysis because, as Figure 3d illustrates, not only is there a shift in the lower detection limit toward hydrogen ions induced by surfactantmediated cation response, but there is also a parallel upward shift in EMF values for the ideal hydrogen ion response segment of the curve. A change in the E°′ value for the electrode response function (eq 12) would cause just such a parallel displacement in the EMF response curve. Such a change in E°′ could, in fact, result from a decrease in the pH carrier’s apparent Ka value (see eq 12) when surfactants are present in the test sample. It is well known that poly(ethylene oxide) derivatives can form complexes with polymeric hydrogen donors that are stabilized by hydrophobic interactions.28,29 It is therefore possible that the (28) Scranton, A. B.; Klier, J.; Aronson, C. L. In Polyelectrolyte gels: properties, preparation, and applications; Harland, R. S., Prud’homme, R. K., Eds.; ACS Symposium Series 480; American Chemical Society: Washington, DC, 1992; pp 171-189. (29) Tsuchida, E.; Takeoka, S. In Macromolecular complexes in Chemistry and Biology; Dubin, P., Bock, J., Davis, R., Schulz, D. N., Thies, C., Eds.; Springer: New York, 1994; pp 183-213.
surfactant molecules are capable of associating with the aminatedPVC chains through hydrogen bonding between the protonated amine sites and the repeating ethylene oxide units (see Figure 4). This multidentate binding could stabilize the protonated amine and thus effectively increase the amine’s apparent basicity by shifting the protonation equilibrium toward the associated form. The cooperativity effect of interpolymer complexation is a key factor that allows for stable complexes to be formed even if the individual interaction energy for each segment of the polymer is relatively small. This appears to be the reason why this basicity shift occurs in the case of membranes formulated with aminatedPVC but not in those membranes containing mobile hydrogen ion carriers. It can be shown that the value of the new apparent basicity constant will depend on the binding constant for the surfactant-protonated amine complex and on the surfactant concentration; however, an explicit expression is not presented here because the stoichiometry of the interpolymer complexation is not known. The magnitude of the EMF shifts obtained from the results shown in Figure 3d (measured at pH values between 5 and 6 in order to separate the effect the surfactant has on the amine site basicity from its effect on interfering cation extraction at higher pH values) allows us to estimate, using eq 12, that under our experimental conditions, Brij 35 and Triton X-100 induced an increase of nearly 1 pKa unit in the apparent basicity of the PIPPVC membrane amine sites. To obtain further evidence that Brij 35 and Triton X-100 surfactants are, indeed, capable of altering the apparent pKa value of the PIP-PVC amine sites in the membrane phase, optical measurements of the hydrogen ion activity in membranes were made in the absence and presence of the nonionic surfactants. This was accomplished by adding small amounts of previously reported lipophilic pH chromoionophores to the PIP-PVC membrane formulation11 (see Experimental Section). Measurements were carried out in distilled deionized water (no ionic background) with or without added surfactant to minimize any effects due to exchange of cations from the sample solution with membrane hydrogen ions. A PIP-PVC membrane doped with the chromoionophore ETH 5294 soaked in water until equilibration yielded an absorption spectrum corresponding to the fully protonated form of the dye, as indicated by two peaks at ∼610 and 660 nm. When this membrane film was transferred to a 0.004 M solution of Brij 35 in distilled water, a gradual drop in the absorbance at these wavelengths was evident immediately, together with the emergence of a broad peak centered around 536 nm. This new band corresponds to the deprotonated form of the chromoionophore. After about 20 min, the bands at 610 and 660 nm had almost disappeared entirely, and the spectrum was similar to that of the fully deprotonated dye, suggesting that, indeed, the pH in the membrane phase increased under the influence of the surfactant. Similar experiments performed with a TDDA-based membrane doped with the more basic pH chromoionophore ETH 5350 (necessary since ETH 5294 is deprotonated by TDDA) did not result in any detectable changes in membrane pH, even after several hours of soaking in Brij 35 solution (protonated form of ETH 5350, with absorption peak at ∼650 nm, did not change to give deprotonated form, with absorption peak at ∼500 nm). No changes in pH were detected either for a membrane containing ETH 2439, a pH chromoionophore itself (with the absorption peak for its protonated form at 660 nm, and that for its deprotonated form at about 520 nm). When a 0.004 M solution of Triton X-100 Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
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Figure 4. Schematic representation of the aminated-PVC membrane/solution interface showing partitioning of a poly(ethylene oxide)-based surfactant into the organic phase (A, hydrophobic portion of the surfactant molecule). Proposed hydrogen bonding between surfactant and protonated amine sites leads to a displacement of the amine protonation equilibrium toward the protonated form, resulting in a pH increase in the membrane phase (lower proton concentration). The dopant pH chromoionophore is present at much lower concentration relative to the PIP-PVC amine sites so as to function as a pH indicator. The change in the membrane’s pH is evidenced by the change in the chromoionophore’s optical spectrum as its protonation equilibrium shifts from the protonated (CH+) to the free amine form (C). R-, lipophilic borate sites, present in concentration equal to that of total positive charges in the membrane phase.
was used instead of Brij 35 in these experiments, the PIP-PVC/ ETH 5294 membrane responded in a manner similar to that observed with Brij 35; however, the membranes containing TDDA/ETH 5350 or ETH 2439 alone did exhibit slow optical changes from the fully protonated to the deprotonated form of the respective pH indicators. A possible explanation for this might be the apparently greater cation-extraction properties of Triton X-100 compared to Brij 35 (i.e., βSJP values, see eqs 8 and 16 above). Indeed, the cationic impurities in the test solution (ICP atomic emission analysis indicated that, at the concentration of Triton X-100 used, a Na+ background of about 3 × 10-5 M is present) appear to be sufficiently high for surfactant-mediated ion exchange to occur with the membrane hydrogen ions, resulting in a pH increase in the membrane phase (the absorbances of TDDA/ETH 5350 and ETH 2439 membranes immersed in a Brij 35 solution with a similar sodium ion background remained unchanged). In fact, when the experiments were repeated using a 0.004 M solution of Triton X-100 buffered at pH 3 (to prevent extraction of interfering cations), the spectrum for the PIP-PVC/ ETH 5294 membrane again shifted from the protonated to the deprotonated form of the dye, whereas no changes were observed for membranes containing TDDA/ETH 5350 or ETH 2439. These results strongly support the notion that the apparent pKa values of amine sites within the PIP-PVC polymer structure are altered by partitioning of nonionic surfactants into the membrane phase. Although not examined experimentally in the present study, another type of pH-selective membrane system that may be considered in relation to its response to poly(ethylene oxide)-based nonionic surfactants is that in which an aminated-PVC matrix is 1630 Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
compounded with a mobile pH ionophore of high basicity (e.g., ETH 5294, TDDA, or ETH 5350). Such membrane compositions have been shown by Buck and Cosofret and co-workers to combine good adhesion characteristics to certain solid substrates with improved potentiometric pH response properties.30,31 For such membranes, provided that there is an excess of free pH ionophore relative to the total concentration of anionic sites, protonation will occur preferentially on the free amine sites owing to their higher basicity. Thus, assuming that the protonated ionophore maintains its mobility in the aminated-PVC matrix, one would expect the response of this type of system to added surfactants to be largely akin to that of membranes containing exclusively free ionophores (i.e., higher detection limits due to enhanced extraction of interfering metal ions but no parallel shift in EMF response function). This would need, however, to be experimentally verified for a given membrane, since multidentate interactions or other effects could occur, depending on the relative proportions of fixed amine, free amine, and anionic sites added, as well as on the physicochemical properties of the membrane. In summary, it has been clearly shown that poly(ethylene oxide)-based nonionic surfactants can interfere with the EMF response of polymer membrane pH electrodes. This interference originates from the ability of such surfactants to partition into the membrane phase and enhance the extraction of interfering metal (30) Cosofret, V. V.; Erdo¨sy, M.; Johnson, T. A.; Buck, R. P.; Ash, R. B.; Neuman, M. R. Anal. Chem. 1995, 67, 1647. (31) Cosofret, V. V.; Lindner, E.; Johnson, T. A.; Neuman, M. R. Talanta 1994, 41, 931.
cations from the sample solution. The resulting decrease in potentiometric selectivity over background cations leads to a shortening of the pH measurement range, the extent of which is dependent on the basicity of the hydrogen ion carrier used to fabricate the pH electrode, as well as the surfactant type and its concentration. In the case of aminated-PVC membranes, a more complex mode of interference has been identified on the basis of the ability of the surfactant molecules to change the apparent basicity of the immobilized amine groups through interpolymer complexation. These possible forms of interference should be taken into consideration when choosing the composition of polymer membrane pH electrodes for applications that involve the presence of surfactants. Further, it is likely that similar interference from nonionic surfactants will occur with polymer
membrane-type ISEs designed to detect other cation species, provided that surfactant-mediated interferent cation extraction competes favorably with the intrinsic extraction of both primary analyte cations and interferent background cations by the specific ionophore employed within the membrane phase of such sensors. ACKNOWLEDGMENT This work was supported in part by a grant from the National Institutes of Health (GM-28882). Received for review October 9, 1995. Accepted February 18, 1996.X AC951017G X
Abstract published in Advance ACS Abstracts, April 1, 1996.
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