Anal. Chem. 1991, 63,596-603
596
Lifetime of Neutral-Carrier-Based Liquid Membranes in Aqueous Samples and Blood and the Lipophilicity of Membrane Components Oliver Dinten, Ursula E. Spichiger, Nicolas Chaniotakis, Peter Gehrig, Bruno Rusterholz, Werner E. Morf, and Wilhelm Simon* Deoartment of Organic Chemistrv. Swiss Federal Institute of Technology (ETH), Universitatstrasse 16, CH-8092 Zuri’ch, Switzerland “ I
On the bask of prevlously reported correlations between the llpophlllclty of membrane components, their partition coefficlent between the membrane and the sample, and the lletbne of corresponding neutral-carrier-based sensors, the lipophllicltles of Ionophores and plastlcizers In analytically relevant ion-selective electrodes, ISFETs, and optodes are analyzed and reported. Equations for the estlmatlon of the ilfetlmes of llquld membranes in contlnuous-flow systems are presented, and the experlmental determlnation of the iipophlllclty values by thln-layer chromatography (TLC) Is described. The required HpopMHctties for the ilfetlmes of llquld membranes over 30 24-h days for dlfferent applications in aqueous solutions as well as In blood are presented. A comparison of the experimental results of lifetime measurements with calculated theoretical values Is given. The experlmental results of the determination of the llpophillclty by TLC are compared with the lipophilicities estimated on the bask of Hansch parameters.
INTRODUCTION Ion-selective liquid-membrane electrodes based on neutral carriers are widely used as integrated devices in clinical chemical analyzers (I). With their availability, a selective determination of different ions in diluted samples as well as in whole blood has become possible. A continuous loss of membrane components to the sample due to an increasing period of exposure finally results in a breakdown of the ionmeasuring capability of the system (2). Therefore, the lifetime of an ion-selective device may be estimated and predicted on the basis of diffusion laws, making use of parameters concerned with the membrane geometry and the characteristics of the membrane components, such as their partition coefficient at the membrane/solution interface. The operational parameters to reach a minimal required lifetime in the worst case for a detector introduced in a continuous-flow analyzer may formally be calculated from equations developed by Dinten and Oesch et al. (2-5). Here we report on the analysis of the lipophilicities that are required for ionophores and plasticizers in analytically relevant ion sensors. Experimentally determined lifetimes are compared with estimated (6) and measured lipophilicities for realistic applications. THEORETICAL SECTION Definition and Theoretical Treatment of the Lifetime of Ion-Selective Liquid Membranes. The lifetime of an ion-selective measuring system may be defined as the time interval between the conditioning of the membrane and the moment when at least one parameter of the functionality characteristics of the device changes detrimentally. For neutral-carrier-based membrane sensors, the lifetime is mainly limited to the following two processes ( 2 ) :
(a) The first process is the leaching out of carrier molecules from the membrane phase to the sample as an effect of a low partition coefficient at the sample/membrane interface, e.g., of an insufficient lipophilicity of the neutral ionophore. Consequently, below a critical carrier concentration, cli,, in the membrane, a breakdown of the selectivity is observed (2, 4).
(b) The second process is the extraction of the plasticizer by the sample and/or the degradation of the mobile ionic site additives of the membrane phase which may catalyze the ion transfer (7) or limit the concentration of charged counterions in optode membranes. As a result, the impedance of membranes in the potentiometric devices and, accordingly, the response time of the sensor increase (7, 8). In the extreme case where the membrane phase has become highly viscous, a complete loss of ion sensitivity may occur (5, 8) (see also refs 9 and 10). For ion-sensitive optode membranes, the average concentration rather than the boundary concentration (e.g., ionophores in ion-selective electrodes (ISEs) (2)) of the relevant components, especially of indicator dyes, in the bulk membrane is decisive (11). General equations for an estimation of the lifetime, as limited by the leaching out of membrane components (processes a and b), are based on Fick’s diffusion laws (4,12). The extended description can be simplified by making use of additional assumptions concerning a continuous-flow system. Firstly, the total effective volume V,,, of the continuously renewed aqueous phase in contact with the membrane can be set equal to infinite. Accordingly, no equilibration between the two bulk phases has to be considered, and the carrier concentration (respectively, plasticizer concentration) in the bulk of the aqueous solution can be neglected. The assumption of a distribution equilibrium of membrane components is nevertheless valid for the two boundary regions at the membrane/solution interface. Secondly, the concentration gradients within the aqueous phase are assumed to be restricted to a steady-state diffusion layer (Nernstian layer) of given average thickness 8. In this case, the most relevant parameter controlling the loss of a membrane component from the membrane is the ratio, L , of the respective permeabilities of the aqueous diffusion layer and the membrane (4): d L = -D,K a
(1)
where D,, is the diffusion coefficient of the component in the aqueous phase, D, is the diffusion coefficient of the component in the membrane phase, K is the partition/distribution coefficient of the component between the aqueous solution and the membrane, d is the thickness of the Nernstian aqueous diffusion layer, and d is the thickness of the membrane. Estimations for ion-selective flow-through electrodes (3) using the representative mean value of D,, = 3.1 X lo4 cm2
0003-2700/91/0363-0596$02.50/00 199 1 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63,
s-l (range of D,,: (2-5) X lo4 cm2 s-l) (2,4), D, = lo-' cm2 s-l, d = 0.024 cm, and a = 0.003 cm led to the following approximate relationship between the permeability ratio and the distribution coefficient of a component:
L = 1031~
(2)
Since membrane components of analytical relevance must have distribution coefficients (respectively, lipophilicities) of K >> IO5 (2), it definitely holds that
L 100 pm, the membrane on a gate in an ion-selective field effect transistor (ISFET) with d I 10 pm, and an ion-selective optode membrane with d 10 pm will result (11). The average residual concentration of a membrane component, c, is given by ( 4 ) c = Coe-D,t/Kda
(4)
Based on this equation, as well as on the knowledge of the limiting concentrations of the membrane components, which guarantee a normal functionality of the sensor, the maximal lifetime, tlim,for various neutral-carrier-based liquid membranes and different configurations of corresponding ionmeasuring systems can be estimated (4).
(5) where clm is the critical limiting concentration of a component in the membrane phase or at the surface of membranes of ISEs or ISFETs. For optodes, c0/climis replaced by the tolerated change in absorbance of 1%and lo%, Ao/Ali, = 1.01 and 1.1, respectively. According to eqs 4 and 5 and the assumptions made, the diffusion rate of components within the membrane and the kinetics of the complexation/decomplexation processes at the interface are obviously not limiting the leaching rate and the lifetime of the sensor, respectively. The thickness of the membrane, d , is preferably replaced by deff,the ratio of the volume, V, of the membrane and the contacted surface area, A, if a window of the bulk membrane surface only is exposed to the sample solution:
Evidently, the lifetime of a membrane-based sensor may be considerably increased if only a portion of the surface is in contact with the sample. Quantitative descriptions of the effective thickness, a, of the Nernstian diffusion layer can be obtained from the fundamental theoretical treatment of hydrodynamics by Levich (13) or from expressions derived for limiting currents in flow-through electrode systems. For example, the following result is valid for a laminar flow through a tubular membrane system of length L and radius R ( 4 ) as it is used in clinical analyzers:
a = ~,,~/3~1/3~1/3~,~-1/3
NO. 6, MARCH 15, 1991 597
values were calculated earlier by Oesch et al. (2) according to eq 5 for the case of ion-selective membrane electrodes. It is also applicable to ISFETs (10, 14), and the same relationship holds for ion-sensitive optode membranes based on ionophores (11).
The minimal lipophilicities required for membrane components to obtain sensors of a specified lifetime were calculated from eq 5 for a lifetime of 30 24-h days; the results are summarized in Table I. The calculations were made for different types of sensors and typical applications, for different membrane components, as well as for different values of the membrane thickness. The Partition Coefficient and the Lipophilicity. To get a representative, standardized quantity that describes the partitioning of a membrane component (ionophore, plasticizer) between an aqueous and an organic liquid phase, the extraction behavior of the two-phase system chosen should be very closely related to that of an ion-selective membrane in contact with the analyzed sample. Hansch et al. (6) used 1-octanol as an apolar model phase comparable to natural lipid membranes, and they suggested introducing the logarithm of the corresponding distribution coefficient (log P) of a compound in the two-phase system 1-octanol/water as a measure of lipophilicity. From extraction experiments, lipophilicities of log P 5 8 can be directly determined with good reliability (6). For the components of liquid membranes studied here, however, the range of log P L. 8 is of special interest (2,4,11). Reversed-phase thin-layer chromatography (TLC) using ethanol/water as the mobile phase is an established technique for the assessment of lipophilicities (log PTLC) over a large range of values (see below). The experimentally determined lipophilicity (log P) of a membrane component is a suitable estimate of the distribution coefficient, K (respectively, of log K ) , of this compound between the membrane and an aqueous solution or serum. It allows for calculations of the time-dependent loss of membrane components and is required for estimating the lifetime of a membrane (see eq 5 ) . With increasing lipophilicity of the membrane components, their loss due to leaching out is obviously reduced, and the life-time of the membrane may finally be increased by several orders of magnitude. Nevertheless, the favorable increase of lipophilicity, is limited to a certain degree (15). When the lipophilicity of an ionophore drastically exceeds an upper value 20, the free diffusion of this species within the of log P membrane is usually hindered and the interfacial reaction rates may also be affected by the slow mobility (8, 15). Correspondingly, there probably exists an optimum lipophilicity. The specification of the lipophilicities of membrane components is obviously essential for predicting the lifetime of a particular membrane. Determination of the Lipophilicity by Reversed-Phase Thin-Layer Chromatography. Reversed-phase thin-layer chromatography has been suggested as a suitable method for the experimental determination of lipophilicity values (log PTX)(16,17). With this technique, lipophilicities in the range log P 2 8 can be obtained by comparison with extrapolated values for the 1-octanol/water (log Po,,) extraction system. In thin-layer chromatography, the retention for a component is characterized by the R, value, which is defined as the ratio of the distance the component has moved to the distance the solvent front has moved. The corresponding results determined with reversed-phase chromatography may be transformed into values log PTLc by the following relationship:
-
(7)
V,, [cm s-l] is the maximum linear flow velocity. Lifetime
where V , is the volume of the mobile phase and Vsis a volume
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
Table I. Required Lipophilicity (PTLC) of Plasticizers and Neutral Carriers for a Lifetime (tlh) of 30 days (24 h a Day) Analyzing Serum and Aqueous Solutions in a Flow-Through Systemn
membrane component neutral carrier
mode of applcn
tlim, S
2.59
X
direct measurement in blood, serum, plasma ISE ISFET OPTODE 2.59
X
c d Climd
4.8 (3.6-6.3) 6.5 (5.1-8.3) 10.4 (8.6-12.8) 9.1 (7.5-11.3)
0.024 0.001 0.0005 0.0005
103 ( 2 , 4 , 9 ) 103 (9, io) 1.01 1.11
11.3 (10.7-11.9) 15.5 (14.9-16.1) 25.0 (24.4-25.6) 21.9 (21.3-22.5)
0.024 0.001 0.0005 0.0005
103 ( 2 , 4 , 9 ) 103 (9, io) 1.01 1.11
7.2 (5.7-9.1) 6.0 (4.8-7.8) 5.9 (4.7-7.7) 7.4 (5.9-9.3) 4.3 (1.9-8.9)
0.024 0.024 0.024 0.001 0.001
1.1 2.0 (9, 10) 3.57 3.57 (IO) 2.0
17.1 (16.6-17.9) 14.3 (14.0-15.2) 10.2 (6.1-17.5) 17.7 (17.1-18.3)
0.024 0.024 0.001 0.001
1.1 (9, 10) 2.0 2.0 (10) 3.57
aq soluns
lo6
ISE ISFET OPTODE
plasticizer
d,’ cm
required log P m b
aq soluns ISE
lo6
ISFET direct measurement in blood, serum, plasma
ISE ISFET
OConditions for the flow-through system: tube radius R = 0.05 cm, length of the electrode surface L = 0.1 cm, flow U‘= 18 cm3h-l, flow velocity V,, = 1.27 cm s-l, diffusion coefficient in the aqueous phase D,, = 3.1, (2-5) X lo4 cm2 s-l, Nernstian layer a = 0.003 cm. bMean value and range of extreme values mainly due to uncertainities in D,, and in log K when applying eqs 5 and 14. cThicknessof the membrane according to refs 1 and 10. dThreshold ratio of the concentrations of ligands and plasticizers at the surface of the liquid membrane applied in electrodes and ISFETs. For optodes, threshold ratio of concentrations of the chromoionophore or indicator within the membrane, resulting in a bias of the absolute value of the absorbance of 1% and lo%, respectively. The absolute value of this bias depends on the molar decadic coefficient of absorption and differs with different indicators. representative of the stationary phase. Bathe-Smith and Westall (16) defined R, as
R, = log and hence log
PTLC
= R,
(k
+ log r,
TO WASTE
t
CELL DIMENSIONS
- 1)
-TO
with
r = V,/V,
.TECTOR
(10)
R, corresponds to the logarithm of the capacity factor k ’, In a calibration procedure, R, values for calibration standards are calculated from the experimental R, data by eq 9 and are correlated by linear regression with the assigned values of log
P& From eq 10, it becomes evident that log r is the intercept. For realistic membrane components, the R, values are experimentally determined, and the R, values are calculated by using eq 9. The corresponding lipophilicities, log P m C , are predicted by using the linear regression equation evaluated by the calibration standards. By this procedure, the resulting lipophilicity, log PTLc,of neutral carriers and plasticizers can be well identified with the basic quantities log Pmt(18). The correlation between the distribution coefficient of membrane components and their lipophilicity is given by the following regression equations ( 2 ) . It was considered to be independent of the membrane composition: For aqueous solutions (2) log K = (0.4 f 0.4) (0.8 f 0.1) log Poct (11)
+
For blood and serum (DOS/PVC membranes) (3, 1, 9) log K = 0.48 0.33 log P T L c (12)
+
For diluted urine (o-NPOE/PVC membranes) (3, 9) log K = 1.42 0.80 log P T L C (13)
+
where DOS and o-NPOE are acronyms for the plasticizers dioctyl sebacate and o-nitrophenyl octyl ether, respectively. Obviously diluted urine as sample can be treated similarly to aqueous solutions. For undiluted urine, this is only true in
t SAMPLE
Figure 1. Geomeby of the flow-through cell for the lifetime experiments by optical detection in the transmission mode. Length of the flowthrough cell, L = 1.20 cm; width of the cell, W = 0.40 cm; optical path length, D = 0.70 cm.
cases where the urine has no lipophilic character. Design of Lifetime Experiments on Ion-Selective Liquid Membranes. The flow-through cells designed for the lifetime experiments reported below have to fulfill different requirements. First, they must be applicable as absorption cells in the transmission mode since optode membranes based on so-called chromoionophores (19-21) (e.g., carriers that exhibit a pronounced absorption change in the visible range upon ion complexation) were selected for these investigations. Second, the geometry should be as close as possible to potentiometric flow-through cells currently in use, e.g., in clinical chemistry. A laminar flow of sample solution in the cell is a further requirement. For the finally designed nontubular flow-through system (see Figure 1)eq 7 becomes
For the experimental geometry of the planar cell, the thick-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
ness, 8, of the Nernstian diffusion layer can be calculated from eq 14, where the kinematic viscosity u = 0.0090 cm2 s-l (22) for a diluted aqueous solution and u = 0.0159 cm2 s-l for a serum sample (23) at 25 "C,respectively. The length of the surface area of the liquid membrane is L = 1.20 cm, and the maximum linear flow velocity of the sample solution a t a flow rate of 0.3 mL min-' is V,, = 0.036 cm s-l as calculated by
V,, = 2U'/ WD
(15)
The width, W, of the cell is 0.40 cm, and the optical path length D = 0.70 cm (Figure 1). The thickness of the Nernstian boundary layer for this very slow flow rate is 8 = 0.0627 cm. An optode membrane (19-21) was selected as most suitable for the planned lifetime studies (see Experimental Section). The gradual loss of a chromoionophore from an optical sensor membrane may easily be monitored by measuring the decrease in absorbance. At the same time, opacity of the membrane originating from interference by proteins may be detected and corrected for by using a blank membrane (same membrane but without chromoionophore) in a reference cell. In order to increase the resulting optical signals and to lower the uncertainty of the measurements, identical membranes can be used on both windows of each cell. The time-dependent change in the corrected absorbance value, A , is therefore related to the decrease in the carrier concentration, c, as
A = 2dtc
(16)
respectively, in view of eq 5
A,/AIim = Co/Clim
(17)
where A , and Alimare the initial and the limiting absorbance values, c is the molar linear decadic absorption coefficient of the chromoionophore in the organic phase, and d is the thickness of each membrane. EXPERIMENTAL SECTION Thin-Layer Chromatography. Standards and Reagents. The calibration standards are given by Ellgehausen (24): pesticides Cl-C8 (Ciba-Geigy AG, Basel, Switzerland); C1 (CGA 10832) log P T L c = 6.34; C2 (GS 19851) log P T L C = 5.10; C3 (G 23992) log P T L c = 4.73; C4 (GS 14260) log P T L C = 3.70; C5 (GS 13529). log P T L C = 3.46; C6 (Diuron) log P T L c = 2.89; C7 (GS 29696) log PTLC= 1.82; C8 (Fenuron) log PTLc= 0.88. Ten milligrams of the compounds was dissolved in 1 mL of methylene chloride (puriss p.a.), Fluka AG, CH-9470 Buchs, Switzerland. Reference Compounds. The reference compounds were ETH 1859, ETH 1778, and ETH 2120 (4,21) (see also Table 11). Ten milligrams each was dissolved in 1 g of methanol (puriss p.a.) (Fluka AG). Mobile Phase. The solvent mixture was 90 mL of ethanol (puriss p.a.) (Fluka AG) and 10 mL of distilled water. Stationary Phase. The stationary phase was octadecylsilane-bonded reversed-phase silica plates (KC 18 F, Whatman, Clifton, NY), 200-wm layer (180-220 pm), 8.0-nm particle size silica gel, cut to 11 X 20 cm. Neutral Carriers and Plasticizers (Compare Table IO. Solutions of the following compounds were investigated in concentrations of 10 mg/mL of ethanol (puriss p.a.): ETH 2220, ETH 5214, TETDS, TTX, B15 C5, DT16 C5, (TBT)Cl, NaTFPB, chloroparaffin (60), and Mesamoll I H81 (Bayer AG, D-5090 Leverkusen, Germany). Three of the eight calibration standards with different lipophilicities were used in the same solution. Three different calibrating solutions were prepared. Of these solutions, l, 2, and 4 fiL were applied on a length of 8 mm by a Linomat I11 (Camag, CH-4132 Muttenz, Switzerland). Of the solutions of the reference compounds, neutral carriers,and plasticizers, 8-12 pL were applied in the same way. The spots of the compounds were detected by ultraviolet spectroscopy (UV) at 254 nm and developed in the iodine vapor or with a spray of concentrated sulfuric acid. Lifetime Experiments. Bovine calf serum, y irradiated (Sigma Chemical Co., St. Louis, MO, Catalog No. C5155, Lot No.
599
87F-0562), kept in a deep freezer (below -20 "C) and pH 7.40 phosphate buffer as well as pH 3.5 acetate buffer were used. Chromoionophores (See also Table I11 (38-40)). The chromoionophores were calibration standard C1 (log PTu: = 6.34), , ,A = 405 nm; ETH 5315 (4-(octadecylamino)azobenzene) (39) (log P T L c = 15, pK, = 3.5),, , ,A protonated = 515 nm, deprotonated = 405 nm; and ETH 2411 (2-amino-4-(octadecanoylamin0)azobenzene) (38) (log P T L C = 11, pK, = 5.75),, ,A protonated = 520 nm, deprotonated = 420 nm. Optode Membrane. The optode membrane was made up of poly(viny1 chloride) (PVC) (high molecular) 30 wt YO, DOS (bis(2-ethylhexyl) sebacate) 60 wt %; KTpClPB (potassium tetrakis(4-chloropheny1)borate)100 mol YO (relative to ionophore), chromoionophore ca. 5 wt YO, and 1.5 mL of tetrahydrofuran (THF) (Fluka AG, CH-9470 Buchs, Switzerland). For the preparation of optode membranes, see ref 40. Instruments. For lifetime experiments and the optode evaluation, we used a Lambda 2 UV/vis spectrophotometer (Bodenseewerke Perkin-Elmer GmbH, D-7770 Ueberlingen, Germany) and a Vario Perpex I1 calibrated flow-through pump (Werner Meyer AG, CH-6000 Luzern, Switzerland). Statistical parameters were estimated with the support of a STATVIEW 11 program (Brainpower Inc., Calabasas, CA), running on a Macintosh I1 CX (Apple Computer, Inc., Cupertino, CA). The decrease in absorbance was measured as follows: The flowing sample solution was first passed through the reference cell with the blank membrane (without chromoionophore) and then through the measuring cell containing the same liquid membrane, however, with the chromoionophore added. Prior to the absorbance measurements (after every 30-60 min depending on the rate of loss of chromoionophore), the cells were flushed and filled with 10 mL of distilled water. The absorption spectra of the reference and the measuring cell were scanned over a wavelength range of 300-700 nm. An increasing opacity due to protein deposition observed for the reference cell and the resulting contribution to absorbance were subtracted from the absorbance obtained from the cell with chromoionophores. Since the lipophilicity of a membrane component is decisive for the lifetime experiments, three types of membranes containing chromoionophores of different lipophilicities were used. To study the influence of the type of sample solutions, lifetime experiments were carried out with undiluted bovine serum and with diluted (1:20) bovine serum buffered to pH 7.4 and 3.5, respectively, as well as with aqueous solutions. RESULTS AND DISCUSSION Calculations of the Required Lipophilicities of Membrane Components. T o calculate the lipophilicities of membrane components, as required for a given minimal lifetime of ion-selective sensors, eq 5 was solved for K using the known experimental parameters. The resulting values of the distribution coefficient were inserted into eqs 8 and 9, respectively, to obtain the corresponding data for log PTLc. The results given in Table I for ISEs and ISFETs indicate that the required lipophilicity of neutral carriers is generally lower than that of plasticizers. This is due to the fact that a considerably lower limiting concentration of carriers in the membrane can be tolerated without risking a detrimental change in the performance of the potentiometric ion-selective device. Accordingly, the actual lifetime of liquid-membrane-based ion sensors is usually limited by the loss of plasticizers and is therefore shorter than the time period expected from the lipophilicity value of the ionophore involved. Since membranes suited for optical ion sensing should have a comparatively small thickness and at the same time a high concentration of chromoionophores, extremely high lipophilicities of membrane components are a prerequisite in this case (see Table I). For carriers with log PTLc 25, however, the practical limit of mobility within the membrane is probably reached (2,15). This suggests that the accessible lifetime of ion-selectiveoptode membranes is considerably shorter than that of corresponding electrochemical devices. For the cal-
-
600
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
Table 11. Lipophilicity of Components for Liquid Membranesa,* lipophilicity acronymh ETH 1778 ETH 1859 TDDA ( 5 ) ETH 1907 ETH 1810 ETH 2137 DM14 C4 (28) ETH 2120 ETH 227 ETH 157 B12 C4 (29) hemisodium (30) valinomycin B15 C5 (31) BME 44 (32) nonactin (33) monactin (33) V 163 ( 5 ) DT16 C5d (34) ETH 322 TETDSd (35) TTXd (36) ETH 1062 ETH 295 (TBT)Cl ( 5 ) ETH 6024 TFABB (37) ETH 6010 ETH 6011 ETH 1001 ETH 129 ETH 5234 ETH 1117 ETH 2220 ETH 5214 ETH 5220 ETH 5282 C1P mesamoll DEHS (41, DOS o-NPOE ( 4 ) NaTFPB
log PTLC
est log PHmuch
15.2; 15.3 f 3‘vd 13.8; 13.8 f 2‘sd 11.6 20.0 7.2; 7.5d 10.3 9.6 8.3; (8.3 f 0.7)c,d 7.8; 7.0d 4.6 6.7 10.4 8.6; 7.8d 11.5c 10.0 5.8 6.5 8.2
9.8 8.4 17.4 11.0 7.9 11.6 7.4 9.9 8.5 7.6 6.3 9.1 (13.6) 6.4 9.6 5.6 6.2 11.8 7.1 20.8
applicn, ionophore for H+ H+ H+ H+ Li+ Li+ Li+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ NH4+ NH4+ Ba2+ Ag+ Pb2+ cu2+ cu2+ CdZ+
UO?+ ClCO?-
co2co2c0:Ca2+ Ca2+ Ca2+ Mg2+ Mg2+ Mg2+ Mg2+ Mg2+ plasticizer plasticizer plasticizer plasticizer anionic site
10.2‘
17.5 3.5c 6.5‘ 6.5 6.2 14; 9.5c 8.0 3.6-5.0 4.3-6.0 6.9-7.6 7.5; 6.gd 7.2; 7.5d 20.0; 23.0d 5.8 ca. 8d 3.4d 11.4 5.5 6.4-9.3‘9
