2422
Anal. Chem. 1982, 54, 2422-2427
Chromatographic Retention Mechanism of Organic Ions on a Low-Capacity Ion Exchange Adsorbent Shapour Afrashlehfar‘ and Frederfck F. Cantwell* Department of Chemlstty, Universl@ of Alberta, Edmonton, Alberta, Canada T6G 2G2
A small electrical charge (ca. loe6 C/cm2) is placed on the surface of a nonionic adsorbent by covalently bonding quaternary ammonium groups. A combination of two mechanisms has been found responsible for the greatly enhanced sorptlon of counterions: ion exchange and surface adsorption. The latter Is dependent on the electrical potential of the surface. An equation relating chromatographic retentlon of a counterion to moblie phase Ionic strength has been derived and verlfied experimentally at “trace” sample concentrations. The dependence of ionic strength of coion sorption Is also predicted, and the abllity of adsorbed perchlorate ion to reverse the surface potential Is demonstrated. Relevance of the model to “ion pair” chromatography Is dlscussed.
It has previously been shown by distribution isotherm and microelectrophoresis measurements that the sorption of organic cations onto a nonionic styrene-divinylbenzene macroporous adsorbent can be quantitatively explained in terms of the Stern-Gouy-Chapman (SGC) theory of the electrical double layer (1). Similar behavior has been demonstrated for organic anions (2). It was shown that linear sorption isotherms and symmetrical chromatographic peaks are obtained for ionic sample species only if the electrical potential of the adsorbent surface is kept constant. This condition is achieved experimentally either by using low sample concentrations or by adding another “auxiliary” potential determining ion to the mobile phase (1). Addition of an auxiliary ion also provides a convenient means of either enhancing or diminishing the chromatographic retention of a sample ion. In such a system, sorption of both the sample ion and the auxiliary ion depends on the ionic strength of the mobile phase. Consequently, the surface charge density of the adsorbent varies with ionic strength. As an alternative to generating a variable surface charge density by adding an auxiliary ion, a constant surface charge density can be produced on the adsorbent via covalent bonding of “fixed-charge groups” onto the surface. Regardless of the way in which the surface charge has come about, there are two principal mechanisms by which a sample ion of opposite charge to that of the surface (i.e., a counterion) can be sorbed: ion exchange of the sample ion for other counterions in the diffuse part of the electrical double layer, and adsorption of the sample ion onto the surface. Surface adsorption is strongly dependent on the electrical potential of the surface (1) while ion exchange is independent of potential. A sample ion of the same charge as that of the surface (i.e., coion) experiences a diminished surface adsorption as a result of the surface potential and, in addition, it tends to be excluded from the diffuse part of the double layer. In the presently reported study a small, fixed, positive electrical surface-charge density no (ca. lo+ C/cm2) has been Present address: Department of Chemistry, McGill University, Montreal, Quebec, Canada H3A 2K6. 0003-2700/82/0354-2422$01.25/0
placed on the surface of a macroporous styrenedivinylbenzene adsorbent via the covalent attachment of quaternary ammonium groups. The nonionic parent adsorbent is Amberlite XAD-2 which is hereafter called “XAD”. The surface-quaternized adsorbent is hereafter called “QXAD”. By measurement of the sorption of a sample counterion S-, a sample coion S+, and a neutral sample species N as a function of ionic strength, it is shown that the dual mechanisms suggested above are, in fact, responsible for the sorption of ionic sample species on the low capacity ion exchanger QXAD.
THEORY Most of the surface area of QXAD lies along the channels within its macroporous structure and, since the average pore diameter is about 90 A, small organic sample species have free access to the surface via diffusion through the pore liquid (3). Bulk liquid phase is present in the pores as well as outside the resin particle. It is assumed that the covalently bound quaternary ammonium fixed-charge groups are more or less evenly distributed on this surface but are not present within the highly cross-linked matrix itself. Hence, there is no “gel” type ion exchange region in QXAD, and all interactions between sample and sorbent occur at the surface. The assumption that chloromethylation and subsequent amination occurs only at the surface of the resin is reasonable even though chloromethylation is carried out in what is usually a swelling solvent. This is because the very highly cross-linked matrix is unlikely to swell extensively and a “shell progressive” reaction mechanism (4) should prevail giving rise to surface quaternization (5). The following nomenclature is employed (1, 6-9): The charge density of the fixed-charge groups is viewed as being uniformly smeared-out on the surface and a plane passing through the charge groups is the “charge surface”. The plane of closest approach by nonadsorbed counterions is the outer Helmholtz plane (OHP). Counterions closer to the surface than the OHP are adsorbed. These are considered to be adsorbed onto the surface in an “inner Helmholtz plane” (IHP) which, for present purposes, is considered to be located at the charge surface so that the electrical potential of the IHP is identical with the surface potential The “compact part” of the double layer extends from the charge surface to the OHP and includes the IHP, while the “diffuse part” includes the OHP and extends from it to the bulk solution. The concentration of a species if in the double layer region is given by its “surface excessn rif,in mol-cm-2. As previously (1, 7), riais defined with respect to the bulk solution concentration of i*, rather than with respect to the surface charge as is often done in ion exchange (8). The bulk solution reference is a more convenient choice in the present case because a nonionic compound was used as an unretained component to measure the column void volume Vw Since a nonionic species has access to all of the liquid phase including that in the diffuse region, the capacity factor of i” (defined below) was de facto measured in a manner that implies that there is no “coion exclusion” from the diffuse region and that the “mobile phase” includes the liquid in the diffuse region. Compensation of these implicit assumptions is made by
+,,.
0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
calculating the “negative sorption” arising from coion exclusion, as will be discussed subsequently. Expressions have been derived (10) which relate I?& to the bulk solution concentration Ci* for the processes of ion exchange, coion exclusion, and surface adsorption. These are discussed below. Ion Exchange of Counterion. Exchange of a sample counterion S- for another counterion such as Cl- in the diffuse layer is represented as
S-
+ (R+CI-)DL F? C1- + (12+S-)DL
(1) in which R+ represents the charged surface and the subscript DL indicates a counterion in the diffuse layer. Ions without subscripts are in the bulk solution. Tho ion exchange equilibrium constant for ey 1has the following form (IO):
KS-,IEX =
c~ s -r, D L ~ c=~ -exp
[-
]
- (~~s-,DLi;~~ci-pL)
in which ~*s-,DL and p 0 , ~ l - p Lare the standard chemical potentials for the transfer of S- and C1- from bulk solution to the diffuse layer and Cs-and Ccl- are concentrations of S- and C1- in the bulk solution. Since both ions involved are univalent, ionic activity coefficients are assumed to have cancelled out of eq 2 (11). In some treatments p ’ ~ - , and ~ ~ Y’C~-,DL are assumed to be equal (8,12,13)which is equivalent to saying that there is no ion exchange selectivity for S- over C1- and that KS-,IEX = 1. Although such an assumption may be reasonable when the surface of the ion exchanger is hydrophilic (12),it is not justified €11, a hydrophobic polystyrene surface which exerts a profound effect on the structure of water near it (1, 14-16). In fact, it is found that low capacity anion exchangers prepared by quaternizing Amberlite XAD adsorbents do exhibit selectivity, even among simple inorganic anions such as F-, C1-, Br-, and I- (5, 17-19), and that the selectivity sequence is similar to that normally observed with high-capacity gel-type exchangers (20). When the number of moles of S- is much less than the number of moles of R+ and C1-, the concentrations of C1- in both the diffuse double layer and the bulk solution are essentially constant and independent of the small amount of S- that is sorbed. These criteria define trace conditions of ion exchange and are achieved experimentally by addition of NaCl swamping electrolyte and by using low concentrations of S- (20). Under trace conditions the total counterion surface excess is due nearly completely to Cl- ion, whose excess can then be related to surface charge density uo and the Faraday constant F (96 487 C/equiv) as follows:
(3) The ionic charge 2..is -1 ]For C1- ion so that both uo and rCl-,DL have positive values. Also, under trace conditions Ccl- is equal to the ionic strength of the bulk solution c in mol/L. For chromatographic purpo,ees it is convenient to define the capacity factor of S- for the ion exchange sorption mechanism, 12 ’s-,lEx, viz. -W%-,IEX
~B-,DLA
~’S-JEX =
s-v,
N
z2vMc
(4)
The constant A is the total surface area of QXAD in the column in cm2, The value of uo in C/cm2 can be calculated from the measured ion exchange ciapacity given in mequiv/g (20) by the equation FQweisht
=(53 103Asp where ASP is the specific surface area of QXAD in cm2/g. Bo
Surface Adsorption of Counterion. Adsorption of Sonto the charge surface (IHP) from bulk solution is represented by the expression (9, 11, 12) CS-,ADSYS-,ADS --- exp cs-Ys-
[
-
Z-Wo + P’S-,ADS
RT
]
(6)
in which q0is the electrical potential of the charge surface, pos-,ms is the standard chemical potential for the transfer of S- from bulk solution to the surface and YS-,ADS and YS- are ionic activity coefficients of S- on the surface and in bulk solution, respectively. The surface concentration of adsorbed S-, CS-,ADs, is given by
(7)
(2)
S Cl-,DL
2423
where d is the thickness of the compact part of the double layer and is estimated as 3.7 X cm from a previous study (1).It has been found that C1- is not surface adsorbed on XAD ( I ) . If the reference state for adsorbed S- is selected as infinite dilution on the surface of QXAD, rather than on the surface of the uncharged XAD, then ys-,ms 1,by definition. The reference state for S- in bulk solution is infinite dilution in pure water. Just as for ion exchange, trace conditions for surface adsorption are achieved when moles of S- is much less than moles of R+ and C1-. Under these conditions uo, +o, and the potential a t the OHP, $OHP, are all independent of the small amount of sorbed S-. Equation 6 can be combined with eq 7 and with the phase ratio A / V , to give an expression for the capacity factor of S- for surface adsorption, k ’S-,ADS kk-,ADS
~S-,ADSA
= --
I
CS-VM
Surface Adsorption of Coion. Equation 8 applies also to k h t , A D S for an adsorbed sample coion s+if z- is changed to Z+, with a value of +1. In this case a higher q0 decreases k ’S+,ADS. Exclusion of Electrolyte. When the bulk solution concentrations are taken as reference for defining surface excesses, then the equivalents of surface charge are balanced by both a positive excess of counterions and a negative excess of coions (1,6,7, 9). If P’C~-,DL H’N~+,DL or, alternatively, if both are I0.025 V so that the conventional small and also if Z++OHP “Debye-Huckel approximation” can be made, then the capacity factor for the exclusion of coion S+ from the diffuse region is given by the expression (10) N
k’S+,EXC
-
FS+,BLA ~
In eq 9, E is the permittivity of the solution in the diffuse region (1, 9, 21) and KSt,IEX is an “ion exchange” equilibrium (selectivity) constant for the “exchange” of coions S+ and Na+, derived in a manner analogous to eq 2 (10). The value of k’st8xC can be only zero or negative. In other words, kh+,EXC is the capacity factor for the negative sorption of S+. The phenomenon corresponds to the well-known process referred to as “electrolyte exclusionn, “coion exclusion”, or “Donnan exclusion” that is observed with gel type ion exchangers (20). When $oHp is very small, k $ t , E x c approaches zero, meaning that the concentration of S+ in the diffuse region is nearly the same as in the bulk solution ( r S t , D L 0).
-
2424
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
k's- = ~ ' S - , I E X+ ~ ' S - , E X C+ ADS (13) Substitution for the three k' components by their c explicit expressions yields the following equation:
Z P U ~ (4.38 x 1 0 3 ) Z P ~ , -c-ljZ
RTCl
d o
0'2
0'4
0'6
0'8
c (moles / L) Flgure 1. = 1.5 X
+
1 0 ,'
RT
I
(14)
1
(16)
The first term on the right-hand side (rhs) of eq 14 is equal to (k's-,I~x+ k'S-,EXC) and accounts for the ion exchange process. The second term is equal to k's-,ADS and accounts for adsorption. In a similar manner, the overall capacity factor for sorption of S+, k'st, is the sum of those arising from adsorption and exclusion, viz. (15) k 's+ = k L+,EXC+ ~ ' S + , A D S I t can be shown to be given by the following equation:
Dependence of +w and q0for QXAD on ionic strength: C F/cm2, uo = 1.31 X lo-' C/cm2.
Coion exclusion influences the sorption of a counterion sample S- as well as a coion sample S. An exclusion "correction" must be made to the ion exchange sorption of S- as a result of the choice of bulk solution concentration as reference in defining rS-,DL. Conceptudy, when a coion such as Na+ is excluded from the diffuse region, it must be accompanied by a counterion (e.g., Cl-, S-) in order that the surface excess of counterions will not change. When Z++oHp I0.025 V the capacity factor for negative sorption of counterions can be shown (IO) to be
Electrical Potentials. In the absence of surface adsorption of C1- and under trace conditions, +omcan be calculated from the SGC theory ( I , I 2 )as a function of c. When Z#oHp I0.025 V, the following expression gives the relationship ( I , 8,9,21):
qOHP
N
(4.38 x ~ O ~ ) U , C - ~ / ~
(11)
+,
The electrical potential of the surface is related to #OHp via the capacitance of the compact part of the double layer, C1, which may be taken as a constant, especially when 00 is constant ( I ) . The equation relating #, and c is as follows (I): $o =
- + (4.38 x ~ o ~ ) u ~ c - ~ / ~(12) "0
C1
The dependencies of both #OHP and +o on c are shown graphically in Figure 1. The exact equations (eq 2 and 11 in ref I),relevant at all potentials, were used to calculate +om and +o in Figure 1using values of uo and C1 that apply to the batch of QXAD used in the present study. Although #OHP tends toward zero at very high c , #, tends toward a nonzero limiting value of uoC1-l. It is also evident from Figure 1that the rate of decrease of both +OHP and +o with increasing c is gradual at values of c between several tenths molar and several molar and that even qOHP is still appreciably above zero a t c I1 M. Figure 1 shows that eq 11can be used a t c L 0.05, since +OHP I0.025 V in this region. Dependence of k' on c . At low values of #OHP, where eq 9-12 are valid, the value of +oHpin eq 9 and 10 can be substituted from eq 11 and the value of in eq 8 can be substituted from eq 12, to give expressions for ADS, ADS, k's-,Exc and k's+,EXC that are explicit in c (IO). A corresponding expression can be developed for k $+,ADS. The overall capacity factor for sorption of S-, k$-, is the sum of the capacity factors arising from the processes outlined above, viz.
+,
Z+Fao (4.38 X 103)Z+Fao -c-ll2 RTCl RT
It should be noted that at low c, where +om 2 0.025 V, the exact equations given in ref 1should be used to calculate +om and +p This produces more complex dependencies on c of k's-,ms,k k+,ms,k k-,Exc, and k kt,EXC than those presented in eq 14 and 16. EXPERIMENTAL SECTION Reagents and Chemicals. The sample compounds (3-nitrobenzy1)trimethylammonium chloride (Aldrich Chemical Co.) and p-phenylenediamine (Fisher Scientific Co.) were used as received. The sample compound p-nitrobenzenesulfonic acid was prepared from practical grade (Eastman Kodak) by recrystallizationfrom methanol. Anhydrous methanol was distilled before use. Water was deionized, distilled, and finally distilled over alkaline permanganate. All other chemicals were reagent grade. Sorbent Preparation. The parent, neutral resin XAD (Amberlite XAD-2, Rohm and Haas, Philadelphia, PA) was ground as previously described (I) and classified on U.S. standard sieves. The 44- to 63-pm portion was washed with 3 M HC1 and then with water and ethanol. It was defined by repeated sedimentation and decantation in methanol and was dried at 50 "C in a vacuum oven. The quaternized sorbent QXAD was prepared from the 44-63 pm XAD by a modification of the procedure of Gjerde et al. (5): A 5.7-g portion of XAD was immersed in a mixture of 10 mL of chloromethyl methyl ether (CAUTION: carcinogen), 40 mL of methylene chloride, and 3 mL of nitromethane for 1 h. The reaction was then initiated by adding 1.1g of zinc chloride. After 10 min the reaction was stopped by adding cold water, and the chloromethylated resin was filtered and washed with 2 L of methanol and 3 L of water. After drying overnight in vacuo at 40 "C, the chloromethylated resin was aminated by suspending it overnight in 100 mL of a 1:3 trimethy1amine:methanolmixture. The product was filtered and washed, in turn, with 1 L of 1 M HC1,l L of 2-propanol, 2 L of methanol, and 4 L of water, and was finally dried overnight in vacuo at 40 O C . This product, QXAD, was a low capacity cation exchanger in the C1- form. The ion exchange capacity of QXAD was determined by equilibrating 1.00-g portions with 50.0 mL of 0.1 M NaOH, neutralizing 20.0-mL aliquots of the supernatant liquid with HN03, and titrating the chloride ion with 0.0100 M AgNOS using a potentiometric end point. The exchange capacity was 45 pequivf g of dry resin. Columns and Apparatus. Each of the two sorbents, XAD and QXAD, was packed into a 30 cm long by 0.28 cm i.d. glass column (Microbore,Laboratory Data Control, Riviera Beach, FL), using a stirred slurry technique (22)with a pressure not exceeding
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14,DECEMBER 1982
600 psig. The weight of resin in each column was 0.55 g, as measured by emptying the column and weighing. The void volume of each column was 0.90 mL,measured as the retention volume of water. A Milton-Roy minipump, Model C-309 (Waters Associates, Milford, MA) was connected to the inlet of the chromatographic column via a Cheminert 10-rL injection valve (Laboratory Data Control) with short lengths of 0.3 mm i.d. Teflon tubing. A Spectra-Physics Model 230 UV absorbance detector set at 254 nm, in combination with a Recordall recorder (Fisher), was used to record the chromatograms. Sample and Mobile Phase Solutions. Solutions of each of the three sample species p-nitrobenzenesulfonate (NBS-), (8nitrobenzy1)trimethylamnonium (NBTA’.), and p-phenylenediamine (PDA) were prepared in M ammonia water at conM, 1.00 X M, 1.00 X M, and centrations of 1.00 X 0.100 M. M, 1-00X M, 2.50 X The mobile phases were 1.00 X M, 0.100 M, 0.250 M, 0.500 M, and 1.00 M M, 2.50 X aqueous solutions of either NaCl or NaC10*and each contained M ammonia. Procedure. The sorbent was equilibrated with the mobile phase of interest by pumping the latter through the column for about 3 h at a flow rate of 1.40 mL/min. This flow rate was used throughout all of the studies. Duplicate injections were made, and if the second injection did not yield an identical chromatogram to the first, then further injections were made until constancy was achieved. (This was only rarely necessary.) Room temperature was constant at 25 i 0.2 OC for all studies. Because chromatographic peaks were asymmetric to a greater or less extent, retention times were measured, not at the peak maximum, but at the “center of gravity“ of the peak (1). The peak center of gravity was locakd after converting the analog recorder tracing to digital form on 8 “Graphitizer-Digitizer” available from the University of Alberta Computing Services. Peak asymmetry decreased with decreasing concentration of the sample injected. Retention time tR was converted to capacity factor k’ via the relationship
in which to is the retention time of an unretained component. For the value of the capacity factor ut truce conditions for a given mobile phase ionic strength, a plot of In k’vs. siample concentration injected was extrapolated to zero sample concentration. Such plots were nearly always straight lines, and tho distance over which the extrapolation was made was usually relatively short. The “trace conditions” values OF k’were used in all comparisons with the theoretical equations. Because retention times were very long (hours) when k’values were high, and because replicate injections were made at several different sample concentrations in order to extrapolate to trace conditions, the experiments described above were tedious and time-consuming. A recently described precolumri equilibration technique (23) was developed to facilitate such measurements in the future.
RESULTS AND DISCUSSION Sorption of Counterion. A plot of k i-vs. c is presented in Figure 2, curve A, for the sample NBS-. According to the theory discussed above, the portion of this curve a t c 2 0.05 should be described by eq 14. In order to test this prediction, we used the nonlinear curve-fitting program KINET (24) with the four solid circle data points on curve A to evaluate the two unknown constants and H ~ S - ~ DinS eq 14. The observed values were (9 :k 1) X lo2, and (-1.85 f 0.02) X lo4 J/mol, respectively. In this process the molar ionic activity coefficient ys-,which varies with c, was assumed to be equal to that for p-toluenesulfonate and was calculated from the molal activity coefficients published for this compound (25). For use in the above calculations the value of C1was estimated as 1.5 X F/cm2 from a previous study of XAD (1). The open circle experimental point at c = 0.025 in Figure 2, curve A, was not included in the data treatment for two reasons: (i) $oHP> 0.025 V, which invalidates eq 14, and (ii)
I \
c
ks- 200 I \
i
‘\.-‘00
0’2
2425
’
D
-----T--
04
06
08
’
10
c (moles / L) Flgure 2. Dependence of k’, for NBS- on Ionic strength using sodium chloride. Curve A Is for overall retention on QXAD; solid line is Galculated by eq 14 using (ro = 1.31 X Clem', C, = 1.5 X F/cm2, A = 1.80 X 10’ cm2, d = 3.7 X lo-*cm, V , = 1.25X L, R = 8.314 (C V)/(mol K), T = 298 K, F = 96487 Clequiv, KS-,IEX = 9 X lo2, fios-,Aos = -1.85 X lo4 J/mol; points are experimental. Curve B is calculated contribution from surface adsorption. Curve C is calculated contrlbutlon from ion exchange. Curve D is for overall retentlon on XAD.
retention times measured for NBS- at c = 0.025 were so long and the chromatographic peak so broad and close to the base line that a retention time could be measured only for the highest concentration injected (0.1 M NBS-). Extrapolation to zero sample concentration was not possible. Curves C and B in Figure 2 are plots of (k’s-,IEX+ k$-,EXC) and 12$-ws vs. c, respectively, which show the contributions of ion exchange and surface adsorption to the retention of NBS-. They were calculated by inserting the values of &-,= and poS-,ADS obtained above from the KINET program into the first term ( k & , m + k&mc)and the second term (k.$-ms) on the rhs of eq 14. The solid line in curve A is the sum of these two terms. The closeness of fit of the solid line to the solid circles in curve A supports the proposed mixed retention mechanism. The value of Ks-,IEx observed for NBS- is similar to values of the selectivity coefficient observed on gel-type anion exchange resins for the exchange of large, poorly hydrated ions such as Reo4-, Mn04, and Au(CN)~-for the C1- ion (14,26). The large value of &-,Ex reflects a large difference between P’S-,DL and P’CI-,DL (eq 2). Curve D in Figure 2 shows the dependence on c of k k- for the sorption of NBS- onto XAD. As expected, k k-on XAD increases gradually with c for values of c 20.01. The anomalous upturn in this plot upon going to values of c I0.01 is observed only when very low concentrations of NBS- are used. At higher NBS- concentrations the expected (I) decrease in k 6- with decreasing c is observed. The anomalous upturn most likely arises from the presence of a very small amount of positively charged ”impurity” sites on the resin giving it, in effect, a very small positive surface charge. Curves A-D in Figure 2 show that the sorption of the counterion NBS- onto a low capacity anion exchanger is substantially greater than onto the nonionic parent resin at all ionic strengths and that it becomes very much greater at low ionic strength. At high c (e.g., c 2 0.5) the enhancement in sorption of NBS- results mainly from its potential-dependent adsorption, while ion exchange plays a minor role. At lower c both ion exchange and surface adsorption make major contributions. These conclusions can be extended only in broad terms t o the sorption of other counterion samples, since K s - ,and ~ pos-ms are free to vary independently. Hence, other sample species S- may exhibit either greater or less relative tendency toward
2426
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
I
I
i
300 1
ks+
j
ks. l
00
02
,
l
04
0.6
c (moles /
- ,
I
08
F are
for PDA on XAD and QXAD using NaCIO,.
ion exchange or surface adsorption, depending on the structure of s-. Sorption of Coions and Neutral Species. In Figure 3, curves A and B are plots of k’s+ vs. c for NBTA+, on both of the resins XAD and QXAD. Examining, first, the portion of curve A at c I 0.1, the same kind of anomalous upturn is seen as was seen for the sorption of NBS- on XAD in Figure 2, suggesting that XAD possesses not only positively charged “impurity” groups but also negatively charged ones. When the ionic sites are so few and far-between, the surface charge cannot be considered to be smeared-out, and a “localized-site” model must be used (27). Adsorption of anionic samples is enhanced near positive surface sites and adsorption of cationic samples is enhanced near negative sites. The presence of very small numbers of both positive and negative surface sites is entirely possible in light of the synthetic scheme of XAD (28). It has previously been shown that XAD sorbs both H30+ and OH- ions (29). As predicted, the sorption of NBTA+ on QXAD (curve B) increases with increasing c and is suppressed compared to its sorption onto XAD (curve A). Although, qualitatively, the sorption of NBTA+ on QXAD is in agreement with theory, it is not in quantitative agreement, since curve B at c 2 0.05 is not described by eq 16. Understanding of this quantitative discrepancy will require further work. Curves C and D in Figure 3 show that the sorption of a neutral compound, PDA, on QXAD is nearly independent of ionic strength and that it is nearly identical with its sorption on the nonionic resin XAD. The similarity of curves C and D suggests that the surface area of QXAD available for adsorption is nearly the same as that for XAD. This is consistent with the fact that only about 5% of the polystyrene phenyl rings on the surface of XAD are quaternized in the synthesis of QXAD with uo = 1.31 X lo4 C/cm2. Perchlorate as Electrolyte. When sodium perchlorate is substituted for sodium chloride as eluent, profound changes are observed in the sorption of the ionic samples NBS- and NBTA+. In Figure 4 are presented plots of k’s- for NBS- and k’s+ for NBTAe, on QXAD and on XAD, vs. ionic strength adjusted with NaC104. Curve A for NBS- on QXAD exhibits the same general shape as curve A in Figure 2 obtained with NaCl eluent, but k h- values at all c values are much lower in the NaC104 case and, at high values of c, k$- is even substantially lower t h a n found o n X A D using N a C l (curve D in Figure 2). It is evident that an increase in concentration of NaC104 decreases k’s- not only because of the generalized ionic strength effect predicted from SGC theory and discussed in connection with NaCl eluent but also because C10, causes
0.2
0.4
;
, 0.6
1
0.8
,
j 1.0
c ( m o l e s / L)
L)
Figure 3. Dependence of k’S+ for NBTA’ and of k’, for PDA on ionic strength. Curves A and B are for NBTA’ on XAD and QXAD using NaCI. Curves C and D are for PDA on XAD and QXAD using NaCI. Curves E and
0.0
10
,
Figure 4. Dependence of k‘s- for NBS- and k ‘ p for NBTA’ on concentration of NaCIO,. Curves A and B are for NBS- on QXAD and XAD. Curves C and D are for NBTA’ on QXAD and XAD.
charge reversal on the surface of QXAD. Unlike C1-, the C104- ion is adsorbed onto the surface as a potential determining ion in excess of the charge density of fixed-charge quaternary ammonium groups. Hence NBS-, which is a counterion for the positively charged QXAD in C1- solution, becomes a coion for the negatively charged QXAD and XAD in the presence of high concentrations of C104-. Comparison of curves A and B in Figure 4 shows that QXAD and XAD must posess similar negative surface potentials at C10, concentrations greater than 0.25 M. The behavior of NBTA+ shown in curves C and D of Figure 4 is consistent with the interpretation presented above. The NBTA+ has evidently become a counterion whose k ’ p increases with the increase in (negative) surface charge and potential at higher concentrations of C104-. In contrast to the behavior of ionic sample species, sorption of the neutral compound PDA, as shown in curves E and F of Figure 3, remains nearly unaffected by an increase in NaC10, concentration, and k/N is virtually the same on XAD and QXAD using both NaCl or NaC104 electrolyte. When one of the ions of the electrolyte, such as C104-, is adsorbed as a potential determining ion, eq 14 and 16 obviously cannot be used. The net surface charge density uo now depends on the concentration of Clod-. In such a system, $om can be estimated if the sorption isotherm of C104-is known or, alternatively, by experimental measurement of the { potential of the sorbent particles. Then $o can be calculated from +OHP via eq 12, if C1 can be estimated ( I , 13, 30). Chromatographic Consequences. Evidently, the low capacity ion exchanger exhibits nearly unaltered sorption properties toward nonionic sample compounds compared to the parent absorbent from which it is made. In contrast, it exhibits a marked increase in sorption affinity for counterion-type sample species and a strong dependence of k’on ionic strength. A high efficiency microparticle packing has previously been prepared from the nonionic adsorbent XAD (22) and the present study suggests that a similar packing prepared from QXAD would exhibit the same retention characteristics for nonionic samples, while at the same time permitting high efficiency separation of organic anions without the need to add a “pairing-ion” (hetaeron) reagent to the mobile phase. In a more general sense, the dual retention mechanism for sample counterions, involving both ion exchange and potential-dependent adsorption onto a charged surface, is relevant to the interpretation of the retention mechanism in so-called “ion-pair” chromatography (30,31). The importance of surface-potential-dependent adsorption in this connection was suggested earlier ( I ) , and its quantitative verification on the constant surface-charge-density adsorbent, QXAD, reported a
Anal. Chem. 1982, 5 4 , 2427-2431
in the present study lends support to this suggestion. The role of C10, in the present study is that of a so-called “pairing-ion”reagent which produces on the adsorbent surface a charge density that depends on its bulk solution concentration and on the overalU ionic strength. Thus, adsorbed C10, transforms the adsorbent into a “dynamic cation exchanger” and, at the same time, imparts to the surface an electrical potential which varies as a result of changes in both surface charge density and bulk solution ionic strength. Studies in the field of so-called “ion-pair” chromatography should not focus solely on the “dynamic ion exchange” contribution to retention (13,32-34) but, should also evaluate the contribution of potential-dependent adsorption. In1 some cases other phenomena, such as ion pairing in the bulk solution, may also make some contribution to the overall retention of an ion. However, the retention of NBS- on QXAD apparently does not involve ion pairing,
Electrochemical Society: Pennington, NJ, 1981; p 16. Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. Deelder, R. S.; van den Berg, J. H. M. J. Chromafogr. 1981, 278, 327. Dlamond, R. M.; Whitney, D. C. I n “Ion Exchange: A Series of Advances”; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1966; Vol. 1, Chapter 8. Reichenberg, D. I n “Ion Exchange: A Series of Advances”; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, Chapter 9. Feltelson, J. I n “Ion Exchange: A Series of Advances”; Marlnsky, J. A., Ed.; Marcel Dekker: New York, 1969; Vol. 2, Chapter 4. Gjerde, D. T.; Fritz, J. S. J. Chromatogr. 1979, 776, 199. Gjerde, D. T.; Fritz, J. S.;Schmuckler, G. J. Chromatogr. 1979, 786, 509. Gjerde. D. T.; Frltz. J. S. Anal. Chem. 1981, 53, 2324. Helfferich, F. “Ion Exchange”; McGraw-HIII: New York, 1962; Chapter 5. Shaw, D. J. “Electrophoresis”; Academlc Press: London, 1969; Chapter 2. Baum, R. G.; Saetre. R.; Cantwell, F. F. Anal. Chem. 1980, 52, 15. May, S.; Hux, R. A.; Cantwell, F. F. Anal. Chem. 1982, 54, 1279. Dye, J. L.; Nlcely, V. A. J. Chem. Educ. 1971, 48, 443. Bonner, 0. D. J. Am. Chem. SOC. 1955, 77, 242. Aveston, J.; Everest, D. A.; Wells, R. A. J. Chem. SOC. 1858, 231. Barlow, C. A., Jr.; MacDonald, J. R. I n “Advances in Electrochemistry and Electrochemlcal Engineerlng”; Delahay, P., Ed.; Interscience: New York, 1967; Vol. 6, Chapter 1. Barrett, J., Rohm and Haas Co., Philadelphia, PA, personal communication, 1982. Puon, S.;Cantwell, F. F. Anal. Chem. 1977, 49, 1256. Bidllngmeyer, B. A. J. Chromatogr. Sn’. 1980, 78, 525. Hearn, M. T. W. I n “Advances in Chromatography”; Giddings, J. C., Grushka, E., Cares, J., Brown, P. R., Eds.; Marcel Dekker: New York, 1980; Vol. 18, Chapter 2. Scott, R. P. W.; Kucera, P. J. Chromatogr. 1979, 175, 51. Knox, J. H.; Hartwlck, R. A. J. Chromatogr. 1981, 204, 3. Hung, C. T.; Taylor, R. B. J . Chromafogr. 1981, 209, 175.
LITERATURE CITED Cantwell. F. F.; Puon, Si. Anal. Chem. 1979, 57, 623. Rotsch, T. D.; Cahlll, W. R., Jr.; Pietrzyk, D. J.; Cantwell, F. F. Can. J. Chem. 1981, 59, 2179. “Amberlite XAD-2”; Technical Bulletin; Rohm and Haas Co.: Phlladelphia, PA, 1972. Schmuckler, G.; Goldstein, S. I n “Ion Exchange and Solvent Extraction”; Marinsky, .I. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1977; Vol. 7, Chapter 1. Gjerde, D. T.; Schmuckller, G.; Frltz, J. S. J. Chromatogr. 1980, 787, 35. Adamson, A. W. “Physlcal Chemistry of Surfaces”, 2nd ed.; Intersclence: New York, 1967; Chapter 4. Van Dolsen, K. M.; Vold, M. J. I n “Adsorptlon from Aqueous Solutions”; Webber, W. J., Matljevic, E., Eds.; American Chemlcal Society: Washington, DC, 1968; Chapter 12. Overbeek, J. Th. I n “Collold Science”; Kruyt, H. R., Ed.; Elsevier: New York, 1952; Vol. 1, Chapter 4. Grahame, D. C. Chem. Rev. 1947, 4 7 , 441. Cantwell, F. F. I n “Ion Exchange and Solvent Extraction”; Marinsky. J. A., Marcus, Y., Eds.; Marcel Dekker: New York, to be published. Buck, R. P. I n “Proceedings of the Symposium on Ion Exchange Transport and Interfacial Properties”; Yeo, R. S., Buck, R. P., Eds.;
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RECEIVED for review April 29,1982. Accepted August 13,1982. This work was supported by an Alberta Heritage Foundation for Medical Research postdoctoral fellowship to S.A., by the Natural Sciences and Engineering Research Council of Canada, and by the University of Alberta. Presented in part at the VI International Symposium on Column Liquid Chromatography, Philadelphia, PA.
Ion Interactiion Chromatography of Inorganic Anions on a Poly(styrene---divinylbenzene)Adsorbent in the Presence of TetraaIky lamimonium SaIts Zlad Iskandaranl and Donald
J. Pletrzyk”
Department of Chemistry, The University of Iowa, Iowa City, Iowa 52242
Two major equiilbria that contribute to the enhanced retention of inorganic anions on a Hamilton PRP-1 column (a poiy(styrene-divlnylbenzene) copolymer) In the presence of a tetraaikyiammonlum salt (R,lM+) are Identified as one involving retention of the R,N+ salt as a double layer on the stationary phase surface and the second as an anion seiectivlty between analyte anlons and thoso occupying the secondary layer of the double layer. The mobile phase variables are structure and concentration of RBI+ salt, mobile phase solvent compositton, type and concemtration of coanlon accompanying the R,N+ salt or introduced for Ionic strength control, and pH. Control of these parameters permits the quantltatlve separation of many complex mixtures of Inorganic anlons at concentrations In the parts per bliiion range. Inorganic monoand multivalent anions were studied.
Several reports (1-3) have shown that inorganic anions are 0003-2700/82/0354-2427$01.25/0
retained on bonded stationary phases when using tetraalkylammonium (R4N+)salts as mobile phase additives. These studies tended to focus on separations, rather than on the mode of retention or a detailed evaluation of elution variables, and represent an alternate to the more familiar two-column ion exchange liquid chromatographic (LC) technique (4)and its variations (5-7) known as ion chromatography. We have recently shown (8) that the enhanced retention of an organic anionic analyte on a nonpolar poly(styrenedivinylbenzene) copolymeric adsorbent, PRP-1, from a mobile phase containing a R4N+salt, its coanion, and a mixed solvent follows a double layer model and not an ion pair or solvophobic model; a review of these models is provided elsewhere (8). Two major equilibria influence the retention. One, which describes retention of the R4N+salt itself on the PRP-1 surface, leads to the formation of a double layer. The R4N+occupies the primary layer producing a positive charge at the stationary phase surface while the coanion occupies a diffuse secondary layer. The second major equilibrium is one that describes the 0 1982 American Chemical Society
