Langmuir 1994,10, 797-801
797
Measurement of the Sele tive Adsorption of Ions at Air/ Surfactant olution Interfaces
B
John D. Morgan,**+Donald H. Napy)er,t Gregory G. Warr,? and Stuart K. Nicolt Division of Physical and Theoretical Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia, and BHP Research, Newcastle Laboratories, P.O. Box 188, Wallsend, New South Wales 2287, Australia Received June 11, 1993. In Final Form: October 21, 199P A convenient flotation technique for measuring the selectivity coefficient of counterions at surfactant solution/air interfaces is presented. The selectivities of C1-, NOa-, I-, and ClOs- versus B r are measured using cetyltrimethylammonium surfactants, and the selectivity of Cl- versus B r is measured using cetylpyridinium surfactants. The selectivity coefficient of B r over C1- is observed to be independent of solution composition, surface excess, and surfactant head group type. The measured selectivity coefficients were found to be multiplicative. We showhow our results differ from thoee obtained in micellar experiments and argue that selectivity coefficients measured by flotation may be interpreted aa thermodynamic ion exchange coefficients.
Introduction The selective adsorption of counterions at charged surfactant interfaces has been observed in a number of systems, including micelles,14 vesicles,7p8and air/solution interfaces.”l5 This phenomenon has also been extensively studied in ion exchange resins.le The tendency of the interface to favor adsorption of one specific counterion over others has usually been described by a selectivity coefficient, K,, defined for univalent ions as
ion exchange model. However, agreement on the value of K, reported for different studies is poor. For instance, literature values of K& range from 2.6 f 0.65 up to 6.3 f 1.O.l’ The values reported in these studies also depend on the degree of ionization of the micelle surface, which is a free parameter in the pseudophase model. Another potential difficulty is that, with one recent exception: selectivity coefficients for most ion pairs have only been determined indirectly. The selectivity of A and B is measured for each ion with respect to a third ion, usually hydroxide. The formal multiplicative identity
where CA and CB are the concentrations of counterions A and B. Quantities with a bar denote the surface phase and those without denote the bulk solution phase. The subscript e denotes the experimental selectivity coefficient, which is distinct from a true thermodynamic equilibrium constant which would be defined in terms of activities. The selectivity coefficient has been measured in micellar systems for a range of competing ions by a number of workers, generally in the context of micellar catalysis. K, may be obtained from observation of reaction rate modification by micelles, interpreted usingthe pseudophase
is then used to calculate K&. This multiplicative relation remains untested experimentally however, and its validity is problematical unless the degree of ionization is the same for the micelles with each different counterion. This is unlikely when very hydrophilic ions such as hydroxide are compared to less hydrophilic ions like bromide and chloride.18 Few studies have been reported on selectivity at air/ surfactant solution interfaces, partly because of the relative difficulty of probing that interface. In this system the selectivity coefficient is rendered as
+ University of Sydney. t
BHP Research.
Abstract published in Aduance ACS Abstracts, January 15,1994. (1) Romated, L. S.; Bunton, C. A.; Nome,F.; Quina, F. H. Ace. Chem. Res. 1991,24, 367. (2) Loughlin, J. A.; Romated, L. S. Colloids Surf. 1990,48, 123. (3) Nome,F.; da Grap, Nascimento, M.; Miranda, S. A. J.Phys. Chem.
(3)
0
1986,90,3366. (4) Abuin, E. B.; Liesi, E. J. Colloid Interface Sei. 1983,96, 293. (6)Drummond, C. J.; Grieser, F. J. Colloid Interface Sci. 1989,127, 281. (6) Sepulveda, L.; Bartet, D.; Gamboa, C. J.Phys. Chem. 1980~34,272. (7) Abuin, E. B.; Lmi, E. J. Phye. Chem. 1989,93,4886. (8) Romted, L. S.; Zanette, D.; et al. J. Phys. Chem. 1989,93,4219. (9) Shinoda, K.;Fujihara, M.Adu. Chem. Ser. 1967, No. 79, 198. (10)Galvin, K.P.; Nicol, S. K.;Waters, A. G. Colloids Surf. 1992,64, 21. (11) Grieves, R. B.; Kyle, R. N.Sep. Sci. Technol. 1982, 17, 466. (12) Grievea, R. B.; Burton, K.E.; Craigmyle, J. A. Sep. Sci. Technol. 1987.22. 1697. (13) Grieves, R. B.; The, P. J. J. Inorg. Nucl. Sci. 1974, 36, 1391. (14) Moraler, M. C.; Waisebluth, 0. L.; Quina, F. H. Bol. SOC.ChiL Quim. 1990,35, 19. (16) MoralescM. C.; Waiesbluth, 0. L.; Figueroa, T.; Pacheco, P. C. Actual. Fh-Quim. Org. 1991, 218. (16) Reichenberg,D.InZonExchunge: A SeriesofAduances;Marinsky, J. A., Ed.; Edward Arnold, La.:London, 1966, Vol. 1, Chapter 7.
where ri is the surface excess of component i. The first experiments that measured K, for univalent-univalent competition were performed by Shinoda and Fujihira in 1967: who used a batch flotation technique to determine the selectivities among bromide, chloride, chlorate, acetate, nitrate, and sulfate counterions of some alkylammonium surfactant ions. A continuous flotation technique was developed by Grieves et al.11-13 to measure selectivity among pairs of univalent anions as well as the alkali-metal cations. A similar technique was also used by Morales et al. for divalent/univalent anion exchange.14Jb However, these approachesare rather laborious. A more rapid batch flotation method was recently used by Galvin et al.l0to measure the selectivity of Au(CN12- over Ag(CN)2- at cetyltrimethylammoniumfilms,although the error in this (17) Abuin, E. B.;Lieei, E. J. Colloid Interface Sei. 1991,149,97. (18) Bunton, C. A.; Moffatt, J. R. Langmuir 1992,8, 2130.
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technique was f25%. Okuda and Ikeda19J0have studied bromide/chloride selectivity using surface tension measurements, although their results are not reported in terms of a selectivity coefficient as defined in eq 1. In each case in the literature where a selectivity coefficient has been obtained, some form of flotation technique was used. The advantage of flotation as a surface probe is that the foam generates sufficiently large interfacial areas for conventional mass balance techniques to be applied to determine the composition of the surface phase. Flotation is also more robust toward the presence of impurities than are surface tension measurements. In this paper we present a flotation technique for determining the selectivity coefficient between competing counterions that is simple, rapid, and precise and is independent of the model used to describe the interface. Direct measurements of Ke for a variety of simple anions are reported and compared to values obtained by other techniques. The effects of surface charge density, solution composition, and headgroup on Ke are examined. We present the simultaneous direct measurement of the three selectivity coefficients in a ternary system as a test of the validity of eq 2 and show how our measurements are connected to measurements in micellar systems. Theory We seek to express Ke in terms of the bulk concentrations. Consider an elementary flotation step, in which a small amount of interfacial area dA is created, perhaps as a bubble, and allowed to come to equilibrium with the bulk solution. We have recently demonstrated that this equilibrium assumption is valid for our particular flotation experiments.lg This area is then removed as foam, along with adsorbed surfactant and counterions. If the volume of the bulk solution is V, the change in the bulk concentrations of the ions for this process is
Counterion B is also adsorbed to the surface and satisfies a similar equation
Taken together we have (5)
and invoking the definition of Ke we arrive at the differential equation
If Ke is constant, this equation is readily integrated to give
In CA = Ki,eIn CB + C
(7) where Cis a constant of integration. Equation 7 describes the way in which the concentrations of the competing counterions change during the course of a flotation experiment. Thus, if a solution containing counterions A and B and a suitable surfactant is foamed, and the concentrations of A and B in the bulk are monitored during the course of an experiment, then the slope of a graph of In CA vs In CB will be the selectivity of the interface for A (19)Okuda, H.;Ikeda, 5. J. Colloid Interface Sci. 1989,292,333. (20)Okuda, H.; Ikeda, S. Colloids Surf. 1987,27, 187.
over B. This result requires no assumptions about the structure of the surface. In comparison to previous flotation techniques which required measurements of four experimental observables,this approach requires only two, thereby reducing the experimental uncertainties. Experiments
All flotation experimenta were carried out on solutions in a glass flotation column fitted with a porous frit at the base through which nitrogen was injected at a flow rate of 200 mL/min to produce a stream of bubbles. The foam which resulted was expelled from the column as it was formed. The column was fitted with a syringe and hollow needle for sampling of the bulk solution and a water jacket for temperature control. All experimenta were carried out at 25 f 1 OC. This apparatus is described in more detail elsewhere.21 Cetyltrimethylammonium bromide (CTAB, Aldrich, 95 % pure) was recrystallized once from ethanol. Cetyltrimethylammonium chloride (CTAC, Aldrich) was provided as a nominal 25% aqueous solution and was quantitatively determined on a chloride basis by ion chromatography. No other anions were observed,and this sample was used asreceived Cetylpyridinium bromide (CPB) and chloride (CPC) were both supplied as the monohydrate (Aldrich,98 % pure) and used as received. All other reagents were analytical grade, and solutions were made up in singly distilled water. As reported below, the selectivity appears to be independent of solution and surface composition, so ultrahigh purity is unnecessary. A typical experiment would require the preparation of a solution containing CTAB at 2.00 X lo-' M and the sodium or potassium salt of another anion at a similar concentration. The solution (900 mL) was then subjected to foaming for 1 to 2 h, and samples (5 mL) were taken from the bulk solution at regular intervals. These intervals varied according to how strongly and for how long the solution foamed. The samples were then analyzed using a Dionex DX-100ion chromatograph fitted with AS-4 analytical anion exchange columns and a Hewlett-Packard integrator. The data obtained was rendered as a log-log plot, the slope obtained by linear regression, and the value reported as K.. All but one of the experimenta were performed on solutions below the cmc of the surfactant, to avoid the possibility of competing adsorption equilibriaat the micellar interface. A lower bound on the concentration range that can be investigated is set by the requirement that the surfactant concentration must be high enough for a stable foam to be generated for enough time for a measurable change in the ion concentrations in the column to occur. For CTAB at 398 K, the lowest concentrations that can be investigated are of the order l(r M. Results and Discussion Selectivity coefficients of bromide over chloride, iodide, nitrate and chlorate were measured at the quaternary ammonium surfactant interface. The experimental loglog concentration graphs are shown in Figure 1,and they confirm the linearity suggested by eq 7. Correlation coefficients were generally greater than 0.99, and the derived selectivity constants were reproducible to within a 5 % error. The measured K, values are tabulated in Table 1,along with the initial counterion concentrations, and comparison with reported values from other flotation experiments and micellar experiments. The precision of the measurements made here compares favorably with most other independent measurements reported for other techniques. They agree with the measurements by Grieves et al.11-13and by Shinoda and Fujihara for bromide versus nitrate and chlorate. Shinoda and Fujihara's value for bromide versus chloride is by far the lowest reported for any system for this pair of counterions and appears to be in error. (21)Morgan, J. D.; Napper,D. H.; Warr, G. G.Langmuir 1992,8,2124. (22)Abuin, E.B.;L h i , E.J. Phys. Chem. 1988,87, 6166.
Selective Adsorption of Ions
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a -8.8
-
4.8
-
la IBrl
-8.8
-8.7
-8.5
-8.8
In t a l
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1 //
4
7
-Q.O
i -9.4
I
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-8.8
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Figure 1. Decay of counterion concentrationsin the bulk solution of a flotation experiment: (a) CNaCll= 2 X lo-' M (b) [KNOd 2 X lo-' M;(c) [KI] = lo-'M (d) [KClOs] = 2 X lo-' M. The upper right point describesthe initial concentrations,which decay to the lower left point. The slope is the selectivity coefficient, K.. Experiments were foamed for 70 min at 25 O C , and [CTAB] = 2 X 10-4 M,with added salts.
One impediment to the interpretation of the experimental data using eq 7 is the possibility that K , depends on the bulk solution composition or on the surface charge density. Both of these change during the course of a flotation experiment as the concentrations of surfactant and counterions decrease. It is therefore desirable to know how K, responds to changes in the ionic strength,
counterion composition, and surface excess of surfactant. This was investigated in the bromidefchloride system. KF,was measured in a series of solutions of CTAB and CTAb at a constant ionic strength of 8 X lo-' M, but with a varying mole fraction of the bromide salt. The results obtained (Figure 2) show that, within experimental error, there is no dependence of selectivity on the relative amounts of the competing counterions. In another set of experiments, we measured K& in CTAB/CTAC solutions over a range of ionic strengths. One of these measurements was made above the critical micelle concentration. The presence of micelles did not affect the results. The results are presented in Figure 3 and show the selectivity to be independent of the solution concentration. The dependence of K& on surface charge density was probed by comparing the selectivity measured in a solution containing CTAB and NaCl at concentrations of 10-4M each, with that measured in a solution containing CTAB and CTAC also at concentrations of lo-' M each. The ionic strengths and the counterion composition are identical in both of these solutions, but because the concentration of the surface active moiety differs, the surface excess is greater in the second solution. The surface exceeaes in these systems may be estimated from adsorption isotherms obtained in surfactant solutions with added salt by Okuda et d.lDJo Their results show that the surface tension of CTAB does not change in the presence of lo-' M added NaBr. Solutions of another surfactant, dodecyldimethylammonium bromide, showed no change in their surface tensions until 103 M NaCl was added, indicating that substitution of bromide by chloride ions has little effect on surface tension at these low concentrations. Therefore we apply their adsorption isotherm for pure CTAB to our two experimental systems. The initial surface excesses were calculated to be 1.4 X 10-8 and 2.0 X 10-8mol m-2,respectively. The values of the selectivity coefficienta measured in the two solutions were identical. In other experiments in which the solution composition was not controlled, the range of the surface excess has been extended to the maximum adsorption density of CTAB, 3.5 X 10-8 mol m-2, without showing any change in the selectivities. To test the validity of the multiplicative relationship, we foamed a solution containing 2 X lo-' M CTAB, 2 X 10-4 M KNO3, and 4 X 10-4 M NaC1. The selectivity coefficients for each pair of ions were measured. We found KE:, = 3.27, K::: = 1.44, and KtE3 = 4.70. If the mdtiplicative relationship is correct, the latter two values would yield K& = 3.26, in excellent agreement with the directly measured value, although the absolute values are marginally greater than those given in Table 1. Morales et al. also observed multiplicativity among the selectivity coefficientarelating chloride, sulfate, and oxalate exchange, although their measurementswere made on three separate binary systems, rather than direct measurements in a ternary system.l' All of the measurementscited in Table 1were performed using quaternary ammonium surfactants. Lindman and Wennerstr6ms have suggested that ion specific interactions at micella interfaces should be particularly marked for alkylpyridinium head groups, due to the possibility of charge-transfer interactions with halide counterions. In order to determine whether the nature of the surfactant head crow has anv effect on the selectivitv. we also measured I?, usingcitylpyridinium surfactants.-bupEcate runs using CPC and CPB, each at an initial concentration (23) Lindman, B.;Wennerstrbm, H.Top. Current Chem. 1980,87,1.
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Morgan et al.
Table 1. Exwrimental8electivitu Conrtantr and Literature Valuer ~~
Ga Gid G2
initial [XI,X10( M
initial [Br], X10( M
K., %
varioue 2.00
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.
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.
,
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,
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Figure 2. Dependence of the bromide/chloride selectivity in the mole fraction of bromide in the bulk phase. The solutione contained varying relative amounts of CTAB and CTAC at a constant total ionic strength of 8 X lo-(M. The horizontal error bars indicate the change in composition that occurs during the
experiment.
0.000
0.001 lonlc Stmngth I
0.002
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Figure 3. Dependence of the bromide/chloride selectivity on ionic strength. The highest concentration tested was above the cmc.
of 4 X lo-' M, gave an average value of K& of 3.11, in agreement with the values obtained by using the cetyltrimethylammonium surfactants. Since K& for both surfactant types are essentially the same, we have no evidence to support Lindman and Wennerstrom's contention, at the air solution interface. For the simple ionic surfactants studied, counterion specificity is entirely determined by the nature of the competing ions. This means that the reported selectivities are very general and may be applied to a wide variety of surfactant systems. The independence of the selectivities toward solution and surface composition also implies that the flotation technique is extremely robust toward the presence of trace contaminants, obviating the need for rigorous purity standards.
Comparison with Micellar Experiments The selectivitycoefficients reportad here agree very well with other results for the air/solution interface and are in fair agreement with those observed in micellar systems. Morales et aZ.l5alsofound fair agreementwith the micellar
~
Ke, % Shinoda and Fujiharas Grieves and Kyle11 1.2 1.3 1.59 i 5%
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micellar measurementa 2.666.311 l.lL1.37'
2.21 f 3.2% 5.85 f 9.4%
3.1: 13"
exchange constants. It is important to note, however, that the micellar counterion competition competition experiments differ from our experiments in several ways. Typically the micellar experimentsare carried out at higher ionic strengths, and usually one of the competing counterions is only present in trace quantities. We have demonstrated, however, that neither of these variables measurably affect the bromidelchloride competitive adsorption equilibrium, so we do not expect any differences between selectivities in the micellar and airlsolution interfaces to arise from different solution compositions. A more interesting difference between the two systems is the high degree of curvature and generallyhigher surface charge densities that characterize the micellar interface. The major consequence of the micellar curvature in consideration of the disposition of the counterions is in the form of the electric field near the charged interface. We have shown for our system,and it is known from studies of mixed ioniclnonionic micelles17 for the micellar solutions, that selectivity is not influenced by the average surface charge density, so differences in the electrostatic properties of plane and curved surfaces are not expected to affect the selectivity. Consequently, the origin of selectivity in micellar solutions should be the same as that at the airlsolution interface. One difficulty remains in the comparisonof the flotation selectivities to micellar selectivities. All the methods of measuring the selectivity in micellar systems have so far relied on some sort of local probing of the micellar interface, interpreted in terms of a model that distinguishesexplicitly between bound and free counterions. Techniques such as fluorescence quenching ex~eriments,4JJ~*~~ in which the fluorophore is located in the plane of the surfactant head groups, and the reactive counterion trapping experimentaz* can only respond to counterions at the immediate micelle interface, while the ions in the diffuse part of the double layer escape detection. Likewise,the modification of reaction rates in micellar solutions is interpreted in the pseudophase model which partitions the counterions into two populations, with the reaction rate only responding to the bound population in the case of micellized reactants,9*6or the unbound population in the case of aqueous reactions.5 In effect, these experiments measure a local selectivity coefficient, defined as
where I'i,st is the>surfaceexcess of bound counterions in a putative Stern layer, explicitly excluding counterions in the diffuse layer. The selectivity coefficient defined by eq 8 therefore depends on the way the Stern layer is defined, and in this respect the problem of measuring a micellar selectivity (24) Nmimento, M. Go;Miranda, 1986,90, 3366.
S.A.; Nome, F.J. Phyu. Chem.
Selective Adsorption of Ions
coefficient is analogous to the problem of measuring the degree of counterion association of a micelle, j3. It is known that values of j3 are quite sensitive to the measurement technique used. Gunnarson e t u L , ~for example, have classified such techniques into (a) spectroscopic methods, which respond to counterions immediately present at the micelle interface, (b) methods which measure transport properties such as conductance or diffusion techniques, and respond to a somewhat larger Stern layer, and (c) methods which measure properties of the bulk phase, from which the degree of binding is inferred. If such methods are used to measure interfacial selectivities, the values obtained should be similarly variable, with the most localized measurements yielding the largest values for Ki, since we assume the ion specific interactions are very short range. Certainly, most reported micellar selectivities are based on local interfacial probing and are larger than the values we obtain. In contrast the selectivity coefficient measured in our experiments is defied simply in terms of the surface excess of counterions, which is the only unambiguous way to describe the selectivity. There is no appeal to a thermodynamic distinction between bound and unbound counterions, a distinction which has been critized in the case of micelles.26*2sRather, eq 3 partitions the counterions into two populations in a manner which is consistent with the various conventions for the thermodynamics of surfaces. Determination of K, by flotation does not require the degree of association to be a free parameter as the interpretation of the experiment is independent of any microscopic model of the double layer structure. ~
(25)Gunnameon, G.; Jbnsson, B.;Wenneretrcm, H.J. Phys. Chem. 1980,84,3114. (26)Evans, D.F.;Ninham, B. W.J. Phya. Chem. 1983,87, 5025.
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Furthermore, both the independence of K, of solution composition and the multiplicativity of the selectivity constants indicate that only the relative strength of counterion-substrate interactions contributes to the selectivity, rather than counterion-counterion interactions in the surface. In this sense the mixing of counterions in the surface is ideal, suggesting that in equating the selectivities obtained in our experiments with the thermodynamic equilibrium constant, no error results from the use of surface excesses rather than thermodynamic interfacial activities. It is not yet certain whether the results from micellar experiments can be interpreted this way, particularly since selectivity has recently been reported to be a function of the solution composition.2
Conclusion We have presented a convenient technique for measuring the selectivity of air/solution interfaces for counterions and have shown the invariance of selectivity with solution and surface composition and surface charge density. Replacing the quaternary ammonium surfactant with a pyridinium surfactant did not affect the selectivity. Determination of the selectivity constant is model independent, and we argue that the quantity that is measured is numerically equal to the thermodynamic ion exchange equilibrium constant. Acknowledgment. We thank BHP and the Department of Industry, Training and Commerce for the award of a PIRA scholarship to J.D.M. and Dr. Kevin Galvin and Mr. Malcolm Engel of BHP Research for support and helpful discussions.
