water interface

191

J. Electroanal. Chem., 345 (1993) 191-203 Elsevier Sequoia S.A., Lausanne

JEC 02373

Successive complex formation of multivalent ions with octaethylene glycol dodecyl ether at the nitrobenzene/water interface Takashi Kakiuchi Department of Agricultural Chemistry, Faculty of Agriculture, Kyoto hive&@, Kyoto 606 (Japan)

Sakyo-ku,

(Received 17 June 1992)

Abstract Transfer of Mg’+ and La3+ ions . across the nitrobenzene (NR)/water (W) interface facilitated by octaethylene glycol monododecyl ether (C12ES) has been studied using cyclic voltammetry under conditions where the concentration of C12E8 in NRf bcc1rE8 NB 1 is much smaller than the concentration of Mg2+ or La3+ ions in W. The shape of the voltammograms is markedly dependent on b~&s. When bNFl cclm > 1 mM, the voltammograms exhibit a clear shoulder on both forward and reverse scans of the potential For L.a3+ transfer, the convolution voltammograms exhibit two-step waves. The dependence of wave shape on ligand concentration indicates the successive formation of 1: 2 (metal: ligand) and 1: 1 complexes in the vicinity of the interface. The first wave is ascribed to the formation of the 1:2 (metal : ligand) complex, and the second wave to the 1: 1 complex. This double wave merges into a single peak, when “c,!:~ < 0.2 mM. From the midpoint potentials and the concentration dependence of the peak separation, stability constants for the 1: 1 and 1: 2 complexes in NR are determined to be log,&, /M-l) = 8.8 and log&,/M-‘) = 3.3 for Mg*+ and log,&, /M-l) = 11.0 and log,& /M-r) = 4.4 for La3+, respectively.

INTRODUCTION

The electrochemical approach has been shown to be a convenient and reliable means to study facilitated ion transfer across an oil(O)/water(W) interface. In most of the studies the condition employed has been that the ligand concentration in O(bcE) is smaller than the concentration of ions in W<“cE:), so that the ion transfer current is limited by diffusion of ligand molecules in 0 [l-161. When only one complex species having a p : s (metal to ligand) stoichiometry is assumed to exist in 0, the analysis of current-potential curves is straightforward [2,17]. However, when the difference in stability constant among complexes with various 0022-0728/93/$06.00

0 1993 - Elsevier Sequoia S.A. Ah rights reserved

192

metal to ligand ratios is not very large, more than one complex species can participate simultaneously in the ion transfer process within the potential window, resulting in distortion of the current-potential curves in comparison with those corresponding to the formation of a single complex species [181. Broadening of the peak separation in cyclic voltammograms due to 1: 2 complex formation has been reported in the facilitated ion transfer of Li+ and Ca*+ ions with octaethylene glycol dodecyl ether (C12E8) and hexaethylene glycol dodecyl ether (C12E6) [19] and in the transfer of Ba*+ ions facilitated by dibenzo-l&crown-6 [20] at the nitrobenzene (NB)/W interface. If the facilitated ion transfer is studied under the condition that boo z+‘cz, the complexity in interpreting voltammograms due to successive compltx formation can be avoided [21]. However, besides practical reasons in favor of the condition, “CL-ZK bc E 1181,there are additional advantages in studying the facilitated ion transfer at “CL-x:c;. First, stability constants of multiple complexation equilibria may be determined simultaneously from voltammograms [18]. Secondly, useful information in elucidating the mechanism of facilitated ion transfer may be obtained. In the present communication, we show that a double wave appears on cyclic voltammograms recorded under the condition boo +zbcE, in facilitated transfer of Mg2+ and La3+ ions with C12E8 at the NBIW triterface and that the double wave can be ascribed to successive complex formation of 1: 1 and 1: 2 complexes in the vicinity of the interface. EXPERIMENTAL

High-purity C12E8 was purchased from Nikko Chemicals (Japan) and used without further purification. The purification of nitrobenzene, water and tetrapentylammonium chloride0’PnACl) and of the preparation of tetrapentylammonium tetraphenylborate(TPnATPB) have been described elsewhere [22]. Other chemicals used were of reagent grade. Cyclic voltammograms were recorded using a computer-controlled voltammetric apparatus [23]. A cell with a flat nitrobenzene/water interface (of area 0.0968 cm*) was employed [22]. A positive feedback method was used for the compensation of potential drop due to solution resistance [22]. The electrochemical cell (I) is represented by I

II

III IV 0.02 M 0.1 M TPnATPB

Ag AgCl TPnACl (water)

+ a M C12ES (nitrobenzene)

V bM MCI,

VI

VII ’

AgCl Ag

(1)

c M MgSO, (water)

The potential of the right-hand side terminal of the cell with respect to the left is denoted as E. After each set of cyclic voltammetric measurements, a few tenths of a millilitre of concentrated tetraethylammonium chloride solution was added to the solution (phase V) and a cyclic voltammogram was recorded, whose half wave potential was employed as a reference of the potential in comparisons between

193

several data sets and in calculating the stability constant. The current was taken to be positive when positively charged species pass through the interface from the aqueous to the nitrobenzene phase. Cyclic voltammograms were analyzed after subtracting the base current which was recorded in the absence of C12E8. At the commencement of recording the base current, an abnormally large positive current amounting to 100 PA was often observed. This current was accompanied by vigorous motion of the aqueous solution adjacent to the interface, and typically lasted several tens of seconds to a few minutes and then subsided suddenly. A scanning of E was often useful to stop the motion of the solution. A cyclic voltammogram recorded after the cessation of abnormal current was usually shifted upward by a few tenths to 1.5 PA. This residual current seemed to be constant over the potential range for cyclic voltammetry measurements. The abnormal current never occurred in the presence of a surfactant, C12E8, in NB, suggesting a similarity in its origin to polarographic maxima 1241.Simple subtraction of the base current from voltammograms of facilitated ion transfer therefore often resulted in a downward shift of the voltammogram. The voltammograms shown below were corrected, if necessary, so that the current at the beginning of the cyclic voltammograms is equal to zero. All measurements were made at 25°C. RESULTS AND DISCUSSION

Figures 1 and 2 show cyclic voltammograms of the transfer of La3+(a = 0.001, b = 0.1, c = 0 in cell (I), Fig. 1) and Mg2+ (a = 0.001, b = 0.1, and c = 2, Fig. 2) at scan rates (v) of 10, 20, 50, 100, and 200 mV s-l. A clear shoulder developed in La3+ ion transfer. A similar shoulder was also discernible, though less conspicuous, in Mg2+ ion transfer. In both cases, the peak potential in the forward scan did not depend appreciably on v. Also, the potential at the shoulder did not vary with Y in La3+ ion transfer. Midpoint potentials E, and peak separations AE, were also virtually independent of u. Figures 3 and 4 show the effect of b~&a on the wave shape at Y = 20 mV s-l when b~NB c12ns was decreased from 2 mM to 0.1 mM. In both La3+ and Mg2+ transfers, the decrease in b~&a resulted in narrowing of AE,. Eventually, at bCNB c12E8 = 0.1 mM, the shoulder disappeared completely. This is more clearly seen in the insets of Figs. 3 and 4. E, remained constant over this range of b~FfiE8 within f 1 mV. When b~NB ci2n8 & 0.5 mM, the peak current for the forward scan NB (I,> was proportional to &‘, e.g. at b cc1zE8 = 1 mM, Y = 10-1000 mV s-l for Mg2+ transfer and v = lo-500 mV s-’ in La3+ transfer. The current at the shoulder in L.a3+ transfer was also proportional to ~~1~. Thus the observed facilitated transfer is a diffusion limited process for both La3+ and Mg2+ ion transfers. At b~NB olzEs = 0.1 mM, the upward deviation of ZP from the straight line in ZP vs. y112 plots was observed with increasing Y in both cases (curve 1 in Fig. 5 and curve 1 in Fig. 6). The magnitude of the deviation is more significant in La3+ ion transfer. A similar deviation from the diffusion-limited current has been found in

1

-40 OW

0 .6

E/V Fig. 1. Cyclic voltammograms of La3’ ion transfer from an aqueous 0.1 M LaCl, solution to a nitrobenzene solution of 0.1 M TF’nATPB facilitated by 1 mM C12E8 in nitrobenzene at 25°C. Scan rates were lO(curve 11, 20 (curve 21, 50 (curve 31, 100 (curve 4) and 200 (curve 5) mV s-l.

the transfer of alkali and alkaline-earth metal ions facilitated by C12En (n = 6 and 8) at the NB/W interface [19] and has been shown to be due to the adsorption of C12En at the interface [19,25-281. The observed deviation in Figs. 5 and 6 is likely to be attributable to the contribution of adsorbed C12E8 molecules to the measured current. In an adsorption wave in the transfer of alkali and alkaline-earth metal ions, the adsorption effect becomes greater with increasing hydrophilicity of the ions to be transferred, which is in agreement with the observed difference in the deviation in Figs. 5 and 6. The observed dependence of the shape of voltammograms of b~&&8 can be interpreted by assuming the presence of a labile equilibrium between M, ML and ML, at the nitrobenzene side of the interface [183, M+L=ML

(1)

ML+L=ML,

(2)

where M and L denote a metal ion and a ligand. We have shown theoretically that in this case the double wave merges into a single wave with decreasing concentra-

195

60

40

20 3 \ l-l

0

-20

-40-----J 0.1 0.2 0.3 0.4 0.5 0.6 E/V Fig. 2. Cyclic voltammograms for transfer of Mg *+ ion facilitated by C12EB in nitrobenzene. Scan rates are the same as in Fig. 1.

tion of ligand in 0. The constancy of the midpoint potential over the change of bit& also agrees with the theoretical prediction [18]. Figure 7(b) shows the convolution voltammogram calculated from the cyclic voltammogram recorded at 0.1 M La3+, b~gr& = 2 mM and Y = 20 mV s-l shown in Fig. 7(a). The convolution voltammogram in the reverse scan traced back along the voltammogram in the forward scan, again indicating that the ion transfer is virtually reversible. The first and second waves have similar wave heights; the limiting value of m for the first step is about 40 PC s-l/*, while that for the second wave is about 80 /.Lcs -‘/* . When a facilitated ion transfer is accompanied by the formation of a single r : s (metal : ligand) complex species, the limiting value of m, md, is given by

El md = ziFA( r/s)(

D0,)“2bcL

(3)

where zi is the charge on ion i in signed units of electronic charge, F is the Faraday constant, A is the area of the interface, and 0: is the diffusion coefficient of the ligand in 0. If the current of the first wave in La3+ transfer is mainly carried by the 1: 2 complex, the diffusion coefficient can be estimated using

196

H

L-

0.1

/I_--

0.2

0.3

0.4

0.5

0.6

E/V Fig. 3. Variation of wave shape with the C12E8 concentration in NB for La3+ transfer. Scan rate is 20 mV s-‘. Concentrations of C12E8 in NB are 0.1 (curve l), 0.5 (curve 21, 1.0 (curve 3) and 4 (curve 4) mM. The inset shows a magnified view of curve 1.

eqn. (1). The observed wave height for the first wave leads to a value of 2.0 X 10e6 cm2 s-l for the diffusion coefficient of C12E8 in NB, when zi = 3, r = 1 and s = 2 in eqn. (3). This value is in good agreement with the literature values of 2.1 X 10e6 cm2 s-l and 2.2 x 10e6 cm2 s-l [19]. This estimation supports the assumption that the current-carrying species in the first wave is the 1: 2 complex. According to the theory of successive complex formation [18], the current of the second wave is carried by the 1: 1 complex and is limited by the backward diffusion of ML, species from the bulk of the 0 phase to the interface. The wave height of the second wave is hence determined by the diffusion coefficient of ML, in 0. In this case, the back diffusion of one ML, molecule supplies two ligand molecules, which are in turn used to form two ML molecules. Calculated convolution voltammograms, when the diffusion coefficient of L in 0 is equal to that of ML,, are shown in Fig. 8 at several different values of 6,/b,, where b, = KJbcL) and 6, = K2(b~[)2. K, and K, are the stability constants of ML and ML, in 0. The values of other parameters are A = 100, lM = lMr_= 0.5, w bcL, ’ “c: being the bulk concentration of the ion &I_ = 5‘MLz = 1.0, where h = bc~/

.

197

40

20

3 \O H

-20

0.1

0.2 0.3 0.4 0.5 0.6 E/V

Fig. 4. Variation of wave shape with the ClZE8 concentration in NB for Mg’+ ion transfer. Concentrations of C12E8 in NB are 0.1 (curve l), 0.5 (curve 21, 1.0 (curve 3), 2.0 (curve 4) and 4 (curve 5) mM. Scan rate is 20 mV s-‘. The inset shows a magnified view of curve 1.

in W. &(i = M, L, or ML) and &(i = ML or ML,) are the ratios of the diffusion coefficient of i in 0 to that of M in W and to that of L in 0, respectively. Calculated curves show two waves with similar wave heights. The agreement with the observed wave heights suggests that the diffusion coefficient of the La3+(C12E8), complex is not very different from that C12E8 in NB. The variations of AE and ZP with the logarithm of “cF& are shown in Fig. 9. From these plots, K, can be estimated roughly as follows. According to the theory of cyclic voltammetry in the presence of successive formation of 1:2 and 1: 1 complexes [X3], the magnitudes of AE, and I,, values are determined by the magnitude of b,. In Fig. 9, the values of lr&$&s/M) where AE, = 50 mV are -8.6 for La3+ and -6.2 for Mg ‘+ . Conversely, in the theoretical curve in the presence of 1: 1 and 1: 2 complex formation shown in Fig. 6 in ref. 18, hr(b,) = 1.6 at zi A E, = 150 mV and 0.2 at zi AE, = 100 mV. Hence, for La3+ ion transfer, In(K,/M-‘1 = 1.6 + 8.6 or log,,(K,/M-‘1 = 4.4. Similarly, for Mg’+ ion transfer, log,&K,/M-‘I= 2.8.

198

80

60 "y \ H"

40

20

0

0

20

10 “J/f

,

30

40

mVlla s-112

Fig. 5. Plots of peak current in the forward scan against square root of scan rate for the transfer of La3+ ion facilitated by C12E8. Concentration of C12E8 in NB is 0.1 (curve 1),0.5 (curve 2), 1 (curve 3) and 2 (curve 4) mM.

K, can be calculated from E, values even in the presence of 1: 2 complex formation, unless the waves for 1: 1 and 1: 2 complex formation are completely separated from each other [18]. First, a value of the standard ion transfer potential of La3+(A&(po,> was estimated by comparing a cyclic voltammogram for a base solution containing 0.1 M LaCI, with that for 0.1 M MgCl,, for which a value of G&%&g is known [29]. Comparison of the two voltammograms at the foot of the final rise at the positive extreme of the potential gave an estimation of 0.35 V for Arw,,cpo,.From this value and E, values, log(K,) was estimated to be lIO.0 for La3+ and 8.8 for Mg ‘+ . In this estimation, diffusion coefficients of L and ML are assumed to have the same value in NB. The log,,(K,/M-‘1 values thus estimated are smaller than the corresponding values of 10.7, 12.3 and 13.4 for Ca*+, Sr*+ and Ba*+ ions, respectively, in the same system [19]. Apparently, the more hydrophilic the ion, the smaller the K, value. Voltammograms were calculated based on the model of successive complex formation of ML and ML, in 0 [18]. Figures 10(a) and (b) show calculated voltammograms at two values of ‘c&s, 2 mM (Fig. 10(a)) and 0.1 mM (Fig.

199

80 -

60 “y \ l-7

40

yl/2

,

.vl/2

s-1/2

Fig. 6. Plots of peak current in forward scan against square root of scan rate for the transfer of Mg*+ ion facilitated by C12FX Concentration of C12E8 in NE3 is OA (curve l), 0.5 kwve 21, 1 (curve 3), 2 (curve 4) and 4 (curve 5) mM.

10(b)), using K, and K, values estimated above for the La3+ ion transfer. Corresponding changes in surface concentrations of L, ML and ML,(‘cF: i = M, ML or ML,) relativized by “CL are shown in Figs. lo(c) (“~:&a = 2 mM) and 10(d) NB =2mM ‘coM,_is small at the maximum value of CbCNB o oc12E8= 0.1 mM). When b cc12EB cMML2, where the first wave approaches a’limiting current. This is consistent with the fact that a value of @Ens estimated above from the limiting value of the first wave approximately agreed with the literature value. On the contrary, ‘ckr_ exceeds ‘coML2at the maximum when “cf = 0.1 mM. In other words, the formation of ML, is significant only at the foot of the voltammogram. Double waves similar to those in Figs. l-4 have also been found in the facilitated transfer of La3+ and Mg2+ ions with hexaethylene glycol dodecyl ether (data not shown). The fact that broadening of AE, in cyclic voltammograms probably due to the participation of 1: 2 complexes occurs in the facilitation of both acyclic and cyclic ligands [20] suggests that the successive complex formation in the vicinity of the interface is a ubiquitous phenomenon in the transfer of relatively hydrophilic ions across an oil/water interface, e.g. the extraction of

200

80-

(b)

0.4

0.5

0.6

E/V Fig. 7. (a) Cyclic voltammogram of transfer of La3+ ion in the presence of 2 mM C12E8 in NB. Scan rate is 20 mV s-l; (b) calculated convolution voltammogram.

heavy-metal ions with a multidentate ligand [30-431. It has been shown in various solvents that poly(ethylene) glycol derivatives form a 1: 1 complex with metal ions when the oxyethylene unit is shorter than ca. 10, while n : 1 (metal to ligand, n 2 2) complex formation prevails when the oxyethylene unit is long enough, e.g. 30 units 144,451. The present study demonstrates that 1: 2 complex formation takes part in the facilitated ion transfer process. The structure of the 1: 2 complex is expected to be significantly different from that of it : 1 (n 2 1) complexes, which take a helical

0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 zi(E

-

E”)

/ V

Fig. 8. Theoretical convolution voltammograms when successive complex formation between ion (M) and ligand (L) having the stoichiometty of ML and ML, is assumed. Values of parameters: A = 100; lM = [1uIL= 0.5; thnL = tMLI = 1.0; b, = 10 lo; b, = 5 X 10’ (curve 11,10’ (curve 2),2X 10” (curve 3) and 5 X 10” (curve 4).

201

-10

I -9

I

I -8

I

-7

-6

In+&IM)

Fig. 9. Variation of peak separation and peak current in the forward scan with concentration of C12E8 in NB for the transfer of La3+ (0, A) and Mg2+ (0, A).

k

-0.4 .... '5. i

..._ 0.8 \ 0.6

\ 1

"2

-1.0

-0.8

: : :

;\ i2 i : : :

/2 : :

-0.6

:*" ( d ) : :

-0.4 r;(E

-0.8 E’)

-0.6

-0.4

/ V

Fig. 10. Calculated voltammograms based on the model of successive complex formation of ML and ML, at two values of “ct, (a) 2 mM and (b) 0.1 mM, using K, and K, values estimated above for La3+ ion transfer: logra(K, /M-l) = 11 and log,,,(K, /M-l) = 4.4. ,y is dimensionless current. Corresponding change in surface concentrations of L, ML and ML,; (c) “c[ = 2 mM and (d) “c! = 0.1 mM. f; &;/bc;; I - M (curves 11, ML (curves 2) or ML, (curves 3). For values of other parameters, see Fig. 8.

202

conformation of poly(oxyethylene) derivatives incorporating metal ions inside the helix [46]. Further structural studies of ion-C12En complexes are of great interest. ACKNOWLEDGMENT

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