Langmuir 1999, 15, 2575-2579
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Complexation Kinetics of 8-Quinolinol Derivatives with Ni(II) and Zn(II) at the 1,2-Dichloroethane-Water Interface As Studied by Electrolyte Ascending Electrode Polarography Takeshi Shioya, Seiichi Nishizawa, and Norio Teramae* Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan Received November 3, 1998. In Final Form: January 21, 1999 Complexation kinetics of 8-quinolinol (HQ) and 5-alkyloxymethyl-8-quinolinols (HCnQ; n ) 1, 2, 3, 5, 8) with metal ions M2+ (M ) Ni, Zn) at the 1,2-dichloroethane (DCE)-water interface is analyzed by dynamic interfacial tensiometry and electrolyte ascending electrode polarography. Polarographic measurements show that the chemical species formed at the interface is a cationic 1:1 complex (M(Q)+ or M(CnQ)+). For Zn2+, the interfacial complexation is very fast, and its kinetics is controlled by the diffusion of ligands from the bulk DCE phase to the DCE-water interface. On the other hand, for Ni2+, the kinetics is interfacial reaction-controlled and is significantly affected by the alkyl chain length of the chelating reagents.
1. Introduction Complexation reactions at liquid-liquid interfaces play an important role in many technological systems such as solvent extraction,1-4 in which interfacial complexation kinetics of chelating reagents has been analyzed by a highspeed stirring method.1 This method, however, is unsuitable for direct monitoring of interfacial reactions. As a direct technique for studying interfacial reactions, dynamic interfacial tensiometry5-7 has been employed. Although this technique can give quantitative information such as interfacial concentration of reaction products and kinetics of reactions, chemical species at the interface cannot be identified explicitly. This disadvantage is expected to be overcome by using both interfacial tensiometry and electrochemical ion transfer measurements.8-13 The latter method can provide powerful insights not only into the identification of reaction products but also into the thermodynamic parameters for the interfacial complexation at polarized liquid-liquid interfaces. In the present paper, 8-quinolinol (HQ) and its derivatives (HCnQ; n ) 1, 2, 3, 5, 8) having various alkyl chain lengths (Figure 1) are used as model compounds for studying interfacial complexation. In bulk aqueous solutions, these 8-quinolinol derivatives show little difference in the thermodynamic and the kinetic parameters such * To whom correspondence should be addressed. Tel: +81-22217-6549. Fax: +81-22-217-6552. E-mail:
[email protected]. ac.jp. (1) Watarai, H.; Cunningham, L.; Freiser, H. Anal. Chem. 1982, 54, 2390. (2) Watarai, H.; Freiser, H. J. Am. Chem. Soc. 1983, 105, 191. (3) Freiser, H. Chem. Rev. 1988, 88, 611. (4) Dietz, M. L.; Freiser, H. Langmuir 1991, 7, 284. (5) Fang, J. P.; Joos, P. Colloids Surf. 1994, 83, 63. (6) Shioya, T.; Tsukahara, S.; Teramae, N. Chem. Lett. 1996, 469. (7) Shioya, T.; Nishizawa, S.; Teramae, N. Langmuir 1998, 14, 4552. (8) Koryta, J. Electrochim. Acta 1979, 24, 293. (9) Yoshida, Z.; Freiser, H. J. Electroanal. Chem. 1984, 179, 31. (10) Samec, Z.; Papoff, P. Anal. Chem. 1990, 62, 1010. (11) Senda, M.; Kakiuchi, T.; Osakai, T. Electrochim. Acta 1991, 36, 253. (12) Girault, H. H. Modern Aspects of Electrochemistry, No. 25; Bockris, J. O., Conway, B. E., White, R. E., Eds.; Plenum Press: New York, 1993; p 31. (13) Shioya, T.; Nishizawa, S.; Teramae, N. J. Am. Chem. Soc. 1998, 120, 11534.
Figure 1. Structures of 8-quinolinol derivatives. n ) 1, 2, 3, 5, 8.
as dissociation constant14 and complexation rate constant15,16 because of less steric hindrance by the alkyl chain. The ligands in Figure 1 are therefore suitable for the investigation of the effect of the chain length on the interfacial complexation kinetics. Another advantage of using these 8-quinolinol derivatives is that their neutral forms are interfacially inactive.17 This means that the ligands dissolved in the organic phase do not adsorb at the liquid-liquid interface, and thus interfacial tension does not change when pH of the aqueous phase is maintained at about 4-10. If the reaction products are interfacially active, a decrease in the interfacial tension is observed as the interfacial concentration of the reaction products increases. In a previous paper,7 the complexation kinetics of HQ and HCnQ with Ni2+ and Zn2+ was analyzed mainly at the heptane-water interface by using dynamic drop volume tensiometry. The decrease in the interfacial tension was caused by the interfacial adsorption of a 1:1 complex (M(Q)+ and M(CnQ)+; M2+ ) Ni2+, Zn2+) formed at the aqueous side of the interface, and the complexation kinetics was governed by the alkyl chain length of the ligands. The purpose of the present paper is to investigate the effects of metal ion and alkyl chain length on the complexation mechanisms at the 1,2-dichloroethane (DCE)-water interface by using electrolyte ascending (14) Ohashi, K. Proceedings of the 10th Colloquium on Inorganic and Analytical Chemistry; Japan, 1993; p 35. (15) Tagashira, S.; Onoue, K.; Murakami, Y.; Sasaki, Y. Bull. Chem. Soc. Jpn. 1992, 65, 286. (16) Tagashira, S.; Onoue, K.; Murakami, Y.; Sasaki, Y. Anal. Sci. 1992, 8, 307. (17) Shioya, T.; Tsukahara, S.; Teramae, N. Chem. Lett. 1997, 695.
10.1021/la9815547 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
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Table 1. Dissociation Constants (Ka) and Partition Coefficients (P) of 8-Quinolinol Derivatives pKa ligand
NH+
OH
log P (DCE-water)
HQ HC1Q HC2Q HC3Q HC5Q HC8Q
4.85 4.80 4.79 4.79 4.85 4.32
9.95 9.42 9.47 9.43 9.45 9.99
2.26 2.56 2.86 3.45 4.52 >5.0
a
Values from ref 14.
electrode polarography,13,18 in addition to the dynamic drop volume tensiometry.7,19 2. Experimental Section Chemicals. Water was doubly distilled and deionized by a Milli-Q Labo system (Millipore) before use. 1,2-Dichloroethane (HPLC grade, Wako Pure Chemical Industries, Osaka, Japan) was used as received. 8-Quinolinol (99%, HQ), 5-octyloxymethyl8-quinolinol (99%, HC8Q), nickel chloride (99.9%), and zinc chloride (99.9%) were purchased from Wako and used without further purification. 5-Alkyloxymethyl-8-quinolinols (HCnQ; alkyl ) methyl (C1), ethyl (C2), propyl (C3), and pentyl (C5))20 and tetrabutylammonium tetraphenylborate (TBATPB)21 were prepared according to the literature. Dissociation constants (Ka)14 and partition coefficients (P) of these compounds are summarized in Table 1. Partition coefficients were determined by UV-vis absorption spectrometry after shaking an organic solution containing the ligands with an aqueous solution of 0.1 M (1 M ) 1 mol dm-3) hydrochloric acid. Dynamic Interfacial Tension Measurements. The dynamic interfacial tension at the DCE-water interface, γ(t) (mN m-1), was measured by a dynamic drop volume method.7,19 Chelating reagents were dissolved in DCE. Droplets of an aqueous solution containing 10 mM divalent metal ion (MCl2; M2+ ) Ni2+, Zn2+) and 70 mM sodium chloride were sent upward, using a syringe pump, into the DCE solution from a glass capillary (0.705 cm o.d.) fixed in the cell. Dynamic interfacial tension was calculated from the volume of droplets as a function of the introduction time. Electrolyte Ascending Electrode Polarography. The compositions of the aqueous and DCE phases were almost the same as those in the interfacial tension measurements except that supporting electrolytes were added for the polarographic measurements. The electrolytic cell is shown below.
(RE 1) Ag/AgCl/0.5 M LiCl//10 mM MCl2 + 0.5 M LiCl (aqueous phase)/x mM HQ or HCnQ + 0.05 M TBATPB (DCE phase)/0.05 M NaTPB + 0.5 M LiCl//0.5 M LiCl/AgCl/Ag (RE 2) where RE 1 and RE 2 denote reference electrodes. Droplets of the aqueous solution moved upward into the DCE solution from a PTFE capillary (0.7 mm i.d.) at a flow rate of 4.2 mg s-1. Potential difference between the aqueous and DCE phases, ∆φ () φw φDCE; mV), was controlled at a scan rate of 2.5 mV s-1 by a conventional four-electrode potentiostat (Hokuto Denko Co. Ltd., Tokyo, Japan, Model HA-501G) and is expressed by
∆φ ) E - ∆φref
(1)
where E (mV) is the potential difference between two reference electrodes (φRE1 - φRE2) and ∆φref (mV) is the sum of all potential differences involved in the cell. The ohmic potential drop of the cell was compensated by using an i-R compensation instrument (18) Samec, Z. Chem. Rev. 1988, 88, 617. (19) Van Hunsel, J.; Bleys, G.; Joos, P. J. Colloid Interface Sci. 1986, 114, 432. (20) Kolobielski, M. J. Heterocycl. Chem. 1966, 3, 275. (21) Kakutani, T.; Nishiwaki, Y.; Osakai, T.; Senda, M. Bull. Chem. Soc. Jpn. 1986, 59, 781.
Figure 2. Dynamic interfacial tension at the DCE-water interface for Zn2+. Aqueous phase: 10 mM ZnCl2 + 70 mM NaCl (pH 5.3-5.4). DCE phase: 2.5 mM HQ (b) or HC8Q (O). Table 2. Equilibrium Interfacial Tension (γe) at the DCE-Water Interface for the Complexation with Zn2+ ligand
γe/mN m-1
ligand
γe/mN m-1
HQ HC1Q HC2Q
28.1 ( 0.2 28.1 ( 0.2
HC3Q HC5Q HC8Q
27.9 ( 0.2 27.8 ( 0.1 27.9 ( 0.1
(Hokuto Denko, model HI-203). The electrical current related to the transfer of cations from the aqueous phase to the DCE phase was defined as positive. All experiments were carried out at 298 ( 0.5 K.
3. Results and Discussion Interfacial Complexation Kinetics for Zn2+. Dynamic interfacial tension was measured for the DCEwater interface, where the aqueous phase is 10 mM ZnCl2 containing 70 mM NaCl (pH 5.3-5.4) and the DCE phase is 2.5 mM HQ or HCnQ. The results for HQ and HC8Q are shown in Figure 2. The broken line denotes the interfacial tension without ligands in the DCE phase (28.5 mN m-1). Under the present experimental conditions, hardly any of these ligands are distributed into the aqueous phase. The complexation of the ligands with Zn2+ therefore takes place only at the DCE-water interface. In Figure 2, a slight decrease in the dynamic interfacial tension is observed when HQ or HC8Q is added to the DCE phase. Since both HQ and HC8Q do not show interfacial adsorptivity, this decrease indicates that the formation of complexes and their interfacial adsorption occur at the DCE-water interface. Interfacial concentration of the complex is zero at t ) 0, and thus the initial interfacial tension should be 28.5 mN m-1 even in the presence of the ligands. However, the interfacial tension is almost constant in the time range of 0-30 s (28.1 ( 0.2 mN m-1 for HQ and 27.9 ( 0.1 mN m-1 for HC8Q). This indicates that the interfacial complexation between these ligands and Zn2+ is very fast and that extraction equilibrium is immediately attained at the DCE-water interface. Similar profiles of the dynamic interfacial tension are obtained for other ligands. The equilibrium interfacial tension for these ligands, γe (mN m-1), is summarized in Table 2. It is easily seen that there is no significant difference in γe irrespective of the alkyl chain length. This
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Figure 3. Ion transfer polarogram for Zn2+: aqueous phase, 10 mM ZnCl2 + 0.5 M LiCl; DCE phase, 2.5 mM HC5Q + 0.05 M TBATPB; dotted line, 0 mM HC5Q.
fact also supports that the extraction equilibrium is immediately attained at the DCE-water interface. Although dynamic interfacial tension profiles give kinetic information on interfacial complexation as discussed above, chemical species formed at the interface cannot be identified. To clarify the chemical species produced by interfacial complexation, the electrolyte ascending electrode polarograms were measured for the transfer of the complexes across the DCE-water interface, where the aqueous phase is 10 mM Zn2+ and the DCE phase is 2.5 mM HQ or HCnQ. The aqueous and DCE phases contain 0.5 M LiCl and 0.05 M TBATPB, respectively, as supporting electrolytes. Figure 3 shows the ion transfer polarograms for HC5Q. The cathodic wave at E < -480 mV and the anodic wave at E > -170 mV are related to the transfer of TBA+ and TPB- from the DCE phase to the aqueous phase, respectively. Though only the wave connected with transfer of TPB- from the DCE phase to the aqueous phase is observed in the absence of HC5Q (dotted line), a new anodic wave appears on addition of 2.5 mM of HC5Q to the DCE phase (solid line). This appearance indicates that the reaction product formed at the DCE-water interface is a cationic complex, not a neutral one such as Zn(C5Q)2. Liu et al.22 proposed a mechanism in which metal ions were transferred across the DCE-water interface as a divalent cationic complex (M(HQ)22+). On the other hand, Sawada and Osakai23 suggested that the polarographic current was caused by the ion transfer of two species, that is, a 1:1 complex (M(Q)+) and protonated 8-quinolinol (H2Q+). In contrast to these studies, the present experiments were done with the concentration of metal ions in the aqueous phase being larger than that of the ligands in the DCE phase. Also, the dynamic interfacial tension measurements showed that the interfacial complexation was very fast. From the assumption that the reaction product is a 1:1 complex (M(Q)+ or M(CnQ)+) and that the reaction is controlled by the diffusion of these ligands from the bulk DCE phase to the DCE-water interface, the following equation is applicable:12
E ) E1/2 +
i 2.303RT log zF il - i
(2)
where E1/2 (mV) is the half-wave potential, z is the charge number, and il (µA) is the limiting current. (22) Liu, X.; Lu, X.; Lin, S. Analyst 1994, 119, 1875. (23) Sawada, S.; Osakai, T. Analyst 1997, 122, 1597.
Figure 4. Relationship between E and log{i/(il - i)}. Same conditions as in Figure 3.
Figure 5. Schematic diagram of the ion transfer mechanism for Zn2+ complexes. K is the stability constant for the interfacial complexation.
The relationship between E and log{i/(il - i)} is shown in Figure 4, where the electrical current was corrected for the base current (dotted line in Figure 3). Figure 4 shows a linear relationship between E and log{i/(il - i)} with a slope of +70 mV. This result indicates that the complex is a monovalent 1:1 complex, that is Zn(C5Q)+, and the complexation kinetics is diffusion-controlled. Other ligands were also studied in the same way as for HC5Q, with same results; the ion transfer species is Zn(Q)+ or Zn(CnQ)+ and the kinetics is diffusion-controlled. The schematic diagram of the ion transfer kinetics for Zn2+ is illustrated in Figure 5, taking the results stated above into consideration. The cationic 1:1 complexes formed at the DCE - water interface are transferred to the DCE phase, accompanied by the release of a proton to the aqueous phase. The interfacial complexation is quite fast and its kinetics is controlled by the diffusion of the ligands from the bulk DCE phase. On the basis of this reaction scheme, the stability constant, K, for the interfacial complexation is expressed by24
E1/2 )
(
)
[H+]w RT ln +C zF K[Zn2+]w
(3)
where C (mV) is a constant involving the partition coefficients both of a free ligand and of a 1:1 complex and is independent of the alkyl chain length of the ligands. (24) Lin, S.; Freiser, H. Talanta 1992, 39, 919.
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Table 3. Half-Wave Potential (E1/2) for the Ion Transfer of Zn2+Complexes ligand
E1/2/mV
ligand
E1/2/mV
HQ HC1Q HC2Q
-207 ( 4 -206 ( 3 -207 ( 5
HC3Q HC5Q HC8Q
-211 ( 1 -210 ( 1 -202 ( 3
The half-wave potential for the ion transfer of Zn(C5Q)+ is estimated as -210 mV from the intercept of the regression curve in Figure 4. The half-wave potentials for the ion transfer of Zn(Q)+ and Zn(CnQ)+ are summarized in Table 3. It can be immediately seen that there is no significant difference in E1/2 between these ligands. Consequently, the alkyl chain length of HCnQ does not affect the stability constants for the interfacial complexation with Zn2+. Interfacial Complexation Kinetics for Ni2+. The complexation kinetics for Ni2+ with HQ or HCnQ at the DCE -water interface was also studied in the same way as described for Zn2+. Previous investigation7 showed that the complexation behavior for Ni2+ was more complicated than that for Zn2+, reflecting the inherent difference in the reaction rate between Ni2+ and Zn2+.25 The complexation kinetics between Ni2+ and 8-quinolinol derivatives at the heptane -water interface is diffusion-controlled for HQ, HC1Q, and HC2Q. On the other hand, the kinetics is interfacial reaction-controlled for HC5Q and HC8Q. These results mean that the complexation rate is faster for the ligands having a shorter alkyl chain than those having a longer one. Dynamic interfacial tension profiles are shown in Figure 6. The aqueous phase is 10 mM NiCl2 containing 70 mM NaCl (pH 5.2), and the DCE phase is 2.5 mM HQ or HCnQ. The broken line denotes the interfacial tension without ligands. The interfacial tension for HQ (Figure 6a) decreases steeply within a short time range of a few seconds. As for HC1Q and HC2Q, the initial decrease in the interfacial tension is relatively slow compared to that for HQ. Comparing the γ(t) profile for HQ-Ni2+ (Figure 6a, closed circles) with that for HQ-Zn2+ (Figure 2, closed circles) shows that the decrease in the interfacial tension is more remarkable for Ni2+ than for Zn2+. This implies that the Ni2+ complexes formed at the interface are poorly extracted into the DCE phase; instead they remain at the DCEwater interface as a result of the slow reaction rate of Ni2+.25 When the interfacial complexation is very slow and the kinetics is controlled by the interfacial reaction, γ(t) is expressed by7
γ(t) ) γe + (γ0 - γe) exp(-kt)
(4)
where γ0 (mN m-1) is the interfacial tension without ligands (28.5 mN m-1) and k (s-1) is the rate constant for the interfacial reaction. The dynamic interfacial tension for HC1Q (Figure 6a, open circles) can be fitted well by eq 4, and the result is shown as the solid curve. The complexation kinetics for HC1Q with Ni2+ is therefore concluded to be reactioncontrolled, indicating that the interfacial reaction is very slow compared to the kinetics for HC1Q with Zn2+ which is diffusion-controlled. (25) For example, rate constants for the hydrated water exchange reaction are 107.5 s-1 for Zn2+ (Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307), and 104.5 s-1 for Ni2+ (Fittipaldi, F.; Petrucci, S. J. Phys. Chem. 1967, 71, 3414).
Figure 6. Dynamic interfacial tension at the DCE-water interface for Ni2+: aqueous phase, 10 mM NiCl2 + 70 mM NaCl (pH 5.2); DCE phase, (a) 2.5 mM HQ (b), HC1Q (O), HC2Q (9); (b) HC3Q (0), HC5Q (2), HC8Q (4).
The decrease in γ (t) (Figure 6b) is neither remarkable nor dependent on the alkyl chain length, implying that the interfacial complexation is too slow for the decrease in γ (t) to be observed. Ion transfer polarograms for HC1Q, HC3Q, and HC5Q are shown in Figure 7, where the aqueous phase is 10 mM Ni2+ and the DCE phase is 2.5 mM HCnQ. The aqueous and DCE phases contain 0.5 M LiCl and 0.05 M TBATPB, respectively. In Figure 7a, the anodic wave is barely observable on addition of HC1Q to the DCE phase. This wave is caused by the ion transfer of a cationic complex, that is Ni(C1Q)+, similar to the case of Zn2+ shown in Figure 3. However, the polarographic wave is quite small in contrast to that for Zn2+, since the interfacial complexation is very slow for Ni2+. In addition, the polarographic wave gradually disappears as the alkyl chain length increases (parts b and c of Figure 7). This trend is in agreement with that obtained by the interfacial tension measurements (parts a and b of Figures 6); that is, the interfacial complexation becomes slower as alkyl chain of the chelating reagent is lengthened. As for HQ, the interfacial complexation kinetics is not completely diffusion-controlled since no reversible polarographic wave was observed. On the other hand, the
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In addition, Table 4 provides an important insight into the reaction pathway of the interfacial complexation. The concentrations of ligands are 2.5 mM in the DCE-water system and 0.25 mM in the heptane-water system. Accordingly the change in the complexation kinetics in the DCE-water system should occur when the partition coefficient is one unit smaller than that in the heptanewater system, if the dependence of the kinetics on P arises from the reaction pathway that the ligands in the organic phase distribute into the aqueous phase and the complexation reaction with Ni2+ takes place in the aqueous phase. However, the kinetics changes at about log P ) 2.2 in both the DCE-water and heptane-water systems. This is powerful evidence that the complexation of these ligands with Ni2+ takes place just at the liquid-liquid interface. The difference in the interfacial complexation rate between 8-quinolinol derivatives can be ascribed to their location at the DCE-water interface.7,26 A less hydrophobic ligand such as HQ may be located on the aqueous side of the interface, and as a result, it can react with Ni2+ on the aqueous side of the interface. Since the complexation rate constants of these ligands in the bulk aqueous phase are almost the same, independent of the chain length,15,16 this location effect on the complexation rate is concluded to be observed specifically at the liquid-liquid interfaces. 4. Conclusions
Figure 7. Ion transfer polarograms for Ni2+: aqueous phase, 10 mM NiCl2 + 0.5 M LiCl; DCE phase, 0.05 M TBATPB + 2.5 mM HC1Q (a), HC3Q (b), and HC5Q (c); dotted line, 0 mM ligands. Table 4. Dependence of the Interfacial Complexation Kinetics for Ni2+ on the Partition Coefficient DCE-water
heptane-watera
ligand
kineticsb
log P
kineticsc
log P
HQ HC1Q HC2Q HC3Q HC5Q HC8Q
mixed reaction reaction reaction reaction reaction
2.26 2.56 2.86 3.45 4.52 > 5.0
diffusion diffusion diffusion mixed reaction reaction
1.35 1.03 1.48 2.22 3.42 4.64
a Reference 7. b [ligand] c DCE ) 2.5 mM. [ligand]heptane ) 0.25 mM. Reaction, reaction-controlled; diffusion, diffusion-controlled; mixed, mixed-controlled.
dynamic interfacial tension for HQ (Figure 6a, closed circles) cannot be fitted by eq 4. This shows that the interfacial complexation kinetics is not completely reaction-controlled as a result from the complexation being relatively fast compared to other ligands. From these results, the kinetics for HQ is considered to be controlled both by the diffusion of HQ from the bulk DCE phase and by its interfacial reaction with Ni2+, namely, the mixedcontrolled kinetics.7 Complexation kinetics between 8-quinolinol derivatives and Ni2+ at the DCE-water and heptane-water interfaces7 is summarized in Table 4 together with the partition coefficient (P) of each ligand. It is noted that the interfacial complexation kinetics can be closely correlated with the partition coefficient; that is, the kinetics changes from diffusion-controlled to reaction-controlled when the partition coefficient has a value of log P ) 2.2.
The complexation kinetics of 8-quinolinol derivatives was analyzed at the DCE-water interface by dynamic interfacial tensiometry and electrochemical measurements. Both methods were demonstrated, for the first time, to provide complementary insights into the mechanism of interfacial reactions. Although the decrease in the dynamic interfacial tension was not remarkable for any of the ligands with Zn2+, the electrochemical ion transfer measurements showed that the interfacial complexation was very fast and formed a 1:1 complex, that is, Zn(Q)+ or Zn(CnQ)+. The complexation kinetics was diffusioncontrolled, and the complexes formed at the DCE-water interface were considered to be extracted immediately into the DCE phase as an ion pair of Zn(CnQ)+Cl-. The kinetics and the stability constant for the interfacial complexation were not significantly affected by the alkyl chain length of the ligands. On the other hand, a remarkable decrease in the interfacial tension was observed in HQ-Ni2+ and HC1QNi2+, since these complexes adsorbed at the DCE-water interface. The kinetics was reaction-controlled, reflecting a slow reaction rate of Ni2+. As the alkyl chain length increased, the complexation rate became extremely slow and the decrease in γ (t) was barely observable. These results were also supported by electrochemical ion transfer measurements. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (B) No. 09440249, from the Ministry of Education, Science, Sports and Culture, Japan, and by The Asahi Glass Foundation. LA9815547 (26) Watarai et al. pointed out that the location of chelating reagents at the liquid-liquid interfaces plays a significant role in the interfacial complexation, based on the results from molecular dynamics simulation studies (Watarai, H.; Gotoh, M.; Gotoh, N. Bull. Chem. Soc. Jpn. 1997, 70, 957).
