J . Phys. Chem. 1992, 96, 11072-11078
11072
(20) Bates, R. G. In Determination of p H , Theory and Practice, 2nd 4.; Wiley: New York, 1973; Chapter 10. (21) Roy, R. N.; Moore, C. P.; White, M. W.; Roy, L.N.; Vogel, K. M.; Johnson, D. A.; Millero, F. J. J. Phys. Chem. 1992, 96, 403. (22) Bates, R. G.; Guggenheim, E. A,; Hamed, H. S.;Ives, D. J. G.; Jam, D. J.; Monk, C. B.;Rues. J. E.; Robinson, R. A.; Stokes, R. H.; WynnaJones,
W. F. K. J. Chem. Phys. 1956, 25, 361; 1957, 26, 222. (23) Pitzer, K.S.J. Solution Chem. 1975, 4 , 249. (24) Pitzer, K. S.;Mayorga, G. J . Phys. Chem. 1973, 77, 2300. (25) Felmy, A. R.; Rai, D. J. Solution Chem. 1992, 21, 407. (26) Brown, P. L.; Ellis, J.; Sylva, R. N. J. Chem. Soc., Dalton Tram 1983, 31.
Kinetics of "Extraction" of Copper( I I ) by Mlceiie-Solubilized Compiexing Agents of Varying Hydrophliic Lipophilic Balance. 1. Stopped-Flow Study
Sung-Gem Son,+ Marc HCbrantt Paolo Tecilla,t Paolo Scrimin,*and Christian Tondre**t
Laboratoire &Etude des Systbmes Organiques et Colloidaux (LESOC), Unit8 Associk au CNRS 406, Facult6 des Sciences, UniversitP de Nancy I, B.P. 239, 54506 Vandoeuvre-18s-Nancy Cedex, France, and Dipartimento di Chimica Organica and Centro CNR, Meccanismi di Reazioni Organiche, Universita di Padova. Via Marzolo 1 , 35131 Padova, Italy (Received: April 8, 1992; In Final Form: July 28, 1992)
Micellar particles can solubilize lipophilic extractants similarly to the organic phase in classical biphasic extraction. A series of 6-(alkylamino)methyl-2-(hydroxymethyl)pyridines (C,NHMePy with n = 1,4,8, 10, 12, 14, and 16), good complexing agents for copper(II), has been used in this work to investigate the role of hydrophobic interactions on the kinetics of complexation in micellar media. Apparent rate constants for complex formation (kp'p) and dissociation (kJPP) obtained from stopped-flow experiments are reported in different micellar systems (CTAB, SDS, and C,,EO,), showing important variations with the value of n. These results which mainly concerned the protonated form of the extractant molecules (pH 3.5) have permitted precise determination of the role played by the electrostatic interactions. k?PP is found to decrease more than 20 times in CTAB and about 10 times in CI2EO6for n changing from 1 to 16. The influence of copper(I1) and surfactant concentrations on the observed rate constants is interpreted in terms of partition coefficients which have been independently determined from dialysis experiments. The reaction is found to be insensitive to surfactant concentration for n = 1,4, and 8 in CTAB and only for n = 1 and 4 for CI2EO6.The activation parameters are determined for n = 4 and 14 in water, in CTAB, and in CI2EO6.The effect of the addition of alcohol and oil to form different o/w microemulsions is finally reported.
Introduction The usefulness of organized systems, such as micelles and microemulsions, in metal extraction processes has been amply demonstrated in the recent literature,'-24 where a constantly growing numter of papers can be observed in this field. The present paper and the following one in this issue will be concerned with an important aspect of the potentialities of these systems, namely, their ability to play the part of the organic phase in a classical solvent extraction pr-. Indeed, the hydrophobic core of the micellar particles can solubilize hydrophobic extractants, and in this respect the micellar pseudophase can act similarly to the organic phase in a classical biphasic extraction. Recent data have shown that although micellar solutions are purely isotropic, phase separation is not a real problem since it can be achieved by means of ultrafiltration t e c h n i q ~ e s . ~ ~ The latter observation is very important as it substantiates the efforts that have been done in the past few years in one of our laboratories to demonstrate the interest of using micelles and microemulsions as model extraction systems.'** From a fundamental point of view, micellar media are very attractive because their microheterogeneous structure guarantees a perfect tranUniversitd de Nancy I. 'Universita di Padova.
sparency to light. This property has invaluable advantages for physicochemical studies, especially for kinetic and mechanistic studies resting on optical methods. On the other hand, from an applied point of view, it becomes thinkable of achieving metal ion extraction in media which are up to 99% aqueous, which, viewed from the environmental side, is also quite attractive. The aim of the present series of papers was two-fold. First, we wanted to investigate the role of the hydrophilic/hydrophobic character of the complexing molecules on the kinetics of complexation of metal ions in micellar media. A series of 6-(alkylamino)methyl-2-(hydroxymethyl)pyridmes, which are goad complexing agents for copper(II),Z5*26 has been used for this purpose, varying their hydrophilic lipophilic balance (HLB) from very hydrophilic to very hydrophobic. Our second objective was to better understand the role of the interfacial kinetics, which constitutes a much debated question in the current literature?'-% As far as micellar systems are concerned this refers to the microscopic interface separating the micellar pseudophase and the bulk aqueous phase, which can be assumed to mimic in some way m a c r m p i c liquid-liquid interfaces. Different approaches of this complicated problem will be considered. The part eventually contributed by interfacial processes to the overall complexation kinetics is expected to be affected by changing the HLB of the complexing molecule and also by varying the nature of the surfactant so as to change the sign of the electric charges borne by the micelles. Both points will be examined in
0022-3654/92/2096-11072503.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 11073
Stopped-Flow Study of "Extraction" of Cu(I1)
TABLE I: Values of tbe Apparent Rate Constant for Complex Formation ( p and ) Dissociation (Pp)la Different Media water CTAB SDS C12E06 n 1 4 8 10 12 14 16
ktP
&tP
ktw
&PP
(S-I)
(M-I I-')
(5-9
2551 2313 1878
1.8 5.3 6.8
1963 2139
6.9 5.1
2744 2213 1942 1119 240 136 128
2.4 6.2 6.0 5.2 2.0 0.2 0.1
(M-I
8-l)
GPp
&PP
(M-l
ktpp
(s-9
5-I)
5780 7006 7734 8417 8250 9079 9000
(M-l
2.7 5.0 3.0 3.6 2.8 1.5 2.5
9-I)
2539 2303 1747 805 236 277 280
&PP
(5-9
1.7 5.0 4.5 3.0 1.8 1.6 1.3
this first part, which will also consider what can be deduced from the values of the activation parameters obtained in different conditions. In part 2 (following paper in this issue), we will use a more theoretical approach to compare the experimental data with the prediction of different kinetic models.
times) of the metal ion concentration over the extractant concentration (usually 10-4 M) was used. Hydrochloric acid was always added in the solution containing the metal ion. All the concentrations given refer to the overall analytical concentrations after rapid mixing, and this is also true for the values of pH.
Experimental Section Reagents. The synthesis of 6-(alkylamino)methyl-2-(hydroxymethy1)pyridines (C,NHMePy with n = 1,4,8, 10, 12, 14, and 16) has been reported in previous publications.2s*26JsTheir purity
Results a d Discussion Kinetic Parameters in the Presence of Cationic, Nonionic, urd Anioaic Micelles. In a previously reported work,% we have shown that, in aqueous solution and in the absence of micelles, the kinetics of copper complexation by the more hydrophilic terms of the series of extractants considered here was most easily studied around pH 3.5. This observation had three reasons: (i) at this pH a large part of the complexhg molecule is protonated on the amino group, which makes the rate of reaction slow enough for the stopped-flow technique to be conveniently used, (ii) the kinetics is strictly first order, which is no longer the case at higher pH; (iii) only the Cu2+ species has to be considered with the exclusion of any hydrolyzed or polynuclear species.39 It was also demonstrated that the expreasion of the reciprocal relaxation time best fitting the data was of the form26
was checked by 'H NMR in CDC13 and elemental analysis. Cetyltrimethylammonium bromide (CTAB) from F l u b was twice recrystallized in methanol/diethyl ether; n-dodocyl hexa(ethy1ene glycol) ether (CI2EO6)obtained from Nikko Chemicals (Japan) and sodium dodecyl sulfate from Roth (Germany) were used as received. CuCl2.2H2O(normapur) was a product from Prolabo (France), and the other salts used were of analytical grade. The pH of the solutions was adjusted with drops of diluted HCl or NaOH (Merck Titrisol) in order to control the value of the final pH in stopped-flow experiments, We thus avoided the use of any kind of buffer because of their likely effect on the kinetics of complex formation. Indeed, most buffer molecules may substitute to water molecules in the inner solvation shell of the metal ions. This is known to have a labilizing effect on the remaining water m ~ l m l e swhich , ~ ~ may induce an enhancement of the reaction rate. Doubly distilled water was used throughout. TecWques, Equilibrium measurements were carried out with a Metrohm pH meter and with a Varian DMS-100 UV-visible spectraphotometer. pH measurements in micellar media have bem performed with a combined glass electrode, following the recommendations given in previous w o r l r ~ (this ~ ~ 3involved in particular a periodic rinsing of the electrode with acetone so as to prevent adsorption of the surfactants). The concentrationof stock solutions of the complexing agents C,NHMePy were systematically controlled by measuring their UV-visible absorption. The stopped-flow technique with optical detection was used for the kinetic measurements. Complex formation at 25 OC was detected at 285 nm (SDS solutions) or 300 nm (CTAB and C12E06 solutions). Two different apparatus were utilized: a Dur" D-110 with data acquisition on a Gould 1602 oscilloscope interfaced with a Victor V286 PC and a Biologic SFM-3 (Grenoble, France) totally computer-controlled (Tandon PC ASL/ 486-1 10) with Biokine software. The latter apparatus was mainly used for automatic variation of the copper chloride concentration at different temperatures. In any case the observed rate constants were computed from least-squares fitting of the experimental curves with exponential functions. Most of time the kinetic curves were perfectly first order. In very seldom cases and due to particular conditionsz (PH above 4.0 or solubility limit of the more hydrophobic complexing molecules for instance), the data were best fitted with a biexponential function. In such c8scs the slower relaxation had usually a small amplitude and was not considered for the purpose of the present work. The reciprocal relaxation time 1/r plotted in the figures is thus equivalent to the observed rate constant, T being the characteristic time of the exponential change of absorption with time. For all the kinetic experiments two identical micellar solutions were mixed, one of them containing the copper ions and the other one containing the complexing agent. A large excess (at least 10
l / r = kfPP[CU2+], + k p
(1)
with
kd[H+] + k-1 (3) where &'PP and &PP are the apparent rate constants for formation and destruction of the complex, respectively, whose values result from the contributions of two simultaneous reactions, one with the neutral form of the ligand (rate constants kl and kI)and the other one with its protonated form (rate constants kfand kd), Kal and Ka2being the acidity constants: hpp
L LH+
k + cu2+ & LCU2+ k-i
k + Cu2+h LCu2+ + H+ kd
L
+ 2H+ & LH+ + H+ &!LH22+
Considering that expression 1 can serve as a basis for understanding the results obtained in micellar media, we expect a linear variation of 1/r versus the analytical concentration of copper ions [Cu2+l0. Such plots are shown in Figures 1-3 for the micellar media considered here: cationic (CTAB), nonionic (C12E06),and anionic (SDS),respectively. For each type of micelles, the effect of varying the HLB of the extractant was investigated in order to determine its influence on the values of Q P P and Q m (see Table I). For CTAB and CI2EO6the behaviors observed are not very different, and the values of &'PP giyen by the slopes of the straight lines can be seen to decrease considerably when going toward the more hydrophobic terms of the series of extractants. Their variations are illustrated in Figure 4, in which the values relative to pure water have been added for the sake of comparison. The fact that the &'PP values reach a lower level with CTAB than for CI2EO6may be tentatively attributed to the fact that the c&06
11074 The Journal of Physical Chemistry, Vol. 96,No. 26, 1992
Son et al. 4
3.5
3
2.5
2
0
2
4
6
0
8
Figure 1. 7-I versus the analytical concentration of Cuz+for the series of C,NHMePy in CTAB micelles: n = 1 (V, dashed line); n = 4 (3y); n = 8, (0);n = 10 (0); n = 12 (a);n = 14 (0); n = 16 (A). [CTAB] = 2.5 X M; [extractant] = lo4 M; pH = 3.5 0.1; T = 25 OC.
10
5
15
Figure 4. Variation of log L$PP for copper complexation versus n in different reaction media: water (3y); SDS (A);CTAB (0); CI2EO6(0).
20
15 I
Ill/
\
\ *
I
\
10
5
0 0 0
0
2
4
6
8
7-I versus the analytical concentration of Cu2+for the series of C,NHMePy in CI2EO6micelles: n = 1 ( X , dashed line); n = 4 (A); n = 8 (0); n = 10 (0); n 3 12 (*); n = 14 (V);n 16 (0). [C1&06] =2X M; [extractant] = lo4 M; pH = 3.5 & 0.1; T = 25 OC.
Figure 2.
0
2
4
6
8
Figure 3. Plots of the quantity 7-I + x versus the analytical concentration of Cu2+for the series of C,NHMePy in SDS micelles: n = 1, x = 0 ( 0 ) ; n = 4, x = 10 (0); n = 8, x = 20 (X); n = 10, x = 30 (0); n = 12, x = 40 (+); n = 14, x = 50 (V); n = 16, x = 60 (A). [SDS] = 0.2 M; [extractant]= lo4 M; pH = 4.0 & 0.1, T = 25 OC.
micelles have some difficulty in accommodating extractant molecules with alkyl chain longer than CI2 In fact the variations observed are almost identical up to n = 12, afterwhich the values for c1zEo6level off, whereas those for CTAB keep decreasing. In the latter case the apparent rate constant decreases by a factor of 21 when going from CINHMePy to C16NHMePy. This behavior is to be put in relation with what was previously 0bserved’J when a comparison is made of the rate of complexation of Ni2+ with 8-hydroxyquinoline (HQ) and with its alkylated analogue (C, I-HQ, the active species of the industrial extractant Kelex 100) in Merent micellar media (factors from 30 to 60 in pH conditions
I
1
5
10
n 15
Figure 5. Variation of M P P versus n in different reaction media: water (m); SDS (A);CTAB (0); C12E06 ( 0 ) .
close to neutrality). In addition, a decreasing factor around 30 times was also measured between the rate of extraction of copper by alkyl-substituted hydroxyoximes when the length of the alkyl substituent changed from C, to C12.32In the last two cases the reduction of rate was attributed to the fact that the reaction goes from almost purely volumic to almost purely interfacial. This point will be discussed in more detail in part 2 with regard to the results reported here. The behavior in the case of SDS is entirely different since a small reverse effect is observed corresponding to only a 50% increase of GPP from the more hydrophilic to the more hydrophobic term of the series. This is not surprising since both reagents are expected to concentrate in the micellar pseudophase whatever the HLB of the extractant, in contradistinction to the previous cases in which a spatial separation of the reagents occurs. Indeed, even for the more hydrophilic terms of the series, electrostatic interactions between the positively charged ligand and the micelles will prevail. For what concerns the values of the apparent dissociation constants GPP (see Figure 5 and Table I), the reason for passing through a maximum is not clear. The C, derivative, although it was thoroughly purified, gave a behavior out of standards. Is this related to the fact that its preparation was slightly different from the others?26We have nothing in mind to support this hypothesis. Looking at the higher terms of the series, we can say that from c 8 to about C12the complex dissociation is influenced by the reaction medium so that it becomes less easy in the order water > CTAB > ClzEOs> SDS From C l 2to c16 the order appears to be completely reversed for the three surfactants: water > SDS > C12E06> CTAB As noticed before for &PPI we find a change of behavior around n = 12. For n < 12 the complex which is positively charged is
logically more stabilized inside the negatively charged SDS mi-
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 11075
Stopped-Flow Study of *Extraction" of Cu(1I)
r-'(C') 20
15
10
0
0
2
4
6 l$xC,y,
8 (mol dm4)
Figure 6. 7-l versus CTAB concentration for the series of C,NHMePy: n = 1 (A);n = 4 (%); n = 8 (0); n = 10 (m); n = 12 (0); n = 14 (A); n = 16 (+). [extractant] = lo4 M; [Cu2+]= 5 X M; pH = 3.5 h 0.1; T = 25 OC.
0 2 4 6 a Figure 8. 7-l versus C12E06concentration for the series of C,NHMePy: n = 4 (%); n = 8 (m); n = 10 (A);n = 12 (0); n = 14 (+); n = 16 (0). [extractant] = M; [Cuz+] = 5 x M; pH = 3.5 & 0.1, T = 25 OC.
T
*
6
10
0
0
" ~ " " " " ' " " 2
4
I~~xc,,,(~oIdm3) 6
-I I
8
O
i
r h + a - - - $
10
x
L0
2
102cc12Eog(md h-31
4
a
8
Figure 7. Signal amplitudes A in OD units associated with relaxation times given in Figure 6 versus CTAB concentration: n = 1 (A);n = 4 (X); n = 8 (0); n = 10 ( 0 ) ;n = 12 (0); n = 14 (V); n = 16 (+).
Figure 9. Signal amplitudes A in OD units associated with relaxation times given in Figure 8 versus C I 2 E o 6concentration: n = 4 (+); n = 8 (0); n = 10 (A);n = 12 (0); n = 14 (%), n = 16 (m).
celles than inside the cationic CTAB micelles. For n > 12 one may assume that this stabilizing effect in SDS micelles becomes less favorable due to steric constraints. The nonionic surfactant is in any case presenting an intermediate situation. influence of Surfactant Concentration and Extractant Partitioning in Micelles. In the preceding experiments, the concentrations of surfactants were chosen largely above the critical micelle concentration (cmc) in order to have a significant concentration for C12E06, of micelles. We used 2.5 X lo-* for CTAB, 2 X le2 and 2 X IO-' M for SDS,i.e. about 27,294, and 24 times the cmc, respectively, the second value being justified by the much larger aggregation number characterizing CI2EO6micelles (60O4Oinstead of 60 to 90 for SDS4' and CTAB42). It was thus important to investigate the effect of the concentrationsof surfactants on the kinetics of complex formation. Figure 6 shows the variations of 1 / r versus the concentration of CTAB, and Figure 7 gives the amplitudes (total absorbance change) of the corresponding relaxation signals. In Figures 8 and 9 we have collected similar information for the case of c12Eo6 micelles. In CTAB, the data can be qualitatively understood as indicating that for short chain extractants (n = I , 4, 8) the reaction is essentially taking place in the water phase since it is almost insensitive to the addition of surfactant (note again the abnormal behavior of C'NHMePy). When the length of the alkyl chain is increased from n = 10 to 16, the effect of CTAB concentration becoma more and more pronounced. This is indicating a stronger and stronger partitioning of the extractants in favor of the micellar pseudophase, as demonstrated from dialysis experiment^.^^ The curves relative to n = IO and 16 are very much reminding those previously obtained for Ni2+complexation by HQ and CII-HQ, respectively." In the present case we are left with two alternative explanations between which we will try to discriminate in the following paper (part 2): either the reaction is always taking place in the bulk water phase or it is partly or totally interfacial. With CI2EO6the partitioning of the extractants in favor of the micellar pseudophase is enhanced (remember that the extractants are in a protonated form at the pH considered26and consequently
electrostatic repulsions in the case of CTAB are thwarting the solubilization). The effect of the electric charge has been found to be possibly counterbalanced by an increase of threeto four CH2 groups in the alkyl chain.43 So in the case of CI2EO6the partitioning effect is already sensitive with C8NHMePy. The behavior of the signal amplitude is totally different for CTAB and C12EOs(see Figures 7 and 9). In the former case the amplitudes are very sensitive to the HLB of the extractants, which is not the case for CIZE06for which they remain in the order of 0.1 f 0.03 optical density (OD)units. In CTAB the signal amplitudes not only increase with the hydrophobicity of the extractant but also with the surfactant concentration, provided that n 1 10. The signal change may be as large as 0.4 OD units for n = 14 or 16. There are two possible explanations: (i) this may be due to the local pH which is known to be more basic than the measured one in the vicinity of CTm (spectroscopic measurements have shown an increase of absorption of the complex at 300 nm when the pH is increased); (ii) it may also be related to the very high concentration of bromide ions (around 3 M)46 close to the micelle surface. The UV-visible spectrum of the complex is indeed considerably accentuated in the presence of bromide ions, which could indicate the formation of species such as [CuBr412-,which are known to exist!' In such an eventuality complexation with C,NHMePy would occur by displacement of the coordinating bromide ions, but this is purely speculative. Figures 6 and 8 clearly show that associating hydrophobic extractants with cationic or nonionic micelles induce an important slowing down effect on the rate of complexation. Applications of such effects for the purpose of selective separation of metal ions have been reported?a The situation encountered with the anionic surfactant SDS,is completely Merent, as demonstrated by Figure 10, which is relative to the derivatives with n = 4 and 12, both giving approximately the same result. In this case an abrupt increase of 1 / is~first observed (for this reason the concentration of metal ion was reduced to M instead of 5 X IO" M for the other surfactants), corresponding to a 25-fold increase comparatively to pure water, after which a change in the reverse way
11076 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
Son et al,
100
w m o l en+)
I
0
I
0
0.06
I
0.2
0.15
0.1
Figure 10. 1-l versus SDS concentration for n = 4 (0) and n = 12 (S). [extractant] = lo4 M;[Cu2+] = M;pH = 3.7 0.2, T = 25 OC.
*
Figure 12. 1-l versus concentration of different additives for n = 4: CTAB (A);NaBr (0); KCl (*); hexa(ethy1ene glycol) EO6 ( 0 ) . [extractant] = lo4 M;[Cu2+] = 5 X M;pH = 3.5 & 0.1; T = 25 OC.
800
1
/
1
1 -0
2
4
6
8
Figure 13. 1 - l versus the analytical concentration of Cu2+for n = 4 in CI2EO6at different temperaturcs. 7,
c
I
I
3 t 2
-B--3-
-e-* -x -
*
0 ‘
0
I
2
I
4
I
6
J
8
Figure 14. 1 - l versus the analytical concentration of Cu2+for n = 14 in CTAB at different temperatures.
place of CI2EO6. The results obtained (see Figure 12) do not support the above idea, since the value of 1 / remained ~ practically unchanged with EO6 concentrations up to 8 X 1V2M. We have no other explanation to offer at this moment. Influence of Temperature and Activation Purwtem. By changing the temperature of the reaction media, we can have a m to the values of the activation parameters, which may help identifying the reaction mechanisms involved. We have not overlooked the fact that changing the temperature of micellar systems is assumed to possibly bring about a modification of the cmc or of the micellar shape and size (this is especially true for nonionic micelles when approaching the critical temperatures4), but we considered these phenomena to be of second order comparatively to the temperature effect on the reaction kinetics. This appeared justified by the fact that the evolution of the kinetic data with temperaturedid not show any unexpected variation but rather a classical Arrhenius-type behavior. This is demonstrated by the data reported in Figure 13, which is relative to C4NHMePy in CI2EO6. The same kind of plot was obtained for C4NHMePy in CTAB as well as in water and also for CI4NHMePyin water.
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 11077
Stopped-Flow Study of "Extraction" of Cu(I1)
TABLE II: Effectof Temperature on GWand Valuea of Activation Parameters M P P (M-I S-I) at given r ("C) (and T (K)) 25 OC 30 OC 35 OC 40 OC 45 o c (298 K) (303 K) (308 K) (313 K) (318 K) CdNHMePy/H,O 2317 2933 4200 5533 7161 5067 7200 C;,NHM&/H~O 2035 2161 3183 C,NHMePy /CTAB 2213 2630 3929 5400 61 50 C14NHMePy/CTAB 190 260 375 510 700 4767 C,NHMePy/C 1 2E06 2304 2911 3633 6161 1200 1515 C ,,NHMePy/C, 2 E 0 6 400 592 161 4
I
I
I
I
I
AH* (kJ-moT1) 43.06 46.86 43.25 49.20 36.25 51.85
As*
AG'
(JaK-ho1-l) -36.25 -24.61 -36.25 -36.25 -58.1 -21.28
(kJ-mol-I)
53.86 54.19 54.05 60.00 53.14 58.19
TABLE 111: Influence of Addition of Butanol and Oil on tbe Value of 7-1 ~
system investigated CTAB/water (NaBr) CTAB/butanol/water (NaBr) CTAB/ butanol/ water (NaBr)/octane CTAB/butanol/ water (NaBr)/dodecane CTAB/ butanol/ water (NaBr)/hexadecane
CTAB (M)
8x 8X 8X
butanol (wt%)
~~
oil (wt %)
lo-*
l/r (s-l)
5.8 5.8
2.9
3.13 3.59 3.33
16 X
11.6
2.9
5.31
16 X
11.6
2.9
4.94
0
3.1
3.15
3.2
3.25
3.3
3.36
3.4
Figure 15. Plot of In (&PP/T) versus l/Tfor n = 4 in water (m),CTAB (0),and CI2EO6(V) and for n = 14 in water (A), CTAB (O), and C12E06(0).
The latter homologue behaved differently in micellar systems. This is shown for CTAB in Figure 14, where we can notice a change of slope in each curve, including the curve obtained at 25 OC (it has thus nothing to do with the temperature rise). The same particularity was also observed with c12Eo6. The reason of this ~ total copper ion concentration, break in the curves giving 1 / vs which shows up only for the more hydrophobic terms of the series, has not been explained yet. One explanation could have been the following one, which is related to the concept of concentration in microheterogeneous systems55(see the following paper in this issue, part 2). In the first part of the curves of Figure 14 (low copper ion concentration), 1 / is~ practically insensitive to the metal ion concentration (slope close to zero whatever the temperature). Such behavior is expected if the extractant concentration (which is kept constant) is in significant excess over the metal ion concentration. This, in fact, could be locally true because a high concentration of the lipophilic extractant may exist in the micellar pseudophase, although its analytical concentration is much lower than that of the metal ion. One way to check this hypothesis was to decrease the extractant concentration and see whether or not the plot of 1 / versus ~ [ C U ~ +becomes ]~ more linear. The results obtained were unfortunately not conclusive, but they cannot completely rule out this explanation, because for reasons of sensitivity it was difficult to investigate what happens when the M. concentration of extractant was decreased below The important point is that, at large copper concentration, a regular Arrhenius-type behavior was obtained, which could be used to determine the activation parameters. We have collected in Table I1 the values of the apparent rate constants for complex formation &PP at temperatures 25, 30,35,40, and 45 "C. These data have been plotted in Figure 15 in the form In (&PP/7') vs 1/T, and the activation parameters obtained from the slopes and intercepts of the straight lines are given in Table 11. It is clear from Figure 15 that the activation enthalpies AH*relative to C,NHMePy in water, CIZE06,and CTAB and to C14NHMePy in water are all practically the same within the experimental uncertainties. Slightly larger values have been obtained for C14NHMePyin micellar systems (note that the downward shift of the corresponding CUNW is totally dependent on the surfactant concentration chosen, so the fact that the curve corresponding to CTAB is below the curve for C&06 has no particular meaning in this case). There is unfortunately a large uncertainty on the
values of AS* for obvious reasons. However the combination of AH* and M*has led to values of AG* at 25 OC, which are all of the order of 57 f 3 kJ.mol-', indicating that the energy barrier remains almost unchanged. This is strongly suggesting that the micelles are mainly acting on the rate of complex formation by modifying the local concentrations of reagents. Assuming that the lines drawn in Figure 15 are parallel would lead to values of AS* (J-K-l-mol-') which may be more significant: -36.2 (water and hydrophilic extractants in micelles); -48.7 (CI4NHMePyin C12E06); -54.9 (C14NHMePyin CTAB). This would mean that the decrease of entropy when going from the uncomplexed extractant to the activated complex is stronger when the extractant partitions into the micelles. This is quite understandable because the complexes stuck into the micelles will obviously loose some degree of freedom compared to the same complexes in water. Unfortunately this does not tell us where is the site of the proper reaction of complex formation. It is just indicating that, on an average, there is a decrease of the degree of freedom due to partitioning of both the free extractant and the complex, each of them contributing to the value of AS'. Effect of Added Alcohol and Oil (Transforming Micelles in Microemulsiom). One last question we wanted to answer in this paper is can we further decrease the rate of complexation of long chain extractants by solubilizing an oil in the hydrophobic core of the micelles? The incorporation of an oil in the CTAB micelles requires the addition of a cosurfactant, as it is well-known from the numerous works concerned with micro emulsion^.^^ Butanol is generally associated with CTAB for this p u r p ~ s e . ~ ' The *~~ presence of a salt is favoring the incorporation of a larger amount of oil, and this contributes to the obtaining of a significant dispersed organic phase (2.9% in weight in the present case). The CTAB concentration was either equal to or larger than the maximum concentration used in Figure 6 (with dodecane and hexadecane complete solubilization was obtained at this condition). We have collected in Table I11 the results obtained for CI2NHMePywith the different micellar, mixed micellar, and microemulsion systems. They demonstrate that additions of al~ rather a cohol or oil never induce a further decrease of l / but slight increase. This is probably indicating that the partitioning of ClzNHMePy (which, at the pH used, is in a protonated form) toward the oil is practically negligible. The slight increase of 1 / ~ may be explained by the effect of the alcohol on the micellar charge. The electrostatic repulsions between the micelles and the metal ions are expected to diminish due to the increase of the area per polar head at the micelle surface when butanol is adsorbed in the surfactant film. The absence of any significant effect when micelles are transformed in microemulsions is in line with previously reported observation^.^^
11078 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
In conclusion, the results presented in this paper have determined with great precision the role of the hydrophilic/hydrophobic character of complexing molecules on the kinetics of copper complexation. The role of electrostatic interactions has also been examined by changing the ionic nature of the micelles solubilizing these molecules. The determination of the activation parameters has led to the conclusion that cationic and nonionic micelles are essentially acting through the modification of the local concentrations of reagents. Transforming micelles in microemulsions has appeared to have almost no effect on the rate of complex formation. In the following paper in this issue (part 2), the results will be examined in a more quantitative manner, in order to establish precisely the nature of the mechanisms involved and especially the role of the interface.
Acknowledgment. The technical assistance of J. L. Fringant is greatly acknowledged.
References and Notes (1) Osseo-Asare, K.; Keeney, M. E. Sep. Sci. Technol. 1980, 15, 999. (2) Fourre, P.; Bauer, D. C. R . Acad. Sci., Ser. 2 1981, 292, 1077. (3) Bauer, D.; Fourre, P.; Lemerle, J. C. R . Acad. Sei., Ser. 2, 1981, 292, 1019. (4) Tondre, C.; Xenakis, A. Faraday Discuss. Chem. Soc. 1984.77, 115. (5) Fletcher, P. D. I.; Robinson, B. H. J . Chem. SOC.,Faraday Trans. 1 1984,80, 2417. (6) Ovejero-Escudero,F. J.; Angelino, H.; Casamatta, G. J. Dispersion Sci. Technol. 1987, 8, 89. (7) Tondre, C.; Boumezioud. M. J. Phys. Chem. 1989, 93, 846. (8) Boumezioud, M.; Kim, H. S.; Tondre, C. Colloids Sutf 1989,41,255. (9) Kim, H. S.; Tondre, C. Sep. Sci. Technol. 1989. 41, 255. (10) Dunn, R. 0.;Scamehom, J. F.; Christian, S. D. Colloids Surf. 1989, 35, 49. (11) Christian, S. D.; Tucker, E. E.; Scamehorn, J. F.; Lee,B.-H.; Sasaki, K. J. Lungmuir 1989, 5, 876. (12) Cavasino, F. P.; Sbriziolo, C.; Pelizzetti, E.; Pramauro, E. J. Phys. Chem. 1989, 93,469. (13) Ganguly, B. (Nandi) J . Photochem. Photobiol. A: Chem. 1990,51, 401. (14) Paatero, E.; SjBblom, J. Hydrometallurgy 1990, 25, 231. (15) Paatero, E.; SjBblom, J.; Datta, S. K. J. Colloid Interface Sci. 1990, 138, 388. (16) Vijayalakshmi, C. S.; Annapragada, A. V.; Gulari, E. Sep. Sci. Technol. 1990, 25, 71 I. (17) Neuman, R. D.; Jones, M. A.; Zhou, N.-F. ColloidsSurf. 1990,46, 45. (18) Derouiche, A.; Tondre, C. Colloids Surf. 1990, 48, 243. (19) Osseo-Asare, K.;Zheng, Y. Colloids Surf. 1991, 53, 339. (20) Savastano, C. A.; Ortiz, E. S . Chem. Eng. Sci. 1991.46, 741. (21) Viiavalakshmi. C. S.: Gulari. E. Sen Sci. Technol. 1991. 26, 291. (22) Kiekc, J.; Simmons, D. L:; Tayior, R. W.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1991, 26, 165. (23) Tondre, C.; Canet, D. J. Phys. Chem. 1991, 95, 4810.
Son et al. (24) Ismael, M.; Tondre, C. Langmuir 1992, 8, 1039. (25) Tondre, C.; Claude-Montigny, B.; Ismael, M.;Scrimin, P.; Tecilla, P. Polyhedron 1991, 10, 1791. (26) Hebrant, M.; Son,S.4.; Tondre, C.; Tecilla. P.; Scrimin,P. J. Chem. Soc., Faraday Trans. 1992,88, 209. (27) Szymanowski, J.; Prochaska, K. J . Radioanal. Nucl. Chem. 1989, 129. 251. (28) Chamupathi, V. G.; Freiser, H. Inorg. Chem. 1989,28, 1658. (29) Osseo-Asare, K. Colloids Sur/. 1988, 33, 209. O&seo-Asare, K . In Hydrometallurgical Process Fundamenrals; Bantista, R. C., Ed.; Plenum: New York, 1984. (30) Albery, W. J.; Choudbery, R. A. 1. Phys. Chem. 1988, 92, 1142. (31) Watarai, H.; Takahashi, M.; Shibata.K. Bull. Chem.Soc. Jpn. 1986, 59, 3469. (32) Szymanowski, J. Polyhedron 1985,4, 269. (33) Akiba, K.; Freiser, H. Anal. Chim. Acra 1982, 136, 329. (34) Danesi, P. R.; Vandegrift, G. F.; Honvitz, E. P.; Chiarizia, R.J. Phys. Chem. 1980, 84, 3582. (35) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1991, 56, 161. (36) Cobb, M. A.; Hague, D. N. J . Chem. Soc., Faraday Trans. 1 1972, 68, 932. (37) Berthod, A.; Saliba, C. Analusis 1985, 13, 437; 1986, 14, 44. (38) Bahri, H.; Letellier, P. J. Chim. Phys. Phys.-Chim. Biol. 1985, 82, 1009. (39) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976. (40) Ravey, J. C. J. Colloid Interface Sci. 1983, 94, 289. (41) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905. (42) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100. (43) Hebrant, M.; Tondre, C. J. Colloid Interface Sci., in press. (44) Kim, H. S.; Tondre, C. Lungmuir 1989,5, 395. (45) Mackay, R. A.; Jacobson, K.; Tourian, J. J. Colloid Interface Sci. 1980, 76, 515. (46) Loughlin, J. A.; Romsted, L. S . Colloids Surf. 1990, 48, 123. (47) Khan, M.A.; Schwing-Weill, M.-J. Bull. Soc. Chim. Fr. 1977,515, 399. (48) James, A. D.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1978, 74, 10. (49) Dieckmann, S.; Frahm, J. J. Chem. Soc., Faraday Trans. 1 1979,75, 2199. (50) Fletcher, P. D. I.; Reinsborough, V. C. Can. J. Chem. 1981,59, 1361. (51) Buschmann, H.-J. Polyhedron 1985,4, 2039. (52) Sakai, Y.; Shinmura, M.; Otsuka, H.; Tahgi, M.Bull. Chem. Soc. Jpn. 1987, 60, 545. (53) Burger-Guerrisi, C.; Tondre, C. J . Colloid Inreflace Sci. 1987. 116, 100; Prog. Colloid Polym. Sci. 1987, 73. 30. (54) Lindman, B.; WennerstrBm, H. J . Phys. Chem. 1991, 95, 6053. (55) Berezin, I. V.; Martinek, K.; Yatsimirskii. A. K. Russ. Chem. Reo. (Engl. Traml.) 1973, 42, 787. (56) Robb, I. D., Ed. Microemulsions; Plenum Pres: New York, 1981. (57) Hermansky, C.; Mackay, R. A. In Solurion Chemisrry of Surfacranrs; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 2, p 723. (58) Tabony, J.; Drifford, M.; De Geyer, A. Chem. Phys. Le??.1983, 96, 119. (59) Fletcher, P. D. I.; Robinson, B. H. J . Chem. Soc., Faraday Trans. 1 1983, 79, 1959.
