Y. Kudo et al. / Journal of Molecular Liquids 194 (2014) 121–129
125
other words, it indicates that the KD,Pic values are sensitive to kinds of the
diluents, while the products of the other three constants are not sensitive
to them, although the equilibrium constants in the right hand side of the
cycle (IIa) can depend on interactions with the diluent molecules
employed. Unfortunately, no KD,Pb, KPb18C6,org and K1,org values at 298 K
were available.
CF N CBu N N DCM (−6.06). The former order for the aromatic diluents
shows that the substitution of –H in a benzene ring by –NO2, –Cl, or –Br
contributes an increase in the interaction between the diluent molecules
and Pic− [19]. On the other hand, the latter order for halo-alkane
diluents shows that the substitutions of –H by –Cl and –Cl by –CH2Cl
or –CH2CH2CH3 increase in the interaction with Pic−. From the above,
the substitution effect of these functional groups on KD,Pic can be in
the order –H b −Br, –Cl b −CH2Cl, –CH2CH2CH3 b −NO2, although
the relation in the aromatic diluents between –CH3 and –H is unclear.
Of course, one must see that such orders are limited with the simulta-
neous distribution of Pb(18C6)Pic+ into the diluents [6,12].
The plot of log Kex versus − log K2,org also yielded a straight line
with a slope of 1.52
0.05 and an intercept of 16.0
0.3 (≈log Kex
on average) at R = 0.978, indicating a good linearity except for the ex-
traction into DCM and TE on which lack of log Kex is present. However,
the plot showed a deviation from the cycle [6,12] of
Similarly, the log KD,18C6 order for the aromatic diluents has been NB
(−1.00) N CBz N BBz ≥ oDCBz N Bz N TE N mX (−1.95) and that for
the halo-alkane diluents CF (0.786) N DCM N DCE N N CBu (−1.93)
(see Table 1) [20]. The former order indicates that the substitution
of –H by –Cl, –Br or –NO2 increases the interaction of the aromatic dilu-
ent-molecules with 18C6, while that of –H by –CH3 decreases its inter-
action. On the other hand, the latter shows that the substitution of –H
by –Cl and –CH2Cl, or –CH2CH2CH3 by –H increases the interactions of
the halo-alkane diluents with 18C6: H–CHCl2 b Cl–CHCl2 and CH3CH2
CH2–CH2Cl ≪ ClCH2–CH2Cl b Cl–CH2Cl.
Considering electronegativities (χX) [21] of the functional groups,
X = −O– (χO = 3.44 [21]) and –CH2– (estimated χCH2 = 2.29), in
18C6, these results suggest any interactions of the –CH2– site in 18C6
with the –Cl, –Br, or –NO2 group in the diluent molecules. The reported
and estimated group electronegativities [22] in Pauling unit were also in
the order of –Cl (χX = 3.16 [21]) N –Br (2.96 [21]) N –NO2 (estimated:
2.72) N –CH2Cl (estimated: 2.47; 2.47 [22]) N –CH2CH2CH3 (estimated:
2.28), –CH3 (estimated: 2.27; 2.27 [22]) N –H (2.20 [21]). This order
seems to be reflected to the log KD,18C6 one. Especially, its log KD,18C6
ꢀ
ꢁ
Kexꢁ ¼ KMLK1K2KD;MLA2=KD;LK2;org ¼ Kex=K2;org
:
ðIIbÞ
From the slope of the plot, it is difficult to conclude the presence of a
correlation based on the cycle (IIb) for all the diluents employed. Howev-
er, the good linearity and the deviation of the slope from unity on the cycle
(IIb) can suggest that the Kex values are controlled by the K2,org ones for
“some” diluents employed. For example, plots except for the TE, DCM,
CBu, and NB systems yielded the relation of log Kex ≈ (−0.98
log K2,org + (12.0 1.0) at R = 0.587, although the R value was low.
The plot of log Kex versus log KD,Pic did not yield a straight line with
a slope of unity {see the cycle (IIa)}. This finding shows the less depen-
dence of the plot on the thermodynamic cycle [6] of
0.22)
Kexꢁ ¼ KMLK1KD;MLA
K
D;A=KD;L
:
ðIIcÞ
Since the condition of KD,MLA (=[MLA+]o/[MLA+]) = KD,A is not
held for the MA2-L extraction systems [6] in general, the cycle (IIc) can-
not be approximated to Kex = KMLK1(KD,A)2/KD,L. Therefore, the find-
ing indicates that there are differences in KD,PbLPic/KD,L at L = 18C6
among the diluents employed. In other words, the distribution proper-
ties of 18C6 do not directly reflect those of ionic Pb(18C6)Pic+, although
the 18C6 properties directly reflect the distribution properties of neu-
tral Pb(18C6)Pic2 (see Section 3.9). On the other hand, the Cd(II) extrac-
order for the halo-alkane diluents reflects well the χX one: Cl–CHCl2
N
H–CHCl2 and/or Cl–CH2Cl N ClCH2–CH2Cl N CH3CH2CH2–CH2Cl. In
spite of a difference between 18C6 and Pic− in the fundamental interac-
tion, dipole–dipole one for the former and ion–dipole one for the latter,
with the aromatic diluent molecules, it is also interesting to us that the
log KD,Pic order is similar to the log KD,18C6 one. This fact can support de-
localization of electrons in Pic−. In other words, Pic− may be behaving
as a neutral molecule in the distribution into the organic phases.
It is interesting to compare these log KD,Pic orders with those for the
tion systems showed dependences to the cycle (IIc): log Kex
≈ log
,Br
KD,Br + constant and log Kex
≈ log KD,Cd18C6Pic + constant [6].
,Pic
From the A−-distribution-point of view, the PbPic2 systems with 18C6
may be close to the CdBr2 ones with it.
CdPic2–18C6 extraction system. The latter orders are CBz (log KD,Pic
=
It is expected in the cycle (IIa) that increases in all the component
equilibrium constants contribute that in Kex . The present Pb(II) extrac-
tion systems correspond to this cycle. In the cycle (IIb) {or (II)}, it is also
−4.87) N TE, mX N Bz (−4.61) and DCM (−3.60) N DCE, CF N CBu
(−5.6) [6,19]. It may be suggested that the PbPic2–18C6 system
weakens the interaction of Pic− with DCM or TE molecules, compared
to the CdPic2–18C6 system.
expected that increases in the component equilibrium constants, KML
,
K1, K2, and KD,MLA2, contribute that in Kex , while increases in KD,L and
K2,org do its decrease. Moreover, it is expected in the cycle (IIc) that in-
creases in the component equilibrium constants, KML, K1, KD,MLA, and
KD,A, contribute that in Kex , while increases in KD,L do its decrease.
This cycle corresponded to the Cd(II) extraction systems [6]. Thus, the
above results indicate that all the component equilibrium constants
which are concerned with the distribution of species into the diluents
employed are not necessarily reflected into the increase of Kex . As
shown above, from the thermodynamic cycles, it is very difficult to pre-
dict a common property in Kex or Kex even among the diluents
employed.
Of course, the above results simply indicate that Kex has the com-
mon dependence on the square of KD,Pic in a series of the diluents
employed. As readers know, the cycle (IIa) does not mean a so-called
reaction mechanism, because the process expressed as KD,Pic does not
necessarily show an elementary reaction.
3.6. For tendency of stabilization of Pb(18C6)Pic2 in the diluents
The log K2,org values listed in Table 1 were in the orders org = BBz
(7.7) ≥ Bz ≥ mX, CBz ≥ oDCBz N NB (4.15) and CBu (8.6) ≥ CF ≥ DCE
(7.0) in the Io range of 1.5 × 10−7 to 1.9 × 10−4 mol dm−3, where Io de-
notes the ionic strength for ionic species in the organic phase. Both or-
ders seem to roughly depend on magnitudes of the dielectric constants
of the pure diluents (see the sequence of the diluents in Table 1). It is
also suggested that the Pb(18C6)Pic2 ion pairs are stabilized in the BBz
and CBu phases. The log K2,org orders of the CdPic2–18C6 system are Bz
(log K2,org = 9.5) ≥ mX ≥ TE ≥ CBz (6.0) and CBu (7.1) ≥ DCE ≥
CF ≥ DCM (5.4) in the Io range of 8.2 × 10−9 to 6.2 × 10−7 mol dm−3
[6]. These orders are similar to those for the PbPic2–18C6 system. The sta-
bilization of the Bz and CBu systems is greater than those of the other
systems, although the data of the CdPic2–18C6 extraction system with
BBz was not available.
3.5. For tendency of the interaction between diluent molecules and Pic− or
18C6
3.7. For a correlation between log KD,Pic and log (Io/I)
The log KD,Pic values listed in Table 1 were in the orders NB (−0.94) N
oDCBz ≥ CBz ≥ BBz ≥ mX ≥ Bz N TE (−3.2) and DCE (−1.89) ≥
The plot of log KD,Pic versus log (Io/I) for the employed diluents yielded
the straight line of log KD,Pic = (0.968
0.050)log (Io/I) + (1.12
0.16)
Kudo, Yoshihiro
