Acid dissociation of (5,10,15,20-tetraphenylporphyrinato)zinc(II) in cetyltrimethylammonium chloride reversed micellar solutions

Article abstract of DOI:10.1246/bcsj.75.1747

The acid dissociation of (5,10,15,20-tetraphenylporphyrinato)zinc(II), Zn(tpp), dispersed in reversed micellar media of cetyltrimethylammonium chloride (CTAC) in chloroform/water and chloroform-cyclohexane/water, was studied using a stopped-flow spectrophotometer. When Zn(tpp) was mixed with the reversed micellar solution containing a concentrated acid, its dissociation, which followed immediate formation of the chloro-coordinated complex Zn(tpp)Cl^- in the interfacial phase of the reverse micelle, rapidly took place to yield the diacid salt of 5,10,15,20-tetraphenylporphyrin, H₄(tpp)²⁺(Cl^-)₂. The rate of acid dissociation increased as acid concentration in the reversed micellar aqueous pool increased. At higher acid concentrations (> 3 mol d/cu m in the pool), the rate constant and the fraction of Zn(tpp)Cl^- formed increased in the order: HNO₃ < HCl ≤ H₂SO₄ < HBr < HClO₄, and decreased as the volume fraction of cyclohexane in the reversed micellar bulk solvent increased.

Full text of DOI:10.1246/bcsj.75.1747

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1747–1753 (2002) 1747
e Chemical
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Acid Dissociation of (5,10,15,20-Tetraphenylporphyrinato)zinc(Ⅱ) in
Cetyltrimethylammonium Chloride Reversed Micellar Solutions
Dissociation of Zn(tpp) in Reverse Micelles
T. Nakashima et al.
Toshiyuki Nakashima, Ikuko Nishida, and Terufumi Fujiwara*
02
47
53
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526
SJA8
09-2673
017
(Received January 22, 2002)
Acid dissociation of (5,10,15,20-tetraphenylporphyrinato)zinc(Ⅱ), Zn(tpp) dispersed in reversed micellar media of
cetyltrimethylammonium chloride (CTAC) in chloroform/water and chloroform–cyclohexane/water has been investigat-
ed using a stopped-flow spectrophotometer. When Zn(tpp) is mixed with the reversed micellar solution containing a con-
centrated acid, spectral data reveal that its dissociation, following immediate formation of the chloro-coordinated com-
plex Zn(tpp)Cl− in the interfacial phase of the reverse micelle, occurs rapidly to produce the diacid salt of 5,10,15,20-
tetraphenylporphyrin, H4(tpp)2+(Cl−)2. The dissociation rate increases with an increase of acid concentration in the re-
versed micellar aqueous pool. At higher acid concentrations (> 3 mol dm−3 in the pool), the rate constant and the frac-
tion of Zn(tpp)Cl− formed increase in the order: HNO3 < HCl ꢀ H2SO4 < HBr < HClO4, and decrease with an increase
in volume fraction of cyclohexane in the reversed micellar bulk solvent. These observations may be explained by assum-
ing selective distribution of the Cl− ion to the interfacial surfactant and bulk organic phases, which is affected by affini-
ties of the acid anions for the surfactant head group and by the Cl− solvation.
02
02
1
.1
Using cetyltrimethylammonium chloride (CTAC) reverse
micelles in 6:5 (v/v) chloroform–cyclohexane as micro-reac-
tors in chemiluminescence (CL) analysis,1–7 we have devel-
oped a flow injection method based on a reversed micellar-me-
diated CL detection following solvent extraction.8–10 When
mixed with the reverse micelle containing luminol, it has been
found that the extracted neutral chelate complexes such as
tris(8-quinolinolato)iron(Ⅲ)2,6 and bis(acetylacetonato)oxova-
nadium(Ⅳ)5,7 are dissociated quickly to serve as a catalyst of
the luminol CL reaction; the metals of these complexes can be
thus transferred selectively from the bulk organic phase to the
inner aqueous core or so-called water pool of the reverse mi-
celle. Such behaviors in the metal uptake by the reverse mi-
celles are an important aspect of the inorganic biochemistry
because of a similarity to the behavior reported for the iron-
store protein, ferritin micelle.11 Since the neutral metal com-
plexes are almost insoluble in water, it is interesting to eluci-
date the driving force responsible for the metal uptake by the
reverse micelles. The reversed micellar interface may be re-
garded as significant in the uptake process. The incorporation
of the oxovanadium(Ⅳ) complex into reverse micelles contain-
ing aqueous basic solution, required for an enhancement of the
CL reaction, appears to arise from formation of the six-coordi-
nate complex by an axial ligation of the hydroxide ion at the
reversed micellar interface.7 However, the nucleophilic attacks
of such oxygen-containing ligands to the vanadium atom were
reported to give small effects in the visible absorption spec-
trum of the complex,12 and in fact, we obtained no observable
spectral shift for the complex in the reversed micellar solu-
tions. On the other hand, it has been observed that the CL
emission with the iron(Ⅲ)6 and oxovanadium(Ⅳ)5 complexes
in reverse micelles containing aqueous acidic solution is en-
hanced, suggesting that the dissociation of the complexes is
promoted by the dispersed acid. To clarify the metal uptake
process involving dissociation of the neutral metal complexes
at the reversed micellar interface, more work is thus required.
The (5,10,15,20-tetraphenylporphyrinato)zinc(Ⅱ) complex,
Zn(tpp) is especially well suited for spectrophotometric studies
because of its large molar absorptivity at Soret band and axial
ligation properties:13 The four-coordinate complex accepts
only one axial ligand to form a five-coordinate complex. Such
adduct formation exhibits a systematic red shift with increas-
ing charge and polarizability of the axial ligand. In our previ-
ous work,14 the solubilization of the neutral Zn(tpp) complex
in the interfacial region of the CTAC reverse micelles was
studied as a model for the first step in the metal uptake process
by the reverse micelles. We concluded that the coordination of
the counter Cl− ion of CTAC to the zinc atom of Zn(tpp) at the
reversed micellar interface leads to an enhancement of its in-
terfacial solubilization. Using aqueous basic solution as a re-
versed micellar aqueous pool, we observed that the interfacial
solubilization of Zn(tpp) was promoted by the formation of the
hydroxide ion adduct at the interface,14 but subsequently no
dissociation of Zn(tpp) occurs. On the other hand, acid disso-
ciation reactions of the neutral zinc porphyrin complexes have
been studied in various media such as methanol/hydrochloric
acid system,15 dimethylformamide/hydrochloric acid system,16
and dodecane–aqueous acid interface.17 The reaction rate and
rate law are highly sensitive to the reaction media,18 and it is
thus interesting to examine the acid dissociation reaction of
Zn(tpp) in the reversed micellar media.
In this work, the metal uptake process by the reverse mi-
celles, which involves interfacial solubilization and subsequent
acid dissociation of their complexes, is examined using the

1748 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
Dissociation of Zn(tpp) in Reverse Micelles
paratus were pushed through the four-jet mixer. After the mixing
of the 1:1 volume ratio, the Zn(tpp) concentration was reduced to
half its initial value, i.e., 5.0 × 10−6 mol dm−3 in the cell, where
the flow was abruptly stopped. The progress of the acid dissocia-
tion reaction was monitored spectrally from 365 to 500 nm at 20
ms intervals over 2 s. The pseudo-first-order rate constants for the
reactions were determined by analyzing the growth of the charac-
teristic absorption band of the diacid salt of 5,10,15,20-tetraphen-
ylporphyrin, H4(tpp)2+(Cl−)2 at 445 nm.
neutral Zn(tpp) complex as an appropriate model compound; a
stopped-flow method is employed for monitoring rapid spec-
tral changes in the dissociation reaction after mixing a Zn(tpp)
solution with a CTAC reversed micellar solution containing a
concentrated acid such as hydrochloric acid, hydrobromic ac-
id, perchloric acid, nitric acid, sulfuric acid, or acetic acid. We
also record the observed influences of added salts and the re-
versed micellar bulk solvents on the dissociation rates. The
knowledge obtained from this work contributes to our under-
standing of the role of the interfacial phase of the reverse mi-
celle in the metal uptake process.
Results and Discussion
Kinetics of Acid Dissociation Reaction of Zn(tpp).
In
the absorption spectrum of Zn(tpp) as observed in our previous
work,14 two B (or Soret) bands at 419 and 433 nm, attributable
to Zn(tpp) and a chloro-coordinated complex, Zn(tpp)Cl−, re-
spectively, appear in a CTAC reversed micellar solution, indi-
cating that the following Zn(tpp)Cl− formation is equilibrated
much more rapidly:
Experimental
Reagents. The Zn(tpp) complex and 5,10,15,20-tetraphen-
ylporphyrin, H2(tpp) were purchased from Aldrich Chemical Co.,
Inc. (Milwaukee, WI, USA). The CTAC surfactant (> 98%) was
obtained from Fluka (Buchs, Switzerland). Chloroform (> 99.7%,
stabilized with pentene) and cyclohexane were of fluorescence re-
agent grade (Cica-Merck) and were obtained from Kanto Chemi-
cal Co., Inc. (Tokyo, Japan). By preliminary experiments, it was
confirmed that the Zn(tpp) spectrum obtained in the reversed mi-
cellar solution is not affected by the presence of pentene.14 Hy-
drochloric acid (36%, electronic grade), hydrobromic acid (47%,
reagent grade), nitric acid (61%, analytical reagent grade), sulfuric
acid (97%, analytical reagent grade) and acetic acid (99.7%, re-
agent grade) were purchased from Kanto Chemical Co., Inc., and
perchloric acid (60%, analytical reagent grade) was obtained from
Katayama Chemical Industries Co., Ltd. (Osaka, Japan). The
nominal concentrations of the acids used were confirmed by a
conventional acid–base titration. Lithium chloride and lithium
perchlorate (> 99.0%, reagent grade) were obtained from Kanto
Chemical Co., Inc., and Katayama Chemical Industries Co., Ltd.,
respectively. All chemicals were used without further purification.
Distilled water (high-performance liquid chromatography reagent
grade) was obtained from Kanto Chemical Co., Inc. and was used
in the preparation of all aqueous solutions.
Zn(tpp) + Cl− ꢀ Zn(tpp)Cl−
(1)
On the other hand, an interesting spectral change was observed
when a Zn(tpp) solution in chloroform was mixed with the re-
versed micellar solution containing 3.0 mol dm−3 HCl in the
water pool (or the HCl concentration of 2.2 × 10−2 mol dm−3
in the final reversed micellar solution, calculated by taking into
account a water pool to final total volume ratio), as shown in
Fig. 1: A rapid decrease in these two peaks was produced, fol-
lowed by the growth of a new band at 445 nm. Finally the
former bands disappeared and the spectrum obtained is identi-
cal with that of the diacid salt of 5,10,15,20-tetraphenylpor-
phyrin, H4(tpp)2+(Cl−)2 in chloroform,19 indicating that the
dissociation of Zn(tpp) into the acid form occurs completely
without re-incorporation of the released Zn2+ ion. Further-
more, Fig. 1 shows the occurrence of a clear isosbestic point at
434 nm during the reaction, strongly suggesting that the
Zn(tpp) and Zn(tpp)Cl− complexes are always in a strictly con-
stant ratio and the H4(tpp)2+(Cl−)2 salt alone is a product. This
also means that Zn(tpp)Cl− is in equilibrium with Zn(tpp) and
Apparatus. Kinetic measurements were carried out by using
an Otsuka Densi model RA-2000 stopped-flow spectrophotome-
ter, equipped with a four-jet mixer and a cylindrical quartz cell of
2.0 mm optical path length thermostated at 25.0 0.1 °C. A pho-
todiode array UV–visible detector (rapid-scan detection system)
was used to measure the spectral change during the reaction. The
water contents in the CTAC solutions were determined coulomet-
rically using a Hiranuma model AQ-7 aquacounter.
Procedure. Stock concentrated acids (1.0–6.0 mol dm−3) of
HCl, HBr, HClO4, HNO3, H2SO4 and CH3COOH were prepared
by serial dilution of an undiluted acid with distilled water. Stock
concentrated aqueous salt solutions (1.0–3.0 mol dm−3) of LiCl
and LiClO4 were prepared in 3.0 mol dm−3 HCl. Chloroform and
its mixture with cyclohexane were used as reversed micellar bulk
solvents. Working reversed micellar solutions were prepared by
dispersing an appropriate amount of the stock concentrated acids
or the stock concentrated aqueous solutions in the reversed micel-
lar bulk solvent containing the CTAC surfactant. By dissolving
solid Zn(tpp) in chloroform or the mixed solvent of chloroform–
cyclohexane, we could make the working solution of 1.0 × 10−5
mol dm−3 Zn(tpp). Both the working solutions were prepared dai-
ly to avoid chloroform decomposition.
Fig. 1. Typical change in the rapid-scan spectra of Zn(tpp)
observed in the chloroform/CTAC/HCl system at Rw = 4.
[Zn(tpp)] = 5.0 × 10−6 mol dm−3, [CTAC] = 0.10 mol
dm−3, [HCl] = 2.2 × 10−2 mol dm−3, and t = 25.0°C.
The working Zn(tpp) solution and the working reversed micel-
lar solution containing a concentrated acid or a concentrated aque-
ous salt solution in a couple of reservoirs of the stopped-flow ap-

T. Nakashima et al.
Bull. Chem. Soc. Jpn., 75, No. 8 (2002) 1749
Cl− throughout the reaction and its formation constant is con-
tinually maintained. In this circumstances, it is likely that the
above step of Eq. 1 is much more rapid than the following step:
Zn(tpp)Cl− + Cl− + 4H+ → H4(tpp)2+(Cl−)2 + Zn2+ (2)
The negatively charged Zn(tpp)Cl− complex may be stabilized
by forming an ion-pair like Zn(tpp)Cl−CTA+ in the interfacial
and bulk organic phases that are low polar media.14 The CTAC
surfactant seems to serve as a phase transfer catalyst of Zn(tpp)
from the bulk organic phase to the interfacial phase of the re-
verse micelle where the irreversible dissociation of Zn(tpp)
may proceed.
In a comparative study, an acid dissociation of Zn(tpp) was
examined in chloroform acidified with HCl, dispersed by using
nitromethane as a solubilizing agent. The interaction between
Zn(tpp) and nitromethane is expected to be small because of its
relatively small donor number, DN = 2.7,20,21 and in fact no
spectral change was observed on the addition of small amounts
of nitromethane. The dissociation of Zn(tpp) in the acidified
chloroform occurs more rapidly to produce H4(tpp)2+(Cl−)2
and Zn(tpp)Cl−, which may exist in an ion-pairing form,
Zn(tpp)Cl−H+:15 The 26% gain in H4(tpp)2+(Cl−)2 was ob-
tained with 3.8 × 10−5 mol dm−3 HCl. However, the reaction
reached an equilibrium and ceased immediately, so the Eq. 2
step may be reversible. In the CTAC reversed micellar medi-
um, all runs have the common characteristic of an initial loss
of the zinc porphyrins followed by a first-order dissociation
process. The initial dissociation was too rapid to be moni-
tored, because of the deadtime (90 ms) of the flow apparatus
used. This kinetic behavior may be explained as follows: The
reactants are in a more rapid equilibrium initially to give
H4(tpp)2+(Cl−)2, likely due to the presence of the acid in mere
trace and limited amounts in the bulk organic phase, that being
the case in chloroform. Once the rapid equilibrium has been
established, the subsequent dissociation reaction proceeds at a
lower rate, probably in the interfacial phase of the reverse mi-
celle. Thus, the starting absorbances are not obtained from the
sum of the zinc porphyrins alone, but include a contribution
from H4(tpp)2+(Cl−)2 initially produced. These results imply
that uptake of the released Zn2+ ion by the reverse micelle into
its water pool may play an important role in the irreversible
dissociation of Zn(tpp). The stabilization of the Zn2+ ion due
to hydration or water coordination in the water pool may prop-
agate the reaction. In addition, the complexing Cl− ion may
facilitate the removal of Zn2+ ion from porphyrin ring in con-
cert with the protonation of the porphyrin.22
Fig. 2. Plots of log [(CP − [H4(tpp)2+(Cl−)2])/(CP − [H4-
(tpp)2+(Cl−)2]0)] vs time for the dissociation of Zn(tpp) at a
fixed HCl concentration of 2.9 × 10−2 mol dm−3 on the
basis of total volume of CTAC reversed micellar solutions
at Rw = 3 (ꢀ, [HCl]pool = 5.3 mol dm−3), 4 (ꢁ, [HCl]pool
= 4.0 mol dm−3), 5 (ꢂ, [HCl]pool = 3.2 mol dm−3), and 6
(ꢃ, [HCl]pool = 2.7 mol dm−3).
where kobs is the pseudo-first-order rate constant and indepen-
dent of CP. The log [(CP − [H4(tpp)2+(Cl−)2])/(CP − [H4(tpp)2+
-
(Cl−)2]0)] values, obtained from the spectral data at various
molar ratios of water-to-surfactant (Rw = [H2O]/[CTAC]) with
a constant CTAC concentration of 0.1 mol dm−3, were plotted
against time in Fig. 2, where the designation [H4(tpp)2+(Cl−)2]0
represents the aforementioned concentration of H4(tpp)2+(Cl−)2
initially produced. Furthermore, the fraction of the Zn(tpp)Cl−
complex produced, FZn(tpp)Cl (= [Zn(tpp)Cl−]/[Zn(tpp)ꢀ]) can
be obtained from the spectral data using the respective molar
absorptivities of Zn(tpp), Zn(tpp)Cl− and H4(tpp)2+(Cl−)2 in
the same manner as reported in our previous paper.14 No
change in the value of FZn(tpp)Cl with time was observed, con-
firming that Zn(tpp)Cl− is almost maintained in constant frac-
tion throughout the reaction. The value will be related to kobs
as described below.
With an increase in Rw, an increase in the size of a reversed
micellar water pool and changes in its physicochemical prop-
erties have been noted.23–26 In the case with the water pool
containing HCl at a fixed concentration, increasing the Rw val-
ue along with an increase in the water content at a constant
CTAC concentration also causes an increase in the total
amount of aqueous HCl dispersed in the reversed micellar so-
lution. However, the rate observed for the first-order dissocia-
tion is gradually reduced as Rw increases. Since an increase in
Rw results in a decrease in the Zn(tpp)-solubilizing ability of
the reverse micelle, probably due to the Cl− hydration in the
interfacial phase,14 this should be reasonable. On the other
hand, a steep decrease in the dissociation rate was observed
when the HCl concentration in the water pool, represented by
the designation [HCl]pool, was decreased with an increase in Rw
or in the water content, but the total amount of HCl in the re-
versed micellar solution was kept constant ([HCl] = 2.9 × 10−2
mol dm−3), as shown in Fig. 2. The concentrations in square
brackets refer to the total volume of reversed micellar solution
unless otherwise noted. Further, the influence of the number
of water droplets in the reversed micellar bulk solvent on the
For the determination of the rate law, we monitored the gain
in H4(tpp)2+(Cl−)2, which is equivalent to the loss of total zinc
porphyrins [Zn(tpp)’] = [Zn(tpp)] + [Zn(tpp)Cl−] = CP
− [H4(tpp)2+(Cl−)2], where CP is the total concentration of
porphyrins. In the presence of large excesses of CTAC and
HCl compared to Zn(tpp) (CP = 5.0 × 10−6 mol dm−3), the re-
action of the H4(tpp)2+(Cl−)2 formation was first order in total
zinc porphyrins and thus can be accommodated by a rate law
of the following form:
d H (tpp)2+(Cl−)2
d Zn(tpp)ꢀ
[
]
=
[
]
= kobs Zn(tpp)ꢀ
4
(3)
[
]
dt
dt

1750 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
Dissociation of Zn(tpp) in Reverse Micelles
dissociation rate was examined while changing both the CTAC
and water concentrations at a fixed Rw value of 4. When 4.0
mol dm−3 HCl was dispersed as a reversed micellar aqueous
core, an increase in the number of reverse micelles resulted in
no change in the kobs value. These observations suggest that
the rate for the step of Eq. 2 is more sensitive to a change in the
local concentration of HCl or the hydrogen ion, likely distrib-
uted to the interfacial phase, compared with its total concentra-
tion in the reversed micellar solution.
Effect of Acid Anions. A comparative study for the aque-
ous acids used as the reversed micellar aqueous core was also
carried out with various concentrations of acids such as
HClO4, HBr, H2SO4, HNO3 and CH3COOH. The use of CH3-
COOH caused no acid dissociation reaction, probably due to
its weak acidity. Despite differences in the acid anions, how-
ever, other initial spectra and changes in the spectra were simi-
lar to those obtained with HCl; for all the other acids, the two
initial bands of Zn(tpp) and Zn(tpp)Cl− and the final band of
Fig. 4. Correlation between kobs and FZn(tpp)Cl for the dissoci-
ation reaction of Zn(tpp) observed with different acids dis-
persed as the aqueous core of the CTAC reverse micelle in
H4(tpp)2+(Cl−)2 appeared. The Zn(tpp)Br− (434 nm) and
chloroform at Rw = 4. [Acid] = 3.6× 10−2 mol dm−3
.
H4(tpp)2+(Br−)2 (449 nm)19 bands and the H4(tpp)2+(ClO4
)
2
−
(440 nm)19 band may be expected but were not observed with
HBr and HClO4, respectively. These observations may reflect
distribution of the Cl− ion alone outside the water pool, so it
seems likely that almost the same situation arise in all the cases
with acids. However, Fig. 3 reveals differences in an accelera-
tion effect of acids on the reaction rate at higher acid concen-
trations ([Acid]pool > 3 mol dm−3 in the water pool): The kobs
increased in the order: HNO3 < HCl ꢀ H2SO4 < HBr <
HClO4 at each acid concentration. This sequence seems to lie
in the order of increasing acidity of concentrated acid solutions
for which the Hammett acidity functions are useful for com-
paring different acid media,27 where no secondary dissociation
of H2SO4 is considered to occur. On the other hand, the frac-
tion of Zn(tpp)Cl−, FZn(tpp)Cl mentioned above is found to in-
crease with an increase in the acid concentration and also with
the acid in the same order at each acid concentration. Figure 4
illustrates a linear relationship between the kobs and FZn(tpp)Cl
values obtained with different aqueous acids present in the re-
spective reversed micellar water pools at the same concentra-
tion of 5.0 mol dm−3. This experimentally obtained relation-
ship is reproduced by the following equation:
kobs = kH FZn(tpp)Cl
(4)
where the rate constant kH may be dependent on the hydrogen
ion concentration as discussed below. Also, from Eqs. 3 and 4,
kobs[Zn(tpp)ꢀ] = kH FZn(tpp)Cl [Zn(tpp)ꢀ] = kH [Zn(tpp)Cl−],
which may be obeyed by Eq. 2. This strongly suggests that the
acceleration effect of acids on the reaction rate is attributed to
an increase in the fraction of Zn(tpp)Cl−, which may be caused
by elevation in the activity of CTAC or the Cl− ion in the step
of Eq. 1. This is likely due to an enhancement in the selective
distribution of the Cl− ion to the interfacial and/or the bulk or-
ganic phase by the presence of the acid anions. A similar se-
lectivity for Cl− was observed for location of a chromoiono-
phore binding with anions in the hydrophobic layer of a cation-
ic vesicle.28 As reported previously,14 an increase in the local
Cl− concentration in the interfacial and bulk organic phases
causes an enhancement in the Zn(tpp)Cl− formation, which
would promote interfacial solubilization of the complex, pre-
sumably bringing about its dissociation. Therefore, the
FZn(tpp)Cl value may be used as a measure of the local Cl− con-
centration.
The observed trend in kobs or FZn(tpp)Cl appears to be fairly
consistent with an increase in the relative binding ability of
acid anions to normal CTA+ micelles: The scales of CTA+ af-
finity of anions29 may be also correlated with a typical lyotro-
2−
pic series for anions:30 SO4 < CH3COO− < Cl− < Br− <
−
NO3−, ClO4 < I− < SCN−. It seems likely that there is a
competition between the counter Cl− ion and the acid anion for
the positively charged head group of the CTA+ surfactant at
the reversed micellar interface. Since the micellar surface can-
not be oversaturated by anions, it is postulated that, for more
strongly CTA+-bond anions such as the ClO4− ion, the Cl− ion
is to some extent exchanged and transferred from the interface
to both the surfactant hydrophobic layer and water pool of the
Fig. 3. Variations of kobs for the dissociation reaction of
Zn(tpp) with the concentrations of HClO4 (ꢁ), HBr (ꢃ),
HCl (ꢄ), H2SO4 (ꢅ), and HNO3 (ꢂ) in the dispersed aque-
ous core of CTAC reverse micelles formed in chloroform
at Rw = 4.

T. Nakashima et al.
Bull. Chem. Soc. Jpn., 75, No. 8 (2002) 1751
Fig. 5. Correlation between kobs and FZn(tpp)Cl for the dissoci-
ation reaction of Zn(tpp) observed with different salts (1.0
mol dm−3) in 3.0 mol dm−3 HCl dispersed as the aqueous
core of the CTAC reverse micelle in chloroform at Rw = 4.
Fig. 6. Correlation between kobs and FZn(tpp)Cl for the dissoci-
ation reaction of Zn(tpp) observed with different volume
ratios of chloroform to cyclohexane used as the CTAC re-
versed micellar bulk solvent at Rw = 4. [HCl] = 3.6 ×
10−2 mol dm−3
.
reverse micelle; this may lead to an increase in the Cl− concen-
tration and thus to an increase in the fraction of Zn(tpp)Cl− in
the hydrophobic layer and the bulk organic phase. Such an ex-
change equilibrium at the interface may be regarded as signifi-
cant in the step of Eq. 1.
To investigate the anion effect on the reaction rate further,
3.0 mol dm−3 HCl containing a 1.0 mol dm−3 inorganic salt
such as LiCl or LiClO4 was dispersed as the reversed micellar
aqueous core. The presence of the salt caused a pronounced
enhancement in FZn(tpp)Cl and the larger magnitude of the in-
crease was obtained with LiClO4 compared with LiCl as
shown in Fig. 5. A similar trend in the anion effect on the
Zn(tpp)Cl− formation was observed in the absence of the acid
in the reversed micellar water pool.31 Also, Figure 5 shows
that there is a reasonable correlation of kobs vs FZn(tpp)Cl in
agreement with that obtained with the different acids, support-
ing the above explanation.
Effect of Bulk Organic Solvent. In our previous studies,
the formation of Zn(tpp)Cl− was found to be affected by the
chloroform fraction in the reversed micellar bulk solvent. It
has been reported that, in the core of an aggregate formed in
CTAC–chloroform solution, chloroform molecules form hy-
drogen bonds directly with the counter Cl− ions of CTAC.32
Therefore, the effect of the bulk solvent due to hydrogen bond-
ing may be expected to be responsible for the variation of the
reaction rate with solvent composition. As shown in Fig. 6,
where the data were obtained using 5.0 mol dm−3 HCl as the
reversed micellar aqueous pool, obviously, the larger the chlo-
roform fraction in the bulk solvent, the greater both the kobs and
FZn(tpp)Cl values become. The plot is also linear, implying that
the reaction process given by the aforementioned scheme is
not altered by the change in solvent composition. The interac-
tion between the counter Cl− ion and chloroform may contrib-
ute to the selective distribution of the Cl− ion to the surfactant
hydrophobic layer and then to the bulk organic phase, thus
leading to the increase in the Cl− concentration in both the
phases.
Fig. 7. Variations of kH for the dissociation reaction of
Zn(tpp)Cl− with the acid concentrations in the dispersed
aqueous core of CTAC reverse micelles at Rw = 4. With
the different acids (ꢅ) and the different compositions of
the reversed micellar bulk solvent (ꢁ), the kH values were
obtained as slopes of the kobs vs FZn(tpp)Cl plots at respective
acid concentrations.
kobs vs FZn(tpp)Cl at each acid concentration used. The logarith-
mic relationship between the obtained kH value and the acid
concentration in the water pool is shown in Fig. 7. The fair
agreement between the plots obtained with different acids and
the different compositions of bulk organic solvent suggests
that the reaction mechanism of Eq. 2 is independent of these
differences. A slope of the linear plots is obtained as 3.4
0.3, indicating that the rate law for Eq. 2 is third order in hy-
drogen ion.
Under the present experimental conditions (Rw = 4 and
[CTAC] = 0.1 mol dm−3 in chloroform), our previous work14
predicts that 26% of the Zn(tpp)Cl− complex will be solubi-
lized in the interfacial phase. Since the molecular dynamics
simulation of a model reverse micelle reveals the water distri-
bution to the surfactant hydrophobic layer,33 it may be reason-
Mechanism of Acid Dissociation. The kH value men-
tioned above was obtained from the slope of the linear plot of

1752 Bull. Chem. Soc. Jpn., 75, No. 8 (2002)
Dissociation of Zn(tpp) in Reverse Micelles
Fig. 8. Schematic representation of the acid dissociation reaction of Zn(tpp) in the interfacial phase of the CTAC reversed micellar
system.
able to consider that hydration of the hydrogen ion should
cause significant penetration of the hydrated proton into the in-
terfacial phase, in which the Eq. 2 step proceeds. A quantita-
tive model for the above situation may be constructed by ap-
plying the pseudophase model,34 which was successfully used
in our previous work.14 With this model (Fig. 8), the reversed
micellar media is assumed to be divided into three
pseudophases, corresponding to the bulk organic phase, inter-
facial phase and water pool. In view of an amphiphilic charac-
teristic of H4(tpp)2+(Cl−)2, this salt appears to be accumulated
in the interfacial phase and thus it is assumed that the partition
coefficient of H4(tpp)2+(Cl−)2, KP between the interfacial and
bulk organic phases is large enough.35 Therefore, the observed
pseudo-first-order rate constant can be obtained as follows:
considered to be a steady-state intermediate and the third at-
tacking of HCl to the intermediate is the rate-determining step.
Hence the mechanism of Eq. 2 is replaced by that shown in
Fig. 8. In a comparative study with the reversed micellar solu-
tions containing the acids, the protonation of the free base por-
phyrin, H2(tpp), to produce H4(tpp)2+(Cl−)2 was observed to
be much faster than the acid dissociation of Zn(tpp) and to
complete immediately, so that it was not monitored. This ob-
servation indicates that H2(tpp) is not formed prior to the rate-
determining step in the Zn(tpp) dissociation process. The
steady-state intermediate represented in Fig. 8 may be regard-
ed as a sitting-atop (SAT) complex, in which the zinc(Ⅱ) ion
sits on the porphyrin plane and two N–H protons are present.
The SAT complex has been reported to exist in the incorpora-
tion process of zinc(Ⅱ) ion into 5,10,15,20-tetraphenylporphy-
rin.38,39 Recently, we spectrophotometrically detected the SAT
complex of copper(Ⅱ) ion with 5,10,15,20-tetrakis[4-(N-trime-
thylammonio)phenyl]porphyrin in CTAC reversed micellar
media,40 and found that the interfacial phase plays an impor-
tant role in formation of the SAT complex. As described
above, on the other hand, the occurrence of the isosbestic point
can imply that appreciable amounts of reaction intermediates
such as an SAT complex are not present, but there may be very
small stationary-state concentrations, presumably reflecting
the stability of the SAT complex formed prior to the zinc atom
dissociation following the third attacking of proton in the inter-
facial phase of the reverse micelle. This view may provide the
reason for the observed third-order dependence of [H+]pool, al-
though further investigations on the formation of the SAT com-
plex in the reverse micelles are required.
3
kobs = kiKiKH [H+]3poolfiFZn(tpp)Cl
(5)
where ki is the rate constant for the rate-determining step in the
interfacial phase, Ki is the formation constant of (Intermedi-
ate)i in the interfacial phase, KH is the partition coefficient of
hydrated proton between the interfacial phase and water pool,
and fiFZn(tpp)Cl is the concentration ratio of the solubilized
Zn(tpp)Cl− relative to the total zinc porphyrins, i.e., fiFZn(tpp)Cl
= [Zn(tpp)Cl−]i/[Zn(tpp)ꢀ].37 From Eqs. 4 and 5, kH = kiKi-
3
KH fi[H+]3pool. Since fi = 0.264 under the experimental condi-
tions used,37 the value of kiKiKH3 for the dissociation in the in-
terfacial phase is obtained as 0.84 dm9 mol−3 s−1 from the in-
tercept of the logarithmic plot as shown in Fig. 7.
It has been pointed out that the reaction order is highly sen-
sitive to the reaction media.18 In dimethylformamide acidified
with HCl, the rate law for the dissociation of Zn(tpp) is sec-
ond-order in hydrogen ion,16 while in methanol acidified with
HCl, the rate law for the zinc etioporphyrin(Ⅲ) complex is
third-order in HCl.15 On the other hand, a first-order depen-
dence of hydrogen ion concentration was obtained for the ki-
netics of the dissociation of Zn(tpp) at a dodecane–aqueous
acid interface by using a centrifugal liquid membrane meth-
od.17 In this work, the observed third-order behavior with in-
creasing [H+]pool suggests that the mechanism of Eq. 2 is simi-
lar to that for the zinc etioporphyrin(Ⅲ) complex in the acidi-
fied methanol,15 in which the zinc porphyrin•2HCl complex is
Conclusion
Acid dissociation of Zn(tpp) occurs to produce a diacid salt,
H4(tpp)2+(Cl−)2 in the CTAC reversed micellar solution con-
taining concentrated acids. With the different acids, an in-
crease in the dissociation rate with an enhancement in the for-
mation of Zn(tpp)Cl− was observed. A good relationship be-
tween them suggests that the dissociation probably proceeds
via the Zn(tpp)Cl− formation which promotes the interfacial
solubilization of Zn(tpp). Also, the acid anions may play an
important role in determining the acid dependence of the rate.

T. Nakashima et al.
Bull. Chem. Soc. Jpn., 75, No. 8 (2002) 1753
20 V. Gutmann, “The Donor–Acceptor Approach to Molecu-
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21 V. Gutmann, G. Resch, and W. Linert, Coord. Chem. Rev.,
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22 P. Hambright and E. B. Fleischer, Inorg. Chem., 9, 1757
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23 J. Lang and G. Mascolo, J. Phys. Chem., 94, 3069 (1990).
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Acceleration effects of the anion and the reversed micellar
bulk solvent may be attributable to an increase in the local Cl−
ion concentrations in the interfacial and bulk organic phases, in
which selective distribution of the Cl− ion may be influenced
by the affinities of the anions for the surfactant head group and
by the Cl− solvation.
This work was partially supported by a Grant-in-Aid for
Scientific Research No. 12640591, from the Ministry of Edu-
cation, Science Sports and Culture.
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