Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 1, pp. 55-60, 2005

Accepted
Ion Transport Across a Bilayer Lipid Membrane in the Presence of a Hydrophobic Ion or
an Ionophore
Osamu Shirai,*, a Akihiro Uehara,a Hajimu Yamana,a Toshihiko Ohnuki,b Yumi Yoshida,c andSorin KiharacaResearch Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japanc
Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japanb

Received: November 15, 2004; In Final Form: April 21, 2005
The ion transport from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) in a cellsystem in the presence of a hydrophobic ion or an ionophore was investigated by voltammetry. The ion transport
current was observed by addition of a small amount of hydrophobic ion such as tetraphenylborate, dipicrylaminate, etc. into W1 or/and W2 containing a hydrophilic salt serving as a supporting electrolyte. The hydrophobic
ion was distributed into the BLM with the counter ion to hold the electroneutrality within the BLM. It waspointed out that the counter ion could transfer between W1 and W2 across the BLM since concentrations of thecounter ion in W1, BLM, and W2 were so high as to cause the ion transfer current while concentrations of thehydrophobic ion were very low. The facilitated transports of alkali ions across a BLM containing valinomycin(Val) used as an ionophore were also investigated by considering the hydrophobicity of both the objective cationand the counter anion and the formation of the alkali metal ion-Val complex.

from the viewpoint of the distribution of ion pairs between the
aqueous phase and the BLM.

1. Introduction
The ion transfer from one aqueous solution (W1) to another(W2) across a bilayer lipid membrane (BLM) has been investigated extensively for a fundamental understanding of thefeatures of ion transfer across biomembranes.1, 2 An ion
transfer current is observed when ion transporters such as ionchannel forming peptides and ion carrier compounds are added
into the BLM or hydrophobic ions are added to the aqueous
phase in the presence of hydrophilic supporting electrolytes,
respectively.2−6 Most of the authors have assumed that the iontransfer current is caused by the transfer of ions associatedwith the transporter or that of the hydrophobic ion, since only
the associated ions or hydrophobic ions can spontaneously
distribute from the aqueous phase to the BLM. However, theelectroneutrality within the BLM is not held in this mechanism.
We observed that the magnitude of the ion transfer current
was proportional, not only to the hydrophobicity of an additivehydrophobic ion but also to that of a counter ion in aqueous
phases.7-9 Then, we discovered that the counter ion spontaneously moved into the BLM with the additive hydrophobic ion
and that the counter ion but not the hydrophobic ion transferred
across the BLM. On the other hand, it has been recognized thatthe ion transfer facilitated by an ionophore is attributed totransfer of the ion associated with the ionophore.2, 10 However,
Wittenkeller et al. showed that the ion pair of a cation associated with valinomycin (Val) and a counter anion was distributed between the aqueous phase and the BLM.11, 12 We foundthe same distribution of the cation-Val complex and a counter
anion as that found by Wittenkeller et al. and observed thecontinuous transfer of the cation and the anion across the BLM
containing Val.13
In the present work, ion transfers across a BLM in the presence of hydrophobic ions and facilitated transfers of alkali ions
across a BLM containing Val as an ionophore were reviewed*Corresponding author. E-mail: shirai@HL.rri.kyoto-u. FAX:
+81-724-51-2634.

2. Experimental
Chemicals. Lipids used to form the BLM were L-α-phosphatidylcholine (Merck, No. 544274), PC, and cholesterol (Wako
Pure Chemical Ind., Ltd., No. 087-21), Ch. Valinomycin waspurchased from the Sigma Chemical Co. (No. 41K4005).
All other reagents were of reagent grade and used without
further purification.
Preparation of BLMs. Two aqueous compartments filledwith 15 mL of aqueous solution were separated by a 0.2 mm
thick tetrafluoroethylene resin sheet (a product of Mitsui
Fluorochemical Co.) as shown in Figure 1. The BLM was
obtained as a black lipid membrane by brushing the n-decane

Ag/AgCl Ag/AgClelectrode 1 electrode 2
Platinum
electrode 1

Electrolyte solution Acrylic cell

Platinum
electrode 2

Teflon sheet

Figure 1. The schematic diagram of the electrochemical cell used in
the present study.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/31/2005

ShiraiJ. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

solution of each lipid or both lipids and Val on a 1 mm diameter
aperture made in the tetrafluoroethylene resin sheet.8, 9 The
formation of a BLM was confirmed by microscopic observation and capacitance measurement. Valinomycin was dissolvedin ethanol at a concentration of 10−3 M. Here, M denotes mol
dm−3. An adequate volume of ethanol solution was added into
a 1 mL flask. After the ethanol was evaporated, the BLMforming solution was prepared by adding a mixture of Val,~ 10 mg of PC and ~ 5 mg of Ch with n-decane to the flask.
Voltammetric measurement. The BLM system employedin the present study is indicated by eq 1.
(W1)

(BLM)

0.1 M Hydrophilic salt
CE1
RE1

0.3
0.2

I/µA cm -2

56

1
2
3

0.1
0
-0.1

(W2)
0.1 M Hydrophilic salt
RE2
CE2 (1)EW1-W2
IW1-W2where a hydrophilic salt such as LiCl, NaCl, KCl, RbCl, CsCl,
LiBr, NaBr, KBr, RbBr, CsBr, Li 2 SO 4 , Na 2 SO 4 , K 2 SO 4 ,Rb2SO4, or Cs2SO4 was used as the supporting electrolyte in
the aqueous phases.
The electrochemical cell used for the voltammetric measurement with a BLM system was essentially the same as that
proposed by Tien,14 and placed in a Faraday cage during themeasurement in order to decrease the background noise. The
voltammogram for the ion transfer from W1 to W2 across theBLM was recorded by scanning the potential difference
between W1 and W2, EW1-W2, and by measuring the current
between W1 and W2, IW1-W2. Two silver-silver chloride electrodes, RE1 and RE2, and two platinum wire electrodes, CE1
and CE2, were used to apply the EW1−W2 and to measure IW1-W2,
respectively.
All voltammograms were measured by scanning EW1-W2 at a
rate of 0.01 – 0.10 Vs−1 at 25 ± 0.5 ˚C, unless otherwise mentioned.
Apparatus. The potentiostat, the function generator and the
X-Y recorder used for the voltammetric measurement were a
Model HA-1010mM1A (Hokuto Denko Co.), Model HB-105(Hokuto Denko Co.) and Model F-5C (Riken Denshi Co.),
respectively.
3. Ion Transport Across a BLM by Addition of Hydrophobic
Ions into W1 and/or W2 in the Presence of Hydrophilic
Electrolytes in W1 and W2
As an example, curve 1 in Figure 2 shows the voltammogram recorded with W1 and W2 containing 0.1 M MgSO4 byscanning EW1-W2 in the region between +0.1 and –0.1 V and,simultaneously, by measuring I W1-W2. No current peak was
observed in the voltammogram. Similar voltammograms wereobserved when KCl, NaCl, MgCl2, K2SO4, and Na2SO4 wereused as hydrophilic salts in W1 and W2 instead of MgSO4.8, 9
By the addition of one of extremely hydrophobic ions ofeither dipicrylaminate, DPA−, or tetraphenylborate, TPhB−,
into W1 of the cell system (1) (Curves 2 and 3 in Figure 2), apair of well established positive and negative peaks symmetrical about the origin (the point of 0 V and 0 A) appeared, eventhough the concentration of the added hydrophobic ion wasvery dilute, less than 10−6 M.7−9 The half-peak potentials ofboth the positive and negative peaks were determined to bearound 0 V.
When DPA− was added into W1, the peak current density
was nearly proportional to the concentration of DPA− in therange between 5 × 10−8 and 10−5 M and to the square root of
the scan rate of EW1-W2 in the range between 0.01 and 0.1 Vs−1.
As shown in Table 1, the current density increased slightly
with the increase of the concentration of the hydrophilic salt,

-0.2
-0.3
-0.15

-0.10

-0.05

0

0.05

0.10

0.15

E/V
Figure 2. Voltammograms for the ion transfer through a BLM composedof PC and Ch. Curve 1: 0.1 M MgSO4 in both W1 and W2. Curves 2
and 3: same as curve 1, but in the presence of 10−6 M DPA− in W1(curve 2) and 10−6 M TPhB− in W1 (curve 3). Scan rate of EW1-W2: 0.01
Vs−1.

TABLE 1: Peak Current Densities in Cyclic Voltammograms Observed with the Addition of DPA–, TPhB–, andCV+ to be 10–6 M9Supporting
electrolyte
in W1 and W2
0.1 M MgSO4
1 M MgSO4
0.1 M K2SO4
0.1 M Na2SO4
0.1 M MgBr2
0.1 M MgCl2
0.2 M KCl
0.2 M NaCl

Peak current density / µA cm−2
Ion added in W1 (10−6 M)
−
TPhB−
CV+
DPA
0.20 ± 0.03
0.23 ± 0.03
0.60 ± 0.10
0.22 ± 0.03
0.21 ± 0.03
0.20 ± 0.03
0.62 ± 0.10
0.19 ± 0.03

0.10 ± 0.02
0.12 ± 0.02
0.25 ± 0.03
0.12 ± 0.02
0.12 ± 0.02
0.11 ± 0.02
0.23 ± 0.03
0.13 ± 0.02

0.04 ± 0.01
0.05 ± 0.01
0.04 ± 0.01
0.04 ± 0.01
0.05 ± 0.01
ca. 0
ca. 0
ca. 0

MgSO 4 , in aqueous phases from 0.1 to 1 M, and increased
when the hydrophilic salt in aqueous phases was changed from
0.1 M MgSO4 to 0.1 M K2SO4 or 0.1 M Na2SO4. By adding
the same amount of TPhB− instead of DPA− into W1 of thesystem in the presence of 0.1 M MgSO4 in W1 and W2, thecharacteristics of the voltammogram were similar to those with
DPA − , except that the peak current density was smaller.
Similar voltammograms were also observed by the addition ofdilute (5 × 10−7 to 10−5 M) crystalviolet, CV+, or ethylviolet,
EV+, into one of two aqueous phases containing 0.1 M MgSO4
or MgBr2, respectively. The peak current densities depended
on the species of the anion in the hydrophilic salt, but werepractically independent from the hydrophilic cation in the salt,
as shown in Table 2.9, 15 Here, standard Gibbs free energy canbe regarded as the measure of the hydrophobicity of the ion. It
is well known in liquid-liquid extraction chemistry that the
distribution coefficient of the ion pair depends on hydrophobicities of the added ion and the counter ion. Similarly, thehydration energy can be also regarded as the measure of thehydrophobicity (Table 3).
These results introduced a new ion transfer mechanism
shown in Figure 3. Not only the hydrophobic ion (A− or C+)
but also the counter ion (i + or j − ) are distributed from theaqueous phase to the BLM. In the case of the addition ofhydrophobic ions such as DPA−, TPhB−, CV+, and EV+, theadded ion and the counter ion are distributed to be about 10−4
M. Therefore, the ion transfer current in the voltammogram is

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Ion Transport Across a Bilayer Lipid Membrane

TABLE 2: Standard Gibbs Free Energies for Transfers, ∆Gtr0,
of Various Ions from Aqueous, W, to nitrobenzene, NB,8 1,2dichloroethane, DCE,8 or Acetonitrile, AN15

DPA−
TPhB−
Pic−
ClO4−
Br−
Cl−
SO42−
EV+
CV+
TPhAs+
TPenA+
TBA+
TPrA+
TEA+
TMA+
K+
Na+
Mg2+

W/NB
∆Gtr0/kJ mol−1

W/DCE∆Gtr0/kJ mol−1i) Addition of hydrophobic anion ii) Addition of hydrophobic cation

W/AN∆Gtr0/kJ mol−1

W1

BLM

W2

W1

i+

i+

i+

i+

j-

j-

j-

C+

C+

j-

-39.4
-35.9
-4.6
8.0
28.4
31.4
>67.3
ca.-44*
-39.5
-35.9
-35.1
-24.0
-10.0
-5.7
3.4
23.4
34.2
69.6

-35.1
17.2
38.5
46.4

-32.4
-4
2
31.3
42.1

A-

A-

BLM

W2
i+
j-

Figure 3. Ion transfer mechanisms with the addition of a hydrophobic
anion and hydrophobic cation. i+: hydrophilic cation, j−: hydrophilic
anion, A−: hydrophobic anion and C+: hydrophobic cation.

0.08

-35.1
-34.7
-21.8
-8.8
4.2
17.6

0.06

-32.8

0.04

-31
-13
-7
3
8.1
15.1

I/µA cm-2

Ion

57

1
2

0.02
0
-0.02
-0.04
-0.06

TABLE 3: The Molar Gibbs Free Energies of the Hydration of Ions18
Cation
+

Li
Na+
K+
Rb+
Cs+
TPhAs+

0tr∆G /kJ mol
475
365
295
275
250
-50

−1

0
tr

Anion

∆G /kJ mol

−

465
340
315
275
205
-50

F
Cl−
Br−
I−
ClO4−
TPhB−

−1

attributable to the transfer of the counter ion, since the concentration of the hydrophobic ion in W1 and W2 is negligible andthe counter ion exists sufficiently in W1, BLM, and W2.
When the hydrophobic ion was added into both aqueous
phases, the peak height became larger than that observed when
the hydrophobic ion was added only in W1 (e.g. about 1.6 times
when 10−6 M DPA− was added in the aqueous phases containing
0.1 M MgSO4).
In the voltammograms that were obtained by the addition of
one of the hydrophobic cations such as tetrapentylammonium,
TPenA+, tetrabutylammonium, TBA+, and tetraphenylarsonium,
TPhAs+, at concentrations of 10−5 to 10−4 M into W1 of the cellsystem, a pair of positive and negative currents of different
magnitudes appeared around the origin. Curve 1 in Figure 4 isthe voltammogram that was recorded by adding 10−4 M TPenA+
into W1 and 0.1 M MgSO4 into both W1 and W2. When oneof the hydrophobic cations was added into both W1 and W2
phases, positive and negative peaks symmetrical about the origin
appeared in the voltammogram (curve 2 in Figure 4). The
magnitudes of the positive and negative peaks (or limiting
currents) were proportional to the concentration of thehydrophobic cation added in the range between 10−5 and 10−4
M, while the ratio of the positive to negative peak (or limiting
current) was almost constant. The ratio depended on the kindof hydrophobic cation that was added. Since the hydrophobicities of these ions are generally smaller than those of DPA−,
TPhB−, CV+, and EV+ as shown in Table 2, the distribution
ratios of an ion pair of the added hydrophobic ion and thecounter ion between the aqueous phase and the BLM phase

-0.08
-0.15

-0.10

-0.05

0

0.05

0.10

0.15

E/V
Figure 4. Voltammograms for ion transfer through a BLM composedof PC and Ch. Curve 1: 10−4 M TPenA+ in W1 and 0.1 M MgSO4 inboth W1 and W2. Curve 2: same as curve 1, but in the presence of
10−4 M TPenA+ in W1 and W2.

were smaller than those for the ions having higher hydrophobicity. As the distribution ratio was smaller, the concentrationof the additive hydrophobic ion needs to be higher to maintaina constant ion transfer current. When the concentration of theadditive hydrophobic ion in aqueous phase was higher than ~10−4 M, a difference in magnitude between the cathodic andanodic current peaks was observed in the voltammogram(Figure 4). The anodic current is caused by the transfer of thecounter ion while the cathodic current is attributed to both thetransfer of the added hydrophobic ion and that of the counterion. These results indicate that the transfer of the addedhydrophobic ion from the aqueous phase to the BLM cannot beignored at concentrations of the hydrophobic ion higher than
10−4 M.
When the hydrophobicity of the added ion is weaker thanthose of the above-mentioned ions, the hydrophobic ion should
be added to reach a concentration greater than 10−4 M in the
aqueous phase. Curve 1 in Figure 5 is a typical result when10−3 M of picrate, Pic-, was added in W1 in the presence of 0.1
M MgSO4 in W1 and W2. Current densities at EW1-W2 = –0.1
(the final descent) and +0.1 V (the limiting current) depended
on the concentrations of both Pic− and Mg2+ in W1. When 10−3
M Pic− was added into both aqueous phases, a symmetry of the
final rise and the final descent about the origin was observed,
as seen as curve 2 of Figure 5.
When a hydrophobic ion was added into one of two aqueous
phases in the presence of a hydrophilic salt, the voltammogram
was transformed depending on the properties and concentration of the added hydrophobic ion. It can be assumed that the
distribution equilibrium of the ion pair between W and BLM issimilar to that between W and organic phase; Org. Equation 2
holds between the distribution ratios, DM and DX, of a cation,
M + , and an anion, X− , and the standard Gibbs transfer free58

ShiraiJ. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005
1.5

0.3
0.2

1
2

0.5

I/µA cm-2

0.1

I/µA cm-2

1
2
3
4

1.0

0

1
2

-0.1

-0.5

3

-0.2

-1.0

-0.3
-0.15

-1.5
-0.3

0

-0.10

-0.05

0

0.05

0.10

0.15

4
-0.2

-0.1

0.1

0.2

0.3

EW1-W2/V

E/V
Figure 5. Voltammograms for the ion transfer through a BLM
composed of PC and Ch. Curve 1: 10 −3 M Pic − in W1 and 0.1 M
MgSO4 in both W1 and W2. Curve 2: same as curve 1, but in thepresence of 10−3 M Pic− in W1 and W2.

Figure 6. Cyclic voltammograms for ion transfer across a BLM
containing Val between W1 and W2 in the presence of 0.1 M KCl.
Potential scanning rate: 0.01 Vs − 1 . Temperature: 298 ± 1 K.
Concentration of Val in the BLM-forming n-decane solution: 10−6
(curve 1); 3 × 10−6 (curve 2); 10−5 (curve 3); 3 × 10−5 M (curve 4).

energies of M+ and X− from W to Org, ∆Gtr, M0 and ∆Gtr, X0,
when W containing a salt, M + ·X − , is equilibrated with anequivolume of Org.16
ln DM = ln DX = 1/2 {–(∆Gtr, M0 + ∆Gtr, X0)/RT

0

4
3
2

Similarly, the distribution ratios of an ion pair of the addedhydrophobic ion and the counter ion between aqueous andBLM phases can be represented. Accordingly, the waveformof the observed voltammogram and the magnitude of ion
transfer current depended on both the hydrophobicity andconcentration of the added hydrophobic and counter ions.
When a hydrophobic ion such as DPA−, TPhB−, CV+, or EV+ isadded into W1 and/or W2, the additive hydrophobic ion
behaves as a carrier of the counter ion within the BLM.
Here, the hydration energy is used as the index of thehydrophobicity. Taking into account the hydration energies ofvarious ions (Table 3) and the transfer energies of various ionsfrom aqueous to Org (Table 2), Cs+ and I− can transfer moreeasily than K+ and Cl−, respectively. Bender and Tien reported
that waveforms of voltammograms obtained by the addition ofI− and I3− were the same as those obtained by the addition of
Pic − and DPA − , respectively. 17 When CsCl was used as ahydrophilic electrolyte instead of KCl, the background current
in the case of CsCl was larger than that in the case of KCl.
This would be one of reasons that 137Cs and 129I, which are
generated by nuclear fission of 235 U, accumulate in livingorganisms more easily than other alkali metal ions and halogen
ions, respectively.

1

4. Ion Transport across a BLM Containing Val as an Ionophore
It has been proposed that an ionophore such as Val or dibenzo18-crown-6 serves as a carrier compound for complex-formingions.2, 10 However, the role of the ionophore in the ion transfer
is still under debate. By considering the electroneutrality
within the BLM and the ion transport mechanism described in
the previous section, the role of the ionophore in the ion transport across the BLM containing Val as an ionophore has beenstudied.
Figure 6 shows cyclic voltammograms for the ion transfer
between W1 and W2 containing 0.1 M KCl across a BLM of
PC and Ch containing Val. When Val was added to a BLM
formed from the n-decane solution containing 10−6 to 3 × 10−5
M Val, the cyclic voltammograms were symmetrical about the

I/µA cm-2

(2)

1
2
3

0
-1
-2
-3
-4
-0.3

-0.2

-0.1

0

0.1

0.2

0.3

EW1-W2 /V
Figure 7. Cyclic voltammograms for the ion transfer across a BLM
containing Val. Potential scanning rate: 0.01 Vs−1. Temperature: 298± 1 K. Concentration of Val in the BLM-forming n-decane solution: 3
× 10−5 M. Electrolyte in W1 and W2: 0.1 M KCl (curve 1); 0.1 M KBr(curve 2); 0.1 M KClO4 (curve 3).

origin, (0 V, 0 A). In the absence of Val in the BLM, no ion
transfer current was observed in the voltammogram by scanning EW1-W2 between –0.1 and 0.1 V and measuring IW1-W2. The
absolute value of IW1-W2, IW1-W2, increased with an increase inEW1-W2. The current density at a specific EW1-W2 was reproducible to about ± 10% on each run. The ion transfer current
at a specific EW1-W2 was not dependent on the scan rate of EW1-W2
in the region from 0.01 to 0.50 Vs−1. This means that the ion
permeation across the BLM is the rate-determining step. The
magnitude of IW1-W2 would be attributed to the electric resistanceof the BLM. As shown in Figure 6, the magnitude of IW1-W2 at
a specific EW1-W2 was proportional to the concentration of Val
in the BLM-forming n-decane solution in the range from 10−6
to 3 × 10−5 M. The molar ratios of PC : Ch : Val in the n-decane
solutions containing 10−6, 3 × 10−6, 10−5, and 3 × 10−5 M Val
were, then, assumed to be about 1 : 1 : 10−3, 1 : 1 : 3 × 10−3, 1 :1 : 10 −2 , and 1 : 1 : 3 × 10 −2 , respectively. Therefore, themagnitude of IW1-W2 at a specific EW1-W2 was also proportional tothe molar ratios of PC : Ch : Val.
Figure 7 shows the cyclic voltammograms for the iontransfer across a BLM containing Val between W1 and W2

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Ion Transport Across a Bilayer Lipid Membraneln DM = ln DX= 1/2 {– (∆Gtr, M0 + ∆Gtr, X0 )/RT + ln (1 + Kst Cligand)}= – (∆Gtr, M* + ∆Gtr, X0 )/2RT,

-3

ClO4-

-3.1
-3.2

log I at 0.1V

containing 0.1 M KCl, KBr and KClO4. These waveforms werealso analogous to that in Figure 6. The magnitude of IW1-W2 at adefinite EW1-W2 was altered by a change in anion species. The
magnitude of IW1-W2 at 0.1 V increased with the increasing ionicradii of the anionic species. This disagrees with the current
magnitudes estimated by the conventional concept that only K+
can transfer across a BLM. In all cases, the magnitude of theion transfer current at a definite EW1-W2 is directly proportional
to the concentration of Val in the BLM. The transfers of alkali
metal ions from W1 and W2 to the BLM are facilitated by theformation of a complex with Val in the BLM by analogy withstudies on facilitated ion transfer at the interface between
aqueous and organic solutions.16, 17 If only the alkali ion is
distributed from W1 or W2 to the BLM, the electroneutrality is
not maintained within the BLM phase. The authors suggested
that the complex-forming ion was distributed with the counterion from W1 and/or W2 to the BLM in the presence of Val in
the BLM.13 Wittenkeller et al. pointed out that the alkali metal
ion was distributed from W1 and W2 to the BLM containing
Val with the counter anion as an ion pair. 11, 12 It can beassumed that the distribution equilibrium of the ion pairbetween W and BLM is similar to that between W and Org
phases as written in eq 2. In this case, the stabilization of thecation by complex formation with Val should be estimated.
The distribution ratio, DM, of a cation, M+, is denoted using thestandard Gibbs transfer free energies of M+ from W to Org,
∆Gtr, M0, and the effect of activity change of the cation in Org,ln (1 + Kst Cligand), when W containing a salt, M+ X−, is equilibrated with an equivolume of Org.16 Therefore, eq 2 can be
rewritten as follows:

59

-3.3

Br-

-3.4

Cl-

-3.5
-3.6
-400

-350

-300

-250

-200

-150

G hyd0/kJ mol-1
Figure 8. The relationship of log I to hydration energy of anion.

Our proposal

Conventional process

W1

BLM

W2

W1

BLM

W2

i+

i+

i+

i+

i+

i+

j-

j-

j-

j-

j-

: Carrier compounds

Electroneutrality holds in the BLM(3)

Figure 9. The ion transfer mechanism of the ion transfer across the
BLM.

where, Cligand indicates the concentration of the ligand in Org,and Kst is the stability constant of the M+-ligand complex inOrg. ∆Gtr, M* is then the seeming transfer energy of M+ from
W to Org, and ∆Gtr, M* is defined by the relation ∆Gtr, M* =∆Gtr, M0 - ln (1 + Kst Cligand). Equation 3 does not involve theformation of ion pairs in either W or Org phases, and the activity
coefficients of M+ and X− in W and Org phases are regarded asunity. This indicates that DM depends not only on ∆Gtr, M0 but
also on ∆Gtr, X0 and the stabilization achieved by forming thecomplex. Similarly, pairs of K+ and the counter ion will be
distributed from the aqueous phase to the BLM. Therefore, itis thought that the currents flowing at both the W1|BLM and
BLM|W2 interfaces are caused by the transferring of not only
K+ but also the counter ions, ClO4−, Br−, and Cl−. Since the ion
permeability depends on the ion concentration, the diffusion
coefficients in the aqueous and BLM phases, the ion absorption, the ion pair formation, and the complex formation, it isthought that the magnitude of the current of the anion transfer
is not always equal to that of the cation transfer. Figure 8
shows the relation between the hydration energy of the anionsin Table 318 and the logarithmic magnitude of the ion transfer
current at 0.1 V in Figure 7. Here, the hydration energies of
ClO4−, Br−, and Cl− are used instead of the transfer energies
from the aqueous phase to the BLM, since the transfer energies
from the aqueous phase to the BLM cannot be evaluated. It issuggested that the logarithmic magnitude of the ion transfer
current depends on the hydrophobicity of the counter anion.
The ion transfer mechanism, shown in Figure 9, is actuallysomewhat more complex. Based on previous findings on thevoltammetry of the ion transfer across the interface between
two immiscible electrolyte solutions, a simplified version of
the ion transfer mechanism can be described as follows: At theinitial condition, the concentration of the ion pair of an alkali
metal ion with a counter anion in the BLM is negligibly low.

In the presence of Val in the BLM, the alkali metal ion spontaneously penetrates into the BLM with the counter anion. By
applying the EW1-W2, the alkali metal ion in the W1 phase thentransfers to the W2 phase across the BLM, and the counter
anion transfers in the opposite direction from W2 to W1
phases at the same time. Sato et al. suggested that the aniontransport occurred independently from the cation transport.10
If their suggestion is correct, the ion transfer current of thecounter anion should be observed in the absence of Val in the
BLM. However, there is no distinct current wave in thevoltammogram in the absence of Val. Accordingly, the aniontransport probably depends on the cation transport.
The cyclic voltammograms for ion transfer across a BLM
containing Val with 0.1 M LiCl, NaCl, RbCl, or CsCl are shownin Figure 10. The magnitude of IW1-W2 at a definite EW1-W2 wasaltered by a change in the cation species. When LiCl, NaCl,KCl, RbCl, or CsCl was used as the supporting electrolyte, themagnitude of IW1-W2 at 0.1 V was 8.7, 13, 350, 1400, or 2100
nA cm−2, respectively. The magnitude of IW1-W2 at a definite
EW1-W2 was also proportional to the concentration of Val in the
BLM.
There are many research reports on the conductivities of
BLMs containing both alkali metal ions in the aqueous phases
and Val. 6, 19, 20 Although several authors reported that the
conductivity of a BLM using RbCl as an electrolyte wasgreater than that using CsCl, the orders of the conductivities inmost of the results were almost consistent with that of thepresent result. It is usually thought that only an alkali metal
ion complexing with Val is distributed from W1 and/or W2 tothe BLM, and the ion transfer current is attributed to thetransfer of the alkali metal ion across the BLM between W1
and W2. The magnitude of the ion transfer current was thenpostulated to depend on the stability constants for the complex
formation, since the alkali metal ions could not transfer across60

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Shiraiutilized instead of those in the BLM, since the stability
constants in the BLM cannot be determined. Based on theseresults, it is suggested that the magnitude of the ion transfer
current depends on both the hydrophobicity of the alkali metal
ion and the stabilization by the complex formation.

10

1
2
3
4
5

I/µAcm-2

5

0

1, 2
3

-5

4

4. Conclusion

5
-10
-0.3

-0.2

-0.1

0

0.1

0.2

0.3

E W1-W2 /V
Figure 10. Cyclic voltammograms for the ion transfer across a BLM
containing Val. Potential scanning rate: 0.01 Vs−1. Temperature: 298± 1 K. Concentration of Val in the BLM-forming n-decane solution: 3
× 10−5 M. Electrolyte in W1 and W2: 0.1 M LiCl (curve 1); 0.1 M
NaCl (curve 2); 0.1 M KCl (curve 3); 0.1 M RbCl (curve 4); 0.1 M
CsCl (curve 5).

0

7

G hyd

6

log Kst

5

-200

4
3

-300

log KstGhyd /kJ mol-1

-100

2
-400
1
-500
-5.5

-5.5

-4.5

-4.0

-3.5

-3.0

-2.5

0
-2.0

log I at 0.1V
Figure 11. The dependence of log I on the hydration energy of cationand on the stability constant of the Val-cation complex.

the BLM in the absence of Val in the BLM.
According to our result on anion transport and findings on
the voltammetry for facilitated ion transfer between W andOrg, the ion transfer energy from W to BLM results from thesum of its own transfer energy and the stability of the cation in
the BLM by complex formation with a ligand as described byeq 3. The third right-hand term of eq 3 represents the potential
shift caused by the stabilization of M+ with the ligand in the
BLM. At the same Cligand, the concentration of M+ in the BLM,
depends on both ∆Gtr, M0 and Kst. Figure 11 shows the relation
between the hydration energy of the alkali metal ions and theion transfer current and that between the stability constant of
the alkali metal ion-Val complex in the acetonitrile and the iontransfer current.21 For the calculation, hydration energies areused instead of transfer energies from the aqueous phase to the
BLM, since the transfer energies of the alkali metal ions fromthe aqueous to the BLM would mainly depends on the hydration energies and the exact values of transfer energies cannot be
evaluated. Similarly, the stability constants in acetonitrile are

In the present paper, ion transport from one aqueous phaseto another across a BLM in the presence of hydrophobic ionsor Val was investigated based on the voltammetric concept andmethod. It was found that not only hydrophobic ions orcomplex-forming ions with an ionophore but also counter ionswere distributed from the aqueous phase to the BLM. By
applying an electric potential between two aqueous phases, thealkali metal ion in one aqueous phase (W1) transfers to anotheraqueous phase (W2) across the BLM and the counter aniontransfers in the opposite direction (from W2 to W1) at thesame time. These results suggest that an ionophore acts as acarrier of cation transport across the BLM.
Acknowledgment. The authors wish to thank Dr. K. Maedaof the Kyoto Institute of Technology for his useful comments.
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