Phase behavior of benzyltributylphosphonium salts in aromatic hydrocarbons or aqueous solutions

Article abstract of DOI:10.1246/bcsj.74.1225

The solubility of benzyltributylphosphonium salt (BTBPX: X = Cl, Br, or I) in aromatic hydrocarbons was examined as a function of the temperature. The solubility curve had specific features, which were characterized by critical values corresponding to the

Full text of DOI:10.1246/bcsj.74.1225

Bull. Chem.
Soc. Jpn.
74
7
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1225–1232 (2001) 1225
The Chemical
Society of Ja-
pan
The Chemical
Society of Ja-
pan
GP
Phase Behavior of Benzyltributylphosphonium Salts in Aromatic
Hydrocarbons or Aqueous Solutions
Phase Behavior of Benzyltributylphosphonium Salt
N. Ohtani et al.
7
15
Noritaka Ohtani* and Daisuke Tsuchimoto
2001
1225
1232
BCSJA8
0009-2673
00404
11
Department of Materials-Process Engineering and Applied Chemistry for Environments,
Faculty of Engineering and Resource Science, Akita University, Akita 010-8502
(Received November 24, 2000)
24
2000
3
30
2001
The solubility of benzyltributylphosphonium salt (BTBPX: X = Cl, Br, or I) in aromatic hydrocarbons was exam-
ined as a function of the temperature. The solubility curve had specific features, which were characterized by critical val-
ues corresponding to the Krafft point and critical micelle concentration. A slight difference in the oil structure exerted a
significant effect on the solubility behavior. The Krafft boundary as well as a liquid–liquid immiscibility gap, on the oth-
er hand, featured the solubility behavior of BTBPX in an aqueous solution. The shape of the gap was assumed to be a
closed loop and the region of the gap was sharply dependent on the concentration of the added metal halide (MX). The
phase behavior of the three- or four-component systems, composed of BTBPX, benzene, water, and MX, has been exam-
ined in detail. According to their constituent, composition, and temperature, the system afforded a single- to four-phase
state form. There were at least three kinds of liquid phases: a BTBPX-rich phase (M), an aqueous solution phase (W),
and an oil solution phase (O). When these three liquid phases coexist, the system affords an O–M–W three-liquid-phase
equilibrium state. The three-liquid phase may be converted to O–M or M–W two-liquid-phase, depending on the tem-
perature and MX concentration in the W phase. In the presence of excess MX (S), BTBPCl gave an O–M–W–S four-
phase state, while BTBPBr or BTBPI never formed a four-phase state. The phase behavior of the four-component sys-
tems is discussed based on phase diagrams of fewer component systems.
1.9
Quaternary phosphonium salts have been used as catalysts
of phase-transfer catalysis (PTC)1 or as catalytic moieties of
the corresponding polymer-supported phase-transfer cataly-
sis.2 Benzyltributylphosphonium salts are quaternary salts that
have been most often used for being attached to an insoluble
polymer matrix in order to prepare immobilized phase-transfer
catalysts.3 The solution behavior and reactivity of the corre-
sponding linear polymers containing the phosphonium salts
(cationic ionomers) have shown that the quaternary salts ag-
gregate in nonpolar oils.4
The recent development of studies on both theoretical and
experimental aspects of microemulsions has provided a better
understanding of their formation, properties, and phase behav-
ior. However, the understanding has not yet progressed to the
point of being able to predict all details of the phase behavior
of PTC reaction systems, because the systems not only contain
so many components, but also involve relatively symmetric
quaternary onium salts. It is important to know the phase be-
havior of quaternary salts in multi-component systems for elu-
cidating the kinetics and mechanism of PTC as well as the mi-
crostructure formed in each layer of PTC systems.
Recently, we reported on the phase behavior of tetrabuty-
lammonium salts (TBAX) in oils or in aqueous electrolyte so-
lutions.5 They afford Krafft boundaries in oils, which are
closely dependent on the oil molecular volume and on their
counter-ions. The behavior was rather similar to the solubility
of common ionic surfactants in water. The quaternary salts af-
ford a lower consolute behavior in aqueous solutions. The be-
havior was very similar to those of micelle-forming quaternary
ammonium surfactants.6 We also disclosed that, under ade-
quate conditions, TBAX/benzene/water/NaBr four-component
systems form microemulsions, with which an oil phase and
aqueous phase may coexist, depending on the temperature and
constituent composition.7 This suggests that the conventional
PTC mechanism of Stark’s ion-pair extraction model is rather
applied to limited systems, and that a wider survey of the phase
behavior of quaternary salts is necessary to understand the
PTC systems. It is also important to clarify whether the find-
ings about TBAX systems can be generally applied to other
quaternary onium salt systems.
In this article, we examine the phase behavior of benzyl-
tributylphosphonium salts (BTBPX: X = Cl, Br, and I) as a
function of the temperature or composition. We show how the
parameters, such as temperature, counter-anion, or electrolyte
concentration, influence the formation of aggregates or micro-
emulsions. The conditions where the phase containing these
aggregates coexists with other phases under the BTBPX/ben-
zene/water/KX four-component systems are given to correlate
to the corresponding PTC systems that involve these quaterna-
ry salts. The similarity and difference between this system and
TBAX system are also described.
Experimental
Materials and Equipments. Decyl methanesulfonate was
prepared by the reaction of methanesulfonyl chloride with 1-de-
canol in pyridine.8 Benzyltributylphosphonium chloride (BTB-
PCl) or benzyltributylphosphonium bromide (BTBPBr) was pre-
pared by the reaction of benzyl chloride or benzyl bromide with

1226 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Phase Behavior of Benzyltributylphosphonium Salt
tributylphosphine in toluene for 72 h at 90 °C under nitrogen.
They were recrystallized twice from benzene. Benzyltribu-
tylphosphonium iodide (BTBPI) was prepared by the reaction of
BTBPCl with iodomethane for 48 h at 40 °C in benzene. The de-
gree of counter-ion substitution was determined by a GLC analy-
sis of 1-iododecane, which was formed through the reaction of
BTBPI with an excess amount of decyl methanesulfonate. Ben-
zene and other aromatic hydrocarbons were purified by distillation
from sodium diphenylketyl under nitrogen. Deionized water was
used throughout the experiments. 1H NMR spectra were recorded
on a Varian Mercury 300 spectrometer. GLC analyses were per-
formed using a Hitachi 163 FID instrument with a 1 m column of
SE-30.
Phase-Equilibrium.5 The solubility of BTBPX in oil was
measured in the following way. Given amounts of a quaternary
salt (BTBPX) and an aromatic oil were added to a 10 mL Teflon-
coated screw-capped test tube with an inside diameter of 10.5 mm.
The tube was transferred to a temperature-variable water bath, and
the mixture was stirred with magnetic stirring. The temperature
was raised at a rate of 1 °C min−1. The temperature was read
when all of the BTBPX crystalline solid or BTBPX-rich liquid
disappeared. The cloud points of quaternary salts in aqueous NaX
solution were measured in such a way that the temperature was
raised at a rate of 1 °C min−1 and the temperature of the phase
transition was read when the solution became turbid.
Phase Composition.7 The amounts of BTBPX, benzene, and
water in a phase were analyzed by 1H NMR. An aliquot of a BTB-
PX-rich phase or benzene-rich phase was added to a prescribed
amount of methanol-d4, and the composition of the phase was de-
termined based on the residual proton in the methanol-d4. The un-
certainty of the composition was within 0.02 in weight fraction.
Fig. 1. Solubility behavior of BTBPX in oils. (a) BTBPCl,
(b) BTBPBr, and (c) BTBPI. The weight fraction of a
component X in a system is represented as f(X) in this
study. The full lines show the boundaries between uni-
phase and liquid–solid two-phase for the systems of Type
I. The dotted lines show the phase boundaries for the sys-
tems of Type II. The symbol O represents an oil phase that
contains few amounts of BTBPX and coexists with other
phase(s), the symbol Q, the solid of BTBPX, and the sym-
bol Om, a homogeneous solution of BTBPX in oil.
Results and Discussion
Solubility and Microstructure of BTBPX in Oils. The
solubility of BTBPX in oils is shown in Figs. 1a–c as a func-
tion of the temperature. The solubility behavior was classified
into two types (ꢀ or ꢁ), depending on BTBPX and oil. The first
one (I) is like that of a number of ionic surfactants in water.9 It
was characterized by a critical solubility temperature (Tc) and
a critical solubility concentration (csc), which correspond to
the Krafft point and the cmc of ionic surfactants, respectively.
A typical example is BTBPCl in benzene. At low tempera-
tures, solid quaternary salt (Q) coexists with a quaternary salt-
poor oil phase (O). Within a very limited range of tempera-
ture, however, the amount of the solid quaternary salt was
sharply decreased as if the quaternary salt melts at the temper-
atures. The solubility suddenly increased and BTBPX was sol-
uble freely in the oil up to a considerably high concentration
above the temperature. The system becomes a homogeneous
solution (Om) where BTBPX is assumed to form a certain ag-
gregate, although, in an O phase, BTBPX may be present as a
monomer or a very small aggregate or a very small aggregate.5
The second type (ꢁ) was the occurrence of liquid–liquid two-
phase separation at high temperatures. The example is BTBP-
Br in ethylbenzene or BTBPI in toluene. BTBPX melts at a
certain temperature. In this case, however, the melt was not
unlimitedly miscible with oil at the temperature. A liquid–liq-
uid two-phase system (O–Om) was formed unless the BTBPX
concentration was extremely high or low. The volume of the
Om phase was much larger than that of added BTBPX, indicat-
ing that the Om phase contained a large volume of oil. It is
noted that the solubility behavior was significantly different
among aromatic oils, though they are often used as a substitute
for benzene, owing to the resemblance of their properties as a
solvent. A similar trend has been observed for TBAX.5 BTB-
PX tends to be more soluble in an oil with a smaller molar vol-
ume, which suggests that such a small oil feasibly penetrates
into the alkyl moiety of BTBPX.
Solubility of BTBPX in Aqueous Solution. The solubili-
ty of BTBPX in water was greatly dependent on its counter-
ion. BTBPCl was very soluble in water. A homogeneous
aqueous solution was obtained irrespective of the BTBPCl
concentration and temperature. The addition of an electrolyte
lowered the solubility of BTBPCl, as shown in Figs. 2a–d.
The solubility rather decreased at a high temperature. BTBPCl
did not precipitate as a solid crystalline, but was liberated as a
liquid phase, except for one case using a 20 wt% LiCl solution.
This means that BTBPCl has a cloud point. When the temper-

N. Ohtani et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1227
Fig. 3. Solubility behavior of BTBPBr in water. The symbol
W represents an aqueous solution that contains few
amounts of BTBPBr and coexists with other phase(s), the
symbol Wm, a homogeneous aqueous solution of BTBPBr,
and the symbol Q, the hydrated solid of BTBPBr.
a solid from the aqueous solution to form a Wm–W–S three-
phase.
Like BTBPCl, the solubility behavior of BTBPBr was great-
ly different from that of TBABr.5 As shown in Fig. 3, the solu-
bility of BTBPBr in water is limited at low temperatures, and
that excess BTBPBr is present as a solid precipitate. Above 40
°C, however, a homogeneous aqueous solution was obtained
irrespective of the concentration of BTBPBr. Although the
critical solubility concentration was rather high (ca. 0.35 mol
dm−3) compared with that of ionic surfactants in water, this
solubility behavior was also characterized by the Krafft bound-
ary. Therefore, BTBPBr may be assumed to form an aggregate
above the csc.
The addition of a minimal amount of NaBr to this system in-
duced a significant influence on the solubility behavior of BT-
BPBr (Fig. 4). In addition to the Krafft boundary, one could
observe an immisibility gap, that is, a region of liquid–liquid
phase separation (Wm–W). The region was enlarged with an
increasing weight fraction of NaBr in water. Thus, the homo-
geneous aqueous solution (W) was restricted only to a very
low BTBPBr concentration region when the weight fraction of
NaBr was high. We also found that the immiscibility gap is a
closed loop and that the gap intersects with the Krafft bound-
ary when the NaBr concentration is higher than 0.5 wt%. It
has been reported that TBABr gives a similar looped immisci-
bility gap when a very high pressure is applied.10 This sug-
gests that the addition of an electrolyte to a symmetric quater-
nary salt system exerts an effect on the phase behavior, some-
what similar to an elevation of the pressure.
The solubility behavior of BTBPI is shown in Fig. 5. The
melting point of BTBPI in water was 85 °C and the crystalline
solid BTBPI (Q) coexists with a BTBPI-poor aqueous solution
(W) below the temperature. Because the melt of BTBPI was
not miscible with water, a liquid–liquid two-phase (Wm–W)
was formed above 85 °C. The addition of NaI to the system
slightly enlarged the Wm–W region, as shown in Figs. 5b and
5c.
Fig. 2. Solubility behavior BTBPCl in aqueous solutions
containing electrolytes. (a) LiCl, (b) NaCl, (c) KCl, and
(d) TMACl. The full lines show the boundaries between
uniphase (Wm) and liquid–liquid two-phase (Wm–W).
The phase boundaries of the system that affords a W–Q
liquid–solid three-phase are given by dashed lines. The
phase boundaries of the systems that afford liquid–liquid–
solid three-phase (Wm–W–S) are given by dotted lines.
The symbol W represents an aqueous solution that contains
few amounts of BTBPCl and coexists with other phase(s),
the symbol Wm, a homogeneous aqueous solution of BTB-
PCl, the symbol Q, the solid of BTBPX, and the symbol S,
the solid of electrolytes.
ature was raised to its cloud point, the aqueous solution sud-
denly became turbid, and most of the BTBPCl eventually
formed a BTBPCl-rich transparent liquid phase (Wm). The
lower consolute boundary was lowered with an increasing
electrolyte concentration. It is noted that BTBPCl is more sol-
uble in water than KCl. KCl was preferentially precipitated as
The influence of NaX over a wide concentration range is
shown in Figs. 6a–c. The Krafft boundary (the boundary line
between W–Q and Wm) was not observed for BTBPCl. The
solid NaCl crystalline was precipitated rather than the BTBPCl

1228 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Phase Behavior of Benzyltributylphosphonium Salt
Fig. 5. Solubility behavior of BTBPI in NaI aqueous solu-
tions. (a) in water, (b) in 1.0 wt% NaI aqueous solution,
and (c) in 10.0 wt% NaI aqueous solution. The symbols
used in the Figures are the similar phases represented in
Fig. 3.
groups. An elevation of temperature weakens the hydrophobic
hydration to promote the hydrophobic interaction among phos-
phonio groups. The size of the aggregate gradually increases
with temperature. When the short-range attractive hydropho-
bic interaction overwhelms the repulsive electrostatic interac-
tion between the phosphonium-ion aggregate, phase separation
(Wm–W) takes place. An electrolyte may also deprive of
some water from the hydrophobic hydration around the alkyl
groups. The aggregate in the Wm of the Wm–W two-phase
state is probably much larger than that of the homogeneous
Wm phase below the cloud point. As shown later in this paper,
the Wm phase coexisting with the aqueous W phase beyond
the cloud point is able to absorb a considerable amount of ben-
zene, indicating that the microstructure of the aggregate is like-
ly to be bicontinuous, rather than of an o/w type. The specific
binding of soft counter-anions (such as Br or I) to phosphoni-
um ions and the increase in the electrolyte concentration both
reduce the electrostatic repulsion among the phosphonium ag-
gregates. Thus, the cloud point of BTBPX was lowered with
adding the electrolyte, particularly for BTBPBr or BTBPI.
The difference in the onium ion structure influences the aggre-
gate structure, which in turn effects the hydrophobic hydration
around the aggregates, to which the variation of lower conso-
Fig. 4. Solubility behavior of BTBPBr in NaBr aqueous so-
lutions. (a) in 0.2 wt% NaBr aqueous solution, (b) in 0.5
wt% NaBr aqueous solution, (c) in 1.0 wt% NaBr aqueous
solution, and (d) in 5.0 wt% NaBr aqueous solution. The
symbols used in the Figures are the same phases represent-
ed in Fig. 3.
precipitation. The boundary line between W–Q and Wm–W of
BTBPBr or BTBPI, which corresponds to the Krafft boundary,
gradually increased with the concentration of NaX until the ex-
cess NaX precipitated as a solid phase (S). The phase equilib-
rium of Wm–W–S was not observed for the BTBPI system.
The presence of the Krafft boundary of BTBPX strongly
suggests that BTBPX forms a certain aggregate in the homoge-
neous Wm phase. The formation of a spherical micelle may be
difficult due to the closely symmetrical structure of the phos-
phonium ions. The candidate may be a rod-like oligomer,
which can at least solubilize some amount of oil. The steric
bulky structure of the phosphonium ions inevitably exposes
some alkyl groups (or the part of the alkyl groups) to water,
leading to the formation of hydrophobic hydration around the

N. Ohtani et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1229
determined in order to distinguish between them.
BTBPX/Benzene/Water Three-Component Systems.
We have suggested the existence of two types of aggregates
that are formed by BTBPX: Om and Wm. The phase Om (or
Wm) is formed when the BTBPX concentration is beyond a
certain critical value (csc) and when water (or benzene) is ab-
sent or only in a minimal quantity in the system. The addition
of water to an Om phase or the addition of benzene to a Wm
phase always leads to the formation of an M phase, which con-
tains benzene and water beyond the amounts of their mutual
miscibility. Because BTBPX possibly aggregates in the Om
and Wm phases, it is reasonable to assume that BTBPX also
aggregates in the M phase. There are two other homogeneous
phases, an O phase and a W phase, in which BTBPX is as-
sumed to be present with a more or less non-aggregated form.
In these phases, BTBPX is probably present almost as an iso-
lated ion-pair in the O phase and as a dissociated form in the W
phase.
The three-component system of BTBPCl/benzene/water at
60 °C gave a phase diagram that was very similar to that of
TBACl/benzene/water.7 As shown in Fig. 7, there was an M-
phase region, under which a broad liquid–liquid two-phase re-
gion (O–M) was observed. The M phase region was attached
to the Wm region of the zero oil line. The region was also at-
tached to the Om region of the zero water line at the part of
small BTBPCl fractions. There was a liquid–solid M–Q re-
gion attached to the zero water line at the higher BTBPCl frac-
tions. The tie lines in the O–M two-phase region indicate that
the M phase is able to coexist with a benzene-rich phase (O
phase), but not with a water-rich phase (W phase). This sug-
gests that the microstructure in the M phase is water-continu-
ous.
The phase diagram of BTBPBr (or BTBPI)/benzene/water
at 60 °C is greatly different from that of BTBPCl. The dia-
gram rather resembles that of TBAI/benzene/water system.7
As shown in Figs. 8 and 9, there is a broad region of a three-
liquid-phase region of O–M–W, above which there are a two-
liquid-phase region (M–W) on the left, and another two-liquid-
phase region (O–M) on the right. The O–M regions of BTBP-
Br and BTBPI are very narrow. There is a distinct O–W region
below the O–M–W region. In addition, we find a narrow M
Fig. 6. Effect of NaX concentration on phase behavior of
BTBPX. (a) BTBPCl, (b) BTBPBr, and (c) BTBPI. To a
mixture of 0.25 mmol of BTBPX and 1.0 mL of water, a
given amount of NaX was added.
Fig. 7. Phase diagram of BTBPCl/benzene/water three-com-
ponent system at 60 °C. Coordinates are in weight frac-
tions of the components. The symbol M represents a mi-
croemulsion phase that contains benzene and water beyond
their mutual miscibility. Tie lines are shown with the total
compositions (round keys) and the observed compositions
of the two phases (square keys).
Fig. 8. Phase diagram of BTBPBr/benzene/water three-com-
ponent system at 60 °C. Coordinates are in weight frac-
tions of the components. Tie lines are shown with the total
compositions (round keys) and the observed compositions
of the two phases (square keys).
lute behavior may be attributable.
On the other hand, the microstructure in the W phase is ei-
ther a non-aggregate form (dissociated monomer) or a very
small aggregate. The csc of each BTBPX should be strictly

1230 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Phase Behavior of Benzyltributylphosphonium Salt
Fig. 9. Phase diagram of BTBPI/benzene/water three-com-
ponent system at 60 °C. Coordinaters are in weight frac-
tions of the components. Tie lines are shown with the total
compositions (round keys) and the observed compositions
of the two phases (square keys). Ambiguous phase-bound-
aries are shown in dashed lines.
Fig. 10. Temperature dependence of phase behavior of BTB-
PBr/benzene/water three-component system. Water was
added to a mixture of BTBPBr (0.6 mmol) and benzene
(1.0 mL).
cause no electrolyte except BTBPBr is present, more water is
also absorbed by the M phase upon warming. Thus, an O–M
two-liquid state is transformed to a homogeneous M phase at
higher temperatures. An O–M–W three-liquid state was trans-
formed to an O–M two-liquid state before the whole amount of
benzene was absorbed by the M phase, because the volume of
the W phase was very small compared with that of the O phase
under these conditions.
BTBPX/benzene/water/NaX Four-Component Systems.
The phase diagrams of BTBPX/benzene/water/NaX system at
60 °C are shown in Figs. 11a–c. The benzene/water volume
ratio was fixed at 50:50. Although the phase diagram of tet-
rabutylammonium salts (TBAX) was similar each other irre-
spective of the counter-ions,7 the counter-ion exerted a signifi-
cant influence on the phase equilibrium of BTBPX. Under the
BTBPCl/benzene/water three-component conditions, the BTB-
PCl system did not afford a three-liquid-state, O–M–W, at 60
°C. When a slight amount of NaCl was added to an O–M two-
phase, however, the three-liquid-state was formed if the BTB-
PCl content was low. It is noted that the O–M–W three-liquid-
phase was converted into a liquid–liquid–liquid–solid four-
phase equilibrium state (O–M–W–S) with a further addition of
phase region that is extended to the benzene corner. The oc-
currence of an O–M–W phase-separation clearly shows that
the presence of inorganic salt is not always a prerequisite for
the formation of a so-called “third liquid phase,” often cited in
PTC systems.11 Thermodynamically, each phase of an O–M–
W three-phase equilibrium of a three-component system
should have constant compositions at a given temperature and
pressure. The composition of each phase (BTBPBr, benzene,
water) by weight fraction was obtained by an NMR analysis:
O(0.06, 0.93, 0.01), M(0.43, 0.47, 0.10), and W(0.04, 0.01,
0.95) at 60 °C. The composition for the BTBPI system was
O(0.07, 0.92, 0.01), M(0.35, 0.64, 0.01), and W(0.006, 0.004,
0.99). The M phase compositions at least reject an o/w micro-
structure being formed in the M phase of the BTBPBr or BTB-
PI system under the O–M–W three-phase equilibrium condi-
tions. The microstructure may be bicontinuous or of a w/o
type, possibly for BTBPI system.
The temperature dependence of the BTBPBr system is
shown in Fig. 10, where a small amount of water is added to a
BTBPBr/benzene two-component system. The results show
that the M phase tends to absorb more oil upon warming. Be-
Fig. 11. Phase diagrams of BTBPX/benzene/water/NaX four-component systems at 60 °C. (a) BTBPCl, (b) BTBPBr, and (c) BTB-
PI. One corner of the pseudo-ternary phase diagrams is an equi-volume mixture of benzene and water. Coordinates are in weight
fractions of the components. Ambiguous phase-boundaries are shown in dashed lines.

N. Ohtani et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1231
zene content in the M phase. The microstructure of the M
phase changes its state from an o/w or bicontinuous type to a
w/o one. Eventually, the miscibility between M with O be-
comes as high as they are completely miscible with each other
at a given temperature, leading to the formation of an M–W
system. On the other hand, the M phase formed by BTBPCl
cannot reduce its water content on the addition of NaCl to such
a level that the microstructure becomes a w/o type to make
complete miscibility with the O phase. Thus, in this case, an
O–M–W–S equilibrium is attained.
The occurrence of an O–M–W–S phase separation is easily
judged by whether an O–M–S area is present or not. The pres-
ence of an O–M–S equilibrium state was easily found for the
BTBPCl system through adding water to a BTBPCl/benzene/
NaCl mixture (Om–S state). Before a NaCl aqueous solution
(W) was formed, a BTBPCl-rich liquid (M) was liberated from
the benzene-solution of BTBPCl (Om). The O–M–S area was
never seen in the BTBPBr system, as shown in Fig. 12. In this
case, the addition of water to a BTBPX/benzene/NaX mixture
led to the phase-separation of an M–W–S. The yielding W
phase was an almost saturated aqueous solution of NaX. A
further increase in water transformed the system to an M–W,
and then to an O–M–W when the NaBr concentration in the W
phase was sufficiently diluted. This may correspond to a
change in the microstructure of the M phase from a w/o type to
a bicontinuous one.
Fig. 12. Temperature dependence of phase behavior of BT-
BPBr under four-component conditions. To a mixture of
0.125 mmol of BTBPBr, 1.0 mL of benzene, and 0.01 g of
NaBr, water was added. The content of water is shown as a
molar ratio of water to BTBPBr (w).
NaCl. This phase transition was the same as those of TBAX.7
Although a liquid–liquid–liquid–solid four-phase equilibrium
state (O–M–W–S) was always observed for the TBAX sys-
tems, irrespective of the counter-ions, only the BTBPCl system
gave four-phase equilibrium in this study. Therefore, if we add
BTBPCl to a mixture of benzene and a NaCl aqueous solution,
of which the NaCl concentration is higher than 0.05 by weight
fraction, the O–W two-liquid state is always transformed into
an O–M–W three-liquid state.
Table 1 shows what kind of phase-transition occurs for BT-
BPBr/oil/NaBr systems upon adding water. As shown in Fig.
1b, BTBPBr was hardly soluble in oils, except benzene at 60
°C. Thus, most of the BTBPBr was present as a solid Q in the
absence of water; the system was a liquid–solid–solid three-
phase (O–Q–S). When toluene was used as oil, the system be-
came an O–M–S state via O–M–Q–S four-phase. This means
that the melting point of BTBPBr in toluene was lowered by
the addition of water. The O–M–S three-phase was trans-
formed to an O–M–W–S four-phase upon adding a further
amount of water, as was the case for the BTBPCl/benzene/wa-
ter/NaCl sysyem. It is noted that the BTBPBr/toluene/water/
NaBr system affords an O–M–W–S because the corresponding
system with benzene never gives the four-phase. The solubili-
ty of BTBPBr in p-xylene or ethylbenzene was very low due to
the high Krafft point. The water added to the BTBPBr/oil/
NaBr rather dissolved NaBr to form an O–Q–W three-phase.
The Krafft point of BTBPBr in aqueous solution was much
higher than 60 °C in the presence of saturated NaBr (Fig. 6b).
Therefore, the formation of the O–Q–W three-phase indicates
that the melting point of BTBPBr in these oils was not lowered
down to 60 °C, even in the presence of a saturated NaBr aque-
On the other hand, we could not find any region of the O–
M–W–S phase separation for the BTBPBr or BTBPI system at
60 °C. Instead, the addition of NaX to an O–M–W system af-
forded an M–W two-liquid phase. If a further amount of the
electrolyte was added to the M–W, an M–W–S liquid–liquid–
solid three-phase was formed. These phase transformations
took place even at very low contents of BTBPX. Figures 10b
and 10c also show that the concentration of NaX in the aque-
ous phase determines what kind of phase transformation is in-
duced upon adding BTBPX to a two-liquid mixture of benzene
and an aqueous solution. At low concentrations of NaX, the
system was transformed from O–W, through O–M–W, to M–
W. At high concentrations of NaX, the system was trans-
formed from O–W to M–W without any visual indication of a
phase-transition. Hypothetical boundaries between O–W and
M–W were drawn in the Figures, assuming that the BTBPX
concentration in the W phase at a given NaX concentration
was equal to the value at the cloud point.
The phase transition of O–M–W to M–W may be qualita-
tively explained as follows. The addition of NaX to an O–M–
W system decreases the water content and increases the ben-
Table 1. Phase Transition of BTBPBr/Oil/Water/NaBr Systems at 60 °C Depending on Water Contenta)
Oil
w (type of phase transition)
Benzene
3.0 (Om–S/M–W–S)
Toluene
p-Xylene
Ethylbenzene
0.35 (O–Q–S/O–M–S)
0.93 (O–Q–S/O–Q–W–S)
0.93 (O–Q–S/O–Q–W–S)
4.4 (O–M–S/O–M–W–S)
28 (O–Q–M–S/O–Q–W)
28 (O–Q–M–S/O–Q–W)
51 (O–Q–W/O–M–W)
38 (O–Q–W/O–M–W)
a) Water was added to a mixture of BTBPBr (0.25 mmol), oil (1.0 mL), and NaBr (0.15 g) at 60 °C. Molar ratio of
water to BTBPBr is shown as w.

1232 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
Phase Behavior of Benzyltributylphosphonium Salt
Reactions in Organic Synthesis;” John Wiley & Sons, New York
(1980).
ous solution. BTBPBr did not melt at 60 °C until the aqueous
solution (W) was diluted with a further addition of water. The
data in Fig. 6b suggest that it may be possible to form an O–
M–W–S four-phase with these oils at temperatures higher than
90 °C.
3
a) W. T. Ford and M. Tomoi, Adv. Polym. Sci., 55, 49
(1984). b) M. Tomoi, Y. Hosokawa, and H. Kakiuchi, J. Polym.
Sci., Polym. Chem. Ed., 22, 1243 (1984). c) N. Ohtani, Y. Inoue,
J. Mukudai, T. Yamashita, in “Phase-Transfer Catalysis,” ed by M.
E. Halpern, ACS Symposium Series 659, American Chemical So-
ciety, Washington DC (1996), p. 248. d) N. Ohtani, S. Murakawa,
K. Watanabe, D. Tsuchimoto, and D. Sato, J. Chem. Soc., Perkin
Trans. 2, 2000, 1851.
Conclusions
Counter-ions and oils exerted significant influences on the
phase behavior of the BTBPX/oil/water NaX four-component
systems. Simple partition of BTBPX between the oil and
aqueous phase could not explain the phase behavior of these
systems. The specific feature was the formation of a micro-
emulsion phase, M. The M phase coexists with other phases to
give several equilibrium states. The change in the microstruc-
ture of the M phase may cause the phase transition of four-
component system. The difference of the phase behavior pri-
marily depended on the BTBPX solubility in oil and in NaX
aqueous solution. The solubility in oils was characterized by
the Krafft boundary. The solubility in aqueous solutions was
characterized not only by the Krafft boundary, but also by the
immiscibility gap that defines the region of liquid–liquid two-
phase coexistence. The shape of the gap was assumed to be a
closed loop, as embodied for the BTBPBr/water/NaBr system.
4
a) N. Ohtani, Y. Inoue, H. Mizuoka, and K. Itoh, J. Polym.
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5
(2000).
6
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8
N. Ohtani, M. Nakaya, K. Shirahata, and T. Yamashita, J.
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