1208 J . Org. Chem., Vol. 64, No. 4, 1999
J acobson et al.
benzyl chloride and potassium bromide
surfactant tail15,16 and thus the phase behavior and
stability of the emulsion, as has been studied for micro-
emulsions.17,18
PhCH2Cl + K+Br- T PhCH2Br + K+Cl-
Exp er im en ta l Section
Even though the area/volume is higher for a microemul-
sion droplet than an emulsions droplet, the volume
fraction of water can be much larger for the emulsion.
Consequently much larger amounts of KBr may be
introduced in the emulsion compared with the micro-
emulsion. We compare the product yield of benzyl bro-
mide for reactions in w/c and c/w emulsions formed with
Gen er a l. AOT (bis(2-ethylhexyl) sodium sulfosuccinate) was
obtained from Aldrich and used as received after drying under
vacuum at 60 °C for at least 10 h. Benzyl chloride, potassium
bromide, anisole, naphthalene, and n-octane were all obtained
from Aldrich and used without any further purification. PBO-
b-PEO (SAM-185, ca. 1500 g/mol) was purchased from PPG
Industries and PDMS-g-PEO (2758 g/mol) was synthesized as
described previously.19 The PFPE COO-NH4+ (940 g/mol) was
a gift from Ausimont, synthesized according to published
procedures.20 All three surfactants were used as received.
Instrument grade carbon dioxide (Proxair Inc.) was purified
with an oxytrap (Oxyclear model no. RGP-31-300, Labclear,
Oakland, CA.). Nanopure II water (Barnstead, Dubuque, IA)
was used in all experiments. Product mixtures were analyzed
by GC in exactly the same manner as before.7
the anionic surfactant CF3O(CF2CF(CF3)O)3CF2COO-NH4
+
(PFPE COO-NH4+) 1 and the nonionic surfactants poly-
(dimethylsiloxane)-g-poly(ethylene oxide) (PDMS-g-PEO)
2 and poly(butylene oxide)-b-poly(ethylene oxide) (PBO-
b-PEO) 3. The results for the emulsions are compared to
those for a w/o emulsion stabilized by the anionic sur-
factant bis(2-ethylhexyl) sodium sulfosuccinate (AOT) 4,
where n-octane is the oil phase.
High -P r essu r e Ap p a r a tu s. The reactions were performed
in the same apparatus as before7 with the following modifica-
tions. A reciprocating minipump (Thermo Separations Prod-
ucts) with a maximum flow rate of 460 mL/h was used to
continuously recirculate the emulsion from the bottom of the
reaction cell through a 6- port 2-position valve (C6W, Valco
Instruments Co., Inc., Houston, TX) equipped with a 100 mL
sample loop and back into the reaction cell though a capillary
(i.d. ) 254 µm, l ) 5 cm). This design allowed samples to be
taken throughout the reaction process and also produced the
high shear necessary to form the emulsion in the cell by
spraying the lower aqueous phase into the upper CO2 phase.
At the desired sampling time the sampling valve was switched
and the sample was trapped in ca. 500 µL of ethanol.
2 (CH3)3SiO[Si(CH3)2O]20[Si(CH3)(R)]2OSi(CH3)3,
where R ) (CH2)3O(C2H4O)∼11
H
3 HO(CH2CH(CH2CH3)O)∼12-b-(CH2CH2O)∼15
H
4 ROCOCH2CH(SO3-Na+)CO2R
where R ) CH2CH(CH2CH3)(n-C4H9)
Low -P r essu r e Exp er im en ts. The experiments for the
H2O/n-octane system at atmospheric pressure, with or without
AOT, were performed in a stainless steel autoclave (Parr, 300
mL) equipped with a mechanical mixer and a stainless steel
stirring blade.
Very low specific conductivities in the range of 20 µS/
cm were measured for emulsions stabilized by 1 with
equal amounts of water and CO2, indicating that water
is the dispersed phase (w/c emulsion).9 In contrast, c/w
emulsions with high conductivities are formed for 3.10 The
emulsion type is unknown for 2, but we have been able
to form both w/c and c/w emulsions for similar surfactants
composed of PDMS and PEO.
Syn th esis of Ben zyl Br om id e. The emulsion was made
with equal weights of water and CO2 or n-octane. Typically,
the cell was loaded with 6 g of H2O, 60 mg (0.5 wt % based on
total weight of H2O and CO2 or oil phase) of surfactant (or 0.1
wt % in the case of PDMS-g-PEO), and an internal standard.
Potassium bromide (1.428 g) was added to give a 2 M
concentration in the H2O phase (V ∼ 6 mL). The cell was then
closed and filled with 6 g of CO2 (or n-octane) and pressurized
to the desired reaction pressure. After the reaction cell was
heated and an emulsion had formed, benzyl chloride was
injected with the 6-port valve to give a concentration of 110
mM in the CO2 (or n-octane) phase (e.g., in the case of CO2, 6
g ) 7.607 mL at 65 °C and 4000 psi, and 96.3 mL of benzyl
chloride was injected). The reaction was monitored by taking
out aliquots of sample with the sample loop in the 6-port valve
and analyzing them by GC.
The nature of the H2O-CO2 interface is likely very
different than that of a water-alkane interface due to
the lower γ for the former and the small size and low
viscosity of CO2. With the low viscosity for this interface
and the high diffusion coefficients in the CO2 phase, it is
conceivable that heterogeneous reactions may occur
faster in w/c and c/w emulsions than in w/o emulsions.
Although numerous studies of organic reactions have
been reported in microemulsions,11 extremely few have
been reported in emulsions,12,13 with the exception of
heterogeneous polymerizations. The difficulty in breaking
an emulsion after reaction is a formidable problem. For
w/c and c/w emulsions, the emulsion may be broken
rapidly simply by reducing the pressure to separate the
water and CO2 phases. The low solubility of water in CO2
is well-known.14 In supercritical fluids, the density of the
fluid, which may be adjusted with temperature and
pressure, has a large effect on the solvation of the
(15) Luna-Barcenas, G.; Gromov, D. G.; Meredith, J . C.; Sanchez,
I. C.; dePablo, J . J .; J ohnston, K. P. Relationship Between Polymer
Chain Conformation and Phase Boundaries in a Supercritical Fluid.
J . Chem. Phys. 1997, 107, 10782.
(16) Meredith, J . C.; Sanchez, I. C.; J ohnston, K. P.; Pablo, J . J . d.
Simulation and Structure and Interaction Forces of Surfaces Coated
with Grafted Chains in
submitted.
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(17) Bartscherer, K. A.; Minier, M.; Renon, H. Microemulsions in
Compressible FluidssA Review. Fluid Phase Equilibria 1995, 107, 93-
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(11) Sjoblom, J .; Lindberg, R.; Friberg, S. E. Adv. Colloid Interface
Sci. 1996, 65, 125-287.
(18) Peck, D. G.; J ohnston, K. P. Theory of the Pressure Effect on
the Curvature and Phase Behavior of AOT Water-in-Oil Microemul-
sions in a Compressible Solvent. J . Phys. Chem. 1991, 95, 9549-9556.
(19) Hill, R. M. Siloxane Surfactants. In Specialist Surfactants; I.
D. Robb, I. D., Ed.; Blackie Acad.: London, 1997.
(20) Chittofrati, A.; Lenti, D.; Sanguineti, A.; Visca, M.; Gambi, C.
M. C.; Senatra, D.; Ahou, Z. Z. Prog. Colloid Polym. Sci. 1989, 79, 218-
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(12) Menger, F. M.; Rhee, J . U.; Rhee, H. K. J . Org. Chem. 1975,
40, 3803-05.
(13) Battal, T.; Siswanto, C.; Rathman, J . F. Langmuir 1997, 13,
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(14) Wiebe, R. The Binary System Carbon Dioxide-Water under
Pressure. Chem. Rev. 1941, 29, 475-481.