Chemistry induced by hydrodynamic cavitation

Full text of DOI:10.1021/ja972171i

J. Am. Chem. Soc. 1997, 119, 9303-9304
9303
In a typical run,12a 60 mL of 1.0 M KI in purified water
saturated with carbon tetrachloride was introduced at a constant
flow rate into the Microfluidizer with a liquid pressure of 1.24
kbar. The reaction solution temperature increased 10 to 12 °C
within 90 s and stabilized at the temperatures reported herein.
Aliquots (4 mL) of the processed solution were periodically
extracted from the reaction system by airtight syringes, analyzed
spectrophotometrically, and returned to the reservoir after
Chemistry Induced by Hydrodynamic Cavitation
Kenneth S. Suslick,* Millan M. Mdleleni, and
Jeffrey T. Ries
Department of Chemistry
UniVersity of Illinois at Urbana-Champaign
601 South Goodwin AVenue, Urbana, Illinois 61801
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analysis. The rate of I3 formation was calculated from the
ReceiVed July 1, 1997
change in absorbance at 353 nm (ꢀ ) 26 400 M-1 cm-1 12b
as
)
a function of reaction time. Initial studies conducted with Ar-
Cavitation (the formation, growth, and implosive collapse of
gas or vapor-filled bubbles in liquids) can have substantial
chemical and physical effects. While the chemical effects of
acoustic cavitation (i.e., sonochemistry and sonoluminescence)
have been extensively investigated during recent years,1-5 little
is known about the chemical consequences of hydrodynamic
cavitation created during turbulent flow of liquids. Hydrody-
namic cavitation is observed when large pressure differentials
are generated within a moving liquid and is accompanied by a
number of physical effects, erosion being most notable from a
technological viewpoint.6,7 In contrast, reports of hydrodynami-
cally induced chemistry or luminescence and direct comparisons
to sonochemistry or sonoluminescence have been extremely
limited.8,9
In aqueous liquids, acoustic cavitation leads to the formation
of reactive species such as OH•, H•, and H2O2. These short-
lived species are capable of effecting secondary oxidation and
reduction reactions. For example, iodide can be sonochemically
oxidized to triiodide by OH• radicals or H2O2 produced during
cavitation. From aqueous solutions containing chlorocarbons,
Cl• and Cl2 are also liberated in high yields and this increases
rates of iodide oxidation.10 The rate of triiodide formation is
easily monitored spectrophotometrically. For many years, this
so-called Weissler reaction has remained the standard dosimeter
for sonochemical reactions.
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sparged water gave relatively low rates of I3 production;
saturation of the Ar sparged H2O with CCl4 resulted in a 20-
fold increase in I3- production, as has been typically observed
for ultrasonic cavitation.10 This is attributed to ready formation
of Cl• and Cl2 from CCl4 under cavitation conditions.
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The effect of upstream liquid pressure on the rate of I3
production was investigated over the range 100-1500 bar. The
reaction rate increases linearly with liquid pressure (Figure 1),
but with a threshold pressure of 150 bar. Below 150 bar of
hydrostatic pressure, no chemical reactions were observed; this
probably represents the minimum jet velocity necessary to
induce cavitation. The resistance of a turbulent flow to
cavitation is given by its cavitation number σ, as defined in eq
1:6
pd - pv pd
pu - pd pu
σ )
≈
(1)
where pd, pu, and pv are the downstream, upstream, and vapor
pressures, respectively, and the approximation holds when pu
. pd . pv, as they do under our experimental conditions. An
increase in upstream pressure should decrease σ and increase
the number of cavitation events. This in turn should increase
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the rate of I3 formation, if the chemistry is cavitation driven,
The recent advent of commercially available high-pressure
jet fluidizers capable of pressure drops as high as 2 kbar and
jet velocities approaching 200 m/s has led to numerous
applications in the physical processing of liquids, for emulsifica-
tion, cell disruption, etc. Chemical consequences of such
processing, however, have received little examination. One
important exception comes from W. R. Moser and co-workers,11
who have shown that such a device can be utilized to prepare
nanostructured catalytic materials. Moser speculated that the
unusual properties of his catalysts resulted from hydrodynamic
cavitation within the fluidizer.11 We describe here conclusive
experimental evidence for chemical reactions caused by hydro-
dynamic cavitation within a jet fluidizer.
consistent with our observations.
The conditions formed during acoustic cavitation and con-
sequently sonochemical rates are known to be affected both by
the polytropic ratio of the dissolved gas (i.e., γ ) Cp/Cv,) and
by the thermal conductivity of the dissolved gas. The former
parameter determines the temperature achieved during bubble
compression, and the latter is responsible for heat dissipation
from the collapsing bubble to the surrounding solution. In the
present study using Ar/He mixtures, the γ of the dissolved gas
was fixed at 1.67, while the thermal conductivity was varied
from 0.017 to 0.142 W m-1 K-1. As shown in Figure 2, the
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I3 formation rate decreases exponentially as the thermal
conductivity of the dissolved gas increases. This observation
is best explained in terms of the hot-spot model for cavitation
which suggests that the maximum temperature (Tmax) realized
(1) Suslick, K. S. MRS Bull. 1995, 20, 29
(2) Mason, T. J., Ed. AdVances in Sonochemistry; JAI Press: New York,
1990-1994; Vols. 1-3.
(3) Price, G. J., Ed. Current Trends in Sonochemistry; Royal Society of
Chemistry: Cambridge, U.K., 1992.
(12) (a) Reagent grade KI and CCl4 were obtained from Aldrich Chemical
Co. and used as received. High-purity water was prepared with a Barnstead
NANOpure. Spectrophotometric measurements were obtained with a Hitachi
U3300 UV-vis double-monochromator spectrophotometer. MKS mass flow
controllers 247C were used to adjust the composition of the Ar/He sparge
gas. Hydrodynamic cavitation studies were performed with an air-driven
model M-110Y Microfluidizer from Microfluidics International Corp., 30
Ossipee Rd., Newton, MA 02164. The reaction solutions were exposed only
to stainless steel or glass. The reaction solution was first sparged with high-
purity argon or Ar/He mixtures and light-proofed to prevent CCl4 photo-
decomposition and then injected into the pressurizing reservoir through a
self-sealing septum. A portion of the reaction solution was pressurized by
a large pneumatically driven pump into an interaction chamber, where two
pulsed flows were redirected at each other through jewel orifices with
velocities of ≈190 m/s11b controlled by a back-pressure regulator. Cavitation
can occur when there is sufficient turbulence upon liquid jet impact or when
there exists a sufficient pressure drop as the streams pass through the orifices.
High-velocity pumping is also accompanied by bulk heating of the flowing
liquid. The reaction chamber, pump, and plumbing were therefore immersed
in a thermally equilibrated water bath. The processed stream was returned
to the solution reservoir for recirculation and analysis. (b) Awtrey, A. D.;
Connick, R. E. J. Am. Chem. Soc. 1951, 73, 1842.
(4) Suslick, K. S. Science 1990, 247, 1439.
(5) Suslick, K. S., Ed. Ultrasound: Its Chemical, Physical, and Biological
Effects; VCH: New York, 1988.
(6) Knapp, R. T.; Daily, J. W.; Hammitt, F. G. CaVitation; McGraw-
Hill: Inc.: New York, 1970.
(7) (a) Young, F. R. CaVitation; McGraw-Hill: Inc.: New York, 1989.
(b) Brennen, C. E. CaVitation and Bubble Dynamics; Oxford University
Press: Oxford, U.K., 1995.
(8) Anbar, M. Science 1968, 161, 1343.
(9) (a) Verbanov, V. S.; Margulis, M. A.; Demin, S. V.; Korneev, Y.
A.; Klimenko, B. N.; Nikitin, Y. B.; Pogodaev, V. I. Russ. J. Phys. Chem.
1990, 64, 1807. (b) Margulis, M. A.; Korneev, Y. A.; Demin, S. V.;
Verbanov, V. S. Russ. J. Phys. Chem. 1994, 68, 828.
(10) (a) Weissler, A.; Cooper, N. W.; Snyder, S. J. Am. Chem. Soc. 1950,
72, 1769. (b) Ibisi, M.; Brown, B. J. Acoust. Soc. Am. 1967, 41, 568. (c)
Clark, P. R.; Hill, C. R. J. Acoust. Soc. Am. 1970, 47, 649. (d) Chendke, P.
K.; Fogler, H. S. J. Phys. Chem. 1983, 87, 1362.
(11) (a) Moser, W. R.; Marshik, B. J.; Kingsley, J.; Lemberger, M.;
Willette, R.; Chan, A.; Sunstrom, J. E.; Boye, A. J. Mater. Res. 1995, 10,
2322. (b) Moser, W. R. Personal communication.
S0002-7863(97)02171-9 CCC: $14.00 © 1997 American Chemical Society

9304 J. Am. Chem. Soc., Vol. 119, No. 39, 1997
Communications to the Editor
Figure 1. Dependence of triiodide formation rate on the hydrodynamic
pressure used to induce cavitation. Conditions: 60 mL of 1 M KI
solution in CCl4-saturated water was recycled under static Ar atmo-
sphere at a constant reaction temperature of 12 °C.
Figure 3. Dependence of triiodide formation rate on bulk temperature
and on solvent vapor pressure during hydrodynamic cavitation. Condi-
tions: 60 mL of 1 M KI solution in CCl4-saturated water was recycled
under static Ar atmosphere at liquid pressure of 1.24 kbar.
pressure. This same dependence is seen in sonochemical
reactions and is attributed to the increase in polyatomic vapor
inside the bubble before collapse, which decreases γ and
cushions the collapse of the cavitating bubble.15 While it is
difficult to make direct comparisons of energy efficiency (e.g.,
moles of product per kilowatt hour of electrical energy), acoustic
cavitation provides significantly higher rates for the Weissler
reaction, at least for the specific source of hydrodynamic
cavitation tested here.
In summary, we have demonstrated that the chemical effects
of hydrodynamic cavitation and acoustic cavitation respond
identically to experimental parameters, notably the bulk tem-
perature and the nature of the dissolved gas. In particular, the
rates decrease with increasing solution temperature, due to the
increased solvent vapor pressure inside the bubble; increasing
solvent vapor pressure attenuates the efficacy of cavitational
collapse, the maximum temperature reached during such col-
lapse, and, consequently, the rates of cavitational reactions. By
reducing the adiabaticity of bubble collapse, the thermal
conductivity of the dissolved gas also has a substantial effect
on the maximum temperature achieved inside a cavitating
bubble; increased thermal conductivity decreased rates of
cavitation reactions.
Figure 2. Dependence of triiodide formation rate on the nature of the
dissolved gas during hydrodynamic cavitation. Conditions: 60 mL of
1 M KI solution in CCl4-saturated water was recycled under static Ar
or Ar/He atmosphere at a constant reaction temperature of 12 °C and
liquid pressure of 1.24 kbar.
in a collapsing bubble decreases linearly with increasing thermal
conductivity of the entrapped gas.13 This inverse relationship
of Tmax to the thermal conductivity of dissolved gases should
Acknowledgment. This work was supported by the National
Science Foundation (CHE 94-20758) and in part by the DOE. The
Microfluidizer used in this study was provided on loan from Catalytica,
Inc., and Microfluidics International Corp. We thank Dr. David L. King
and Prof. W. R. Moser for useful discussions.
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lead to an exponential decrease in the I3 formation rate
(assuming Arrhenius behavior) with increasing thermal con-
ductivity, as observed.
JA972171I
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The influence of bulk solution temperature on the I3
(14) (a) Sehgal, C.; Sutherland, R. G.; Verrall, R. E. J. Phys. Chem.
1980, 84, 525. (b) Didenko, Y. T.; Nastich, D. N.; Pugach, S. P.; Polovinka,
Y. A.; Kvochka, V. I. Ultrasonics 1994, 32, 71.
production rate was investigated to probe the effect of vapor
-
pressure. The I3 production rates decrease sharply with
(15) (a) Suslick, K. S.; Gawienowski, J. J.; Schubert, P. F.; Wang, H.
H. Ultrasonics 1984, 22, 33. (b) Suslick, K. S.; Gawienowski, J. J.; Schubert,
P. F.; Wang, H. H. J. Phys. Chem. 1983, 87, 2299.
(16) (a) Washington, C.; Davis, S. S. Int. J. Pharm. 1988, 44, 169. (b)
Lidgate, D. M.; Fu, R. C.; Fleitman, J. S. Biopharm 1989, 2, 28. (c) Lidgate,
D. M.; Trattner, T.; Shulz, R. M.; Maskiewicz, R. Pharm. Res. 1992, 9,
860.
increasing temperature, as shown in Figure 3. This same
behavior has been reported for many previous acoustic cavitation
studies.10b,14,15 Figure 3 also demonstrates that the observed
rates decrease exponentially with increasing solvent vapor
(13) Young, F. R. J. Acoust. Soc. Am. 1976, 60, 100.