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THE CHEMISTRY OF ULTRASOUND
by Kenneth S. Suslick
from The Yearbook of Science & the Future 1994;
Encyclopaedia Britannica: Chicago, 1994; pp 138-155.
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For a recent popular press account of our
work, see "Sonochemistry" Chemistry, Summer
2000, pp. 3, 17-22.
Chemistry is a free quarterly magazine
published by the American Chemical Society and
distributed to its more than 160,000 members
and student affiliates.
For other press clippings, click here.
For a listing of commercially available
sonochemical equipment, click here.
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Exec. Summary:
Sonochemistry
Ultrasound can produce temperatures as high as those on the surface of the
Sun and pressures as great as those at the bottom of the ocean. In some
cases, it can also increase chemical reactivities by nearly a millionfold.
Ultrasound is simply sound pitched above human hearing. It has found many uses in
many areas. At home, we use ultrasound for dog whistles, burglar alarms, and
Exec. Summary:
Smell-Seeing
Exec. Summary:
Porphyrin Research
Introduction to
Sonochemistry
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for Visiting
jewelry cleaners. In hospitals, doctors use ultrasound to remove kidney stones
without surgery, to treat cartilage injuries (such as "tennis elbow"), and to image fetal
development during pregnancy. In industry, ultrasound is important for emulsifying
cosmetics and foods, welding plastics, cutting alloys, and large-scale cleaning. None
of these applications, however, take advantage of the effects that ultrasound can
have on chemical reactivity.
The chemical applications of ultrasound, "sonochemistry", has become an exciting
new field of research during the past decade. The history of sonochemistry, however,
begins in the late 1800s. During field tests of the first high-speed torpedo boats in
1894, Sir John I. Thornycroft and Sydney W. Barnaby discovered severe vibrations
from and rapid erosion of the ship's propeller. They observed the formation of large
bubbles (or cavities) formed on the spinning propeller and postulated that the
formation and collapse of these bubbles were the source of their problems. By
increasing the propeller size and reducing its rate of rotation, they could minimize this
difficulty of "cavitation". As ship speeds increased, however, this became a serious
concern and the Royal Navy commissioned Lord Rayleigh to investigate. He
confirmed that the effects were due to the enormous turbulence, heat, and pressure
produced when cavitation bubbles imploded on the propeller surface. In the same
work, he explained that cavitation was also the origin of teakettle noise!
This phenomenon of cavitation occurs in liquids not only during turbulent flow but also
under high-intensity ultrasonic irradiation. It is responsible for both propeller erosion
and for the chemical consequences of ultrasound. Alfred L. Loomis noticed the first
chemical effects of ultrasound in 1927, but the field of sonochemistry lay fallow for
nearly 60 years. The renaissance of sonochemistry occurred in the 1980's, soon after
the advent of inexpensive and reliable laboratory generators of high-intensity
ultrasound.
Scientists now know that the chemical effects of ultrasound are diverse and include
substantial improvements in both stoichiometric and catalytic chemical reactions. In
some cases, ultrasonic irradiation can increase reactivities by nearly a millionfold.
The chemical effects of ultrasound fall into three areas: homogeneous
sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or liquid-solid
systems, and sonocatalysis (which overlaps the first two). Because cavitation can
take place only in liquids, chemical reactions do not generally occur during the
ultrasonic irradiation of solids or solid-gas systems.
Frontpiece.This micrograph shows interparticle collisions induced by ultrasound
between tin and iron particles about 20 microns in size. The velocity of such collisions
can be as high as 500 m/s (1100 mph). The elemental composition dot map was
produced by scanning Auger electron spectroscopy and show tin in orange and iron
in blue.
Ultrasonic irradiation differs from traditional energy sources (such as heat, light, or
ionizing radiation) in duration, pressure, and energy per molecule (Figure 1). Because
of the immense temperatures and pressures and the extraordinary heating and
cooling rates generated by cavitation bubble collapse, ultrasound provides an
unusual mechanism for generating high-energy chemistry. As in photochemistry, very
large amounts of energy are introduced in a short period of time, but it is thermal
rather than electronic excitation. High thermal temperatures are reached.
Furthermore, sonochemistry has a high-pressure component, which suggests that it
might be possible to produce on a microscopic scale the same large-scale conditions
produced during explosions or by shock waves (a shock wave is a compressional
wave formed whenever the speed of a body or fluid relative to a medium exceeds
that at which the medium can transmit sound).
Figure 1. Chemistry: the interaction of energy and matter. The three axes represent
duration of the interaction, pressure, and energy per molecule. The labeled islands
represent the nature of the interaction of energy and matter in various different kinds
of chemistry.
Sound, Ultrasound, and Cavitation
Sound is nothing more than waves of compression and expansion passing through
gases, liquids or solids. We can sense these waves directly through our ears if they
have frequencies from about Hertz to 16 kHz (the Hertz unit is cycles of compression
or expansion per second; kiloHertz, abbreviated kHz, is thousands of cycles per
second). These frequencies are similar to low frequency radio waves, but sound is
intrinsically different from radio or other electromagnetic radiation. For example,
electromagnetic radiation (radio waves, infrared, visible light, ultraviolet, x-rays,
gamma rays) can pass through a vacuum without difficulty; on the other hand, sound
cannot because the compression and expansion waves of sound must be contained
in some form of matter.
High intensity sound and ultrasound are generally produced in a similar fashion:
electric energy is used to cause the motion of a solid surface, such as a speaker coil
or a piezoelectric ceramic. Piezoelectric materials expand and contract when an
electric field is applied. For ultrasound a high frequency alternating electric current is
applied to a piezoelectric attached to the wall of a metal container (as in an
ultrasonic cleaning bath of the kind used, for example, by jewelers) (Figure 2).
Figure 2. Diagram shows a typical sonochemical apparatus. Ultrasound can easily
be introduced into a chemical reaction in which there is good control of temperature
and ambient atmosphere. The titanium rod shown immersed in the reaction liquid is
driven into vibration by a piezoelectric, which vibrates when subjected to an
alternating current electric field. The usual piezoelectric cerramic is PZT, a lead
zirconate titanate material.
Ultrasound has frequencies pitched above human hearing (above roughly 16 kHz).
Scientists can make narrow beams of "silent" ultrasound far more intense than the
roar of a jet engine, but completely unheard by our ears. Ultrasound has wavelengths
between succession compression waves measuring roughly 10 cm to 10-3
centimeters. These are not comparable to molecular dimensions. Because of this
mismatch, the chemical effects of ultrasound cannot result from a direct interaction
of sound with molecular species.
Nonetheless, the ultrasonic irradiation of liquids does produce a plethora of high
energy chemical reactions. This occurs because ultrasound causes other physical
phenomena in liquids that create the conditions necessary to drive chemical
reactions. The most important of these is cavitation: the formation, growth, and
implosive collapse of bubbles in a liquid. The dynamics of cavity growth and collapse
are strikingly dependent on the local environment. Cavity collapse in a homogeneous
liquid is very different from cavitation near a liquid-solid interface, which will be
considered later.
As ultrasound passes through a liquid, the expansion cycles exert negative pressure
on the liquid, pulling the molecules away from one another. If the ultrasound is
sufficiently intense, the expansion cycle can create cavities in the liquid. This will
occur when the negative pressure exceeds the local tensile strength of the liquid,
which varies according to the type and purity of liquid. (Tensile strength is the
maximum stress that a material can withstand from a stretching load without tearing.)
Normally, cavitation is a nucleated process; that is, it occurs at pre-existing weak
points in the liquid, such as gas-filled crevices in suspended particulate matter or
transient microbubbles from prior cavitation events. Most liquids are sufficiently
contaminated by small particles that cavitation can be readily initiated at moderate
negative pressures.
Once formed, small gas bubbles irradiated with ultrasound will absorb energy from
the sound waves and grow. Cavity growth depends on the intensity of the sound. At
high intensities, a small cavity may grow rapidly through inertial effects. If cavity
expansion is sufficiently rapid during the expansion half of a single cycle, it will not
have time to recompress during the compression half of the acoustic cycle.
At lower acoustic intensities cavity growth can also occur by a slower process called
rectified diffusion (Figure 3). Under these conditions a cavity will oscillate in size over
many expansion and compression cycles. During such oscillations the amount of gas
or vapor that diffuses in or out of the cavity depends on the surface area, which is
slightly larger during expansion than during compression. Cavity growth during each
expansion is, therefore, slightly larger than shrinkage during the compression. Thus,
over many acoustic cycles, the cavity will grow. The growing cavity can eventually
reach a critical size where it can efficiently absorb energy from the ultrasonic
irradiation. Called the resonant size, this critical size depends on the liquid and the
frequency of sound; at 20 kHz, for example, it is roughly 170 micrometers. At this
point the cavity can grow rapidly during a single cycle of sound.
Figure 3. Liquids irradiated with ultrasound can produce bubbles. These bubbles
oscillate, growing a little more during the expansion phase of the sound wave than
they shrink during the compression phase. Under the proper conditions these
bubbles can undergo a violent collapse, which generates very high pressures and
temperatures. This process is called cavitation.
Once the cavity has overgrown, either at high or low sonic intensities, it can no longer
absorb energy as efficiently. Without the energy input the cavity can no longer sustain
itself. The surrounding liquid rushes in, and the cavity implodes. It is the implosion of
the cavity that creates an unusual environment for chemical reactions.
The Sonochemical Hot-Spot
Compression of a gas generates heat. On a macroscopic scale, one can feel this
when pumping a bicycle tire; the mechanical energy of pumping is converted into
heat as the tire is pressurized. The compression of cavities when they implode in
irradiated liquids is so rapid than little heat can escape from the cavity during
collapse. The surrounding liquid, however, is still cold and will quickly quench the
heated cavity. Thus, one generates a short-lived, localized hot spot in an otherwise
cold liquid. Such a hot spot is the source of homogeneous sonochemistry; it has a
temperature of roughly 5000 C (9,000 F), a pressure of about 1000 atmospheres, a
lifetime considerably less than a microsecond, and heating and cooling rates above
10 billion C per second. For a rough comparison, these are, respectively, the
temperature of the surface of the sun, the pressure at the bottom of the ocean, the
lifetime of a lightning strike, and a million times faster cooling that a red hot iron rod
plunged into water! Thus, cavitation serves as a means of concentrating the diffuse
energy of sound into a chemically useful form. Alternative mechanisms involving
electrical microdischarge have been proposed (most recently by M.A. Margulis of the
Russian Institute for Organic Synthesis), but they do not appear fully consistent with
observed data.
Determination of the temperatures reached in a cavitating bubble has remained a
difficult experimental problem. The transient nature of the cavitation event precludes
direct measurement of the conditions generated during bubble collapse. Chemical
reactions themselves, however, can be used to probe reaction conditions. The
effective temperature of a system can be determined with the use of competing
unimolecular reactions whose rate dependencies on temperature have already been
measured. This technique of "comparative-rate chemical thermometry" was used by
K.S. Suslick, D.A. Hammerton and R.E. Cline, Jr., at the University of Illinois to
determine the effective temperature reached during cavity collapse. For a series of
organometallic reactions, the relative sonochemical rates were measured. In
combination with the known temperature behavior of these reactions, the conditions
present during cavity collapse could then be determined. The effective temperature of
these hot spots was 5,200 K. Of course, the comparative rate data represent only a
composite temperature: during the collapse, the temperature has a highly dynamic
profile, as well as a spatial gradient in the surrounding liquid.
When a liquid is subjected to ultrasound, not only does chemistry occur, but light is
also produced (Figure 4). Such "sonoluminescence" provides an alternate measure
of the temperature of the high-energy species produced during cavitation. Highresolution sonoluminescence spectra were recently reported and analyzed by E.B.
Flint and Suslick. From a comparison of synthetic to observed spectra, the effective
cavitation temperature of the emitting species is 5,100 K. The agreement between
this spectroscopic determination of the cavitation temperature and that made by
comparative rate thermometry of sonochemical reactions is surprisingly close.
Figure 4. High intensity ultrasound creates localized hot spots in liquids through the
process of cavitation. Local heating produces excited states of molecules that emit
light, just as they do in a flame. The image shown is such sonoluminescence seen
from a vibrating titanium rod (about 0.4 inch) in diameter. False color is used to
enhance contrast. The temperature created in cavitation hot-spots, determined from
the spectrum of this emission, is ~5000 K.
Cavitation in Liquid-Solid Systems
When cavitation occurs in a liquid near a solid surface, the dynamics of cavity
collapse changes dramatically. In pure liquids, the cavity remains spherical during
collapse because its surroundings are uniform. Close to a solid boundary, however,
cavity collapse is very asymmetric and generates high-speed jets of liquid (Figure 5).
The potential energy of the expanded bubble is converted into kinetic energy of a
liquid jet that moves through the bubble's interior and penetrates the opposite bubble
wall. Werner Lauterborn at the Technische Hochschule in Darmstadt, Germany,
observed liquid jets driving into the surface with velocities of roughly 400
kilometers/hour (Figure 6). These jets hit the surface with tremendous force. This
process can cause severe damage at the point of impact and can produce newly
exposed, highly reactive surfaces; it has great importance for understanding the
corrosion and erosion of metals observed in propellers, turbines, and pumps where
cavitation is a continual technological problem.
Figure 5. A bubble in a liquid irradiated with ultrasound implodes near a solid
surface. The presence of the solid causes the implosion to be asymmetrical, forming
a high-speed jet of liquid that impacts the surface. The cavity is spherical at first, but
as it collapses the jet develops opposite the solid surface and moves towards it. (L.A.
Crum)
Figure 6. High-speed microcimemagraphic sequence of laser-induced cavitation
near a solid surface shows the formation of a microjet impact with a velocity of
approximately 400 kilometers (250 miles) per hour. (W. Lauterborn)
Distortions of bubble collapse depend on a surface several times larger than the
resonant size of the bubble. The presence of fine powders, therefore, does not
induce jet formation. In the case of liquid-powder slurries, the shock waves created
by homogeneous cavitation can create high-velocity interparticle collisions. The
turbulent flow and shock waves produced by intense ultrasound can drive metal
particles together at sufficiently high speeds to cause effective melting at the point of
collision (Figure 7). Such interparticle collisions are capable of inducing striking
changes in surface texture, composition, and reactivity, as discussed later.
S. J. Doktycz and K. S. Suslick used metal powders to estimate the effective
maximum temperatures and speeds reached during interparticle collisions (Figure 8).
When chromium, molybdenum, and tungsten powders of a few micrometers in size
are irradiated in decane at 20 kHz and 50 watts per square centimeter, one observes
agglomeration and welding of particles for the first two metals but not for the third. On
the basis of the melting points of these metals, the effective transient temperature
reached at the point of impact during interparticle collisions is roughly 3000 C. On
the basis of the volume of the melted region of impact, the amount of energy
generated during collision was determined. From this, the velocity of impact is
estimated to be roughly 1800 kilometers per hour, which is half the speed of sound in
liquids. It should be noted that the conditions reached during interparticle collisions
are not directly related to the temperatures reached during cavitational collapse of
bubbles.
Figure 7. Scanning electron micrograph reveals zinc powder after ultrasonic
irradiation. The neck formation from localized melting or plastic deformation was
caused by high-velocity collisions of the zinc particles.
Figure 8. Scanning electron micrographs reveal slurries of metal powders both
before and after ultrasonic irradiation. Chromium has a melting point of 1857 C
(3,374.6 F), and its particles both agglomerate and are deformed; molybdenum
melts at 2617 C (4,742.6 F), and its particles are slightly agglomerated but not
smoothed or deformed; tungsten melts at 3410 C (6,170 F) and is unaffected.
Sonochemistry in Homogeneous Liquids
High-intensity ultrasonic probes (10 to 500 watts per square centimeter) are the
most reliable and effective sources for laboratory-scale sonochemistry. A typical
laboratory apparatus permits easy control over ambient temperature and atmosphere
(Figure 2). Lower acoustic intensities can often be used in liquid-solid heterogeneous
systems because of the reduced liquid tensile strength at the liquid-solid interface.
For such reactions a common ultrasonic cleaning bath will often be adequate. The
low intensity available in these devices ( about one watt per square centimeter) can,
however, prove to be a limitation. On the other hand, ultrasonic cleaning baths are
easily accessible, comparatively inexpensive, and usable on moderately large scales.
Finally, for large-scale irradiations, flow reactors with high ultrasonic intensities are
commercially available in modular units as powerful as 20 kilowatts.
The chemical effect of ultrasound on aqueous solutions have been studied for many
years. The primary products are molecular hydrogen (H2) and hydrogen peroxide
(H2O2). Other high-energy intermediates may include HO2 (superoxide), H (atomic
hydrogen), OH (hydroxyl), and e-(aq) (solvated electrons). Peter Riesz and
collaborators at the National Institutes of Health used electron paramagnetic
resonance with chemical spin-traps to demonstrate definitively the generation of H
and OH . The extensive recent work in Arne Henglein's laboratory at the HahnMeitner Institute involving aqueous sonochemistry of dissolved gases has established
analogies to combustion processes. As one would expect, the sonolysis of water,
which produces both strong reductants and oxidants, is capable of causing
secondary oxidation and reduction reactions, as often observed by Margulis and
coworkers.
In contrast, the ultrasonic irradiation of organic liquids has been little studied. Suslick
and co-workers established that, as long as the total vapor pressure is low enough to
allow effective bubble collapse, almost all organic liquids will generate free radicals
(uncharged, reactive intermediates possessing an unpaired electron) when they
undergo ultrasonic irradiation. The sonolysis of simple hydrocarbons creates the
same kinds of products associated with very high temperature pyrolysis. Most of
these products - H2, CH4 (methane), and the smaller 1-alkenes, derive from a wellunderstood radical chain mechanism. Relatively large amounts of acetylene (C 2H2)
are also produced, which is explained by the stability of this gas at very high
temperatures.
The sonochemistry of solutes dissolved in organic liquids also remains largely
unexplored, though that of metal carbonyl compounds is an exception. In 1981, P. F.
Schubert, J. W. Goodale and Suslick reported the first sonochemistry of discrete
organometallic complexes and demonstrated the effects of ultrasound on metal
carbonyls. Detailed studies of these systems led to important understandings of the
nature of sonochemistry. Unusual reactivity patterns have been observed during
ultrasonic irradiation, including novel metal cluster formation and the initiation of
homogeneous catalysis at low ambient temperature, with rate enhancements greater
than 100,000-fold.
Polymers and Biomaterials: Bond Making and Breaking
The effects of ultrasound on polymers (giant molecules formed by the coupling of
small molecules-monomers) have been thoroughly studied over the past 30 years.
The controlled cleavage of polymers in solutions irradiated with ultrasound has been
examined in detail. Polymer degradation produces chains of smaller lengths with
relatively uniform molecular weight distributions, with cleavage occurring primarily in
the center of the polymer chain. Several mechanisms have been proposed for this
sonochemical cleavage, which is usually described as a mechanical breakage of the
chains induced by shock waves or solvent flow created by cavitation during the
ultrasonic irradiation of liquids.
This polymer fragmentation has been used by G. J. Price at the University of Bath to
synthesize block copolymers of various sorts. Block copolymers are long chain
polymers with two different, but linked, parts. As an analogy, imagine a train made up
in front by passenger cars and in back by freight cars. In this fashion, block
copolymers can do double-duty in their properties. Peter Kruus at Carleton University,
Ottawa, reported the use of ultrasound to initiate polymerization in solutions of
various monomers.
Applications of ultrasound to the synthesis of biomaterials are under rapid
development. While the chemical effects of ultrasound on aqueous solutions have
been studied for many years, the development of aqueous sonochemistry for
biomaterials synthesis is very recent. The area of protein microencapsulation has
proved especially interesting. Microencapsulation, the enclosing of materials in
capsules a few micrometers in size, has diverse important applications; these include
uses with dyes, flavors and fragrances, as drug delivery systems, and as medical
diagnostic agents.
One recent example is the use of high intensity ultrasound to make aqueous
suspensions of long-lived proteinaceous microspheres filled with air or with waterinsoluble liquids for medical applications (Figure 9). By itself, emulsification is
insufficient to produce these long-lived microspheres; chemical reactions requiring
oxygen are critical in forming them. Specifically, the sonolysis of water produces
hydrogen atoms that react with oxygen to produce superoxide. Suslick and M. W.
Grinstaff demonstrated that the proteinaceous microspheres are held together by
disulfide bonds between protein cysteine residues and that superoxide is the crosslinking agent.
Figure 9. Protein microspheres filled with the oily hydrocarbon dodecane were
formed by the ultrasonic irradiation of albumin solutions. Such microspheres may
prove useful for drug delivery and medical diagnostic imaging.
Sonoluminescence: Microscopic Thunder and Lightning
A few years after the discovery of sonochemical reactions, H. Frenzel and H.
Schultes in 1934 first observed sonoluminescence from water. As with
sonochemistry, sonoluminescence derives from acoustic cavitation. Although
sonoluminescence from aqueous solutions has been studied in some detail, only
recently has significant work been reported on sonoluminescence from non-aqueous
liquids containing no water. In both cases, the emission of light comes from the high
temperature formation of reactive chemical species in electronic excited states. The
emitted light from these excited states provides a spectroscopic probe of the
cavitation event.
High resolution sonoluminescence spectra from hydrocarbons and silicone oil were
recently analyzed by Flint and Suslick. The observed emission comes from excited
state diatomic carbon which are the same transitions responsible for the blue color of
a hydrocarbon flame (from the kitchen stove, for example). The details of this
emission depend on the temperature of the emitted C2 and can be accurately
modeled with synthetic spectra as a function of presumed temperature. From a
comparison of synthetic to observed spectra, the average effective temperature of the
excited state of C2 is about 5,100 K, as mentioned above.
Recently, it was discovered that sonoluminescence can be observed, quite
remarkably, in a single, oscillating gas bubble. In 1990 E. Gaitan and L. A. Crum at
the University of Mississippi discovered conditions under which a single, stable gas
bubble could produce sonoluminescent emission on each acoustic cycle, and
continue this process essentially indefinitely. Seth J. Putterman at the University of
California Los Angeles examined these bubbles with a time resolution in
picoseconds. Gaitain, Crum, and Putterman were able to use sophisticated light
scattering techniques to measure the radius-time curve of the luminescing bubble
and to correlate the optical emissions with a particular phase of the sound field. As
expected, the emissions occurred during cavity collapse. Quite surprisingly, the
duration of the sonoluminescence emissions was less than a hundred picoseconds,
roughly one millionth the duration of the acoustic cycle used. This very short emission
appears to originate from the formation of shock waves within the collapsing bubble
during the first stages of compression.
Heterogeneous Sonochemistry: Reactions of Solids with Liquids
The use of high-intensity ultrasound to enhance the reactivity of metals as
stoichiometric reagents has become an important synthetic technique for many
heterogeneous organic and organometallic reactions, especially those involving
reactive metals, such as magnesium, lithium, and zinc. This development originated
from the early work of Pierre Renaud in France in the 1950's and the more recent
breakthroughs of J.-L. Luche at the University of Grenoble, France. This application
of sonochemistry grew rapidly during the past decade in a large number of
laboratories across the world. The effects are quite general and apply to reactive
inorganic salts as well. Reactivity rate enhancements of more than 10-fold are
common, yields are often substantially improved, and by-products avoided. A few
simple examples of the sonochemistry of reactive reagents are shown below (where
))) indicates ultrasonic irradiation), taken from the work of Takashi Ando, Philip
Boudjouk, Luche, Timothy J. Mason, and Suslick, among others.
The mechanism of the rate enhancements in reactions of metals has been unveiled
by monitoring the effect of ultrasonic irradiation on the kinetics of the chemical
reactivity of the solids, examining the effects of irradiation on surface structure and
size distributions of powders and solids, and, determining depth profiles of the
surface elemental composition. The power of this three-pronged approach has been
proved in studies of the sonochemistry of transition metal powders. Doktycz and
Suslick found that ultrasonic irradiation of liquids nickel, zinc, and copper powders
leads to dramatic changes in structure. The high-velocity interparticle collisions
produced in such slurries cause smoothing of individual particles (Figure 10) and
agglomeration of particles into extended aggregates (Figure 8). Surface composition
was probed by Auger electron spectroscopy and mass spectrometry to generate
depth profiles of these powders; they revealed that ultrasonic irradiation effectively
removed the inactive surface oxide coating. The removal of such passivating coatings
dramatically improves reaction rates.
Figure 10. The effect of ultrasonic irradiation on the surface texture of nickel
powder. High-velocity interparticle collisions caused by ultrasonic irradiation of
slurries is responsible for these effects.
Considerably less work has been done on the activation of less reactive metals. This
goal continues to attract major efforts in both synthetic organometallic chemistry and
heterogeneous catalysis. Ultrasound can be used at room temperature and pressure
to promote heterogeneous reactions that normally occur only under extreme
conditions of hundreds of atmospheres and hundreds of degrees. For example, R. E.
Johnson and Suslick found good results with the use of ultrasound to drive some of
the most difficult reactions known for transition metals: the attack of carbon monoxide
on the very unreactive early transition metals such as vanadium, tantalum,
molybdenum and tungsten.
Another application of ultrasound in materials chemistry involves the process of
intercalation, which is the adsorption of organic or inorganic compounds as guest
molecules between the atomic sheets of layered solid hosts, such as graphite or
molybdenum sulfide. Intercalation permits the systematic change of optical,
electronic, and catalytic properties. Such materials have many technological
applications (for example, lithium batteries, hydrodesulfurization catalysts, and solid
lubricants). The kinetics of intercalation, however, are generally extremely slow, and
syntheses usually require high temperatures and very long reaction times. M.L.H.
Green at University of Oxford, Suslick and their students discovered that highintensity ultrasound dramatically increases the rates of intercalation of a wide range
of compounds (including amines, metallocenes, and metal sulfur clusters) into
various layered inorganic solids such as ZrS2, V2O5, TaS2, MoS2, and MoO3.
Scanning electron microscopy of the layered solids in conjunction with studies of
chemical kinetics demonstrated that the origin of the observed rate enhancements
comes from particle fragmentation (which dramatically increases surface areas), and
to a lesser extent from surface damage. Because high-intensity ultrasound can
rapidly form uniform dispersions of micrometer-sized powders of brittle materials, it is
useful for a wide range of liquid-solid reactions.
Another application of heterogeneous sonochemistry involves the preparation of
amorphous metals. If one can cool a molten metal alloy quickly enough, it can be
frozen into a solid before it has a chance to crystallize. Such amorphous metallic
alloys lack long range crystalline order and have unique electronic, magnetic, and
corrosion resistant properties. The production of amorphous metals, however, is
difficult because extremely rapid cooling of molten metals is necessary to prevent
crystallization. Cooling rates of approximately 106 K per second are required; for
comparison, plunging red hot steel into water produces cooling at only about 2500 K
per second. Very recently, the use of ultrasound to synthesize amorphous metal
powders by using the sonochemical decomposition of volatile organometallics was
reported by Suslick, S.-B. Choe, A. A. Cichowlas, and M. W. Grinstaff. This exciting
discovery opens new applications of ultrasound for the low temperature synthesis of
unusual phases. For example, the sonolysis of iron pentacarbonyl produces nearly
pure amorphous iron, which was characterized by a variety of techniques to prove its
lack of long-range order. Scanning electron micrographs show conchoidal fractures
(those with smoothly curved surfaces, which are typical of an amorphous material),
and at higher magnification reveals a coral-like porosity coming from the
agglomeration of small clusters of iron (Figures 11 and 12).
Figure 11. Amorphous iron powder is formed from the ultrasonic irradiation of iron
carbonyl. The micrograph shows the porous, coral-like structure formed from
nanometer-sized clusters created during acoustic cavitation. The amorphous iron is
an extremely soft ferromagnetic material with high catalytic activity. The heating and
cooling produced by cavitation are so rapid that the iron atoms cluster and solidify
before they can form a well-ordered crystal.
Figure 12. A transmission electron micrograph of amorphous iron powder, in falsecolor to enhance contrast. Because of their excellent magnetic properties, amorphous
metals have important technological applications; these can include electrical
transformer cores and magnetic tape recorder heads.Magnification of the cover
image is approximately 100,000.
The sonochemically synthesized amorphous powders may have important
technological applications. For example, the amorphous iron powder is an active
catalyst for several important reactions, including the synthesis of liquid fuels from
CO and H2 (which can be produced from coal). In addition, magnetic measurements
reveal the amorphous iron to be a very soft ferromagnet, that is, a material that very
quickly forgets its magnetization once an magnetic field has been turned off. While
such materials would be very bad for making permanent magnets, they are very good
for making magnetic shielding, electrical transformer cores, or magnetic media
recording heads.
Sonocatalysis
Catalytic reactions are of enormous importance in both laboratory and industrial
applications. Catalysts increase the rates of chemical reactions without being
consumed themselves; they are generally divided into two types. If the catalyst is a
molecular or ionic species dissolved in a liquid, then the system is "homogeneous"; if
the catalyst is a solid, with the reactants either in a percolating liquid or gas, then it is
"heterogeneous." In both cases, it is often a difficult problem either to activate the
catalyst or to keep it active.
Ultrasound has potentially important applications in both homogeneous and
heterogeneous catalytic systems. Heterogeneous catalysis is generally more
industrially important than homogeneous systems. For example, virtually all of the
petroleum industry is based on a series of catalytic transformations. Heterogeneous
catalysts often require rare and expensive metals. The catalytic converters used on
automobiles to lessen pollution, for example, use platinum or rhodium, which are
enormously expensive; rhodium costs about $1500 dollars per ounce!
Using ultrasound offers some hope of activating less reactive, but also less costly,
metals. Some early investigations of the effects of ultrasound on heterogeneous
catalysis can be found in the Soviet literature. In this early work, increases in turnover
rates were usually observed upon ultrasonic irradiation, but were rarely more than
10-fold. In the case of modest rate increases, it appears likely that the cause is
increased effective surface area; this is especially important in the case of catalysts
supported on brittle solids.
More impressive accelerations, however, have been recently reported, including
hydrogenations (catalytic reactions of hydrogen with unsaturated organic
compounds) by nickel, palladium, or platinum. For example, D. J. Casadonte and
Suslick discovered that hydrogenation of alkenes by nickel powder is enormously
enhanced (about 100,000-fold) by ultrasonic irradiation. A very interesting effect on
the surface morphology was observed (Figure 10). Ultrasonic irradiation smoothes,
at a macroscopic scale, the initially crystalline surface and causes agglomeration of
small particles. Both effects are probably due to interparticle collisions caused by
cavitation-induced shock waves. Auger electron spectroscopy reveals that there is a
considerable decrease in the thickness of the oxide coat after ultrasonic irradiation.
The removal of this layer is probably responsible for the great increase observed in
catalytic activity.
A Sound Future
Acoustic cavitation results in an enormous concentration of energy. If the energy
density in an acoustic field that produces cavitation is compared with that in the
collapsed cavitation bubble, there is an amplification of almost one trillion. The
enormous local temperatures and pressures of cavitation result in sonochemistry and
sonoluminescence. Cavitation produces an unusual method for fundamental studies
of chemistry and physics under extreme conditions, and sonochemistry provides a
unique interaction of energy and matter.
In addition, ultrasound is well suited to industrial applications. Since the reaction
liquid itself carries the sound, there is no barrier to its use with large volumes. In fact,
ultrasound is already heavily used industrially for the physical processing of liquids,
such as emulsification, solvent degassing, solid dispersion, and sol formation. It is
also extremely important in solids processing, including cutting, welding, cleaning,
and precipitation.
The extension of ultrasound to the chemical processing of liquids is underway. The
future uses of ultrasound to drive chemical reactions will be diverse. It is becoming a
common tool in nearly any case where a liquid and a solid must react. In the
synthesis of pharmaceuticals, for example, ultrasound may permit improved yields
and facilitate reactions run on larger scale. In the development and use of catalysts,
ultrasound also has potential applications. Its ability to create highly reactive surfaces
and thereby increase their catalytic activity has only just now been established.
Ultrasound can produce materials with unusual properties. The extraordinary
temperatures and pressures reached during cavitational collapse, combined with the
exceptionally high rates of cooling, may allow researchers to synthesize novel solid
phases difficult to prepare in other ways. One may be optimistic that the unusual
reactivities caused by ultrasound will find important industrial application in the years
to come.
THE
SCIENCE
THE
GROUP
THE
MAÎTRE D'
LAGNIAPPE:
A LITTLE
EXTRA
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©2006, K.S. Suslick; all rights reserved.
Comments and suggestions: ksuslick@uiuc.edu