Sonochemistry is a new interdisciplinary subject developed in the middle and late 1980s. It uses ultrasonic cavitation effect to form local hot spots. It can induce chemical reactions in the extreme micro environment of 4000-6000k, pressure of 100MPa and rapid cooling rate of 109K / s. In recent years, there are a lot of research reports on the application of ultrasound in the field of chemistry. Many scholars introduce ultrasound into their own research, trying to use the special environment created by ultrasound to change and improve the chemical reaction process, or to find some strange reaction phenomena.
Sonochemistry is a member of the sonic family
Power characteristics - when sound waves propagate in the air, they push the particles in the air to vibrate back and forth and do work on the particles. Sound wave power is the physical quantity that indicates the speed of sound wave work. At the same intensity, the lower the frequency of the sound wave, the greater the power it has. Due to the high frequency of ultrasonic wave, the power of ultrasonic wave is very large compared with the general sound wave.
Cavitation: when the ultrasonic wave propagates in the liquid, small cavities will be produced in the liquid due to the violent vibration of the liquid particles. These small cavities expand and close rapidly, resulting in violent impact between the liquid particles, resulting in thousands to tens of thousands of atmospheres of pressure. The intense interaction between the particles will make the temperature of the liquid rise suddenly and play a good role in stirring, so that the two immiscible liquids (such as water and oil) will be emulsified, and the solute dissolution and chemical reaction will be accelerated. This kind of effect caused by ultrasonic action in liquid is called ultrasonic cavitation.
The application of ultrasound in chemical reaction can increase the reaction rate, shorten the reaction time, improve the selectivity of reaction, and stimulate the chemical reaction which can not occur without ultrasound. Due to its unique reaction characteristics, sonochemistry has attracted wide attention and is one of the most important and active research fields in synthetic chemistry.
Sonochemistry has been widely used in every field of chemistry, such as organic synthetic chemistry, nano material preparation, biochemistry, analytical chemistry, polymer chemistry, polymer materials, surface processing, biotechnology and environmental protection.
Ultrasonic chemistry is a phenomenon that uses cavitation phenomenon of power ultrasound to accelerate and control chemical reaction, improve reaction rate and initiate new chemical reaction. The frontier interdisciplinary science, which emerged in the 1980s, has the characteristics of accelerating chemical reaction, reducing reaction conditions, shortening reaction induction time, and being able to carry out some chemical reactions which are difficult to carry out by traditional methods. It is a unique interaction between sound energy and matter.
There is an alternating period of positive and negative pressure in the process of ultrasonic propagation in the medium. In the positive pressure phase, the original density of the liquid medium increases due to the extrusion of the medium molecules by the ultrasonic wave, while in the negative pressure phase, the density of the medium decreases.
When the ultrasonic wave with large enough amplitude is applied to the liquid medium, the average distance between the molecules in the negative pressure region will exceed the critical molecular distance to keep the liquid medium unchanged, and the liquid medium will fracture and form microbubbles, which will grow into cavitation bubbles. In the following compression process, these cavitation bubbles are compressed, their volume is reduced, and some even disappear completely.
When the resonance phase is out, the cavitation bubble is no longer stable. At this time, the pressure in the cavitation bubble can no longer support its own size, that is, it begins to collapse or disappear. This process is called cavitation, or pitting.
The change of reaction conditions caused by cavitation leads to the change of thermodynamics of chemical reaction, which improves the speed and yield of chemical reaction.
In the process of propagation, ultrasonic wave interacts with the medium, and the phase and amplitude change, which can change the state, composition, structure, function and property of the medium. This kind of change is called ultrasonic effect. The interaction between ultrasound and medium can be divided into thermal mechanism, mechanical mechanism and cavitation mechanism
(1) Thermal mechanism: when the ultrasonic wave propagates in the medium, its vibration energy is continuously absorbed by the medium and transformed into heat, which increases the temperature of the medium. This effect of raising the temperature of the medium is called the thermal mechanism of ultrasound.
(2) Mechanical mechanism: when the frequency is low, the absorption coefficient is small, and the ultrasonic action time is very short, the ultrasonic effect is not accompanied by obvious thermal effect. At this time, the ultrasonic effect can be attributed to the mechanical mechanism, that is, the ultrasonic effect comes from the contribution of the mechanical quantity representing the acoustic field. Ultrasonic is also a form of mechanical energy transmission. The mechanical parameters such as origin displacement, vibration velocity, acceleration and sound pressure can describe the ultrasonic effect.
(3) Cavitation mechanism: one of the main mechanisms of ultrasonic sonochemical effect is acoustic cavitation (including bubble formation, growth and disintegration). The phenomenon includes two aspects, that is, the bubbles in liquid produced by high intensity ultrasound and the special movement of bubbles under the action of high intensity ultrasound.
Ultrasonic wave is a kind of high frequency mechanical wave, which has the characteristics of concentrated energy and strong penetration. The ultrasonic wave is composed of a series of longitudinal waves with density alternating, and propagates around through the liquid medium. When the sound energy is high enough, the intermolecular attraction in the liquid phase will be broken in the loose half period, and the cavitation nucleus will be formed. The lifetime of cavitating nucleus is about 0.1 μ s. It can produce local high temperature and pressure environment of about 4000-6000 K and 100 MPa at the moment of explosion, and produce micro jet with strong impact force and velocity of about 110 m / s. This phenomenon is called ultrasonic cavitation.
Sonochemical reaction mainly comes from the mechanism of acoustic cavitation, which is the main force of sonochemical reaction. These conditions are sufficient to cause chemical bond breaking, aqueous combustion, pyrolysis or radical reaction in cavitation bubbles.