introduction of ultrasound

2 .3 .1 Introduction of Ultrasound

Ultrasound is simply sound pitched above human hearing . Ultrasound is the part of the sonic spectrum which ranges from about 20 kHz to 10 MHz and can be roughly subdivided in three main regions (A ) low frequency high power ultrasound (20-100 kHz (B ) high frequency , medium power ultrasound (100 kHz-1 MHz (C ) and high frequency , low power ultrasound (1-10 MHz . The frequency level is inversely proportional to the power output . High power , low frequency ultrasound does alter the state of the medium and is the type of [banner_entry_middle]

ultrasound typically used for sonochmical applications . Table A shows the application of ultrasound (1

2 .3 .2 Theory

Two theories exist to explain the chemical effects due to cavitation hot-spot theory (2 ) and the electrical theory (3 4 . Hot spot theory has been experimentally shown that the cavitational collapse creates drastic conditions inside the medium for an extremely short time temperatures of 2000-5000 K and pressures up to 1800 atm inside the collapsing cavity . A remarkable event during the cavitation collapse is the emission light under certain conditions ( HYPERLINK “http /www .fb-chemie .uni-rostock .de /ess /sonochem_intro .htm ” \l “1 1 sonoluminescence . Furthermore , the collapse causes a couple of strong physical effects outside the bubble : shear forces , jets and shock waves The electrical theory postulates that an electrical charge is created on the surface of a cavitation bubble , forming enormous electrical field gradients across the bubble which are capable of bond breakage upon collapse

2 .3 .3 Sonochemistry in Aqueous Solution

The reactive species formed during the sonolysis of water are similar to those observed radiolysis (Table B . Among the most extensively studies species are OH radical and hydrogen peroxide (H2O2 , produced by the thermolysis of water molecules in the gas phase of the bubble , and recombination of the resulting free radicals H2O2 is formed in the cooler interfacial area of the cavitation bubble (5

2 .3 .3 .1 Kinetic Analysis

The chemical transformation which occur during sonolysis may occur in several different regions of the cavitation bubble (Fig C . Three regions of sonochemical activity in sonicated systems (6 . Attack by oxidizing species such as hydroxyl radical (OH ) or oxygen atom or thermolysis of chemical bonds within the substrate can occur in either the gas phase or interfacial region . OH is most concentrated in the gas phase of the cavitation bubble . It is presumed that aromatic substrates are attacked by addition of OH whereas non-aromatic molecules are attach by hydrogen atom abstraction (7 ) due to much stronger C-H bond in aromatic system

2 .3 .4 Acoustic cavitation

Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of liquid motion into heating of the contents of the bubble . The high local temperatures and pressures , combined with extraordinarily rapid cooling , provide a unique means for driving chemical reactions under extreme conditions

The origin of sonochemical effects in liquids is the phenomenon of acoustic cavitation . Ultrasonic waves traveling through a solution impose… [banner_entry_footer]