J.P. Lorimer

Quantifying sonochemistry : casting some light on a 'black art'

Despite the ever increasing usage of power ultrasound to enhance reactivity in synthetic chemistry, many experimentalists experience difficulty in reproducing the work of other groups. Although such problems are most commonly encountered with chemical reactions which have been performed in ultrasonic baths, even reactions involving probe systems have sometimes proved difficult to reproduce. The origin of many of tehse problems can be traced to a failure of the original report to specify the exact sonication conditions, e.g. the frequency of ultrasonic irradiation, the precise power entering the reaction system, the geometry of the reaction vessel, the presence of a bubbled gas or even the temperature of the reaction∗. In this article we report the results of some model reactions performed using the newly developed Undatim Sonoreactor probe system. This particular instrument is equipped with an automatic transducer resonance frequency search device, enabling the power input to a system to be maintained accurately throughout a reaction, thereby offering considerable advantages over the normal commercially available equipment in terms of both the monitoring and control of irradiation parameters. Using this facility we are able to report how ultrasonic energy input to a chemical reaction is affected by the following important reaction parameters: ultrasonic power used, the presence of bubbled gas, temperature, solvent composition and reaction volume. In addition, a comparison is made of the ultrasonic power available from three different laboratory sonicators operating at a nominal frequency of 20 kHz.

The development and evaluation of ultrasound in the biocidal treatment of water

The effect of ultrasound upon the destruction of micro-organisms has been studied and reported here. The results obtained from the work carried out has shown that ultrasound can be used effectively for water disinfection and has several advantages. When used in conjunction with chlorine it significantly reduces the number of bacteria present in water samples. Ultrasound also reduces the amount of chlorine required for disinfection. Increasing the power of ultrasound leads to greater efficiency in the destruction of bacterial cells. High frequency ultrasound is more beneficial than low frequency in the disinfection of water.

Enhancement of chemical reactivity by power ultrasound: an alternative interpretation of the hot spot

Abstract 20 kHz ultrasound has been found to enhance the decomposition of the non-volatile initiator, potassium persulphate. The usual sonochemical correlation with solvent vapour pressure did not apply. The data was analysed by assuming that reaction took place at an elevated temperature in the interface between the cavitation bubble and the bulk solvent. The analysis indicated that the enhanced decomposition was proportional to both the acoustic pressure and the square root of the acoustic intensity. The analysis was also found to be applicable to the solvolysis of 2-chloro methylpropane, a volatile substrate.

Sonochemically enhanced Ullmann reactions

Abstract During the Ullmann coupling reaction of 2-iodonitrobenzene in the presence of a four-fold excess of copper in DMF, the reaction rate increases 50-fold when ultrasound is introduced via a tapered microtip (diameter 1 16 in). If a stud (diameter 1 2 in) is used the rate increases 64-fold. Electron microscopy and particle size analysis show that sonication reduces the average particle size of copper by a factor of four. However, this alone is insufficient to explain the increase in reaction rate. Replacement of DMF by decalin or toluene was found to lead to substantial reductions in rate.

The effect of ultrasound on the solvolysis of 2-chloro-2-methylpropane in aqueous ethanol

Abstract Ultrasonic irradiation has been found to accelerate the solvolysis of 2-chloro-2-methylpropane in aqueous ethanol mixtures from 20 to 60% w/w in ethanol at temperatures between 10° and 25°. The effects are more marked at lower temperatures and higher alcohol compositions with a 20 fold rate acceleration occurring at 10° and 60% w/w. The results also lend independent support to two previous observations that (a) the position of maximum in solvent structuredness is 50% w/w and (b) a logarithmic relationship exists between solvent vapour pressure and ultrasonically enhanced rate constant. At solvent compositions of 50 and 60% w/w ethanol the absolute rate of reaction under irradiation increases with decreasing reaction temperature below 20°.

Effect of ultrasound on the encapsulation of titanium dioxide pigment

In this paper we describe the effect of 20 kHz power ultrasound on the encapsulation of titanium dioxide pigment with poly (methyl methacrylate) in an otherwise conventional emulsion-polymerisation process. In all instances, sonication of the reaction during the mixing period led to an improvement in the extent and uniformity of the coverage of the pigment. Low-power ultrasound produced a more uniform coverage than high-power ultrasound, but the latter provided greater particle dispersion.

Effect of ultrasound on the solvolysis of 2-chloro-2-methylpropane in aqueous alcoholic solvents

Abstract Ultrasonic irradiation was found to acclerate the solvolysis of 2-chloro-2-methylpropane in aqueous ethanol mixtures. The effects were more marked at lower temperatures and higher alcohol compositions, with a 20-fold rate acceleration occurring at 10°C and 60 wt%. The kinetic data were analysed with reference to the variation in solvent structure and vapour pressure. Enthalpies and heat capacities of activation in both the presence and absence of ultrasound were also determined. The result spresented in this Paper confirm the existence of a region of maximum structure in the binary mixture.

Sonochemical enhancement of electrochemiluminescence

Abstract The electrochemiluminescence of the tris- (2,2′-bipyridine) ruthenium (II) dication in aqueous oxalate and in acetonitrile is substantially enhanced by simultaneous irradiation with 40–60 kHz ultrasound. Edge effects and electrode patchiness are diminished, quantum efficiency is increased and lower cell voltages are required. Reproducibility and stability are improved and electrode fouling is minimized. Electrochemiluminescence (ECL) and sono-electrochemiluminescence (SECL) seem to be of similar origin but a weak sonoluminescence (SL), also observed, is of a different origin. Preliminary experiments upon the luminol and 1-aminopyrene electrochemiluminescence systems suggest that different mechanisms for ultrasonic influence may operate in each case.

Investigation of the consumption of diphenylpicrylhydrazyl in solution in the absence and presence of ultrasound

The reaction by which diphenylpicrylhydrazyl (DPPH) is decolorised in trichlorobenzene, toluene and benzene solution in the absence of ultrasound and at elevated temperatures has been demonstrated to conform to first-order kinetics. This reaction is best rationalised as resulting from an abstraction reaction between DPPH and the solvent. The activation energy for this reaction is in good agreement with those deduced for the abstraction reactions between a polystyrene macro-radical and either toluene or benzene.The reaction of DPPH in the presence of ultrasound has also been shown to occur by an abstraction reaction and not by reaction with radicals resulting from the extremes of temperature and pressure generated as the cavitation bubbles collapse. The rate constants for this reaction, determined over a wide range of sonochemical conditions, appear to be dependent on the power dissipated by the ultrasonic horn, the diameter of the irradiating horn and the vapour pressure of the solvent.

A method for the determination of the activation energy for a reaction from a single kinetic run

Abstract A FORTRAN 77 program is described for the determination of activation energy ( E a ) and Arrhenius pre-exponential factor ( A ) from a single kinetic run. The program may used on an Apple 11 microcomputer, and represents a substantial saving in experimental time over the conventional method of determining these parameters.