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Historical perspective
The first report of the chemical effects of ultrasound was in 1927 which described the use of ulrasound for a range of processes including emulsification and surface cleaning [1]. There were occasional references to chemistry influenced by ultrasound after this and the two uses of ultrasound for chemists were considered to be in analysis and synthesis [2]. In the 1950,s thought was being given to the reasons why acoustic cavitation could induce chemical changes and this resulted in the concept of cavitation bubbles acting as “hot-spots” [3]. Primary ultrasonic reactions were seen as gas phase reactions, probably of a thermal nature, taking place inside the bubbles themselves. Sonochemistry itself as a subject in its own right was not really recognised until the 1980’s when the first ever international conference was held at Warwick University in the UK in April 1986 [4]. This was followed by the first two major reviews of the subject divided into physical aspects [5] and synthetic applications [6].
1. The chemical effects of high frequency sound waves - a preliminary survey,Richards, W.T. and A.L. Loomis, Journal of the American Chemical Society, 1927. 49(12): p. 3086-3100. 2. Ultrasonics in chemistry Weissler, A., . J Chem Educ., 1948. 25: p. 28-30. 3. Chemical Effects of Ultrasonics-``Hot Spot'' Chemistry", Fitzgerald, M.E., V. Griffing, and J. Sullivan, J, J. Chem. Phys., 1956. 25: p. 926 - 933. 4. Sonochemistry symposium, annual chemical congress., Mason, T.J., Ultrasonics, 1987. 25(1): p. 5. 5. Sonochemistry. Part 1 - The physical aspects, Lorimer, J.P. and T.J. Mason, . Chemical Society Reviews, 1987. 16: p. 239 - 274. 6. Part2 - Synthetic applications, Lindley, J. and T.J. Mason, Sonochemistry. Chemical Society Reviews, 1987. 16: p. 275 - 311. General Overview The chemical effects of ultrasound cannot be the result of any direct coupling of the sound field with the chemical species involved on a molecular level since the sound frequencies most commonly employed (20‑40kHz) are several orders of magnitude too low even for the excitation of rotational motion. Thus, there is no direct interaction between the ultrasonic wave and matter and so there must be an indirect interaction via a process of energy concentration that can then affect molecules. Such a process is acoustic cavitation which provides bubbles which act as microreactors (see introduction to sonochemistry). Here there are four possible reaction sites associated with the collapse (Table):
Based on the physical effects just described and observed during bubble collapse (or to some extent during stable cavitation), there are several possible chemical effects at the reaction sites. These may be divided into two groups: radical effects and mechanical effects. (1) Radical formation The drastic conditions inside the bubble just described stimulate the generation of radicals. The source of the radical formation may be the solvent vapour or volatile compounds in the liquid. Primary radicals may induce secondary reactions: they may be converted to other radicals, may initiate a radical chain, can react with substrates (including the solvent itself) or attack other radicals (including recombination). Such radical reactions may occur in the bubble interior, in the bubble liquid shell/interface or in the bulk solution. Examples are the generation of OH-radicals in water, the formation of hydrogen peroxide in water, the hydroxylation of aromatic compounds by OH-radicals or the acceleration of the initiation of radical polymerisation. Furthermore, volatile substrates may be thermally degraded inside or at the interface of a collapsing bubble. Organic solvents will also slowly decompose on sonication but solvent decomposition normally provides only a minor contribution to any sonochemical reaction that is taking place in the medium. However, it should be noted that the primary formation of radicals inside the bubble is a relatively slow process having reaction rates that are of the order of 10-4 to 10-5 mol.l-1.min-1. Moreover, secondary radical attacks on a substrate are not influenced by ultrasound. Consequently, only those radical reactions causing a radical chain or those requiring only a small number of radicals compared to the substrate (e.g. destruction of polymers) are of interest to a synthetic chemist. (2) Mechanical effects Results indicate that ionic chemical reactions in solution are not influenced by sonication unless they involve solids, metals or multi-phase conditions however the physical effects generated in solution around the collapsing bubble can effect chemical species in solution, solid particles in suspension or surfaces.
Homogenous systems The following effects in a homogeneous solution are of relevance: · enhanced mass and heat transfer due to microstreaming or acoustic streaming · degradation of large molecules such as polymer chains due to shear forces induced by shock waves and microstreaming · disturbance of the solvation layer around neutral and charged molecules · degassing of liquids occurs if dissolved gas is present or if gaseous reaction products are generated in the course of a chemical reaction.
Heterogenous systems The most successful applications of ultrasound have been found in the field of heterogeneous chemistry involving solids and metals. This is due to the mechanical impact of ultrasound on solid surfaces. In conventional chemistry there are several problems associated with conventional reactions involving solids or metals · small surface area of the solid/metal, · penetration of reactants into deeper areas is not possible, · oxide layers or impurities can cover the surface, · reactants/products have to diffuse onto and from the surface, · reaction products can act as deposit on the surface and prevent further reactions.
The mechanical effects of ultrasound offer an opportunity to overcome these problems: · hard oxide layers on soft metals are broken by plastic deformation of the surface · oxide layers on hard metals (low cohesion) are removed · impurities are removed in the same manner (surface cleaning) · “break up“ of the surface structure allowing penetration of reactants and/or release of materials from surface · degradation of large solid particles due to shear forces induced by shock waves and microstreaming leads to reduction of particle size and increase of surface area · microstreaming produces collisions of those particles smaller in size than a cavitation bubble · accelerated motion of suspended particles · erosion of solid surfaces due to jetting, cleaning of solid surfaces due to shock waves and/or jetting · intensification of mass transfer from and onto the surface by microstreaming (removing of impurities and small particles, accelerated transportation of reactants/products) · reduction of the induction time
A problem when dealing with syntheses in heterogeneous systems involving immiscible liquids (e.g. aqueous/organic mixtures) is that the reagents are often dissolved in different phases. Any reaction between these species can only occur in the interfacial region between the liquids and this is a very slow process. Sonication can be used to produce very fine emulsions from immiscible liquids. This is the result of cavitational collapse at or near the interface which causes disruption and impels jets of one liquid into the other to form the emulsion. The normal method of inducing a reaction between species dissolved in different immiscible liquids (usually water and an organic solvent) is through the use of a phase transfer catalyst (PTC) which will bring both reactants into the same, usually organic, phase. There are however two drawbacks to the use of such catalysts in that some of the more specialised PTC reagents are expensive and all PTC's are potentially dangerous since they can, by their very nature, transfer chemicals from water into human tissue. Sonication of immiscible liquids generates extremely fine emulsions which result in very large interfacial contact areas between the liquids and a corresponding dramatic increase in the reactivity between species dissolved in the separate liquids. This effect can be used to either replace the need for a PTC or reduce dramatically the quantity required.
The following effects in liquid/liquid or gas/liquid systems are of relevance: · rapid emulsification due to intense mixing by microstreaming/jetting/shock waves and interaction of the sound field with the liquid/liquid boundary · accelerated transportation of reactants or products onto/from the phase boundary due to radiation forces and microstreaming · intense mixing of bubbled gas and liquid · intensification of mass and heat transport Sonochemistry has some important environmental connotations. Thus, the accelerating effect of sound waves often reduces the formation of side products (waste minimisation), and the enhanced activation of catalysts and reagents enables the replacement or control of hazardous, highly reactive substances. Overall this results in simplified and milder procedures accompanied by energy savings. Essentially ultrasound can often be considered to offer a cheaper “green” route. General References
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