Ultrasound solvent viscosity

The nature and strength of solvent intermolecular interactions (especially the presence of hydrogen bonding) can greatly influence the physical and chemical outcomes of ultrasound irradiation. These interactions determine the intensity of bubble collapse and the ease with which bubble nucleation can occur (recall that nucleation occurs when the pressure amplitude of the ultrasound wave exceeds the natural cohesive forces in the liquid). Increases in solvent viscosity and surface tension reduce the rate of bubble nucleation (i.e., fewer microbubbles are formed) but increase the intensity of bubble collapse (i.e., higher temperatures and pressures). Some of the adverse effects of high surface tension can be overcome with the addition of small amounts of surfactants, which reduce the solvent surface tension and facilitate bubble nucleation. ... 

Marken F, Akkermans RP, Compton RG (19%) Voltammetry in the presence of ultrasound the limit of acoustic streaming induced diffusion layer thinning and the effect of solvent viscosity. J Electroanal Chem 415 55-63... 

Viscosity is an important factor during ultrasound-induced bulk polymerizations as the long polymer chains formed upon reaction cause a drastic increase in the viscosity of the reaction mixture, thereby hindering cavitation and consequently reducing the production rate of radicals. Precipitation polymerization forms a potential solution to this problem, because a constant viscosity and hence a constant radical formation rate can be maintained. In this perspective, high-pressure carbon dioxide is an interesting medium as most monomers have a high solubility in CO2, whereas it exhibits an anti-solvent effect for most polymers. ... 

At increasing polymer concentration, the scission process becomes less effective and eventually stops. This is a drawback for the development of a scission process based on ultrasound, because concentrated polymer systems are favored in industry. The addition of an anti-solvent for the polymer can prevent the increase in viscosity at higher polymer concentrations. To determine the influence of CO2 as an anti-solvent on the ultrasound-induced scission rate,... 

As described in Section 21.3.3.1, ultrasound-induced bulk polymerizations are limited to relatively low conversions, because a strong increase in viscosity upon reaction hinders cavitation. In order to obtain higher conversions, precipitation polymerization forms a potential solution. Because the polymer produced is precipitated from the reaction medium, the viscosity and consequently the radical formation rate are expected to remain virtually constant. From this perspective, liquid carbon dioxide is a suitable reaction medium, because most monomers have a high solubility in CO2, whereas it exhibits an anti-solvent effect for most polymers. Moreover, CO2 is regarded as an environmentally friendly compound, which is nontoxic, nonflammable, and naturally abundant. Since higher pressures are required for CO2 to act as an anti-solvent [56, 149], the possibility of ultrasound-induced cavitation in pressurized carbon dioxide systems has been studied [73,150]. 

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