Studies on the spatial distribution of polymeric reagents in sonochemical reactions - application of competitive kinetics

Intense ultrasound may initiate chemical reactions in aqueous solutions. The reactivity is not uniform in the whole sonicated volume, but rather concentrated in the close vicinity of oscillating or collapsing gas bubbles formed by the action of ultrasound (cavitation bubbles). It has been shown that certain low-molecular-weight substrates, due to their partially hydrophobic properties, tend to accumulate at the surface of cavitation bubbles, thus being particularly susceptible to ultrasound-induced chemical reactions. In this paper, using an approach based on competition kinetics method, we demonstrate that this effect takes place also in the case of polymeric substrates. Relatively hydrophobic water-soluble polymer, poly(ethylene oxide), and its oligomer poly(ethylene glycol) accumulate in the close vicinity of the bubbles. Their local concentrations in these zones may be two orders of magnitude higher than the average concentration in solution. In contrast, such effect is observed neither for strongly hydrophilic polyelectrolyte chains exemplified by poly(acrylic acid), nor for dissociated sodium acetate used as a low-molecular-weight hydrophilic model. The ultrasound-induced processes employed in the competition kinetics study in this work were the reactions of substrates with hydroxyl radicals emerging from the cavitation bubbles. In order to provide quantitative comparison with a system of uniform reactivity distribution, the same reactions were studied using ionizing radiation for OH-radicals generation.