Toward a new paradigm for sonochemistry: Short review on nonequilibrium plasma observations by means of MBSL spectroscopy in aqueous solutions

Abstract

This review summarizes recent studies of multibubble sonoluminescence (MBSL) in aqueous media in order to highlight new insights into the origin of the sonochemical activity. The observation of OH(CΣ-AΣ) emission band and a spectroscopic analysis of OH(AΣ-XΠ) emission band in MBSL of water pre-equilibrated with noble gases revealed the formation of a nonequilibrium plasma inside the collapsing bubble (T>T>T, where T is an electron temperature, T is a vibrational temperature and T is a rotational (gas) temperature). The T and T estimated using OH(AΣ-XΠ) emission band increase with ultrasonic frequency. In Xe the T of OH(AΣ) state is much higher than in Ar most probably due to the lower ionization potential of Xe. The MBSL of C Swan band (dΠ-aΠ) measured in aqueous tert-butanol (t-BuOH) solutions correlates with the data obtained for OH(AΣ-XΠ) emission band. Analysis of the gaseous products of t-BuOH sonolysis revealed a significant sonochemical activity even at high t-BuOH concentration when MBSL is totally quenched, indicating that drastic intrabubble conditions (plasma) are not necessarily accompanied by sonoluminescence. The nonequilibrium plasma model of cavitation allows to explain the reverse carbon isotope effect observed during the sonolysis of water in the presence of Ar/CO gas mixture.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…Recent spectroscopic studies have shown a more complex situation with non-equilibrium plasma formation in the form of excited •OH radicals in different energy states [80]. In this instance the electron temperature (T e ) is much greater than the vibrational (T v ), rotational (T r ) and translational (T g ) temperatures in the order T e > T v > T r ≈ T g [81].…”

“…Spectral emissions from multibubble sonoluminescence in argon atmosphere have shown that vibrational excitations of the plasma inside the bubble can change with frequency [81]. A disparity in the population of vibrational levels can have significant effects on the reactions taking place for several sonochemical systems [80]. As frequency is increased bubbles experience a shorter growth time and sonochemical yield decreases as observed between 585 and 1140 kHz [106].…”

Sonocatalytic oxidation of EDTA in aqueous solutions over noble metal-free Co3O4/TiO2 catalyst

“…In aqueous solutions, cavitation causes splitting of water molecules leading to formation of OH % radicals and H atoms, and to the products of their mutual recombination: H 2 O 2 and H 2 , respectively [11]. Recent spectroscopic studies of light emitted by collapsing bubbles, called sonoluminescence, revealed formation of nonequilibrium plasma inside the bubble [12]. Therefore, each cavitation bubble can be considered as microscopic plasma reactor producing highly reactive species in liquids at almost room bulk temperature.…”

Leuchtende Blasen

“…Auch die Frage, was da genau strahlt, ist weiterhin aktueller Forschungsgegenstand. So zeigen zum Beispiel neuere Untersuchungen an Emissionslinien unterschiedliche Temperaturen von Elektronen und Ionen im Sonolumineszenz‐Plasma , und es wurde von unerwartet hoch ionisiertem Plasma in speziellen Einzelblasen‐Experimenten berichtet . Es gibt auch Hinweise, dass es womöglich einen sehr heißen Kern im Innern des Hot Spots geben könnte, den man aber nicht sehen kann – die beobachteten Temperaturen würden dann nur von der Oberfläche stammen, ähnlich wie bei der Sonne.…”