La unión entre preposición y sustantivo: un doble proceso de

the cavitation bubbles. The increasing of the external pressure hampers the formation and growth of the cavitation bubble [179, 184]. However, it increases the violence of the bubble collapse [179, 184].

Thus, if sufficient ultrasonic intensity is delivered to the system to form cavitation bubbles, an increasing pressure will result in increasing sonochemical effects.

I.5.2.1.3.4 SOLVENT

The effect of the solvent in the cavitation phenomenon is similar to the effect of the temperature, since different solvents will have different vapour pressures. The effect of cavitation will be harder to produce with low vapour pressure and high surface tension solvents, since the forces between molecules are stronger [179, 184]. However, at an adequate ultrasonic intensity, the effects resulting from the bubbles implosion will be greater [179, 184].

I.5.2.1.3.5 NATURE OF THE DISSOLVED GAS

Gases with higher specific heat capacity originate greater cavitational effects and, consequently, monoatomic gases are preferred to diatomic gases since they dissipate more energy upon cavitation [184].

I.5.2.1.4 ULTRASOUND SOURCE

The ultrasonic source that causes the mechanical disturbance in the liquid medium is called a transducer. The transducer converts mechanical or electrical energy into ultrasound waves. There are three major types of transducers, gas driven, liquid driven and electromechanical. Electromechanical transducers are the most commonly employed in sonochemistry devices [179]. Within this type of transducers, the use of piezoelectric or the magnetostrictive effects are the most explored. Although magnetostrictive transducer was historically one of the first transducers used, piezoelectric transducers are nowadays the most employed to generate ultrasound waves as well as detecting them [179]. The piezoelectric transducers make use of the piezoelectric properties of some materials to generate ultrasounds. Piezoelectric materials have the ability to convert mechanical stress into an electrical signal and the opposite, transform a potential difference into mechanical strain [181]. Another important feature is that upon mechanical strain, opposite charges are generated on each face of the piezoelectric material, which means that by alternating the polarity of the potential difference applied to the piezoelectric material it will expand or contract. Thus, by changing rapidly the polarity charge

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applied to the piezoelectric material is possible to induce ultrasonic vibrational motion into a medium [179, 181]. Figure I.27 shows both direct and reverse piezoelectric effects.

Figure I.27 – (a) Direct piezoelectric effect in which the application of mechanical stress generates a charge separation. (b) Reverse piezoelectric effect in which the application of a differential potential induces mechanical stress (Adapted from K. Kirk Shung, Diagnostic Ultrasound: Imaging and Blood Flow Measurements, Taylor & Francis, 2006).

I.5.2.1.5 APPLICATIONS

Ultrasounds have a wide range of applications in a variety of scientific fields. In medicine it is a valuable tool in medical imaging, providing a complementary method to X-ray and magnetic resonance diagnostic tools [179]. Concerning therapeutic purposes, it has been used, mainly, in massages, to improve blood circulation and in sporting injuries treatment. More recently, it is also being used to dissolve blood clots together with enzymatic catalysis and to enhance the action of chemotherapy agents in cancer [179]. In dental medicine, ultrasounds are being implemented as well for imaging purposes and more recently for dental tissue repair [192, 193]. In engineering, it is being used to assist drilling, gridding and cutting a variety of materials, such as stainless steel, ceramics, glass and carbide. In food industry, the use of ultrasounds is being implemented to destroy all kind of bacteria, fungi and viruses [179]. In geography, and this is perhaps the best known application of ultrasounds, it is used in navigation for localisation and communication purposes.

Regarding chemistry and biochemistry, several applications have been reported highlighting the role of cavitation bubbles to enhance a variety of reactions. In solid-liquid extraction reactions, the use of ultrasounds represents a great advance over conventional techniques [194]. The implosion of cavitation bubbles near the solid surface, generates micro jets capable to force the liquid to penetrate the solid surface and most important capable to disrupt the solid surface, which results in an increasing contact area between the liquid and the solid [179, 194]. As mentioned above, ultrasounds are an indispensable tool in cleaning processes and are being also implemented for a variety of processes,

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such as filtration, mixing and formation of stable emulsions [195-197]. In cell extraction, ultrasonic energy provides efficient microbial cell wall disruption and, for that reason, is an essential tool to extract components from microorganisms to be used for both research and industrial aims [198, 199].

Despite its growing application in recent years, the use of ultrasounds to enhance chemical reaction dates back to 1938 [200]. From then, numerous authors, mainly organic chemists, have highlighted its effect in the reaction rate of a variety of organic synthesis [201-203]. In recent years the successful use of ultrasonic energy to enhance the reactivity of numerous chemical and biochemical reactions involved in analytical methodologies led to a growing interest of its application within the scientific community for which Professor Capelo et al. have greatly contributed with an extensive work comprising a broad field of ultrasonic applications [194, 204-212].

I.5.2.1.6 COMMON INSTRUMENTATION

The most common ultrasonic devices employed by sono-chemists comprises the conventional ultrasonic bath, which is the most common device providing ultrasonic irradiation in chemical and biochemical laboratories all over the world, the ultrasonic probe and the cup horn system [213]. These devices present distinct characteristics and hence the choice of the ultrasonic device must be done carefully in accordance with the intended application and purpose.

I.5.2.1.6.1 ULTRASONIC BATH

The conventional ultrasonic baths are, as mentioned above, the most available devices within chemical laboratories. These devices normally consist of a stainless steel container with one or more transducers on the bottom, depending on its volume. Due to their larger volume, the power density provided by these devices ranges between 1 and 5 Wcm^-2, which is a low intensity value [200, 213]. The temperature inside these devices is difficult to control and the frequency applied varies significantly with the supplier brand, which make it difficult to reproduce the same experimental conditions from laboratory to laboratory [179]. Furthermore, the position and shape of the vessel inside the tank, as well as the amount of water filling the tank, have influence on the amount of ultrasonic power that reaches the sample [179]. For this reason the majority of ultrasonic baths is essentially applied for cleaning and degassing purposes and fail to enhance chemical reactions.

Recently, new ultrasonic baths have been developed to overcome some of the issues mentioned above by including a heater, which allows temperature control, dual frequency, ultrasonic power regulation and different operation modes [213]. Nevertheless, its use for sonochemistry is very limited.

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I.5.2.1.6.2 ULTRASONIC PROBE

The ultrasonic probe or sonic horn is a popular device amongst sono-chemists. Unlike, the ultrasonic baths, these devices are immersed on the liquid medium and are able to supply a large amount of ultrasonic power to the medium [179]. The majority of these systems work at fixed frequency, usually ranging from 20kHz to 40kHz, allowing the control of the vibrational amplitude of the tip, i.e. the ultrasonic power. In most commercially available ultrasonic probes, this function is controlled either by controlling the input power of the transducer or by changing the tip horn attached to the transducer.

The tip horn works like an amplifier and so, changing its shape will have great influence on the power transmitted [179].

Despite its enormous potential, ultrasonic probes allow low throughput and since it must be immersed into the liquid medium, the possibility of contamination is higher, both from previous applications and from the metal material from the tip.

I.5.2.1.6.3 CUP HORN

An alternative to avoid the contamination issue associated with the ultrasonic probe is the cup horn.

The cup horn is a high ultrasonic power system that allows indirect ultrasonication, which is a great advantage for analytical purposes, since the reaction vials are handled closed. Due to its characteristics, it may be seen as a small powerful ultrasonic bath. Like the ultrasonic probe, this device works at a predetermined frequency. The input power can be controlled as well as the cycle duty. In addition, this device can handle more than one sample at the same time, which results in higher throughput when compared with the ultrasonic probe.

Nevertheless, since the ultrasonic waves are transmitted through water and need to cross the wall of the vials, the ultrasonic power density provided by these devices is lower when compared to the probe. Furthermore, to achieve higher ultrasonic power, the container volume must be small, which imposes a limitation to the vial size. In Figure I.28 the relative power density provided by the three devices presented here are showed.

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In document UNIVERSIDAD COMPLUTENSE DE MADRID (página 166-170)