Liquid-solid systems, ultrasonic effects

Ultrasonic cavitation in liquid-solid systems produces interesting effects. Physical effects include (a) improvement of mass transfer in turbulent mixing and acoustic flow, (b) generation of surface damage at the liquid-solid interfaces due to shock waves, (c) generation of high-velocity interparticle collisions in the suspension (slurry), and (d) fragmentation of solids to increase their surface area. 

Some unexpectedly complex liquid solid interactions have been detected and studied by ultrasonic impedance measurements (ultrasonic impedometry). Small amounts of water and alcohols have pronounced effects on the physical state of hydrophilic polymers specifically, the high frequency shear modulus and crystallinity index of a poly (vinyl alcohol) film increases with water content to a maximum before normal solution phenomena occur. These effects are attributed to the increased molecular order owing to water hydrogen bonded between polymer chains. The unusual effects of moisture on a novel poly(vinyl chloride)/plasticizer system and on hydrophilic polymers other than poly (vinyl alcohol) are also described. 

Commercially available high-intensity ultrasonic probes (10 to 500Wcm ) are the most effective sources for laboratory scale sonochemistry. A typical system operates at 24 kHz with an adjustable total power output of up to 200W and also adjustable irradiation times per pulse of a few tenths of a second. Lower intensities can often be used in liquid-solid heterogeneous systems of interest here because of the reduced liquid tensile strength at the liquid-solid interface, and a common ultrasonic cleaning bath (about lWcm ) can often be adequate for SAE. 

Ultrasonic cavitation has particularly important effects on biphasic systems, emulsification of immiscible liquid-liquids, particle breakage and dispersion, and surface cleaning in liquid-solid mixtures. These mechanical effects, even if sometimes oversimplified, appear to be understandable and predictable by nonexperts in the field, who exploited them in a variety of heterogeneous reactions. Sonochemistry of biphasic systems then developed rapidly, and synthetic applications were reviewed in recent articles. In many cases, the presence of a phase transfer catalyst becomes unnecessary, and sonication can be considered as a "physical substitute to PTC", according to the expression of Ando. a... 

Temperature of the medium. Ultrasonic irradiation warms up the solid-liquid system. The resulting adverse effects of the temperature changes can be avoided in various ways, namely ... 

All comments above about common and specific variables for ultrasonic baths and probes also apply to continuous US-assisted digestion, which additionally involves dynamic variables. The most influential dynamic variable is the flow-rate of the liquid phase, which should be set in such a way as to avoid compaction of the solid in the chamber and ensure effective contact of the two phases. Consequently, the flow-rate of choice in each case will depend on the particular solid-liquid system. 

The same apparatus was used to measure the kinetics of emulsion crystallization under shear. McClements and co-workers (20) showed that supercooled liquid n-hexadecane droplets crystallize more rapidly when a population of solid n-hexa-decane droplets are present. They hypothesized that a collision between a solid and liquid droplet could be sufficient to act as a nucleation event in the liquid. The frequency of collisions increases with the intensity of applied shear field, and hence shearing should increase the crystallization rate. A 50 50 mixture of solid and liquid n-hexadecane emulsion droplets was stored at 6 -0.01 °C in a water bath (i.e., between the melting points and freezing points of emulsified n-hexadecane). A constant shear rate (0-200 s ) was applied to the emulsion in the shear cell, and ultrasonic velocities were determined as a function of time. The change in speed of sound was used to calculate the percentage solids in the system (Fig. 7). Surprisingly, there was no clear effect of increased shear rate. This could either be because increase in collision rate was relatively modest for the small particles used (in the order of 30% at the fastest rate) or because the time the interacting droplets remain in proximity is not affected by the applied shear. 

Microstreaming, shock waves, and liquid microjets in the vicinity of solid surfaces lead to very efficient cleaning. This effect has been used in industry for more than forty years. Insoluble layers of inorganic salts, polymers, or liquids can be removed by the ultrasonic cleaning effect. In heterogeneous systems such a clean reactive surface leads to improved dissolution rates of metals in acids and enhanced reaction rates. Chemical reactions giving insoluble products are freed from these mass-transport-limiting layers and react rapidly. 

Currently, suspensions prepared from micronised active substances are the only marketed dehvery system for nebulisation of poorly water soluble substances such as steroids and cyclosporine [53]. Several problems are inherent in nebulising micro-suspensions and they vary from non-optimised lung deposition for the active substance to heterodispersity of the active substance concentration in the aerosol droplets and poor compatibility with different types of nebulisers, particularly ultrasonic devices. Suspensions may also have poor stability and the two components (solid and liquid) tend to separate with time within the formulation by sedimentation or flocculation, depending on the particle density relative to that of the liquid. Several jet nebulisers can deliver suspensions quite effectively, even independently of the primary particle size [54], but ultrasonic devices may convert primarily the continuous phase into aerosol whereas vibrating mesh inhalers can be blocked by particles being larger than the pore diameter of the membrane. 

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