Phase ultrasonic properties

For ideal mixtures there is a simple relationship between the measurable ultrasonic parameters and the concentration of the component phases. Thus ultrasound can be used to determine their composition once the properties of the component phases are known. Mixtures of triglyceride oils behave approximately as ideal mixtures and their ultrasonic properties can be modeled by the above equations [19]. Emulsions and suspensions where scattering is not appreciable can also be described using this approach [20]. In these systems the adiabatic compressibility of particles suspended in a liquid can be determined by measuring the ultrasonic velocity and the density. This is particularly useful for materials where it is difficult to determine the adiabatic compressibility directly, e.g., powders, biopolymer or granular materials. Deviations from equations 11 - 13 in non-ideal mixtures can be used to provide information about the non-ideality of a system. 

In non-ideal mixtures, or systems where scattering of ultrasound is significant, the above equations are no longer applicable. In these systems the ultrasonic properties depend on the microstructure of the system, and the interactions between the various components, as well as the concentration. Mathematical descriptions of ultrasonic propagation in emulsions and suspensions have been derived which take into account the scattering of ultrasound by particles [20-21]. These theories relate the velocity and attenuation to the size (r), shape (x) and concentration (0) of the particles, as well as the ultrasonic frequency (co) and thermophysical properties of the component phases (TP). 

In non-scattering systems, ultrasonic properties and the volume fraction of the disperse phase are related in a simple manner. In practice, many emulsions and suspensions behave like non-scattering systems under certain conditions (e.g. when thermal and visco-inertial scattering are not significant). In these systems, it is simple to use ultrasonic measurements to determine 0 once the ultrasonic properties of the component phases are known. Alternatively, if the ultrasonic properties of the continuous phase, 0and p2 are known, the adiabatic compressibility of the dispersed phase can be determined by measuring the ultrasonic velocity. This is particularly useful for materials where it is difficult to measure jc directly in the bulk form (e.g. powders, granular materials, blood cells). 

Equation 9.13, where k =(o/c + is the wave number of the continuous phase, illustrates how the scattering coefficients of a single particle are related to the ultrasonic properties of an ensemble of particles. 

Multiple scattering theory also assumes that the particles are randomly distributed in the continuous phase ( . e. that they are uncorrelated). In some systems, this is not true and ultrasonic properties can depend on the radial distribution function of the particles. 

Afifi and El-Wakil measured the ultrasound velocity in NR/NBR (50/50) blends compatibilized with different weight percentages of NR-g-MA. It was found that at moderate concentrations of compatibilizer (2-6 phr) the ultrasonic velocity was increased. However, incorporating more NR-g-MA (up to 10 phr) decreased this property. This phenomenon was correlated with the degree of compatibility of the blend components. Increased ultrasound velocity relates to stronger interactions at the interfaces, thus better compatibility. The reason behind this is that the ultrasonic properties of the rubber blends are affected by the occurrence of phase inversion and variation of micro-voids within the blend. 

Konvergenzpunkt, Lehmann convention 416 Kosterlitz-TTioules mechanism, dislocations 291 Kozhevnikov theory, ultrasonic properties 551 f Kramers-Krohnig relation 223, 245 Krat-Kennedy equation, phase transitions 359 Kratky camera 627 Kroehnke reaction, synthesis 98 Kronecker delta, tensor properties 194... 

In the previous section we demonstrated the use of ultrasonic velocity measurements to characterise creaming, and indirectly to characterise flocculation. However, there is more information to be obtained from an emulsion using ultrasonic spectroscopy. This involves measurement of phase velocity and attenuation of ultrasound as a function of frequency after propagation through the emulsion. There are a number of mechanisms by which ultrasound is attenuated by the emulsion, resulting in characteristic ultrasonic properties. Figure 4.15 shows the prineipal mechanisms of absorption. 

These emulsions contained 1-bromohexadecane at 5% v/v as the dispersed phase, stabilised against coalescence using a nonionic surfactant. Figure 4.16(a) shows the microscopic apfiearance of the emulsion. The ultrasonic properties of the emulsion were measured using a broad-band spectrometer at 25 °C. Figure 4.16(b) shows the... 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

Ultrasonic irradiation has also been employed for chemical remediation of water but the mode of sonochemical degradation of organic compounds in aqueous solution depends upon their physical and chemical properties. This is because there are two ways in which the cavitation bubble can function. In the case of volatile chemicals which enter the bubble, destruction occurs through the extreme conditions generated on collapse. In the case of chemicals remaining in the aqueous phase the bubble acts as a source of radicals (H, HO and HOO ) which enter the bulk solution and react with pollutants. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

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