Polysaccharide Applications - American Chemical Society

Chapter 18

Ultrasonic Techniques to Characterize Concentrated Colloidal Dispersions D. J. Hibberd, M. J. Garrood, J. M. Leney, and M. M. Robins

Downloaded by UNIV OF ALBERTA on August 29, 2013 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0737.ch018

Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, United Kingdom

The principles and applications of ultrasonic techniques to colloidal dispersions are described. The techniques are arranged as a hierarchy, showing firstly the major effects, requiring comparatively simple apparatus, and then more sophisticated techniques where complex theories and sensitive instrumentation are required. Applications to model suspensions and emulsions are shown, with reference to non -ideal food dispersions.

Many foods and pharmaceutical products are concentrated dispersions, whose properties are critically dependent on the colloidal structure. There is a need for nonintrusive techniques to characterise such systems. In many cases it is desirable to monitor colloidal properties on-line during processing. The important properties include the dispersed phase concentration and state, the particle size distribution of the dispersed phase, and whether aggregation (flocculation) has occurred. Further information on the structure of the dispersion, such as the composition of the particle surfaces, would also be of value in the control of product quality. In principle ultrasonic techniques can be used to determine all these properties of dispersions, and without the dilution needed for most alternative techniques. When ultrasound is propagated through a colloidal dispersion, the beam is attenuated by a number of mechanisms, depending on the contrast in thermophysical properties between the dispersed and continuous phases. Here we summarise the principal effects of dispersion composition and structure on the propagation of ultrasound, showing a hierarchy from the major phenomena to more subtle effects requiring sensitive instrumentation. The instrumentation required for each application is also described.

262

© 1999 American Chemical Society

In Polysaccharide Applications; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

263


First-Order Effect: Dispersion Composition Characterisation of Creaming or Sedimentation In most dispersions, the overall composition is the dominating influence on the propagation of ultrasound. Each component contributes to the velocity and attenuation of the wave by an amount that depends on the volume fraction and the characteristics of propagation in the isolated component. In many cases, there is significant contrast in the velocity of ultrasound between the dispersed and continuous phases, and simple mixing theory can be used to estimate the velocity through a dispersion of known volume fraction (7). Conversely, a measurement of the velocity through a dispersion can be used to measure the particle volume fraction reasonably accurately, provided the ultrasonic velocity (and density) of each phase is known.(2). This technique has been applied very successfully to monitor the vertical distribution of dispersed phase in dispersions that are sedimenting or creaming in a gravitational field (3,4). Figure l a shows the oil concentration profiles measured ultrasonically for an emulsion during creaming. Each line represents a vertical scan of over 80 data points collected at a particular sample age. Initially, the line (dashed) was horizontal, showing uniform composition. After a few days the oil had begun to move up the sample cell, forming a concentrated cream layer at the top. As time progressed the base slowly cleared of oil droplets, which all collected in the cream layer (dotted line). The emulsion was visually uniform for 14 days, but the ultrasonic instrument detected droplet movement between one and two days. The emulsion contained polydisperse droplets, and the smaller fraction creamed more slowly than the larger droplets, maintaining opacity at the base even though the majority of the droplets had already moved towards the top. The form of the creaming profiles allowed the effective hydrodynamic size distribution of the droplets to be estimated (5), and the resulting size distribution is shown in Figure lb. The effective hydrodynamic size distribution was very similar to that measured at the start of the experiment, using a Malvern Mastersizer, and shown in Figure lc. The ultrasonic instrument is thus able to determine the effective size distribution of creaming (or sedimenting) particles in concentrated dispersions. This can be used to detect droplet coalescence, or aggregation due to added flocculants. Figure 2a shows creaming profiles for the oilin-water emulsion shown in Figure 1, but after the addition of 0.02%w/w hydroxyethyl cellulose (HEC). The hydrodynamic size distribution of the creaming particles is shown in Figure 2b. The form of the profiles, and the derived size distribution, showed two distinct populations of droplets in the emulsions, corresponding to individual droplets and small floes. At certain concentrations nonadsorbing polymers, such as H E C in the presence of a surfactant, cause partial flocculation of the droplets, which is readily detected and quantified using the ultrasonic instrument (4). At higher polymer concentrations the emulsion was fully flocculated, and the creaming behaviour, shown in Figure 3, indicated that the droplets moved as i f they were part of a single network, with a sharp lower boundary. The behaviour of the network is the subject of an associated paper in the current volume (6). Crystallisation Processes The contrast in ultrasonic velocity between phases in dispersions can also be used to detect crystallisation. In general, when a material



264

oil volume fraction (%) 0.09 days

80

2.1 60

5.9 9.13

40

12.84 20

68.81 s-

0

C)

~t 50

100

150

height (mm)

Figure la. Profiles of oil volume fraction with height during creaming of an alkane-in-water emulsion, obtained using the IFR ultrasonic sedimentation/creaming monitor.



265

Figure lb. The hydrodynamic droplet size distribution derived from the creaming data of Figure la.

Figure lc. The droplet size distribution measured by the Mastersizer light diffraction sizer.


266


oil volume fraction (%)

50

100

height (mm)

Figure 2a. Profiles of oil volume fraction with height during creaming of an alkane-in-water emulsion in the presence of 0.02%w/w hydroxyethyl cellulose.

d i a m e t e r (microns)

Figure 2b. The hydrodynamic droplet size distribution derived from the creaming data in Figure 2a, showing floes and individual droplets.



267

oil volume fraction (%) 0.05

80

days

0.07 60

0.13

m

40

on zU

|

0.19 0.95 5.96

J . 0

50

100

150

height (mm)

Figure 3. Profiles of oil volume fraction with height during creaming of an alkane-in-water emulsion in the presence of 0.04%w/w hydroxyethyl cellulose showing collective movement of the flocculated droplets.



268 solidifies its ultrasonic velocity is significantly higher than in the liquid state, and measurement of the velocity is a simple method to determine the degree of crystallinity (7). Figure 4 shows an application of ultrasonic velocimetry to monitor crystallisation in honey, a super-saturated solution of fructose in glucose. The liquid honey solution was seeded with a small amount of fructose crystals and the ultrasonic velocity measured over a period of 72 days. The data show the changes in ultrasonic velocity during the crystallisation process. With associated calibration experiments using samples of known crystallinity, the data in Figure 4 can be converted to values of crystal concentration. In the case of natural products such as honey it is also important to allow for variations in moisture in the samples, and to collect data from a wide range of sources in order to confirm absolute crystallinity values. However, the simplicity of this application makes it particularly suitable for an industrial environment. Instrumentation Required To apply either of these techniques to a dispersion requires a measure of the velocity of ultrasound through the material. The simplest techniques determine the time-of-flight for a pulse of ultrasound to propagate from a transmitting transducer, through the sample and be received by a second transducer (2). A variant on this method uses a single transducer for both transmission and reception, with a reflector at the far side of the sample (8). The technique requires timing electronics, and good temperature control. If absolute values of velocity are required, then the instrument needs calibration using fluids of known velocity.

Ultrasonic Spectroscopy Second-Order Effect: Dispersion Composition and Particle Size. A much more sophisticated application of ultrasonic methods involves measuring the velocity and the attenuation of ultrasound of known frequency as it propagates through a dispersion. The variation with frequency of the ultrasonic velocity and attenuation in a simple dispersion are related to the concentration of dispersed phase, its particle size distribution and a number of thermophysical parameters of both phases. In principle, measurement of the ultrasonic properties provides information on the volume fraction and particle size of the dispersed particles. The theory most commonly used to relate ultrasonic properties to dispersion composition is based on the scattering of ultrasound for single particles, formulated in detail by Allegra and Hawley (°). Essentially, the theory considers the loss mechanisms in operation when an ultrasonic wave travelling in a liquid medium (the continuous phase) encounters a (spherical) particle of a different material. A n amount of energy is converted to heat and therefore lost from the ultrasonic wave. Other losses from the straight-through wave arise from scattering of the ultrasound at oblique angles, and mode conversion to shear or surface waves. The amount of the loss, which affects both the overall attenuation and propagation speed of the wave, is dependent on the contrast between the thermal properties of the two phases, and on their differences in density and ultrasonic velocity. A key factor in the overall loss is



269 the particle size and the number of particles. Thus, i f the relevant properties of the two phases are known, the ultrasonic propagation at a range of frequencies can be used to determine the volume fraction and size of the particles. In practice, it is not straightforward to infer the dispersion properties from ultrasonic measurements, due mainly to uncertainty in the thermophysical properties. However, it is possible to distinguish overall differences in size distribution using ultrasonic attenuation, even for irregular, polydisperse particles. Figure 5 shows the ultrasonic spectra of suspensions of 5%w/w sucrose crystals in a saturated sucrose/glucose solution (JO). The smaller crystals passed through a 25jum sieve, and the larger fraction passed through a 45 Lim mesh but were retained on a 32jLim mesh sieve. Despite the high polydispersity of the samples (10), and their irregular shape resulting from grinding, there were still significant differences in their ultrasonic properties. The increases in velocity and attenuation occurred at a lower frequency for the larger particle size distribution, which is consistent with the predictions of the scattering theory, although the non-spherical particle shape and lack of thermophysical parameters precluded full application of the theory. Third-order Effects: Distribution of Components in a Dispersion. Adsorbed layers. The application of scattering theory to a simple two-phase dispersion is highly complex, as shown above, but it is possible to detect further effects using ultrasonic techniques. Since these effects are in general of much smaller magnitude than the first- and second-order phenomena, they can only be identified in systems where the concentration, primary particle size and temperature are very well-controlled. The basic scattering theory of Allegra and Hawley (9) has been modified by several workers (77, 72) to include a third component, a shell on the surface of the particles. This development opens the possibility to detect adsorbed layers of surfactant, or polymer, on the surfaces of particles or droplets. Figure 6 shows the attenuation spectra from polystyrene latex suspensions in the absence and presence of adsorbed layers of non-ionic surfactant or polymer (HEC). The lines show the predicted effects, using scattering theory, i f the added components were uniformly dispersed in the continuous phase (13,14). The experimental results show a higher attenuation than predicted, indicating that the distribution of the added components (on the surfaces of the particles instead of as a background) is detectable using ultrasonic techniques (14). To fit the proposed shellcore models requires considerable knowledge of the thermophysical properties of the layers, which is not currently available. However, the data are shown as preliminary evidence that such layers are detectable and clearly more work is needed on a wide range of systems to be confident of the results. Flocculation. A related third-order effect is the distribution of particles in the dispersions. If particles become flocculated, the majority of them are surrounded by other particles, not by continuous phase, and this is similarly predicted to affect the ultrasonic properties (72). To examine this effect, polystyrene suspensions were flocculated in two ways, both by the addition of HEC (13,14). At low polymer concentrations, the HEC was adsorbed on to the particles but insufficiently to obtain full surface coverage. The latex particles were thus flocculated by polymer bridges,


270

ultrasonic velocity (mis)


2,200

500

1,000

1,500

2,000

time (hours)

Figure 4. The ultrasonic velocity of honey during crystallisation.

attenuation (Np/m) 25,000

20

40 60 80 frequency (MHz)

100

20

40 60 80 frequency (MHz)

Figure 5. Ultrasonic velocity (a) and attenuation (b) spectra obtained from 5%w/w sucrose crystals dispersed in saturated sugar solution. Key: • , crystals

Polysaccharide Applications - American Chemical Society

Recommend Documents