T

FIGURE 10.10 Turbidity versus wavelength spectra for emulsions with different droplet sizes.

300 400 500 600 700 800 900 Wavelength / nm

FIGURE 10.10 Turbidity versus wavelength spectra for emulsions with different droplet sizes.

used to determine ^ if the droplet size remains constant. Turbidity versus wavelength spectra for emulsions with different droplet sizes are shown in Figure 10.10.

The emulsion to be analyzed is placed in a cuvette, and its turbidity is measured over a range of wavelengths (typically between 200 to 1000 nm). The droplet size distribution is then determined by finding the best fit between the experimental measurements of turbidity versus wavelength and those predicted by the Mie theory. The spectroturbidimetric technique can be carried out using the UV-visible spectrophotometers found in most research laboratories, which may circumnavigate the need to purchase one of the expensive commercial light-scattering instruments mentioned above. Nevertheless, some samples adsorb light strongly in the UV-visible region, which interferes with the interpretation of the turbidity spectra. In addition, the refractive indices of the dispersed and continuous phases must be known, and these vary with wavelength (Walstra 1968).

Reflectance Methods. An alternative method of determining the droplet size of emulsions is to measure the light reflected back from the emulsion droplets (O = 180°). The intensity of backscattered light is related to the size of the droplets in an emulsion (Lloyd 1959, Sherman 1968b). This technique has been used much less frequently than the spectroturbidimetric or angular scattering techniques but may prove useful for the study of more concentrated emulsions which are opaque to light.

10.3.2.3. Applications

The principal application of static light-scattering techniques in the food industry is to determine droplet size distributions (Dickinson and Stainsby 1982). A knowledge of the particle size of an emulsion is useful for predicting its long-term stability to creaming, flocculation, coalescence, and Ostwald ripening (Chapter 7). Measurements of the time dependence of the particle size distribution can be used to monitor the kinetics of these processes (Chapter 7). Light-scattering instruments are widely used in research and development laboratories to investigate the influence of droplet size on physicochemical properties, such as stability, appearance, and rheology. They are also used in quality control laboratories to ensure that a product meets the relevant specifications for droplet size.

It should be noted that the data from any commercial light-scattering technique should be treated with some caution. To determine the droplet size distribution of an emulsion, com mercial instruments have to make some a priori assumption about the shape of the distribution in order to solve the scattering theory in a reasonable time. In addition, the solution of the scattering theory is often particularly sensitive to the refractive indices and adsorptivities of the continuous and dispersed phases, and these values are often not known accurately (Zhang and Xu 1992). Finally, the mechanical design of each commercial instrument is different. All of these factors mean that the same emulsion can be analyzed using instruments from different manufacturers (or even from the same manufacturer) and quite large variations in the measured droplet size distributions can be observed, even though they should be identical (Coupland and McClements 1998). For this reason, commercial light-scattering instruments are often more useful for following qualitative changes rather than giving absolute values.

One must be especially careful when using light scattering to determine the particle size distribution of flocculated emulsions. The theory used to calculate the size distribution assumes that the particles are isolated homogeneous spheres. In flocculated emulsions, the droplets aggregate into heterogeneous "particles" which have an ill-defined refractive index and shape. Consequently, the particle size distribution determined by light scattering gives only an approximate indication of the true size of the flocs. In addition, emulsions often have to be diluted in continuous phase (to eliminate multiple scattering effects) and stirred (to ensure they are homogeneous) prior to measurement. Dilution and stirring are likely to disrupt any weakly flocculated droplets but leave strongly flocculated droplets intact. For these reasons, commercial light-scattering techniques can only give a qualitative indication of the extent of droplet flocculation. Another important limitation of light-scattering techniques is that they cannot be used to analyze optically opaque or semisolid emulsions in situ (e.g., butter, margarine, and ice cream).

10.3.3. Dynamic Light Scattering 10.3.3.1. Principles

Dynamic light scattering is used to determine the size of particles which are below the lower limit of detection of static light-scattering techniques (e.g., small emulsion droplets, protein aggregates, and surfactant micelles) (Hallet 1994, Dalgleish and Hallet 1995, Horne 1995). Instruments based on this principle are commercially available and are capable of analyzing particles with diameters between about 3 nm and 3 | m. Dynamic light-scattering techniques utilize the fact that the droplets in an emulsion continually move around because of their Brownian motion (Hunter 1986). The translational diffusion coefficient (D) of the droplets is determined by analyzing the interaction between a laser beam and the emulsion, and the size of the droplets is then calculated using the Stokes-Einstein equation (Horne 1995):

where n1 is the viscosity of the continuous phase. 10.3.3.2. Measurement Techniques

A number of experimental techniques have been developed to measure the translational diffusion coefficient of colloidal particles (Hunter 1993). The two most commonly used methods in commercial instruments are photon correlation spectroscopy (PCS) and Doppler shift spectroscopy (DSS).

Photon Correlation Spectroscopy. When a laser beam is directed through an ensemble of particles, a scattering pattern is produced which is a result of the interaction between the electromagnetic waves and the particles (Horne 1995). The precise nature of this scattering pattern depends on the relative position of the particles in the measurement cell. If the scattering pattern is observed over very short time intervals (approximately microseconds), one notices that there are slight variations in its intensity with time, which are caused by the change in the relative position of the particles due to their Brownian motion. The frequency of these fluctuations depends on the speed at which the particles move, and hence on their size. The change in the scattering pattern is monitored using a detector that measures the intensity of the photons, I(t), which arrive at a particular scattering angle (or range of scattering angles) with time. If there is little change in the position of the particles within a specified time interval (t), the scattering pattern remains fairly constant and I(t) ~ I(t + t). On the other hand, if the particles move an appreciable distance within the time interval, the scattering pattern is altered significantly and I(t) ^ I(t + t). The correlation between the scattering patterns can be expressed mathematically by an autocorrelation function (Horne 1995):

where N is the number of times this procedure is carried out, which is typically of the order of 105 to 106. As the time interval (t) between which the two scattering patterns are compared is increased, the autocorrelation function decreases from a high value, where the scattering patterns are highly correlated, to a constant low value, when all correlation between the scattering patterns is lost (Figure 10.11). For a monodisperse emulsion, the autocorrelation function decays exponentially with a relaxation time (tc) that is related to the translational diffusion coefficient of the particles: tc = (2Q2D)-1, where Q is the scattering vector, which describes the strength of the interaction between the light wave and the particles (Horne 1995):

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