Single heat source
Single heat source
Differential Scanning Calorimeter s
Separate heat sources
instrument consists of two sample holders which are connected to the same furnace, which is a device for varying the temperature in a controlled fashion (Figure 10.23). A few milligrams of sample is accurately weighed into a small aluminum pan and placed into one of the sample holders. A reference, usually an empty aluminum pan or a pan containing a material that does not undergo a phase transition over the temperature range studied, is placed into the other sample holder. The sample and reference pans are then heated or cooled together at a controlled rate. The difference in temperature (AT) between the two pans is recorded by thermocouples located below each of the pans. The output from the instrument is therefore a plot of AT versus T. If the sample absorbs heat (an endothermic process), then it will have a slightly lower temperature than the reference, but if it releases heat (an exothermic process), it will have a slightly higher temperature. Thus measurements of the difference in temperature between the sample and reference pans can be used to provide information about physical and chemical changes which occur in the sample. An exothermic process leads to the generation of a negative peak, and an endothermic process leads to the generation of a positive peak (Figure 10.24).
Differential Scanning Calorimetry. DSC records the energy necessary to establish a zero temperature difference between a sample and a reference material which are either heated or cooled at a controlled rate. Thermocouples constantly measure the temperature of each pan, and two heaters below the pans supply heat to one of the pans so that they both have exactly the same temperature. If a sample were to undergo a phase transition, it would either absorb or release heat. To keep the temperature of the two pans the same, an equivalent amount of energy must be supplied to either the test or reference cells. Special electrical circuitry is used to determine the amount of energy needed to keep the two sample pans at the same temperature. DSC data are therefore reported as the rate of energy absorption (Q) by the sample relative to the reference material as a function of temperature.
Thermal analysis can be used to determine the temperature range of a phase transition, as well as the amount of material involved in a phase transition (Figure 10.24). When an emulsion
that contains solid droplets is heated above the melting point of the dispersed phase, the droplets melt and an exothermic peak is observed. The melting temperature can therefore be ascertained by measuring the position of the peak. The area under the peak is proportional to the amount of material which undergoes the phase transition: A = kAHfm, where AH is the heat of fusion per gram, m is the mass of material undergoing the phase transition in grams, and k is a constant which depends on the instrument settings used to make the measurement. The value of k is determined by measuring the peak areas of a series of samples of known mass and heat of fusion. Thus the mass of material which melts can be determined by measuring the peak area. When an emulsion that contains liquid droplets is cooled, an endothermic peak is observed when the droplets crystallize. The crystallization temperature and the amount of material which has crystallized can be determined in the same way as for the melting curve. As mentioned earlier, the droplets tend to crystallize at a much lower temperature than they melt because of supercooling. In addition, the melting range of edible fats is much wider than that shown in Figure 10.24 because they contain a mixture of triacylglycerols.
Thermal analysis has been used to monitor the influence of oil type, emulsifier type, cooling rates, catalytic impurities, polymorphic changes, and droplet size on the nucleation and crystallization of droplets in oil-in-water emulsions (Dickinson and McClements 1995). It has also been used to study the factors which influence the melting and crystallization of water droplets in water-in-oil emulsions (Clausse 1985) and to study phase transitions in the continuous phase of emulsions.
The ultrasonic properties of a material change significantly when it melts or crystallizes, and so ultrasound can be used to monitor phase transitions in emulsions (Dickinson et al. 1990, 1991c; McClements 1991). The temperature dependence of the ultrasonic velocity of an oil-in-water emulsion in which the droplets crystallize is shown in Figure 10.25. As the emulsion is cooled to a temperature where the droplets crystallize, the ultrasonic velocity increases steeply, because the velocity of ultrasound is larger in solid fat than in liquid oil. As the
emulsion is heated to a temperature where the droplets melt, the velocity decreases rapidly. The droplets crystallize at a temperature which is much lower than the melting point of the bulk oil because of supercooling effects (Section 4.2).
To a first approximation, the fraction of crystalline material (^SfC) in an emulsion can be determined using the following equation (Dickinson et al. 1991):
where ceL and ceS are the ultrasonic velocities in the emulsion if all the droplets were either completely liquid or completely solid, respectively. These values are determined by extrapolating measurements from higher and lower temperatures into the region where the fat is partially crystalline or by using ultrasonic scattering theory to calculate their values.
The ultrasonic properties of an emulsion can be measured using the same techniques as used for determining the droplet size distribution (Section 10.3). The measurements are usually carried out in one of two ways: isothermal or temperature scanning. In an isothermal experiment, the temperature of the emulsion is kept constant and the change in the ultrasonic velocity is measured as a function of time. In a temperature scanning experiment, the ultrasonic velocity is measured as the temperature is increased or decreased at a controlled rate.
The solid contents determined using ultrasound are in good agreement with those determined using traditional techniques such as dilatometry (Hussin and Povey 1984) and NMR (McClements and Povey 1988). Ultrasound has been used to monitor phase transitions in nonfood oil-in-water emulsions (Dickinson et al. 1990, 1991), triacylglycerol oil-in-water and water-in-oil emulsions (McClements 1989, Coupland et al. 1993), margarine and butter
(McClements 1989), shortening and meat (Miles et al. 1985), and various triacylglycerol/oil mixtures (McClements and Povey 1987, 1988).
It should be noted that in some systems, large increases in the attenuation coefficient and appreciable velocity dispersion have been observed during melting and crystallization because of a relaxation mechanism associated with the solid-liquid phase equilibrium (McClements et al. 1998). The ultrasonic wave causes periodic fluctuations in the temperature and pressure of the material, which perturb the phase equilibrium. When a significant proportion of the material is at equilibrium, a large amount of ultrasonic energy is absorbed due to this process. This effect depends on the ultrasonic frequency, the relaxation time for the phase equilibrium, and the amount of material undergoing phase equilibrium and can cause large deviations in both the velocity and attenuation coefficient. In systems where this phenomenon is important, it is not possible to use Equation 10.14 to interpret the data. Nevertheless, it may be possible to use ultrasound to obtain valuable information about the dynamics of the phase equilibrium.
The emulsion to be analyzed is placed in a measurement cell, and a static electrical field (E) is applied across it via a pair of electrodes (Figure 10.26). This causes any charged emulsion droplets to move toward the oppositely charged electrode (Hunter 1986, 1993). The sign of the charge on the emulsion droplets can therefore be deduced from the direction they move. When an electrical field is applied across an emulsion, the droplets accelerate until they reach a constant velocity (v) where the electrical pulling force is exactly balanced by the viscous drag force exerted by the surrounding liquid. This velocity depends on the size and charge of the emulsion droplets and can therefore be used to provide information about these parameters. Experimentally, particle velocity is determined by measuring the distance they move in a known time or the time it takes to move a known distance. Droplet motion can be monitored using a number of different experimental methods. The movement of relatively large particles (>1 |im) can be monitored by optical microscopy or static light scattering, whereas the movement of smaller particles can be monitored by an ultramicroscope or dynamic light scattering.
Mathematical expressions have been derived to relate the movement of a droplet in an electric field to its zeta potential (Hunter 1986). These are based on a theoretical consideration of the forces that act on a particle when it has reached constant velocity (i.e., the electrical pulling force is balanced by the viscous drag force). The mathematical theory that describes this process depends on the droplet size and charge, the thickness of the Debye
layer, the viscosity of the surrounding liquid, and the strength of the applied electric field. The general solution of this theory leads to a complicated expression that relates all of these parameters. Nevertheless, under certain experimental circumstances, it is possible to derive simpler expressions:
where n is the viscosity of the surrounding liquid, u is the electrophoretic mobility (= particle velocity divided by electric field strength), £0 is the dielectric constant of a vacuum, and rR is the relative dielectric constant of the material. In practice, the latter equation is the most applicable to emulsions, because the droplet size is much greater than the Debye length (k-1). Even so, there are many practical examples where the particle size is comparable to the Debye length, and so there are considerable deviations between Equation 10.16 and experimental measurements. In these cases, it is necessary to solve the full theory.
A more sophisticated instrument for measuring both the zeta potential and size of droplets in emulsions is the Zetasizer© developed by Malvern Instruments (Hunter 1986). Two coherent beams of light are made to intersect with each other at a particular position within a measurement cell so that they form an interference pattern which consists of regions of low and high light intensity. The charged emulsion droplets are made to move through the interference pattern by applying an electrical field across the cell. As the droplets move across the interference pattern, they scatter light in the bright regions, but not in the dark regions. The faster a droplet moves through the interference pattern, the greater the frequency of the intensity fluctuations. By measuring and analyzing the frequency of these fluctuations, it is possible to determine the particle velocity, which can then be mathematically related to the zeta potential (e.g., using Equation 10.16 for larger particles). The sign of the charge on the particles is ascertained from the direction they move in the electric field. The same instrument can also be used to determine droplet concentration and size distribution (from 10 nm to 3 |im) of an emulsion by a dynamic light-scattering technique. Consequently, it is possible to determine the droplet size, concentration, and charge using a single instrument, which is extremely valuable for predicting the stability and bulk physicochemi-cal properties of emulsions.
Recently, analytical instruments based on electroacoustics have become commercially available for measuring the size, concentration, and zeta potential of droplets in emulsions (Hunter 1993, O'Brien et al. 1995, Carasso et al. 1995, Dukhin and Goetz 1996). The sample to be analyzed is placed in a measurement cell and an alternating electrical field is applied across it via a pair of electrodes. This causes any charged droplets to rapidly move backward and forward in response to the electrical field (Figure 10.27). An oscillating droplet generates a pressure wave with the same frequency as the alternating electric field which emanates from it and can be detected by an ultrasonic transducer. The amplitude of the signal received by the transducer is known as the electrokinetic sonic amplitude and is proportional to the
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