Emulsion dielectric spectroscopy

Y. Feldman, T. Skodvin, and J. Sjoblom, Dielectric spectroscopy on emulsion and related colloidal systems—A review, in Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York, 2001, pp. 109-168. 

Sjoblom J, Foredal H, Skodvin T. Flocculation and coalescence in emulsions as studied by dielectric spectroscopy. In Ref. 1 393-435. 

In aqueous systems in which particles are surrounded by a well-developed double layer, such as in sols and emulsions, sharp increase in dielectric constant is observed at particular frequencies of external field. The observed unusually high values of dielectric constants typical for such systems (Fig. V-12) are due to the fact that particles move relatively to the surrounding ionic atmosphere as charges of high magnitude. At high frequencies of external field such motion becomes impossible, and dielectric constant assumes its normal values. The studies of such trends in dielectric constant are in the basis of dielectric spectroscopy, which is an effective method for investigation of disperse systems, and in particular of emulsions [15]. 

Dielectric Spectroscopy on Emulsion and Related Colloidal Systems—A Review... 

There is a constant challenge for improved techniques in order to make accurate predictions on the colloidal stability of various sytems. In this section we demonstrate how dielectric spectroscopy can be applied as a technique to follow the breakdown of water-in-oil emulsions and to monitor the sedimentation of particle suspensions. Dielectric spectroscopy, combined with statistical test design and evaluation, seems to be an appropriate technique for the study of these problems. However, one should continue to seek satisfactory theoretical models for the dielectric properties of inhomogeneous systems. 

Dielectric Spectroscopy on Technical Emulsions (Gas Hydrate Formation)... 

SjOblom et al. used dielectric spectroscopy to study emulsions over a period of years (42). It is concluded that the stabilizing fraction in water-in-oil emulsions is the asphaltenes and not the resins. However, it is noted that some resins must be present to give rise to stability. It is suggested that the greater mobility of the resins is needed to stabilize the emulsions until the asphaltenes, which migrate slowly, can align at the interface and stabilize the emulsions. 

One of the methods used to study emulsions has been the use of dielectric spectroscopy. The permittivity of the emulsion can be used to characterize an emulsion and assign a stability (1,42,48—54). The Sjoblom group has measured the dielectric spectra using time-domain spectroscopy (TDS) technique. A sample is placed at the end of a coaxial line to measure total reflection. Reflected pulses are observed in time windows of 20 ns, Fourier transformed in the frequency range from 50 MHz to 2 GHz, and the complex permittivity calculated. Water or air can be used as reference sample. The total complex permittivity at a frequency (co) is given by ... 

Skodvin and SjOblom used dielectric spectroscopy in conjunction with rheology to study a series of emulsions (54). A close cotmection was found between the viscosity and dielectric properties of the emulsions. The large effects of shear on both the static permittivity and the dielectric relaxation time for the emulsion was ascribed, at least in part, to the degree of flocculation in the emulsion system. At high shear rates, at which emulsions are expected to have a low degree of flocculation and high stability, the dielectric properties still varied from those expected from a theoretical model for spherical emulsion droplets. 

Fordedal and Sjoblom used dielectric spectroscopy to study several real erode oil emulsions and model systems stabilized with either separated asphaltenes and resins from crude oil or by commercial surfactants (55). Emulsions could be stabilized by the asphaltene fraction alone, but not by the resin fraction alone. A study of a combination of mixtures shows an important interaction between emulsifying components. F0rdedal et al. used dielectric spectroscopy to study model emulsions stabilized by asphaltenes extracted from crude oils (56). Analysis showed that the choice of organic solvent and the amount of asphaltenes, as well as the interaction between these variables, were the most significant parameters for determining the stability of the emulsions. 

Ese et al. found similar results on model emulsions stabilized with resins and asphaltenes extracted from North Sea oil (57). The dielectric spectroscopy results showed that the stability of model emulsions could be eharaeterized. Stability was found to depend mainly on the amount of asphaltenes, the degree of aging of asphaltenes and resins, and the ratio between asphaltenes and resins. 

Emulsion properties and stability can be measured by rheological studies and dielectric spectroscopy. Rheological studies include forced oscillation experi ments. The formation of stable emulsions is marked by a sharp increase in the elastic modulus. Water con tent is not a good indicator of emulsion characteristics other than that low water contents (<50%) indicate that an emulsion has not been formed and that the product is entrained water-in-oil. Interfacial measure ments are useful for measuring the film strength of... 

Dielectric spectroscopy has proved to be an important tool to describe emulsion and microemulsion systems. Professor Yuri Feldman and his Norwegian colleagues review in Chapter 6 the fundamentals of this technique and its plausible applications. The span of applications is very wide and includes flocculation processes in emulsions, diffusion processes and porosity measurements in solid materials, characterization of particulate biosuspensions, and percolation phenomena in microemulsions. 

Gas hydrate formation is a well-known obstacle in the transport of gas, oil, and water. The formation of such chlatrates and their agglomeration will eventually plug pipes and prevent transport. One way to overcome this problem is to form the gas hydrates in a water-in-oil emulsion. The chapter by Tore Skodvin summarizes some current research at the University of Bergen in this field. It is stated that dielectric spectroscopy is a convenient technique to follow the formation of gas hydrates inside the water droplets, and because of this formation the dielectric properties of water change remarkably. It is also shown that when the gas hydrate particles are emulsified in a water-in-oil matrix one can transport up to about 30 weight% of water without any inhibitors present. 

---