Phase behavior colloids

In the simplest emulsions just described, the final separation is into two Hquid phases upon destabilization. The majority of emulsions are of this kind, but in some cases the emulsion is divided into more than two phases. One obvious reason for such a behavior is the presence of a material that does not dissolve in the oil or the water. One such case is the presence of soHd particles, which is common in emulsions for food, pharmaceuticals, and cosmetics. Another less trivial reason is that the surfactant associates with the water and/or the oil to form a colloidal stmcture that spontaneously separates from the two hquid phases. This colloidal stmcture may be an isotropic Hquid or may be a semisoHd phase, a Hquid crystal, with long-range order. 

E. ten Grotenhuis, M. Paques, G. A. van Aken 2000, (The application of diffus-ing-wave spectroscopy to monitor the phase behavior of emulsion-polysaccharide systems),/. Colloid Interface Sci. 227, 495. 

This paper reviews the experiences of the oil industry in regard to asphaltene flocculation and presents justifications and a descriptive account for the development of two different models for this phenomenon. In one of the models we consider the asphaltenes to be dissolved in the oil in a true liquid state and dwell upon statistical thermodynamic techniques of multicomponent mixtures to predict their phase behavior. In the other model we consider asphaltenes to exist in oil in a colloidal state, as minute suspended particles, and utilize colloidal science techniques to predict their phase behavior. Experimental work over the last 40 years suggests that asphaltenes possess a wide molecular weight distribution and they may exist in both colloidal and dissolved states in the crude oil. 

One major question of interest is how much asphaltene will flocculate out under certain conditions. Since the system under study consist generally of a mixture of oil, aromatics, resins, and asphaltenes it may be possible to consider each of the constituents of this system as a continuous or discrete mixture (depending on the number of its components) interacting with each other as pseudo-pure-components. The theory of continuous mixtures (24), and the statistical mechanical theory of monomer/polymer solutions, and the theory of colloidal aggregations and solutions are utilized in our laboratories to analyze and predict the phase behavior and other properties of this system. 

Leal-Calderon et al. [13] have proposed some basic ideas that control the colloidal interactions induced by solvent or a mixture of solvent and solute, when varying their length from molecular to colloidal scale. They have investigated the behavior of water- and glycerol-in oil emulsions in the presence of linear flexible chains of various masses. Figure 3.7 shows the phase behavior of both water and glycerol droplets of diameter 0.4 pm when dispersed in a linear aliphatic solvent of formula C H2 +2, from n = 5 to n = 30. Because, for n larger than 16, solvent crystallization occurs at room temperature, a second series of experiments... 

An analogy may be drawn between the phase behavior of weakly attractive monodisperse dispersions and that of conventional molecular systems provided coalescence and Ostwald ripening do not occur. The similarity arises from the common form of the pair potential, whose dominant feature in both cases is the presence of a shallow minimum. The equilibrium statistical mechanics of such systems have been extensively explored. As previously explained, the primary difficulty in predicting equilibrium phase behavior lies in the many-body interactions intrinsic to any condensed phase. Fortunately, the synthesis of several methods (integral equation approaches, perturbation theories, virial expansions, and computer simulations) now provides accurate predictions of thermodynamic properties and phase behavior of dense molecular fluids or colloidal fluids [1]. 

When two similarly charged colloid particles, under the influence of the EDL, come close to each other, they will begin to interact. The potentials will detect one another, and this will lead to various consequences. The charged molecules or particles will be under both van der Waals and electrostatic interaction forces. The van der Waals forces, which operate at a short distance between particles, will give rise to strong attraction forces. The potential of the mean force between colloid particle in an electrolyte solution plays a central role in the phase behavior and the kinetics of agglomeration in colloidal dispersions. This kind of investigation is important in these various industries ... 

Bourrel M, Salager JL, Schechter RS, Wade WH (1980) A Correlation for Phase Behavior of Nonionic Surfactants. J Colloid Interface Sci 75 451-461... 

Graciaa A, Barakat Y, El-Emary M, Fortney L, Schechter RS, Yiv S, Wade WEI (1982) HLB, CMC and phase behavior as related to hydrophobe branching. J Colloid Interface Sci 89 209-216... 

Ysambertt F, Anton RE, Salager JL (1997) Retrograde Transition in the phase behavior of surfactant-oil-water systems produced by an oil EACN scan. Colloid Surf A 125 131-136... 

Mori F, Lim JC, Raney OG, Elsik CM, Miller CA (1989) Phase behavior, dynamic contact angle and detergency in systems containing triolene and nonionic siufactants. Colloid Surf 40 323-345... 

P. N. Pusey and W. Van Megen, Phase behavior of concentrated suspensions of nearly hard colloidal sphres. Nature 320, 340-342 (1986). 

Proteins are both colloids and polymers. Therefore, attempts have been made to understand the phenomenon of protein aggregation with the help of models from the polymer and colloid fields such as DLVO theory, describing the stability of colloidal particles, or phase behavior and attraction-repulsion models from polymers (De Young, 1993). For faster progress, more phase diagrams for equilibrium protein precipitation, in both the crystalline and the non-crystalline state, as well as more data on observations of defined protein oligomers or polymers, are required. 

Figure 8.8. Schematic illustrating the analogy between colloid flocculation behavior and phase behavior of the stabilizer in bulk solution. As density is lowered, separation of solvent from chains in bulk solution resembles separation of solvent from chains on surfaces, which produces flocculation.

Effect of Pressure on Micelles. While temperature studies of the phase transitions of bilayers and micelles have been performed for some time now, the utilization of pressure as a variable is a more recent development. Variation in temperature of a colloidal aggregate such as a bilayer causes simultaneous changes in thermal energy and volume, whereas isothermal variation in pressure (up to 50 kbar) yields spectroscopic changes due only to volume effects. A review of high pressure vibrational spectroscopy of phospholipid bilayers has recently appeared (74). in which the surprisingly rich barotropic phase behavior of these compounds is explored in detail. 

Schambil, F. and Schwuger, M. J. (1987) Correlation between the phase behavior ofternary systems and removal of oil in the washing process. Colloid Polym. Scl, 265( 11), 1009-17. 

Phase Behavior of Mixtures of Sterically Stabilized Colloidal Dispersions and Free Polymer... 

The phase behavior of nonaqueous colloidal suspensions containing nonadsorbing polymer was investigated by Gast et al. [3] on the basis of statistical mechanics. In their theory, a second-order perturbation approach was used to calculate the free energy. Rao and Ruckenstein [4,5] examined the phase behavior of systems involving steric, depletion, and van der Waals interactions. 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

Alany et al. [11,35] reported on the phase behavior of two pharmaceutical ME systems showing interesting viscosity changes. The viscosity of both systems increased with increasing volume fraction of the dispersed phase to 0.15 and flow was Newtonian. However, formation of LC in one of the two systems, namely the cosurfac-tant-free system, resulted in a dramatic increase in viscosity that was dependent on the volume fraction of the internal phase and a change to pseudoplastic flow. In contrast, the viscosity of the bicontinuous ME was independent of water volume fraction. The authors used two different mathematical models to explain the viscosity results and related those to the different colloidal microstructures described. 

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