Rheological methods, stabilization emulsions

This chapter outlines emulsion characterization techniques ranging from those commonly found infield environments to those in use in research laboratories. Techniques used in the determination of bulk emulsion properties, or simply the relative amount of oil, water, and solids present, are discussed, as well as those characterization methods that measure the size distribution of the dispersed phase, rheological behavior, and emulsion stability. A particular emphasis is placed on optical and scanning electron microscopy as methods of emulsion characterization. Most of the common and many of the less frequently used emulsion characterization techniques are outlined, along with their particular advantages and disadvantages. 

Emulsion Pipeline Operations. Prediction of pipeline pressure gradients is required for operation of any pipeline system. Pressure gradients for a transport emulsion flowing in commercial-size pipelines may be estimated via standard techniques because chemically stabilized emulsions exhibit rheological behavior that is nearly Newtonian. The emulsion viscosity must be known to implement these methods. The best way to determine emulsion viscosity for an application is to prepare an emulsion batch conforming to planned specifications and directly measure the pipe viscosity in a pipe loop of at least 1-in. inside diameter. Care must be taken to use the same brine composition, surfactant concentration, droplet size distribution, brine-crude-oil ratio, and temperature as are expected in the field application. In practice, a pilot-plant run may not be feasible, or there may be some disparity between pipe-loop test conditions and anticipated commercial pipeline conditions. In these cases, adjustments may be applied to the best available viscosity data using adjustment factors described later to compensate for disparities in operating parameters between the measurement conditions and the pipeline conditions. 

A very important part of emulsion study is the availability of methodologies to study emulsions. In the past ten years, both dielectric methods (1) and rheological methods (2) have been exploited to study formation mechanisms and the stability of emulsions formed from many different types of oils. Standard techniques, including NMR, chemical analysis techniques, microscopy, interfacial pressure, and interfacial tension, are also being applied to emulsions. These techniques have largely confirmed findings noted in the dielectric and rheological mechanisms. 

Interfacial rheology is a very important tool in understanding the formation, stability and other properties of emulsions and foams. It also contributes to the characterization of monolayers, in addition to spectroscopic, electric and other methods. Hence, there is a clear motive for considering it in some detail. 

Emulsions are used extensively in the food, pharmaceutical, and cosmetics industries, where their most important properties include stability (both physical and qualitative), rheology, reactivity, shelf life, texture, appearance, and flavor. All of these properties are affected by droplet size and/or size distribution [107,108], which in turn are functions of the method of production. Because emulsions are thermodynamically unstable, they require energy input for production and usually rely on a stabilizing agent (emulsifier, surfactant) to remain stable over long periods [109]. 

The mechanical properties of asphaltene films at interfaces can be probed by a variety of rheological techniques. These methods provide valuable insight into the origins of stability of asphaltene emulsions and into the role of concentration, and solvation by resins and aromatic solvents on the adsorption and self-assembly of asphaltenes. Miller et al. provide a comprehensive review of methods for probing interfacial dilational and shear properties of adsorption layers at liquid interfaces (72). They describe devices that measure surface velocity profiles (indirect methods) or determine torsional stress values (direct methods). Indirect... 

Adsorbed protein molecules interact at the interfaces to form viscoelastic films. The viscoelastic properties of protein films adsorbed at fluid interfaces in food emulsions and foams are important in relation to the stability of such systems with respect to film rupture and coalescence. Interfacial rheology techniques are very sensitive methods to measure the viscoelastic properties of proteins, thereby evaluating the protein-protein or protein-surfactant interactions at the interfaces. There was an excellent review about the principal and methods of interfacial rheology [17]. 

Measurement of the droplet size distribution, rates of flocculation, Ostwald ripening and coalescence are also described in Chapter 6. These methods should be applied for agrochemical emulsions to ensure their long-term physical stability. In addition, the bulk rheology of the system should be investigated after storage at various temperatures. 

Binary mixtures of a flexible polymer and a rigid rod-like molecule (nematogen or liquid crystal) play an important role in electro-optical devices, such as light shutters and displays. Since the miscibility or phase separation controls the performance of the materials, the phase behavior and phase separation kinetics have been of fundamental and practical interests. Liquid crystalline domains dispersed in a polymer matrix are called polymer dispersed-liquid crystals (PDLCs), or polymer-stabilized liquid crystals (PSLCs), where the polymer forces the liquid crystals to phase separate into droplets surrounding by the polymer matrix [2]. Practically, there are many ways to create PD LCs by mixing polymers and liquid crystals the emulsion method [37] and phase separation method [38], including polymerization-, thermally-, and solvent-induced phase separations. The reader is referred to text books [1, 2] for details of PDLC and a review [39] for the rheological and mechanical properties. 

In this chapter, novel method for microencapsulation by coacervation is presented. The method employs polymer-polymer incompatibility taking place in a ternary system composed of sodium carboxymethyl cellulose (NaCMC), hydroxypropylmethyl cellulose (HPMC), and sodium dodecylsulfate (SDS). In the ternary system, various interactions between HPMC-NaCMC, HPMC-SDS and NaCMC-(HPMC-SDS) take place. The interactions were investigated by carrying out detailed conductometric, tensiometric, turbidimetric, viscosimetric, and rheological study. The interactions may result in coacervate formation as a result of incompatibility between NaCMC molecules and HPMC/SDS complex, where the ternary system phase separates in HPMC/SDS complex rich coacervate and NaCMC rich equilibrium solution. By tuning the interactions in the ternary system coacervate of controlled rheological properties was obtained. Thus obtained coacervate was deposited at the surface of dispersed oil droplets in emulsion, and oil-content microcapsules with a coacervate shell of different properties were obtained. Formation mechanism and stability of the coacervate shell, as well as stability of emulsions depend on HPMC-NaCMC-SDS interaction. Emulsions stabilized with coacervate of different properties were spray dried and powder of microcapsules was obtained. Dispersion properties of microcapsules, and microencapsulation efficiency were investigated and found to depend on both properties of deposited coacervate and the encapsulated oil type. 

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