Emulsions thermodynamic instability

Multiple emulsions may be interesting ways for releasing bioactive compounds in a controlled rate, useful in cosmetic, pharmacy, agricultural, and industrial chemicals nevertheless, their commercial applications have been limited due to their thermodynamic instability and unexpected fast release of encapsulated bioactive molecules (Yoshida et al., 1999 Beer et al., 2013). 

Emulsions are thermodynamically instable systems. The work AW which is necessary for dispersion is determined by the inier/uce tension y and the interface magnification of the disperse phase AA ... 

Notwithstanding their thermodynamic instability, many emulsions are kinetieally stable and do not change appreciably for a prolonged period. These systems exist in the metastable state (8-15). The fundamentals of emulsion sta-... 

Double emulsions have been known for over three decades and were extensively studied in the last 15 years. The internal phase is an excellent reservoir for active matter that needs protection and can be released at a controlled rate. However, the sizes of the droplets and the thermodynamic instability was a significant drawback of this technology. 

An interdisciplinary team of leading experts from around the world discuss recent concepts in the physics and chemistry of various well-studied interfaces of rigid and deformable particles in homo- and hetero-aggregate dispersed systems, including emulsions, dispersoids, foams, fluosols, polymer membranes, and biocolloids. The contributors clearly elucidate the hydrodynamic, electrodynamic, and thermodynamic instabilities that occur at interfaces, as well as the rheological properties of interfacial layers responsible for droplets, particles, and droplet-particle-film structures in finely dispersed systems. The book examines structure and dynamics from various angles, such as relativistic and non-relativistic theories, molecular orbital methods, and transient state theories. 

In the above systems AAyjL TAS and AAyow —TAS and hence AG > 0. This implies thermodynamic instability and the production of suspension or emulsions by the dispersion process is non-spontaneous, i.e. energy is required to produce the smaller particles or droplets from the larger ones. In the absence of any stabilisation mechanism (which will be discussed below), the smaller particles or droplets tend to aggregate and/or coalesce to reduce the total interfadal area, hence reducing the total surface energy of the system. 

Two types of thermodynamic instabilities are known to be responsible for the evolution of W/OAV multiple emulsions (Ficheux et al., 1998). The first type involves coalescence of the small inner droplets with the outer droplets interface, which is due to the rupture of the thin nonaqueous film that forms... 

The first chapter by J. Jiao and D. J. Burgess discusses the thermodynamic instability of multiple emulsions as a result of the excess of free energy caused by the formation of the emulsion droplets. In multiple emulsions consisting of three distinct liquid phases, counteracting the effect of the Laplace pressure by electrolyte addition to the inner dispersed aqueous phase will increase the destabilization of the system owing to osmotic pressure. In addition the authors discuss the effects of both osmotic and Laplace pressure as well as the interfacial rheological properties of these complex systems and their stability. 

Microemulsions are distinctly different from emulsions in that the former are thermodynamically stable one-phase systems whereas the latter are kinetically stabilized dispersions. Thus, microemulsions require no work for their formation, and once formed they are infinitely stable (cf. previous chapters). Emulsions, on the other hand, require work for their formation and display a kinetically controlled instability. This fundamental difference between emulsions and microemulsions is not always appreciated within the pharmaceutical literature. 

Macro- and miniemulsions are thermodynamically unstable. If not stabilized, the droplets tend to fiocculate, coalesce, sediment or cream [2-4]. Other instabilities, such as Ostwald ripening and phase inversion, are also known. At worst, an emulsion will break, i.e. the two phases will separate completely. A product becoming unstable will lose its quality within a short period of time and thus cannot be commercialized. Therefore, even in natural emulsion-based products, amphiphilic molecules are found (e.g. lecithin and proteins in egg yolk and milk and artificial surfactants and emulsifiers in cosmetics and chemical products. They adsorb at the droplets interfaces and stabilize them against flocculation and coalescence. Adsorption and stabilization mechanisms depend on the molecular structure of a surfactant or an emulsifier as depicted in Figure 20.1. Stabilization mechanisms are summarized in... 

Multiple emulsions made of low-molecular-weight emulsifiers (the so-called monomeric emulsifiers) are mostly unstable thermodynamically. This is mainly because in the second stage of the emulsification severe homogenization or shear are not recommended, and as a result large droplets are obtained. During years of research attempts have been made to find proper and more suitable combinations of emulsifiers to reduce droplets sizes and to improve the emulsion stability. Aggregation, flocculation, and coalescence (occurring in the inner phase and between the multiple-emulsion droplets) lead to rupture of droplets and separation of the phases and thus are major factors behind the instability of the emulsions. 

The coalescence of liquid droplets, therefore, is intimately related to the nature of the thin lamellar film formed between them as they are brought into close encounters as a result of thermal convection. Brownian motion, or mechanical agitation. It is important to understand the nature of the forces acting across the film in order to obtain information about the thermodynamic stability, metastability, or instability of the film, and the kinetic processes that will control the rate of film breakdown. Comprehensive reviews of those aspects of emulsion stability can be... 

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