Emulsion classical theories

The conductivity of a dilute emulsion can be treated by classic theory (see Maxwell [6]) assuming spherical droplets... 

In conclusion, we may say that one of the most Inportant problems which still remain to be solved is to clarify the mechcinism by which polymer particles are formed and to establish a pcirticle nucleation model by which the number of polymer particles produced can be estimated, because the classical theory of Smith md wcu t does not serve for this purpose in the emulsion polymerization of vinyl acetate. In the succeeding section, a new reaction model is... 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

It was observed that intermediate compositions dried slower than the unblended constituents. The mechanical properties of latex blends comprising low and high particles, the film formation ability, and the comparison of results to classical theory of polymer blends containing hard particles have been reported by Lepizzera et al. [1997], This is one of the few studies in the literature that has attempted to correlate emulsion blend properties with empirical/theoretical relationships previously available in the open literature. 

The precise mechanism of emulsion polymerisation remains a matter for debate. Whilst theoretical studies have given rise to the classical theories of Harkins and Smith-Ewait, commercial polymerisations do not always behave as those theories predict. There are a number of reasons for this. For example the water solubility of various monomers has a profound effect on the mechanism by which the monomer undergoes polymerisation, any one of several options being possible. 

Again, where a pre-emulsion of monomer and surfactant is used (as is often the case with commercial polymerisations), the classical theories do not adequately explain the practical results. 

Kinetics and Mechanisms. Early researchers misunderstood the fast reaction rates and high molecular weights of emulsion polymerization (11). In 1945 the first recognized quaHtative theory of emulsion polymerization was presented (12). This mechanism for classic emulsion preparation was quantified (13) and the polymerization separated into three stages. 

In this chapter we present the theory involved in developing sustained- and controlled-release delivery systems and applications of these systems as therapeutic devices. Although suspensions, emulsions, and compressed tablets may demonstrate sustaining effects within the body compared with solution forms of the drug, they are not considered to be sustaining and are not discussed in this chapter. These systems classically release drug for a relatively short period, and their release rates are strongly influenced by environmental conditions. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

The electrical conductivity of dilute emulsions can be treated by classical electrodynamic theory and the conductivity is given by... 

Most papers involving the theory of emulsion polymerization begin with a brief discussion of the classical work of Smith-Ewart (1). Their Case II theory, tested extensively with styrene batch reactions, yields the following prediction for polymerization rate (R ) and peurticle number (N). ... 

All of the petroleum emulsion applications or problems just discussed have in common the same basic principles of colloid science that govern the nature, stability, and properties of emulsions. The widespread importance of emulsions in general and scientific interest in their formation, stability, and properties have precipitated a wealth of published literature on the subject. This chapter provides an introduction and is intended to complement the other chapters in this book on petroleum emulsions. A good starting point for further basic information is one of the classic texts Becher s Emulsions Theory and Practice (4) or Sumner s Claytons Theory of Emulsions and... 

The use of emulsions and their range of practical application has been expanded enormously. As a result, the field of the theory of emulsions and technical emulsion science, as a part of classical colloid chemistry, can use a lot of theory developed there. 

The power to dissolve resins is the foremost requirement of a solvent except in cases involving dispersions in nonaqueous solvents (NADs) or dispersions in water (latices, emulsions, and dispersions). Theories of solvency and solution are covered by Rider in the preceding chapter. The classic books by Hildebrand and Scott (18) and Hildebrand, Prausnitz, and Scott (19) discuss solubility and solutions in considerable depth. The monumental book by Doolittle ( covers both theoretical and applied aspects of solvents. Several chapters in the Mattiello series published in 1941-46 deal with solvents the chapter on lacquer solvents by Bogin... 

Tauer, K. and Ktihn, I. (1995) Modeling particle formation in emulsion polymerization - an approach by means of the classical nucleation theory. Macro-molecules, 28, 2236. 

Classically, emulsion polymerizations involve monomers that are solvents for the homopolymer. Strictly speaking only these systems follow the theory of Harkins as quantified by Smith and Ewart. According to this theory, the propagation rate is a function of the number of polymer particles that are... 

Capek and Chudej [87] studied the emulsion polymerization of styrene stabilized by polyethylene oxide sorbitan monolaurate with an average of 20 monomeric units of ethylene oxide per molecule (Tween 20) and initiated by the redox system of ammonium persulfate and sodium thiosulfite. It is interesting to note that the constant reaction rate period is not present in this polymerization system. The maximal rate of polymerization is proportional to the initiator and surfactant concentrations to the -0.45 and 1.5 powers, respectively. The final number of latex particles per unit volume of water is proportional to the initiator and surfactant concentrations to the 0.32 and 1.3 powers, respectively. In addition, the resultant polymer molecular weight is proportional to the initiator and surfactant concentrations to the 0.62 and -0.97 powers, respectively. Some possible reaction mechanisms may explain the deviation of the polymerization system from the classical Smith-Ewart theory. Lin et al. [88] investigated the emulsion polymerization of styrene stabilized by nonylphenol polyethoxylate with an average of 40 monomeric units of ethylene oxide per molecule (NP-40) and initiated by sodium persulfate. The rate of polymerization versus monomer conversion curves exhibit two nonsta-tionary reaction rate intervals and a vague constant rate period in between. 

Recently, Durant et al. [55] developed a mechanistic model based on the classic Smith-Ewart theory [48] for the two-phase emulsion polymerization kinetics. This model, which takes into consideration complete kinetic events associated with free radicals, provides a delicate procedure to calculate the polymerization rate for latex particles with two distinct polymer phases. It allows the calculation of the average number of free radicals for each polymer phase and collapses to the correct solutions when applied to single-phase latex particles. Several examples were described for latex particles with core-shell, inverted core-shell, and hemispherical structures, in which the polymer glass transition temperature, monomer concentration and free radical entry rate were varied. This work illustrates the important fact that morphology development and polymerization kinetics are coupled processes and need to be treated simultaneously in order to develop a more realistic model for two-phase emulsion polymerization systems. More efforts are required to advance our knowledge in this research field. 

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