Emulsion kinetics

Fig. 2. Emulsification kinetics. All the emulsions have the same composition but different initial sizes. Empty symbols correspond to emulsions kinetics composed of two regimes whereas filled symbols (see insert) correspond to kinetics presenting only the slow regime (see text for details). Initial sizes are oil pm 8.2 pm A 6 pm 4.7 pm and 4.1 pm. The lines are fits to experimental data of the second regime, using Eq. (1). Note the unique fit for all the open symbols...

It is possible to calculate k via the emulsion kinetics outlined previously. Some results are listed in Table IV. 

The frequency distribution of diameters is the most widely used way of presenting population size data. It contains useful information which aids the prediction of emulsion kinetic behavior e.g., sedimentation and diffusion are functions of droplet size. Also, one can follow flie evolution of the DSD as a function of time, the shift towards fewer/larger droplets being evidence of droplet-depletion mechanisms, such as coalescence and Ostwald ripening. From the distribution, the kinetic coefficients can be calculated, allowing prediction of how the DSD will develop (e.g., 48, 55). This is described in detail by Dukhin et al.. Chapter 4, this volume. Figure 11 shows how the addition of a demulsifier can destabilize an emulsion and bring about emulsion resolution. The example is a water-in-crude oil emulsion, the demulsifier a phenolic resin alkoxy-late. 

Luckily, although thermodynamics will be the factor controlling the ultimate long-term stability of an emulsion, kinetics can play an important role over the short term, and it is through kinetic pathways that most useful emulsions achieve their needed stability. It is clear, then, that while lowering the interfacial tension between phases is an important factor in the formation and stabilization of emulsions, that may not always represent the most important factor in their preparation and ultimate application. 

From the foregoing, grafting (eq. (13)) is expected to show a 3/4 power dependence on [M] and is half order in initiator and leather amount, respectively. If emulsion kinetics prevail, the exponential dependence is 0.4 in fl] and 0.6 in [Si, but zero in monomer disappearance from the stabilized reservoirs in the aqueous phase (2,47-50). Consequently the exponential intensity factor, a, of the general equation for rate, R,... 

Emulsifiers play two roles in emulsion kinetic stability. They lower interfa-... 

The use of ttaditional disperse systems, e.g., macroemulsions, in the pharmaceutical industry has been limited due to manufacturing complexity and stability problems [117]. The characteristic properties of nano-emulsions (kinetic stability, small and controlled droplet size, etc.) make them interesting systems for pharmaceutical applications. Indeed, nano-emulsions are used as drug delivery systems for administration through various systemic routes. There are numerous publications on nano-emulsions as drug delivery systems for parenteral [17,18,28,29,118-124], oral [25,125-129], and topical administration, which includes the administration of formulations to the external surfaces of the body skin [32,130,131] and to the body cavities nasal [30,132] as weU as ocular administration [31,133-136]. Moreover, many patents concerning pharmaceutical applications of nano-emulsions have been registered [17,18,25,137-145]. An application of nano-emulsions in this field has been in the development of vaccines [33,146-147]. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

There are two approaches to the kinetics of emulsion flocculation. The first stems from a relationship due to Smoluchowski [52] for the rate of diffusional encounters, or flux ... 

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

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. 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

A kinetic model for the particle growth stage for continuous-addition emulsion polymerization has been proposed (35). Below the monomer... 

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

However, the kinetics of PVC emulsion does not foUow the above theory. The rate shows the same increasing behavior with conversion as mass polymerization (94,95). [N depends on [3], but the relationship varies with the emulsifier type (96,97). However, the rate is nearly independent of [N (95). The average number of radicals per particle is low, 0.0005 to 0.1 (95). The high solubiUty of vinyl chloride in water, 0.6 wt %, accounts for a strong deviation from tme emulsion behavior. Also, PVC s insolubiUty in its own monomer accounts for such behavior as a rate dependence on conversion. 

Emulsions stabilized with a nonionic surfactant and catalyzed with a monomer soluble initiator were found to foUow kinetics dependent on initiator concentration (17). 

The activity of antioxidants in food [ 1 ] emulsions and in some biological systems [2] is depends on a multitude of factors including the localisation of the antioxidant in the different phases of the system. The aim of this study is determining antioxidant distributions in model food emulsions. For the purpose, we measured electrochemically the rate constant of hexadecylbenzenediazonium tetrafluorborate (16-ArN,BF ) with the antioxidant, and applied the pseudophase kinetic model to interpret the results. 

The effectiveness of a fluidized bed as a ehemical reactor depends to a large extent on the amount of convective and diffusive transfer between bubble gas and emulsion phase, since reaction usually occurs only when gas and solids are in contact. Often gas in the bubble cloud complex passes through the reactor in plug flow with little back mixing, while the solids are assumed to be well mixed. Actual reactor models depend greatly on kinetics and fluidization characteristics and become too complex to treat here. 

In this chapter, the polymerization methods used for the production of uniform latex particles in the size range of O.I-lOO /Ltm are described. Emulsion, swollen emulsion, and dispersion polymerization techniques and their modified forms for producing plain, functionalized, or porous uniform latex particles are reviewed. The general mechanisms and the kinetics of the polymerization methods, the developed synthesis procedures, the effect of process variables, and the product properties are discussed. 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

As a conclusion, the basic kinetic features of the emulsion polymerization system may be summarized as follows ... 

According to the other kinetic model proposed for the soapless emulsion process, the growing macroradicals may also form micelle structures at earlier polymerization times since they have both a hydrophilic end coming from the initiator and a hydrophobic chain [74]. 

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