Emulsion importance

The pulsed field gradient NMR technique can be readily used for the determination of the water droplet size distribution in W/O emulsions or the oil droplet size distributions in O/W emulsions. Important advantages are the non-invasive nature, the ease of sample preparation, and the feet that pulsed field gradient NMR measures the droplet size distribution of the bulk in contrast with microscopic methods which estimate the size distribution of the surface. Both the proposed matrix method and the iterative curve fitting procedure can be successfully applied in a factory environment. The method can be implemented on a high as well as on a low resolution NMR spectrometer. 

OAV emulsion Important aqueous phase gel-formed. Fatty phase esters and different oils A light slip and spread texture, less rich, rather mat... 

W/O emulsion Important fatty phase gel-formed. Fatty phase ester, silicone oils, small quantity of wax. Contains electrol54es Good slip and spread, rich in texture, comfortable, some what thick film... 

Apart from chemical composition, an important variable in the description of emulsions is the volume fraction, outer phase. For spherical droplets, of radius a, the volume fraction is given by the number density, n, times the spherical volume, 0 = Ava nl2>. It is easy to show that the maximum packing fraction of spheres is 0 = 0.74 (see Problem XIV-2). Many physical properties of emulsions can be characterized by their volume fraction. The viscosity of a dilute suspension of rigid spheres is an example where the Einstein limiting law is [2]... 

Other important characterization techniques include electrophoresis measurements of droplets [11, 12] (see Section XIV-3C), infrared absorption of the constituent species [13], and light or x-ray scattering. NMR self-diffusion measurements can be used to determine droplet sizes in W/0 emulsions [14]. 

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. 

The importance of steric factors in the formation of penetration complexes is made evident by the observation that although sodium cetyl sulfate plus cetyl alcohol gives an excellent emulsion, the use of oleyl alcohol instead of cetyl alcohol leads to very poor emulsions. As illustrated in Fig. XIV-3, the explanation may lie in the difficulty in accommodating the kinked oleyl alcohol chain in the film. 

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

An important industrial example of W/O emulsions arises in water-in-crude-oil emulsions that form during production. These emulsions must be broken to aid transportation and refining [43]. These suspensions have been extensively studied by Sjoblom and co-workers [10, 13, 14] and Wasan and co-workers [44]. Stabilization arises from combinations of surface-active components, asphaltenes, polymers, and particles the composition depends on the source of the crude oil. Certain copolymers can mimic the emulsion stabilizing fractions of crude oil and have been studied in terms of their pressure-area behavior [45]. 

There have been some studies of the equilibrium shape of two droplets pressed against each other (see Ref. 59) and of the rate of film Winning [60, 61], but these are based on hydrodynamic equations and do not take into account film-film barriers to final rupture. It is at this point, surely, that the chemistry of emulsion stabilization plays an important role. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The production of organic polymeric particles in tire size range of 30-300 nm by emulsion polymerization has become an important teclmological application of surfactants and micelles. Emulsion polymerization is very well and extensively reviewed in many monographs and texts [67, 68], but we want to briefly illustrated tire role of micelles in tliis important process. 

Although the remainder of this contribution will discuss suspensions only, much of the theory and experimental approaches are applicable to emulsions as well (see [2] for a review). Some other colloidal systems are treated elsewhere in this volume. Polymer solutions are an important class—see section C2.1. For surfactant micelles, see section C2.3. The special properties of certain particles at the lower end of the colloidal size range are discussed in section C2.17. 

An important step in tire progress of colloid science was tire development of monodisperse polymer latex suspensions in tire 1950s. These are prepared by emulsion polymerization, which is nowadays also carried out industrially on a large scale for many different polymers. Perhaps tire best-studied colloidal model system is tliat of polystyrene (PS) latex [9]. This is prepared with a hydrophilic group (such as sulphate) at tire end of each molecule. In water tliis produces well defined spheres witli a number of end groups at tire surface, which (partly) ionize to... 

Several polymerization techniques are in widespread usage. Our discussion is biased in favor of methods that reveal additional aspects of addition polymerization and not on the relative importance of the methods in industrial practice. We shall discuss four polymerization techniques bulk, solution, suspension, and emulsion polymerization. 

The fourth and most interesting of the polymerization techniques we shall consider is called emulsion polymerization. It is important to distinguish between suspension and emulsion polymerization, since there is a superficial resemblance between the two and their terminology has potential for confusion A suspension of oil drops in water is called an emulsion. Water-insoluble monomers are used in the emulsion process also, and the polymerization is carried out in the presence of water however, the following significant differences also exist ... 

Hydrophobically Modified Alkali-Swellable Emulsions. The hydrophobe modification of acryHc acid represents an important class of... 

Higher alkyl acrylates and alkyl-functional esters are important in copolymer products, in conventional emulsion appHcations for coatings and adhesives, and as reactants in radiation-cured coatings and inks. In general, they are produced in direct or transesterification batch processes (17,101,102) because of their relatively low volume. 

Emulsion Polymerization. Emulsion polymerization is the most important industrial method for the preparation of acryhc polymers. The principal markets for aqueous dispersion polymers made by emulsion polymerization of acryhc esters are the paint, paper, adhesives, textile, floor pohsh, and leather industries, where they are used principally as coatings or binders. Copolymers of either ethyl acrylate or butyl acrylate with methyl methacrylate are most common. 

High Water-Base Fluids. These water-base fluids have very high fire resistance because as Httle as 5% of the fluid is combustible. Water alone, however, lacks several important quaUties as a hydrauHc fluid. The viscosity is so low that it has Httle value as a sealing fluid water has Httle or no abiHty to prevent wear or reduce friction under boundary-lubrication conditions and water cannot prevent mst. These shortcomings can be alleviated in part by use of suitable additives. Several types of high water-based fluids commercially available are soluble oils, ie, od-in-water emulsions microemulsions tme water solutions, called synthetics and thickened microemulsions. These last have viscosity and performance characteristics similar to other types of hydrauHc fluids. 

The ionic nature of the radicals generated, by whatever technique, can contribute to the stabilisation of latex particles. Soapless emulsion polymerisations can be carried out usiag potassium persulfate as initiator (62). It is often important to control pH with buffets dutiag soapless emulsion p olymerisation. 

The vast majority of commercial apphcations of methacryhc acid and its esters stem from their facile free-radical polymerizabiUty (see Initiators, FREE-RADICAl). Solution, suspension, emulsion, and bulk polymerizations have been used to advantage. Although of much less commercial importance, anionic polymerizations of methacrylates have also been extensively studied. Strictiy anhydrous reaction conditions at low temperatures are required to yield high molecular weight polymers in anionic polymerization. Side reactions of the propagating anion at the ester carbonyl are difficult to avoid and lead to polymer branching and inactivation (38—44). 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

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