Droplet stabiliser

In a suspension polymerisation monomer is suspended in water as 0.1—5-mm droplets, stabilised by protective coUoids or suspending agents. Polymerisation is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle sise is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

In the detergency process, fatty materials (i.e. dirt, often from human skin) are removed from surfaces, such as cloth fibres, and dispersed in water. It is the surfactants in a detergent which produce this effect. Adsorption of the surfactant both on the fibre (or surface) and on the grease itself increases the contact angle of the latter as illustrated in Figure 4.7. The grease or oil droplet is then easily detached by mechanical action and the surfactant adsorbed around the surface of the droplet stabilises it in solution. 

The polymeric adsorbents are usually prepared by variations of two-phase suspension processes. These refer to systems where microdroplets of monomers and solvent are converted into solid beads upon polymerisation. In the case where the monomers are not water soluble, as in the case of styrene-based polymers and many methacrylate-based polymers, the monomers, a solvent and a droplet stabiliser are suspended as droplets by stirring in water and then polymerised (o/w suspension polymerisation). The particle size and dispersity can be influenced by the stirring speed and the type of stabiliser. So far, only a few examples of the preparation of imprinted polymers in bead format have been described [4-8] and these are thoroughly reviewed in Chapter 12. In non-covalent imprinting, the main limitation to the use of these techniques is that the imprinting method often requires the use of polar partly water soluble monomers or templates in combination with less polar water insoluble components. Use of the o/w suspension method... 

An important technique is that in which it is the precursor of the final colloidal particle that is reduced to a colloidal size. Thus a liquid reactant may be emulsified and then caused to react to form a colloidal dispersion of solid particles whose particle size distribution is related to that of the emulsion precursor. The commonest application of this method is in suspension polymerisation, in which an emulsion of monomer droplets, stabilised by a surfactant, is polymerised by adding an initiator which is soluble in the monomer. Polymerisation occurs within the monomer droplet, leading to the formation of a polymer latex. 

Investigations on water-fuel emulsions using different types of fuels such as gasoline, diesel fuel, fuel oil are discussed in [250], based on results on emulsion droplet stabilisation against flocculation and coalescence [50 - 58]. Water-gasoline fuels containing up to 10% of water... 

A study was made of the deformation and break-up of water droplets dispersed in an epoxy resin phase under shear in terms of microrheology and the interaction and coalescence dynamics occurring between the water droplets stabilised by the emulsifier molecules examined theoretically. A phase inversion model is proposed to account for the effects of some variables on phase inversion and the structure of the waterborne particles. 29 refs. 

In the present study, the fibril solutions exhibited a far better emulsifying activity with no pH-dependent differences in oil droplet size, which indicates that a pick-ering type of droplet stabilisation may exist [56]. Slight changes in the oil droplet... 

Figure 6 shows the detailed mechanism as the first step we prepared the inner solution (I-solution) containing the enzyme ([urease] = 10.3 U/mL) and the fluorescent probe ([pyranine] = 650 p.M). Next, we dispersed the I-solution in mineral oil containing [POPC] = 0.5 mM, in order to form the w/o macroemulsion. The lipid formed a monolayer around w/o droplets, stabilising the water/oil interface and preventing phase separation or coalescence (Fig. 6a). A layer of mineral oil containing [POPC] =0.5 mM was also stratified over the aqueous outer solution (0-solution), in order to prepare an interfacial phase. In this way, a continuous POPC monolayer self-assembles at the oil/water interface (Fig. 6b). Note that I-and 0-solutions are isotonic, but their densities (p) have been adjusted by adding [sucrose] = 150 mM (p 1.24 g cm ) to the I-solution and [glucose] = 150 mM...

Emulsion polymerisation is initiated using a water-soluble initiator, such as potassium persulfate. This forms free radicals in solution which may initiate some growing chains in solution. These radicals or growing chains pass to the micelles and diffuse into them, which causes the bulk of the polymerisation to occur in these stabilised droplets. 

Electrical coalescers, in which a high voltage field is used to break down the stabilising film surrounding the suspended droplets, are used for desalting crude oils and for similar applications see Waterman (1965). 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

A well-studied example of a bioemulsifier is emulsan, a cell surface-exposed molecule that allows Acinetobacter calcoaceticus RAG-1 to attach to crude oil droplets [123]. Upon depletion of the short-chain alkanes utilised by this strain, the emulsan molecules were released from the bacterial surface, thereby allowing the cells to leave the oil droplet and to find a new substrate. Important positive side-effects of this mechanism seem to be that the remaining emulsan hydrophilises the droplet and prevents both the reattachment of A. calcoaceticus RAG-1 and the coalescence of the used oil droplet with other droplets that still contain unexploited alkanes. Bredholt et al. [124] studied the oil-emulsifying activity of Rhodococcus sp. strain 094. When exposed to inducers of crude-oil emulsification, the cells developed a strongly hydrophobic character, which was rapidly lost when crude-oil emulsification started. This indicated that the components responsible for the formation of cell-surface hydrophobi-city acted as emulsion stabilisers after release from the cells. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

Figure 4.8 Diagram of how surfactant molecules stabilise water droplets in oil.

The contribution of double-layer forces to the osmotic pressure of HIPEs was also investigated [98], These forces arise from the repulsion between adjacent droplets in o/w HIPEs stabilised by ionic surfactants. It was observed that double-layer repulsive forces significantly affected jt for systems of small droplet radius, high volume fraction and low ionic strength of the aqueous continuous phase. The discrepancies between osmotic pressure values observed by Bibette [97] and those calculated by Princen [26] were tentatively attributed to this effect. 

Very recently, ESR techniques have been employed to study the packing of surfactant molecules at the oil/water interface in w/o HIPEs [102,103], By including an amphiphilic ESR probe, which is adsorbed at the oil/water interfaces, it is possible to determine the microstructure of the oil phase from the distribution of amphiphiles between the films surrounding the droplets and the reverse micelles. It was found that most of the surfactant is located in the micelles, over a wide range of water fraction values. However, when the water content is very high (water droplets of the emulsion, to stabilise the large interfacial area created. 

Kizling and coworker [21] suggested that salts in the aqueous phase stabilised w/o HIPEs by two means. First, the Ostwald ripening process is inhibited due to the decreased solubility of the aqueous solution in the continuous oil phase. Secondly, the attractive forces between adjacent aqueous droplets are lowered, as a result of the increase in refractive index of the aqueous phase towards that of the oil phase. When the refractive indices of the two phases are matched, the attractive forces are at a minimum and highly stable, transparent emulsions are formed. The attractive force, A, is given by ... 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

Finally, some studies have been performed on the addition of salt to the aqueous phase of oil-in-water HIPEs [109]. For systems stabilised by ionic surfactants, increasing salt concentration reduces the double-layer repulsion between droplets however, stability is more or less maintained, probably due to steric and polarisation repulsions. Above a sufficiently high salt concentration, emulsions become unstable due to salting-out of the surfactant into the oil-phase. For nonionic surfactants, the situation is similar, except that there are no initial double-layer forces. In addition, Babak [115] found that increasing the electrolyte concentration reduced the barrier to coagulation between emulsion droplets, and therefore increased coalescence. Generally, therefore, stability of o/w HIPEs is not enhanced by salt addition. 

Figure 4 (A) A spherical reversed micelle of a negatively charged micro droplet of water stabilised by cationic surfactant molecules. (B) Schematic representation of the steric interactions in the reversed micelle which favors the formation of linear alkyl rhodium intermediates.

Although mechanical parameters are important, this is not the only area of control. The selection of the correct mix of stabiliser components is also critical. Beverage emulsions are essentially different from food emulsions. Their application in a mobile, liquid phase at concentrations in the region of 0.1% results in the formation of a uniform dispersion of the component droplets, and in order to remain stable and to avoid the effects already mentioned, these droplets must remain discrete from each other and also not interact with other... 

Suspension polymerisation is arguably the simplest and most widely employed preparation method for polymers in beaded form [6], In this method, polymerisation occurs entirely within monomer droplets that are dispersed inside a monomer-immiscible phase the stability of the droplet dispersion is often increased by addition of suitable stabilisers. 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

The stability of emulsions stabilised by proteins arises from the mechanical protection given by the adsorbed films around the droplets rather than from a reduction of interfacial tension. 

Figure 6.21 The influence of emulsifier concentration on the relative viscosity of sorbitan mono-oleate stabilised W/O emulsions in paraffin. The emulsions had dispersed phase volume fractions in the range 0.37 to 0.68 and mean droplet diameters, am, as plotted along the x-axis. From data in Sherman [215].

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

VCM and an emulsifier. These components are circulated through a mixing pump (homogeniser) which causes the mixture to disperse into very fine droplets, before being placed in the autoclave. The droplets are coated with surfactant which stabilises them during the reaction. Initiation and polymerisation occur within the droplets. After polymerisation, the autoclave contains a stable dispersion of fine particles of PVC in water. Thereafter the subsequent operations for obtaining the final product are similar to the emulsion polymerisation process. 

The critical concentration Cc for formation of foam and emulsion bilayers of Do(EO)22 are 4-10 6 mol dm 3 and 1.6 10 5 mol dm 3, respectively, and are in good correlation with the lowest concentrations, 2-31 O 6 mol dm 3 and 10 5 mol dm 3 [421] at which maximum filling of the surfactant adsorption monolayer is attained. It should also be noted that in the case of the emulsion bilayers, CMC < Ce which implies that it is not possible to obtain infinitely stable (i.e. with r = °°) bilayers of Do(EO)22 between two droplets of nonane under the described conditions. For this reason, it may be thought that thermodynamically stable nonane-in-water emulsions stabilised with Do(EO)22 do not exist. 

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