Solid-stabilised emulsions

I. Aranberri, B. P. Binks, J. H. Clint, and P. D. I. Fletcher, "Synthesis of macroporous silica from solid-stabilised emulsion templates," J. Porous. Mater.,16 (2009) 429-437. 

Sinks, B.P. and Kirkland, M. (2002) Interfadal stmcture of solid-stabilised emulsions studied by scanning electron microscopy. Physical Chemistry Chemical Physics, 4, yin-ilii. 

Microbes were frequently found to synthesise surface-active molecules in order to mobilise hydrophobic organic substrates. These biosurfactants, which are either excreted by the producing organisms or remain bound to their cell surfaces, are composed of a hydrophilic part making them soluble in water and a lipophilic part making them accumulate at interfaces. With respect to their physical effects, one can distinguish two types of biosurfactants firstly, molecules that drastically reduce the surface and interfacial tensions of gas-liquid, liquid-liquid and liquid-solid systems, and, secondly, compounds that stabilise emulsions of nonaqueous phase liquids in water, often also referred to as bioemulsifiers. The former molecules are typically low-molar-mass... 

AKDs are waxy, water-insoluble solids with melting points around 50 °C, and ASAs are viscous water-insoluble liquids at room temperature. It is necessary to prepare them as stabilised emulsions by dispersion in a cationic polymer (normally cationic starch). Small amounts of retention aid and surfactants may also be present. Particle size distributions are around 1 fim, and addition levels around 0.1% (of pure AKD or ASA) by weight of dry fibre. This is an order of magnitude lower than the amount of rosin used in rosin-alum sizing (1-2%). Emulsions of AKD are more hydrolytically stable than ASA, and the latter must be emulsified on-site and used within a few hours. 

Pal, R. and Hwang. C-Y. J. 1999. Loss coefficients for flow of surfactant-stabilised emulsions through pipe components. Trans. IChemE, Vol.77 (Part A) 685-691 Pienaar, V.G. Slatter, P.T. (2004). Interpretation of experimental data for fittings losses 12th International Conference on Transport and Sedimentation of Solid Particles, pp. 537 - 546, ISBN 80-239-3465-1, Prague, Czech Republic September 20-24, 2004... 

Fillers are often employed to reduce the surface tack of the final product. Examples are talc and china clay. If powdered materials are added directly to a latex they compete for the emulsion stabiliser present and tend to coagulate the latex. They are therefore added as an aqueous dispersion prepared by ball milling the filler with water and a dispersing agent, for example a naphthalene formaldehyde sulphonate at a concentration of about 1% of the water content. Heat and light stabilisers which are solids must be added in the same way. 

Electrodeposition This method of paint application is basically a dipping process. The paint is water-based and is either an emulsion or a stabilised dispersion. The solids of the paint are usually very low and the viscosity lower than that used in conventional dipping. The workpiece is made one electrode, usually the cathode, in a d.c. circuit and the anode can be either the tank itself or suitably sized electrodes sited to give optimum coating conditions. The current is applied for a few minutes and after withdrawal and draining the article is rinsed with de-ionised water to remove the thin layer of dipped paint. The deposited film is firmly adherent and contains a minimum of water and can be stoved without any flash-off period. This process is used for metal fabrications, notably car bodies. Complete coverage of inaccessible areas can be achieved and the corrosion resistance of the coating is excellent (Fig. 14.1). 

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. 

The idea of the preparation of porous polymers from high internal phase emulsions had been reported prior to the publication of the PolyHIPE patent [128]. About twenty years previously, Bartl and von Bonin [148,149] described the polymerisation of water-insoluble vinyl monomers, such as styrene and methyl methacrylate, in w/o HIPEs, stabilised by styrene-ethyleneoxide graft copolymers. In this way, HIPEs of approximately 85% internal phase volume could be prepared. On polymerisation, solid, closed-cell monolithic polymers were obtained. Similarly, Riess and coworkers [150] had described the preparation of closed-cell porous polystyrene from HIPEs of water in styrene, stabilised by poly(styrene-ethyleneoxide) block copolymer surfactants, with internal phase volumes of up to 80%. 

We have already referred to the use of stabilisers in the production of water-dispersible flavourings and/or emulsion based cloudifying systems. Such additives, as well as contributing to stability, are also used, where appropriate, in soft drink formulations to impart stability to natural clouds, for example, dispersions of fruit solids, and to improve mouthfeel characteristics by increasing the viscosity of the dr ink. 

Several theories relating to emulsion type have been proposed. The most satisfactory general theory of emulsion type is that originally proposed for emulsions stabilised by finely divided solids (see Figure 10.1). If the solid is preferentially wetted by one of the phases, then more particles can be accommodated at the interface if the interface is convex towards that phase (i.e. if the preferentially... 

Figure 16.1 Stabilisation of emulsions by finely divided solids (a) preferential wetting by water leading to an O/W emulsion (b) preferential wetting by oil leading to a W/O emulsion...

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

The use of finely dispersed solid particles as stabilisers of O/W and W/O emulsions has been known since last century. In a number of works [73-77] it has been established that there is a close relationship between the type and stability of emulsions, stabilised by solid particles and the value of the contact angle at the solid body/water/organic liquid interface. A... 

The homogeneity of solutions and gas mixtures is also easy to maintain or to reconstitute. The vast majority of calibration CRMs are pure solid substances or solutions. The only difficulty encountered by the manufacturer is the availability of adapted preparation tanks in which large volumes of solutions can be handled, with sufficient stirring capabilities, and without any risk of contamination. Emulsions need to be stabilised through adequate addition of emulsifiers or dispersion products. Such additives may pose the problem of representativeness of the final material. 

Demulsification can be usefully broken down into a series of stages. In the first instance, droplets must approach one another to form a loose aggregate. At this point, it is important to sweep up small droplets before the next stage, coalescence, progresses too far. During coalescence, there is a dramatic reduction in interfacial area, resulting in the concentration of solids and other debris at the interface. These materials, which may have contributed to the stabilisation of the original emulsion, must be dealt with by dispersal into the bulk of one or other of the liquid phases. The formation of distinct phases can then be completed and the water removed. 

Aveyard R, Clint JH, Horozov TS (2003) Aspects of the stabilisation of emulsions by solid particles effects of line tension and monolayer curvature energy. Phys Chem Chem Phys 5(ll) 2398-2409... 

Conditions can be deduced from the energy balance mentioned above under which the rewetting tension is positive, a precondition for the mechanism shown in Fig. 1.12. It should be said that re-wetting is the first step in the complex process of removal of hydrophobic layers from a solid. The oil droplets formed must also be sufficiently stabilised by the surfactant to prevent them coalescing. This takes place in many cleaning processes. When a water-insoluble substance is to be dispersed in water we can distinguish between thermodynamically stable and unstable dispersions. Thermodynamically unstable dispersions are the usual emulsions or dispersions of solids. Solubilisation systems and optically transparent emulsions, so-called micro-emulsions, are in a metastable state where drop growing by collision and coalescence cannot be completely suppressed. These systems are frequently called thermodynamically stable. 

Essential criteria for the preparation of emulsions, solubilisates and micro-emulsions will be dealt with in more detail below. On the subject of stabilisation and flocculation we will restrict ourselves to emulsions rather than dispersions in general as the basic laws are transferable. However, dispergates of solid particles are more difficult to treat than emulsions because of their often rough and inhomogeneous surface structure. With emulsions experimental complications arise over drop size distribution caused by the mechanical work input during emulsification, the nature of the emulsifier and by time, which is of considerable importance too. 

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. 

Emulsions, like foams, can also be stabilised by finely divided solids, provided the properties of the solid/liquid/liquid interface are appropriately adjusted. These properties may also determine whether an O/W or a W/O emulsion is formed. For example, shaking water and benzene together with finely divided calcium carbonate yields a benzene-in-water emulsion. On the other hand, if the calcium carbonate is made hydrophobic by treatment with oleic acid solution, a water-in-benzene emulsion results. 

High solids polymer latices up to 70% solids content are prepared by conventional emulsion polymerisation from different monomer systems. Emulsifier but also polyvinyl alcohol stabilisation is used. Polymerisation process is executed as a two-step seeded but also as a single-step process. When using known concepts for the control of viscosity for high solids, a high shear viscosity below 50 mPas is achieved even for 70% solids. Quantitative mathematical evaluation of the viscosity depending on bimodal particle size distribution and on the hydrodynamic particle surface layer is possible. 8 refs. 

Another technique for overcoming the solids/viscosity conundrum is demonstrated in a patent by American Cyanamid. Here a conventional water soluble acrylic is prepared as a dispersion and is then used as a colloid stabiliser for further emulsion polymerisation. This polymerisation occurs within the dispersion droplets that have already been formed. This allows higher solids solutions to be formed with low viscosities, and low cosolvent contents ... 

The fact that the emulsions are free of added surfactant or polymeric stabilisers is of some significance in that the droplet/solution interface is in this case a truly fluid one, whereas those bearing adsorbed stabilisers are invariably viscoelastic, exhibiting Gibbs-Marangoni effects. This should be of interest to those wishing to carry out experiments to test the various hydrodynamic theories of liquid droplets (e.g. diffusion, sedimentation, viscosity, electrophoresis), compared to solid parti-... 

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