Emulsion stabilisation

An example of the first type is the emulsion stabiliser as exemplified by sodium oleyl sulphate, cetyl pyridinium chloride and poly(ethylene oxide) derivatives. For a number of applications it is desirable that the latex be thickened before use, in which case thickening agents such as water-soluble cellulose ethers or certain alginates or methacrylates may be employed. Antifoams such as silicone oils are occasionally required. 

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. 

The chemical resistance of group (b), frequently used in emulsion or latex paints, is often upset by the presence of water-soluble emulsion stabilisers and thickeners, which remain water soluble in the dried paint film. 

N. Monfreux, P. Perrin, F. Lafuma, and C. Sawdon. Invertible emulsions stabilised by amphiphilic polymers and application to bore fluids (emulsions inversables stabilisees par des polymeres amphiphiles et application a des fluides de forage). Patent WO 0031154, 2000. 

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. 

B. Binks and S.O. Lumsdon Stability of Oil-in-Water Emulsions Stabilised by Sihca Particles. Phys. Chem. Chem. Phys. 1, 3007 (1999). 

B.P. Binks, A.K.R. Dyab, and P.D.I. Rletcher Multiple Emulsions Stabilised Solely by Nanoparticles. In Proceedings of the 3rd World Congress on Emulsions, Lyon, Prance (2002). 

Nugent noted that caustic soda broke a benzene water emulsion stabilised with gelatine, and that the breaking was preceded by an inhibition period which increased with the age of the emulsion as instanced by the following figures. 

An experimental study was performed to determine the applicability of the theory. Oil-in-water (o/w) emulsions, stabilised with anionic surfactants, were prepared, with known quantities of added electrolyte, and were creamed by either gravitation or centrifugation. The results can be summarised as follows at low electrolyte concentrations, where h would have a finite value, <(> was less than 0.74. Over a range of concentrations, where it was assumed that both 0 and h were negligible, = 0.74 ( 0.02). The emulsions were found to be polydis-perse, so this did not appear to affect the volume fraction to a great extent. In addition, < > was found to be independent of the method of cream formation. 

Water-in-fluorocarbon emulsions, stabilised with fluorinated nonionic surfactants, were investigated by small angle neutron scattering (SANS) spectroscopy [8,99]. The results indicated that the continuous oil phase comprised an inverse micellar solution, or water-in-oil microemulsion, with a water content of 5 to 10%. However, there was no evidence of a liquid crystalline layer at the w/o interface. A subsequent study using small angle x-ray scattering (SAXS) spectroscopy gave similar results [100]. 

Fechner, A., Knoth, A., Scherze, I., Muschiolik, G. (2007). Stability and release properties of double-emulsions stabilised by caseinate-dextran conjugates. Food Hydrocolloids, 21, 943-952. 

A few studies have reported the embedding of an MIP film between two membranes as a strategy for the construction of composite membranes. For example, a metal ion-selective membrane composed of a Zn(II)-imprinted film between two layers of a porous support material was reported [253]. The imprinted membrane was prepared by surface water-in-oil emulsion polymerisation of divinylbenzene as polymer matrix with 1,12-dodecanediol-0,0 -diphenylphosphonic acid as functional host molecule for Zn(II) binding in the presence of acrylonitrile-butadiene rubber as reinforcing material and L-glutamic acid dioleylester ribitol as emulsion stabiliser. By using the acrylonitrile-butadiene rubber in the polymer matrix and the porous support PTFE, an improvement of the flexibility and the mechanical strength has been obtained for this membrane. 

B.W. Barry and G.M. Eccleston, Influence of gel networks in controlling consistency of o/w emulsions stabilised by mixed emulsifiers, J. Texture Stud. 4 (1973) 53-81. 

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. 

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... 

Breaking of Emulsions Emulsions can be broken or demulsified to get the constituent liquids by heating, freezing, centrifuging or by addition of appreciable amounts of electrolytes. They are also broken by destroying the emulsifier. For example, an oil-water emulsion stabilised by soap is broken by the addition of a strong acid. The acid converts soap into insoluble free fatty acids. 

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. 

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... 

Zn(II) was employed as a print molecule because of its strong interaction with the bifunctional monomer, DDDPA. Divinylbenzene, L-glutamic acid dioleylester ribitol and toluene were used as matrix-forming monomer, emulsion stabiliser and diluent, respectively. After polymerisation, the print molecules were removed from the resin, upon which selective recognition sites were formed. The schematic illustration of surface template polymerisation with DDDPA is shown in Scheme 9.8. The Zn(II)-imprinted resins were ground into particles, whose volume-averaged diameters were ca. 40 pm. The yield was ca. 80%. 

Figure 7.14 The change in critical HLB values as a function of added salt concentration, where the salt is either NaCl or Nal. Results were obtained from measurements of particle size, stability, viscosity and emulsion type as a function of HLB for liquid paraffin-in-water emulsions stabilised by Brij 92-Brij 96 mixtures. Data from different experiments showed different critical values hence, on each diagram hatching represents the critical regions while data points actually recorded are shown. Results in (a) show particle size and stability data those in (b) show the HLB at transition from pseudoplastic to Newtonian flow properties (see section 7.3.10) and emulsion type (o/w— w/o) transitions.

Figure 7.26 The relative viscosities of w/o emulsions stabilised with sorbitan trioleate four emulsions have been studied with different mean particle diameters, D ,.

The ability of some macromolecules to adsorb at interfaces is made use of in suspension and emulsion stabilisation (see Chapter 7). Gelatin, acacia, poly(vinyl alcohol) and proteins adsorb at interfaces. Sometimes such adsorption is unwanted, as in the case of insulin adsorption onto glass infusion bottles and poly(vinyl chloride) infusion containers and tubing used in giving sets. Adsorption of insulin to glass bottles and plastic i.v. tubing at slow rates of infusion is well documented. It... 

Long-term stability of emulsified systems is usually provided by proteins or polysaccharides. The role of a good emulsion stabiliser is to keep the droplets apart once they have been formed. This protects the emulsion against processes such as creaming, flocculation and coalescence during long-term storage. 

Low juice content is defined by a content of 1 to 20% by weight of juice in the final drink. These products are mainly based on citrus juices. As mentioned before, citms juices provide good turbidity and stability even with low juice content. In combination with special raw materials made from citrus, like cloudy concentrates or peel oils, very stable low-juice drinks can be obtained. Turbidity comes from a complex mixture of dispersions of fruit cells and oil-in-water emulsions, stabilised by fruit pulp particles and hydrocolloid stabilisers, such as locust bean gum [9]. 

Emulsion coalescence This can occur if the emulsifier is not strongly adsorbed at the O/W or W/O interface, and results in its partial or complete displacement by the suspension dispersant which is not a good emulsion stabiliser. This in turn... 

Model suspoemulsions of polystyrene latex (mean volume diameter 1.84 pm) were prepared using the surfactant-free method, and then stabilised with Synperonic PE P94 (an ABA block of PEO-PPO-PEO with 47 PPO units and 42 PEO units) and isoparaffinic O/W emulsion (stabilised with Synperonic PE L92 with 47 PPO units and 16 PEO units). 

Figure 12.8 Viscosity-volume fraction curves for W/O emulsions stabilised with PHS-PEO-PHS block copolymer.

Figure 12.10 Variation of C and C" with

Microemulsions are a special class of dispersions (transparent or translucent) which actually have little in common with emulsions, and are better described as swollen micelles. The term microemulsion was first introduced by Hoar and Schulman [1,2] who discovered that the titration of a milky emulsion (stabilised by a soap, such as potassium oleate) with a medium-chain alcohol such as pentanol or hexanol led to the production of a transparent or translucent system. A schematic representation of the titration method adopted by Schulman and coworkers is given below ... 

ZnS nano crystals were synthesised in ternary w/o micro emulsion stabilised by non-ionic or cationic surfactants [44]. Several morphologies, e.g. nanorods or spherical or ellipsoidal ZnS particles were obtained by varying the to values. The product morphology was also found to be function of the absolute reactant concentration and concentration ratio of Zn2+ to S2, the incubation time and the ambient temperature. 

Fig. 1.14. Mechanism of emulsion stabilisation a) electric repulsion, b) steric hindrance According to Hamaker (1937) the Free Energy of the attraction forces is approximately...

In determining the emulsification temperature for emulsions stabilised with EO containing nonionics, the consideration of the phase inversion temperature (PIT or HLB-temperature) suggested by Shinoda and co-workers [193] can be also important in order to select the surfactant of optimum HLB. The PIT of an emulsion depends not only on the structure of surfactant(s), but also on many other parameters, such as the surfactant concentration, nature of the oil, phase ratio, or the presence of salts. The lowest interfacial tension at the PIT is the important factor for obtaining emulsions with small average droplet size and hence good stability. 

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