Macroemulsions stability

RD.I. Eletcher and D.I. Horsup Droplet Dynamics in Water-in-Oil Microemulsions and Macroemulsions Stabilized by Non-Ionic Surfactants. J. Chem. Soc. Earaday Trans. I 88, 855 (1992). 

A.S. Kabalnov and H. Wennerstrom Macroemulsion Stability The Oriented Wedge Theory Revisited. Langmuir 12, 276 (1996). 

A.S. KabalnovandJ. Weers Macroemulsion Stability Within the Winsor 111 Region Theory Versus Experiment. Langmuir 12, 1931 (1996). 

E. Ruckenstein Thermodynamic insights on macroemulsion stability, ADVANCES IN COLLOD AND INTERFACE SCIENCE 79 (1999) 59-76. 

Keywords Macroemulsion stability Hydrophilic-hydrophobic balance (HLB) Interfacial tension vs. HLB Surface excess vs. HLB Surface excess vs. temperature Phase inversion temperature Bancroft rule... 

The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability in terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10- dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. 

Physical Nature of the Interfacial Film The droplets of dispersed liquid in an emulsion are in constant motion, and therefore there are frequent collisions between them. If, on collision, the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures, the two droplets will coalesce to form a larger one, since this results in a decrease in the free energy of the system. If this process continues, the dispersed phase will separate from the emulsion, and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. 

Discuss the changes in interfacial tension that occur in the conversion of an O/W macroemulsion stabilized by a POE nonionic surfactant to a W/O macroemulsion upon raising the temperature above the cloud point. 

Fletcher, P.D.I. and Horsup, D.I. (1992) Droplet dynamics in water-in-oil microemulsions and macroemulsions stabilized by non-ionic surfactants - correlation of measured rates with monolayer bending elasticity. /. Chem. Soc. Faraday Trans., 88, 855-864. 

Kabalnov A, Wennerstrom H. Macroemulsion stability the oriented wedge theory revisited. Langmuir 1996 12 276-292. 

Perrin P, Monfreux N, Thierry F, Lafuma F, Lequeux F. Concentrated direct and inverse macroemulsions stabilized by amphiphilic polyelectrolytes. Proc ACS Div Polym Mater Sci Eng 1999 81 492-494. 

Droplet size plays an important role in macroemulsion stability. Forming a macroemulsion requires energy input because of the increased interfacial area between the oil and water phases (Fig. 12). Conversely, as the emulsion breaks and the system returns to the original state, energy is released. In the example in Fig. 12, 31 J of work is required to form one mieron droplet of 100 mLs of heptane in 1 L of water. If smaller droplets were... 

A Kalbanov, J Weers. Macroemulsion stability within the Winsor III region Theory versus experiment. Langmuir 12 1931—1935, 1996. 

A. Kabalnov and H. Wennerstrom, Macroemulsion stability The oriented wedge theory revis-... 

Macroemulsion stability between aqueous alkaline and acidic oil systems The rates of flocculation and/or coalescence can be inferred from plots of the ratio of the emulsion volume to the total volume, V /V.p, versus time as shown in Figures 11 and 12. 

Fig. 9.3. Macroemulsion stability diagram of cyclohexane-water-polyoxyethylene (9.7) nonyl phenol ether system.

Ruckenstein, E., Thermodynamic insights on macroemulsion stability, Adv. Colloid Interface Sci., 79, 59 (1999). 

This chapter has the following outline. First, an introduction to the behavior of thermodynamically stable equilibrium oil-water-surfactant (O-W-S) systems is given. The main discussion topics are the phase equilibria, surfactant monolayer elasticity, and the interfacial tensions between the co-existing phases. This section is necessary to introduce the terminology and theoretical formalism which will then be used throughout the chapter. In the next section, an overview of experimental trends in macroemulsion stability is made. Next, the theory of hole nucleation in emulsion films is presented, which is followed by comparison of the theory with experiment. 

Since the basic ideas in surfactant science have been introduced, one can now proceed with the main topic of this chapter macroemulsion stability. Macroemulsions are formed by mechanical mixing of oil and water in the presence of surfactants, e.g. by mixing the phases of the Winsor 1 equilibrium in each other. As a result of mixing, one of the phases breaks into macroscopic droplets, while the other stays continuous. Macroemulsions are thermodynamically unstable and gradually resolve with time into two distinct layers. However, in some cases they show a remarkable kinetic stability. Most experimental trends in macroemulsion stability were established a long time ago and will be outlined below. 

There are two more subtle features of the nonionic macroemulsion stability to be discussed. Firstly, within the Winsor III region, the stability of macroemulsions is very temperature sensitive. Although exactly in the balanced state, the macroemulsions are very unstable and break within minutes, the system becomes stable only several tenths of a degree away from the balanced point, while still being within the Winsor III region. Secondly, the macroemulsion stability pattern is not completely symmetric. W/O emulsions reach maximum stability at ca. 20 C above the balanced point, after which the stability starts to decrease. On the other... 

The Kabalnov-Weimerstrom theory provides a qualitative explanation for the oriented wedge trend in macroemulsion stability, i.e. it explains the PIT and Optimal Surfactant Formulation behavior of macroemulsions, as well as the multilamellar stabilization. For a quantitative comparison, the data for only one system is currently available.A detailed comparison will be made below. 

In all the experimental studies discussed below, the macroemulsion stability was characterized by the time for the resolution of one half of the emulsified disperse phase Ti/2, similar to how it had been done before by Salager et a/. All the macroemulsions contained 50 vol% of disperse phase and were prepared by handshaking in a jacketed beaker under careful temperature control. The emulsification was quite easy to perform because of the very low interfacial tensions in the system. The experimental reproducibility in r,/2 values was fairly good (within 20%) in the Winsor III experiment, provided that the temperature was controlled to within 0.01 °C. In other words, the way in which the macroemulsions were prepared... 

A characteristic example of the macroemulsion stability dependence on the surfactant concentration is shown in Fig. 7.25a and involves almost the same system as the one discussed above, with the oil changed for -heptane. The experiments were conducted at 20 C, where the system above the CMC is in the Winsor I state and tends to form 0/W emulsions. The transition from very... 

After many years of emulsion research, several reliable empirical trends in macroemulsion stability to coalescence have been established. The most powerful rules of thumb are (i) the correlation between the surfactant spontaneous curvature and the macroemulsion stability and (ii) the tight monolayer packing concept. Other empirical correlations, HLB scale and Bancroft s rule, can be substantially reduced to these two. In rare cases when the oriented wedge trend and the HLB scale, or Bancroft s rule, disagree, nature usually follows the oriented wedge trend. Thus, Bancroft s rule often fails for nonionic surfactants of the ethylene oxide type below the balanced point, while the oriented wedge trend does not. 

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