Phase behaviour microemulsions

Shinoda, K., Araki, M.,Sadaghiani, A., Khan, A., and Lindman, B. (1991), Lecithin-based microemulsions phase behaviour and microstructure,/. Phys. Chem., 95, 989-993. 

Salt and cosuifactants are common additives used to tune microemulsion phase behaviour. Less common is the addition of polymers, i.e. additives that increase the viscosity in addition to influencing the phase behaviour. Polymers of interest may be water-soluble, oil-soluble, or contain hydrophilic and hydrophobic groups and thus be interfacially active. 

Burauer, S., Sachert, T., Sottmann, T. and Strey, R., On microemulsion phase behaviour and the monomeric solubility of surfactant, Phys. Chem. Chem. Phys., 1, 4299 (1999). 

Microemulsion structure is strongly linked to phase behaviour and therefore in this context it is useful to also consider the microemulsion phase behaviour. Oil-in-water droplet microemulsions are... 

Emulsions, foams and defoaming, microemulsion phase behaviour, coagulation theory Interplay of adsorption of polymers-surfactants-proteins, rheological properties, electrophoresis... 

Robbins, M. L., "Theory for the Phase Behaviour of Microemulsions", Paper No 5839, presented at the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, Tulsa, Oklahoma, March 22-24 (1976). 

The choice of surfactant, which is mostly constrained by the choice of the oil and the resulting phase behaviour of the microemulsion, can have different effects on the enzyme stability and activity. In general we have to differentiate between ionic and nonionic surfactant types ... 

In contrast to nonionic surfactants, ionic surfactants build up a high zeta-po-tential at the water-oil interface which can also can influence the enzyme activity. Most investigated systems used AOT as the surfactant because its phase behaviour is well understood. However, AOT is often not very suitable, because it can totally inhibit enzymes (e.g. the formate dehydrogenase from Candida bodinii). The usage of lipases in AOT-based microemulsions is generally unfavourable as AOT is an ester that is hydrolysed itself. 

The majority of the published investigations are concentrated onto the reaction conditions of enzymes in reverse micelles at low substrate concentrations, because high substrate concentrations in microemulsions influence their phase behaviour. Additionally, high substrate and enzyme concentrations often lower the enzyme stability to uneconomical values. At high enzyme concentrations the activity can be lowered due to the formation of protein aggregates. 

The conditions required to form an emulsion of oil and water and a microemulsion. The complex range of structures formed by a microemulsion fluid. Emulsion polymerization and the production of latex paints. Photographic emulsions. Emulsions in food science. Laboratory project on determining the phase behaviour of a microemulsion fluid. 

The Winsor microemulsion classification system distinguishes among three different types based on their phase behaviour ... 

Several types of phase behaviour occur in microemulsions they are denoted as Nelson type IT, type II+, and type III. These designations refer to equilibrium phase behaviours and distinguish, for example, the number of phases that can be in equilibrium and the nature of the continuous phase. Winsor-type emulsions are similarly identified, but with different type numbers. 

Several categories of microemulsions that refer to equilibrium phase behaviours and that distinguish, for example, the number of phases that can be in equilibrium and the nature of the continuous phase. They are denoted as Winsor Type I (oil-in-water), Type II (water-in-oil), Type III (most of the surfactant is in a middle phase with oil and water), and Type IV (water, oil, and surfactant are all present in a single phase). The Winsor Type III system is sometimes referred to as a middle-phase microemulsion , and the Type IV system is often referred to simply as a microemulsion . An advantage of the Winsor category system is that it is independent of the density of the oil phase and can lead to less ambiguity than do the lower-phase or upper-phase microemulsion type terminology. Nelson type emulsions are similarly identified, but with different type numbers. 

E. Ruckcnstcin Thermodynamics of microemulsions relation between their structure and phase behaviour, FLUID PHASE EQUILIBRIA 20(1985) 189-206. 

THERMODYNAMICS OF MICROEMULSIONS RELATION BETWEEN THEIR STRUCTURE AND PHASE BEHAVIOUR... 

Schurtenberger, P., Peng, Q., Leser, M. E., and Luisi, P. L. (1993), Structure and phase behaviour of lecithin-based microemulsions A study of chanin length dependence, J. Colloid Interface Sci, 156,43-51. 

M.J. Barlow, D.J. Prediction of phase behaviour in microemulsion systems using artificial neural networks. J. Coll. 

Warisnoicharoen W, Lansley AB, Lawrence MJ. Non-ionic oil-in-water microemulsions the effects of oil type on phase behaviour. Int J Pharm 2000 198(1) 7-27. 

Surfactants find apphcation in almost all disperse systems that are utilised in areas such as paints, dyestulfs, cosmetics, pharmaceuticals, agrochemicals, fibres, and plastics. Therefore, a fundamental understanding of the physical chemistry of surface-active agents, their unusual properties, and their phase behaviour is essential for most formulation chemists. In addition, an understanding of the basic phenomena involved in the application of surfactants, such as in the preparation of emulsions and suspensions and their subsequent stabilisation, in microemulsions, in wetting, spreading and adhesion, is vitally important to arrive at the correct composition and control of the system involved [1, 2]. This is particularly the case with many formulations in the chemical industry mentioned above. 

Phase Behaviour, Interfacial Tension and Microstructure of Microemulsions... 

The extensive research on microemulsions was prompted by two oil crises in 1973 and 1979, respectively. To optimise oil recovery, the oil reservoirs were flooded with a water-surfactant mixture. Oil entrapped in the rock pores can thus be removed easily as a microemulsion with an ultra-low interfacial tension is formed in the pores (see Section 10.2 in Chapter 10). Obviously, this method of tertiary oil recovery requires some understanding of the phase behaviour and interfacial tensions of mixtures of water/salt, crude oil and surfactant [4]. These in-depth studies were carried out in the 1970s and 1980s, yielding very precise insights into the phase behaviour of microemulsions stabilised by non-ionic [5, 6] and ionic surfactants [7-9] and mixtures thereof [10]. The influence of additives, like hydro- and lyotropic salts [11], short- and medium-chain alcohols (co-surfactant) [12] on both non-ionic [13] and ionic microemulsions [14] was also studied in detail. The most striking and relevant property of micro emulsions in technical applications is the low or even ultra-low interfacial tension between the water excess phase and the oil excess phase in the presence of a microemulsion phase. The dependence of the interfacial tension on salt [15], the alcohol concentration [16] and temperature [17] as well as its interrelation with the phase behaviour [18, 19] can be regarded as well understood. 

The fact that microemulsions have gained increasing importance both in basic research and in industry is reflected in the large number of publications on microemulsions. A survey of paper titles reveals that the number of papers on the subject of microemulsions increased within the last 30 years from 474 in 1976-1985 to over 2508 in 1986-1995 and to 6691 in 1996-2005.1 The fact that micro emulsions also provide the potential for numerous practical applications is mirrored in the number of patents filed on this topic. A survey of patents on microemulsions2 shows an increase from 159 in 1976-1985 to over 805 in 1986-1995 and to 2107 in 1996-2005. In the following the basic properties of microemulsions will be presented concentrating on the close connection between the phase behaviour and the interfacial tensions as well as on the fascinating microstructure. 

The primary aim of microemulsion research is to find the conditions under which the surfactant solubilises the maximum amounts of water and oil, i.e. the phase behaviour has to be studied. As the effect of pressure on the phase behaviour is (in general) rather weak [30 ], it is sufficient to consider the effect of the temperature. Furthermore, it hasbeen shown that simple ternary systems consisting of water, oil and non-ionic n-alkyl polyglycol ethers (QEj) exhibit all properties of complex and technically relevant systems [6]. Therefore, we will first describe the phase behaviour of ternary non-ionic microemulsions. 

Considering now the variation of the phase behaviour with increasing mass fraction y of surfactant one can see that the volume of the respective microemulsion phase increases (see test tubes in Fig. 1.3(b)) until the excess phases vanish and a one-phase microemulsion is found. The optimal state of the system is the so-called X-point where the three-phase body meets the one-phase region. It defines both the minimum mass fraction y of surfactant needed to solubilise water and oil, i.e. the efficiency of the surfactant, as well as the corresponding temperature f, which is a measure of the PIT. 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

Figure 1.7 Vertical sections T(wb) and 7 (wA) through the phase prism which start at the binary water-surfactant (wb = 0) and the binary oil-surfactant (wA = 0) corner, respectively. These sections have been proven useful to study the phase behaviour of water- and oil-rich microemulsions, (a) Schematic view of the sections T wg) and T(wA) performed at a constant surfactant/fwater + surfactant) mass fraction ya and at a constant surfactant/(oil + surfactant) mass fraction 7b, respectively, (b) T(wb) section through the phase prism of the system FhO-n-octane-CioEs at ya = 0.10. Starting from the binary system with increasing mass fraction of oil wg, the oil emulsification boundary (2- 1) ascends, while the near-critical phase boundary (1 - 2) descends, (c) T(wA) section through the phase prism of the system EbO-n-octane-QoEs at 7b = 0.10. The inverse temperature behaviour is found on the oil-rich side With increasing fraction of water wA the water emulsification boundary (1 - 2) descends, whereas the near-critical phase boundary (2 —> 1) ascends.

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