Surfactant phase behaviour

The quantitative determination of surfactant concentration in solution is an essential part of any experimental work on surfactant adsorption or phase behaviour. In the field of experimental enhanced oil recovery the technique employed should be capable of determining surfactant concentrations in sea water, and in the presence of oil and alcohols, the latter being frequently added as a co-surfactant. 

Figure 2. Phase Behaviour or Bonnie Glen Crude, Distilled Water, and Surfactant System...

Figure 4. Phase Behaviour of Bradford Crude, Brine, and Surfactant (PRL-5A) System...

It is important for the theoretical understanding of the formation of various topologies that these aggregates have entropic contributions on the scale of the objects, i.e. on a much larger scale than set by the molecules. These cooperative entropic effects should be included in the overall Helmholtz energy, and they are essential to describe the full phase behaviour. It is believed that the mechanical parameters discussed above kc,k and J0, control the phase behaviour, where it is understood that these quantities may, in principle, depend on the overall surfactant (lipid) concentration, i.e. when the membranes are packed to such a density that they strongly interact. 

In comparison with System A, we thus find that different surfactants would give divergent phase behaviour, due to the dependence of Yow surfactant characteristics. Further, the addition of n-butanol gives rise to a lowering of yow by about 2 mN/m (Fi-... 

Older compilations about the state of the art can be found in several review articles [41 -47]. It is surprising that most work is carried out with the surfactant bis-ethylhexyl-sulfosuccinate (tradename AOT or Aerosol OT). The reasons seem to be the variability of the obtained reverse micelles (from very low up to high water concentrations) and the well-known phase behaviour of AOT with water and several oils [48,49]. AOT is approved for medical application, e.g. as an additive in suppositories, but not for food engineering. 

Regarding the phase behaviour of ternary mixtures of water, oil and a nonionic surfactant. Fig. 3 shows schematically the so-called one-phase channel. This sec-... 

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. 

Experiment 5.2 Determination of the phase behaviour of concentrated surfactant solutions... 

Equilibrium Phase Behaviour. Phase studies were performed using approximately 10 g samples of oil-surfactant mixture diluted sequentially by the weighed addition of water. The initial binary mixture contained 5-70 w/w surfactant at 5 intervals. Phase boundaries were determined to + 0.5 water. The ternary mixtures in Pyrex glass tubes fitted with PTFE lined caps were equilibrated to the required temperature (20-65 0.1°C) for 2 hours and then thoroughly mixed for 5 minutes using a Fisons orbitsil whirlimixer. The tubes were then returned to the waterbath and left undisturbed for 48 hours before identification of the phase type using a crossed polarised viewer and an optical microscope. 

Phase Behaviour. The differences in the self-emulsifying behaviour of Tagat TO - Miglyol 812 binary mixtures can, in part, be explained from considerations of their phase behaviour. Figures 4a-4d show the representative equilibrium phase diagrams obtained when binary mixtures containing 10,25,30 and 40J surfactant were sequentially diluted with water. The phase notation used is based on that of Mitchell et ai (li). 

During the studies of phase behaviour two types of liquid crystalline phases were identified. LC material was viscous and exhibited intense "white" birefingence. material was apparently homogeneous but of low viscosity and exhibited "multi-coloured" birefringence. The liquid crystalline phases observed in the equilibrium studies of surfactant concentrations up to 25 are unlikely to take part in the self-emulsification process due to the presence of two-phase regions between L2 and liquid crystalline phases however, LC material may account for the improved stability of emulsions formed by 25 surfactant systems (Table II). Figure 4c indicates that by increasing the surfactant concentration to 30 the... 

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. 

As a practical example for the phase behaviour of surfactants, Figure 3.18 shows the phase diagram of a pure non-ionic surfactant of the alkyl polyglycol ether type C Em. n denotes the length of the hydrocarbon chain and m the degree of ethoxylation [20]. 

The phase behaviour can have a significant impact on detergency [21] but, if there is no phase change for the surfactant-water system, a linear dependence of detergency on temperature is observed (Figure 3.19). 

Figure 3.20 Phase behaviour of the polyoxyethylene alcohol C12E3 and detergency, 2 gl 1 surfactant (reproduced with permission [22]).

Figure 3.22 (right) represents the three-phase temperature intervals for Q2E4 and Q2E5 vs the number n of carbon atoms of n-alkanes (for the phase behaviour of ternary systems see Section 3.4.2, Figure 3.26). The left part of Figure 3.22 shows the detergency of these surfactants for hexadecane. Both parts of Figure 3.22 indicate that the maximum oil removal is in the three-phase interval of the oil used (n-hexadecane) [22]. This means that not only the solubilisation capacity of the concentrated surfactant phase, but probably also the minimum interfacial tension existing in the range of the three-phase body is responsible for the maximum oil removal. Further details about the influence of the polarity of the oil, the type of surfactant and the addition of salt are summarised in the review of Miller and Raney [23].

The Kahlweit fish, however, is only a special case for a fixed water/oil ratio of an even more complex phase behaviour of the ternary system water/oil/surfactant. The more general... 

Undoubtly, phase diagrams are the most convenient. The above discussed is in fact the first attempt to construct such a diagram for foam bilayers. Phase diagrams of surfactant solutions indicating the system state can be found in the monograph of Laughlin The Aqueous Phase behaviour of Surfactants [455] and are of major practical importance. 

R.G. Laughlin, The Aqueous Phase Behaviour of Surfactants, Academic Press, London,... 

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