Phase behaviour micelles

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

Fig. 67 Schematic of phase behaviour for blend of novolac epoxy resin with nearly symmetric poly(methyl acrylate-co-glycidylmelhacrylate)-0-polyisoprene. Ordered L can be swollen with up to about 30% of resin before macroscopic phase separation occurs, producing heterogeneous morphologies containing various amounts of L, C, worm-like micelles and pristine epoxy. At lower concentrations, disordered worm-like micelles transform into vesicles in dilute limit. According to [201]. Copyright 2003 Wiley...

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. 

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 phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

The phase behaviour of PEO PBO has recently been determined in detail, including the effect of addition of the salt K2S04 (Deng et al. 1995). Increasing the concentration of aqueous K2S04 reduces the upper sol-gel transition as shown in Fig. 4.15 however, it has a much weaker effect on the lower gel boundary. This is because the effect of salt in reducing the micellar expansion factor (<5t) is compensated at the lower boundary by more favourable conditions for formation of micelles in the poorer solvent (i.e, a lower cmc) whereas no such compensation is possible at the upper boundary. Regions of clear and cloudy,... 

Valdes-Diaz G, Rodriguez-Calvo S, Perez-Gramatges A, Rapado-Paneque A, Femandez-Lima FA, Ponciano CR, da Silveira EF (2007) Effects of gamma radiation on phase behaviour and critical micelle concentration of Triton X-100 aqueous solutions. J Colloid Interface Sci 311(1) 253—261... 

The phase behaviour of biomimetic polypeptide-based copolymers in solution was described and discussed with respect to the occurrence of secondary structure effects. Evidently, incorporation of crystallisable polypeptide segments inside the core of an aggregate has impact on the curvature of the corecorona interface and promotes the formation of fibrils or vesicles or other flat superstructures. Spherical micelles are usually not observed. Copolymers with soluble polypeptide segments, on the other hand, seem to behave like conventional block copolymers. A pH-induced change of the conformation of coronal polypeptide chains only affects the size of aggregates but not their shape. The lyotropic phases of polypeptide copolymers indicate the existence of hierarchical superstructures with ordering in the length-scale of microns. 

Ionic surfactants with only one alkyl chain are generally extremely hydrophilic so that strongly curved and thus almost empty micelles are formed in ternary water-oil-ionic surfactant mixtures. The addition of an electrolyte to these mixtures results in a decrease of the mean curvature of the amphiphilic film. However, this electrolyte addition does not suffice to drive the system through the phase inversion. Thus, a rather hydrophobic cosurfactant has to be added to invert the structure from oil-in-water to water-in-oil [7, 66]. In order to study these complex quinary mixtures of water/electrolyte (brine)-oil-ionic surfactant-non-ionic co-surfactant, brine is considered as one component. As was the case for the quaternary sugar surfactant microemulsions (see Fig. 1.9(a)) the phase behaviour of the pseudo-quaternary ionic system can now be represented in a phase tetrahedron if one keeps temperature and pressure constant. 

W. Actually, the surfactant does not like to be in water nor in oil because one part of the molecule is always lyophobic, which is why micelles are formed to hide it away from the solvent. Hence, it may be said that in type I phase behaviour the surfactant dislikes more oil than water, and in type II it dislikes more water than oil. Then, in type III phase behaviour, the surfactant equally dislikes both phases and would seek a third alternative, e.g. forming a bicontinuous microemulsion. In thermodynamic terms, it simply means that the chemical potential of the surfactant in such a microemulsion phase is lower than when it is adsorbed at the curved interface of a drop. 

So far, all theoretical models are based on surfactant solutions with a distribution of surfactant molecules as monomers. One of the specific properties of surfactants is that they form aggregates once a certain concentration, the critical micelle concentration CMC, is reached. The shape and size of such aggregates are different and depend on the structure and chain length of the molecules. At higher concentrations, far beyond the CMC, the phase behaviour is often complex giving rise to novel physical properties (Hoffmann 1990). 

Another option is extractive crystallisation. Here, the tendency of particular aqueous-solvent mixtures such as water-propanol, water-amines, water-micelles, water-polar polymers to split into two liquid phases upon small variations in temperature is used to dehydrate solutions of crystallisable solutes. At low temperatures, these systems form homogeneous mixtures, whereas at high temperatures, a solvent rich phase is created. The aqueous solute becomes concentrated in a smaller volume and consequently crystallises, whereas the pure solvent is recycled. Also, alternative schemes may be used depending on the exact phase behaviour of the component. For instance, a solute such as amino acids and peptides may crystallise from an aqueous solution upon introducing a fully miscible component, such as in water-ethanol mixtures. In a second stage, after the separation of the crystals, the conditions may be altered to induce an L - L phase split that allows easy recovery of the auxiliary component. Maurer and co-workers [25] described the use of high pressure CO2 in water-alkanol systems. At low pres-... 

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