Phase transition temperature mixed micelles

Polymerization studies were performed on multilamellar dispersions prepared at low concentration, 2 mg/ml. The dispersions were gel filtered on Sephadex G 50-150 column, so that any of the short chain lipid component that might be present in micelle form would be separated from the mixed lipid vesicles. The vesicle preparations from the 1 1, 1 2 and 2 1 DC 8,9 PC mixtures were used in the polymerization study. The UV irradiated samples showed no evidence of participation by the acetylenic lipid in the polymerization reaction, as indicated by measuring the phosphorus content after separation of monomer by TLC. Diacetylenes polymerize when they are aligned, - and in bilayer systems this situation arises only below the lipid phase transition temperature. Thus, temperature may be expected to play an important role in polymerization. Indeed, after UV irradiation at room temperature brown or yellow dispersions resulted which contained a substantial fraction of unreacted monomer. However, polymerization of the mixtures for 1 minute at -5 C resulted in extensive polymerization (more than 90% elimination of monomer). Figure 4 shows that visible spectra of both the samples at 1 minute irradiation contained two major peaks at 532 and 494 nm. In contrast, pure DC 8,9 PC dispersion, prepared and irradiated under identical conditions, showed no clear signature of a polymer spectrum (defined peak around 500 nm) and very little monomer participation. Schoen and Yager, on the other hand, have demonstrated that the diacetylene lipid in tubular morphology shows a defined spectrum, but that the participation of monomers in polymerization process is low. ... 

The maximum additive concentration (MAC) is defined as the maximum amount of solubilisate, at a given concentration of surfactant, that produces a clear solution. Different amounts of solubilisates, in ascending order, are added to a series of vials containing the known concentration of surfactant and mixed until equilibrium is reached. The maximum concentration of solubilisate that forms a clear solution is then determined visually. This same procedure can be repeated for the different concentrations of surfactant in a known amount of solubilisate in order to determine the optimum concentration of surfactant (Figure 4.24). Based on this information, one can construct a ternary phase diagram that describes the effects of three constituents (i.e., solubilisate, surfactant, and water) on the micelle system. Note that unwanted phase transitions can be avoided by ignoring the formulation compositions near the boundary. In general, the MAC increases with an increase in temperature. This may be due to the combination of the increase of solubilisate solubility in the aqueous phase and the micellar phase rather than an increased solubilization by the micelles alone. 

Non-ionic surfactants do not exhibit Krafft points. Rather the solubility of non-ionic surfactants decreases with increasing temperature and the surfactants begin to lose their surface active properties above a transition temperature referred to as the cloud point. This occurs because above the cloud point, a separate surfactant rich phase of swollen micelles separates the transition is visible as a marked increase in dispersion turbidity. As a result, the foaming ability of, for example, polyoxyethylenated non-ionics decreases sharply above their cloud points. The addition of electrolyte usually lowers the cloud point while the addition of ionic surfactant usually increases the cloud points of their non-ionic counterparts, this increase being dependent on the composition of the mixed micelle. 

The liquid crystal phases discussed so far are called thermotropic liquid crystals and the transitions from one phase to another are driven by varying temperature. There is another type of liquid crystals, called lyotropic hquid crystals, exhibited by molecules when they are mixed with a solvent of some kind. The phase transitions from one phase to another phase are driven by varying the solvent concentration. Lyotropic liquid crystals usually consist of amphiphilic molecules that have a hydrophobic group at one end and a hydrophilic group at the other end and the water is the solvent. The common lyotropic liquid crystal phases are micelle phase and lamellar phase. Lyotropic liquid crystals are important in biology. They will not be discussed in this book because the scope of this book is on displays and photonic devices. 

Phase transitions are not limited to boiling, freezing, and mixing. Several different forms of molecular organization of surfactant molecules as a function of their concentration in water are shown in Figure 25.21. At very low temperatures, they crystallize. At higher temperatures, low concentrations dissolve in water. Increasing their concentration to beyond the critical micelle concentration leads to spherical micelles, then cylindrical micelles, then lamellar phases such as bilayers. 

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