Structure formation liquid crystalline structures

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

It has been shown (Friberg, 2003 Birdi, 2002, 2008) that there exists a correlation between foam stability and the elasticity [E] of the film (i.e., the monolayer). In order for E to be large, surface excess must be large. Maximum foam stability has been reported in systems with fatty acid and alcohol concentrations well below the minimum in y. Similar conclusions have been observed with -C12H25S04Na [SDS] + -C12H25OH systems that give minimum in y versus concentration with maximum foam at the minimum point (Chattoraj and Birdi, 1984). Because of mixed mono-layer formation it has been found that SDS + C12H25OH (and some other additives) make liquid-crystalline structures at the surface. This leads to a stable foam (and liq-... 

While the emphasis of this section has been on kinetics of liquid crystal formation, the rate of liquid crystal dissolution may also be of interest—e.g., in connection with breaking foams and emulsions by adding materials which destroy the liquid crystalline structure. 

Several theories attempt describe the critical conditions for the formation of a lyotropic liquid crystalline structure. 

The viscosity of the continuous phase affects the stability of the concentrated emulsion. The viscosity of the continuous phase can be modified either by adding thickeners or by increasing the surfactant concentration. For instance, the formation of a liquid-crystalline structure in the continuous phase when the surfactant concentration is sufficiently large can increase the stability of the emulsion. However, a too high viscosity of the continuous phase caused by a high surfactant concentration can hinder the formation of a concentrated emulsion because it generates resistance to the dispersion of the dispersed phase. 

The structure of the blue phase is of some importance. Among the lipoproteins carrying lipids in the blood, low-density lipoproteins (LDL) have attracted much attention. They are the factors mainly responsible for plaque formation, which ultimately leads to atheriosclerotic changes and heart disease. The major components of the LDL-particles are cholesterol fatty acid esters. A remarlmble property is the constant size of LDL particles [28], which indicates that the interior must possess some degree of order. It seems probable that the structure proposed above for cholesterol esters in the cholesteric liquid-crystalline structure should occur also in the LDL-particle. In that case the LDL particle can be viewed as a dispersed blue phase, whose size is related to the periodicity of the liquid-crystalline phase, and the protein coat at the surface is oriented parallel to adjacent specific crystallographic planes of the blue phase. These amphiphilic proteins will expose lipophilic segments inwards emd expose hydrophilic groups towards tiie enviroiunent. 

In the case of symmetric coil-rod-coil molecule, the rod segment is connected with coil segments at both ends. This gives rise to the formation of the liquid crystalline structure with higher interfacial area in comparison with rod-coil diblock systems at similar coil volume fraction. For example, the... 

A wide variety of polymeric liquid-crystalline structures have been prepared by the synthetic formation of covalent bonding [3, 75]. Recently, the use of... 

The fourth design feature is the formation of a liquid crystalline structure. During the last fifteen years, liquid crystalline polymers (LCP) have become one of the most exciting polymer families synthesized by chemists. Recently, the syntheses of LCPs were reviewed by Griffin (J ). Interesting physical properties of liquid crystalline polyesters were described by Huynh-ba and Cluff (13) To the interest of tribologists, some of LCPs are rather wear resistant. 

In the past few years a considerable number of papers were published which were concerned with liquid crystalline structures in polymeric systems. Different routes were employed to obtain polymers with liquid crystalline structures or even thermodynamically stable liquid crystalline phases 1,, In general monomers containing mesogenic groups - groups which are known to have a tendency towards the formation of liquid crystalline structures, or rigid groups were used. Cases are known where the monomers exhibit liquid crystalline phases Z) In that case the polymerization can be performed in anisotropic melts frozen-in liquid crystalline structures and textures can be obtained in many instances i . In other cases the monomers do not display liquid crystalline phases. The formation of liquid crystalline polymer structures may nevertheless be possible due to the restriction of the motions of the individual repeat units -3),... 

Even though hydrotropes exhibit a resemblance to surfactants, a number of differences are obvious. The amount of hydrotrope needed to facilitate solubilization of the solute in water is usually much higher than that needed for surfactants. The reason for this is that the shorter carbon chain of the hydrotrope will result in a higher concentration for self-association, which is a requirement for solubilization (12). In the case of hydrotropes, the maximum solubilization will usually be higher than for surfactants. This might be explained by the fact that the micellar solution of a surfactant will be transformed into an inverse micellar solution via the formation of a lamellar liquid crystalline structure and the solubilization of the solute in water will be... 

Twelve oxyethylated fatty alcohols with various lengths of alkyl chain and ethylene oxide were used in this investigation. In view of numerous literature data available, physicochemical tests were limited to measurements of surface tension, wettability and viscosity. Microscope photographs were taken in polarized light in order to confirm the appearance of liquid crystalline structures. As expected, formation of micelles was observed at low concentrations, whereas mesophases (hexagonal and lamellar) were identifled at concentrations of about 50% to 70%. 

The physicochemical test results presented in this chapter also indicate that micelles form in aqueous solutions of SML/ESMIS mixtures when the concentration is around 1% (figs. 18.2 and 18.5). The presence of aggregates in the bulk phase increases the viscosity of the compositions (figs. 18.5-18.7, table 18.1). The possibility of producing liquid crystalline structures in aqueous solutions of SML/ESMIS mixtures is a consequence of micelle formation (figs. 18.8 and 18.9). 

One of the earliest methods for reducing coalescence is to use mixed surfactant films. These will increase the Gibbs elasticity and/or interfacial viscosity. Both effects reduce film fluctuations and, hence, reduce coalescence. In addition, mixed surfactant films are usually more condensed and hence diffusion of the surfactant molecules from the interface is greatly hindered. An alternative explanation for enhanced stability using surfactant mixture was introduced by Friberg and coworkers [67] who considered the formation of a three-dimensional association structure (liquid crystals) at the oil/water interface. These liquid crystalline structures prevent coalescence since one has to remove several surfactant layers before droplet-droplet contact may occur. 

The mono-olein-water phase diagram (Figure 15c) shows the formation of lamellar liquid crystalline structure at room temperature (20 °C) at water content between 2 and 20%. At higher water concentrations, a cubic phase is formed, which above 40% water exists in equilibrium with water. If the temperature of the cubic phase is increased above 90 °C, a hexagonal II phase is produced. 

---