Lamellar liquid crystalline phases

Orienting systems of quasi-ternary mixtures composed of cetylpyridinium chloride/hexa-nol/NaCl [26] and cetylpyridinium bromide/hexanol/NaBr [27] have been reported to form lamellar liquid crystalline phases that allow a large temperature range over which dipolar couplings can be measured. The optimum condition for protein alignment consists of a 1 1.33 (w/w) ratio of cetylpyridinium bromide/hexanol. The residual 2H quad-rupolar splitting of the HOD resonance increases from 5 Hz to 20 Hz as the concentration of the mixture is varied from 30 to 65 mg/mL. These quasi-ternary mixtures are positively charged. 

The phase condition for concentrations in the range close to the cmc are found in Fig. 4A. For the lowest soap concentrations, a liquid isotropic alcohol solution separated, when the solubility limit of the alcohol was exceeded. This was changed at concentrations approximately one half the cmc, when a lamellar liquid crystalline phase appeared Instead. After the relatively narrow three-phase region had been transversed, this liquid crystalline phase was the only phase in equilibrium with the aqueous solution. Solubilization of the long chain alcohol Increased at the cmc, as expected. 

As indicated above, miscibiUty gaps are small and intermediate lamellar liquid crystalline phases dissolve rapidly into the aqueous phase if the surfactant or surfactant mixture is rather hydrophihc with a high spontaneous curvature (low (v/la)), for instance at temperatures below Tq for pure nonionic surfactants. In this case dissolution, which converts lamellae of zero curvature to aggregates with significant curvature as surfactant concentration decreases, occurs spontaneously because it reduces system free energy. 

L micellar solution phase L lamellar liquid crystalline phase V viscous isotropic phase H2 reverse hexagonal phase... 

Region I. Relative to lamellar liquid crystalline phase Region IIl. Relative to hexagonal liquid crystalline phase Region IV. Relative to isotropic micellar solution... 

For mixtures of lecithin plus Na cholate it appears possible to infer the molecular arrangement in the dispersed micelles from the most likely structure of the liquid crystalline phase suggested by x-ray analysis. However, there are cases where dispersion is not possible because neither component is sufficiently hydrophilic to be dispersed even when alone in water. This is shown by the association of cholesterol and lecithin in the presence of water. The ternary diagram of Figure 4 is relative to these systems. Here only the lamellar liquid crystalline phase is obtained (region 1< in Figure 4). This phase is already given by lecithin alone, which can absorb up to 55% water. Cholesterol can be incorporated within this lamellar phase up to the proportion of one molecule of choles-... 

Figure 1 demonstrates the drastic influence on the stability region of a lamellar liquid crystalline phase when an aromatic hydrocarbon is substituted by an aliphatic one. The lamellar phase formed by water and emulsifier is stable between 20 and 60 wt % water. Addition of an aromatic hydrocarbon (p-xylene) to the liquid crystalline phase increased the maximum amount of water from 45 to 85% (w/w) (Figure 1 left). Inclusion of an aliphatic hydrocarbon (n-hexadecane) gave the opposite result the maximum water content in the liquid crystalline state was reduced (right). Some of the factors which govern the association behavior of these surfactants and cause effects such as the one above are treated below. 

The presence of a liquid crystalline phase at high surfactant concentrations has been shown by Shinoda (31), but the method of presentation renders the evaluation of the temperature dependence of necessary emulsifier concentrations to obtain the liquid crystalline phase difficult. Although several phase diagrams of the system (water, emulsifier, and nonionic surfactant) have been published (4, 45, 46, 47, 48), no results have been given on the relation between the surfactant phase and the lamellar liquid crystalline phase in these systems. 

Figure 9. Changes of the phase regions in the PIT value range are more pro-nounced for the micellar solutions than for the lamellar liquid crystalline phase formed by water and emulsifier...

The behavior of a series of polyoxyethylene alkyl ether nonionic surfactants is also illustrative. According to Figure 11 the dioxyethylene (A) compound does not form liquid crystals when combined with water. Its solutions with decane dissolve water only in proportion to the amount of emulsifier. The tetraoxyethylene dodecyl ether (B) forms a lamellar liquid crystalline phase and is not soluble in water but is completely miscible with the hydrocarbon. The octaoxyethylene compound (C) is soluble in both water and in hydrocarbon and gives rise to three different liquid crystals a middle phase, an isotropic liquid crystal, and a lamellar phase containing less water. If the hydrocarbon p-xylene is replaced by hexadecane (D), a surfactant phase (L) and a lamellar phase containing higher amounts of hydrocarbon are formed in combination with the tetraoxyethylene compound (B-D). 

In the insulin-L/CL complexes, it is evident from the dimensions of the lamellar liquid crystalline phase that insulin is simply associated electrostatically with the L/CL bilayer and that it replaces water (13). The amounts depend on the number of charges (see Figure 8). The limit of protein association is reached when no more surface is available at the bilayer-water interface. 

FIGURE 7.32. Phase diagram of PMOXA-b-PDMS-b-PMOXA in water and Cryo-TEM image of the lamellar phase formed at x = 50. x = polymer fraction in water in % w/w La = lamellar liquid crystalline phase. 

Miller, C.A. and Ghosh, 0.(1986) Possible mechanism for the origin of lamellar liquid crystalline phases of low surfactant content and their breakup to form isotropic phases. Langmuir, 2(3), 321-9. 

In order for studies with model membranes to be biologically relevant, it is important that the fluid phase, the lamellar liquid crystalline phase (L ), is... 

It is essential to realize that any thermodynamic evaluation of this solubility "maximum" with standard reference conditions in the form of the three pure components in liquid form is a futile exercise. The complete phase diagram. Fig. 2, shows the "maximum" of the solubility area to mark only a change in the structure of the phase in equilibrium with the solubility region. The maximum of the solubility is a reflection of the fact that the water as equilibrium body is replaced by a lamellar liquid crystalline phase. Since this phase.transition obviously is more. related to packing constraints — than enthalpy of formation — a view of the different phases as one continuous region such as in the short chain compounds water/ethanol/ethyl acetate. Fig. 3, is realistic. The three phases in the complete diagram. Fig. 2, may be perceived as a continuous solubility area with different packing conditions in different parts (Fig. 4). 

However, experimental evidence has shown (7) that inverse micellar systems are rarely in equilibrium with aqueous micellar solutions but rather with a lamellar liquid crystalline phase. The presence of an electrolyte will influence the stability of both the inverse micelles and the lamellar liquid crystalline phase. This influence will be estimated now. 

The discussion of the relative stability of solutions with inverse micelles and of liquid crystals containing electrolytes may be limited to the enthalpic contributions to the total free energy. The experimentally determined entropy differences between an inverse micellar phase and a lamellar liquid crystalline phase are small (12). The interparticle interaction from the Van der Waals forces is small (5) it is obvious that changes in them owing to added electrolyte may be neglected. The contribution from the compression of the diffuse electric double layer is also small in a nonaqueous medium (II) and their modification owing to added electrolyte may be considered less important. It appears justified to limit the discussion to modifications of the intramicellar forces. 

The energy of the electric double layer is directly dependent on the square of the surface potential (Equation 4) and the observed increase of the potassium oleate alcohol ratio should enhance the stability of the inverse micelle. The stability of the inverse micelle is not the only determining factor. Its solution with a maximal amount of water is in equilibrium with a lamellar liquid crystalline phase (7) and the extent of the solubility region of the inverse micellar structure depends on the stability of the liquid crystalline phase. 

Certain dilute lamellar liquid crystalline phases having relatively low apparent viscosities can propagate through this porous medium micromodel without plugging it. Their behavior followed the trends established with isotropic phases. 

Membrane lipids are invariably polymorphic that is, they can exist in a variety of kinds of organized structures, especially when hydrated. The particular polymorphic form that predominates depends not only on the stmcture of the lipid molecule itself and on its degree of hydration, but also on such variables as temperature, pressure, ionic strength and pH (see References 11 and 12 and article Lipids, Phase Transitions of). However, under physiologically relevant conditions, most (but not all) membrane lipids exist in the lamellar or bilayer phase, usually in the lamellar liquid-crystalline phase but sometimes in the lamellar gel phase. It is not surprising, therefore, that the lamellar gel-to-liquid-crystalline or chain-melting phase transition has been the most intensively studied lipid phase transition... 

From a biologic viewpoint, of greatest interest are the transitions that involve the physiologically important lamellar liquid-crystalline phase, namely, the gel- liquid-crystalline (melting) transition, and the lamellar- nonlamellar mesomorphic transitions. 

Formation of lamellar liquid crystalline phases at the O/W interface This mechanism, as suggested by Friberg and coworkers [37], proposed that surfactant or mixed surfactant film can produce several bilayers that wrap the droplets. As a result of these multilayer structures, the potential drop is shifted to longer distances, thus reducing the van der Waals attractions. A schematic representation of the role of Hquid crystals is shown in Figure 10.32, which illustrates the difference between having a monomolecular layer and a multilayer, as is the case with hquid crystals. 

W = micellar solution or O/W microemulsion 1 = lamellar liquid crystalline phase 0 = oil phase. 

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