Neat mesophases

Formation of the Vi and V2 Mesophases and of the Non-Amphiphilic Cubic Mesophase "Smectic D . In Figures 1 and 5, the formation of the amphiphilic cubic mesophases Vi and V2 is attributed to transitional globular micellar forms which arise intermediate between the indefinitely extended fibrous (M) and lamellar (G) forms which constitute the middle and neat mesophases, respectively. With a few non-amphiphilic mesogens (19, 20, 21, 22) a cubic mesophase "smectic D is found intermediate in the thermal succession of mesophases between smectic A... 

Recently, Sato and Hatano 67 69) found a new type of chiral lyotropic mesophase composed of Tween 80, sorbitan mono-9-octadecenoate poly(oxy-l,2-ethanediyl), and water, and discussed the ICD of achiral solute molecules intercalated into the lyotropic mesophase. As the concentration of Tween 80 is increased, three distinct phases are obtained micelle, neat phase, and reversed micelle, in that order. In the region of the volume ratio of Tween 80/(Tween 80 + water) of 0.40 to 0.63 under crossed Nicol-prisms, a focal conic texture was observed. This result indicates that the... 

Figure 1. Possible structural arrangement of a bimolecular lipid membrane (BLM) separating two aqueous solutions. Open circles and zig-zag lines denote polar groups and hydrocarbon chains, respectively. The structure of BLM is akin to a neat or smectic mesophase found in liquid crystals...

An even more dramatic example of the potential lack of selectivity afforded to the Norrish II reactions of ketones by supposedly very ordered systems than that described in the 76 systems is provided by neat samples of the mesomorphic alkanophenones (81) [278]. These molecules are capable of existing in nematic and smectic B mesophases (see Figure 16) as shown in Scheme 42. The instability of the monotropic smectic B phase of 81a and smectic B phase of 81b did not allow their photoreactions to be examined these smectic phases became solids soon after the initiation of irradiation. 

The cubic amphiphilic mesophases (Sic, Vi, and V2) from their interposition in the succession of mesophases Sic, Mi, Vi, G, V2, and M2, have generally been termed liquid crystalline like the optically anisotropic amphiphilic mesophases Mi, G, and M2. The cubic mesophases formed by non-amphiphilic globular molecules have however usually been termed plastic crystals. This nomenclature has obscured the fact that these plastic crystals are fundamentally liquid crystals rather than solid cyrstals and bear a relationship to the optically anisotropic non-amphiphilic smectic and nematic liquid crystals similar to that born by the amphiphilic cubic mesophases to the optically anisotropic neat (G) and middle (Mi and M2) liquid crystalline phases. 

T extures of lyotropic mesophases have been the object of numerous observations by optical (1,2,3) and electronic (4, 5, 6,7) microscopy. Except for the pioneering work of Lehmann (1) and Friedel (2) who intended to identify the various kinds of defects which constitute the textures, the purpose of these observations was to recognize the different existing phases—lamellar, hexagonal (or in the soaps language neat phase, median phase, etc.)—in correlation with x-ray data. 

Irradiation procedures. Mesophase solutions and neat solid samples of BN were prepared and sealed under N2 or vacuum in Kimax capillary tubes. Isotropic samples were either degassed (freeze-pump-thaw techniques) and sealed in pyrex tubes or saturated with N2 in pyrex tubes. Nitrogen was bubbled through the latter solutions during irradiation periods. When ther-mostatted, samples were placed in a temperature controlled ( 1°) water bath. All samples were irradiated with a 450 W Hanovia medium pressure Hg arc and were stored at -30°C until their futher use. Usually, a "dark sample was prepared and treated in an identical fashion to the irradiated samples except that it was shielded from the light. JSN from each tube was recovered by either column chromatography (silica or alumina and pentane eluant) at 4°C followed by solvent removal at 0°C and reduced pressure or by hplc (tr-hexane) at room temperature followed by solvent removal at 0°C and reduced pressure. Neat solid samples were dissolved in one of either benzene, tetrahydrofuran or toluene and were frozen until analyzed. 

The junction zones of CAQ organogel networks are lyotropic microdomains that vary from hexagonal in decane to more compact, lamellar-like ordering in 1-octanol 94. Schlieren optical textures confirm the inhomogeneity of the orientation of the threadlike structures (Fig. 16), and CD spectra resemble those of neat CAQ solid, cooled from its mesophase. However, the solid morph obtained from solvent crystallization is packed differently from the gel strands 114. ... 

This chapter deals almost exclusively with neat, or pure, diblock copolymer melts. Polymer blends are discussed in Chapter 9, micellar solutions in Chapter 12, and stabilized suspensions in Chapter 6. In the following, Section 13.2 briefly reviews the thermodynamics of block copolymers, and Section 13.3 describes the rheological properties and flow alignment of lamellae, cylinders, and sphere-forming mesophases of block copolymers. More thorough reviews of the thermodynamics and dynamics of block copolymers in the liquid state have been written by Bates and Fredrickson (1990 Fredrickson and Bates 1996). The processing of block copolymers and mechanical properties of the solid-state structures formed by them are covered in Folkes (1985). Biological applications are discussed in Alexandridis (1996). 

The closed loop is not the only characteristic of the nonionic surfactant-water binary phase diagram. Like the ionic surfactant-water mixture, nonionic surfactants, at higher concentration in water, exhibit lyotropic mesophases. Figure 3.14 shows a typical binary phase diagram exhibiting the full lyotropic mesophase sequence II, cubic isotropic phase HI, direct hexagonal phase (middle phase) VI, special cubic ( viscous phase) La, lamellar phase (neat phase). Note the presence of the two-phase domains surrounding each mesophase, the critical point on top of each, and the zero-variant three-phase feature. 

The potential of nuclear magnetic resonance spectroscopy for studying liquid crystalline systems is discussed. Typical spectra of nematic, smectic, and cholesteric mesophases were obtained under high resolution conditions. The observed line shape in the cholesteric phase agrees with that proposed on the basis of the preferred orientation of this phase in the magnetic field. The line shapes observed in lyotropic smectic phases appear to be the result of a distribution of correlation times in the hydrocarbon portions of the surfactant molecules. Thermotropic and lyotropic phase transitions are easily detected by NMR and agree well with those reported by other methods. The NMR parameters of the neat and middle lyotropic phases allow these phases to be distinguished and are consistent ivith their proposed structures. 

Of all the lines encountered in this system only those from the isotropic liquid phase show any structure. The shapes of the lines from the lyotropic mesophases are similar to that of superwaxy NaS shown in Figure 5. These shapes are discussed in more detail below. There are also small but definite differences between the line widths from the neat and middle phases which are not visible on the scale used in Figure 10. 

The two most common lyotropic mesophases are designated neat and middle 26), Both are members of the family of mesomorphic structures derived by Hermann 12). The neat phase structure is generally accepted as being made up of parallel, equidistant sheets of double molecules separated by solvent 4, 21, 22). Neat phase is optically anisotropic and flows easily. Figure 11 indicates the structure 22) and a cross-sectional view of one of the layers. 

A third lyotropic mesophase which occurs frequently in surfactant-water systems is normally designated viscous isotropic. This phase is very viscous, sometimes brittle, but unlike neat and middle it is not bire-fringent. The structure of the viscous isotropic phase is still not known with certainty. In some systems x-ray studies have indicated that the structure consists of spherical units packed in a face-centered arrangement 4, 22). It has been proposed that the polar groups of the molecules cover the outside surfaces of the spherical units and that the hydrocarbon chains are essentially liquid in their arrangement inside the units. In this respect the structure is similar to one of the proposed middle phase structures (4). As in the other lyotropic phases, the solvent probably fills the voids among the spherical units of surfactant. 

By careful measurement of line widths, second moments, and the R(8/2) parameter (the ratio of the line width at eighth-height to that at half-height) the neat and middle lyotropic mesophases can be distinguished from each other. The R(8/2) parameter is shown to be sensitive to changes in both phase structure and molecular structure. The NMR parameters of the neat and middle phases are consistent with the structural pictures proposed for these phases (4, 22) but do not define the details of the middle structure. The NMR spectra observed in viscous isotropic mesophases are surprising since they are essentially the same as those obtained from dilute, micellar solutions. This type of spectrum does not appear to be consistent with the proposed structure of this phase. 

Fig. 6.1.3. Quadrupole splittings for the aromatic and a-aliphatic vj deuterons of deuterated hexa-n-hexyloxytriphenylene (THE6) as functions of temperature (r— 7 ) in the mesophase region, where 7 is the mesophase-isotropic transition point. The open circles correspond to measurements on neat THE6-ard, and THE6-adi2 separately, while the filled circles correspond to a 2 1 mixture of the two isotopic species. The scale on the upper right-hand side gives the orientational order parameter of the aromatic part. The curve at the bottom gives the ratio of the quadrupole splittings for the a-aliphatic and aromatic deuterons (Goldfarb, Luz...

Helpful tools for this structurization of liquid crystal research were temperature dependent X-ray investigations [36] of natural and synthetic lipids, and the discovery that mesophases may be identified by their different textures appearing in the microscope using crossed polarizers [37]. In the decade starting in about 1957 systematic screening of the concentration and temperature dependency of the major lyotropic mesophases was done and models of the molecular arrangement in the different phases were developed [38-45] (e.g., the so-called middle or neat phases [38], the cholesteric phase of polypeptides and nucleopep-tides [44]). 

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