Liquids, diffraction structure

It was Ziman [77] who has noted that there is little hope, at least at present, to develop an experimental technique permitting the direct measurement of these correlation functions. The only exception are the joint densities x / (r> ) information about which could be learned from the diffraction structural factors of inhomogeneous systems. On the other hand, optical spectroscopy allows estimation of concentrations of such aggregate defects in alkali halide crystals as Fn (n = 1,2,3,4) centres, i.e., n nearest anion vacancies trapped n electrons [80]. That is, we can find x mK m = 1 to 4, but at small r only. Along with the difficulties known in interpretating structure factors of binary equilibrium systems (gases or liquids), obvious specific complications arise for a system of recombining particles in condensed media which, in its turn, are characterized by their own structure factors. 

The three fundamental lyotropic liquid crystal structures are depicted in Figure 1. The lamellar structure with bimolecular lipid layers separated by water layers (Figure 1, center) is a relevant model for many biological interfaces. Despite the disorder in the polar region and in the hydrocarbon chain layers, which spectroscopy reveals are close to the liquid states, there is a perfect repetition in the direction perpendicular to the layers. Because of this one-dimensional periodicity, the thicknesses of the lipid and water layers and the cross-section area per lipid molecule can be derived directly from x-ray diffraction data. 

Figure 22.6. (A) Temperature variation profile during, and (B) three-dimensional plots of the results obtained from, a time-resolved synchrotron X-ray diffraction experiment on SOS (1,3-distearoyl-2-oleoyl-.sn-glycerol) without annealing, a and (3 polymorphic forms LC1 liquid crystalline structure 1. (Reproduced with permission from Sato et al., 1999.)...

An X-ray liquid diffraction experiment on a solution corresponding to the stoichiometry of the trimethylamine decahydrate [807], 4(CH3)3N 40H2O, provided the radial distribution function shown in Fig. 21.14, which could be fitted equally well with models based on the hydrate crystal structure or on the ice I structure with interstitial amine molecules. This result clearly illustrates the insensibility of X-ray liquid diffraction data alone for distinguishing between competing models for aqueous solutions which have similar first- and second-order coordinations. 

Structure and Polymorphism of Lipopeptides. Amphiphilic lipopep-tldes Cjj(AA)p exhibit mesophases in aqueous solution for water concentrations smaller than about 60 %, The structure of the mesophases and of the dry lipopeptides obtained by evaporation of the mesophase water at a slow rate was determined by X-ray diffraction. Lipopeptides X-ray diagrams obtained are similar to those exhibited by classical amphiphiles (11). They have allowed us to establish the existence of three types of liquid-crystalline structures Isunel-lar, hexagonal and cubic. 

Fig. lla and b. Cholesteric liquid crystalline structure of PBLG in m-cresol a, striation patterns observed under a polarizing microscope b, optical diffraction pattern with a beam from the He—Ne gas laser. Concentration is 17% volume fraction of polymer, and cell thickness is 2 mm... 

The linear H-bond imposes a very open tetrahedral structure on the water molecules throughout the liquid. Diffraction experiments show each water molecule to be tetrahedraUy surrounded by about four other water molecules at a distance of 0.28 nm with a next layer at... 

Li L, Lieser G, Rosenau-Ek hin R, Fischer EW (1987) An electron diffraction study of crystalline modifications and the liquid-crystalline structure of p-hydroxybenzoate/m-l droxy-benzoate copolymers. Makromol. Chem. Rapid Conunun. 8 159... 

The introduction of liquid Cl in amounts less than 5 wt% affects Ot, negligibly and may even slightly elevate it. According to X-ray diffraction structural analysis, this also leads to the growth of crystallinity of the film materials and stresses in the crystalline lattice of PE at 5-10 wt% [24]. Cl particles evidently behave like additional crystallization centers in PE. Increasing the concentration of Cl above 7-10% noticeably impairs the strength of inhibited films. 

In COlL-1, all of the main aspects of the liquid-state structure of room-temperature ionic liquids were already laid out in contributions by several groups, although sometimes still in an incipient way. Hardacre and co-workers reported neutron diffraction studies of short-chain dialkylimidazolium ionic liquids, perfectly illustrating the charge ordering and n-interactions [17]. They also used different spectroscopic and simulation methods to study how the solvation of aromatic and polar molecules (benzene and ethanenitrile, respectively) in the ionic liquids modifies the structure of the media [46] and the balance between the different terms in the interactions coulombic, van der Waals (dispersive), hydrogen bonds and multipolar. 

The most qualitative techniques for identification of the various liquid crystalline phases are based on diffraction studies, either light. X-ray or neutron. Liquid crystalline structures have a repetitive arrangement of aggregates and observation of a diffraction pattern can give evidence of long-range order and so distinguish between alternative structures. 

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