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  • 1
    ISSN: 0947-6539
    Keywords: alcohols ; amphiphiles ; GID ; ice nucleators ; monolayers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A systematic analysis of grazing incidence synchrotron X-ray diffraction data of uncompressed amphiphilic alcohols CnH2n+1 OH (n = 31, 30, 23, 20, 19, 18, 16, 14, 13) on a water subphase at 5°C is presented. Pronounced structural changes were observed on reduction of chain length from n = 31 to 13. The relative amount of two dimensional (2-D) crystalline material formed fell drastically; shorter crystalline coherence lengths were also observed. For n-31-18 the molecules are arranged in a rectangular cell (a ≈ 5 Å, b increases from ca. 7.4 to ca. 8.2 Å) with plane symmetry p1 g1. For n 〈 18 a tilted free-rotator phase is probably adopted. The two glide-related molecules in the unit cell form a herringbone arrangement in which the chain axes are parallel and separated by [(a + b)/2]. The molecular chains are tilted from the vertical in the b direction; the tilt angle increases from ca. 7° to ca. 21° over the range n = 31 to 19, and then drops to 12° for n = 13. There is a continuous increase in molecular cross-sectional area from 18.4 to 20.3 Å2 and in the atomic displacement parameter parallel to the water surface; it increases from 0.1 Å2 for n = 31 to 0.30 Å2 for n = 19. We explain the preference for chain tilt along the b axis, rather than a, in terms of hydrogen bonding to the water subphase. The various structural properties of the CnH2n+2OH (n = 31-13) monolayer series, such as degree of crystallinity and coherence length, lattice dimensions, chain orientation, and molecular motion, may be correlated with the ice-nucleating efficiency of these alcohol monolayers as a function of n.
    Additional Material: 17 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0947-6539
    Keywords: cryo-transmission electron microscopy ; monolayers ; oligothiophenes ; polymorphism ; scanning force microscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The formation of Langmuir monolayers at the air-water interface has long been believed to be limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup. Here we report the formation of crystalline mono- and multilayer self-assemblies of oligothiophenes, a class of aromatic nonamphiphilic molecules, self-aggregated at the air-water interface. As model systems we have examined the deposition of quaterthiophene (S4), quin-quethiophene (S5), and sexithiophene (S6) from chloroform solutions on the water surface. The structures of the films were determined by surface pressure-area isotherms, by scanning force microscopy (SFM) after transfer of the films onto atomically smooth mica, by cryo-transmission electron microscopy (Cryo-TEM) on vitreous ice, and by grazing incidence synchrotron X-ray diffraction (GID) directly on the water surface. S4 forms two polymorphic crystalline multilayers. In polymorph α, of structure very similar to that of the three-dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23° from the normal to the water surface. In polymorph β the long molecular axis is perpendicular to the water surface. S5 self-aggregates at the water surface to form mixtures of monolayers and bilayers of the β polymorph; S6 forms primarily crystalline monolayers of both α and β forms. The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports. The relevance of the present results for the understanding of the early stages of crystal nucleation is presented.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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