Crystal structures of self-aggregates of insoluble aliphatic amphiphilic molecules at the air-water interface. An X-ray synchrotron study

D. Jacquemain, F. Leveiller, S.P. Weinbach, M. Lahav, L. Leiserowitz, K. Kjær, J. Als-Nielsen

    Research output: Contribution to journalJournal articleResearchpeer-review

    Abstract

    Uncompressed insoluble amphiphilic molecules possessing linear hydrocarbon chains CnH2n+1X (n = 23, 30, 31, X = OH; n = 29, X = COOH; and n = 19, X = CONH2) spontaneously form large two-dimensional (2-D) crystalline clusters over pure water at low temperature (5 °C). These 2-D crystallites were detected and their structures were solved using grazing incidence X-ray diffraction (GID). Their packing arrangements are described in terms of 2-D space-group symmetry and hydrocarbon-chain packing. All the crystal structures display rectangular unit cells containing two molecules that are probably related by glide symmetry in the 2-D space group pg for the alcohol (X = OH) and the acid (X = COOH) and by translation symmetry in the 2-D space group p1 for the amide (X = CONH2). The alcohol molecules are tilted by 8–11° from the vertical toward next-nearest neighbors, the tilt angle being dependent on the chain length. The amide and the acid molecules are tilted toward nearest neighbors by 18° and 26°, respectively. The positional correlation lengths of the crystallites were found to be anisotropic; they extend over only 35–95 spacings parallel to the molecular tilt direction, but over 135–270 spacings perpendicular to it. The similarity of chain packing in the 2-D crystallites and in three-dimensional (3-D) crystals of aliphatic amphiphilic molecules is clearly established. These crystallites may therefore, on the water surface, mimic crystallization mechanisms observed in 3-D systems.

    Original languageEnglish
    JournalJournal of the American Chemical Society
    Volume113
    Issue number20
    Pages (from-to)7684-7691
    ISSN0002-7863
    DOIs
    Publication statusPublished - 1991

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