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 language | English |
---|---|
Journal | Journal of the American Chemical Society |
Volume | 113 |
Issue number | 20 |
Pages (from-to) | 7684-7691 |
ISSN | 0002-7863 |
DOIs | |
Publication status | Published - 1991 |