The results could be explained by increased retention of toothpaste by smaller diameter filaments and denser tufts on soft brushes and the greater flexion of filaments increasing the area of contact with the surface. Calculations on the clinical outcome of these data in vitro indicate that toothbrushing with toothpaste alone would produce minimal damage to dentine even over many years. Differences between brushes therefore are probably of little clinical significance. Certainly, the data do not support the use of hard brushes, particularly in view of the potential detrimental effects to gingival tissues.
Pasteur's brilliant observation, using optical microscopy, that crystals of a salt of "racemic acid" actually existed in two enantiomorphous forms ['] not only uncovered the existence of enantiomers, but indeed stimulated the development of modern structural theory. The direct (without resorting to diffraction techniques) observation of supermolecular chirality and the relationship between supermolecular chirality and molecular structure have fascinated chemists ever since. Starting with early studies of the morphology of chiral crystals by optical microscopy, the scale for the observation of chiral supermolecular assemblies has been approaching that of the molecules themselves. McConneli and Weis first recorded the formation of chiral crystalline monolayer domains in Langmuir films of enantiomerically pure lipids by epifluorescence optical microscopy.[zl More recently, chiral monolayers have been observed at molecular resolution by scanning probe microscopy. Thus, chiral symmetry breaking from achiralc31 and racemicf4I molecules has been detected in Langmuir films by atomic force microscopy (AFM), and enantiomorphous monolayer domains from achiral liquid crystal (LC) molecules on graphite have been observed by scanning tunneling microscopy (STM) .Is1 In related work, amino acids have been spontaneously resolved by crystallization in two dimensions at the air-water interface.16]We report here on the molecular resolution STM observation of enantiomorphous images of crystal monolayers grown from pure enantiomers. Furthermore, a racemic mixture of the same molecules produced images of co-existing enantiomorphic domains indistinguishable from those obtained from the enantiomerically pure materials, providing strong evidence for chiral symmetry breaking to give a two-dimensional (2-D) conglomerate. These results suggest a complete 2-D analog of Pasteur's famous system.The experiments were performed on enantiomerically enriched and racemic biphenylbenzoates 1. These have one tetra-1 hedral stereogenic center (indicated by an asterisk). Both enantiomers were prepared in approximately 99 YO ee; the racemate was also prepared."] All three compounds were liquid crystalline with a smectic A phase at room temperature.Each STM sample was prepared by placing a small amount (< 1 mg) of the liquid crystal on a freshly cleaved surface of highly oriented pyrolytic graphite. Scanning tips were prepared from mechanically cut 0.025 cm platinum/iridium (80: 20) wire. Images were recorded in air at room temperature using a commercially available STM (Nanoscope 11, Digital Instruments, Inc.) with tip positive bias.
Steric effects on the adsorption of self-assembled monolayers (SAMs) formed by alkylthiolates on the Au-(111) surface were investigated using density functional theory. Based on the ( 3 × 3) R30°structure, the current results on methylthiolate (CH 3 S) show that the adsorption prefers the face-centered cubic-bridge and hexagonal close-packed-bridge sites. Furthermore, the adsorption energy decreases slightly compared to the CH 3 S adsorption on the p(2 × 2) structure due to lateral interactions. Comparison between the results on CH 3 S and 1-propylthiolate (C 3 H 7 S) illustrates that the adsorption energy increases with chain length. Strong steric effects were found due to the chain length of the alkylthiolates and the hydrogen atoms in the CH 2 unit adjacent to the S atom (R hydrogens). The energetically favored tilt angle is 20°for C 3 H 7 S. The preferred geometry for both CH 3 S and C 3 H 7 S adsorptions has the two R hydrogens pointing toward the bridge Au atoms of the surface. The results suggest a flat potential energy surface, which correlates well with the dynamic nature of alkylthiolate SAMs observed experimentally.
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