GROWTH SHAPES OF A 2-DIMENSIONAL SOLID IN A LANGMUIR MONOLAYER - ROLE OF IMPURITIES

Two-dimensional solid domains in a monolayer of a fluorescent amphiphile, 12-NBD-stearic acid, are observed with a optical microscope. When oxidation products of the molecule are absent, domains have a rodlike shape showing two long, parallel facets. The introduction of even a few oxidized molecules (less than 1%) leads to needlelike shapes. These needles can be understood as the result of a gradient of impurity concentration, appearing in the beginning of growth. This explanation, based on isotherm measurements and structural observations, implies that impurities segregate in the solid with the growth of the domains. The fluid becomes more pure; this feature is confirmed by observations. The faceted rods are probably growth shapes. Since concentration gradients transform rods into needles, it is proposed that needles are growth shapes as well as rods.