A temperature-dependent two-dimensional condensation transition during Langmuir-Blodgett deposition

Langmuir-Blodgett monolayers, deposited from a Langmuir monolayer (on the water surface) in a two-dimensional liquid (LE) state, were examined using atomic force microscopy (AFM), transmission infrared (IR) spectroscopy, and contact angle goniometry. Pentadecanoic and hexadecanoic acid films condensed into densely-packed islands on the mica substrate. IR spectra and contact angle data for these films suggested that molecules within islands are well-ordered-similar to those within a monolayer deposited from a condensed monolayer phase (LC). Tetradecanoic acid monolayers, however, were uniformly (no condensation) covered by a monolayer of disordered molecules if deposited below 17 degrees C and displayed increasing numbers of dense, compact islands between 17 and 20 degrees C (resembling those in the longer chain acids). At slightly higher temperatures, lower porous islands were observed in addition to these dense, high islands. By about 22 degrees C, the surface was completely covered by the porous aggregates. We believe this represents a sequence of two transitions of the surface phase after transfer. We also present AFM images of 12-NBD octadecanoic acid monolayers deposited from the LE phase or the LE/LC coexistence region. Orientational alignment of the needle-shaped LC domains in the direction of growing islands lends support to the hypothesis of a surface tension gradient driven island growth mechanism.