FLUORESCENCE MICROSCOPY STUDY OF LANGMUIR MONOLAYERS OF STEARYLAMINE

The liquid-expanded (LE) to liquid-condensed (LC) coexistence region of monolayers of stearylamine deposited on water acidified to pH 2.5 with HCI has been observed using fluorescence microscopy and surface pressure isotherm measurements. Protonation of the amine headgroup on acidification with HCI shifts the LE-LC region down more than 30-degrees-C in temperature, widens the coexistence region, and leads to the observation of numerous shape transitions of the LC phase domains. The LC domain shapes become progressively less compact on compressions at a series of increasing temperatures. Image analysis of the domain shapes at two temperatures is used to obtain a critical domain radius for transition to a noncircular shape. The LE-LC line tension is estimated from the observed critical radius.