A light scattering study of phase transitions in monolayers of n-pentadecanoic acid

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Abstract
The phase transitions of monolayers of carefully purified n-pentadecanoic acid at the air/water interface have been investigated using both classical and laser scattering methods. The equilibrium pi -A isotherms showed flat coexistence regions between the liquid-expanded and liquid-condensed states. Such clear first-order transitions appear only to be observable for pure pentadecanoic acid. In transitions at temperatures above the triple point ( approximately 17 degrees C) the light scattering clearly showed up phase separation within the monolayer, again demonstrating the first-order nature of the transitions. The liquid domains in the liquid/vapour transition were about 1 cm across. In the liquid-expanded/liquid-condensed transition the denser phase regions were of the order of 0.4 mm in size. These estimates are compatible with surface potential fluctuations which have been observed for this system. In the condensed/vapour transition below the triple point the monolayer behaved quite differently, no fluctuations being observed. At a surface concentration such that half of the pentadecanoic acid was in each surface phase, the viscoelastic properties of the film changed abruptly from close to those of the clean subphase to those of a viscoelastic medium. It is hypothesized that the molecular aggregates formed in this transition interact, when sufficiently close together, to form some kind of surface superstructure, which acts as a homogeneous surface phase.