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

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.

Hole dynamics in polymer Langmuir films

“…The authors suggest that their liquid phase might not be sufficiently incompressible for their model to apply. Significant compressibility is unlikely for any liquid monolayer for the slow speeds they observe, and indeed, the elasticity of their liquid phase is measured to be ϳ20 mN/ m, 13 more than four orders of magnitude greater than required for incompressibility ͑see Sec. II A͒.…”

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant

“…These are gaseous (G), liquid expanded (LE), liquid condensed (LC), and solid (S) phases. The phase transitions G-LC, G-LE, and LE-LC all are of first order [17][18][19]. The G-LE phase transition occurs at very low pressure, whereas the LE-LC phase transition takes place at relatively high surface pressure.…”

“…Comparing with the results for spread monolayers [16][17][18][19], an attempt is made, here, to characterize these phases and phase transitions. During the formation of Gibbs monolayers, the concentration of the surface molecules should be very low just after sweeping of the solution surface by the Teflon barriers.…”

“…In another report, the existence of only a G-LC phase transition is concluded [24]. Although the conditions under which these three phase transitions occur in spread monolayers are detailed [16][17][18][19], such work is still absent for medium-chain water-soluble amphiphiles in Gibbs adsorption layers. The principal aim of this article is to study the details of the conditions for these phase transitions in Gibbs monolayers.…”

Picosecond Studies of Chemical Intermediates

“…[8][9][10][11] Hence we want to use SHG to investigate surface heterogeniety in the gas-liquid coexistence region. A heterogeneous surface in the gas-liquid region consists of two diffearent types of molecular arrangements.…”