Solid–liquid phase transition in a Gibbs monolayer of melissic acid at the n-hexane–water interface

Abstract

A sharp phase transition from a crystalline state with the area per molecule A = (17 ± 1)Å 2 to a liquid state with A = (23 ± 1)Å 2 at the n-hexane -water interface in a Gibbs monolayer of melissic acid has been revealed in data of X-ray reflectometry with the use of synchrotron radiation.

Diffuse X-ray scattering near a two-dimensional solid–liquid phase transition at the n-hexane–water interface

“…In this work, the temperature dependence of the intensity of diffuse (nonspecular) scattering of 15-keV photons at the n-hexane -water interface, where such a transition occurs in the adsorbed layer of melissic acid (C 30 -acid) [3], is studied. It is shown below that the diffuse scattering intensity in the low-temperature crystal phase at the interface is one or two orders of magnitude higher than that in the high-temperature phase, which indicates the existence of an extended transverse structure with a thickness of ∼ 200Å in the former phase.…”

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AbstractAccording to experimental data on X-ray scattering and reflectometry with the use of synchrotron radiation, a two-dimensional crystallization phase transition in a monolayer of melissic acid at the n-hexane -water interface with a decrease in the temperature occurs after a wetting transition.The observation of a two-dimensional solid -liquid phase transition at the oil -water interface was reported in [1,2,3]. In this work, the temperature dependence of the intensity of diffuse (nonspecular) scattering of 15-keV photons at the n-hexane -water interface, where such a transition occurs in the adsorbed layer of melissic acid (C 30 -acid) [3], is studied.

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“…The observation of a two-dimensional solid -liquid phase transition at the oil -water interface was reported in [1,2,3]. In this work, the temperature dependence of the intensity of diffuse (nonspecular) scattering of 15-keV photons at the n-hexane -water interface, where such a transition occurs in the adsorbed layer of melissic acid (C 30 -acid) [3], is studied.…”

“…On the other hand, the data shown in Fig. 3 indicate a gradual decrease in the scattering intensity I diff to ≈ 2 · 10 −3 in the liquid phase with an increase in the temperature from T c to T * ≈ 320 K. In [3,5], the phases of the monolayer of melissic acid were described within a qualitative two-layer model with the structure factor (7) of the form…”

“…The reflectometry data obtained with synchrotron radiation previously reported for this system show that molecules of C 30 -acid are adsorbed at the n-hexane -water interface in form of the Gibbs monolayer with the thermodynamic parameters (p, T, c) [3]. A sharp phase transition from a crystal state with the area per molecule A = 17 ± 1Å 2 to a liquid state with the area per molecule A = 23 ± 1Å 2 occurs in the monolayer at a pressure of p = 1 atm and a temperature of T c ≈ 293.5 K. The density of the low-temperature solid phase of the Gibbs monolayer corresponds to the packing in the crystal phase of Langmuir monolayer of C 30 -acid on the surface of water and is close to the volume density of the corresponding crystal [6,7].…”