Thermotropic phase transition in an adsorbed melissic acid film at the n-hexane–water interface

Abstract

A reversible thermotropic phase transition in an adsorption melissic acid film at the interface between n-hexane and an aqueous solution of potassium hydroxide (pH ≈ 10) is investigated by X-ray reflectometry and diffuse scattering using synchrotron radiation. The experimental data indicate that the interface freezing transition is accompanied not only by the crystallization of the Gibbs monolayer but also by the formation of a planar smectic structure in the 300-Å-thick adsorption film; this structure is formed by 50-Å-thick layers.

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

“…The intensity I0 of the incident monochromatic photon beam at a wavelength of λ = 0.825±0.002 Å was ∼ 10 10 photon/s. This method was recently used to study thermotropic transitions in protonated and ionized melissic acid absorption films at the n-hexane -water interface [18,19].…”

“…The method of I b determining is described in detail in [18]. The most intense peak on the curves corresponds to specular reflection at β = α, and the peak against the diffuse background at β → 0 illustrates an increase in the intensity of scattering atβ = αc (≈ 0.05 • ) [19,20].…”

“…The intensity of diffuse surface scattering In(β) on the thermal fluctuations (capillary waves) at the interface was calculated by the method presented in [18,19,26,27].…”

X-Ray Study of Thermotropic Mesophases of an Adsorbed n-Triacontanol Film at the n-Hexadecane–Water Interface

“…From data for R(qz) and In(β), we obtain information on the surface normal structure of the interface using the distorted wave Born approximation [20]. According to the model approach described in [16,18], the interpretation of experimental data is reduced to determining the parameters of the structure factor function of the interface Φ(q), which is in turn specified by the chosen model of the electron density distribution ρ(z) across the interface. Symmetric model profiles ρ(z) are constructed with the error function erf(x), which is used in the standard theory of capillary waves [21].…”

“…The parameters of the optical measurement scheme were considered in detail in [9,15,18]. At small glancing angles, the vertical dimension of the incident beam is determined by slits spaced from the center of the cell by a distance of ∼ 120 mm and the natural divergence of the beam ∼ 10 −4 rad is reduced to ∼ 2 • 10 −5 rad by two input slits with a gap of ∼ 10 µm at a distance of ∼ 600 mm.…”