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

2018

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The electric density profile along the normal to the phase interface between aromatic hydrocarbon toluene and water has been studied by X-ray reflectometry using synchrotron radiation. According to the experimental data, the width of the interface under normal conditions is (5.7±0.2)Å. This value is much larger than a theoretical value of (3.9±0.1)Å predicted by the theory of capillary waves with an interphase tension of (36.0 ± 0.1) mN/m. The observed broadening of the interface is attributed to its own diffuse near-surface structure with a width no less than 4Å, which is about the value previously discussed for high-molecular-weight n-alkane -water and 1,2-dichloroethane -water interfaces.

“…About 50 mL of toluene was used as the upper bulk phase. Before the measurements of the reflection coefficient R, the sample was annealed and was aged for several hours [11].…”

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confidence: 99%

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confidence: 99%

On the Diffuse Structure of the Toluene–Water Interface

2018

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“…In the coordinate system where the origin O lies at the center of the illuminated region, the plane xy coincides with the interface, the Ox axis is perpendicular to the beam direction, and the axis Oz is normal to the surface and is directed opposite to the gravitational force, the components of the scattering vector q = kinksc in the interface plane are qx ≈ k0φ and qy ≈ k0(α 2 − β 2 )/2 and the normal component is qz ≈ k0(α + β) (α, β << 1, φ ≈ 0). According to the method described in [9,16], the interface sample was prepared in a thermostatic cell, which was then placed on an optical table with active vibration isolation. Deionized water (Barnstead, NanoPureUV) with a volume of ∼ 100 mL was used as the bottom bulk phase.…”

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confidence: 99%

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

2017

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Using synchrotron radiation with a photon energy of 15 keV, the molecular structure of an adsorbed n-triacontanol layer at the nhexadecane -water interface in different its phase states has been studied by the method of diffuse X-ray scattering. The analysis of the experimental data shows that a transition to the multilayer adsorption occurs at a temperature below the two-dimensional vapor -liquid transition at the interface. This transition has been attributed to a feature in the temperature dependence of the concentration of micelles in a surface layer ∼ 100 ÷ 200Å thick.Various reversible two-dimensional transitions between surface mesophases of fatty alcohols and acids (lipids) are possible in an adsorbed film at the n-alkane -water interface [1][2][3][4][5][6][7]. In particular, a solid -vapor phase transition is observed in a fluoroalkanol Gibbs monolayer [5,8] and a solid -liquid phase transition is observed in a carbon acid monolayer [9]. It was briefly reported in [10] that a liquid -vapor thermotropic phase transition occurs in adsorbed n-triacontanol (C30-alcohol) film at the n-hexadecane -water interface. In this work, the molecular structure of the neutral surface mesophases of this lipid is studied by diffuse (nonspecular) X-ray scattering and reflectometry with the use of synchrotron radiation. It is shown that a transition from the structure with a width of ∼ 3Å to a monolayer with a thickness of ≈ 27Å and, then, to a structure 100 ÷ 200Å thick occurs with decreasing temperature in a relatively narrow temperature range. We attribute the latter transition to an increase in the concentration of micelles in the surface layer.An adsorption film at the planar oilwater interface can be considered as a two-dimensional thermodynamic system with the parameters (p, T, c), where p is the pressure and c is the concentration of the lipid in the volume of the hydrocarbon solvent [11][12][13]. According to [10], the liquidvapor transition in the adsorbed C30-alcohol film at the n-hexadecane -water interface at p = 1 atm and c ≈ 0.6 mmol/kg is observed at Tc ≈ 300 K. The corresponding temperature dependence of the interfacial tension γ(T ), measured by the Wilhelmy plate method, is shown by closed circles in Fig. 1 [10 , 14]. A change in the slope of is due to a change in the surface enthalpy at the transition ∆H = −Tc∆(∂γ/∂T )p,c = 0.42 ± 0.04 J/m 2 . At the same time, the transition to the C30-alcohol monolayer at the n-hexanewater interface (open circles in Fig. 1) is characterized by the tripled value ∆H = 1.3 ± 0.1 J/m 2 .The reflection coefficient R and the intensity of diffuse surface scattering In of X rays at the n-hexadecane -water interface were measured at the X19C beamline of the National Synchrotron * tikhonov@kapitza.ras.ru

“…It was reported earlier that melissic acid (C30-acid) is adsorbed at the interface between nhexane and an aqueous solution of sulfuric acid (pH ≈ 2) as a protonated multilayer in which there exist three thermotropic mesophases [5]. On one hand, an increase in the temperature T (at a pressure of p = 1 atm) leads to a solid -liquid phase transition in the monolayer immediately located at the interface (Gibbs monolayer), the transition temperature Tc being determined by the concentration of the surfactant in the bulk phase of n-hexane, which serves as a reservoir for the former [6]. On the other hand, upon a decrease in the temperature, the two-dimensional crystallization phase transition at the interface is preceded by a transition at T * > Tc to multilayer adsorption.…”

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confidence: 99%

“…In the qualitative two-layer model suggested in [6] for the parametrization of the phases of the Gibbs monolayer of melissic acid, the structure factor has the form…”

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confidence: 99%

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

2017

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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.