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

Abstract

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

Model-Independent X-Ray Scattering Study of a Silica Sol Surface

“…This deviation is due to the contribution from roughnesses with low spatial frequencies ν < 10 −4 nm −1 and to the interference of diffuse scattering from different layer interfaces of the surface structure. The deviation of the integral characteristic σ of the spectrum C(ν) from the values calculated within the capillary-wave theory was previously reported, e.g., for oilwater interfaces in [42,43]. Usually, the spectral features of C(ν) responsible for this deviation were not indicated in the cited works, where the interpretation was reduced to the assumption of the existence of a certain intrinsic structure.…”