Structure and kinetics of fatty acid Langmuir monolayers on zinc salt solutions

Influence of Unsaturations on the Organization and Air Reactivity of Triglyceride Monolayers

The behavior of Langmuir monolayers of saturated and unsaturated C18 triglycerides (TAGs) was studied as two-dimensional (2D) models for paint systems at the air-water interface or transferred onto solid films. The organization of saturated tristearin (C18:0) monolayer was probed thanks to grazing incidence X-ray diffraction: one observes a hexagonal packing of the chains perpendicular to the air-water interface, in contrast to what is described in the literature. Conversely, the mono- and polyunsaturated TAGs (triolein C18:1, trilinolein C18:2, and trilinolenin C18:3) monolayers do not present any organization at the air-water interface but do exhibit peculiar reactivity regarding the air atmosphere. The obtained results derived from the evolution of surface pressure-molecular area isotherms and monolayer compressibility under different atmospheres over time, combined with ultrahigh-vacuum infrared (UHV-FTIR) spectroscopy, showed the adsorption of O₂ molecules in the monolayer together with chemical reactions with hydrocarbon chains. The kinetic effect of lead ions, known to be efficient siccative agents in oil paints, was also assessed: the addition of Pb²⁺ in the subphase induces an increase of the O₂ adsorption.

Spontaneous collapse of palmitic acid films on an alkaline buffer containing calcium ions

Understanding the interaction of ions with fatty acids is important to identify their roles in various bioprocesses and to build novel biomimetic systems. In this study, the molecular organization of palmitic acid (PA) films on alkaline buffer solutions (pH 7.4) with and without divalent Ca²⁺ was measured at a constant surface area using Langmuir troughs coupled with microscopy and X-ray interfacial techniques. Without Ca²⁺, PA molecules remained a monolayer organization; however, with Ca²⁺, formation of the inverted bilayers of PA-Ca²⁺ superstructures caused a spontaneous 2D to 3D transformation under no compression due to the strong interaction between PA and the divalent cation. Self-assembly of this highly-organized inverted bilayer superstructure involved a two-step process of nucleation and nuclei growth. During nucleation, densely packed PA and Ca²⁺ monolayer firstly corrugated and some of PA and Ca²⁺ molecules ejected out from the monolayer; the ejected molecules then reorganized and formed the inverted bilayer nuclei. Nucleation was followed by nuclei growth, during which PA and Ca²⁺ in the monolayer kept integrating into the inverted bilayer structure through molecule migration and PA rotation around Ca²⁺.

Inorganic mixed phase templated by a fatty acid monolayer at the air-water interface: the Mn and Mg case

We studied by means of Grazing Incidence X-ray Diffraction (GIXD) coupled with X-ray fluorescence spectroscopy the structure of a behenic acid monolayer spread at the surface of Mg²⁺/Mn²⁺ mixed aqueous solutions. For the pure Mg²⁺ and Mn²⁺ aqueous solutions, the cations induce at the surface different 2D lattice superstructures of the organic monolayer. These superstructures correspond to an inorganic organized monolayer anchored to the hydrophilic group of the ordered behenic acid monolayer. Among the various diffraction peaks, we focused on those characteristics of the behenic acid oblique cell. As the Mg²⁺ mole fraction x increases in the Mg²⁺/Mn²⁺ mixed subphase, a continuous evolution of the oblique cell parameters is observed indicating the insertion of Mg²⁺ cations in the Mn²⁺ ordered monolayer. Then, a further increase leads to the appearance of a coexistence between two oblique surface phases. The cell parameters of both phases evolve continuously along the x range of the transition until a single Mg-rich ordered phase is detected. However, although the intensities of the peaks in the coexistence region are in agreement with a first-order phase transition, the cell parameters evolve simultaneously. Considering a thermodynamics analysis, this evidences that, apart from the concentration, another unidentified intensive parameter is varying. We suggest that it is the ionic strength, which appears to be strongly related to the concentrations.

Cation-induced monolayer collapse at lower surface pressure follows specific headgroup percolation

A Langmuir monolayer can be considered as a two-dimensional (2D) sheet at higher surface pressure which structurally deform with mechanical compression depending upon the elastic nature of the monolayer. The deformed structures formed after a certain elastic limit are called collapsed structures. To explore monolayer collapses at lower surface pressure and to see the effect of ions on such monolayer collapses, out-of-plane structures and in-plane morphologies of stearic acid Langmuir monolayers have been studied both at lower (≈6.8) and higher (≈9.5) subphase pH in the presence of Mg^{2+},Ca^{2+},Zn^{2+},Cd^{2+}, and Ba^{2+} ions. At lower subphase pH and in the presence of all cations, the stearic acid monolayer remains as a monolayer before collapse, which generally takes place at higher surface pressure (π_{c}>50mN/m). However, at higher subphase pH, structural changes of stearic acid monolayers occur at relatively lower surface pressure depending upon the specific dissolved ions. Among the same group elements of Mg^{2+},Ca^{2+}, and Ba^{2+}, only for Ba^{2+} ions does monolayer to multilayer transition take place from a much lower surface pressure of the monolayer, remaining, however, as a monolayer for Mg^{2+} and Ca^{2+} ions. For another same group elements of Zn^{2+} and Cd^{2+} ions, a less covered bilayer structure forms on top of the monolayer structure at lower surface pressure, which is evidenced from both x-ray reflectometry and atomic force microscopy. Fourier transform infrared spectroscopy confirms the presence of two coexisting conformations formed by the two different metal-headgroup coordinations and the monolayer to trilayer or multilayer transformation takes place when the coverage ratio of the two molecular conformations changes from the critical value (p_{c}) of ≈0.66. Such ion-specific monolayer collapses are correlated with the 2D lattice percolation model.