The structure dynamics in the temperature range of the melting phase transition of a dimyristoyl-phosphatidylserine multilayer on the surface of a colloidal silica solution with a particle diameter of 5 nm has been investigated by X-ray reflectometry and grazing diffraction of 71-keV photons. The joint model and model-free analysis of the reflectometry data revealed a structure consisting of a surface lipid monolayer and a set of lamellar bilayers sandwiched between water layers, with a period of 150 Å. With an increase in temperature above the critical value one can observe a surface monolayer transition from a crystalline phase with a minimum area per lipid molecule of 40 ± 1 Å2 to a disordered (liquid) phase with a calculated area per molecule of 52 ± 2 Å2. At low temperatures, the data indicate that from five to eight H2O molecules are tightly bound to the PS fragment of the lipid in both the monolayer and the bilayer structures. However, above the transition temperature, approximately 14 water molecules are attached to the headgroups of the bilayer: this is almost twice as many molecules as the eight H2O molecules-per-headgroup in the surface monolayer.

Structure of adsorption layer of long-chain monoatomic alcohols: 1-dodecanol and 1-tetracosanol at the interfaces n-hexane – water and n-hexadecane – water in the vicinity of “liquid – vapor” thermotropic phase transition is investigated by the method of X-ray reflectometry at synchrotron source. Model-independent structural data obtained on the adsorption layers under investigation deviate considerably from the structural parameters which have been proposed previously within a model-based representation and discussed in previous publications on said systems. It is shown that in the low-temperature mesophase the adsorption film consists of a Gibbs monolayer, a transitional liquid region with thickness of two to three monolayers ~50 Å and an extended (wide up to 200 Å) layer of micelles. Presence of a plane of the closest approach of micellar layer to the adsorption film at the interface is established. Transition to the high-temperature mesophase is followed by liquefying and partial evaporation of alcanol film along with observed depletion of micellar layer down to its complete disappearance.

In the present work, the intramolecular O-H···O hydrogen bonding in 3-hydroxy-1,3-diphenylprop-2-en-1-one (keto-enol form of dibenzoylmethane, DBM) was investigated. For this purpose, the Raman spectra of polycrystalline samples of ordinary (H-DBM) and deuterated (D-DBM) 3-hydroxy-1,3-diphenylprop-2-en-1-one in the temperature range of 5–300 K were measured. It was found that low-temperature hydrogen bonding is extremely strong, the proton and deuteron are located in the midpoint of the O···O segment, and their ground and first excited vibrational states are located above the barrier U0 between the local minima. The vibrational frequencies, in this case, are 1543 and 1709 cm−1 for the proton and 1045 and 1087 cm−1 for the deuteron. As the temperature rises and the barrier height increases in H-DBM, the zero-point vibrational state of the proton begins to move into one of the local minima at T > 50 K, while the excited state remains in the broad single-well potential. The same is observed in D-DBM, but with a significant temperature delay. Compounds with donor (−OCH3) and acceptor (−NO2) substituents in the phenyl ring were also synthesized and their spectra were obtained. Both results confirm existing ideas about the nature of the extremely strong hydrogen bond. The quantum-chemical calculation of the vibrational spectrum of H-DBM and D-DBM is consistent with the experimental results.

The possibilities of X-ray reflectometry for studying the structure of planar liquid-phase membranes are demonstrated by the example of polyester films formed on the surface of deionized water from solutions of polylactoglycolide (PLG) in chloroform and tetraglycol (TG). It is found that the use of solutions with PLG concentrations ranging from 1 to 4 wt % or above 6 wt % leads to a proportional increase in the density of these films with preservation of their structure and thickness up to 25 Å. At a PLG concentration close to 5 wt % the PLG/TG system transits to an unstable state, characterized by intense penetration of PLG aliphatic chains into the water substrate bulk to a depth up to 100 Å.

The spectral properties of the correlation function of the heights of interlayer boundaries in a lamellar 1,2‑distearoyl-sn-glycero-3-phosphocholine film deposited on the surface of the hydrosol of amorphous silica nanoparticles are investigated within the scope of a model-independent approach based on the data of nonspecular X-ray scattering. The data analysis shows that the spectrum of interlayer roughnesses in a multilayer is qualitatively different from the prediction of the capillary wave theory in the entire considered interval of spatial frequencies.

The results of a systematic study on the adsorption of polylysine molecules of different lengths on the surface of a 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) monolayer in the liquid (LE) and condensed (LC) states are presented. A compressibility diagram and the Volta potential were recorded with the Langmuir monolayer technique and further analyzed with the empirical approach. The structure of the monolayer films with adsorbed polypeptides was studied with synchrotron X-ray reflectometry. Two- and three-layer slab models describe the reflectivity data fairly well and reveal both the significant structural changes and the dehydration of the polar groups induced by all polylysines used at the maximal coverage of the monolayer interface in both the LE and LC states. On the one hand, in the LE phase of the monolayer (area per molecule A ≅ 70 Ǻ2), the integrated electron density of the lipid headgroup region is approximately half the density contained in the clean monolayer. This indicates both significant compaction and dehydration in the polar groups of the lipids, caused by the adsorption of polypeptides. On the other hand, in the LC state (A ≅ 40 Ǻ2), the degree of the hydration of the polar region is similar to that for the initial DMPS monolayer. However, both the electron density and the thickness of the head group region differ significantly from the values of these parameters for the clean monolayer in the LC state.

An X-ray reflectivity analysis has shown that the composition of liquid substrates affects the structure of deposited Nafion (Teflon copolymer) films. A model of Nafion monomer, developed based on small-angle X-ray scattering data, is used to interpret the results of X-ray reflectivity analysis.

The authors present a review of the systematic studies of the structure of macroscopically planar thin films at the air-liquid interface (water, alkali solution and silica hydrosol). A common feature of the considered works is the application of a model-independent approach to the analysis of X-ray reflectometry data, which does not require a priori assumptions about the structure of the object under study. It is shown that the experimental results obtained with the laboratory source in some cases are qualitatively on par with the results of those obtained with the use of synchrotron radiation source. The reproducibility of the effect of spontaneous ordering in films of amphiphilic organic molecules (phospholipids) at the surface of the colloidal solution of silica nanoparticles is demonstrated. The possibility of influencing the kinetics of the in situ formation of a phospholipid film by enriching the liquid substrate with alkali metal ions is also discussed.

The structure of an adsorption octadecanamide film at the planar toluene-water interface is studied by X-ray reflectometry using synchrotron radiation with photon energy of 15 keV. The electron density (polarizability) profiles, according to which the interface structure is determined by the pH level in the water subphase, are reconstructed from experimental data with the help of a model-independent approach. For a high pH~11, the adsorption film is a crystalline octadecanamide monolayer with a thickness of about 2.6 nm, in which aliphatic tails of surfactant are extended along the normal to the surface. For low pH ~ 2, the thickness of the surface structure consisting of the crystalline monolayer directly on the toluene-water interface and a thick layer of deposited octadecanamide micelles reaches about 50 nm. In our opinion, the condensation of nonionogenic surfactant micelles for which the surface concentration of the surfactant increases significantly is caused by a change in the polarization direction upon a decrease in the pH level in the electric double layer at the interface between the water subphase and the octadecanamide monolayer. The shape of the reconstructed electron density profiles also indicates the existence of a plane of the closest approach of surfactant micelles to the interface at a distance of about 7 nm from it.

An integrated approach is applied to reveal fine changes in the surface-normal structure of 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS) monolayers at the air-lipid-water interface occurring in a liquid expanded (LE)-liquid condensed (LC) transition. The combination of the Langmuir monolayer technique, X-ray reflectometry, and molecular dynamics (MD) modeling provides new insight into the molecular nature of electrostatic phenomena in different stages of lipid compression. A homemade setup with a laboratory X-ray source (λ = 1.54 Å) offers a nondestructive way to reveal the structural difference between the LE and LC phases of the lipid. The electron density profile in the direction normal to the interface is recovered from the X-ray reflectivity data with the use of both model-independent and model-based approaches. MD simulations of the DMPS monolayer are performed for several areas per lipid using the all-atom force field. Using the conventional theory of capillary waves, a comparison is made between the electron density profiles reconstructed from the X-ray data and those calculated directly from MD modeling, which demonstrates remarkable agreement between the experiment and simulations for all selected lipid densities. This confirms the validity of the simulations and allows an analysis of the contributions of the hydrophobic tails and hydrated polar groups to the electron density profile and to the dipole component of the electric field at the interface. According to the MD data, the dependence of the Volta potential on the area per lipid in the monolayer has a different molecular nature below and above the phase transition. In the LE state of the monolayer, the potential is determined mostly by the oriented water molecules in the polar region of the lipid. In the LE-LC transition, these molecules are displaced to the bulk, and their effect on the Volta potential becomes insignificant compared with the contribution of the hydrophobic tails. The hydrophobic tails are highly ordered in the state of the liquid crystal so that their dipole moments entirely determine the growth of the potential upon compression up to the monolayer collapse.

The structure of an adsorption octadecanamide film at the planar toluene–water interface is studied by X-ray reflectometry using synchrotron radiation with photon energy of 15 keV. The electron density (polarizability) profiles, according to which the interface structure is determined by the pH level in the water subphase, are reconstructed from experimental data with the help of a model-independent approach. For a high pH ≈ 11, the adsorption film is a crystalline octadecanamide monolayer with a thickness of about 26 Å, in which aliphatic tails of surfactant are extended along the normal to the surface. For low pH ≈ 2, the thickness of the surface structure consisting of the crystalline monolayer directly on the toluene–water interface and a thick layer of deposited octadecanamide micelles reaches about 500 Å. In our opinion, the condensation of nonionogenic surfactant micelles for which the surface concentration of the surfactant increases significantly is caused by a change in the polarization direction upon a decrease in the pH level in the electric double layer at the interface between the water subphase and the octadecanamide monolayer. The shape of the reconstructed electron density profiles also indicates the existence of a plane of the closest approach of surfactant micelles to the interface at a distance of about 70 Å from it.

We present a brief review of application of a model-independent analysis of X-ray refleetometry data to our recent investigations of thin films of phospholipids DSPC and DMPS on a surfaces of water and colloidal silica hydrosol. Comparison of the results against analytical models of phospholipid membranes known in literature allowed us to extract more extensive information on the samples in question. The effect of spontaneous ordering of lipid films on a surface of colloidal silica is discussed. We also demonstrate the possibilities to influence the structure of prepared membranes by enriching the substrate with alkali ions.

The effect of the adsorption of a polypeptide on the lateral interaction of dimyristoylphosphatidylserine molecules in different phase states on the surface of a 10 mM KCl aqueous solution has been studied. Changes in the surface pressure and Volta potential induced by the adsorption of large poly-D-lysine molecules (about 200 links in a chain) have been determined at different areas per lipid molecule in a monolayer. The adsorption of macromolecules noticeably increases the elasticity of the monolayer under lateral compression in the liquid expanded state of lipid and reduces the effective dipole moment from 0.48 to 0.38 D. These properties are in qualitative agreement with X-ray reflectometry data for the lipid monolayer obtained with synchrotron radiation with a photon energy of ≈70 keV. The electron density profiles perpendicular to the surface of the aqueous subphase have been reconstructed from reflectometry data within a model approach to the structure of an interface with two and three layers. These profiles indicate the existence of a wide diffuse polymer layer (150 ± 40) Å in width at the interface of the monolayer in both the liquid expanded and liquid condensed states. A decrease in the area per molecule in the monolayer by a factor of 2 results in the doubling of the surface density of the macromolecule film. The adsorption of the polymer also affects the integral density of the layer of polar phospholipid groups, which decreases by a factor of ≈2 in the liquid expanded phase and by ∼30% in the liquid condensed phase.

Whispering galleries propagating along large-radius concave meniscuses at the surfaces of rotating deionized water as well as hydrosol of similar to 10 nm amorphous silica particles enriched by CsOH were probed by both x-ray reflectometry and x-ray fluorescence for the first time. The measurements were carried out at a wavelength of 1.5405 angstrom of Cu-K-alpha radiation by using a homemade diffractometer with a moving tube-sample-detector system. According to the experimental results, the x-ray beam deflection angle at the sol's surface reaches 4 degrees, which is roughly four times higher than that obtained on the water surface. The rigorous solution of the Helmholtz equation for the whispering gallery reflection mode at the concave liquid surface agrees well with experimental observations. We attribute the difference in the x-ray beam deflection angle for the studied liquids to the difference in their viscosity, which presumably is inversely proportional to the effective surface roughness. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

The structure of the soluble protonated (pH = 2) octadecanoic acid film adsorbed on the saturated hydrocarbon (n-hexane)–water and aromatic hydrocarbon (toluene)–water interfaces is studied by X-ray reflectometry using synchrotron radiation. The experimental data demonstrate that a solid phase of a Gibbs monolayer 26 ± 1 Å thick, in which aliphatic tails are perpendicular to the surface and the area per molecule is A = 18 ± 2 Å2, is formed in the film at the n-hexane–water interface. The solid monolayer on the toluene–water interface in the adsorbed film melts when temperature increases, and this transition is caused by disordering of the hydrocarbon tails of the acid. During the solid–liquid transition, the Gibbs monolayer thickness remains almost the same, 22 ± 1 Å. In the solid phase, we have A = 20 ± 2 Å2, and the angle of deviation of the molecular tails from the normal to the surface is about 30°. The density of the liquid monolayer phase with A = 24 ± 2 Å2 corresponds to liquid n-octadecane.