Angelina Angelova

Infrared spectra of Langmuir—Blodgett multilayers of docosylammonium phosphate

Abstract The composition and bonds in Langmuir—Blodgett (LB) films of docosylammonium phosphate are analyzed by means of transmission infrared spectroscopy. The study is carried out at pH values at which the amine is completely or partially protonated and where mono- and divalent phosphate anions are present in the subsolution. Comparison of the multilayer spectra with those of model ammonium and butylammonium mono- and dihydrogen phosphates shows that divalent counterions are predominantly included in the LB structures. The shift of the characteristic bands of NHand (P)OH vibrations indicates a formation of NH⋯O and (P)OH⋯O hydrogen bonds in the multilayers. These bonds probably determine the Y-type of the deposition and the stability of the three-dimensional structures. The spectroscopic data suggest that the Langmuir—Blodgett systems studied do not contain hydration water and that they are built up of uncorrelated (in the normal direction) monolayers of docosylammonium phosphate.

Wetting of heterogeneous monolayer surfaces

Abstract A three-phase system with a chemically heterogeneous solid/liquid interface and a homogeneous liquid/gas interface is built up by means of the Langmuir-Blodgett technique. The miscibilities of two cationic surfactants with the same hydrocarbon chain length but different chemical structure and dimension head groups are studied to form the heterogeneous monolayers. The application of the miscibility criterion reveals partial phase separation of the two components. Static contact angles are determined after the downward LB deposition of a first heterogeneous monolayer, and after the upward deposition of a second homogeneous monolayer. In the latter case, resembling wetting of a chemically heterogeneous solid by monocomponent liquid, an independence of the receding static contact angles on the heterogeneous composition is found. The insensitivity of wetting of the composition may result from the intermolecular interactions established for the partially immiscible monolayers. These interactions probably quench the difference in the properties of the heterogeneous mixtures at the high surface pressure at which the LB system is formed.

Swelling of bilayer lipid membranes

Abstract Short-range repulsive (“hydration”) forces dominate the interactions of many bilayer membranes, as they approach a close contact, and play an important role in the membrane swelling. The physical origin of these forces has still not been completely understood, especially for ionic bilayers. This study presents an extension of the osmotic stress method for evaluation of the bilayer membrane swelling in humid environment. Zwitterionic dimyristoylphosphocholine (DMPC) and ionic dioctadecyldimethylammonium bromide (DODAB) multilayers are studied by means of low-angle X-ray diffraction and Fourier transform infrared spectroscopy. The effect of humidity on the swelling and organization of the DMPC and DODAB multilayers is compared. The physical origin of the established strong short-range repulsive forces in both lipid systems is discussed.

Equilibrium surface properties of lipid mixtures of retinal, phosphatidylcholine and fatty acid derivatives at the air/water interface

Abstract The miscibilities of phosphatidylcholine, retinal and saturated fatty acid derivatives in surface phases at the air/water interface are investigated on the basis of the thermodynamic two-dimensional phase rule. The latter is applied to the ‘collapse’ pressure and the equilibrium surface pressure characteristics of binary lipid monolayers or spread amphiphilic mixtures, respectively. The equilibrium surface pressures (ESPs), at which insoluble lipid monolayers are in equilibrium with three-dimensional lipid phases, are determined by spreading of single-component or binary solutions of lipids in organic solvent up to supersaturation at the air/water interface. The kinetics of establishment of steady surface pressure values at supersaturation is followed depending on the nature of the lipid samples. ESPs and ‘collapse’ pressures of mixtures of dilaur-oylphosphatidylcholine (DLPC), all- trans retinal ( t -R) and lauric acid (LA) are studied at various lipid molar ratios. The compositional phase diagrams of the ESPs and ‘collapse’ pressures, obtained at a constant temperature, indicate that the interfacial miscibilities of both DLPC and t -R and DLPC and LA are non-ideal. Owing to its ‘bulky’ molecular structure and the tendency towards self-aggregation, dominated by intermolecular π-π interactions, the t -R component could be accommodated in the hydrophobic portion of the phospholipid membrane at mole fractions less than 0.5. The accommodation of the other neutral, rod-like fatty acid component in the DLPC matrix is probably favoured by the formation of intermolecular hydrogen bonding. Phase separation between DLPC and LA is evident from the thermodynamic results at high LA mole fractions (> 0.75) in the surface mixtures.

Does the tail-to-head molecular arrangement exist in asymmetrically deposited fatty acid alkylester Langmuir-Blodgett multilayers?

Abstract A major problem in the X-ray diffraction investigations of asymmetrically deposited fatty acid alkylester Langmuir-Blodgett (LB) films is whether the molecular overturning to a symmetrical multi-bilayer arrangement could always be detected. The conclusion for the occurrence of molecular overturning is related to the specificity of the electron density profiles (EDP) of the ester multilayers. The problem arises from the similarity of the EDP of the “tail-to-head” and the “head-to-head” arranged alkylester films. To establish the true structural arrangement, two approaches are applied in the present study for modification of fatty acid alkylester film EDP: (i) by using a ω-bromopentadecanoic acid methyl ester (BP) as a “label” of the molecular arrangement in propyl stearate (PS) and methyl arachidate (MA) multilayers, and (ii) by mixing of PS and MA. The structural arrangements, normal to the solid substrate, of X-type deposited pure and mixed LB films of PS, MA, and BP are studied by means of low-angle X-ray diffraction. The pure PS and MA films are characterized by a same single-layer periodicity. The X-ray patterns of mixed MA/PS films and of PS films, labeled by relatively low BP molar fractions, evidence a bilayer periodicity (head-to-head arrangement) of these fatty acid alkylester multilayers. Therefore, the multilayers studied possess symmetrical multi-bilayer structures, which could be detected by means of X-ray diffraction only under particular conditions.

On the origin of the long-range attraction between surface-confined DNA bases

Abstract Surface forces apparatus measurements (Phys. Rev. Lett., 73 (1994) 2780) have established strongly attractive long-range forces between thymine- and adenine-terminated ester monolayers approaching in an aqueous environment. The origin of these forces has not yet been completely understood. We report here a molecular modelling investigation of the forces between single adenine and thymine DNA bases and a molecular dynamics simulation of monolayers of the same amphiphilic esters. The tail regions of the monolayers are in a fluid, disordered state. However, the thymidine- and adenosine-terminated head groups show a tendency toward aggregation and domain formation. The simulation reveals that the orientation of the water molecules is well expressed only at close distances to the DNA bases. The hydrogen bonds between the nucleotides themselves, and with water, are explicitly considered in the performed simulations.

Solid-on-solid model applied to surface pressure of binary surfactant mixtures spread at the air-water interface

Abstract A theoretical approach is presented to relate the equilibrium surface pressure (ESP) of binary mixtures of water-insoluble surfactants to the equilibrium surface pressures of the pure components. The approach is based on the application of a two-dimensional (2D) solid-on-solid model for an exact computation of the surface pressure. The Hamiltonians describing the energetic cost for the creation of “mixed” interfaces are constructed considering configurations with one random walk. The performed numerical computations use a single parameter for an evaluation of the interfacial intermolecular interactions between like and unlike molecules. The predictions of the 2D solid-on-solid model in the case of non-ideal mixing are compared with experimental results for ESP of two-component phospholipid-alkanoic acid surfactant mixtures.

Comment on “Molecular organization of amphiphilic disc-shaped penta-alkynes in LB mono- and multilayers”, D. Janietz, D. Hofmann and J. Reiche, Thin Solid Films, 244 (1994) 794

Abstract The molecular arrangement of disc-like penta-alkynes in Langmuir-Blodgett films is discussed on the basis of the deposition conditions and the X-ray diffraction data. It is concluded that the films deposited at low transfer ratios exhibit the same crystalline structure as the bulk discotic materials, while those obtained at transfer ratios higher than 0.5 are of a bilayer arrangement.

A new type of arrangement of disc-shaped molecules in Langmuir-Blodgett films

Abstract The molecular arrangement induced in Langmuir-Blodgett films of a disc-like mesogene is studied by means of various structural and optical methods. The disc-like molecules show an “edge-on” arrangement with a bilayer periodicity normal to the solid substrate and a nematic arrangement in the monolayers planes.