Cholesterol-dipalmitoyl cephalin and cholesterol-dilauroyl cephalin monolayers spread on silicic acid substrates

Mixed cholesterol-dipalmitoyl cephalin and cholesterol-dilauroyl cephalin monolayers are slightly more expanded on silicic acid substrates than on silica-free substrates. Plotting the mean molecular area of the mixed monolayers against the mole fraction of cephalin shows that cholesterol produces condensation of the cephalin monolayer whether or not the substrate contains silicic acid, and the more expanded the pure phospholipid film, the greater is the condensation produced. These phenomena have been tentatively interpreted in terms of hydrophobic interactions between hydrocarbon chains and electrostatic interactions between the horizontally oriented polar groups of the cephalin molecules.     

Properties of mixed monomolecular films of poly(benzyl-methacrylate) and arachidic acid

Binary mixed monomolecular films of poly(benzyl-methacrylate) and arachidic acid at the water/air interface have been studied with respect to compatibility and stability. The surface pressure — area isotherms indicate compatibility of the two components. However, the miscible state is unstable at high surface pressures. This is demonstrated by the constant pressure relaxation of the mixtures. There is a mechanism of separation and nucleation of the arachidic acid from the film for mixtures with high polymer content at a surface pressure of 30 mN/m. For lower concentrations of polymer in the mixtures and at a surface pressure of 20 mN/m the fatty acid is stabilized by the polymer.     

Simulation of a monolayer adsorbed on a heterogeneous surface: effect of the size and shape of the adsorbate on the structure of the interface and the distribution of the adsorption energies

At the low temperature limit, the effect on the size and shape factors of the adsorbate molecule is shown in a simulation of a monomolecular phase adsorbed on a heterogeneous surface. This factor is reflected both in the theoretical distribution of the adsorption energies and the packing of the adsorbed phase.     

Effect of polymerized silicic acid on mixed lipid-protein monolayers used as cell membrane models. I. Hemoglobin-stearoyl erythroceramide films

The effect of polysilicic acid on mixed monolayers of hemoglobin and stearoyl erythroceramide used as cell membrane models has been studied. The presence of the sphingolipid at the interface stabilizes the hemoglobin monolayer, hindering expulsion of its residues from the surface when the collapse pressure (ca. 23 mN/m) is reached. The interaction between the components of the mixed film results in non-additivity of their molecular areas to a degree depending on film composition and substrate pH. The presence of polysilicic acid in the substrate increases the miscibility of the components in the mixed monolayer as a result of ionic interaction between the silicic acid and the protein.     

Gas adsorption by a crystalline silicic acid

Crystalline silicic acids are prepared from alkali layer silicates by exchanging protons for the alkali ions. The acid H₂Si₂₀O₄₁ · xH₂O (parent material K₂Si₂₀O₄₁ · xH₂O) exhibits some outstanding gas adsorption properties which are related to the layer structure and the interlamellar microporosity. The external surface, about 20 m² g^–1, is estimated from nitrogen adsorption data after blocking the micropores. Slit-shaped ultramicropores (with diameters similiar to that of the nitrogen molecule) between the layers are widened to supermicropores near the crystal edges. During an adsorption run the nitrogen molecules penetrate more deeply into the ultramicropores. Nitrogen molecules strongly adsorbed in the ultramicropores are not desorbed at 77 K. Additional amounts of nitrogen are adsorbed by widening of the slit-shaped micropores at the crystal edges when pressure increases. This process proceeds slowly and is reversible.     

Azeotropic transformation in mixed monolayers of octadecylurea and hexadecylurea

The surface pressure of mixed monolayers of octadecylurea and hexadecylurea has been measured as a function of mean area per molecule at various temperatures and compositions. The surface pressure of the phase transition obtained was observed to decrease both with an increase in temperature and with an addition of another component. With the aid of the thermodynamic method developed previously, the apparent molar entropy and energy changes associated with the phase transition were found to be positive. These positive values were explained by the rupture of the intermolecular hydrogen bonding. Furthermore, the activity coefficients of film-forming components with reference to their respective pure components were considered in connection with the mutual interaction between octadecylurea and hexadecylurea molecules. It was concluded that the system exhibits the negative azeotropy as a result of the difficulty in forming the hydrogen bonding in the mixed monolayer.     

Monolayers of neutral and charged polyamino-acids spread on silicic acid substrates

Surface pressure-area isotherms (-A) of poly(L-alanine) (PA), poly(L-glutamic acid) (PG), and poly(L-arginine) (PArg) monolayers spread on substrates of varous pH values containing silicic acid were recorded and compared with those obtained on silica-free substrates. The compression curves of PA showed a pH-independent plateau region which was assumed to correspond to a monolayer to bilayer transition. The inflection observed in the -A curves of PG was attributed to the formation of loops made up of the terminal heads of the polymer and lying under the monolayer. Polysilicic acid interacted with the PArg films at pH 6; such films were more rigid and stable and had larger specific areas than those obtained in the absence of silica, which prompts the occurrence of an ionic interaction. The other two polyamino-acids did not interact with silicic acid at any pH value, so we may rule out the formation of hydrogen bonds between the silanol groups of polysilicic acid and the peptide or carboxyl groups of these amino acids.     

Phase and miscibility behavior of binary lipid systems

Differential scanning calorimetry was applied to study the phase diagrams of the following binary lipid systems: myristic acid (C₁₃COOH) / pentadecanoic acid (C₁₄COOH); palmitic acid methyl ester (C₁₅COOMe) / heptadecanoic acid methyl ester (C₁₆COOMe); palmitic acid methyl ester (C₁₅COOMe) / stearic acid methyl ester (C₁₇COOMe); palmitic acid methyl ester (C₁₅COOMe) / arachidic acid methyl ester (C₁₉COOMe). A distinct succession in the phase diagram types and phase regions was observed, according to the chemical structure of the mixing components.In the systems C₁₃COOH/C₁₄COOH; C₁₅COOMe/C₁₆COOMe and C₁₆COOMe/ C₁₇COOMe, both components are completely miscible in the low- and high-temperature phase. Contrasting with these three binary lipid systems, the system C₁₅COOMe/ C₁₄COOMe shows complete miscibility only in the high-temperature phase, but almost complete demixing in the solid state. In the phase diagram an incongruent melting 11 complex is built up. This complex forms an eutectic mixture with the phase of C₁₅COOMe and a peritectic system with C₁₉COOMe.     

Stabilization of alkylated azacrown ether by fatty acid at the air-water interface

The adsorbed amount of partially deuterated dihexadecyl-diaza-18-crown-6 ether (d-ACE16) in the presence of different chain length fatty acids as a function of surface pressure was determined by neutron reflectometry technique. The highest adsorbed amount of the azacrown ether was observed for the mixture of ACE16 with hexadecanoic (palmitic) acid, pointing to the importance of chain length matching between the two species for optimum stabilization of the mixed monolayer. The contrast variation technique was used to estimate the contribution to the total adsorbed amount from stearic acid and ACE16. It was found that the mixed Langmuir monolayer is stable against dissolution up to a surface pressure of 20 mN m(-1). Above this pressure, however, the spread and adsorbed amounts start to deviate, indicative of partial dissolution into the aqueous subphase. The consequences of this behavior for the transport of metal ions through the interfaces of permeation liquid membranes (PLMs) are discussed.     

Incorporation and exclusion of long chain alkyl halides in fatty acid monolayers at the air-water interface

Mixed monolayers of deuterated palmitic acid C(15)D(31)COOH (dPA) and deuterated stearic acid C(17)D(35)COOH (dSA) with 1-bromoalkanes of different alkyl chain length (C(4) to C(16)) at the air-water interface were investigated. Alkanes and 1-chlorohexadecane ClC(16)H(33) (ClHex) were also studied to compare the effects of the halogen on the mixed monolayers. Surface pressure-area isotherms and Brewster angle microscopy (BAM) were used to obtain the organization and phase behavior, providing a macroscopic view of the mixed monolayers. A molecular-level understanding of the interfacial molecular organization and intermolecular interactions was obtained by using vibrational sum frequency generation (SFG) spectroscopy and infrared reflection-absorption spectroscopy (IRRAS). It was found that from the alkyl halide molecules investigated 1-bromopentadecane, BrC(15)H(31) (BrPent), 1-bromohexadecane, BrC(16)H(33) (BrHex), and ClHex incorporate into the fatty acid monolayers. Alkanes of 15- and 16-carbon chain length do not incorporate into the fatty acid monolayer, which suggests that the halogen is needed for incorporation. Isotherms and spectra suggest that BrHex molecules are squeezed out, or excluded, from the fatty acid monolayer as the surface pressure is increased, while BAM images confirm this. Additionally, SFG spectra reveal that the alkyl chains of both fatty acids (dPA and dSA) retain an all-trans conformation after the incorporation of alkyl halide molecules. BAM images show that at low surface pressures BrHex does not affect the two-dimensional morphology of the dPA and dSA domains and that BrHex is miscible with dPA and dSA. We also present for the first time BAM images of BrHex deposited on a water surface, which reveal the formation of aggregates while the surface pressure remains unchanged from that of neat water.     

Melittin and ionic surfactant interactions in monomolecular films

The interaction between the anionic and cationic surfactants with Melittin spread monolayers at the air-water interface was investigated. The addition of anionic Cl^–, under the films of Melittin gives rise to a change in both surface pressure and surface potential. These interactions are different when surfactants are present, due to specific interactions between Melittin and the ionic-surfactants.     

Changes in electrokinetic properties of natural rubber latex after surface chemical modifications

High-ammonia latex concentrate prepared from doubly-concentrifuged fieldHevea latex was exhaustively dialyzed to remove any residual water-soluble non-rubber constituents. The specially purified latex was then treated with specific chemical reagents to modify the surface ionogenic groupings originally present on the latex particle surface. The electrophoretic mobility of the modified latexes was investigated as a function of pH. The change in electrokinetic properties of the surface-modified latex was explained in terms of chemical modification to the ionogenic groups of the adsorbed layer of proteins and long chain fatty acid soaps on the latex particle surface, the negative charges of which are primarily responsible for the colloidal stability of the latex. For comparison direct extraction of the long-chain fatty acid soaps from the specially purified latex by solvent was also carried out. Present results indicate that the number of carboxyl groups from the adsorbed long-chain fatty acid soaps plays a major role in the stabilization of the latex concentrate. In comparison the contribution of negative charges from the adsorbed proteins towards the stability of the latex is of less importance.     

Lipolysis of didecanoyl-lecithin/triolein mixed monolayers by phospholipaseA 2

A study of enzyme lipolysis by pancreatic phospholipaseA ₂ and by vipera berus phospholipaseA ₂ on monomolecular mixed films of didecanoyl-lecithin and triolein on an aqueous subphase of pH 8 has been carried out. The influence of the composition of the mixed film, the surface pressure of the film and the amount and type of the injected enzyme on the lipolysis rate were studied.In order to relate the lipolytic activity with the monolayer state, the compression isotherms of the didecanoyl-lecithin/triolein mixed monolayers have also been obtained.The resuls are compared to observations on lipolytic activity of phospholipaseA ₂ on the didecanoyl-lecithin/cholesterol mixed monolayers. Triolein improves the kinetic conditions of the lipolysis of lecithin films in a higher degree than cholesterol. Probably it increases the enzyme penetration by the fluidifying effect exerted on the lecithin monolayers.     

Molecular properties of acetone and some of its chloroderivatives adsorbed at the surface of water

The relations between the electric surface potential (V) and the surface tension () of aqueous solutions of acetone, chloroacetone, 1,3-dichloroacetone, and their concentration were investigated. The vertical components of dipole moments of the above mentioned compounds were determined using the Helmholtz equation. The calculations were carried out on the basis of surface excess values, which were obtained from surface tension measurements and surface potential changes. Once the vertical component of dipole moments were found and the orientation of adsorbed molecules was assumed, the local dielectric permittivities of the surface monolayer were estimated.     

Effects of simulated acid rain on the surface tension of selected leaves

In this study the effect of simulated acid rain on the surface tension of leaves of selected crop plants is reported. The contact angle measurements of liquids (water and n-propanol mixtures) on various plant leaves (bean, mustard, lettuce, cress) showed that the acid rain effect can be investigated when analyses of the surface tension of leaves are estimated. Acid rain was found to give rise to a change in the polar surface tension of leaves, while the apolar surface tension was unaffected. This effect was strongest in bean leaves, which suggests that this is related to the polar surface tension of leaves.     

Behavior of pure and mixed DPPC liposomes spread or adsorbed at the air-water interface

Liposomes from pure dipalmitoylphosphatidylcholine (DPPC) and mixed DPPC: distearoylphosphatidylcholine (DSPC): soybean lecithin (SL) prepared by the Bangham method with sonication were dispersed into solution or spread at the interface and the kinetics of the surface film formation was studied by measuring and recording the evolution of superficial tension, surface potential, and superficial (¹⁴C labeled) DPPC density.A simple theoretical approach can describe these kinetics by two processes: irreversible diffusion of closed vesicles into or from the bulk phase, and irrevers ible transformation of closed spherical vesicles into destroyed ones which form the surface film. Diffusion controls the phenomenon for small initial amounts of liposomes.Transformation controls the phenomenon for important initial amounts of liposomes. The kinetic constant of the transformation,K, does not depend on the technique used to form the surface film (spreading or adsorption).The equilibrium and rheological properties of surface films formed after liposome spreading are compared to those of monolayers     

Solutions of alkali soaps and water in fatty acids XIII. Circumstances that regulate the extension of theL 2-phase and the role of the acid-soaps rich in fatty acid for the occurrence of the phase

A basic requirement for that type ofL ₂-phase which exists in the system sodium octanoate-octanoic acid-water is the formation of acid-soaps. In order for the phase to be formed at all, the temperature must lie above the melting point of the fatty acid so that a reaction in non-aqueous milieu between neutral soap and fatty acid is possible. In order to obtain the characteristic shape and complete extension of the phase in direction of high water content the temperature must be so high that also the hydrated acid-soaps occur in fluid state. On the other hand the temperature cannot be so high that the acid-soaps become unstable.At temperatures at which the phase has obtained its full extension those circumstances differs which in different regions regulate the location of the phase borders; they depend on the composition of the acid soaps and on their amounts. In that part of the phase where the molar ratio between octanoic acid and sodium octanoate lies between 2 and 3 and where one has a continuous transition from reversed to normal structure only the two acid octanoates 1 NaC₈ 2 HC₈ x H₂O and 1 NaC₈ 3 HC₈ x H₂O occur and both are at 20 °C in fluid state.At water contents from about 22 % to 40 % the hydrate-water molecules belonging to the first mentioned soap are capable of contributing actively to the formation of large aggregates of acid-soap, a process which however is counteracted by the inmixing of the latter acid-soap. This mixture of the two acid-soaps decides in this region where the border of the phase will lie in direction towards an increased content of sodium octanoate; the result is that in spite of the fact that the hydration is increased, the border is only slowly displaced towards a higher content of fatty acid. As soon as the hydration of the acid octanoates has been completed and the additional water occurs as unbound bulkwater, the location of the phase boundary will no longer be influenced by the water content — now it will be the amphiphilic composition of the acid-soaps that determines the location of the border and it remains at the molar ratio 2.5 between octanoic acid and sodium octanoate at water contents from about 40% and up to 82%.In the direction of decreasing content of neutral sodium octanoate and increased content of water theL ₂-phase both at the highest content of fatty acid and the highest contents of water will be in equilibrium with the water-richL ₁-phase; in the first mentioned region with theL ₁-phase below the lac where at the border it is saturated with octanoic acid and in the latter region with theL ₁-phase just above the lac, where the dilute sodium octanoate solution contains dissolved 1 NaC₈ 1HC₈ x H₂O. In the large central part of theL ₂-phase, from about 20 % to about 86 % of water, the location of the border is dominated by the acid octanoate 1 NaC₈ 3 HC₈ x H₂O and that makes an equilibrium with theL ₁-phase impossible; instead one has an equilibrium via a two-phase zone between the amphiphile-rich region of theL ₂-phase and its water-rich region. In the first region the location of the border is regulated by the decreasing capability of the hydrated acid octanoate 1 NaC₈ 3 HC₈ x H₂O to dissolve octanoic acid; in the latter it is regulated by the fact that 1 NaC₈ 3 HC₈ x H₂O is the most fatty acid-rich acid-soap that is formed and that the octanoic acid is very little soluble in water and in the aqueous solution of this acidsoap.The middle part of theL ₂-phase, especially the region between about 55 % and 82 % of water, constitutes a direct continuation of the liquid crystalline lamellarD-phase. The liquid crystalline character of theD-phase is lost at the transition, but the lamellar organization is retained. That the molecules at least up to a water content of about 40 % are of the original reversed type and have an elongated shape with a central part of hydrated polar groups, from which core the hydrocarbon chains extend in two opposite directions, is the reason to that they, at crowding, form transient layer-like agglomerates of tightly packed more or less parallel molecules; this facilitates the transformation to coherent double amphiphilic layers, in which all molecules lie with the hydrated polar groups outwards toward coherent domains of bulk-water, without another liquid phase occurs.     

Competition between atmospherically relevant fatty acid monolayers at the air/water interface

Competition and oxidation of fatty acids spread at the air/water interface were investigated using surface-specific, broad-bandwidth, sum frequency generation spectroscopy. At the air/water interface, a monolayer of oleic acid replaced a monolayer of deuterated palmitic acid at equilibrium spreading pressure. Subsequent oxidation of the oleic acid monolayer with ozone resulted in products more water soluble than the palmitic acid; therefore, the palmitic acid monolayer reformed at the surface. Results indicate that the surfactants on the surface of fat-coated tropospheric aerosols will only possess oxidized acyl chains after all less soluble species in the aqueous subphase have been removed through the processes of replacement at the surface and atmospheric oxidation.     

Adsorption of polar substances from binary and ternary mixtures on silica gel

The adsorption isotherms of acetone and methyl ethyl ketone from binary and ternary mixtures in benzene and n-heptane on silica gel were measured. The experimental adsorption data are discussed on the basis of changes of the composition of mixed solvent (benzene + n-heptane) in ternary mixtures. It has been found that the different structures of the surface phase correspond to the system investigated. The marked dependence of the adsorption on the solvent character is demonstrated. For benzene and ternary (ketone + benzene + n-heptane) mixtures a mixed character of the surface phase is observed whose composition is determined by competition of liquid components for silica surface as well as its tendency to complex. Bilayer model of the surface phase gives a good representation of the experimental data for binary systems benzene + ketone.     

Solutions of alkali soaps and water in fatty acids XI. Correlation between the acid sodium octanoate in the most water-rich part of the L2-phase and the acid soaps in two adjacent phases

The special nature of the outer-most water-rich region of theL ₂-phase in the ternary system sodium octanoate-octanoic acid-water is evidenced by its somewhat turbid appearance and by the character of its equilibria with adjacent phases. The phase contains aggregated acid sodium octanoate which is dispersed in a very dilute aqueous solution of sodium octanoate. The acid octanoate has the composition 1 NaC₈2 HC₈x H₂O and is composed of closely packed amphiphilic units, all with the polar groups in the same direction. This acid soap obviously forms double-layered aggregates with the lipophilic hydrocarbon chains pointing inwards and the polar groups pointing outwards towards the surrounding bulk-water. The phase is formed when octanoic acid is added to theL ₁-phase of the system just above the l.a.c.; in this aqueous solution, the acid reacts with dissolved acid octanoate 1 NaC₈1 HC₈x H₂O and that results in the formation of the slightly soluble acid soap 1 NaC₈ 2 HC₈x H₂O that separates as a new phase, the turbidL ₂ phase. On further addition of octanoic acid, the content of the mentioned acid soap increases until the solution phase is transformed into a liquid crystalline lamellarD-phase with the same acid soap composition. This formation of acid soap 1 NaA2 HA on addition of fatty acid to the dilute soap solution just above the l.a.c., has been known for a long time to occur in various systems containing a long-chain sodium soap. However, at suitably low temperatures, the reaction in these systems does not result in separation of the acid soap in the liquid crystalline, but in the solid crystalline state.