Interactions In Mixed Monolayers Research Articles

Interfacial characterization of the molecular interactions in mixed monolayers of coumarin and phospholipids

Coumarin (CMR) is a small molecule with diverse biological functions as anti-tumor, anti-fungal, anticoagulant and antimicrobial activities. CMR is poorly water soluble molecule with low bioavailability. Lipid-based colloidal carriers play an important role in the delivery of hydrophobic compounds. Langmuir technique is an effective method to evaluate interactions between drugs and lipids. In this study, we used 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) lipids. Interfacial characterization was carried out based on surface pressure (π)-area (A) isotherms of pure and mixed films. Topographical analysis was performed using atomic force microscopy. The miscibility of the mixed films was dependent on their composition. The mixed films were stable due to the intermolecular attractive interactions. Our results contribute to the understanding of CMR-lipid interactions aiming to obtain stable colloidal carriers for drug delivery.

Physical Characterization of Synthetic Phosphatidylinositol Dimannosides and Analogues in Binary Systems with Phosphatidylcholine

Native phosphatidylinositol mannosides (PIMs) from the cell wall of Mycobacterium bovis (M. bovis) and synthetic analogues have been identified to exert immunostimulatory activities. These activities have been investigated using particulate delivery systems containing native mannosylated lipids or total lipid extracts. Limited work has been carried out examining the incorporation of individual PIM lipids into suitable particulate formulations such as liposomes. The present study explored the possibility of constructing phosphatidylcholine (PC)-based liposomes containing synthetic PIM analogues. A series of six phosphatidylinositol dimannosides (PIM(2)s) and phosphatidylglycerol dimannosides (PGM(2)s) was characterized in this study. Binary Langmuir monolayers are a useful approach for establishing pharmaceutical properties, such as lipid-lipid interactions in mixed monolayers, to facilitate the design of liposome-based delivery systems. In mixed films the phosphoglycolipids were found to be miscible with PC based on evaluation of collapse pressures and deviations of experimental molecular areas from calculated ideal values. Concanavalin A (ConA) agglutination confirmed the presence of mannosylated lipids on the surface of the liposomes. Physicochemical properties of liposomes were affected by the presence of 2% (w/w) of phosphoglycolipids with liposome stability being increased by incorporation of long-chain PIM(2) and PGM2. Overall, while membrane stability of the liposomes was found to be dependent on incorporation of the phosphoglycolipids, all formulations retained proteins in amounts making them suitable for delivery.

Stability and interactions in mixed monolayers of fatty acid derivatives on Artificial Sea Water

We studied the formation and stability of fatty acid and derivatives films on aqueous sub-phases by means of Langmuir trough experiments. Films were prepared from pure stearic acid (SA), stearyl amine (SAm) and 12-phenyldodecanoic acid (PDA), and from binary systems of SA with either SAm or PDA. For the aqueous sub-phase, multicomponent salt solutions (‘Artificial Sea Water’) at various concentrations (cASW, 0–100%) and pH values (3–7) were explored. SAm and SA differ most strongly at pH 7, where they can dissociate into oppositely charged species. For SA:SAm mixtures at this pH, pressure-area isotherms indicate mixing on a molecular scale. Increase of salt concentration from 0.05 to 0.5M causes significant contraction of the layer, which is ascribed to (enhanced) electrostatic attractions between the head-groups. Relaxation experiments with these films indicate that dissolution of SAm into the sub-phase is suppressed by SA. The most stable films are formed at xSA=0.5, in agreement with the calculated excess free energy of mixing. PDA is much less amphiphilic than SA. (Meta-) stable mixed films of SA and PDA are formed only at high salt concentration and low pH, where the solubility of PDA into the sub-phase is the lowest. Under these conditions, SA appears to have a stabilizing effect, which is however not strong enough to prevent expulsion of PDA from interface. Isobars of SA:PDA films corroborate this picture. The weak interactions between SA and PDA are confirmed by the excess free energies of mixing, which are close to those of ideal mixtures.

Structure and Lateral Interaction in Mixed Monolayers of Dioctadecyldimethylammonium Chloride (DOAC) and Stearyl Alcohol

π-A isotherms, atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy are employed to investigate the molecular structure and lateral interactions in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol (SA) at air/water and air/solid interfaces. To avoid possible interference between the two molecules in the SFG spectroscopic measurements, perprotonated DOAC and perdeuterated SA (dSA) were used. The thermodynamic analyses for the π-A isotherms show that DOAC is miscible with dSA. SFG observations reveal that DOAC molecules become conformationally ordered as dSA molecules are introduced into the monolayer. AFM observations demonstrate coexistence of DOAC-rich and dSA-rich domains in the mixed monolayer with ratios different from their initial composition in the subphase. The present study suggests that DOAC molecules in the mixed monolayer are condensed by mixing with dSA in which the repulsive interactions between positively charged head groups of the DOAC molecules are largely reduced along with an increase of van der Waals interactions with dSA.

Hybrid lipid/polymer giant unilamellar vesicles: effects of incorporated biocompatible PIB–PEO block copolymers on vesicle properties

Self-assembly of lipids and block copolymers into mixed giant unilamellar vesicles (GUVs) and the underlying phase-behavior are reported, based on the well-known assembly of natural lipids and synthetic di- and triblock copolymers in dilute solutions. In this publication the formation of mixed vesicles containing DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and newly prepared PIB–PEO diblock copolymers (BCP) was investigated viaconfocal laser scanning microscopy and Langmuir films. Polyisobutylene (PIB) and poly(ethylene oxide) (PEO) are highly biocompatible polymers and their incorporation into DPPC vesicles therefore leads to interesting structural modifications upon the formation of hybrid GUVs. Results of the confocal microscopy studies prove the mixing/demixing behavior of lipid and polymer molecules in the corresponding bilayer membranes. In particular a homogenously mixed phase below 20 mol% and above 30 mol% of PIB87-b-PEO17, but also the formation of demixed phases of DPPC with 20 to 28 mol% of BCP were observed. Lipid/polymer interactions in mixed monolayers were further investigated by Langmuir balance techniques coupled with fluorescence microscopy. It is shown that the typical behavior of DPPC monolayers at the air/water interface is strongly disturbed by the presence of the diblock copolymers, further proving phase separation.

Microphase Separation in Mixed Monolayers of DPPG with a Double Hydrophilic Block Copolymer at the Air−Water Interface: A BAM, LSCFM, and AFM Study

Phase separation and interactions in mixed monolayers of dipalmitoylphosphatidylglycerol (DPPG) with the rhodamine B end-labeled double-hydrophilic block copolymer (DHBC), poly(N,N-dimethylacrylamide)-block-poly(N,N-diethylacrylamide) (RhB-PDMA(207)-b-PDEA(177)), was studied at the air-water interface. Surface pressure versus area isotherms indicate that both components behave almost independently. Brewster angle microscopy (BAM) images show a random distribution of liquid condensed (LC) domains of DPPG in an apparent homogeneous matrix of DHBC, excluding the macroscopic phase separation. The laser scanning confocal fluorescence microscopy (LSCFM) of the rhodamine dye at the end of the PDMA chain showed how the DHBC is distributed in Langmuir-Blodgett (LB) mixed monolayers. The high spatial resolution of atomic force microscopy (AFM) combined with the LCSFM images indicates that DHBC incorporates in the expanded phase of DPPG to form mixed domains, being excluded from the condensed regions. Upon compression, nanosized LC domains of DPPG nucleate inside the mixed domains corralled in the nanopatterning of pure DHBC. The negatively charged polar group of DPPG inhibits rhodamine aggregation, while the long polymer chains promote the formation of corralled nanodomains of DPPG in two dimensions.

Surfactant−Surfactant Molecular Interactions in Mixed Monolayers at a Highly Hydrophobic Solid/Aqueous Solution Interface and Their Relationship to Enhanced Spreading on the Solid Substrate

Spreading of mixed aqueous hydrocarbon-chain surfactant solutions on the solid polyethylene (PE) surface has been studied. Synergistic effect on the spreading of the mixed surfactant solution on the PE film has been observed, and the obtained spreading is comparable to superspreading normally obtainable from trisiloxane-based surfactants. Some other interfacial phenomena related to surfactant spreading, such as surfactant−surfactant molecular interactions in the mixtures adsorbed at various interfaces, dynamic contact angle change of the mixed surfactant solutions during the process of spreading on the hydrophobic PE substrate, and surfactant adsorption at the solid/liquid and air/liquid interfaces have been investigated. It is suggested that stronger surfactant−surfactant attractive interactions and greater adsorption at the PE powder/aqueous solution interface than at the air/aqueous solution interface account for the observed spreading enhancement in the mixed hydrocarbon-chain surfactant systems, which is also accompanied by lower dynamic contact angles, implying greater dynamic spreading coefficients.

Molecular Interactions in Mixed Monolayers of Octadecanoic Acid and Three Related Amphiphiles

Binary mixed monolayers of octadecanoic acid and three related amphiphilic compounds (octadecanamide, octadecylamine, octadecylurea) have been investigated at the air/water interface by surface pressure–area (Π–Â) isotherms and their resistances to water evaporation (r). In addition, the excess free energies of mixing (ΔGE) were calculated using the Goodrich method. Both the ln r vs x and ΔGE vs x plots exhibit marked deviations from linearity, indicating a high degree of miscibility and nonideal behavior of the components in the mixed films. For all of these binary systems the excess free energies of mixing have been found to be minimum for a certain composition corresponding almost to a maximum in evaporation resistances. Weak interactions were detected in octadecanoic acid/octadecanamide monolayers, whereas significant condensation effects were observed in 1 : 1 mixed films containing octadecanoic acid and octadecylamine. This is attributed to an acid–base equilibrium followed by the formation of a well-ordered arrangement of COO− and NH3+ head groups bound to each other by electrostatic forces. The unusual polymorphism of octadecylurea monolayers could be influenced by adding small amounts of octadecanoic acid. The formation of the low-temperature phase (β-phase) is completely suppressed, if the acid content exceeds 8 mol%. The octadecanoic acid seems to induce the formation of the high-temperature phase (α-phase), which is characterized by a vertical orientation of the hydrocarbon chains.

Detection of Peptide-Lipid Interactions in Mixed Monolayers, Using Isotherms, Atomic Force Microscopy, and Fourier Transform Infrared Analyses

To improve the understanding of the membrane uptake of an amphipathic and positively charged vector peptide, we studied the interactions of this peptide with different phospholipids, the nature of whose polar headgroups and physical states were varied. Three lipids were considered: dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and dioleoylphosphatidylglycerol (DOPG). The approach was carried out by three complementary methods: compression isotherms of monolayers and atomic force microscopy observations associated with Fourier transform infrared investigations. From analysis of the compression isotherms, it was concluded that the peptide interacts with all lipids and with an expansion of the mean molecular area, implying that both components form nonideal mixtures. The expansion was larger in the case of DOPG than for DPPC and DPPG because of an α to β conformational transition with an increase in the peptide molar fraction. Atomic force microscopy observations showed that the presence of small amounts of peptide led to the appearance of bowl-like particles and that an increase in the peptide amounts generated the formation of filaments. In the case of DOPG, filaments were found at higher peptide molar fractions than already observed for DOPC because of the presence of negatively charged lipid headgroups.

Molecular interaction in mixed monolayers at the air/water interface

Different approaches used to characterize molecular interaction in mixed monolayers, formed either by adsorption or by spreading at the air/water interface, are reviewed in this paper.

Synergism in Binary Mixture of Surfactants: 11. Mixtures Containing Mono- and Disulfonated Alkyl- and Dialkyldiphenylethers

Interaction and synergism at 25°C of some alkyl- and dialkyldiphenylether mono- and disulfonates with a second surfactant containing a single hydrophilic and a single hydrophobic group (a nonionic, a betaine, or an amine oxide type of surfactant) in aqueous solutions containing swamping amounts of NaCl were studied by calculating interaction (β) parameters from surface tension-concentration data. Attractive interaction in mixed monolayers at the aqueous solution/water interface increased in the order: monoalkyl monosulfonate (MAMS) < monoalkyl disulfonate (MADS) < dialkyl disulfonate (DADS). In mixed micelles, by contrast, the DADS interaction was always weaker than that of either the MAMS or MADS. This is believed to be due to greater steric inhibition of micelle formation by the DADS structure. As a result of these differences in interaction, the DADS (gemini-type) structure is more prone than the other two types of structures to show synergism in both surface tension reduction efficiency and effectiveness, and less prone to show synergism in mixed micelle formation.

Detection of phase transitions in monolayers using retardation of evaporation of water

The role of cholesterol in inducing the phase transitions in mixed monolayers with arachidyl alcohol and oleic acid has been studied by surface pressure and evaporation resistance measurements of the monolayers. The resistance of the monolayers to water evaporation indicates the effect of molecular interactions in mixed monolayers. The evaporation resistance of monolayers has been found to be a more sensitive parameter for studying the molecular interactions in mixed monolayers than the average area/molecule. A minimum of 15–20 mol% of cholesterol is required in a cholesterol-arachidyl alcohol mixture to completely fluidize the two-dimensional solid alcohol film. The evaporation resistance sharply decreases to a lower value at a molar ratio of 20:80 for cholesterol:arachidyl alcohol due to the phase transition in the monolayer. Cholesterol fluidizes the arachidyl alcohol monolayer by disrupting the association of saturated hydrocarbon chains of the alcohol. In the case of oleic acid-cholesterol mixed monolayers, 75 mol% of cholesterol is required to show a maximum solidifying effect. The evaporation resistance has been found to be maximum at this particular molar mixture. It is proposed that cholesterol molecules occupy the molecular cavities present in two-dimensional liquid oleic acid monolayers and thus reduce the evaporation of water.

Lipid-Protein Interaction in Mixed Monolayers from Phospholipids and Proteins

Abstract The thermotropic behavior of different proteins with phospholipids in monolayers was investigated by measuring the surface pressure and the surface viscosity. When phospholipids, such as dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC), were mixed with the hydrophobic proteins, such as hemoglobin and β-casein, below their phase transition temperatures (Tc), a considerable expansion of monolayers was observed. On the other hand, when phospholipids were mixed with the hydrophilic proteins, such as bovine serum albumin (BSA) and lysozyme below Tc, no expansion was observed. When DMPC was mixed with different proteins at Tc or above, all mixtures formed slightly deviated monolayers from ideal mixing, independent of a variety of proteins. This type of behavior suggests that the hydrophobic proteins perturb the structure of boundary lipids near the protein molecule below Tc, while the hydrophobic proteins do not perturb the phospholipid structure below Tc because of their low hydrophobicities. At Tc or above the membrane properties are independent of a variety of proteins, because all lipid hydrocarbon chains cause a disordering, even without proteins. In addition, when DPPC was mixed with a hydrophobic protein, such as hemoglobin, the fluidity of monolayer increased significantly.

The effect of pH on the interactions in mixed monolayers between phosphatidylserine and all- Trans retinal

The interactions in mixed monolayers between phosphatidylserine and all- trans retinal have been studied at the nitrogen/water interface at 21.0 ± 0.5°C. The monolayers were spread on a 1 × 10 −3 M phosphate buffer containing in addition 2.25 m M NaCl. Three pH values of the subphase were investigated, 5.6, 7.0, and 8.1. Two different phosphatidylserine samples were used. One was the highly unsaturated lipid component extracted from the disk membrane, the other being a commercial, much less unsaturated lipid sample (main components: stearic and oleic acids). The results show that the protonation state of the lipid films does not modify to any measurable extent the mixing pattern of PS with all- trans retinal. On the other hand, the difference in aliphatic composition between the two PS samples has small, but definite, bearing on the miscibility of these components at the interface. These results are consistent with the fact that hydrophobic interactions dominate the thermodynamics of mixing these constituents at the nitrogen/water interface.

Thermotropic behavior of binary mixtures of diplamitoylphosphatidylcholine and glycosphingolipids in aqueous dispersions

The thermotropic behavior of mixtures of dipalmitoylphosphatidylcholine (DPPC) with natural glycosphingolipids (galactosylceramide, phrenosine, kerasine, glucosylceramide, lactosylceramide, asialo-G M1, sulfatide, G M3, G M1, G D1a, G T1b) in dilute aqueous dispersions were studied by high sensitivity differential scanning calorimetry over the entire composition range. The pretransition of DPPC is abolished and the cooperativity of the main transition decreases sharply at mole fractions of glycosphingolipids below 0.2. All systems exhibit non-ideal temperature-composition phase diagrams. The mono- and di-hexosylceramides are easily miscible with DPPC when the proportion of glycosphingolipids in the system is high. A limited quantity (1–6 molecules of DPPC per molecule of glycosphingolipid (GSL) can be incorporated into a homogeneously mixed lipid phase. Domains of DPPC, immiscible with the rest of a mixed GSL-DPPC phase that shows no cooperative phase transition, are established as DPPC exceeds a certain proportion in the system. One negative charge (sulfatide) or four neutral carbohydrate residues (asialo-G M1) in the oligosaccharide chain of the glycosphingolipids results in phase diagrams exhibiting coexistence of gel and liquid phases over a broad temperature-composition range. Systems containing gangliosides show complex phase diagrams, with more than one phase transition. However, no evidence for phase-separated domains of pure ganglioside species is found. The thermotropic behavior of systems containing DPPC and glycosphingolipids correlates well with their interactions in mixed monolayers at the air/water interface.

Interactions in mixed monolayers between distearoyl- l-phosphatidylethanolamine, rod outer segment phosphatidylethanolamine and all- trans retinal. Effect of pH

The interactions in mixed monolayers between distearoyl- l-phosphatidylethanolamine, natural phosphatidylethanolamine purified from bovine rod outer segments and all- trans retinal have been studied at the nitrogen/water interface at 21.0 ± 0.5° C . Seven mixtures of each phospholipid with all- trans retinal, covering the whole range of molar fractions, were studied. The monolayers were spread on a 1·10 −3 M phosphate buffer subphase at three different pH values, 5.5, 7.1 and 8.2. The results for the two series of mixtures are strikingly different. The surface phase rule shows that all- trans retinal is miscible with the natural phospholipid at the interface. Small, negative deviations with respect to the additivity rule are observed in this case. The excess free energies of mixing were also calculated as a function of concentration for this system at four different surface pressures, 5, 7, 10 and 13 mN·m −1. They are negative for the four surface pressures considered and symmetrical with respect to the mole fraction. On the other hand, when distearoyl- l-phosphatidylethanolamine is mixed with all- trans retinal, the components are no longer miscible at the interface. This marked difference in behaviour between the two lipids reflects the importance of hydrophobic interactions in the mixed monolayers of phospholipids with retinals. Furthermore, for the two series of mixtures, the surface pressure isotherms do not show any significant shift when the subphase pH is changed from 5.5 to 8.2. This behaviour raises questions about the formation of a Schiff base between phosphatidylethanolamine and retinal at the interface. It is suggested that, owing to the nature of the disk membranes, such an effect would also be observed in vivo. The possible implications of this are discussed, particularly with respect to questions pertaining to the stability of the retinal chromophore.

Interactions in mixed monolayers between dioleoyl-l-phosphatidylcholine and 11-cis retinal. A comparison with all-trans retinal

The surface pressure isotherms of 11-cis retinal, diolcoyl-l-phosphatidylcholine and seven mixtures of these components covering the whole range of molar fractions have been studied at the nitrogen-water interface at 20.5°C. For all the mixtures, an almost constant collapse pressure at 44 ± 1 mN m−1 is observed. The surface pressure isotherms also show an apparent collapse pressure at 14, 16, 20, 24, and 28 mN m−1 at a molar fraction of dioleoyl-l-phosphatidylcholine of 0.100, 0.200, 0.400, 0.500, and 0.600, respectively. Below this apparent collapse pressure, it is shown that the components are completely miscible at the interface. However, this pressure represents the onset of the rejection of 11-cis retinal from the monolayer, this rejection appearing to be total at the true collapse pressure. The miscibility region of the isotherms is studied in terms of the additivity rule. Negative deviations to ideality are observed and negative excess free energies of mixing are calculated for all the systems at four different surface pressures. A comparison between 11-cis retinal and all-trans retinal mixtures with dioleoyl-l-phosphatidylcholine shows that the two isomers have about the same thermodynamics of interaction with this phospholipid.

Interactions in mixed monolayers between dioleoyl- l-phosphatidylcholine and all- trans retinal

The surface pressure isotherms of all- trans retinal and dioleoyl- l-phosphatidylcholine at the nitrogen-water interface were studied at 20.5°C. The extrapolated limiting areas per molecule are 49 and 83 Å 2 molecule −1, respectively. The properties of seven mixtures of all- trans retinal and dioleoyl- l-phosphatidylcholine, covering the whole range of molar fractions were also measured. The results show that an almost constant collapse pressure is observed at 44 ± 1 mN m −1 for all the mixtures studied. Furthermore, an apparent collapse pressure is also observed at 16, 18, 22, 26, 32, and 35 mN m −1 at a molar fraction of dioleoyl- l-phosphatidylcholine of 0.100, 0.200, 0.400, 0.500, 0.600, and 0.800, respectively. An analysis of the results in terms of the surface phase rule shows that below the apparent collapse pressure, the components are completely miscible. Negative deviations from ideality were observed in the region of miscibility for all the systems, as shown by the negative deviations to the additivity rule and the negative excess free energies of mixing calculated for all the systems at 5, 7, 10, and 13 mN m −1. Furthermore, it is shown that above the apparent collapse pressure, all-trans retinal is rejected from the monolayer, the rejection being total at the collapse pressure.

Mixed monolayers of simple saturated and unsaturated fatty acids

The conventional surface pressure-area per molecule isotherms of single and binary films in the acid series, (Z)-octadec-9-enoic (?oleic?), its (E) isomer (?elaidic?), octadecanoic (?stearic?) and eicosanoic (?arachidic?), have been obtained up to and including collapse. While the films formed from binary mixtures of saturated fatty acids were ideal at all mole fractions, the binary films formed with a saturated acid and the (E) isomer of octadec-9-enoic acid produced negative deviations from ideality; films of a saturated acid and the (Z) isomer produced positive deviations from ideality. Mixed films of the two unsaturated acids showed negative deviations at low mole fraction of (Z) isomer and positive deviations at low mole fraction of the (E) isomer. The role of chain stereochemistry and the effect of (Z) and (E) double bonds on hydrophobic interactions in mixed monolayers is considered.

Interactions in mixed monolayers. II. Effect on equilibrium surface properties, with application to fatty acid soap solutions

Existing surface equation of state theory for ideal surface mixtures is extended to cover mixed monolayers with both solvent-surfactant interactions and surfactant-surfactant interactions. The numerical effects of these interactions are illustrated for a first order interaction model. Equilibrium surface behavior for all surface compositions, then, is governed by a number of constant parameters, only one of which ( H 23) is determined by specific surfactant-surfactant interactions. Surface tension-concentration curves for sodium laurate-lauric acid solutions at pH values ranging from 7 to 11 are adequately described by this theory with H 23 = −2.5 RT. This type of interaction, especially when coupled with positive (“repulsive”) solvent-surfactant interaction parameters H 12 and H 13 decreases the surface pressure at given adsorptions, but increases the surface pressure at given bulk concentrations. Under certain conditions, the treatment is consistent with a 1:1 complex formation of the two surfactants, superimposed on the surfactant-solvent interactions existing in the pure monolayers. The value of the surface complex formation constant inferred from the surface tension measurements on laurate-lauric acid mixtures is about 20, and of the same order of magnitude as that for complex formation in the solid phase.