Isotherms Of Monolayers Research Articles

Pressure versus length isotherms of homogenous and mixed one-dimensional dipolar monolayers.

We demonstrate a novel method for compressing and expanding microscopic one-dimensional monolayers consisting of a finite number of aligned magnetic dipoles using a pair of microscopic magnetic barriers. By measuring the interaction between the beads and the barriers, we are able to determine the pressure of the dipolar monolayers. Our sensor can measure one-dimensional pressure in the femto and piconewton regime and is used to probe both homogeneous and mixed monolayers consisting of magnetic beads with diameters 1.0 microm and 2.8 microm. The larger beads appear to be well-described by a formalism taking into account magnetic dipolar interactions, whereas for smaller beads, such a simple picture does not hold. Upon compressing the monolayer above a certain density, it forms a bilayer. This process is governed by steric interactions or dipolar interactions, depending on the applied magnetic field. We also found odd-even effects, where the number of beads in the monolayer determines the initial structure of the bilayer.

Amphiphilic Block Copolymers of Poly(ethylene oxide) and Poly(perfluorohexylethyl methacrylate) at the Water Surface and Their Penetration into the Lipid Monolayer

Poly(ethylene oxide) (PEO) and poly(perfluorohexylethyl methacrylate) (PFMA) containing amphiphilic block copolymers have been investigated for their interfacial behavior and their penetration into 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) monolayer at the air/water interface by measuring surface pressure (π)/area (A) isotherms in conjunction with infrared reflection absorption spectroscopy (IRRAS). The π/A isotherms of the block copolymers show pressure regimes corresponding to different conformations of the polymer chains, i.e., a pancakelike conformation at low surface coverage and a brush conformation at high surface pressures. The plateau in π/A isotherms of the block copolymers, (i.e. a phase transition from the pancakelike conformation to the brush conformation) becomes less and less pronounced with a decrease in the PEO block length. The π/A isotherm of the DPhPC monolayer shows only a liquid expanded phase state at the air/water interface at all surface pressures. The DPhPC monolayer pe...

Supramolecular Assemblies between a New Series of Gemini-Type Amphiphiles and TPPS at the Air/Water Interface: Aggregation, Chirality, and Spacer Effect

Supramolecular assemblies between a series of novel gemini amphiphiles (BisImC17Cn) and TPPS (tetrakis(4-sulfonatophenyl)porphine) at the air/water interface were investigated. It was found that the gemini amphiphiles could form stable complex monolayers at the air/water interface with TPPS through electrostatic interaction and π−π stacking. There is an inflection point in each of the surface pressure−area isotherms of the complex monolayers. At a certain pH value of the subphase, TPPS could complex with the gemini amphiphiles as a J-aggregate. AFM measurements revealed that nanofibers were formed in the transferred films onto solid substrates by a horizontal lifting method. Interestingly, all the complex films containing the J-aggregate of TPPS showed CD signals in the J-band although both of the amphiphiles and TPPS were achiral. Helical stacking of the TPPS underneath the gemini monolayers was suggested to be responsible for the supramolecular chirality of the multilayer films. The spacers in the gemin...

Surface behaviour and peptide–lipid interactions of the antibiotic peptides, Maculatin and Citropin

Surface behaviour of Maculatin 1.1 and Citropin 1.1 antibiotic peptides have been studied using the Langmuir monolayer technique in order to understand the peptide–membrane interaction proposed as critical for cellular lysis. Both peptides have a spontaneous adsorption at the air–water interface, reaching surface potentials similar to those obtained by direct spreading. Collapse pressures ( Π c, stability to lateral compression), molecular areas at maximal packing and surface potentials (Δ V) obtained from compression isotherms of both pure peptide monolayers are characteristic of peptides adopting mainly α-helical structure at the interface. The stability of Maculatin monolayers depended on the subphase and increased when pH was raised. In an alkaline environment, Maculatin exhibits a molecular reorganization showing a reproducible discontinuity in the Π– A compression isotherm. Both peptides in lipid films with the zwitterionic palmitoyl-oleoyl-phosphatidylcholine (POPC) showed an immiscible behaviour at all lipid–peptide proportions studied. By contrast, in films with the anionic palmitoyl-oleoyl-phosphatidylglycerol (POPG), the peptides showed miscible behaviour when the peptides represented less than 50% of total surface area. Additional penetration experiments also demonstrated that both peptides better interact with POPG compared with POPC monolayers. This lipid preference is discussed as a possible explanation of their antibiotic properties.

Thermodynamic characteristics of mixed monolayers of amphotericin B and cholesterol

Surface–pressure ( Π) and surface–area isotherms as a function of surface area were measured for monolayers of amphotericin B (AmB) and cholesterol mixtures at the air/water interface at 10, 20, and 30 °C. When chloroform/methanol was used as a spreading solvent, the Π– A isotherms of the mixed monolayers exhibited characteristic transitions from the gas to liquid-expanded, then liquid-condensed, and finally the solid state. The expanding effect in monolayers was accompanied by a large Π– A hysteresis and a positive excess of free energy of mixing at high Π. At low Π, a condensing effect was observed with the most significant deviation from ideality occurring at a mole fraction of AmB ( X AmB) of 0.67. Free energy calculations revealed a condensing effect at low Π and an expanding effect at high Π except at 30 °C, where a condensing effect was observed for X AmB around 0.33. In contrast, when 2-propanol/water was used as spreading solvent, the mixed monolayers at 20 °C exhibited Π– A isotherms which obey van der Waals equation of state, with no visible transitions, low hysteresis, a condensing effect, and a negative free energy of mixing. The most stable monolayers were produced from mixtures of AmB and cholesterol with a 2:1 stoichiometry.

Analysis of GM3-Gg3 interaction using clustered gycoconjugate models constructed from glycolipid monolayers and artificial glycoconjugate polymers.

Carbohydrate-carbohydrate interactions between clustered GM3 on the Langmuir monolayer and clustered Gg3 trisaccharide along a polystyrene chain were investigated using surface pressure-area (pi-A) isotherms and surface plasmon resonance (SPR). The pi-A isotherm of the GM3 monolayer was expanded substantially and specifically by the Gg3-trisaccharide-bearing glycoconjugate polystyrene [PN(Gg3)] even at 10(-12) M. The PN(Gg3)-induced expansion of the GM3 monolayer required no calcium ion, and the expansion was strongly inhibited in the presence of urea and acetamido sugars. SPR studies of the GM3-Gg3 interaction were carried out to estimate the affinity constant and specificity of the interaction quantitatively. PN(Gg3) was adsorbed onto the GM3 monolayer strongly and specifically with an apparent affinity constant of Ka = 2.5 x 10(6) M(-1). The mechanism of the GM3-Gg3 interaction was discussed on the basis of the relationship between affinity and structure. We found that the NHAc groups of N -acetylneuraminic acid in GM3 and of GalNAc in Gg3 play an important role in the GM3-Gg3 interaction and that PN(Gg3) recognizes not only some specified portions of GM3 but also the trisaccharide as a whole. Published in 2003.

Study of the π–A isotherms of miltefosine monolayers spread at the air/water interface

Miltefosine (hexadecylphosphocholine), an anticancer drug based on a phospholipid-like structure, was spread and investigated at the aqueous solution/air interface as Langmuir monolayers by means of surface pressure–area (π–A) isotherms in addition to Brewster angle microscopy. The influence of such factors as subphase temperature, ionic strength, speed of compression, number of molecules spread at the surface on the characteristics of the π–A isotherms was studied. Miltefosine was found to form stable Langmuir monolayers which are nearly not influenced by experimental conditions. The liquid-expanded character of miltefosine films was confirmed with both compressibility modulus values and homogeneous BAM images. Ellipsometric measurements were performed to measure the thickness of miltefosine monolayer, and were used to calculate the orientation angle of miltefosine hydrophobic tail, which was found to change from 78.4 to 65.5 upon full monolayer compression. These results prove that the expanded character of miltefosine monolayer is due to the inclination of the molecule's hydrocarbon tail.

The covalent coupling of HAV-VP3 (110–121) synthetic peptide to liposomes: physicochemical studies

In this work we have studied the conjugation of the immunogenic peptide sequence (110–121) belonging to the VP3 capsid protein of hepatitis A virus to the surface of preformed liposomes by means of an amide bond between the vesicles and the synthetic peptide. The surface activity of the conjugate at air/water interface was determined. Moreover, the interaction of the conjugate with lipids was also studied recording the pressure increases produced after the injection of the liposome–peptide preparation under dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG) and stearylamine (SA) monolayers at different initial surface pressures. As expected, due to the negative net charge of the liposome–peptide complex, the higher interaction was found with positive charge monolayers (SA). However, the conjugate was also able to incorporate to zwitterionic and anionic lipids. This behaviour was also confirmed performing compression isotherms of monolayers of these lipids spread on subphases containing the conjugate. These results suggest that the coupling of VP3 (110–121) to liposomes does not influence its ability to interact with membrane lipids such as DPPC and DPPG. Then it can be assumed that its immunogenicity will be preserved or even increased after this modification. All these results are also useful in the preparation of liposome-based synthetic peptide vaccines.

Role of lipid interactions in autoimmune demyelination.

A morphological transformation involving loss of adhesion between myelin lamellae and formation of myelin vesicles has been described as a mechanism for demyelination in multiple sclerosis and marmoset experimental allergic encephalomyelitis (EAE). Although protein interactions are involved in maintaining normal myelin structure, we describe here how lipids contribute to myelin stability and how lipid changes in EAE, including increases in lipid polyunsaturation and negatively charged phosphatidylserine (PS), promote demyelination. Three physico-chemical techniques were used to identify these changes: (1) Langmuir monolayer isotherms indicated that EAE white matter lipids were significantly more "expanded" (fluid) than controls. (2) NMR spectroscopy indicated that EAE myelin lipids were more polyunsaturated than controls. (3) High-performance liquid chromatography (HPLC) with an evaporative light scattering detector indicated increased PS in EAE compared to controls, while sphingomyelin (SM), sulfatides and phosphatidylcholine (PC) were decreased. We present a physical model considering electrostatic, van der Waals and undulation forces to quantify the effect of these changes on myelin adhesion at the extracellular interface. Taken together, the isotherm, NMR, HPLC and modeling results support a mechanism for autoimmune demyelination whereby the composition of myelin lipids is altered in a manner that increases myelin fluidity, decreases myelin adhesion, increases membrane curvature, and promotes vesiculation.

New Monolayer Architecture Constructed by Competitive Hydrogen-Bonding Force and Compression Pressure Characterized by Infrared Multiple-Angle Incidence Resolution Spectroscopy

A unique monolayer that has a novel film architecture has been constructed by controlling a balance of hydrogen-bonding force and monolayer compression pressure, and the new film architecture has been characterized by infrared multiple-angle incidence resolution spectroscopy (MAIRS) that has recently been developed. The compound synthesized for the study consists of a single hydrocarbon chain and a headgroup that has three trans amide moieties. The limiting molecular area of the monolayer isotherm was approximately double that typically obtained for a single alkanoic-acid chain molecule, which suggested unusual film architecture. Then, the monolayer was transferred on a germanium substrate by the Langmuir−Blodgett film technique, and it was subjected to the infrared MAIRS analysis. The in-plane and out-of-plane mode spectra of MAIRS clearly suggested a flat alignment of interdigitated hydrogen-bonding networks and standing hydrocarbon chains with a disordered kink, which were totally consistent with a corresponding infrared reflection−absorption spectrum and the results of the isotherm measurement. The study suggests that the strongly correlated physical parameters are useful to develop a new film architecture.

Orientational changes in dipalmitoyl phosphatidyl glycerol Langmuir monolayers

Dipalmitoyl phosphatidyl glycerol (DPPG) as Langmuir monolayers at the air/water interface was investigated by means of surface pressure measurements in addition to Brewster angle microscopy (BAM) during film compression/expansion. A characteristic phase transition region appeared in the course of surface pressure–area ( π– A) isotherms for monolayers spread on alkaline water or buffer subphase, while on neutral or acidic water the plateau region was absent. This phase transition region was attributed to the ionization of DPPG monolayer. It has been postulated that the ionization of the phosphatidyl glycerol group leads to its increased solvation, which probably provokes both a change in the orientation of the polar group and its deeper penetration into bulk phase. Film compression along the transition region provokes the dehydration of polar groups and subsequent change of their conformation, thus causing the DPPG molecules to emerge up to the interface. Quantitative Brewster angle microscopy (BAM) measurements revealed that along the liquid-expanded to liquid-condensed phase transition the thickness of the ionized DPPG monolayer increases by 4.2 Å as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface.

Interaction of a laminin derived peptide with phosphatidyl choline/phosphatidyl glycerol

The physicochemical interactions between an active peptide sequence derived from laminin and phospholipids have been studied. The main aim is to determine the suitability of this peptide fragment to be linked to liposome's with the purpose to develop targeting devices. Results indicate that this peptide is able to insert in lipid monolayers and also to form monomolecular layers at an air/water interface. Besides, miscibility studies carried out through compression isotherms of mixed monolayers [dipalmitoyl phosphatidylcholine (DPPC)/peptide], indicate a strong interaction at 60–80% DPPC molar composition. Studies carried out with lipid bilayers indicate that the interaction is restricted to the external face of the vesicles. Moreover, the presence of this peptide in the incubation media promotes a low level of carboxyfluorescein (CF) leakage and no fusion of vesicles. These results indicate that the association of this sequence to vesicles do not produce damage of the bilayer and can be used as potential targeting vector.

Interaction of ibuprofen with eukaryotic membrane lipids

With the versatility of the molecular mechanism of amphiphilic drugs there is the possibility that ibuprofen could interact with eukaryotic model membrane lipids. Using the Langmuir technique, we first determined the pressure/molecular area isotherms of glycerophospholipids monolayers at 37°C, and, second, using differential scanning calorimetry (DSC), phase transition parameters in liposomes of the same lipids. Ibuprofen interacted in a concentration‐independent manner with monolayers of saturated phosphatidylcholines (PC, i.e. markers of the outer membrane leaflet of eukaryotic cells). Ibuprofen was found to interact with liposomes of saturated and unsaturated phosphatidylcholines and ‐serines (PS, i.e. markers of the inner membrane leaflet of eukaryotic cells), and saturated ethanolamines (PE, i.e. markers of the inner membrane leaflet of eukaryotic cells). A lowering of the lipid melting temperature (Tm) and a change of enthalpy (ΔH) of the gel to liquid‐crystalline phase transitions of liposomes were detected.

Computer Simulation of Pressure-Area Isotherms of Monolayers of Charged Fatty-Acid Molecules

We present a new Monte Carlo algorithm for simulating the conformational behaviour of surfactant soap molecules tethered to a planar interface. The extension to other geometrical arrangements is straightforward. The algorithm makes use of the superposition of local structural rearrangements for the description of conformational changes in the chains. The displacements of pairs of atoms subject to the geometrical constraints can be achieved in a computationally efficient way by the application of least-squares search methods. It is shown that a balance of the attractive interactions between the methylene chains and longer-range repulsive interactions between the charged headgroups determines the form of the spreading pressure isotherms. The experimental isotherms cannot be reproduced if the headgroups interact through purely repulsive potentials. We find it necessary either to modify these potentials by invoking a short-range attraction or to assume that the charge of a headgroup decreases with average area-per-molecule in the monolayers. Our model tracks the experimentally observed spreading pressure-area isotherm in Langmuir monolayers of ionised and non-ionised fatty acids. Importantly, we were able to reproduce the dependence of the isotherms on the chain length.

Amphotericin B and Cholesterol in Monolayers and Bilayers

The antimycotic activity of amphotericin B depends on its ability to make complexes with cell membrane sterols. Surface pressure (π) as a function of surface area (A) and π−A hysteresis were measured for monolayers of amphotericin B/cholesterol mixtures on the water/air interface. Specific area per molecule of amphotericin B and free energy of mixing were calculated as a function of concentration of amphotericin B. When chloroform/methanol was used as a spreading solvent, the π−A isotherms of the mixed monolayers exhibited characteristic transitions from the gas to liquid-expanded, then liquid-condensed, and finally the solid state. The mean molecular area of the mixed monolayers was significantly higher than the calculated sum of the molecular areas of the pure components. This expanding effect was accompanied by a large π−A hysteresis and a positive excess of free energy of mixing at high π. In contrast, when 2-propanol/water was used as spreading solvent, the mixed monolayers at 20 °C exhibited π−A isotherms with no visible transitions, low hysteresis, a condensing effect, and a negative free energy of mixing. The most stable monolayers were produced from molecules of amphotericin B and cholesterol with a 2:1 stoichiometry. At this ratio, amphotericin B and cholesterol form ion channels in lipid bilayers with conductance of 4−400 pS. These results provide a better understanding of the biological activity of amphotericin B. Artificial amphotericin B/cholesterol ion channels having large conductance could be useful in nanotechnology.

Interfacial synthesis of noble metal nanoparticles

Palladium and gold metallic nanoparticles were generated two-dimensionally via the chemical decomposition of precursor molecules in a mixed Langmuir monolayer with arachidic acid (AA) or octadecyl amine (ODA) used as surfactants at the gas/aqueous borohydride solution interface in a gaseous phase of the monolayer (at low or no surface pressure). Water-insoluble Pd 3(CH 3COO) 6 and Au(P(C 6H 5) 3)Cl molecules were used as precursors. Surface pressure–monolayer area isotherms of control and particulate Langmuir monolayers were studied. It was found that the generation of nanoparticles can change the compression isotherm of the mixed monolayer and that synthesis of nanoparticles without surfactant also resulted in characteristic compression isotherm of the gas/sodium borohydride solution interface with floating nanoparticles. The morphology of synthesized nanoparticles was characterized by transmission electron microscopy (TEM). The size and shape of grown nanoparticles, and the structure of two-dimensional (2-D) aggregates and nanostructures, which resulted from the time-dependent self-organization processes, were dependent substantially on the monolayer composition and state during the growth process.

Selective Expansion of the GM3 Glycolipid Monolayer Induced by Carbohydrate−Carbohydrate Interaction with Gg3 Trisaccharide-Bearing Glycoconjugate Polystyrene at the Air−Water Interface

Carbohydrate−carbohydrate interactions between GM3 and Gg3 were investigated using surface pressure−area (π−A) isotherms of glycolipid monolayers in contact with glycoconjugate polystyrenes at the air−water interface. The GM3 monolayer was greatly expanded by the Gg3-trisaccharide (GalNAcβ1−4Galβ1−4Glc)-bearing glycoconjugate polystyrene [PN(Gg3)] and moderately expanded by lactose-bearing polystyrene, while little or no expansion was observed with GalLac (Galβ1−4Galβ1−4Glc)- or cellobiose-bearing polystyrenes or Gg3-trisaccharide-bearing styrene monomer. The PN(Gg3)-induced expansion of π−A isotherms of sphingolipid monolayers was maximal with GM3 and decreased in the following order: (KDN)GM3 > GM2 ≈ LacCer > GD3 ≈ 2,6-isoGM3 ≈ GM4 ≈ GM1 ≈ sphingomyelin (SM). The extent of expansion reflected the strength of carbohydrate−carbohydrate interactions with Gg3, which were amplified by the clustering effect of carbohydrates in the glycoconjugate polystyrenes. The expansion of various proportions of GM3/SM mixed monolayers by PN(Gg3) was smaller than that of the GM3 monolayer, indicating the importance of clustered carbohydrates in the monolayer. The PN(Gg3)-induced expansion of the GM3 monolayer required no calcium ion, and the expansion was strongly inhibited in the presence of urea and N-acetamido sugars. These observations reveal a new aspect of the mechanism of the carbohydrate−carbohydrate interactions where the N-acetyl groups and hydrogen bonds between the carbohydrate moieties play important roles. The π−A isotherms using glycoconjugate polystyrenes have been demonstrated to be useful as a molecular probe for the carbohydrate−carbohydrate interactions.

Polyelectrolytes. 2. Intrinsic or Extrinsic Charge Compensation? Quantitative Charge Analysis of PAH/PSS Multilayers

Floating multilayers of polyallylamine/polystyrenesulfonate (PAH/PSS) adsorbed to a dimethyldioctadecylammonium bromide (DODAB) monolayer at the air/water interface (at pH 5.5) are analyzed quantitatively with respect to the internal charge stoichiometry of the polyelectrolytes. To this end the adsorbed amount of polymer is translated into charge per area. It is postulated that the strong acid PSS is fully charged (fully dissociated). The weak polyelectrolyte PAH is also assumed to be fully dissociated. This is vindicated by theoretical considerations and deduced from pH-dependent studies of the isotherms of Langmuir monolayers of negatively charged dimyristoylphosphatidic acid (DMPA) with PAH adsorbed from the subphase. With both polyelectrolytes fully charged the PAH charges significantly overcompensate the PSS charges. The polyelectrolyte charge stoichiometry is not 1:1, and the multilayer must contain some small counterions (Cl-) for charge compensation. It is suggested that this “extrinsic compensation” occurs when the charge density of the polyelectrolytes exceeds a certain threshold. If the distance between the charges is less than the Bjerrum length, the strong binding of the counterions (“Manning condensation”) prevents their release upon multilayer formation. This explains why multilayers with PAH with its unusually high charge density contain counterions whereas other polyelectrolyte films are ion-free. Literature data support this hypothesis.

Surface Pressure Isotherms, Dilatational Rheology, and Brewster Angle Microscopy of Insoluble Monolayers of Sugar Monoesters

Surface pressure isotherms of insoluble monolayers of high purity, enzymatically synthesized sucrose, lactose, glucose, and galactose stearate at the air−water and n-tetradecane−water interfaces were obtained. The dilatational moduli of these surfactant monolayers were also determined by oscillations of the interface at 0.1 Hz. Results have indicated similarities in the properties of sucrose and lactose stearate. Brewster angle microscopy has shown the transitions from gaseous to liquid expanded to liquid condensed phases upon compression of these monolayers at the air−water (A−W) interface. It was also shown that the interfacial phase separation is strongly dependent on the rate of deformation of the interface. Glucose and galactose stearate formed more aggregated monolayers at the A−W interface and (particularly galactose stearate) had higher dilatational moduli, compared to sucrose and lactose stearate. Brewster angle microscopy confirmed the extensive aggregation and the formation of a solid phase in ...

An efficient algorithm in the grand canonical ensemble: constructing adsorption isotherms

A new computational method is presented that efficiently describes open thermodynamic systems within the grand canonical ensemble formalism. The method is based on the j-walking algorithm, which circumvent sampling difficulties by coupling random walkers in different thermodynamic states. By imposing detailed balance, a new acceptance probability is derived and applied to the construction of adsorption isotherms for atomic monolayers. The method converges much faster than the standard grand canonical Monte Carlo method and permits the construction of accurate adsorption isotherms and the identification of phase transitions occurring in the adsorbed material.