Development of a new Langmuir-type pendant-drop film balance

A new Langmuir-type pendant-drop film balance has been developed for the determination of surface pressure–molecular area isotherms of insoluble monolayers deposited on the surface of a pendant-drop . The determination of surface tension as a function of surface area is performed using the Axisymmetric Drop Shape Analysis (ADSA). The complete setup, i.e. the image capturing and microinjector system is fully computer controlled by a user-friendly and fully Windows-integrated program, including the ADSA surface tension calculation algorithm. Two monolayer deposition methods were studied, deposition on capillary and deposition on drop. The latter was found to be more successful. The compression isotherms obtained on monolayers of four amphiphilic substances with different behavior were found to be reproducible and in agreement with conventional film balance isotherms found in literature.     

Chitosan as a removing agent of beta-lactoglobulin from membrane models

Many chitosan biological activities depend on the interaction with biomembranes, but so far it has not been possible to obtain molecular-level evidence of chitosan action. In this article, we employ Langmuir phospholipid monolayers as cell membrane models and show that chitosan is able to remove beta-lactoglobulin (BLG) from negatively charged dimyristoyl phosphatidic acid (DMPA) and dipalmitoyl phosphatidyl glycerol (DPPG). This was shown with surface pressure isotherms and elasticity and PM-IRRAS measurements in the Langmuir monolayers, in addition to quartz crystal microbalance and fluorescence spectroscopy measurements for Langmuir-Blodgett (LB) films transferred onto solid substrates. Some specificity was noted in the removal action because chitosan was unable to remove BLG incorporated into neutral dipalmitoyl phosphatidyl choline (DPPC) and cholesterol monolayers and had no effect on horseradish peroxidase and urease interacting with DMPA. An obvious biological implication of these findings is to offer reasons that chitosan can remove BLG from lipophilic environments, as reported in the recent literature.     

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 (pi-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 A as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface.     

Miscibility of phospholipids and cholesteryl acetate in mixed monolayers on aqueous surfaces

Summary The surface pressure — area per molecule curves (F A curves) of mixed monolayers of phosphatidyl serine (PS) and cholesteryl acetate (CA), and those of dimyristoyl phosphatidyl choline (DMPC) and CA were measured on aqueous surfaces. ThoseF — A curves showed kink points, which were considered to be the collapse point of the monolayers. Then, the collapse pressure was determined as the surface pressure at the collapse points. On the basis of the phase diagrams, drawn by plotting the measured values of the collapse pressure as a function of the composition, the miscibility of the lipids in the mixed monolayers was discussed. Thus, it has been concluded that PS and CA, and also DMPC and CA are completely miscible in the monolayers on water. On the other hand, it has been also found that, on aqueous solutions of 100 mM CaCl₂, PS and CA are immiscible in the monolayers because of the aggregation of PS molecules induced by Cat²⁺.     

Properties of lipophilic nucleoside monolayers at the air–water interface

The capability of self-assembly and molecular recognition of biomolecules is essential for many nanotechnological applications, as in the use of alkyl-modified nucleosides and oligonucleotides to increase the cellular uptake of DNA and RNA. In this study, we show that a lipophilic nucleoside, which is an isomer mixture of 2′-palmitoyluridin und 3′-palmitoyluridin, forms Langmuir monolayers and Langmuir–Blodgett films as a typical amphiphile, though with a smaller elasticity. The nucleoside may be incorporated into dipalmitoyl phosphatidyl choline (DPPC) monolayers that serve as a simplified cell membrane model. The molecular-level interactions between the nucleoside and DPPC led to a remarkable condensation of the mixed monolayer, which affected both surface pressure and surface potential isotherms. The morphology of the mixed monolayers was dominated by the small domains of the nucleoside. The mixed monolayers could be deposited onto solid substrates as a one-layer Langmuir Blodgett film that displayed UV–vis absorption spectra typical of aggregated nucleosides owing to the interaction between the nucleoside and DPPC. The formation of solid films with DNA building blocks in the polar heads may open the way for devices and sensors be produced to exploit their molecular recognition properties.     

Semiempirical quantum mechanical calculations of dipolar interaction between dipyridamole and dipalmitoyl phosphatidyl choline in Langmuir monolayers

Recent studies have shown that dipalmitoyl phosphatidyl choline (DPPC) monolayers respond cooperatively to the presence of dipyridamole (DIP) guest molecules even at small concentrations, which is a signature of molecular recognition. Using semiempirical quantum mechanical calculations for the DIP-DPPC system, we show that the incorporation of DIP causes large changes in the vertical dipole moment of the DIP-DPPC system, which can explain why measurable changes in surface potential are observed experimentally even at very low DIP concentrations. The calculations are also consistent with the anomalous concentration dependence of the surface pressure and surface potential isotherms for DIP-DPPC monolayers. Rather than saturation or a continuous increase in the effects caused by the incorporation of increasing amounts of DIP, the experimentally observed inversion in the behavior of the surface potential as the DIP concentration reaches 0.5 mol % would be caused by a change in DIP conformation, from a vertical arrangement for the DIP rings to a horizontal or intermediate arrangement. The strong dipolar interactions indicated in the calculations may also be the origin of the drastic changes in monolayer morphology seen in fluorescence microscopy images, with triskellion-shaped domains being formed for condensed DIP-DPPC monolayers.     

Viscoelastic properties of insoluble amphiphiles at the air/water interface

The effects of the presence of a molecular monolayer on the dilatational properties of the air/water interface have been investigated. Two water insoluble amphiphiles, dipalmitoyl phosphatidyl choline and quercetin 3-O-palmitate, were spread onto a pendant drop and the dynamic surface pressure was measured by means of drop shape analysis. The surface dilatational elasticity and viscosity of the spread monolayers were also determined by the oscillating drop technique. Constraints on the range of measuring conditions were investigated and we demonstrated that the pressure-area isotherms derived from oscillatory dynamic measurements display phase behaviour similar to that found in equilibrium measurements, albeit at reduced resolution. Both the amphiphiles formed purely elastic films that were characterised by a dilatational modulus that depended on the surface concentration and obeyed a power scaling law. The exponent of the relationship could be related to the thermodynamic conditions prevailing at the interface. The phospholipid monolayer scaling exponent was 2.8 in a temperature range of 20-26 degrees C indicates a favourable solvency of molecules in the bidimensional matrix. A very high scaling exponent (11.8 at 7 degrees C) for quercetin palmitate was interpreted assuming that molecules self-organise in fibre-like structures. This interface structure and the phase behaviour was found consistent with observations of the surface film obtained by Brewster angle microscopy. The structured quercetin 3-O-palmitate monolayers are disrupted by temperature increase or by adding a 0.2 molar fraction of the immiscible dipalmitoyl phosphatidyl choline.     

水溶性菁染料与类脂物混合单分子膜中J-聚集体形成的研究

报道了具有相同发色团,并有不同电荷的二种菁染料单分子膜中J-聚集体的形成。测定了单分子膜的π/A,△V/A曲线,反射光谱及吸收光谱。从实验结果得出:带正电荷的染料与花生酸混合形成单分子膜后,膜中染料的二聚体与J-聚集体随着表面压力改变存在着一个平衡。而带负电荷的染料与二十烷基胺成膜后,即使在很低的表面压下也有J-聚集体形成。同时染料分子在水面上的取向是各向异性。     

Excess fibrinogen adsorption to monolayers of mixed lipids

Adsorption of fibrinogen to the monolayers of mixed lipids, dipalmitoyl phosphatidyl choline (DPPC) and eicosylamine (EA) was measured at a surface pressure of 20 mN/m by an in situ surface plasmon resonance technique. Pressure–area isotherms of DPPC + EA mixtures on water and buffer subphases indicated good lipid miscibility and some contraction of the monolayers at intermediate and higher surface pressures. Surface electric potential of the DPPC + EA monolayers showed excess values for intermediate DPPC:EA ratios. Fibrinogen adsorption and its adsorption rates from a dilute solution (0.03 mg/ml) were proportional to the fraction of EA in the monolayer indicating that protein binding was primarily driven by electrostatic interactions between positive EA charges in the monolayer and a net negative protein charge. At a higher protein concentration (0.06 mg/ml) both the fibrinogen adsorbed amount and its maximum adsorption rate showed excess values relative to the pure EA for 1:1, 2:1 and 3:1 DPPC + EA monolayers. This excess adsorption could be explained, in part, by the contraction of the monolayers with intermediate DPPC:EA ratios which resulted in an excess surface electric potential.     

Langmuir and Langmuir-Blodgett films of poly(ethylene oxide)-b-poly(epsilon-caprolactone) star-shaped block copolymers

Self-assembly of poly(ethylene oxide)-block-poly(epsilon-caprolactone) five-arm stars (PEO-b-PCL) was studied at the air/water (A/W) interface. The block copolymers consist of a hydrophilic PEO core with hydrophobic PCL chains at the star periphery. All the polymers have the same number of ethylene oxide repeat units (9 per arm), and the number of epsilon-caprolactone repeat units ranges from 0 to 18 per arm. The Langmuir monolayers were analyzed by surface pressure/mean molecular area isotherms, compression-expansion hysteresis experiments, and isobaric relaxation measurements, and the Langmuir-Blodgett (LB) films' morphologies were investigated by atomic force microscopy (AFM). PCL homopolymers crystallize directly at the A/W interface in a narrow surface pressure range (11-15 mN/m). In the same pressure region, the star-shaped block copolymers undergo a phase transition corresponding to the collapse and the crystallization of the PCL chains as shown by the presence of a pseudoplateau in the isotherms. The LB films were prepared by transferring the Langmuir monolayers onto mica substrates at various surface pressures. AFM imaging confirmed the formation of PCL crystals in the LB monolayers of the PCL homopolymers and of the copolymers, but also showed that the PCL segments can undergo additional crystallization after monolayer transfer during water evaporation. The PCL crystal morphologies were also strongly influenced by the surface pressure and by the PEO segments.     

Surface behavior of oleoyl palmitoyl phosphatidyl ethanolamine (OPPE) and the characteristics of mixed OPPE-miltefosine monolayers

Langmuir monolayers of oleoyl palmitoyl phosphatidyl ethanolamine (OPPE) were investigated at the air/water interface by means of surface pressure (pi)-area (A) isotherms complemented with Brewster angle microscopy images upon film compression/expansion. The characteristic phase transition appearing in the course of pi/A isotherms was attributed to the coexistence of two liquid-expanded phases of different molecular ordering. The interactions between OPPE and hexadecylphosphocholine (miltefosine) were studied at different subphase pHs (2, 6, and 10) at 20 degrees C and analyzed with mean molecular area (A12)-, excess area of mixing (Aexc)-, and excess free energy of mixing (DeltaGexc)-composition plots. The obtained results indicate that at pH 10, where both OPPE and miltefosine polar groups are negatively charged, attractive interactions are observed (reflected by negative deviations from ideality), contrary to expectation. This peculiar behavior is explained as being due both to water molecules, which surround negatively charged polar groups and increase the distance between them, weakening in this way the electrostatic repulsion forces; and to positively charged counterions present in the diffuse double layer, neutralizing their charge. In this way, the van der Waals attraction forces between hydrocarbon tails of both molecules predominate and are responsible for the observed negative deviations from ideal behavior. Similar explanations are given for the observed negative deviations at pH 2 where both polar groups are positively charged. At pH 6, the observed negative deviations at low surface pressures and positive deviations at high pressures are interpreted as being due to a change in orientation of polar groups upon monolayer compression.     

Monolayers of a solvent-free polymer brush: Part 1. Thermodynamics at the air/water interface

Hydrophobie polymers with low glass transition temperature (polyisoprenes) and a single head group (sulfonate) have been synthesised and characterised as insoluble monolayers on a water surface. A single ionic head group is sufficient to make these hydrophobic polymers surface-active and isotherms of an expanded type have been recorded. These isotherms can be described quantitatively by using a theory based on constant density of the hydrophobic region, Gaussian chain statistics and affine deformation: at a fixed area per head group, the surface pressure is proportional to the length of the hydrophobic tail. The slope of this relationship is proportional to the third power of the area per head group. Chains shorter than 300 repeat units exhibit a systematic deviation from the theory.     

Surface-pressure isotherms of monolayers formed by microsize and nanosize particles

The thermodynamic model of a 2D solution developed earlier for protein monolayers at liquid interfaces is generalized for monolayers composed of micro- and nanoparticles. Surface pressure isotherms of particle monolayers published in the literature for a wide range of particles sizes (between 75 microm and 7.5 nm) are described by the theoretical model with one modification. The calculations of surface pressure pi on area A provide satisfactory agreement with the experimental data. The theory also yields reasonable cross-sectional area values of the solvent molecule water in the range between 0.12 and 0.18 nm2, which is almost independent of particle size. Also, the area per particle in a closely packed monolayer obtained from the theory is quite realistic.     

Intermolecular forces in spread phospholipid monolayers at oil/water interfaces

The lateral intermolecular forces between phospholipids are of particular relevance to the behavior of biomembranes, and have been approached via studies of monolayer isotherms at aqueous interfaces, mostly restricted to air/water (A/W) systems. For thermodynamic properties, the oil/water (O/W) interface has major advantages but is experimentally more difficult and less studied. A comprehensive reanalysis of the available thermodynamic data on spread monolayers of phosphatidyl cholines (PC) and phosphatidyl ethanolamines (PE) at O/W interfaces is conducted to identify the secure key features that will underpin further development of molecular models. Relevant recourse is made to isotherms of single-chain molecules and of mixed monolayers to identify the contributions of chain-chain interactions and interionic forces. The emphasis is on the properties of the phase transitions for a range of oil phases. Apparent published discrepancies in thermodynamic properties are resolved and substantial agreement emerges on the main features of these phospholipid monolayer systems. In compression to low areas, the forces between the zwitterions of like phospholipids are repulsive. The molecular model for phospholipid headgroup interactions developed by Stigter et al. accounts well for the virial coefficients in expanded phospholipid O/W monolayers. Inclusion of the changes in configuration and orientation of the zwitterion headgroups on compression, which are indicated by the surface potentials in the phase transition region, and inclusion of the energy of chain demixing from the oil phase will be required for molecular modeling of the phase transitions.     

Hydrolytic behavior of enantiomeric poly(lactide) mixed monolayer films at the air/water interface: stereocomplexation effects

The Langmuir film balance technique was used to determine the hydrolytic kinetics of monolayers of the stereocomplex formed from mixtures of enantiomeric polylactides, poly(L-lactide) (L-PLA) and poly(D-lactide) (D-PLA), spread at the air-water interface. The present study investigated parameters such as degradation medium, mixture composition, and time on the relative degradation rate. The pi-A isotherms of monolayers of the mixtures provide clear evidence for the presence of a stereocomplex; the isotherms of monolayers of individual polyenantiomer show a transition at about 8.5 mN/m, whereas the transition of monolayers containing a stereocomplex formed from the equimolar mixture shifted to higher surface pressure, about 11 mN/ m. The rate of hydrolysis was recorded by a change in occupied area when the monolayer is maintained at a constant surface pressure. The hydrolysis of the mixture monolayers under basic conditions was slower than that of individual polyenantiomer monolayers, depending on the composition or the degree of complexation. In the presence of proteinase K, the enzymatic hydrolysis rate of mixture monolayers with >50 mol % l-PLA was much slower than that of the single-component L-PLA monolayer. The monolayers formed from mixtures with < or =50 mol % L-PLA did not show any change of occupied areas. This result is explained by the inactivity of D-PLA and stereocomplexed chains to the enzyme. From both results, it can be concluded that the retardation of the hydrolysis of mixture monolayers is mainly due to a strong interaction between D- and L-lactide unit sequences, which prevents the penetration of water or enzyme into the bulk.     

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.     

Behaviour of a protein isolate from rapeseed (Brassica napus) and its main protein components - globulin and albumin - at air/solution and solid interfaces, and in emulsions

The behaviour of a rapeseed protein isolate (RI) and its main protein components - globulin (RG) and albumin (RA) - in adsorbed and spread monolayers, as well as in emulsions was investigated. Tensiometry, film-pressure area and Langmuir-Blodgett-techniques, and emulsion parameters were used to characterise the behaviour of the rapeseed proteins at various interfaces. The adsorption isotherms for albumin showed a plateau value for the surface pressure (Pi(e)) of 11.6 mN/m at the low critical association concentration (CAC) of 5.6x10(-8) g/ml. Both values were found to be distinctly higher for the globulin and the protein isolate. The isotherms of a mixture of RG and RA, which corresponds to the composition of RI, seems to be a superimposition of the isotherms of RA and RG. Contact angle measurements showed that all samples used were able to form LB-layers and to make hydrophilic glass surfaces more hydrophobic and vice versa. The changes in contact angle were more pronounced on hydrophobic glass surfaces. Monolayer and emulsion characteristics are dominated by the interfacial properties of albumin. The maximum film pressure reached by globulin was only about 8 mN/m. The globulin also possesses the lowest emulsifying activity. From the mean molecular area calculated for spread globulin, it is concluded that the globulin maintains its globular conformation at surfaces, which explains the low surface activity. Albumin and the protein isolate were highly surface active in monolayers and emulsion formation. The slightly different interfacial behaviour of the protein isolate compared with the corresponding mixture is probably due to additional effects of non-protein components and a partially denatured state of the protein.     

Langmuir monolayer properties of perfluorinated double long-chain salts with divalent counterions of separate electric charge at the air-water interface

The novel perfluorinated double long-chain salts with divalent counterions of separate electric charge, 1,1-(1,omega-alkanediyl)-bispyridinium perfluorotetradecane- carboxylate [CnBP(FC14)2 : n = 2, 6, 10, 14], were newly synthesized and their interfacial behavior was investigated by Langmuir monolayer methods. Surface properties [surface pressure (pi)-, surface potential (DeltaV)-, dipole moment (micro perpendicular)-area (A) isotherms] and morphological images of CnBP(FC14)2 monolayers on a subphase of water and on various NaCl concentrations were measured by employing the Wilhelmy method, the ionizing electrode method, fluorescence microscopy (FM), and Brewster angle microscopy (BAM). CnBP(FC14)2 formed a stable monolayer on water at 298.2 K, where these pi-A isotherms shifted to a larger molecular area with increasing charge separation and had no transition point from a disordered phase to an ordered one. On the contrary, the pi-A isotherms on NaCl solutions moved to the smaller areas, showed the transition and higher collapse pressures compared to the pi-A isotherms on water. These results suggested that a sodium chloride subphase induced the condensation of CnBP(FC14)2 molecules upon compression. In addition, it is quite noticeable that a dissociation of CnBP counterion from CnBP(FC14)2 occurs on NaCl solutions, depending on the extent of charge separation. This phenomenon was supported by the changes of the limiting area, transition pressure, collapse pressure, repeated compression-expansion cycle curve, and DeltaV behavior of perfluorotetradecanoic acid (FC14). Furthermore, temperature dependence of these monolayers was investigated, and an apparent molar quantity change on the phase transition was evaluated on 0.15 M NaCl. The morphological behavior of CnBP(FC14)2 and FC14 monolayers was also confirmed by FM and BAM images.     

Packing, flipping, and buckling transitions in compressed monolayers of ellipsoidal latex particles

The behavior of monolayers of monodisperse prolate ellipsoidal latex particles with the same surface chemistry but varying aspect ratio has been studied experimentally. Particle monolayers at an air-water interface were subjected to compression in a Langmuir trough. When surface pressure measurements and microscopy observations were combined, possible structural transitions were evaluated. Ellipsoids of a sufficiently large aspect ratio display a less abrupt increase in the compression isotherms than spherical particles. Microscopic observations reveal that a sequence of transitions is responsible for this more gradual increase of the surface pressure. When a percolating aggregate network is used as the starting point, locally ordered regions appear progressively. When it reaches a certain surface pressure, the system "jams", and in-plane rearrangements are no longer possible at this point. A highly localized yielding of the particle network is observed. The compressional stress is relieved by flipping the ellipsoids into an upright position and by expelling particles from the monolayer. The latter does not occur for spherical particles with similar dimensions and surface chemistry. In the final stage of compression, buckling of the monolayer as a whole was observed. The effect of aspect ratio on the pressure area isotherms and on the obtained percolation and packing thresholds was quantified.     

Miscibility of mixed stereoregular PMMA/PVPh monolayers at the air/water interface

The mixed monolayer behavior of stereoregular poly(methyl methacrylate) (PMMA) and poly(vinyl phenol) (PVPh) was investigated from the measurements of surface pressure–area per molecule (π–A) isotherms. The π–A isotherms indicated that isotactic PMMA (iPMMA) and PVPh were miscible at the air/water interface. The miscibility and non-ideality of the mixed monolayers were examined by calculating the excess area as a function of composition, and negative deviations from ideality were observed, which suggest the existence of attractive interactions between iPMMA and PVPh. However, the π–A isotherms of mixed syndiotactic PMMA (sPMMA)/PVPh monolayers showed positive deviation from ideality, which might suggest that non-favorable interactions exist between sPMMA and PVPh.The π–A isotherms of mixed atactic PMMA (aPMMA)/PVPh monolayers exhibited complicated excess area behavior. Both positive and negative deviations from ideality were observed at various surface pressures. These isotherm results of mixed polymers correlate approximately well with the miscibility of the corresponding mixtures in the bulk state. The formation of hydrogen bonding between PMMA and PVPh was substantiated in the bulk state by means of Fourier transform infrared (FTIR). Regardless of tacticity, an increase of hydrogen-bonded carbonyl fraction was observed.