Long-range nanometer-scale organization of semifluorinated alkane monolayers at the air/water interface

We have determined the structure formed at the air-water interface by semifluorinated alkanes (C(8)F(17)C(m)H(2m+1) diblocks, F8Hm for short) for different lengths of the molecule (m = 14, 16, 18, 20) by using surface pressure versus area per molecule isotherms, Brewster angle microscopy (BAM), and grazing incidence x-ray experiments (GISAXS and GIXD). The behavior of the monolayers of diblocks under compression is mainly characterized by a phase transition from a low-density phase to a condensed phase. The nonzero surface pressure phase is crystalline and exhibits two hexagonal lattices at two different scales: a long-range-order lattice of a few tens of nanometers lateral parameter and a molecular array of about 0.6 nm parameter. The extent of this organization is sufficiently large to impact larger scale behavior. Analysis of the various compressibilities evidences the presence of non organized molecules in the monolayer for all 2D pressures. At room temperature, the self-assembled structure appears generic for all the F8Hm investigated.     

Semifluorinated chains in 2D-(perfluorododecyl)-alkanes at the air/water interface

Langmuir monolayers of a homologous series of perfluorododecyl-n-alkanes (general formula F(CF2)12(CH2)nH, abbreviated as F12Hn, where n = 6-20) are investigated through isotherms of surface pressure (pi) and electric surface potential (DeltaV) versus area (A) and quantitative Brewster angle microscopy. The investigated monolayers are found to be liquid in nature. The negative sign of the measured surface potential evidences the orientation of all the investigated molecules with their perfluorinated parts directed toward the air regardless of the length of the hydrogenated unit. Analysis of the direction of the molecular dipole moment with respect to the main axis indicates that the minimum effective dipole moment is achieved for a molecule oriented at an angle of about 35 degrees to the surface normal. The film thickness was evaluated from the relative intensity measurements. The results suggest that the F12Hn molecules are tilted to the interface in the vicinity of collapse, which is in accordance with the liquid character of their monolayers.     

Electron hopping dynamics in Au38 nanoparticle langmuir monolayers at the air/water interface

The electron-transfer dynamics between nanoparticles has been studied as a function of interparticle distance by in situ voltammetry of well-defined monolayers of a metal quantum dot nanoparticle Au38(hexanethiolate)24. The interparticle distance is precisely controlled by the Langmuir technique and addition of various lengths of dithiol linkers (HS(CH2)nSH; n = 5, 6, 8, and 9). Voltammograms of Au38 monolayers display a well-defined single-electron charging peak that increases remarkably with decreasing the interparticle distance. The diffusion coefficient and rate constant calculated from the peak current for core-core electron hopping reaction both exponentially increase respectively, from 3.3 x 10-10 to 5.2 x 10-9 cm2/s and 2.2 x 104 to 5.0 x 105 s-1 as the distance decreases from 13.3 to 9.5 A and then levels off at 8.0 A. These rate constants are in good agreement with the literature values, demonstrating that the present experimental approach provides a powerful way to investigate the correlation between the electron-transfer dynamics and nanoparticle assembly structure.     

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.     

Molecular dynamics simulation of amphiphilic bistable [2]rotaxane langmuir monolayers at the air/water interface

Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT(4+)@mpTTF) is that in which the CBPQT(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT(4+) ring surrounding the DNP (CBPQT(4+)@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, we report here the structure and surface pressure-area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxanes. For both co-conformations, we found that as the area per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co-conformation. Thus we find that the necessity of allowing the tetracationic ring to become solvated by water leads to differences in the structures for the two co-conformations in the LM. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT(4+) ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find the surface pressure-area isotherms for each co-conformation in which the CBPQT(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally.     

Cosurfactant effect of a semifluorinated alkane at a fluorocarbon/water interface: impact on the stabilization of fluorocarbon-in-water emulsions

Previous work has demonstrated that semifluorinated alkanes CnF2n+1CmH2m+1 (FnHm diblocks), when used in conjunction with phospholipids, strongly stabilize fluorocarbon (FC)-in-water emulsions destined to be used as oxygen carriers. Although the presence of FnHm diblocks in the emulsion's interfacial phospholipid film was suggested to account for the observed stabilization, no direct proof of the diblock's location has been provided so far. We now report definite experimental evidence of the diblock's presence at the interfacial film, both on a macroscopic level by investigating the FC/water interface using the pendant drop method and directly on emulsions by monitoring their stability for various phospholipid chain lengths. We first establish that F8H16 has a strong cosurfactant effect with phospholipids [dimyristoylphosphatidylcholine (DMPC), dilaurylphosphatidylcholine (DLPC), dioctanoylphosphatidylcholine (PCL8)] at a perfluorooctyl bromide (PFOB)/water interface, as evidenced by a dramatic F8H16-concentration-dependent decrease of the interfacial tension. Where FC emulsions are concerned, we show that the stabilization effect, which consists of a decrease of the rate of molecular diffusion of the FC, depends strongly on the length of the phospholipid's fatty chain as compared to the length of the hydrocarbon segment, Hm, of the diblock. Stabilization is maximized when the Hm length is similar to that of the phospholipid's fatty chains. A strong mismatch between Hm and the phospholipid chain length can actually destabilize the emulsion. A different destabilization mechanism is then at work: coalescence. The presence of F8H16 at the interfacial film is further supported by the fact that perfluorodecyl bromide, a heavy analogue of PFOB that stabilizes PFOB emulsions by lowering the solubility and diffusibility of the emulsion's dispersed FC phase, exercises its stabilizing effect similarly for all the phospholipids investigated.     

Nystatin in Langmuir monolayers at the air/water interface

The paper presents a thorough characteristics of Langmuir monolayers formed at the air/water interface by a polyene macrolide antibiotic-nystatin. The investigations are based on the analysis of pi/A isotherms recorded for monolayers formed by this antibiotic at different experimental conditions. A significant part of this work is devoted to the stability and relaxation phenomena. It has been found that nystatin forms at the air/water interface monolayers of the LE state. A plateau region, observed during the course of the isotherm compression, is suggested to be due to the orientational change of nystatin molecules from horizontal to vertical position. Quantitative analysis of the desorption of the monolayer material into bulk water indicates that the solubility of nystatin monolayers increases with surface pressure. At low surface pressures, the desorption of nystatin from a monolayer is controlled both by dissolution and by diffusion. However, at the plateau and in the post-plateau region, the desorption does not achieve a steady state and the monolayer is less stable than in the pre-plateau region. However, the presence of membrane lipids, even at a low mole fraction, considerably increases the stability of nystatin monolayers. This enables the application of the Langmuir monolayer technique to study nystatin in mixture with cellular membrane components, aiming at verifying its mode of action and the mechanism of toxicity.     

Insoluble monolayers of irisresorcinol at the air/water interface

Monolayer properties of irisresorcinol [5-(cis-10-heptadecenyl) resorcinol] were measured at the air/water interface. TheA-T isobars of the monolayers at 10 and 15 mN/m gave two-dimensional thermal expansivities of 1.4 × 10^–4/K and 1.3 × 10^–4/K at a temperature span from 7–40 C, respectively. The- A isotherms of the material showed only a little dependence on temperature from 5–35 C and onpH except at highpH, where monolayers expanded by ionization of resorcinol headgroups. Some types of saccharose in the subphase exhibited a characteristic interaction with irisresorcinol in monolayers, and there is a possibility that this material will be used for molecular recognition of some saccharoses.     

Contact forces at the sliding interface: mixed versus pure model alkane monolayers

Classical molecular dynamics simulations of an amorphous carbon tip sliding against monolayers of n-alkane chains are presented. The tribological behavior of tightly packed, pure monolayers composed of chains containing 14 carbon atoms is compared to mixed monolayers that randomly combine equal amounts of 12- and 16-carbon-atom chains. When sliding in the direction of chain cant under repulsive (positive) loads, pure monolayers consistently show lower friction than mixed monolayers. The distribution of contact forces between individual monolayer chain groups and the tip shows pure and mixed monolayers resist tip motion similarly. In contrast, the contact forces "pushing" the tip along differ in the two monolayers. The pure monolayers exhibit a high level of symmetry between resisting and pushing forces which results in a lower net friction. Both systems exhibit a marked friction anisotropy. The contact force distribution changes dramatically as a result of the change in sliding direction, resulting in an increase in friction. Upon continued sliding in the direction perpendicular to chain cant, both types of monolayers are often capable of transitioning to a state where the chains are primarily oriented with the cant along the sliding direction. A large change in the distribution of contact forces and a reduction in friction accompany this transition.     

A study of the interaction of dodecyl sulfobetaine with cationic and anionic surfactant in mixed micelles and monolayers at the air/water interface

The miscibility and interactions between components in mixed adsorbed films and micelles containing zwitterionic (dodecyl sulfobetaine--DSB) and cationic (dodecyltrimethylammonium bromide) or anionic (sodium dodecyl sulfonate) surfactant, respectively, have been investigated. The molecular interactions have been quantified by the values of the excess free energy of adsorption (DeltaGS,Exc) and micelle formation (DeltaGM,Exc). The obtained results indicate nonideal behavior of the investigated mixtures since the values of DeltaGS,Exc and DeltaGM,Exc) are negative. Moreover, it has been found that DSB interact more strongly with anionic surfactant as compared to cationic surfactant owing to different structure of mixed monolayers and micelles.     

Surface viscoelasticity of phospholipid monolayers at the air/water interface

The surface viscoelastic properties of monolayers of two phospholipids DPPC (L--dipalmitoylphosphatidylcholine) and DMPE (L--dimyristoylphosphatidyl-ethanolamin), at the air-water interface have been investigated. Two techniques were used for the investigation. One involved use of an interfacial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity _s as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodie dilation and compression so as to measure the dilational elastic modulusE as a function of surface area.Results indicate a strong dependence of _s andE upon monolayer phases. This suggests that the viscoelastic relaxation of monolayers can be understood as molecular rearrangements, domain exchange and molecular reorientations between different monolayer states.     

Anomalous spreading behaviour of polyethyleneglycoldistearate monolayers at air/water interface

Spreading behaviour of the dimeric surfactant polyethylene-glycoldistearate (PEGDS) monolayer at air/water interface has been studied using surface pressure-area (π-A) isotherms as a function of temperature. The isotherms show a plateau suggesting a transition between a liquid expanded (LE) and a condensed state. The condensed state possibly arises due to nucleation and growth of multilayers from the monolayer. Isobaric measurements of bothA-T and π-T at constant area show transitions atT = 295 K. These plots suggest a melting followed by formation of condensed microcrystallites. Structure optimization carried out using various angles of orientation of the alkyl tails with respect to the backbone in PEGDS reveals tilt transitions of the tails in different states which can be related to the packing behaviour seen in the isotherms. Optical microscopy has been used to confirm the structures in these states.     

Interaction of rabbit C-reactive protein with phospholipid monolayers at air/water interface

C-reactive protein (CRP) is a major acute phase reactant in most mammalian species. The structure of CRP has been previously established by crystallography, and the significance of its interaction with lipid membranes is accepted in the literature. However, the nature of the interaction between CRP and phospholipids is not yet well understood. In this paper we use monolayer technique to study the characteristics of the interaction of rabbit C-reactive protein (rCRP) with the phospholipid membranes. The results show that rCRP is surface active and can spontaneously insert into the lipid monolayers. The critical pressure for rCRP inserting into the phospholipid monolayers is about 34.5 mN/m, which is not sensitive to the types of the lipid headgroups and the presence of calcium ions in the subphase. The findings of this paper may provide a clue to the further understanding of the mechanism of the interactions between rCRP and the biological membranes.     

Influence of phytohormones on polar and hydrophobic parts of mixed phospholipid monolayers at water/air interface

Surface parameters--the limiting area, collapse pressure, and compressibility modulus for monolayers of phospholipids containing a determined hydrophobic part (16:0) but different polar parts (PPL) and a determined polar part (PC) but different fatty acids (HPL), characteristic of nonembryogenic (NE) and embryogenic (E) winter wheat calli--were evaluated at 15 degrees C. These parameters were dependent on the kind of hydrophilic group and the size of the hydrophobic part of phospholipids. In the case of PPL, higher values of Alim and picoll were noticed for NE phospholipids. In the case of HPL, lower Alim and higher picoll were detected for NE than for E monolayers. All investigated phospholipid systems stimulated the adsorption of phytohormones from the water subphase. The influence of phytohormones of anionic (IAA, 2-4-D), cationic (kinetin, zeatin), and nonionic character (zearalenone) was examined. It appeared that the surface activity of phytohormones depended strongly on the kind of tissue from which the phospholipid mixture was extracted and, in a lesser degree, on their charge. In PPL systems with a determined hydrophobic part (16:0), no great differences in phytohormone influence on NE and E monolayers were observed (except of IAA). The greatest phytohormone influence on NE monolayers in HPL systems was related to the structure of the hydrophobic part of phospholipids. IAA, the most active (with the highest Alim values) among the phytohormones examined, influenced both HPL and PPL monolayers. This indicated the interactions of IAA with polar groups of phospholipids. Phytohormones also changed the monolayer stability against collapse process and the direction of changes depended on the kind of tissue (embryogenic or nonembryogenic).     

Interactions between phosphatidylcholines and cholesterol in monolayers at the air/water interface

Mixtures of cholesterol and synthetic phospholipids, differing in saturation of phosphatidylcholine (PC) acyl chains, such as distearoyl phosphatidylcholine (DSPC), stearoyl-oleoyl phosphatidylcholine (SOPC) and dioleoyl phosphatidylcholine (DOPC) have been studied as floating Langmuir monolayers at the air/water interface. In order to examine the influence of a polar group, distearoyl phosphatidylethanolamine (DSPE) was chosen. The films were spread at room temperature on aqueous subphases and characterized by the surface pressure-area (pi-A) isotherms and compression modulus (C(s)(-1)) values. The interactions were examined by analyzing the mean molecular areas and quantified by the excess free energy of mixing values. The obtained results indicate that the affinity of cholesterol to saturated/unsaturated phosphatidylcholines does not differ significantly, and revealed strong influence of the kind of a polar group on the cholesterol-phospholipid interactions. On the other hand, the apolar group structure was found to modify the stoichiometry of sterol-PC complexes.     

Spectroscopic studies of the stability of monolayers of 2-docosylamino-5-nitropyridine at the air/water interface

Monolayers of 2-docosylamino-5-nitropyridine (DCANP) at the air/water interface were investigated by UV/Vis spectroscopy. The combination of this method with the classic constant-area relaxation technique yields insight into the longtime stability and the collapse behavior of monolayers. We have demonstrated that monolayers of DCANP are certainly stable under standard deposition conditions. At surface pressures above 20 mN/m monolayer instabilities lead to the formation of a three-dimensional head-to-head multilayered structure.     

Lateral quantized charge transfer across nanoparticle monolayers at the air/water interface

Lateral quantized charge transfer was observed with gold nanoparticle monolayers at the air/water interface. The electronic conductivity was measured by using an interdigitated arrays (IDA) electrode perpendicularly aligned at the air/water interface where a particle ensemble was trapped between the IDA fingers. The overall voltammetric responses were analogous to that of the Coulomb blockade with a relatively flat central gap. This gap was found to shrink with increasing surface pressure. Differential pulse voltammetry revealed a series of well-defined voltammetric peaks within this central gap, which are ascribed to the single electron transfer of the particle ensemble. This observation was interpreted on the basis of relatively weak electronic coupling between neighboring particles where the particles behave more individually.     

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.     

Catalase monolayers at the air/water interface I. Reversibilities and irreversibilities

/A-isotherms of catalase monolayers established at the air/water-interface are discussed quantitatively on the basis of molecular data: A relationship between a critical value of the surface pressure, the corresponding molecular area, and the molecular dimension of the molecules at the interface is proposed. It is shown that the unfolding of molecules at the water surface is pH-dependent. For each pH-value there is a distinct degree of unfolding; the molecules keep their globular state at neutral pH. Establishment at the surface of bulk solutions corresponding to globular and partly unfolded states, respectively, catalase molecules keep their original configuration on changing the pH-value of bulk-phase. The monolayers are confirmed to show reversibility with regard to lateral changes of state as well as irreversibility with respect to desorption of molecules.A model is proposed to explain the nature of the critical/A-value occurring in the/A-isotherms: on compression beyond _c, molecular segments are transferred from the surface into the bulkphase via a subsurface layer. From the experiments it is concluded that the surface pressure is determined, not only by the surface itself, but also by this subsurface layer.