Photochemical behaviour of polyacryloylacetone;and polyethylacrylacetate monolayers at the air–water interface

The surface organization of enol units of polyacryloylacetone (PAA) and polyethylacrylacetate (PEAA) monolayers at the air–water interface is examined using surface pressure, surface potential and rheological measurements and theoretical calculations based on molecular models. The mechanism and kinetics of the photochemical enol–keto tautomerization of PAA and PEAA polymers organized in a monolayer of closely packed monomer units are studied by measuring the surface area increase at constant surface pressure. The results indicate an increase in the area per unit during the consecutive enol-to-keto photoconversion and the slow interfacial reorganization of these ¶forms to a more favourable state.     

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.     

Infrared and Raman spectroscopies of monolayers at the air–water interface

Infrared and Raman spectroscopies are now currently used to obtain molecular information (orientation, conformation, organization) on monolayers at the air–water interface. In the past year, several original studies were performed on peptides and proteins and their interaction with phospholipidic monolayers.     

Monolayers of γ-globulin at the air–water interface

Monolayer formation, of γ-globulin at the air–water interface has been investigated under varying subphase compositions. At pH 7.4, it is found that a stable monolayer is obtained only when the ionic strength is greater than 0.5 M. The magnitude of the collapse pressure increases with increasing ionic strength of the subphase. These data are analyzed in comparison to the literature data. Received: 2 March 1999 Accepted in revised form: 16 June 1999     

Protein–emulsifier interactions at the air–water interface

The main interfacial physico-chemical characteristics and the kinetics of the formation of protein and emulsifier mixed films at the air–water interface are reviewed. Recent advances include the development of new molecular resolution and spectroscopic techniques coupled with surface rheological instruments and the incipient development of computer simulation of the displacement of proteins by emulsifiers.     

Interfacial assembly and host-guest interaction of anthracene-conjugated L-glutamate dendron with cyclodextrin at the air/water interface

The interfacial assembly of photo-induced dimerization of atypical anthracene-containing amphiphilic dendron and host-guest interaction with γ-cyclodextrin has been investigated.It has been proved that even without long alkyl chain the amphiphilic dendron could still form stable Langmuir monolayer at the air/water interface.Through the host-guest interaction,γ-cyclodextrin can be used to encapsulate two headgroups of amphiphilic dendron in the antiparallel direction.However,the formed host-guest complex was sensitive to the surface pressure.Slight compression of surface pressure led amphiphilic dendron to reassemble into nanofibers through the strong π-π stacking between headgroups.On the other hand,under in sitv irradiation,the amphiphilic dendron was stabilized in the cavity of γ-cyclodextrin through headgroup dimerization and the host-guest complex further irregularly aggregated to nanoparticles.Meanwhile,γ-cyclodextrin,as a silencer,blocked the supramolecular chirality transfer.Our conclusion was demonstrated through UV/vis,FT-IR,CD spectrum and AFM images,respectively.     

Coadsorption of sodium dodecyl sulfate and medium-chain alcohols at the air–water interface

The adsorption of sodium dodecyl sulfate (SDS) and n-dodecanol from aqueous solutions of the pure and mixed surfactants at the air–water surface is studied by equilibrium surface pressure measurements, surface pressure transients and Brewster angle microscopy. The adsorption layers of SDS and n-dodecanol show fundamental differences in the phase behaviour. The adsorption parameters of both components are determined. Under appropriate conditions, a phase transition at which condensed phase textures are formed, occurs in the adsorption layers of n-dodecanol. The adsorption layers of surface-chemically purified SDS exist only in a fluid-like state without a phase transition under formation of condensed phase domains. Coadsorption of both surfactants is only investigated in the range of trace amounts of n-dodecanol. Depending on the mixing ratio and the system conditions (bulk concentration, temperature), a phase transition can or cannot occur. At absence of a phase transition, comparable surface concentrations of both components are calculated based on a orthogonal collocation solution for a two-component system. The adsorption properties are completely different when a phase transition occurs. Condensed phase domains of n-dodecanol formed, after the phase transition point, grow finally to a homogeneous condensed phase which replaces completely SDS.     

Interfacial dilational properties of partly hydrolyzed polyacrylamide and gemini surfactant at the decane–water interface

The dilational viscoelastic properties of partly hydrolyzed polyacrylamide (HPAM) and surfactant (C₁₂COONa-p-C₉SO₃Na) in the absence or presence of electrolyte were investigated at the decane–water interface by means of longitudinal method and the interfacial tension relaxation method. The polymer plays different roles in influencing the structure of HPAM–surfactant mix-adsorbed layer at different surfactant concentration. At low surfactant concentration, the addition of polymer could sharply decrease the dilational elasticity mainly due to the weakening of the “entanglement” among long alkyl chains in surfactant molecules, while the addition of the polymer may enhance the dilational elasticity due to the slow diffusivity of the polymer chains at higher surfactant concentration. And the added electrolyte, which results in screening of electrostatic interactions between the ionized groups, generally decreases the interfacial dilational elasticity and increases the dilational viscosity. The data obtained on the relaxation processes via interfacial tension relaxation measurement can explain the results from dilational viscoelasticity measurements very well.     

Synthesis of amphiphilic V-type silica nanogels and study of their self-assembling at the air–water interface

The methods for the synthesis of silica nanogels and a modifying agent, as well as related amphiphilic V-type silica nanogels are presented. Self-assembly of the synthesized amphi philic nanogels during the formation of Langmuir monolayers at the air–water interface is considered. Aggregate structures were revealed to form during ordering of the silica V-type nanogels at the air–water interface after collapse of the Langmuir monolayer. For the low-molecular-weight fraction of the silica nanogels, the aggregates do not completely decompose upon the expansion of the Langmuir layer since are formed by mutually penetrating macromolecules. For the highmolecular- weight fraction, they are reversibly formed and decompose in consecutive compression– expansion cycles, which is characteristic of Langmuir layers of nanoparticles.     

Ultrathin dynamic networks formed by the surfactant SPAN 65 at the air–water and oil–water interface

Some surfactants tend to form ultrathin films at the surface of water or at the interface between oil and water. A representative of these surface-active compounds is SPAN 65 (trioctadecanyl ester of sorbic acid). Induced by attractive interactions these molecules can self-associate to form temporary networks. At the planar surface we measured the two-dimensional relaxation modulus, the storage modulus, the loss modulus and the relaxation spectrum. In addition to these measurements, we have also investigated the molecular structure of these networks with “Brewster-angle-microscopy”. The results indicate that temporary cross-linking points, which have lifetimes of the order of a few seconds, tend to stabilize these films. This dynamic network formation is also interesting for numerous technical applications, and it might be used for the preparation of emulsions, foams or microcapsules. Received: 23 October 2000 Accepted: 3 November 2000     

Adsorption characteristics of N’-undecylenamidopropyl-N”-trimethylammonium methyl sulfate at the air–water interface

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Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.     

Molecular organization and aggregation of mucin at air–water interface in the presence of chromium(III) complexes

The interfacial organization of mucin (glycoprotein) in the presence of chromium(III) complexes has been assessed from the surface pressure–molecular area (π–A) isotherms in Langmuir films at air–water interface and the surface energy of their LB films through contact angle measurements. At pH 7.0, the electrostatic interaction of [Cr(en)₃]Cl₃ with mucin was found to bring about changes in the average surface area from 3.26 to 1.47 nm²; suggesting the possible formation of large aggregates of mucin. Adsorption experiments using surface potential measurements reveal that [Cr(en)₃]Cl₃ binds at a much faster rate to the available binding sites in mucin when compared to [Cr(salen)(H₂O)₂](ClO₄) which binds coordinatively to mucin.     

A comparative study of the thermodynamic properties at the air–water interface and in the bulk of structurally related phenothiazine drugs aqueous solutions

Surface tension, conductivity, density, and ultrasound velocity measurements have been performed in order to determine in a systematic manner some of the aggregation properties of the phenothiazine drugs promazine and triflupromazine hydrochlorides. Both drugs are structurally related, differing in an extra CF₃ group in the triflupromazine molecular structure. Surface tension data showed that the presence of an extra CF₃ in the molecular structure of triflupromazine involves a higher hydrophobicity of this drug and a restriction in the number of conformations molecules can adopt due to the presence of this bulkier atomic group. This involves a larger surface area in order to accommodate triflupromazine molecules at the interface. From conductivity measurements at different temperatures, the thermodynamic quantities of the micellization process of these drugs indicate that the aggregation is a spontaneous process, mainly enthalpic, where the London-dispersion forces play an active role. Using density and ultrasound velocity measurements, apparent molar volume and adiabatic compressibility of aqueous solutions of the amphiphilic cationic drugs have been determined. Positive deviations from the Debye–Hückel limiting law of the apparent molar volume were obtained from both drugs over the whole temperature range, which provides evidence of possible pre-association at concentrations below the critical concentration. Apparent molar adiabatic compressibility of the aggregates formed by these drugs was typical of those corresponding for an aggregate formed by a stacking process.     

Dilational properties of gemini surfactant/polymer systems at the air–water surface

The surface properties of mixed system containing gemini anionic surfactant 1,2,3,4-butanetetracarboxylic sodium, 2,3-didodecyl ester and partly hydrolyzed polyacrylamide were investigated by surface tension measurements and oscillating bubble methods. The influences of surfactant concentration, dilational frequency, temperature, pH, as well as salts on dilational modulus were explored. Meanwhile, the interfacial tension relaxation method was employed to obtain the characteristic time of surface relaxation process. The polymers play important roles in changing the interfacial properties especially at lower surfactant concentration. The possible mechanism of the polymer in changing the interfacial properties is proposed. Both the hydrophobic and electrostatic interaction among the surfactants and polymers dominate the surface properties of mixed system. These dynamic properties are of fundamental interest in understanding the structure of adsorption layers, dynamics of surfactant molecules, and their interaction with polymers at the surface.     

Studies of temperature influence on adsorption behaviour of nonionic polymers at the zirconia–solution interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO₂ surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.     

Cratylia mollis lectin at the air–aqueous solution interface: adsorption and lectin–lipid interactions

The interfacial behaviour of Cratylia mollis (Cra) at the air/water interface and its penetrant ability into spread phospholipid monolayers (Lipoid E80 and Epicuron 200) has been monitored by surface tension measurements. The first-order rate constants defining adsorption and rearrangement obtained from surface tension kinetics data reveal that Cra is a rather stable protein which exhibits characteristic protein adsorption patterns in which the breaking points separating diffusion–penetration and rearrangement profiles could have been easily distinguished. The penetration of Cra into Lipoid E80 and Epicuron 200 phospholipid monolayers has been inferred in terms of penetration pressure increments (ΔΠ) versus time relationships. The data clearly showed that penetrant ability of the lectin was, to a large extent, dependent on monolayer compressibilities. Thus, for Lipoid E80, which contained a rather high percentage of phosphatidylethanolamine (DPPE) in the mixture with phosphatidylcholine (DPPC), penetration of Cra at the high monolayer compression (20 mN m⁻¹) was lower than that observed for Epicuron 200, which did not contain DPPE. Indeed, in the middle of the Π-A isotherm, DPPE was markedly less compressible than DPPC. However, at the low monolayer surface coverage (3 mN m⁻¹), the rates of Cra penetration into both Lipoid E80 and Epicuron 200, although much higher for the latter at the beginning of adsorption, yielded similar limiting values of ΔΠ. This has been attributed to the occurrence of a hydrophobic interaction between the lectin and hydrophobic phospholipid chains that have the same length for both Lipoid E80 and Epicuron 200.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.     

Dynamics of water and oil at the solid–liquid interface in macroporous media and reservoir rocks

Nuclear magnetic relaxation dispersion experiments at different temperatures, using the magnetic field-cycling method, are reported. These experiments allow the obtaining of original information about water or oil molecular dynamics at solid–liquid interface in porous media, such as grain packing or reservoir rocks. These results on molecular dynamics at the pore surface are of real interest for oil-recovery. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones.     

Molecular dynamics simulations on the adsorption of 4-n-octyl-4′-cyanobiphenyl (8CB) at the air/water interface

The influence of surface coverage to the structural properties of 4-n-octyl-4´-cyanobiphenyl (8CB) monolayer at the air/water interface was studied by full atomistic molecular dynamics simulations. These properties include density profiles, interface thickness, monolayer width, orientational order parameters, and atom-pair radial distribution functions. The calculated tilt angles of the cyanobiphenyl and alkyl parts are in fairly good agreement with the experiments. The simulation results exhibit the general trends in the previous experimental and simulation data.