Isotherms of phospholipid monolayers measured by a pendant drop technique

The Axisymmetric Drop Shape Analysis (ADSA) has been used to study the surface pressure/area isotherms of insoluble surfactant monolayers. The continuous measurement of surface tension as a function of surface area by increasing and decreasing the drop volume allows to investigate the phase transitions in monolayers. The isotherms of two phospholipids, dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl ethanolamine (DMPE), show good agreement with those measured by using a conventional Langmuir-Blodgett film balance, except in the coexistence region. The observed disagreements are discussed in terms of differences in compression rate, curvature of the surface and effect of impurities. Evidence of possible geometric effects on monolayer domain formation and growth is given on the basis of BAM images.Due to the small total surface area, the ADSA technique provides advantages as regards homogeneity of temperature, surface pressure, surface concentration and the symmetry of area changes.     

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

Surface phase behavior of n-hexadecyl phosphate (n-HDP) and its mixture with L-arginine (L-arg), which behaves as L-argininium cation (L-arg(+)) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at 20 degrees C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at > or =10 degrees C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the pi-t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg(+).     

INFLUENCE OF pH AND PRESENCE OF Cu2+ IONS ON THE PROPERTIES OF POLYACRYLOYLACETONE (PAA)MONOLAYERS

The influence of pH on the properties of polyacryloylacetone (PAA) monolayers at air/water interface was examined by using surface pressure-area isotherm measurement,a new rheological approach and AFM imaging. The isotherms and AFM-imaging detect phase transitions in PAA monolayers and growth of microdomains during compression in the case of pH 6.6 and missing of such kind of structures at pH 9. The rheological study below and above the phase transition shows an existence of a large relaxation time related to the process of formation of microdomains.

The influence of divalent ions on the properties of a PAA monolayer was studied. The presence of Cu²⁺ ions leads to polymer - ion interaction which results in the formation of microdomains.     

Self-assembled monolayers at electrode metal surfaces

Neutral organic compounds, dissolved in an electrolyte in contact with an electrode, adsorb and form different monolayers which may range from dilute to compact films. In some instances, non-electroactive organic molecules are highly associated and form 2D condensed phases which are characterized by the presence of phase transitions. The occurence of these self-assembled monolayers is discussed on the basis of experimental results obtained at equilibrium as well as under dynamic conditions. Self-assembling depends on the relative magnitude of the interactions involving the surfactant, the solvent and the electrode. Adequate potential-step programmes have been successfully used to trigger the formation of the ordered phase. It is found that the kinetics are controlled by a nucleation and growth mechanism. According to the experimental conditions, a deterministic or stochastic behaviour is observed. The amplitude of the supersaturation, given by the surface free energy gap between the final and metastable states, is independently controlled by the potential, temperature and surfactant concentration. The classical nucleation theory allows the determination of key parameters such as the line tension, the radius and free energy of formation of the critical nucleus. Ion and electron transfer processes through condensed monolayers are also briefly described.     

Infrared imaging of a solid phase surfactant monolayer

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.     

Polysorbate20 adsorption layers below and above the critical micelle concentration over aluminum; cloud point and inhibitory role investigations at the solid/liquid interface

Non‐ionic polysorbate20 surfactant was used to produce adsorption protective layers below and above its critical micelle concentration (CMC) at the liquid/solid interface. The well‐ordered accumulation of surfactant molecules on the metal surface below the CMC led to the formation of oriented surfactant monolayers. On the other hand, as the surfactant concentration increased above the CMC, the monodisperse micelles, free surfactant molecules and oriented surfactant monolayers undergo aggregate formation and produce a turbid solution. The gradual increase in the number and size of aggregates leads to phase separation and hence disassembled protective layers that allow easier penetration of corrosive HCl at a metal surface. This was demonstrated by inhibition efficiency, activation energy, enthalpy and entropy of activation values. Two‐dimensional irregular crystalline sheets accumulated at the surface of aluminum, as shown by scanning electron micrographs. Adsorption of polysorbate20 at the aluminum surface exhibited a Temkin isotherm fit. Larger desorption processes at the cloud point demonstrate aggregate formation and phase separation, and hence poorer adsorption layers at the metal surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Kinetic appearance of first-order gas-liquid expanded and liquid expanded-liquid condensed phase transitions below the triple point

Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure-time (pi-t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 degrees C. Above this triple point, a first-order G-LE phase transition occurring at 0 surface pressure is followed by another first-order LE-LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the pi-t measurements show only one first-order phase transition that should be G-LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G-LC phase transition is, in fact, a combination of the G-LE and LE-LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G-LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.     

Kinetics of Complexation in the Monolayers of Amphiphilic Dicetylcyclene on the Surface of Aqueous Copper(II) Chloride Solutions

The kinetics of complexation in monolayers of dicetycyclene at the surface of aqueous copper(II) chloride solutions was studied. It was shown that the changes in the phase state of monolayer related to the conformational transitions of macrocycle are responsible for the differences in the rate and binding mechanism of copper ions. It was concluded that the sterically more advantageous (for the coordination with the metal ion) conformation of macrocyclic polyamine is ensured in a monolayer. In addition, it was established that the rate and mechanism of complexation in such monolayers greatly depend on the degree of protonation of ligands, the latter being dependent on the subphase pH. It was also demonstrated that an increase in subphase pH to 7 and higher results in an almost total suppression of metal ion binding due to strong conformational distortion of dicetylcyclene macrocycles in a monolayer and the hydrogen bonding between macrocycles.     

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant

We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate (n-TDP) at the air-water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure (pi)-time (t) adsorption isotherm is found to occur from 2 to 20 degrees C with appropriate concentrations of the amphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the surface pressure is caused by a first-order phase transition. Only this phase transition is observed at <6.5 degrees C and it is considered as a gas (G)-liquid condensed (LC) phase transition. Above 6.5 degrees C, the phase transition at zero surface pressure is followed by another phase transition, which is indicated by the presence of cusp points in the pi-t curves at different temperatures. Each of the cusp points is followed by a plateau, which is accompanied by two surface phases, indicating that the latter transitions are also first-order in nature. At >6.5 degrees C, the former transition is classified as a first-order G-liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE-LC phase transition. The critical surface pressure (pi(c)) necessary for the G-LC and G-LE phase transitions is zero and remains constant all over the studied temperatures, whereas that for the LE-LC transition increases linearly with increasing temperature. Based on these results, we construct a rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 degrees C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air-water interface.     

Molecular simulations of surface forces and film rupture in oil/water/surfactant systems

We use dissipative particle dynamics (DPD) and molecular models to simulate interacting oil/water/surfactant interfaces. The system comprises sections of two emulsion droplets separated by a film. The film is in equilibrium with a continuous phase, in analogy with the surface force apparatus. This is achieved by combining DPD with a Monte Carlo scheme to simulate a muVT ensemble. The setup enables the computation of surface forces as a function of the distance between the two interfaces, as well as the detection of film rupture. We studied monolayers of nonionic model surfactants at different densities and compared oil-water-oil and water-oil-water emulsion films. Between surfactant monolayers facing each other tails-on (water-oil-water films), we observed repulsive forces due to the steric interaction between overlapping hydrophobic tails. The repulsion increases with surfactant density. Conversely, no such repulsion is observed between surfactant monolayers facing each other heads-on. Instead, the film ruptures, the monolayers merge, and a channel forms between the two droplet phases. Film rupture can also be induced in the water-oil-water films by forcing the interfaces together. The separation at rupture increases for oil-water-oil films and decreases for water-oil-water films when the surfactant density increases. The results are in qualitative agreement with existing theories of emulsion stability in creams, in particular with the channel nucleation theory based on the natural curvature of surfactants.     

Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant

The effect of hydrophobic alkylated gold nanoparticles (Au NPs) on the phase behavior and structure of Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) and Survanta, a naturally derived commercial pulmonary surfactant that contains DPPC as the main lipid component and hydrophobic surfactant proteins SP-B and SP-C, has been investigated in connection with the potential implication of inorganic NPs in pulmonary surfactant dysfunction. Hexadecanethiolate-capped Au NPs (C(16)SAu NPs) with an average core diameter of 2 nm have been incorporated into DPPC monolayers in concentrations ranging from 0.1 to 0.5 mol %. Concentrations of up to 0.2 mol % in DPPC and 16 wt % in Survanta do not affect the monolayer phase behavior at 20 °C, as evidenced by surface pressure-area (π-A) and ellipsometric isotherms. The monolayer structure at the air/water interface was imaged as a function of the surface pressure by Brewster angle microscopy (BAM). In the liquid-expanded/liquid-condensed phase coexistence region of DPPC, the presence of 0.2 mol % C(16)SAu NPs causes the formation of many small, circular, condensed lipid domains, in contrast to the characteristic larger multilobes formed by pure lipid. Condensed domains of similar size and shape to those of DPPC with 0.2 mol % C(16)SAu NPs are formed by compressing Survanta, and these are not affected by the C(16)SAu NPs. Atomic force microscopy images of Langmuir-Schaefer-deposited films support the BAM observations and reveal, moreover, that at high surface pressures (i.e., 35 and 45 mN m(-1)) the C(16)SAu NPs form honeycomb-like aggregates around the polygonal condensed DPPC domains. In the Survanta monolayers, the C(16)SAu NPs were found to accumulate together with the proteins in the liquid-expanded phase around the circular condensed lipid domains. In conclusion, the presence of hydrophobic C(16)SAu NPs in amounts that do not influence the π-A isotherm alters the nucleation, growth, and morphology of the condensed domains in monolayers of DPPC but not of those of Survanta. Systematic investigations of the effect of the interaction of chemically defined NPs with the lipid and protein components of lung surfactant on the physicochemical properties of surfactant films are pertinent to understanding how inhaled NPs impact pulmonary function.     

Penetration behaviour of alkylbetainate chlorides into lipid monolayers

In this paper, the penetration behaviour of the alkylbetainate chloride surfactants (C(n)BC, n=10-16) into lipid monolayers of dipalmitoylphosphatidylserine (DPPS), dipalmitoylphosphatidic acid (DPPA), dipalmitoylphosphatidylethanolamine (DPPE), palmitoyoleoylphosphatidylcholine (POPC) and cholesterol (CHOL) is investigated using the Langmuir trough technique. The penetration of C(n)BC is followed by measurement of the surface pressure increase (Δπ) at a constant surface area after the injection of C(n)BC into the aqueous phase, underneath the lipid monolayer previously spread at the air-water interface at 25°C and at different initial surface pressures (π(i)). The influence of both the lipid head group and the surfactant hydrocarbon chain length on the effectiveness of C(n)BC penetration into these monolayers is discussed. The results have shown that C(n)BC adsorb at the air-water interface giving evidence of their surface-active properties. The adsorption kinetics of C16BC into different lipid monolayers are lipid head charge and lipid head volume-dependent. The magnitude of the surface pressure increase (Δπ) arises in the following order: DPPA>DPPS?CHOL≈DPPE>POPC. C(n)BC penetration into negatively-charged (DPPS and DPPA) monolayers does not seem to depend on surfactant alkyl-chain length compared to uncharged (CHOL) and zwitterionic (DPPE and POPC) monolayers for which Δπ increases with a larger alkyl-chain length. Electrostatic interactions are mainly involved in the affinity of C(n)BC with monolayers but the hydrophobic effect plays also a role.     

Effect of mycolic acid on surface activity of binary surfactant lipid monolayers

In pulmonary tuberculosis, Mycobacterium tuberculosis lies in close physical proximity to alveolar surfactant. Cell walls of the mycobacteria contain loosely bound, detachable surface-active lipids. In this study, the effect of mycolic acid (MA), the most abundant mycobacterial cell wall lipid, on the surface activity of phospholipid mixtures from lung surfactant was investigated using Langmuir monolayers and atomic force microscopy (AFM). In the presence of mycolic acid, all the surfactant lipid mixtures attained high minimum surface tensions (between 20 and 40 mN/m) and decreased surface compressibility moduli <50 mN/m. AFM images showed that the smooth surface topography of surfactant lipid monolayers was altered with addition of MA. Aggregates with diverse heights of at least two layer thicknesses were found in the presence of mycolic acid. Mycolic acids could aggregate within surfactant lipid monolayers and result in disturbed monolayer surface activity. The extent of the effect of mycolic acid depended on the initial state of the monolayer, with fluid films of DPPC-POPC and DPPC-CHOL being least affected. The results imply inhibitory effects of mycolic acid toward lung surfactant lipids and could be a mechanism of lung surfactant dysfunction in pulmonary tuberculosis.     

Mixed monolayers of bovine serum albumin with a nonionic surfactant (Tween 80)

Mixed monolayers of bovine serum albumin (BSA) with a nonionic surfactant (Tween 80) are obtained by spreading solutions containing both components over the surface of a subphase (water with pH 6) over a wide range of solution compositions. According to compression-expansion isotherms, the mixed monolayers are of the condensed type when the BSA concentrations in the solution are far higher than or equal to the surfactant concentration. Such monolayers mostly consist of BSA-Tween 80 (1: 1) complexes. In contrast, a BSA monolayer is of the expanded type. When Tween 80 in the solution prevails over Tween 80, the monolayers become unstable. The results of this work pertain to the monitoring of the properties of protein-surfactant mixtures and design of Langmuir-Blodgett (LB) films.     

Hydrolysis and Condensation of Alkyltrimethoxysilanes in Monomolecular Films

Hydrolysis and condensation of monomolecular alkyloxysilane films by the Langmuir technique is presented. Octadecyltrimethoxysilane formed monolayers on aqueous subphases with different properties depending on the bulk pH. At pH 1 a solid condensed film was directly formed with a molecular area of 23 Ų and a surface pressure/surface area variation similar to that on non-ionized stearic acid. At pH 5.6 and 11 several phase transitions were observed during the compression of the monolayer with a final collapse at a molecular area of 20 Ų. Relaxation measurements confirmed the stability of the films for longer than 12 hours at different surface pressures below a critical value.     

Phase behavior and viscoelastic properties of trisilanolcyclohexyl-POSS at the air/water interface

A trisilanol polyhedral oligomeric silsesquioxane (POSS), trisilanolcyclohexyl-POSS (TCyP), has recently been reported to undergo a series of phase transitions from traditional Langmuir monolayers to unique rodlike hydrophobic aggregates in multilayer films that are different from "collapsed" morphologies seen in other systems at the air/water interface. This paper focuses on the phase transitions and morphology of films varying in average thickness from monolayers to trilayers and the corresponding viscoelastic properties of trisilanolcyclohexyl-POSS molecules at the air/water interface by means of surface pressure-area per molecule (Pi-A) isotherms, Brewster angle microscopy (BAM), and interfacial stress rheometry (ISR) measurements. The morphology studies by BAM reveal that the TCyP monolayer can collapse into different 3D structures by homogeneous or heterogeneous nucleation mechanisms. For homogeneous nucleation, analysis by Vollhardt et al.'s nucleation and growth model reveals that TCyP collapse is consistent with instantaneous nucleation with hemispherical edge growth at Pi = 3.7 mN.m(-1). Both surface storage (Gs') and loss (Gs") moduli obtained by ISR reveal three different non-Newtonian flow regimes that correlate with phase transitions in the Pi-A isotherms: (A) A viscous liquidlike "monolayer"; (B) a "biphasic regime"between a liquidlike viscous monolayer and a more rigid trilayer; and (C) an elastic solidlike "trilayer". These observations provide interesting insights into collapse mechanisms and structures in Langmuir films.     

Surface phase transitions in foams and emulsions

Surface phase transitions in surfactant adsorption layers are known to affect the dynamic properties of foams and to induce surface nucleation in freezing emulsion drops. Recently, these transitions were found to play a role in several other phenomena, opening new opportunities for controlling foam and emulsion properties. This review presents a brief outlook of the emerging opportunities in this area. Three topics are emphasized: (1) the use of surfactant mixtures for inducing phase transitions on bubble surfaces in foams; (2) the peculiar properties of natural surfactants saponins, which form extremely viscoelastic surface layers; and (3) the main phenomena in emulsions, for which the surface phase transitions are important. The overall conclusion from the reviewed literature is that surface phase transitions could be used as a powerful tool to control many foam and emulsion properties, but we need deeper understanding of the underlying phenomena to fully explore these opportunities.     

Liquid–liquid interfaces: studied by X-ray and neutron scattering

Major recent advances include the development of new experimental techniques that enabled the first precise measurements of interfacial widths at water–oil interfaces and of the ordering of surfactants adsorbed to these interfaces, studies of phase transitions and domain formation in surfactant monolayers, and studies of interfacial fluctuations confined by and coupled across thin liquid films.     

Line tension of alkane lenses on aqueous surfactant solutions at phase transitions of coexisting interfaces

Alkane droplets on aqueous solutions of surfactants exhibit a first-order wetting transition as the concentration of surfactant is increased. The low-concentration or “partial wetting” state corresponds to an oil lens in equilibrium with a two-dimensional dilute gas of oil and surfactant molecules. The high-concentration or “pseudo-partial wetting” state consists of an oil lens in equilibrium with a mixed monolayer of surfactant and oil. Depending on the combination of surfactant and oil, these mixed monolayers undergo a thermal phase transition upon cooling, either to a frozen mixed monolayer or to an unusual bilayer structure in which the upper leaflet is a solid layer of pure alkane with hexagonal packing and upright chains while the lower leaflet remains a disordered liquid-like mixed monolayer. Additionally, certain long-chain alkanes exhibit a surface freezing transition at the air–oil interface where the top monolayer of oil freezes above its melting point. In this review, we summarize our previous studies and discuss how these wetting and surface freezing transitions influence the line tension of oil lenses from both an experimental and theoretical perspective.     

Insoluble mixed monolayers : II. Protolytic equilibria and the influence of the pH on the collapse pressure

A general thermodynamic treatment is given for the protolytic equilibria in an insoluble monolayer, containing surfactant molecules with n ionizable protons and able to accept m more protons, and being spread at the liquid/gas interface. The correlation between the pH of the subphase liquid and the collapse pressure of the monolayer is discussed. By using the approximation of perfect solutions and of binary surface systems (protonated and deprotonated molecular species of the surfactant) several methods are proposed for deriving apparent surface acidity constants from experimental collapse pressure vs pH curves, in the case of both miscible in monolayer miscible in collapsed bulk phase and miscible in monolayer immiscible in collapsed bulk phase, type systems. Some of these methods are based on a complete perfect solution approximation (CA) taking into account the molar fraction of the subphase liquid in the monolayer and the others use “surfactant” approximation (SA), neglecting this molar fraction. The methods proposed are tested on monolayers of carotenoid pigments spread at aqueous solution/air interfaces. Results obtained by the different methods are rather close to each other, but the approximation CA is better than SA. The apparent surface acidity constants of different carotenoids are compared with each other and discussed in terms of molecular structure and electronic effects.