Liquid to hexatic to crystalline order in Langmuir-Blodgett films

Two-Dimensional Nanoparticle Supracrystals: A Model System for Two-Dimensional Melting

In a Langmuir trough, successive compression cycles can drive a two-dimensional (2D) nanoparticle supracrystal (NPSC) closer to its equilibrium structure. Here, we show a series of equilibrated 2D NPSCs consisting of gold NPs of uniform size, varying solely in the length of their alkanethiol ligands. The ordering of the NPSC is governed by the ligand length, thus providing a model system to investigate the nature of 2D melting in a system of NPs. As the ligand length increases the supracrystal transitions from a crystalline to a liquid-like phase with evidence of a hexatic phase at an intermediate ligand length. The phase change is interpreted as an entropy-driven phenomenon associated with steric constraints between ligand shells. The density of topological defects scales with ligand length, suggesting an equivalence between ligand length and temperature in terms of melting behavior. On the basis of this equivalence, the experimental evidence indicates a two-stage 2D melting of NPSCs.

Two-dimensional melting under quenched disorder

We study the influence of quenched disorder on the two-dimensional melting behavior of superparamagnetic colloidal particles, using both video microscopy and computer simulations of repulsive parallel dipoles. Quenched disorder is introduced by pinning a fraction of the particles to an underlying substrate. We confirm the occurrence of the Kosterlitz-Thouless-Halperin-Nelson-Young scenario and observe an intermediate hexatic phase. While the fluid-hexatic transition remains largely unaffected by disorder, the hexatic-solid transition shifts to lower temperatures with increasing disorder. This results in a significantly broadened stability range of the hexatic phase. In addition, we observe spatiotemporal critical(like) fluctuations, which are consistent with the continuous character of the phase transitions. Characteristics of first-order transitions are not observed.

Effect of cholesterol nanodomains on monolayer morphology and dynamics

At low mole fractions, cholesterol segregates into 10- to 100-nm-diameter nanodomains dispersed throughout primarily dipalmitoylphosphatidylcholine (DPPC) domains in mixed DPPC:cholesterol monolayers. The nanodomains consist of 6:1 DPPC:cholesterol "complexes" that decorate and lengthen DPPC domain boundaries, consistent with a reduced line tension, λ. The surface viscosity of the monolayer, ηs, decreases exponentially with the area fraction of the nanodomains at fixed surface pressure over the 0.1- to 10-Hz range of frequencies common to respiration. At fixed cholesterol fraction, the surface viscosity increases exponentially with surface pressure in similar ways for all cholesterol fractions. This increase can be explained with a free-area model that relates ηs to the pure DPPC monolayer compressibility and collapse pressure. The elastic modulus, G', initially decreases with cholesterol fraction, consistent with the decrease in λ expected from the line-active nanodomains, in analogy to 3D emulsions. However, increasing cholesterol further causes a sharp increase in G' between 4 and 5 mol% cholesterol owing to an evolution in the domain morphology, so that the monolayer is elastic rather than viscous over 0.1-10 Hz. Understanding the effects of small mole fractions of cholesterol should help resolve the controversial role cholesterol plays in human lung surfactants and may give clues as to how cholesterol influences raft formation in cell membranes.

Effect of confinement on melting behavior of cadmium arachidate Langmuir-Blodgett multilayer

The effect of confinement between two metallic layers on the melting behavior of a 13 monolayer cadmium arachidate (CdA) Langmuir-Blodgett (LB) multilayer has been studied. Temperature dependent diffraction measurements provide information about structural changes occurring in the film plane as well as in the out-of-plane direction. X-ray standing waves have been used to achieve depth selectivity in diffraction measurements. It is found that the difference in melting behavior of the surface and the bulk, which is observed in the film with free surface, disappears in the case of confined films; while the free surface transforms to hexaticlike phase via an intermediate smectic phase, confinement results in disappearance of this phase, and the sequence of transformations in the bulk and the interfacial regions becomes identical. Some anisotropy between (01 + 11¯) and (10) directions remains, with coherence along (10) direction decreasing at a faster rate. The confinement between metallic layers also significantly reduces the tilting of the chains observed at higher temperature. Further, both in the case of film with free surface and confined films, melting at the surface/interface occurs at a lower temperature as compared to the bulk.

Active microrheology and simultaneous visualization of sheared phospholipid monolayers

Two-dimensional films of surface-active agents-from phospholipids and proteins to nanoparticles and colloids-stabilize fluid interfaces, which are essential to the science, technology and engineering of everyday life. The 2D nature of interfaces present unique challenges and opportunities: coupling between the 2D films and the bulk fluids complicates the measurement of surface dynamic properties, but allows the interfacial microstructure to be directly visualized during deformation. Here we present a novel technique that combines active microrheology with fluorescence microscopy to visualize fluid interfaces as they deform under applied stress, allowing structure and rheology to be correlated on the micron-scale in monolayer films. We show that even simple, single-component lipid monolayers can exhibit viscoelasticity, history dependence, a yield stress and hours-long time scales for elastic recoil and aging. Simultaneous visualization of the monolayer under stress shows that the rich dynamical response results from the cooperative dynamics and deformation of liquid-crystalline domains and their boundaries.

Overcoming rapid inactivation of lung surfactant: analogies between competitive adsorption and colloid stability

Lung surfactant (LS) is a mixture of lipids and proteins that line the alveolar air-liquid interface, lowering the interfacial tension to levels that make breathing possible. In acute respiratory distress syndrome (ARDS), inactivation of LS is believed to play an important role in the development and severity of the disease. This review examines the competitive adsorption of LS and surface-active contaminants, such as serum proteins, present in the alveolar fluids of ARDS patients, and how this competitive adsorption can cause normal amounts of otherwise normal LS to be ineffective in lowering the interfacial tension. LS and serum proteins compete for the air-water interface when both are present in solution either in the alveolar fluids or in a Langmuir trough. Equilibrium favors LS as it has the lower equilibrium surface pressure, but the smaller proteins are kinetically favored over multi-micron LS bilayer aggregates by faster diffusion. If albumin reaches the interface, it creates an energy barrier to subsequent LS adsorption that slows or prevents the adsorption of the necessary amounts of LS required to lower surface tension. This process can be understood in terms of classic colloid stability theory in which an energy barrier to diffusion stabilizes colloidal suspensions against aggregation. This analogy provides qualitative and quantitative predictions regarding the origin of surfactant inactivation. An important corollary is that any additive that promotes colloid coagulation, such as increased electrolyte concentration, multivalent ions, hydrophilic non-adsorbing polymers such as PEG, dextran, etc. added to LS, or polyelectrolytes such as chitosan, also promotes LS adsorption in the presence of serum proteins and helps reverse surfactant inactivation. The theory provides quantitative tools to determine the optimal concentration of these additives and suggests that multiple additives may have a synergistic effect. A variety of physical and chemical techniques including isotherms, fluorescence microscopy, electron microscopy and X-ray diffraction show that LS adsorption is enhanced by this mechanism without substantially altering the structure or properties of the LS monolayer.

Mechanisms of polyelectrolyte enhanced surfactant adsorption at the air-water interface

Chitosan, a naturally occurring cationic polyelectrolyte, restores the adsorption of the clinical lung surfactant Survanta to the air-water interface in the presence of albumin at much lower concentrations than uncharged polymers such as polyethylene glycol. This is consistent with the positively charged chitosan forming ion pairs with negative charges on the albumin and lung surfactant particles, reducing the net charge in the double-layer, and decreasing the electrostatic energy barrier to adsorption to the air-water interface. However, chitosan, like other polyelectrolytes, cannot perfectly match the charge distribution on the surfactant, which leads to patches of positive and negative charge at net neutrality. Increasing the chitosan concentration further leads to a reduction in the rate of surfactant adsorption consistent with an over-compensation of the negative charge on the surfactant and albumin surfaces, which creates a new repulsive electrostatic potential between the now cationic surfaces. This charge neutralization followed by charge inversion explains the window of polyelectrolyte concentration that enhances surfactant adsorption; the same physical mechanism is observed in flocculation and re-stabilization of anionic colloids by chitosan and in alternate layer deposition of anionic and cationic polyelectrolytes on charged colloids.

Low-dimensionality effects in the melting of a Langmuir-Blodgett multilayer

Low-dimensionality effects in the melting behavior of a cadmium arachidate Langmuir-Blodgett multilayer have been studied. Depth resolved information about structural changes occurring with temperature is obtained using in-plane X-ray diffraction under standing wave conditions. The surface region exhibits a distinctly different melting behavior as compared to the bulk of the film. While in the bulk of a 13-monolayer cadmium arachidate multilayer, the crystalline phase directly transforms to a tilted hexaticlike phase at 360 K, in the near surface region transformation occurs via an intermediate smectic phase. This behavior of the surface region is similar to that observed in two-dimensional crystals. Thus even in a thick Langmuir-Blodgett multilayer, the surface region exhibits low-dimensionality effects.

Thermal response of Langmuir-Blodgett films of dipalmitoylphosphatidylcholine studied by atomic force microscopy and force spectroscopy

The topographic evolution of supported dipalmitoylphosphatidylcholine (DPPC) monolayers with temperature has been followed by atomic force microscopy in liquid environment, revealing the presence of only one phase transition event at approximately 46 degrees C. This finding is a direct experimental proof that the two phase transitions observed in the corresponding bilayers correspond to the individual phase transition of the two leaflets composing the bilayer. The transition temperature and its dependency on the measuring medium (liquid saline solution or air) is discussed in terms of changes in van der Waals, hydration, and hydrophobic/hydrophilic interactions, and it is directly compared with the transition temperatures observed in the related bilayers under the same experimental conditions. Force spectroscopy allows us to probe the nanomechanical properties of such monolayers as a function of temperature. These measurements show that the force needed to puncture the monolayers is highly dependent on the temperature and on the phospholipid phase, ranging from 120+/-4 pN at room temperature (liquid condensed phase) to 49+/-2 pN at 65 degrees C (liquid expanded phase), which represents a two orders-of-magnitude decrease respective to the forces needed to puncture DPPC bilayers. The topographic study of the monolayers in air around the transition temperature revealed the presence of boundary domains in the monolayer surface forming 120 degrees angles between them, thus suggesting that the cooling process from the liquid-expanded to the liquid-condensed phase follows a nucleation and growth mechanism.

A freeze-fracture transmission electron microscopy and small angle x-ray diffraction study of the effects of albumin, serum, and polymers on clinical lung surfactant microstructure

Freeze-fracture transmission electron microscopy shows significant differences in the bilayer organization and fraction of water within the bilayer aggregates of clinical lung surfactants, which increases from Survanta to Curosurf to Infasurf. Albumin and serum inactivate all three clinical surfactants in vitro; addition of the nonionic polymers polyethylene glycol, dextran, or hyaluronic acid also reduces inactivation in all three. Freeze-fracture transmission electron microscopy shows that polyethylene glycol, hyaluronic acid, and albumin do not adsorb to the surfactant aggregates, nor do these macromolecules penetrate the interior water compartments of the surfactant aggregates. This results in an osmotic pressure difference that dehydrates the bilayer aggregates, causing a decrease in the bilayer spacing as shown by small angle x-ray scattering and an increase in the ordering of the bilayers as shown by freeze-fracture electron microscopy. Small angle x-ray diffraction shows that the relationship between the bilayer spacing and the imposed osmotic pressure for Curosurf is a screened electrostatic interaction with a Debye length consistent with the ionic strength of the solution. The variation in surface tension due to surfactant adsorption measured by the pulsating bubble method shows that the extent of surfactant aggregate reorganization does not correlate with the maximum or minimum surface tension achieved with or without serum in the subphase. Albumin, polymers, and their mixtures alter the surfactant aggregate microstructure in the same manner; hence, neither inhibition reversal due to added polymer nor inactivation due to albumin is caused by alterations in surfactant microstructure.

Enhanced surfactant adsorption via polymer depletion forces: a simple model for reversing surfactant inhibition in acute respiratory distress syndrome

Lung surfactant adsorption to an air-water interface is strongly inhibited by an energy barrier imposed by the competitive adsorption of albumin and other surface-active serum proteins that are present in the lung during acute respiratory distress syndrome. This reduction in surfactant adsorption results in an increased surface tension in the lung and an increase in the work of breathing. The reduction in surfactant adsorption is quantitatively described using a variation of the classical Smolukowski analysis of colloid stability. Albumin adsorbed to the interface induces an energy barrier to surfactant diffusion of order 5 k(B)T, leading to a reduction in adsorption equivalent to reducing the surfactant concentration by a factor of 100. Adding hydrophilic, nonadsorbing polymers such as polyethylene glycol to the subphase provides a depletion attraction between the surfactant aggregates and the interface that eliminates the energy barrier. Surfactant adsorption increases exponentially with polymer concentration as predicted by the simple Asakura and Oosawa model of depletion attraction. Depletion forces can likely be used to overcome barriers to adsorption at a variety of liquid-vapor and solid-liquid interfaces.

Structure of beta-casein layers at the air/solution interface: atomic force microscopy studies of transferred layers

We report the nanoscale structural changes associated with the interfacial gelation of adsorbed beta-casein layers as a function of aging time. Adsorbed layers were transferred to solid supports and imaged by atomic force microscopy. The aging of the layer was accompanied by the formation of distinct disk-shaped protein nanoparticles ( approximately 20 nm in diameter). Under conditions where a gelled layer was expected (from previous interfacial rheology experiments), we observed ordering of the particles and the formation of elongated aggregates or linear rows. Brewster angle microscopy images were also obtained during the adsorption and gelation processes and during the degradation of the protein layer following addition of the surfactant sodium dodecyl sulfate (SDS). If SDS was added prior to interfacial protein gelation, the layer developed a foamlike morphology consistent with a fluid interfacial protein layer. However, if SDS was added after gelation, the protein layer was observed to fracture, consistent with the behavior of a solid phase.

Two-dimensional melting of an anisotropic crystal observed at the molecular level

A distinctive two-dimensional (2D) melting transition occurring at nearly 100 degrees Celsius ( degrees C) has been observed in Langmuir-Blodgett films by in situ atomic force microscopy (AFM). A 2D orthorhombic crystal phase melted to a 2D smectic phase at about 91 degrees C. The smectic phase was characterized by 1D molecular periodicity with short-range correlations (about 40 angstroms). At 95 degrees C, the smectic order melted to form a hexatic phase. Infrared spectroscopy measurements were consistent with the AFM observations. These observations support the dislocation-mediated melting scenario for an anisotropic 2D crystal predicted by Ostlund and Halperin. A longer wavelength height modulation was also observed in the smectic and hexatic phases.

Spatial resolution of the variable-period x-ray standing-wave method as applied to model membranes

A series of model membranes as Langmuir-Blodgett (LB) films composed of long-chain zinc alkanoates (saturated fatty acid salts) was used to evaluate the spatial resolution of the variable-period x-ray standing-wave (XSW) technique. The chain length dependence of the zinc mean position (z) above the supporting substrate demonstrates that it is possible to detect differences in (z) of 1-2 A. Thus 1-2 A is the spatial resolution of the method in the current application. The data show that the chain tilt angle is chain length dependent, varying from 40 degrees to 0 degrees for alkanoates 18 and 24 carbon atoms long, respectively. The spread about the mean position of the zinc in the film, sigma(in), was found to be independent of chain length at 10.0 A for all members of the series. Sigma(in) was shown to be insensitive to the presence of a "spacer" omega-tricosenoic acid (omegaTA) bilayer placed between the zinc alkanoate LB film and the coated gold mirror. However, an overlayer of omegaTA sharpened the zinc ion distribution and lowered the chain tilt angle. This study provides important information regarding sample composition and constitution that facilitates membrane structure determination by XSWs.