Laser-induced local collapse in a langmuir monolayer

Heating of a two-dimensional, methyloctadecanoate, Langmuir monolayer with a focused laser induces the local collapse of the monolayer. We observe the growth of a three-dimensional collapse aggregate that is fed by an inward flow of the two-dimensional monolayer surroundings. The experiments are explained with a hydrodynamic model describing the dynamics of the local collapse. From this theory we predict that local collapse can be induced if the collapse pressure of the monolayer decreases faster with temperature than with the surface tension of the pure air/water interface. Such conditions are fulfilled for lung surfactants, and it should therefore be possible to perform time-resolved local studies of the collapse of lung surfactants at those temperatures.     

Nonequilibrium bubbles in a flowing langmuir monolayer

We investigate the nonequilibrium behavior of two-dimensional gas bubbles in Langmuir monolayers. A cavitation bubble is induced in liquid expanded phase by locally heating a Langmuir monolayer with an IR-laser. At low IR-laser power the cavitation bubble is immersed in quiescent liquid expanded monolayer. At higher IR-laser power thermo capillary flow around the laser-induced cavitation bubble sets in. The thermo capillary flow is caused by a temperature dependence of the gas/liquid line tension. The slope of the line tension with temperature is determined by measuring the thermo capillary flow velocity. Thermodynamically stable satellite bubbles are generated by increasing the surface area of the monolayer. Those satellite bubbles collide with the cavitation bubble. Upon collision the satellite bubbles either coalesce with the cavitation bubble or slide past the cavitation bubble. Moreover we show that the satellite bubbles can also be produced by the emission from the laser-induced cavitation bubbles.     

Nanocrystal induced organization of a langmuir phospholipid monolayer

The influence of crystal surface charge on the thermodynamic and structural behavior of phospholipid monolayers has been investigated. We present how charged nanocrystals in the vicinity of an inherently nonordered lipid membrane provoke strong effects on the molecular arrangement within the monolayer. Apart from the induction of phase shifts and nucleation processes, the molecules were forced to adopt an ordered phase. A very recently developed X-ray scattering method is used for the first time to replace time-consuming specular reflectivity measurements. We conclude on the potential effects of crystal surface charge on cellular membranes.     

Headgroup percolation and collapse of condensed langmuir monolayers

We present a study of Langmuir isotherms and 2D bulk moduli of binary lipid mixtures, where changes in monolayer collapse pressure (Pic) are followed while varying the relative amounts of the two components. For monolayers containing dipalmitoylphosphocholine (DPPC) with either hexadecanol (HD) or palmitic acid (PA), a distinctly non-monotonic change in Pic is observed with varying composition. At low mole fractions, there is a slight decrease in Pic as films get richer in DPPC, while a sharp increase to pure DPPC-like values is observed when the mole fraction exceeds approximately 0.7. The sudden transition in collapse pressure is explained using the principles of rigidity percolation, and important ramifications of this phenomenon for biological surfactant are discussed.     

Surface chemistry and in situ spectroscopy of a lysozyme langmuir monolayer

Surface pressure and surface potential-area isotherms were used to characterize a lysozyme Langmuir monolayer. The compression-decompression cycles and stability measurements showed a homogeneous and stable monolayer at the air-water interface. Salt concentration in the subphase and pH of the subphase were parameters controlling the homogeneity and stability of the Langmuir monolayer. In situ UV-vis and fluorescence spectroscopies were used to verify the homogeneity of the lysozyme monolayer and to identify the chromophore residues in the lysozyme. Optimal experimental conditions were determined to prepare a homogeneous and stable lysozyme Langmuir monolayer.     

Atomistic monte carlo simulations of polymethylene fluids

Atomistic Monte Carlo simulations of polymethylene fluids in various environments have been of great importance for improving our understanding of the molecular arrangements and conformations in systems of chain molecules. This paper describes models, calculations and principal results of a number of simulations performed in the last two decades for liquid alkanes in bulk and in the proximity of solid surfaces, and for systems of long chain amphiphilic molecules in monolayers, double layers, micelles and droplets.     

Atomistic molecular dynamics simulations of chemical force microscopy

Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects.     

Atomistic simulations of biologically realistic transmembrane potential gradients

We present all-atom molecular dynamics simulations of biologically realistic transmembrane potential gradients across a DMPC bilayer. These simulations are the first to model this gradient in all-atom detail, with the field generated solely by explicit ion dynamics. Unlike traditional bilayer simulations that have one bilayer per unit cell, we simulate a 170 mV potential gradient by using a unit cell consisting of three salt-water baths separated by two bilayers, with full three-dimensional periodicity. The study shows that current computational resources are powerful enough to generate a truly electrified interface, as we show the predicted effect of the field on the overall charge distribution. Additionally, starting from Poisson's equation, we show a new derivation of the double integral equation for calculating the potential profile in systems with this type of periodicity.     

Atomistic molecular dynamics simulations of shock compressed quartz

Atomistic non-equilibrium molecular dynamics simulations of shock wave compression of quartz have been performed using the so-called BKS semi-empirical potential of van Beest, Kramer, and van Santen [Phys. Rev. B 43, 5068 (1991)] to construct the Hugoniot of quartz. Our scheme mimics the real world experimental set up by using a flyer-plate impactor to initiate the shock wave and is the first shock wave simulation that uses a geometry optimised system of a polar slab in a three-dimensional system employing periodic boundary conditions. Our scheme also includes the relaxation of the surface dipole in the polar quartz slab which is an essential pre-requisite to a stable simulation. The original BKS potential is unsuited to shock wave calculations and so we propose a simple modification. With this modification, we find that our calculated Hugoniot is in good agreement with experimental shock wave data up to 25 GPa, but significantly diverges beyond this point. We conclude that our modified BKS potential is suitable for quartz under representative pressure conditions of the Earth core, but unsuitable for high-pressure shock wave simulations. We also find that the BKS potential incorrectly prefers the β-quartz phase over the α-quartz phase at zero-temperature, and that there is a β → α phase-transition at 6 GPa.     

Sequential collapse transitions in a Langmuir monolayer

Unusual sequential collapse transitions are investigated in a lignoceric acid Langmuir monolayer. The nucleation of monolayer collapse is first initiated in the solid, S, phase but at remarkably low surface pressure where small three-dimensional (3D) granular dots appear. The density of nucleation centers increases, and the 3D dots prevail over the monolayer (surface roughening regime) as the surface pressure increases, but individual dots neither grow very much in size nor evolve into other shapes such as stripes or elongated dots. On further compression the second collapse mode manifests itself by highly anisotropic, global crack arrays (anisotropic cracking regime) where the surface pressure "kink" appears in the isotherm. In the latter regime, various forms of 3D curved filaments develop in the crack regions, and they break into smaller fragments with a typical relaxation time (approximately 60 ms).     

Molecular simulation of langmuir monolayer formation by Gd(III) stearate complexes

Clusters simulating the formation of Gd(III) stearate complex monolayers on the surface of an aqueous phase are studied by means of molecular mechanics. From a comparison of the calculated and experimental data, it concluded that Langmuir monolayers are produced by Gd(III) aqua complexes with one and two stearate anions.     

Atomistic simulations of rare events using gentlest ascent dynamics

The dynamics of complex systems often involve thermally activated barrier crossing events that allow these systems to move from one basin of attraction on the high dimensional energy surface to another. Such events are ubiquitous, but challenging to simulate using conventional simulation tools, such as molecular dynamics. Recently, E and Zhou [Nonlinearity 24(6), 1831 (2011)] proposed a set of dynamic equations, the gentlest ascent dynamics (GAD), to describe the escape of a system from a basin of attraction and proved that solutions of GAD converge to index-1 saddle points of the underlying energy. In this paper, we extend GAD to enable finite temperature simulations in which the system hops between different saddle points on the energy surface. An effective strategy to use GAD to sample an ensemble of low barrier saddle points located in the vicinity of a locally stable configuration on the high dimensional energy surface is proposed. The utility of the method is demonstrated by studying the low barrier saddle points associated with point defect activity on a surface. This is done for two representative systems, namely, (a) a surface vacancy and ad-atom pair and (b) a heptamer island on the (111) surface of copper.     

Molecular dynamic simulations of eicosanoic acid and 18-methyleicosanoic acid langmuir monolayers

The conformational and dynamical properties of Langmuir monolayers of 18-methyleicosanoic acid (18-MEA) and the parent material, eicosanoic acid (EA), are compared using molecular dynamics simulations. The effects on various properties, including film thickness, tilt angle, and order parameter, of the methyl group at the 18 position in 18-MEA were investigated as a function of film-packing density. NVT simulations were run as a function of decreasing areal-packing density similar to experimental Langmuir-Blodgett film compressions and expansions. We find that the order parameters and film thickness for 18-MEA monolayers were markedly different from those of EA. The order parameters for methylene groups for both 18-MEA and EA are greater in the middle region of the chain than at the ends in high-density films. This trend becomes reversed in lower density films. Significantly, our simulations show that the order parameters for methylene groups near the CH3 and carboxyl termini in 18-MEA are comparatively independent of film density in contrast with those of EA. Our findings show that the presence of the methyl group at the 18-position in 18-MEA induces unique intermolecular structural correlations compared to EA.     

Infrared reflection-absorption spectroscopy and polarization-modulated infrared reflection-absorption spectroscopy studies of the aequorin langmuir monolayer

The Langmuir monolayer of aequorin and apoaequorin was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS techniques. The alpha-helices in the aequorin Langmuir monolayer were parallel to the air-water interface at zero surface pressure. When the surface pressure increased to 15 mN.(m-1), the alpha-helices became tilted and the turns became parallel to the air-water interface. As for apoaequorin, the alpha-helices were also parallel to the air-water interface at 0 mN.m(-1). However, the alpha-helix became tilted and the turns became parallel to the air-water interface quickly at 5 mN.m(-1). With further compression of the apoaequorin Langmuir monolayer, the orientation remained the same. The different behaviors of aequorin and apoaequorin at the air-water interface were explained by the fact that aequorin formed dimers at the air-water interface but apoaequorin was a monomer. It is more difficult for a dimer to be tilted by the compression of the Langmuir monolayer.     

String defects connecting pairs of half-integer disclinations and tilting transition of a langmuir monolayer

The static and dynamic string defect textures connecting pairs of half-integer disclinations have been observed by Brewster angle microscopy in the solid phase of pentacosadiynoic acid Langmuir monolayers. The static string defect structures have appeared coexisting with two kinds of point disclinations that have four and two black brushes. The use of local laser heating has allowed one to observe kinetics of creation and annihilation of string defects connecting the two-half-integer disclinations in the splitting process of an s = 1 point disclination into fractional disclinations. These kinetics have been analyzed by studying the competition between the orientational elasticity of the molecules and the line tension of the string and the drag force of the disclinations.     

Study of molecular aggregation of artificial amyloid in a langmuir monolayer by infrared spectroscopy

A synthesized peptidolipid (C18IIGLM-NH2) comprised of a single C18-saturated hydrocarbon chain connected to the amino acid sequence IIGLM terminated with the NH2 group was spread on water, which formed a stable Langmuir monolayer. The Langmuir and Langmuir-Blodgett (LB) films have been characterized by measurements of surface pressure-area (pi-A) and surface potential-area (DeltaV-A) isotherms and infrared multiple-angle incidence resolution spectrometry (MAIRS). The Langmuir monolayer had a significantly larger limiting molecular area than that of a similar molecule of C18IIGLM-OH, which was reported in our previous study. The surface dipole moment analysis coupled with the pi-A isotherm suggested that the C18IIGLM-NH2 monolayer was extraordinarily stiff and the fundamental structure of the monolayer was brought about before the monolayer compression. The infrared MAIRS analysis of the C18IIGLM-NH2 LB film revealed that the backbone structure of the monolayer was the 'antiparallel' beta sheet aligned parallel to the substrate. Since the C18IIGLM-OH LB film was made of 'parallel' beta sheet with a random orientation, it has been found that the present C18IIGLM-NH2 Langmuir monolayer has a largely different monolayer structure, although the chemical structures are slightly different from each other by the terminal group only.     

Effect of surface pressure on the insulator to metal transition of a langmuir polyaniline monolayer

A remarkable change in the conductivity of a polyaniline (PAN) Langmuir monolayer in the conducting state, as a function of surface pressure, has been observed using scanning electrochemical microscopy (SECM). The film conductivity, as expressed by the SECM current response of a redox mediator, was measured in-situ in a Langmuir film balance. The conductivity of the film increases significantly with surface pressure, above a threshold value of ca. 20 mN m-1.     

Atomistic simulations of calcite nanoparticles and their interaction with water

Molecular dynamics (MD) simulations have been used to study the stability of calcite nanoparticles ranging in size from 18 to 324 f.u., both in vacuo and in the presence of explicit water molecules. In vacuo, the smallest particles become highly disordered during the MD simulation due to rotation and translation of the undercoordinated CO(3) (2-) anions at the edges of the particles. As the nanoparticle size increases, the influence of the fully coordinated bulk ions begins to dominate and long-range order is seen both in the Ca-C pair distribution functions and in the degree of rotational order of the CO(3) (2-) anions. However, when explicit water is added to the system, the molecules in the first hydration layer complete the coordination shell of the surface ions, preserving structural order even in the smallest of the nanoparticles. Close to particle surface, the structure of the water itself shows features similar to those seen close to planar periodic (1014) surfaces, although the molecules are far less tightly bound.     

Transformation diagrams for the collapse of a phospholipid monolayer

The kinetics of phase transitions in three-dimensional bulk materials are commonly presented in transformation diagrams. Time-temperature transformation (TTT) and continuous-cooling-transformation (CCT) diagrams plot the time required to transform specific fractions of the material to the new phase by cooling below a transition temperature. Transformation occurs isothermally for the TTT diagrams and during continuous cooling through a range of temperatures for CCT curves. Here we present analogous transformation diagrams for two-dimensional monolayers, which collapse at the equilibrium spreading pressure (pi e) to form a three-dimensional bulk phase. Time-surface pressure-transformation (TpiT) diagrams give the time required for specific fractions of the film to collapse when surface pressure is constant, and continuous-compression-transformation diagrams give the same information when surface pressure varies continuously. The diagrams, constructed here from previously reported data for 1-palmitoyl-2-oleoyl phosphatidylcholine, provide insights into the behavior of the films. The TpiT diagrams successfully predict the existence and approximate magnitude of a threshold rate for compressing the films to high surface pressures above pi e and the approximate shape of isotherms obtained with different rates of interfacial compression. The diagrams also caution that the behavior of mixed monolayers, explained previously in terms of compositional changes, can instead result from collapse that varies with surface pressure. Finally, the similarity between the shapes of the TTT and TpiT diagrams, with the time for transformation passing through a minimum and then increasing as the systems deviate further from equilibrium, suggests that analogous mechanisms determine the behavior of both systems.     

Atomistic molecular dynamics simulations of peptide amphiphile self-assembly into cylindrical nanofibers

Relaxation of a self-assembled structure of 144 peptide amphiphile (PA) molecules into cylindrical nanofibers is studied using atomistic molecular dynamics simulations including explicit water with physiological ion concentration. The PA for these studies includes a hydrophobic alkyl chain that is attached to the N-terminus of the sequence SLSLAAAEIKVAV. The self-assembly is initiated with PA molecules in a roughly cylindrical configuration, as suggested from previous experimental and theoretical investigations, and the cylindrical configuration that results is found to be stable during 40 ns simulations. In the converged structure of the resulting nanofiber, the cylinder radius is ～44 ?, a result that is consistent with experimental results. Water and sodium ions can penetrate into the peptide portion of the fiber but not between the alkyl chains. Even though each PA has an identical sequence, a broad distribution of secondary structure is found in the converged structure of the nanofiber. The β-sheet population for the SLSL and IKV segments of the peptide is ～25%, which is consistent with previous circular dichroism results. We also found that the epitope sequence IKVAV is located on the surface of the nanofiber, as designed for the promotion of the neurite growth. Our findings will be useful for designing new PA fibers that have improved bioactive properties.