Monolayer Transition Research Articles

Kinetics of phase transitions in monolayers: collapse

Although monolayers at the air/water interface have been thesubject of intense study over the past 15 years, they continueto be actively investigated as evidenced by the topicsdiscussed in this workshop. Such films continue to be employedfor investigations of subjects as diverse as chain packing,critical phenomena, two-dimensional flow and wetting, biological catalysis, membrane physics and the kinetics of phasetransitions. We examine the kinetics of monolayer collapse, thetransition between two and three dimensions, for2-hydroxytetracosanoic acid. The studies include measurementsof surface pressure-area isotherms and imagingin situ by light-scattering microscopy. Three mechanisms ofcollapse have been observed: (equation 1) slow collapse by thenucleation and growth of multilayer islands; (equation 2) theformation of giant folds into the subphase; and (3) long multiple folds of small amplitude. Both folding modes arereversible. The slow collapse occurs at low surface pressuresand the folding at high pressures, with the giant foldsappearing only at low compression rates.

Kinetics of Overall Phase Transition in Changing Size System

Kinetics of phase transition is studied for the more general case when the size of the system is assumed time dependent. In the three dimensional case typical examples in this respect could be connected with the solution of cosmological problems. In the two dimensional case could be the spreading and crystallization of undercooled water on oil or the experiments concerning phase transition in monolayers in a Langmuir balance. The time dependence of the degree of overall transformation α(t) is formulated in a generalized form corresponding to above assumption. Explicit solutions are given for a constant rate of expanding (or shrinking) and for a constant acceleration of the size change of the system under the assumption that both nucleation rate I and linear growth rate G are constants. It is demonstrated that the process of phase transition proceeds much faster in a shrinking systems and is impeded in expanding ones. The reason for this effect is that parts of the new phase, formed in the initial parts of the system, are transferred into the diminishing volume (or surface). Thus the concentration of the new phase is mechanically increased.

The crossover between quantum and classical phase transitions in monolayers: a discrete φ4 model study

The crossover between quantum and classical phase transitions in monolayers is studied in the framework of the quantum discrete φ 4 model by the quantum Monte Carlo simulations and analytical approximations. There are three parameters in the quantum discrete φ 4 model: the reduced mass, the normalised temperature and the parameter determining the type of the transition. The crossovers between the quantum and the classical phase transitions and between the displacive and the order–disorder transition are controlled by these parameters. The critical values of the parameters of the quantum discrete φ 4 model are extracted from the dependencies of the order parameter on the temperature at various values of other parameters of the model. The phase diagrams are obtained for a wide range of all parameters. The independent-mode and the mean-field approximation are used as analytical methods and show a qualitative agreement with numerical results.

Crystal lattice transition of behenic acid monolayer on pure water surface observed by polarization modulation infrared spectroscopy

The crystal lattice of CH3(CH2)20COOH (behenic acid) monolayer on pure water subphase was investigated in situ by polarization modulation infrared reflection absorption spectroscopy. Two kinds of centered rectangular unit cells were found: rectangular parallel and orthorhombic packing of the trans zigzag planar–(CH2)20–chain. In the former cell the chain was tilted on the water surface and the C–C–C plane at the rectangle center was parallel to that at the corner. In the latter cell the chain was normal to the water surface and the C–C–C plane at the rectangle center was perpendicular to that at the corner. A rectangular parallel-to-orthorhombic transition was discovered for two kinks from 0.260 to 0.220 nm2/molecule along the surface pressure-area isotherm at 283 K, evidenced by splitting of the CH2 scissoring mode. By holding the barrier at 0.240 nm2/molecule and cyclically varying the monolayer temperature, a partially reversible transition from rectangular parallel to orthorhombic unit cells was observed.

Simulation of an order-disorder transition in monolayerN2/NaCl(001)

Monte Carlo simulations of ${\mathrm{N}}_{2}$ molecules physisorbed on a NaCl(001) surface show that at low temperature a monolayer forms an ordered $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure which, upon heating, undergoes a continuous order-disorder phase transition around 25 K. This transition is characterized by a logarithmically divergent heat capacity and the formation of pairs of counter rotating vortices.

Nanoscale Topographic Instabilities of a Phospholipid Monolayer

Light scattering microscopy reveals previously undetected topographic instabilities in phospholipid monolayers at the air/water interface far below the collapse pressure. Following compression through the fluid → condensed phase transition in monolayers of dipalmitoyl phosphatidylcholine, after the disappearance of the fluid phase, the contact regions between condensed domains acquire static roughness as indicated by enhanced light scattering. With further compression, a nanoscale budding process occurs within the roughened regions, while the interiors of the condensed domains remain flat and retain their domain shapes. At monolayer collapse, the buds proliferate across the entire interface, suggesting that the buds detected at lower pressures represent spatially confined fluctuations into the collapse phase. The confinement of static roughness formation and budding to domain contact regions indicates that these topographic instabilities originate from packing defects created where adjacent domain edges w...

Effect of dodecyl dimethyl phosphine oxide penetration into phospholipid monolayers: morphology and dynamics

The penetration kinetics of dodecyl dimethyl phosphine oxide (C 12DMPO) into a Langmuir monolayer of dipalmitoylphosphatidylcholine (DPPC) and the corresponding morphology were experimentally studied. It was found that under proper conditions the penetration of dodecyl dimethyl phosphine oxide could induce a first-order phase transition in the fluid-like lipid Langmuir monolayer. The transition is indicated by a break point in the Π( t) penetration kinetics curves and can be visualized by BAM. The dynamics of the penetrated layer and the subphase effect on the Langmuir monolayer were also investigated. The experimental results show that the adsorption of soluble molecules increases the surface density, which in turn causes the first-order phase transition in the monolayer. When the surface density reaches a certain critical value, the main phase transition sets in.

Phase Transitions in Monolayers of PS−PEO Copolymer at the Air−Water Interface

The structures and phase transitions occurring during the compression of polystyrene−poly(ethylene oxide) (PS−PEO) diblock copolymer monolayers at the air−water interface have been studied by surface pressure isotherms, neutron reflectivity, and Brewster angle microscopy. At low coverage, the EO monomers adsorb at the air−water interface, but the PEO layer is not purely bidimensional. At high coverage, the EO−interface interaction becomes repulsive. A brush structure has been observed with differences between the long and the short PEO chains attributed to the difference in the accessible range of surface concentration. For the longest chain, the PEO concentration profiles exhibit a superposition of a depletion layer near the interface and discontinuous profile characteristics of a two-phase brush described by the “n-cluster” theory. For the shortest chains, the profiles are “pseudoparabolic”. Finally, Brewster angle microscopy shows that, for the long chains, the transition between the adsorbed and the b...

Phase Transitions in Monolayers of Human Apolipoprotein C-I

Human apolipoprotein C-I was deposited onto a highly ionic water subphase to form a monolayer in a Langmuir trough. Pressure−area isotherms of the human apolipoprotein C-I monolayer, as well as direct observations with Brewster angle microscopy, were carried out. Two first-order phase transitions were found. The first phase transition involves the coexistence of a fluid condensed phase and a gas phase, whereas the second one is a transition between two condensed phases. A protein model is proposed, where the apolipoprotein C-I is made of two amphiphilic α-helices bonded with a poorly structured polypeptide fragment. With this model, we have been able to explain our results, in particular the second phase transition, which seems to correspond to a conformational change in the protein. This conformational change could be due to the desorption of one amphiphilic α-helix from the subphase, when lateral pressure is applied to the protein monolayer. The relevance of this conformational change for lipid binding activity is discussed.

Phase transition of lipid-like monolayer characterized by second harmonic generation

Phase transition of a lipid-like hemicyanine compound characterized by second harmonic generation is studied carefully. The phase transition is assigned as the first order transition between solid state and liquid state. The transition temperature increases with an increase in the surface molecular concentration. A monolayer structure parametera which is very sensitive to the phase transition is introduced.

Unequivocal Evidence for a Liquid−Gas Phase Transition in Monolayers of Decanol Adsorbed at the Air/Water Interface

Unequivocal Evidence for a Liquid−Gas Phase Transition in Monolayers of Decanol Adsorbed at the Air/Water Interface

An Interfacial Stress Rheometer To Study Rheological Transitions in Monolayers at the Air−Water Interface

An interfacial stress rheometer has been constructed to study the rheology of Langmuir films subjected to time-dependent flows. A magnetized rod resides at the air−water interface and is set into oscillation by applying a sinusoidal magnetic field gradient. Analysis of the amplitude and phase of the resulting rod motion relative to the applied force allows the determination of the dynamic surface modulus, Gs*(ω), and measurement of the relative elastic and viscous contributions of the monolayer. Measurements at 22 °C were conducted on eicosanol (C20) and mixtures of a rigid-rod polymer, phthalocyaninatopolysiloxane (PcPS), dispersed in eicosanol. The surface pressure dependence of the rheology for eicosanol reveals the presence of a maximum in the loss modulus, Gs‘ ‘(ω), within the L2‘ phase at Π = 6 mN/m. In the LSI phase at pressures above 15 mN/m, the monolayer is Newtonian and has a surface viscosity of 0.03 mN·s/m. The mixtures of PcPS with eicosanol are known to have two-dimensional nematic behavior. The presence of PcPS in the film increased ||Gs*(ω)|| 100-fold, creating a non-Newtonian interface with a measurable elasticity. As the polymer rod concentration was increased further, Gs*(ω) became less dependent on frequency, and above the isotropic−nematic transition, the storage modulus, Gs‘(ω), exceeded the loss modulus, Gs‘ ‘(ω). The results on eicosanol and the mixtures of the rigid rod with eicosanol demonstrate that the rheometer is capable of detecting microstructural transitions in a Langmuir monolayer.

On the commensurate–incommensurate transition in adsorbed monolayers

A Monte Carlo simulation method is used to study the commensurate–incommensurate phase transition in monolayers and the formation of bilayer films on the (100) face of an fcc crystal. The phase diagram for the system which forms the registered (1×1) and high density incommensurate phases in the monolayer has been determined. It is shown that the registered phase undergoes the transition to a denser incommensurate solid phase when the film density increases. The mechanism of melting of the monolayer film is found to depend on the film density. In particular, the melting of dense incommensurate solid monolayer film is found to be accompanied by the transfer of adsorbed molecules into the second layer.

Molecular twist transition in chiral and racemic phospholipid monolayers detected by Maxwell-displacement-current measurements

The physicochemical properties of chiral and racemic phospholipid (alpha-phosphatidylcholine dipalmitoyl) monolayers at the air-water interface were investigated using Maxwell-displacement-current (MDC) and surface pressure measurements by monolayer compression. The chiral and racemic phospholipid monolayers exhibited the same pressure-area isotherm with a pressure plateau at a temperature of 20 degrees C. However, some critical MDC peaks were generated with different amplitude and sign depending on the chirality of the phospholipids in the plateau region. This anomalous MDC generation is theoretically interpreted on the basis of the molecular twist transition induced by monolayer compression.

Phase transitions in self-assembled monolayers of alkanethiols containing the polar group

Equilibrium states of the system of self-assembled monolayers (SAMs) of n-alkanethiol molecules HS(CH2) n−1(X) with polar group X chemiabsorbed on the Au(111) crystal surface are considered. The couplings between the atoms (C, H) of the n-alkanethiols are approximated by the Lennard-Jones potential. The couplings between the n-alkanethiols and the crystal surface are approximated by the 12-3 potential. The interactions of polar groups and the self-images with the metal substrate are taken into consideration. The phase-transition temperatures, the structural order and equilibrium tilt, and the twist and azimuthal angles of the macromolecules, which depend on the dipole moments, are found.

Polar orientational phase transition in smectic monolayers induced by monolayer compression

Taking into consideration the mean electric field produced by a molecular configuration, a self-consistentfield state equation is derived to describe the behavior of polar smectic phase for monolayers. A general equation of orientational phase transition between a normal-director phase and a tilted-director phase induced by monolayer compression for monolayers is obtained. It was revealed that smectic monolayers show a possible second-order orientational phase transition under a necessary condition 4216/gy,gz,gy/4. Here gy and gz are two parameters proportional to the internal electric fields in the directions parallel and perpendicular to the monolayer surface, respectively. The two-dimensional orientational phase transition theory developed here may reveal such a transition in smectic monolayers by some techniques, for example, the Maxwelldisplacement-current measuring technique. @S1063-651X~98!03605-8#

An order–disorder phase transition in monolayer CO/LiF(001)

Monte Carlo simulations of CO physisorbed on a LiF(001) surface show that a monolayer of CO molecules forms an ordered p(2√×√)R45 herringbone structure which undergoes an order–disorder phase transition around 30 K. The CO molecules sit near the Li+ sites (C atom down) with a tilt of ∼40° from the surface normal.

Morphology and structural characterization of organized monolayers by Brewster angle microscopy

Brewster angle microscopy has been involved in some of the recent advances in the field of monomolecular layers, such as the detection of a phase transition in monolayers formed by a surfactant at the solution surface, the determination of the line tension of condensed phase domains coexisting with a liquid expanded phase, as well as new insights in the role of head group interactions (e.g. hydrogen bonds) and in the organisation of biological materials in thin films.

Molecular dynamics and phase transitions in phospholipid monolayers at liquid-liquid interfaces

Stablen-hexadecane/water andn-tetradecane/water macroemulsions containing monolayers of natural (egg yolk lecithin, EY) and synthetic (dimyristoylphosphatidylcholine, DMPC) phospholipids at liquid-liquid interfaces were prepared. The existence of the monolayers was proved by studying the reduction kinetics of a surface-active spin probe with ascorbate anions. Spin labeled derivatives of stearic acid in which the nitroxide group is locared at different distances from the polar head (5-, 12-, and 16-doxylstearic acids) were used to study the temperature dependences of the molecular ordering, rotational mobility, and local polarity in the monolayers in emulsions and also in bilayers in liposomes obtained from the same lipids. In the EY monolayers, the degree of spin probe solubilization is higher, while the order parameters (S) and rotational correlation times (τ) are lower than those in EY bilayers. The differences between these parameters for mono- and bilayers increase with an increase in the distance of the reporter group from the aqueous phase. In the DMPC monolayers, a first-order phase transition was detected by measuring the temperature dependences ofS and τ. The temperature region of the phase transition in monolayers is shifted to lower temperatures with respect to that for bilayers and depends on the nature of the oil phase. It was concluded that the phospholipid monolayers in emulsions incorporate hydrocarbon molecules, whose concentration in the DMPC monolayers increases on going from the low-temperature (gel) to the high-temperature (liquid crystal) phase.

On the Nature of The Liquid Expanded/Liquid Condensed Phase Transition in Monolayers of Polar Molecules

During the liquid expanded/liquid condensed (LE/LC) phase transition, the long range repulsive interactions among fatty acid or phospholipid molecules induce the formation of a dispersion of LC islands (of almost the same size) as a superlattice in a continuous phase of LE molecules. Assuming that the transformation occurs at thermodynamic equilibrium, one demonstrates that, as the surface fraction of LC phase increases, the surface pressure π increases. Consequently, the LE/LC transition can have characteristics different from those of the conventional first order transitions.