Phase Transitions In Langmuir Monolayers Research Articles

Origin of lipid tilt in flat monolayers and bilayers.

This paper continues the series of our works devoted to the liquid-gel phase transition in lipid membranes. Previously we described a variation of area per lipid, membrane thickness, and diffusion coefficient at the temperature-driven liquid-gel phase transition in bilayers. Here we expand the application of our analytic model approach to include a description of the lipid tilt and also extend the investigation to include Langmuir and self-assembled monolayers. The theory describes tilt formation at the temperature-driven liquid-gel phase transition in bilayers and the pressure-driven phase transition in Langmuir monolayers. Neither uniform tilt nor liquid-gel phase transition is found in self-assembled monolayers chemically bonded to the substrate.

Emergence of a linear slope region of the isotherm in the first-order liquid-expanded–liquid-condensed phase transition in Langmuir monolayers

A nonhorizontal slope in the isotherm has been observed in the two-phase coexisting region of the first-order liquid-expanded (LE)-liquid-condensed (LC) phase transition in Langmuir monolayers for many decades. We show that the simple analysis of a phenomenological Landau free energy involving the coupling-energy contributions of molecular lateral density (ρ) with spontaneous collective chain tilt (θ) and two-dimensional strain (ɛ_{s}) inside the LC domain enables one to understand the origin of a nonhorizontal straight-line slope in the LE-LC phase coexistence region of the isotherm. The presence of ρ-ɛ_{s} coupling must be essential for the appearance of the straight-line shape of a nonhorizontal plateau in the isotherm. Moreover, it is found from the comparison of the two-dimensional contour plots of the free energy that an LE phase may persist significantly even at the later stage of the straight-line regime beyond a transition midpoint surface pressure in the presence of this coupling. The persistence of the LE phase may lead to the delay of transition progress as manifested more clearly by the appearance of a compressibility plateau in the coexistence region that indicates the existence of persistent equilibrium density fluctuations in the monolayer.

Phase transitions in Langmuir monolayer of long chain attached fluorescein studied by second harmonic generation technique

Optical second harmonic generation (SHG) measurements coupled with Π-A isotherms have been shown to be helpful, for the following and comprehension of orientational orders and phase transitions in Langmuir monolayers. Using the SHG-(Π-A) measurements, monolayers of 5-hexadecanoylaminofluorescein on the water surface were examined by monolayer compression. The phase transitions were noticeably revealed. Dependence of the square root of the intensities polarizations quotient in the molecules surface density, allowed establishing tilting orientation alignment phases. In addition, change in the monolayer symmetry C ∞ v → C 2 v as it goes through the LE–LC phase transition, was clearly recognized. It was concluded that a possible change in β is taking place due to aggregate formation.

Ising-like model of tilted phase transitions in Langmuir monolayers

Two Ising models on a rectangular lattice (${\ensuremath{\sigma}}_{1}$ model and ${\ensuremath{\sigma}}_{2}$ model) linked to three- and four-spin interactions are used to describe the sequence of tilted phase transitions in monolayers of amphiphilic long-chain molecules on an air-water surface. Starting from Morse atom-atom potentials self-consistent equations for basic thermal averages are derived in terms of a mean-field approximation. We employ the model for $\mathrm{C}{\mathrm{H}}_{3}{(\mathrm{C}{\mathrm{H}}_{2})}_{18}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$ at two values of the molecule density and obtain all known tilted phases, including an intermediate one.

Thermodynamic distribution functions associated to the isothermal phase transition in Langmuir monolayers

The theoretical basis for statistical mechanical deconvolution of an isothermal two-dimensional, surface pressure-induced surfactant phase transition is presented. It is demonstrated that all the thermodynamic quantities characterizing a Langmuir monolayer phase transition can be obtained from the variation of the average molecular area of the system, as a function of the surface pressure, without the need for any assumption of the particular model or mechanism. The cooperativity of the phase changes becomes clearly evident and it is possible to discriminate the behaviour of each coexisting phase state in terms of the average size and statistical distribution functions of thermodynamically correlated molecules undergoing the liquid-expanded to condensed transition in the film.

Anisotropic aggregation and phase transition in Langmuir monolayers of methyl/ethyl esters of 2,3-dihydroxy fatty acids

We studied interfacial properties of a series of methyl and ethyl esters of enantioenriched syn-2,3-dihydroxy fatty acids with different chain lengths at the air–water interface, using a Langmuir type film balance and a Brewster angle microscope (BAM). After analyzing their surface pressure ( Π)–area ( A) isotherms, we inferred that these molecules existed as an E conformation in the liquid-expanded (LE) phase of monolayers, and the E conformation of molecules changed into a Z conformation during the LE–LC transition in a monolayer. BAM images evidenced the formation of elongated LC aggregates. This is possibly induced by the intermolecular hydrogen bonds, leading to the anisotropic growth of LC domains, on the basis of the FT-IR spectroscopy data. The enthalpy change of the LE–LC phase transition is considered to result from the three types of intermolecular interactions at the air–water interface during compression of these amphiphiles. These findings are discussed in terms of various physical factors that influenced intermolecular interactions and macroscopic aggregations of these amphiphiles.

Surface phase behavior in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface

The surface phase behavior of condensed-phase domains formed during a first-order phase transition in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface has been investigated by Brewster angle microscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). A variety of two-dimensional (2D) structures are observed just after the appearance of the phase transition at different temperatures. At 10 and 15 degrees C, the domains are found to be small nuclei of irregular structures. Spiral structures are observed at 20 and 22 degrees C, while striplike structures at 24 degrees C. The spiral domains attain increasingly compact shape with increasing temperature, and finally become circular at >or=26 degrees C. Increases in temperature result in dehydration in the ethylene oxide chain, which increases the hydrophobicity, and impart to the molecules a longer-chain-like character. As a result line tension increases with increasing temperature, which probably outweighs the dipole-dipole repulsions showing circular domains at higher temperatures. The PM-IRRAS measurement reveals that the nu(as)(CH(2)) mode moves to lower wave numbers indicating that the LE-LC (liquid expanded-liquid condensed) phase transition during the compression of the monolayer involves changes in the conformational order of the molecules with a preferential increase in the planner trans zigzag conformation of the hydrocarbon chains. The nu(as)(CH(2)) mode in the LC region of the isotherm shows a constant value around 2917.8 cm(-1) indicating a stable state of the monolayer with an almost all-trans conformation of the hydrocarbon chains. The downward band at 1124 cm(-1) assigned to the nu(as)(C-O-C) mode indicates that the corresponding transition dipole moment is oriented perpendicular to the water surface.

Thermodynamics of Phase Transitions in Langmuir Monolayers Observed by Vibrational Sum Frequency Spectroscopy

Vibrational sum-frequency spectroscopy (VSFS) was used to study gauche defects in octadecylamine (ODA) monolayers at the air/water interface. The VSFS spectra provide unique insights into phase transitions that occur as a result of changes in the structure of the monolayer's hydrophobic region. These changes can be attributed to the increased presence of gauche conformers in the ODA alkyl chains during the monolayer's transition from the solid to liquid phase. Temperature-dependent spectra from monolayers at several different pressures were used to assign the phase transition temperature based on the observed changes in microscopic structure. Through application of a two-dimensional form of the Clapeyron equation, the first in situ measurements of the entropy and enthalpy changes associated with gauche conformers in a monolayer were made.

Phase transitions in Langmuir monolayers of a rhodamine dye as studied by a second harmonic generation technique

Phase transitions in the Langmuir monolayer of a xanthene dye alkylated with two long hydrophobic chains have been studied by a second harmonic generation (SHG) technique at various temperatures. Owing to the sensitivity of the SHG technique to the molecular ordering at an interface, transitions between phases of different orientational organizations of dye chromophores have been observed in the shape of SHG signals. The method of the derivative of an SHG signal has been proposed to determine the points of phase transitions in the Langmuir monolayer. Obtained results are in good agreement with those derived from ( π– A) isotherms. Several additional orientational phase transitions have been found upon compression of a liquid phase of the layer. Detailed comparison of the SHG signals with the ( π– A) isotherms has allowed us to conclude that the phase transitions at a low surface pressure may not involve the rearrangement of dye chromophores. Formation of a denser phase likely occurs by means of the orientational rearrangement of flexible alkyl chain network of the layer. However, a transition to the solid phase is accompanied by profound changes in the chromophore arrangement. The amplitude of SHG signals generated by the solid phase increases with a decrease in the temperature, suggesting the formation of more ordered layers at lower temperatures.

Phase transition in Langmuir monolayers

The main features of several theoretical models which describe the main phase transition and correspondingly the non-horizontal shape of the Π-A isotherms of the Langmuir monolayers, are discussed. New equations of state are based on the generalised Volmer's equation and consider the coexistence monomers and large clusters as bimodal distribution. A further generalisation for the case of quasi-bimodal distribution allows the consideration of monomers with small aggregates on one side of the spectrum and large clusters on the other side. The new theoretical model is corroborated by the Π-A isotherms of various amphiphilic monolayers the condensed phases of which have various gradual differences in the crystalinity and packing density data. The large variety of shapes of the Π-A isotherms in the region A<A c can be determined by a single general equation whose parameters, except A c, correspond to the isotropic fluid-like monolayer region. The application of the generalised equation of state on the experimental Π-A isotherms indicates the formation of small aggregates in the region A>A c.

2D Phase Transitions in Langmuir Monolayers of the Aromatic 4‘,5‘-Diphenyl-1,1‘:3‘,1‘ ‘-terphenyl-4-carboxylic Acid

Langmuir monolayers of 4‘,5‘-diphenyl-1,1‘:3‘,1‘ ‘-terphenyl-4-carboxylic acid (in short DTCA) were studied under different experimental conditions. The π−A isotherms displayed a broad plateau on a pure water subphase and two plateaus on a neutral aqueous subphase containing Cd2+ ions. DTCA monolayers were transferred as LB multilayer films and characterized by FTIR, X-ray diffraction, and electric surface potential measurements. From the analysis of surface potentials for LB films transferred at surface pressures corresponding to different parts of the π−A isotherm it was possible to infer that molecular orientational changes are the probable cause for the plateau in the isotherm.

Orientational Phase Transitions in Langmuir Monolayers

A statistical mechanics approach is applied to studying orientational phase transitions in Langmuir monolayers. The orientational phase transition of rodlike molecules in the monolayer is attributed to a competition between two types of forces experienced by the dipolar molecules. One is the electrostatic attractive force between the dipolar molecules and water surface, the other is the steric repulsive force among the molecules. The consideration of finite steric repulsive potentials makes the present theoretical result in qualitative agreement with the existing experiments.

A Study on Two-Dimensional Phase Transitions in Langmuir Monolayers of a Carboxylic Acid with a Symmetrical Triphenylbenzene Ring System

A detailed investigation on phase transitions in Langmuir monolayers of 5‘-phenyl-1,1‘:3‘1‘ ‘-terphenyl-4-carboxylic acid (PTCA) at the air−water interface is presented. Surface pressure and surface potential isotherms were obtained using varying experimental parameters such as spreading volume, subphase pH, ionic strength, subphase temperature, and compression speed. The isotherms exhibit two liquid-condensed regions separated by a broad plateau with a 2-fold decrease in area/molecule. PTCA monolayers were visualized by Brewster angle microscopy, transferred as multilayer LB films and characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and surface potential measurements. On the basis of the analysis of surface pressure and surface potential isotherms, it was not possible to distinguish between bilayer formation and conformational changes of the triphenylbenzene ring system upon compression as the origin of the plateau. The LB transfer from monolayers in the plateau was unsuccessful and thus characterization of LB films does not help in elucidating the possible cause for the transition.

Phase transitions in Langmuir monolayers

Some fatty acids, lipids, polymers and mesogenic molecules which are amphiphilic in nature spread at the air-water interface to form stable Langmuir monolayers. They exhibit a rich variety of two-dimensional (2D) phases. In this article, we briefly review some of the novel features we have found in these monolayers. For example, we find transition from a 2D monolayer to three-dimensional structures possessing liquid crystalline order, induced liquid condensed phase, demixing of liquid expanded phase, critical points and pattern formation.

Structure and phase transitions in Langmuir monolayers

Lipid monolayers on the surface of water have been studied for over a hundred years, but in the last decade there has been a dramatic evolution in our understanding of the structures and phase transitions of these systems, driven by new experimental techniques and theoretical advances. In this review, dense monolayers of simple lipids are described in detail, including structures revealed by x-ray-diffraction experiments, computer simulations, molecular models, and a phenomenological theory of phase transitions. The effects of chirality and the structures of phospholipid monolayers are considered. Open questions and possible approaches to finding answers are discussed.

Coupling of phase transitions in Langmuir monolayers

A Ginzburg–Landau theory is presented to study coupled first-order gas–liquid phase transition and second-order orientational phase transition in Langmuir monolayers. We show that the coupling between the two phase transitions has a strong influence on the orientational phase transition, making the polar order phase separated from the isotropic one. Such a phase separation will substantially suppress the onset of the transition and the growth of the polar order parameter, which accords with the existing experiments.

Ellipsometric model for two-dimensional phase transition in Langmuir monolayers

A microscopic ellipsometric model of dioctadecyldimethylammonium bromide monolayer compression has been proposed. The suggested model explains the dependence of the ellipsometric parameters on the surface density throughout the entire range of compression, comprising the liquid expanded (LE) and liquid condensed (LC) phases, as well as their coexistence region. The model uses unique microscopic parameters, such as molecular polarizabilities, Lorentz factors and geometric molecular dimensions. Molecules are treated as rods whose cross-section decreases linearly during the LE–LC phase transition. During compression the rods change their orientation from parallel to the interface to almost perpendicular. It was shown that the volume density of the LE phase is 10–15% lower than that of the LC phase.

Phases and Phase Transitions in Langmuir Monolayers by Second-Harmonic Generation

We have studied phase transition phenomena in Langmuir monolayers of a molecule incorporating NPP dye by optical second-harmonic generation. From these measurements we obtain information on the molecular orientation and the molecular conformation in the film.

Symmetry and phase transitions in Langmuir monolayers: The Landau theory.

Condensed phases of Langmuir monolayers are treated in the framework of the Landau theory of phase transitions as a result of successive ordering of the hexatic phase. All types of ordering allowed by symmetry and the couplings between them are described. The phase diagrams and x-ray structural data on monolayers of various long-chain amphiphilic molecules can be explained by using three coupled order parameters. One of the parameters governs the collective tilt of the molecules, while the other two describe one-dimensional ``weak crystallization'' involving herringbone ordering of the molecular backbone planes along the bond direction and normal to it. Possible manifestations of other order parameters are discussed.

Textures and phase transitions in Langmuir monolayers of fatty acids. A comparative Brewster angle microscope and polarized fluorescence microscope study

Brewster angle microscopy (BAM) and polarized fluorescence microscopy (PFM) are used to observe the distinctive textures of and the transitions between condensed phases in Langmuir monolayers of n-alkanoic acids. BAM is sensitive to film anisotropy even when the molecules are not tilted as long as the unit cell is anisotropic. Every transition is visible with one or both of the techniques, either as a dramatic change in the degree of contrast or as a sudden alteration of the mosaic domain texture. The two techniques are generally consistent, although the presence of the fluorescent probe impurity (for PFM) causes a subtle difference in the appearance of one transition and small shifts in transition surface pressures.