A new surface phase in liquid normal-alkanes

A new surface phase is found in liquid normal-alkanes. X-ray and surface tension measurements show the formation of a crystalline monolayer on the surface of the liquid at temperatures up to 3°C above the bulk solidification temperature, T_f. The molecules in the monolayer are hexagonally packed and oriented normal to the surface. A single solid monolayer persists down to T_f, thus exhibiting a very limited partial wetting. The new surface phase is obtained only for chain lengths 14<n50. Its vanishing for n14 is interpreted as a possible transition from surface freezing to surface melting behaviour. The measurements are satisfactory accounted for within a simple theory based on surface energy cosiderations.     

X-Ray Reflectometry of DMPS Monolayers on a Water Substrate

The molecular structure of dimyristoyl phosphatidylserine (DMPS) monolayers on a water substrate in different phase states has been investigated by X-ray reflectometry with a photon energy of ~8 keV. According to the experimental data, the transition from a two-dimensional expanded liquid state to a solid gel state (liquid crystal) accompanied by the ordering of the hydrocarbon tails C₁₄H₂₇ of the DMPS molecule occurs in the monolayer as the surface pressure rises. The monolayer thickness is 20 ± 3 and 28 ± 2 Å in the liquid and solid phases, respectively, with the deflection angle of the molecular tail axis from the normal to the surface in the gel phase being 26° ± 8°. At least a twofold decrease in the degree of hydration of the polar lipid groups also occurs under two-dimensional monolayer compression. The reflectometry data have been analyzed using two approaches: under the assumption about the presence of two layers with different electron densities in the monolayer and without any assumptions about the transverse surface structure. Both approaches demonstrate satisfactory agreement between themselves in describing the experimental results.     

Superfluid molecular hydrogen

In order to produce a supercooled liquid phase of molecular hydrogen that may possibly change at a sufficiently low temperature to a superfluid state, it is suggested to reduce the temperature of its equilibrium coexistence with the solid phase by means of developing different pressures in these phases through the use of linear mechanical pressure on the solid phase or of external electric field. The thermodynamic functions of hydrogen are calculated in both the stable and metastable regions; its phase diagram and the region of possible transition to a superfluid state are also found. The values of excess pressure on the solid phase and of external electric field intensity are estimated, which are necessary for the stabilization of this state.     

Étude comparée de la première couche d'adsorption de méthane sur nitrure de bore et graphite

The adsorption of methane on crystalline boron nitride has been studied between 77 and 90 K by the volumetric method, especially in the first monolayer domain. It is shown that the thermodynamical properties of the layer are similar to those previously obtained on graphite. It can be inferred from the results that below 77 K, the layer undergoes two first-order phase transitions: a 2D gas-2D liquid transition and a 2D liquid-2D solid transition. In its solid state the localization of the layer is certainly the same on both substrates.     

Evidence for a Mott-Hubbard transition in a two-dimensional 3He fluid monolayer

The heat capacity and magnetization of a fluid 3He monolayer adsorbed on graphite plated with a bilayer of HD have been measured in the temperature range 1-60 mK. Approaching the density at which the monolayer solidifies into a sqrt[7]xsqrt[7] commensurate solid, we observe an apparent divergence of the effective mass and magnetization corresponding to a T=0 Mott-Hubbard transition between a 2D Fermi liquid and a magnetically disordered solid. The observations are consistent with the Brinkman-Rice-Anderson-Vollhardt scenario for a metal-insulator transition. We observe a leading order T2 correction to the linear term in heat capacity.     

Randomly crosslinked macromolecular systems: Vulcanization transition to and properties of the amorphous solid state

As Charles Goodyear discovered in 1839, when he first vulcanized rubber, a macromolecular liquid is transformed into a solid when a sufficient density of permanent crosslinks is introduced at random. At this continuous equilibrium phase transition, the liquid state, in which all macromolecules are delocalized, is transformed into a solid state, in which a non-zero fraction of macromolecules have spontaneously become localized. This solid state is a most unusual one: localization occurs about mean positions that are distributed homogeneously and randomly, and to an extent that varies randomly from monomer to monomer. Thus, the solid state emerging at the vulcanization transition is an equilibrium amorphous solid state: it is properly viewed as a solid state that bears the same relationship to the liquid and crystalline states as the spin glass state of certain magnetic systems bears to the paramagnetic and ferromagnetic states, in the sense that, like the spin glass state, it is diagnosed by a subtle order parameter.

In this article we give a detailed exposition of a theoretical approach to the physical properties of systems of randomly, permanently crosslinked macromolecules. Our primary focus is on the equilibrium properties of such systems, especially in the regime of Goodyear's vulcanization transition. This approach rests firmly on techniques from the statistical mechanics of disordered systems pioneered by Edwards and co-workers in the context of macromolecular systems, and by Edwards and Anderson in the context of magnetic systems. We begin with a review of the semi-microscopic formulation of the statistical mechanics of randomly crosslinked macromolecular systems due to Edwards and co-workers, in particular discussing the role of crosslinks as quenched random variables. Then we turn to the issue of order parameters, and review a version capable, inter alia, of diagnosing the amorphous solid state. To develop some intuition, we examine the order parameter in an idealized situation, which subsequently turns out to be surprisingly relevant. Thus, we are motivated to hypothesize an explicit form for the order parameter in the amorphous solid state that is parametrized in terms of two physical quantities: the fraction of localized monomers, and the statistical distribution of localization lengths of localized monomers. Next, we review the symmetry properties of the system itself, the liquid state and the amorphous solid state, and discuss connections with scattering experiments. Then, we review a representation of the statistical mechanics of randomly crosslinked macromolecular systems from which the quenched disorder has been eliminated via an application of the replica technique. We transform the statistical mechanics into a field-theoretic representation, which exhibits a close connection with the order parameter, and analyse this representation at the saddle-point level. This analysis reveals that sufficient crosslinking causes an instability of the liquid state, this state giving way to the amorphous solid state. To address the properties of the amorphous solid state itself, we solve the self-consistent equation for the order parameter by adopting the hypothesis discussed earlier. Hence, we find that the vulcanization transition is marked by the appearance of a non-zero fraction of localized monomers, which we compute, the dependence of this fraction on the crosslink density indicating a connection with random graph theory and percolation. We also compute the distribution of localization lengths that characterizes the ordered state, which we find to be expressible in terms of a universal scaling function of a single variable, at least in the vicinity of the transition. Finally, we analyse the consequences of incorporating a certain specific class of correlations associated with the excluded-volume interaction.     

Grain Boundary Phase Transitions and their Influence on Properties of Polycrystals

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.     

Heat capacity of Ar monolayer adsorbed on basal plane graphite

The specific heat of Ar monolayer film adsorbed on basal planes of graphite has been measured for film coverage ranging from 0.31 to 0.97 monolayers for 4.2 < T < 68 K. It is found that a broad specific heat peak centered around 50 K exists over most of the submonolayer coverages until reaching monolayer completion. A comparison with recent neutron scattering and LEED studies relates the anomaly to a possible higher order phase transition between 2D solid and 2D fluid.     

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.     

The NaNO3–KNO3 phase diagram

Many papers have been published in relation to the NaNO₃–KNO₃ phase diagram determination in the last 160 years. These papers fall in two categories: (1) the solid–liquid equilibrium is assumed to be of the eutectic type, and (2) the solid–liquid equilibrium is considered as a loop with a minimum. The discordance between the two views is related to the slow transition kinetics that complicate the assessment of thermal ‘fluctuations’, and also to the appearance of a metastable form of potassium nitrate. The main result of this paper is the experimental phase diagram constructed with new experimental data so that we can assure that the second option is correct. This phase diagram is defined by a eutectoid invariant, an asymmetric immiscibility gap and a continuous solid solution with a minimum of melting point. Additionally, the ABθ model simulates correctly the experimental piece of evidence.     

Isotropic to nematic liquid crystalline phase transition of F-actin varies from continuous to first order

We report that the properties of the isotropic to nematic liquid crystalline phase transition of F-actin depend critically on the average filament length. For average filament lengths longer than 2 microm, we confirm previous findings that the phase transition is continuous in both alignment and concentration. For average filament lengths shorter than 2 microm, we show for the first time a first order transition with a clear discontinuity in both alignment and concentration. Tactoidal droplets of coexisting isotropic and nematic phases, differing in concentration by approximately 30%, form over the course of hours and appear to settle into near equilibrium metastable states.     

Application d'un modéle de l'état liquide à une monocouche de xénon ou de krypton adsorbées sur la surface (0001) du graphite

A phenomenological model of three-dimensional (3D) liquids is applied to a xenon or krypton monolayer adsorbed on (0001) graphite. The rare-gas atoms are assumed to be mobile with a short-range order as in 3D liquids. From the study of the equilibrium 3D gas ? 2D solid and 3D gas ? 2D liquid, one can find with reasonable agreement the pressures and temperatures of the 2D triple point determined experimentally by other authors.     

Anchoring and structural transitions as a function of molecular length in confined liquid crystals

Using deuteron nuclear magnetic resonance to study liquid crystals confined to cylindrical pores, an anchoring transition has been found. The transition exhibits an unexpected sharp dependence of the anchoring strength on cyanobiphenyl liquid crystal molecular length. A structural transition from a parallel axial to a planar radial configuration occurs due to an anchoring transition from planar to weakly homeotropic orientation at the walls. The anchoring strength is at a minimum near the decylcyanobiphenyl (10CB) liquid crystal length. Long chain liquid crystal configurations depend on thermal cycling and on the equilibrium atmosphere leading to a bistable SmA structure. Orientational order wetting in the isotropic phase also depends on molecular length.     

Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation

We report a molecular simulation study of freezing transitions for simple fluids in narrow slit pores. A major stumbling block in previous studies of freezing in pores has been the lack of any method for calculating the free energy difference between the confined solid and liquid phases. Conventional thermodynamic integration methods often fail for confined systems, due to the difficulty in choosing a suitable path of integration. We use a different approach that involves calculating the Landau free energy as a function of a suitable order parameter, using the grand canonical Monte Carlo simulation method. The grand free energy for each phase can then be obtained by one-dimensional integration of the Landau free energy over the order parameter. These calculations are carried out for two types of wall—fluid interaction, a hard wall and a strongly attractive wall modelled on carbon. The grand free energy results for both cases clearly indicate a first order fluid to solid transition. In the case of the attractive carbon wall, there are three phases. Phase A corresponds to all layers having a liquid-like structure; phase B corresponds to the contact layers (the layers adjacent to the two pore walls) being frozen and the rest of the layers being fluid-like; phase C corresponds to all the layers being frozen. Our results for the angular structure function in the individual molecular layers show strong evidence of a transition from a two-dimensional liquid phase to a hexatic phase. This is followed by a transition from the hexatic to a crystal phase.     

Pyroelectric investigation of the surface polarization in the isotropic phase of a nematic liquid crystal

Measurements of the temperature dependence of the pyroelectric coefficients in cells with homeotropic and planar boundary conditions were performed in order to study the surface polarization and determine its nature in the isotropic phase of a nematic liquid crystal. Analysis of these data established that the ordering polarization arising as a result of the nonuniformity of the order parameter in the near-boundary region of the liquid crystal and the polar monolayer due to the bifilar properties of the molecules of the liquid crystal at the boundary with the solid wall of the cell are responsible for the surface polarization in the isotropic phase of a nematic liquid crystal. The values of the pyroelectric coefficients of monolayers and the coefficients of the ordering polarization for planar and homeotropic orientations are estimated.     

Monte Carlo values for the radial distribution function of a system of fluid hard spheres

The perturbation theory recently developed by Weeks, Chandler, and Andersen is applied to the solid phase of a rare gas near its melting line. The potential is separated into two parts: a reference part containing all the repulsive forces and a perturbation part containing all the attractions. We show that the expansion of the free energy in the perturbation potential is, as in the liquid phase, rapidly convergent for a temperature of the order of the triple-point temperature. On the contrary, the representation of the reference system by hard spheres with an appropriate diameter is less accurate than in the liquid phase. This representation requires the knowledge of the radial distribution functions of the hard-sphere solid for which we give a tabulation as well as an analytical representation. The perturbation theory is applied to the determination of the fluid-solid transition.     

Continuous crystallization in hexagonally ordered materials

We demonstrate that the phase transition from columnar-hexagonal liquid crystal to hexagonal-crystalline solid falls into an unusual universality class, which in three dimensions allows for both discontinuous transitions as well as continuous transitions, characterized by a single set of exponents. We show by a renormalization group calculation (to first order in =4-d) that the critical exponents of the continuous transition are precisely those of the XY model, giving rise to a continuous evolution of elastic moduli. Although the fixed points of the present model are found to be identical to the XY model, the elastic compliance to deformations in the plane of hexagonal order, mu, is nonetheless shown to critically influence the crystallization transition, with the continuous transition being driven to first order by fluctuations as the in-plane response grows weaker, micro-->0.     

A theory of phase transitions in solid krypton and xenon monolayers on graphite

A statistical mechanical model of solid Kr and Xe monolayers on graphite is described. A phase transition in a Kr monolayer is interpreted as a transition from a solid localised in the surface potential wells of the graphite, to an unlocalised solid having a higher density. Comparison with experimental data enables the graphite well depth to be estimated. Explicit calculations are presented for four different interatomic potential models. The density of the unlocalised phase, the isosteric heat, and the gas-solid transition are given as a function of temperature and pressure for both Kr and Xe. It is shown that Xe first condenses into an unlocalised phase, and its ability to undergo a transition into localisation is examined. It is found that this transition is unlikely at p ? 10^?3 Torr where volumetric measurements have been made, but may occur at lower pressures. The gas-solid transition data is re-interpreted in terms of a transition to the unlocalised phase. A combination of the solid-solid and gas-solid transition data is shown to be a good test of interatomic potential models. The comparison is made with particular reference to the magnitude of the Sinanoglu-Pitzer potential.     

Monolayer ice

We report results from molecular dynamics simulations of water under confinement and at ambient conditions that predict a first-order freezing transition from a monolayer of liquid water to a monolayer of ice induced by increasing the distance between the confining parallel plates. Since a slab geometry is incompatible with a tetrahedral arrangement of the sp(3) hybridized oxygen of water, the freezing is coupled to a linear buckling transition. By exploiting the ordered out-of-plane displacement of the molecules in the buckled phase the distortion of the hydrogen bonds is minimized.     

Multilayer adsorption and wetting of acetone on graphite

Adsorption/desorption isotherms of acetone on highly oriented pyrolytic graphite have been measured by ellipsometry for temperatures above the bulk triple point. The behavior in the monolayer and submonolayer regime is conventional, with 2D gas-liquid and 2D liquid-solid coexistence regions. Further liquid monolayers grow on top of the completed monolayer. The growth is basically layer-by-layer. For temperatures between 190 K and the triple point a prewetting-type transition occurs with a thin-thick jump of the layer thickness on adsorption but a layer-wise removal of the film on desorption. In this temperature regime the first monolayer is solid and its molecules are oriented perpendicular to the substrate whereas the higher layers are orientationally disordered polar liquid.