Size and surface effects on phase transitions

Size effects on liquid crystal phase transitions are investigated by scanning calorimetry of samples absorbed in porous silica. For pore diameter of order 100 Å, the liquid crystal transition depressions are of order 0.1°C and the solid melting points of order 10°C.     

First-order phase transitions in repulsive rigid k-mers on two-dimensional lattices

In a previous paper [F. Roma?, A. J. Ramirez-Pastor, and J. L. Riccardo, Phys. Rev. B 72, 035444 (2005)], the critical behavior of repulsive rigid rods of length k (k-mers) on a square lattice at half coverage has been studied by using Monte Carlo (MC) simulations. The obtained results indicated that (1) the phase transition occurring in the system is a second-order phase transition for all adsorbate sizes k; and (2) the universality class of the transition changes from 2D Ising-type for monomers (k = 1) to an unknown universality class for k ≥ 2. In the present work, we revisit our previous results together with further numerical evidences, resulting from new extensive MC simulations based on an efficient exchange algorithm and using high-performance computational capabilities. In contrast to our previous conclusions (1) and (2), the new numerical calculations clearly support the occurrence of a first-order phase transition for k ≥ 2. In addition, a similar scenario was found for k-mers adsorbed on the triangular lattice at coverage k/(2k+1).     

Positron annihilation and phase transitions in argon intercalated n-nonadecane

Positron annihilation method was applied to study phase transitions in argon intercalated n-nonadecane. Particular phases are easily recognizable due to large differences in ortho-positronium lifetimes. It was found that melting temperature is a non-monotonous function of pressure. In the range of temperatures 301.5–303 K with the rise of pressure the phase sequence rotator – liquid–rigid crystal was observed. After the pressure induced transition to the rigid phase the sample structure became porous due to escape of dissolved argon. The triple point in intercalated samples is located at much lower pressure and temperature than in pure ones. An admixture of alkane with other length of carbon chain increases the ortho-positronium lifetime in rigid phase, indicating Ps location in the interlamellar gaps.     

Effect of particle geometry on phase transitions in two-dimensional liquid crystals

Using a version of density-functional theory which combines Onsager approximation and fundamental-measure theory for spatially nonuniform phases, we have studied the phase diagram of freely rotating hard rectangles and hard discorectangles. We find profound differences in the phase behavior of these models, which can be attributed to their different packing properties. Interestingly, bimodal orientational distribution functions are found in the nematic phase of hard rectangles, which cause a certain degree of biaxial order, albeit metastable with respect to spatially ordered phases. This feature is absent in discorectangles, which always show unimodal behavior. This result may be relevant in the light of recent experimental results which have confirmed the existence of biaxial phases. We expect that some perturbation of the particle shapes (either a certain degree of polydispersity or even bimodal dispersity in the aspect ratios) may actually destabilize spatially ordered phases thereby stabilizing the biaxial phase.     

Simulation of water cluster assembly on a graphite surface

The assembly of small water clusters (H2O)n, n = 1-6, on a graphite surface is studied using a density functional tight-binding method complemented with an empirical van der Waals force correction, with confirmation using second-order M?ller-Plesset perturbation theory. It is shown that the optimized geometry of the water hexamer may change its original structure to an isoenergy one when interacting with a graphite surface in some specific orientation, while the smaller water cluster will maintain its cyclic or linear configurations (for the water dimer). The binding energy of water clusters interacting with graphite is dependent on the number of water molecules that form hydrogen bonds, but is independent of the water cluster size. These physically adsorbed water clusters show little change in their IR peak position and leave an almost perfect graphite surface.     

Packing effects on argon and methanol adsorption inside graphitic cylindrical and slit pores: a GCMC simulation study

Using Grand Canonical Monte Carlo simulation, we have studied the effects of confinement on argon and methanol adsorption in graphitic cylindrical and slit pores. Linear chain, zigzag and incomplete helical packing are observed for argon adsorption in cylindrical pores. However, for methanol adsorption different features appear because the electrostatic interactions favour configurations that maximize the hydrogen bonding among methanol molecules. We have found zigzag chains with hydrogen-bonded structures for methanol adsorption in cylindrical and slit pores. To investigate how dense the adsorbed phase is and how many molecules could be packed per unit physical volume of the solid, we consider two different definitions of pore density; one based on the physical volume and the other on the accessible volume. That based on accessible volume gives a measure of the fluid density, while that based on the physical volume gives a measure of how much adsorbate can be stored per unit volume of the adsorbent. It is found that the adsorbate is denser in cylindrical pores, but that slit pores can pack more molecules per unit solid volume. We also discuss the effects on the isosteric heat of argon and methanol of pore size, pore geometry and loading.     

Calorimetry by immersion into liquid nitrogen and liquid argon: a better way to determine the internal surface area of micropores

The aim of this work is to assess the internal surface area of a set of samples (either carbons or oxides, either porous or nonporous, either microporous or mesoporous) by microcalorimetry via immersion into liquid nitrogen or argon. We have made use of an isothermal, heat-flux microcalorimeter, initially designed and built in our laboratory for the sake of gas adsorption experiments at 77 or 87 K. It seems that immersion calorimetry into liquid nitrogen and argon makes it possible to go one step further in the determination of the internal surface area of micropores.     

Monte Carlo simulations of phase transitions and adsorption isotherm discontinuities on surface compression

Low temperature, Grand Canonical Monte Carlo simulations were used to study the adsorption of fluid layers on cubic, hexagonal, and atomically smooth substrates to determine the effects of registry and surface compression on the system. The size of the fluid molecules was fixed to be 20% larger than the substrate molecules in order to observe the transition from an expanded to commensurate and finally to an incommensurate monolayer. For relatively weak fluid-substrate interactions, the cubic system underwent a first-order phase transition. As the strength of the fluid-substrate interactions increased, the molecules became fixed at commensurate locations and the transition from low density to commensurate packing became continuous. The strong fluid-substrate interactions lead to the development of a kink in the adsorption isotherm that showed the increased stability of the commensurate phase. This kink became more pronounced as the system temperature was decreased. The hexagonal system showed less dramatic results due to a decrease in the substrate well depth of the relative to the cubic system. The system did experience a first-order phase transition for a weak fluid-substrate interactions and the transition became much more gradual as the fluid-substrate interaction increased. The molecules became fixed to commensurate substrate locations, but the surface was not corrugated sufficiently to have a stable commensurate phase. The atomically smooth substrate showed the first-order phase transition expected of a low temperature system with no effects of registry.     

Phase transitions and nucleation in diamond and graphite

Conclusions It has been shown that the region of diamond thermodynamic stability depends on the dimensions of the carbon particles in small-particle systems. At the high supersaturation realized in vacuum condensation and chemical decomposition, there is an increase in the ratio of the probabilities for diamond and graphite nucleation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1184–1188, June, 1979.     

Calorimetric study of phase transitions in a liquid-crystal-based microemulsion

A lyotropic inverse micelle phase composed of water, thermotropic liquid-crystal octylcyanobiphenyl (8CB), and surfactant (DDAB) was studied by using high-resolution calorimetry on several mixtures with 3%, 8%, and 15% micelle concentration. Calorimetric results show strong depression of the isotropic to nematic (I-N) phase-transition temperature. Broad heat-capacity anomalies show the existence of a wide coexistence range of isotropic, nematic, and smectic-A phases, which mimics the behavior of a new nearly stable thermodynamic phase. An observation of the rather sharp almost bulklike nematic to smectic-A (N-A) transition at low-temperatures indicates that our heat capacity results are consistent with the phase separation scenario in which significant number of micelles is expelled during I-N conversion leaving almost pure nematic phase at lower temperatures. It was found that micelles get almost completely remixed on heating the mixture back to the isotropic phase.     

Spectroscopic study of phase transitions in natural calcite mineral

The process and the formation of new minerals upon heating the carbonate rocks containing clay minerals, together with calcite are determined with thermal analysis, X-ray diffraction, infrared and Raman spectroscopy. The calcite-calcium oxide phase transition sequence was followed up to 947 degrees C in naturally occurring limestone samples. The spectral variations of the internal modes of the carbonate trigonal (nu(1), nu(2), nu(3) and nu(4)) were used to probe the structural phase transitions. The calcium oxide phase (which on reaction with atmospheric water forms portlandite) with an onset temperature of around 950 degrees C was also characterized by the appearance of the infrared mode around 450 cm(-1). The minerals, which were formed upon heating the calcite, were calcium oxide and wollastonite.     

AC calorimetric study of phase transitions in phosphatidylcholine-cholesterol systems

Using an ac calorimetric method, detailed behaviour of the heat capacity in dipalmitoyl-phosphatidylcholine-cholesterol system was studied in the cholesterol concentration less than 5 mol%. It was revealed that the heat capacity near the main transition was composed of at least four anomalies, i.e., multipeak took place in the heat capacity. This fact indicates that a simple theory explaining coexistence of two phases in two component systems does not work in the multipeak region. Then, relation between the multipeak heat capacity and the change of the ripple structure with the cholesterol concentration should be taken into account, when we consider thermodynamical behaviour of the systems.

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Zusammenfassung Mittels AC-Kalorimetrie wurde bei Cholesterol-Konzentrationen von weniger als 5 mol% das Verhalten der Wärmekapazität im System Dipalmitoylphosphatidylcholin-Cholesterol untersucht. Es wurde gezeigt, daß sich die Wärmekapazität in der Nähe der Hauptumwandlung aus mindestens vier Anomalien zusammensetzt, d.h. bei der Wärmekapazität kann ein Multipeak beobachtet werden. Diese Tatsache zeigt, daß eine einfache Theorie, welche die Koexistenz zweier Phasen in einem Zweikomponenten-System erklärt, für die Multipeakregion nicht geeignet ist. Weiterhin sollte bei Überlegungen zum thermodynamischen Verhalten von Systemen eine Beziehung zwischen der Multipeak-Wärmekapazität bzw. der Welligkeitsstruktur und der Cholesterol-Konzentration berücksichtigt werden.

     

Molecular dynamics simulation of benzene in graphite and amorphous carbon slit pores

It is well known that confining a liquid into a pore strongly alters the liquid behavior. Investigations of the effect of confinement are of great importance for many scientific and technological applications. Here, we present a study of the behavior of benzene confined in carbon slit pores. Two types of pores are considered–graphite and amorphous carbon ones. We show that the effect of different pore structure is of crucial importance for the benzene behavior. © 2013 Wiley Periodicals, Inc.     

A positron annihilation study on phase transitions in trans-stilbene single crystal

Positron lifetime spectroscopic and Dopplerbroadening measurements have been carried out on trans-stilbene single crystals. The results (including the complementary DSC measurement) give new evidence for the existence of phase transitions in trans-stilbene at temperatures between 20 and 300 K.     

A study of molecular dynamics and phase transitions in solid MBBA

Abstract

The solid state polymorphism of liquid crystalline MBBA was investigated by temperature dependent NMR spin-lattice relaxation time measurements. The sensitivity of the method could be utilized because the correlation time of the measurement is in the correlation time region of molecular motion. Motional correlation time and activation energy values were determined and the results show some interesting changes between the different solid phases. Non-trivial variation in the end-chain rotational motion in two crystalline phases has been observed. Conclusions were drawn on the relationship between rotational molecular dynamics, intermolecular order and phase transitions.     

Thermodynamic analysis of ordered and disordered monolayer of argon adsorption on graphite

We presented a detailed thermodynamic analysis of argon adsorption on a graphitized carbon black with a kinetic Monte Carlo scheme. In this study, we particularly paid attention to the formation of a hexagonal two-dimensional molecular layer on a graphite surface and discuss conditions of its stability and thermodynamic properties of the adsorbed phase as a function of loading. It is found that the simulation results are substantially affected by the dimensions of the simulation box when the monolayer forms a hexagonal ordered structure. This is due to the fact that the lattice constant is constrained by the dimensions of the surface. To circumvent this, we presented a thermodynamic technique, which allows for the variation of the box size as a function of loading, to determine the "intrinsic" lattice constant (rather than apparent average value because of the fixed dimensions of the simulation box) and the thermodynamic functions for the adsorbed phase: the Helmholtz free energy, the chemical potential, and the surface tension. The tangential and normal pressures as a function of the distance from the surface are also discussed.     

Confinement and orientational study at liquid crystal phase transitions

Abstract

We have studied the heat capacity of the thermotropic liquid crystal, octylcyanobiphenyl (8CB), confined to the nearly cylindrical, 0·2 μm diameter pores of Anopore membranes. Orientation of the nematic director within the pores can be controlled with surface treatment. It is known from NMR measurements that the nematic director is aligned parallel to the pore axis in the untreated membrane. A perpendicular alignment is obtained when the pore surface is treated with lecithin. The second order smectic A to nematic (S_A–N) and the weakly first order nematic to isotropic (N–I) phase transitions of 8CB were studied in these pores, for both director orientations, using an AC calorimetry technique. Effects on heat capacity amplitudes, transition temperature shifts, rounding and broadening of these phase transitions will be presented and contrasted with bulk measurements.     

Calorimetric study of phase transitions for octylphenylthiolpentyloxybenzoate in silica aerogels

An a.c. calorimetric study has been carried out on octylphenylthiolpentyloxybenzoate (8S5) in three silica aerogels with mass densities rho 0.08, 0.17 and 0.36g cm . Results for the least porous aerogel (rho 0.36) are completely consistent with those reported previously for 8CB in the same aerogel. Freezing of 8S5 damaged the more porous rho 0.08 and 0.17 aerogels by creating internal voids and cracks that are large compared with the intrinsic pore size. As a result, overlapping sharp bulk-like Cp features and rounded features due to 8S5 in residual aerogel pores were observed, and no quantitative separation of these features could be achieved. 3     

Chemical tools for study and modulation of biomolecular phase transitions

Biomolecular phase transitions play an important role in organizing cellular processes in space and time. Methods and tools for studying these transitions, and the intrinsically disordered proteins (IDPs) that often drive them, are typically less developed than tools for studying their folded protein counterparts. In this perspective, we assess the current landscape of chemical tools for studying IDPs, with a specific focus on protein liquid–liquid phase separation (LLPS). We highlight methodologies that enable imaging and spectroscopic studies of these systems, including site-specific labeling with small molecules and the diverse range of capabilities offered by inteins and protein semisynthesis. We discuss strategies for introducing post-translational modifications that are central to IDP and LLPS function and regulation. We also investigate the nascent field of noncovalent small-molecule modulators of LLPS. We hope that this review of the state-of-the-art in chemical tools for interrogating IDPs and LLPS, along with an associated perspective on areas of unmet need, can serve as a valuable and timely resource for these rapidly expanding fields of study.

Chemical tools provide the ability to illuminate and manipulate the behavior of intrinsically disordered proteins and their phase transitions.