Rotator Phase Transitions Research Articles

Instabilities, thermal fluctuations, defects and dislocations in the crystal-RI-RII rotator phase transitions of n-alkanes.

The theoretical study of instabilities, thermal fluctuations, and topological defects in the crystal-rotator-I-rotator-II (X-RI-RII) phase transitions of n-alkanes has been conducted. First, we examine the nature of the RI-RII phase transition in nanoconfined alkanes. We propose that under confined conditions, the presence of quenched random orientational disorder makes the RI phase unstable. This disorder-mediated transition falls within the Imry-Ma universality class. Next, we discuss the role of thermal fluctuations in certain rotator phases, as well as the influence of dislocations on the X-RI phase transition. Our findings indicate that the herringbone order in the X-phase and the hexatic order in the RII-phase exhibit quasi-long-range characteristics. Furthermore, we find that in two dimensions, the unbinding of dislocations does not result in a disordered liquid state.

Theoretical study of the effects of carbon nanotubes on the rotator phase transitions of pentacosane

ABSTRACT The effects of carbon nanotubes (CNT) on the rotator phase transitions of pentacosane have been studied theoretically. A model has been developed using Flory–Huggins theory of isotropic mixing and Landau theory. The effects of CNT on the lattice distortion and tilt angle on the rotator phases have been discussed. The decrease of lattice distortion and increase of tilt angle in the CNT+pentacosane mixtures have been observed. The first order character of the R I − R I I and R V − R I phase transitions both in pure pentacosane and CNT+pentacosane mixtures have been observed.

Effect of silica nanoparticles on the heat capacity of the rotator phase transitions of alkanes

ABSTRACT Experimental studies have shown that quenched disorder induced by silica nanoparticles changes the hysteresis behavior and the heat capacity of the rotator phase transitions of n-alkanes. We theoretically study the effect of silica nanoparticles on the heat capacity near the Liquid–R, R–R and R–R phase transitions of normal alkanes by combining Flory–Huggins free energy of isotropic mixing and Landau free energy. This is followed by theoretical calculations and plotting figures. A qualitative agreement between theoretical and experimental results is observed.

Determination of phase transition temperatures (melting, crystallization, rotator phases) of n-alkanes by the optical method

The temperatures of phase transitions (melting, crystallization, rotator phases) were determined for a number of individual n-alkanes from C19H40 (n-nonadecane) to C28H58 (n-octacosane) by the optical method, a modified droplet technique. For this research, n-alkanes were presented as an emulsion in water without the addition of surfactants. Emulsions were prepared by ultrasonic dispersion of a small amount of n-alkane in water at a temperature above the melting point of the investigated n-alkane. The hydrodynamic radius of the dispersions measured by the dynamic light scattering (DLS) method was about 100 nm and did not change within a wide temperature range within the measurement accuracy. The temperatures of melting, crystallization, and rotator phase transitions of pure n-alkanes in the form of emulsion were determined from temperature-dependent measurements of the light scattering by the dispersions. The supercooling values for studied n-alkanes were also determined.

Analysis of rotator phase transitions in the linear alkanes hexacosane to triacontane by adiabatic scanning calorimetry and by photopyroelectric calorimetry

The study of the nature of various phase transitions between rotator phases in several linear alkanes was performed by analyzing the hysteretic behavior of the specific heat between heating and cooling measurements. The investigations have been carried out by both adiabatic scanning calorimetry and photopyroelectric calorimetry techniques, whose combined use has provided complementary information concerning the changes occurring in the samples’ structure during their temperature change. The study enabled to establish that, unlike what previously reported, the Riii-Riv and the Rii-Riv transitions are of first order, despite them not showing any sharp peak profile in the temperature dependence of the specific heat. The first order of the Rv-Rii transition was confirmed also in the present study and, finally the Rv-Riii transition could be observed by calorimetric detection for the first time and shown to be of second order. The obtained results were discussed in terms of order parameters power terms in the Landau free energy expansion.

Effect of nanoparticles on the [formula omitted] rotator phase transitions of alkanes

Experimental studies have shown that nanoparticles play an important role on the rotator phase transitions of n-alkanes. A phenomenological model for predicting the RII-RI-RV phase transitions in mixtures of alkanes and nanoparticles has been proposed by combining Flory-Huggins free energy of isotropic mixing and Landau free energy. The impact of nanoparticles on the RII-RI-RV phase transitions and their transition temperatures is discussed by means of phenomenological theory. The possibility of the tricritical behavior of the RI-RV phase transition in the mixtures of alkanes and nanoparticles is discussed. The theoretical predictions are in good qualitative agreement with available experimental results.

Effect of the liquid crystal solute on the rotator phase transitions of n-alkanes

Recent experimental studies have shown that the liquid crystal substance plays an important role in determining the structures and the phase transitions of the different rotator phases in binary mixtures of n-alkane and liquid crystals.

Effect of Gauche Molecular Conformations and Molecular Flexibility on the Rotator Phase Transitions of Alkanes

Experimental studies have shown that the gauche molecular conformations and the chain configurations play an important role in determining the structures and the phase transitions of the different rotator phases of n-alkanes. We extend our previous phenomenological theory to describe the effect of gauche defects and molecular flexibility on the various rotator phase transitions of n-alkanes. The roles of gauche defects and molecular flexibility are discussed by means of phenomenological theory. The effect of gauche defects and molecular flexibility is to modify the Landau coefficients and to shift the transition lines in the phase diagram. The theoretical predictions are found to be in good qualitative agreement with available experimental results.

Simple Landau Model of the Liquid-RII-RI Rotator Phases of Alkanes

Simple Landau free energy function is presented to describe the Liquid-RII-RI phase sequence of alkanes and transitions between them. The order parameters necessary to describe these rotator phase transitions are identified. We present a mean-field description of the Liquid-RII and Liquid-RI transitions. General arguments are presented for the topology of the phase diagram in the vicinity of the Liquid-RII-RI triple point. Within this model the Liquid-RII and Liquid-RI transitions are found to be always strongly first order. Calculations based on this model agree qualitatively with experiments.

Simulation of multiple ordered phases in C23 n-alkane

Normal alkanes display multiple ordered phases, including an orthorhombic crystal (X) and two partially ordered rotator phases (RI and RII). The rotator phase transitions X-RI and RI-RII are of interest because they are weakly first-order, and because experiments suggest that crystalline polyethylene may nucleate via a metastable rotator phase. We have performed heating and cooling scans of all-atom NσT (isothermal, isostress) simulations of a pure C(23) solid. We find a sequence of phases, transition temperatures, structural and thermodynamic properties, all reasonably consistent with experiment, except that a monoclinic crystal is more stable in our simulations than the experimental orthorhombic structure. We find that the RI phase is well described as an orthorhombic crystal disordered by random ±90° rotations of molecules about their stem axis, and the RII phase can be represented as a loose hexagonal packing of parallel chain stems, which tend to orient with the in-plane projection of C-C bonds pointing between neighbors. To measure local orthorhombic, RI, or RII order, we define Potts- and Ising-like order parameters, from which global order parameters and correlation functions can be computed. We observe modest pretransitional fluctuations of local RI order in the RII phase near T(RI-RII), characteristic of this weakly first-order transition.

Confinement-Driven Weakening of the Rotator Phase Transitions in an Alkane through a Possible Tricritical Point

We report calorimetric and X-ray measurements in the R1, R2, and R5 rotator phases of a long chain alkane (n-tetracosane) in bulk and confined to porous matrices (PTFE and Anopore) of two different length scales. Probing the order within and normal to the layers, in the Anopore case having a mesoscopic length scale (200 nm), drastic weakening of the R2-R1 and R1-R5 transitions is seen. The effect on the latter is to such an extent that it results in the first observation of a confinement-driven second order transition in these systems. A significant reduction of the temperature range of the R1 phase is also seen in the Anopore case, a feature argued to cause the change in the order of the transition. Comparisons are also made on the recent prediction of such a point in a Landau model. These findings, while paving a new means of realizing a tricritical point, will lead to better understanding of finite size effects in alkanes.

Simple Landau model of the RIV-RIII-RV rotator phases of alkanes

Simple Landau-type free energy function is presented to describe the R(IV)-R(III)-R(V) rotator phase sequence and the transitions between them. The order parameters necessary to describe the above rotator phase transitions are identified. Varying the coupling between the order parameters, the R(IV)-R(III) and R(III)-R(V) transitions occur. Available experimental results are consistent with our model.

Thermal and Hyperthermal Collision-Energy Depositions of Transition Metal−Benzene Sandwich Complexes onto a Self-Assembled n-Octadecanethiol Monolayer

Gas-phase synthesized transition metal−benzene sandwich complexes of M(benzene)2 (M = Ti, V, and Cr) are soft-landed onto a self-assembled monolayer of n-octadecanethiol (C18−SAM) at a collision energy of 10−20 eV. The resulting adsorption states and thermal desorption kinetics of the soft-landed complexes are studied with infrared reflection absorption spectroscopy and temperature-programmed desorption. The complexes keep their native sandwich structure intact on the SAM substrate even after the “hyperthermal” deposition event. The soft-landed complexes are oriented with their molecular axes largely tilted off the surface normal of the SAM substrate and exhibit unusually large desorption activation energies (Ed = ∼130 kJ/mol). For comparison, thermal deposition (∼25 meV) of Cr(benzene)2 vapor onto the C18−SAM, carried out using a physical vapor deposition technique, showed that the complexes are weakly physisorbed (Ed = ∼70 kJ/mol) on the SAM with a random orientation. Only a hyperthermal collision event allows the incident complexes to penetrate into the SAM matrix. The desorption of the embedded complexes in the SAM is then suppressed to around room temperature and may be associated with the crystal−rotator phase transitions of the SAM matrix.

Elastic properties of the rotator phases of pentacosane C25H52

Pentacosane C25H52 exhibits two different rotator to rotator phase transitions. A phenomenological theory of the elastic properties of the rotator phases is developed on the basis of a free energy expansion. The temperature dependence of the elastic constants is calculated for the various rotator phases. We discuss the temperature variation of the heat capacity in the rotator phases in terms of the elastic constants. The theoretical predictions are found to be in good qualitative agreement with available experimental results.

Picojoule and submillisecond calorimetry with micromechanical probes

The experimental limits of micromechanical calorimetry at ambient conditions are demonstrated. In the investigation of rotator phase transitions of n alkanes we have obtained a heat sensitivity of 500 pJ for a sample mass of about 7 pg with a time resolution of 0.5 ms. The time resolution is restricted by the thermal response of the bimetallic cantilever. A problem in the precise quantification of the transition enthalpies is heat transport through the atmospheric environment. A calibration of the apparatus constant is proposed to overcome this problem.