Phase Transitions Of N-alkanes Research Articles

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.

An investigation of the thermodynamic properties of long-chain molecular crystals and the possibility of their application as phase change materials

The thermodynamic properties of n-alkanes have been investigated by using differential scanning calorimetry. The impact of the chain length on the thermodynamic characteristics of the first order phase transitions in n-alkanes has been estimated. The n-alkanes homologous series has been analyzed using the theory of diffused phase transitions. The nuclei sizes of a new phase within the bulk of the initial phase have been estimated.

Ortho-Positronium in Alkanes under High Pressure

Phase transitions in n-alkanes were observed by using the positron annihilation lifetime method. In the case of alkanes, application of pressure is equivalent to temperature lowering (1 K corresponds to ≈ 4 MPa). When argon at high pressure penetrates the sample one observes a non-monotonous dependence of the melting point on pressure. This effect is observed also in even-numbered alkanes, in which there is no rotator phase.

The phase transitions of n-alkanes in mesoscopic pores of graphite

The phase transitions of two n-alkanes of different parity, C 16H 34 and C 17H 36, confined in the mesoscopic pores of exfoliated graphite have been studied by differential scanning calorimetry. For samples from one statistical monolayer up to about four layers of paraffin, microcalorimetry shows a characteristic signature from the “surface phase” alone. At higher fillings the melting and phase transition temperatures and enthalpies of the additional “included” phase were depressed compared to those of the bulk paraffin. The depressions in the phase transition temperatures and the neperian logarithm of the enthalpies are to a good approximation inversely proportional to the number of included layers. Depending on the coverage, competing effects between the graphite surface and the degree of filling of the pores are observed for both the surface phase and the included phase. The results for the included phase are consistent with the Gibbs–Thompson equation down to pore sizes of the order of a few nanometres.

Phase transitions of n-alkanes as rotator crystals

Phase transitions of n -alkane homologues with n =17–24, their solid solutions, two-phase mixtures and multicomponent mixtures with n =17–37, of biological, geological and technological origin have been studied by high-temperature X-ray powder diffraction. In keeping with the rotator nature of n -alkanes, their thermal deformations and polymorphic transformations are discussed as a function of the thermal torsional motion of the molecules. We have shown that not only orthorhombic n -alkanes but also triclinic n -alkanes undergo consistent phase transitions from the crystal state (cryst) to the low-temperature (rot.1) and high-temperature (rot.2) rotator states. When molecules (atoms) of different kinds combine in a structure one more rotator state of n -alkanes (rot.1+2), intermediate between the low-temperature (rot.1) and high-temperature (rot.2) rotator states, was identified. Each of these states is characterized by a specific form of the molecular thermal oscillation motion. The existence of crystal phase V and rotator phase RV is discussed on the basis of X-ray powder diffraction and literature data. Phase transitions of the mixtures of two crystal phases were shown to depend on the molecular symmetry (parity) of the mixed components and the difference in chain length (Δ n ). The distinguishing feature of the phase transition to the rotator state of multicomponent mixtures is step-wise phase separation of the solid solution during heating.

High-resolution calorimetry: new perspectives for the study of phase transitions.

Only 7 pg of substance are needed to measure solid-solid phase transitions in n-alkanes with a new type of calorimeter (see schematic picture). This corresponds to a transition heat of only 500 pJ. The small time constant of the calorimeter additionally permits the observation of dynamic effects in the examined phase transitions.

Thermal analysis using a micromechanical calorimeter

A fast and extremely sensitive method for reversible thermal analysis of picoliter volumes of solid samples is presented. Using a micromechanical calorimeter based on the deflection of a bimetallic cantilever, enthalpy changes at phase transitions in n-alkanes (paraffins) are determined. The key role of volume changes associated with phase transitions is discussed together with a method to separate thermal and volume-induced stresses associated with these phase transitions.

Correlation of the solubility of even-numbered paraffins C 20H 42, C 24H 50, C 26H 54, C 28H 58 in pure hydrocarbons

Solubilities of C 20, C 24, C 26, C 28, n-paraffins (eicosane, tetracosane, hexacosane, octacosane) in heptane and cyclohexane have been measured for temperatures ranging from their melting points to 275 K. The experimental results are compared with values calculated by means of the Wilson, UNIQUAC and two modified NRTL equations, utilizing parameters taken from solid—liquid equilibria. The root mean square deviations of the solubility temperatures for all measured data vary from 0.11 to 1.0 K and depend on the particular equation used. The best correlation was obtained with the UNIQUAC and the modified NRTL2 equations. The existence of solid—solid first-order phase transitions in n-alkanes has been taken into consideration in the solubility curve calculations.

高級奇数 n-アルカンの固相転移にともなう振動スペクトルの変化

The temperature dependences of the longitudinal acoustic mode (LAM) frequency and the splitting of the methylene rocking mode accompanied with the A-B-C solidsolid phase transitions of higher n-alkanes, C_ , C_ , C_ , C_ and C_ , were measured by Raman and infrared spectra. The LAM frequency became constant in the B phase after a gradual decrease with the increase of temperature and fell in a lower constant value near the B-C transition temperature. The temperature dependence of the splitting of the methylene rocking mode was essentially the same with that of the distance between the nearest neighbor chains in a sub-cell. The character of the phase transitions ofn-alkanes was discussed.

Theory of Orientational Order in Chain Molecules –Phase Transitions in n-Alkanes and Lipid Membranes–

By assuming an attractive potential between molecules which depends on their orientations and configurations, an orientational phase transition accompanied with a configurational change of molecules are studied in mean field approximation. A “two state model”, in which only the state of “long” configuration causes the orientational order, is assumed. The phase transition temperature T c , the orientational order parameter S , the averaged extension of molecules, and the transition entropy are calculated as functions of the statistical weight of “long” configuration. The results are investigated with respect to the chain-length dependences. By analysing thermal data, the orientational order parameters adjacently below the transition temperature are estimated as S ∼0.95 for n-alkane crystals and as 0.7–0.8 for lipid membranes.