Search results for "Phase Transition"
showing 10 items of 1281 documents
Molecular dynamics in liquid crystalline N-acylated oligoethylenimines as studied by broadband dielectric spectroscopy
1996
The molecular dynamics and the charge transport in six liquid crystalline N-acylated oligoethylenimines were investigated by dielectric spectroscopy. The dielectric spectra in the temperature range from 125 K up to 400 K and in the frequency range between 10 -2 Hz and 10 7 Hz could be described by a conductivity contribution and three relaxation processes. The molecular assignment of the α-process is a relaxation of the rigid benzamide unit. Its mean relaxation time is influenced by the length of the alkoxy side chains, the spacer length (ethylene or propylene groups) and the neighboring repeating units. In the columnar mesophase this process is cooperative due to sterical hindrance. A norm…
Ellipsometric studies of behenic acid at the air-water interface
1991
The rich phase behaviour of monolayers of behenic acid (=docosanoic acid) has been analyzed by null ellipsometry in isothermic and isobaric measurements. By using structural data known from x-ray measurements the anisotropy and refractive indices in the vicinity of the S-CS phase transition have been calculated with a minimum of model assumptions. The calculated values are: nsx=1.47. ncsx=1.48. nsz=1.54. ncsz=1.55. and anisotropy γ=nx/nz ≈0.95. It is also shown that ellipsometry is a very sensitive and convenient method of detecting phase transitions. Its value for quantifying the roughness of the monolayer in the close vicinity of phase transitions due to thickness and density fluctuations…
Optical measurements of an acylated azacrown at the air-water interface
1991
In this study, a cyclame derivative bearing four aliphatic chain substituents shows, like many amphiphiles, solid-condensed as well as liquid-expanded phases. In contrast to the classical amphiphiles, the onset of its phase transition is characterized by a bump-like shape whose amplitude is a function of the compression speed. Ellipsometry which is very sensitive to the monolayer physical state changes, and fluorescence microscopy which has contributed significantly to the understanding of the phenomena occurring in the phase transition region have been used to investigate the monolayer behavior of this compound. This study shows that in the liquid-expanded state, the film is homogeneous an…
Water-Hydrophobic Zeolite Systems
2012
Water intrusion-extrusion in hydrophobic microporous AFI, IFR, MTW and TON pure silica zeolites (zeosils) has been investigated through molecular dynamics (MD) simulations. It was found that intruded water volumes correlate with the free volume of the zeosil unit cells. Calculated adsorption isotherms allowed us to estimate the amounts of water intruded, and deviations from experiments (lower experimental with respect to calculated intrusion pressures) have been;explained in terms of connectivity defects in the synthesized materials. Water phase transitions in defectless zeosils occur in a narrow range at high pressure. On the basis of a simple model, we derived a thermodynamic equation tha…
Guest Modulation of Spin-Crossover Transition Temperature in a Porous Iron(II) Metal Organic Framework: Experimental and Periodic DFT Studies
2014
The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin-crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)(4)]} (1) with five-membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin-crossover transition with hysteresis loops 14-29 K wide and average critical temperatures T-c=201 K (1.fur), 167 K (1.pyr), and 114.6 K (1.thio) well below that of the parent compound 1 (T-c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1.fur, while 1.pyr and 1.thio show 50% spin transition. For 1.fur the transformation between the HS and IS (mi…
Transitions of tethered chain molecules under tension
2014
An applied tension force changes the equilibrium conformations of a polymer chain tethered to a planar substrate and thus affects the adsorption transition as well as the coil-globule and crystallization transitions. Conversely, solvent quality and surface attraction are reflected in equilibrium force-extension curves that can be measured in experiments. To investigate these effects theoretically, we study tethered chains under tension with Wang-Landau simulations of a bond-fluctuation lattice model. Applying our model to pulling experiments on biological molecules we obtain a good description of experimental data in the intermediate force range, where universal features dominate and finite…
Grafted Rods: A Tilting Phase Transition
1996
A tilting phase transition is predicted for systems comprising rod like molecules which are irreversibly grafted to a flat surface, so that the non interacting rods are perpendicularly oriented. The transition is controlled by the grafting density $\rho$. It occurs as $\rho$ increases as a result of the interplay between two energies. Tilt is favoured by the van-der-Waals attraction between the rods. It is opposed by the bending elasticity of the grafting functionality. The role of temperature is discussed, and the tilting mechanism is compared to other tilting transitions reported in the literature.
Independent ion migration in suspensions of strongly interacting charged colloidal spheres
1999
We report on sytematic measurements of the low frequency conductivity in aequous supensions of highly charged colloidal spheres. System preparation in a closed tubing system results in precisely controlled number densities between 1E16/m3 and 1E19/m^3 (packing fractions between 1E-7 and 1E-2) and electrolyte concentrations between 1E-7 and 1E-3 mol/l. Due to long ranged Coulomb repulsion some of the systems show a pronounced fluid or crystalline order. Under deionized conditions we find s to depend linearily on the packing fraction with no detectable influence of the phase transitions. Further at constant packing fraction s increases sublinearily with increasing number of dissociable surfac…
<title>Dielectric response to low and infra-low frequency in layered ferroelectrics</title>
2006
Features of dielectric response to frequencies in the 0.25.1000 Hz range in ceramic samples of BaBi2Nb2O9, Na0,5Bi8.5Ti2Nb4O27, Na0,5Bi8.5Ti2Ta4O27, and K0,5Bi8.5Ti2Nb4O27 compounds are reported. Considerable relaxation of e'(T) and e"(T) maximums has been observed at all frequencies in BaBi2Nb2O9. Na0,5Bi8.5Ti2Nb4O27 and Na0,5Bi8.5Ti2Ta4O27 are characterised by broad maximums on e'(T) at frequencies below 100 Hz and a weak anomaly in K0,5Bi8.5Ti2Nb4O27 at temperatures well below the paraelectric phase transition. Increasing values of e' are observed at low temperatures in compounds containing sodium. The observed features of dielectric response are explained by largescale charge fluctuatio…
Dielectric Response in (K0.5Na0.5) (Nb1-xSbx)O3+0.5 mol%MnO2Ceramics to Weak Sinusoidal Fields of Low and Infra-low Frequencies
2011
The thermal anomalies exhibited as “shoulders” at infra-low frequencies and maximums at low frequencies related to structural transitions in the polar phase of (K0.5Na0.5)(Nb1-xSbx)O3+0.5 mol%MnO2 are found to shift from T ∼ 185°C to T ∼ 140°C at changing concentration x from 0 to 0.05. The range of temperature ΔТ(х) corresponding to the difference between positions of the anomalies on the ϵ′(T) curve at frequencies 1000 Hz and 1 Hz changes from 0.5 degrees at x = 0 to 2.5 degrees at x = 0.05, which may point to relaxor properties of the given phase transition being present only at rather high concentrations of Sb.