Communication: Identical temperature dependence of the time scales of several linear-response functions of two glass-forming liquids

The frequency-dependent dielectric constant, shear and adiabatic bulk moduli, longitudinal thermal expansion coefficient, and longitudinal specific heat have been measured for two van der Waals glass-forming liquids, tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl-4-ether. Within the experimental uncertainties the loss-peak frequencies of the measured response functions have identical temperature dependence over a range of temperatures, for which the Maxwell relaxation time varies more than nine orders of magnitude. The time scales are ordered from fastest to slowest as follows: Shear modulus, adiabatic bulk modulus, dielectric constant, longitudinal thermal expansion coefficient, and longitudinal specific heat. The ordering is discussed in light of the recent conjecture that van der Waals liquids are strongly correlating, i.e., approximate single-parameter liquids.     

Intermolecular hydrogen bonds: Comparison of associates in crystals and liquids

The density and sound velocity were measured within wide ranges of temperature for a number of liquids (water, formamide, diols, aliphatic alcohols, cellosolves, ketones), in which various types of H-associate can exist. The temperature dependences of the adiabatic and molar adiabatic compressibility, density, and molar volume are analyzed. The values of the molar adiabatic compressibility permit one to evaluate the dimensionality of H-associates existing in liquids; the values of adiabatic compressibility do not offer this possibility. The terms responsible for the similarity and difference between H-associates in crystals and liquids are discussed.     

Small scale adiabatic dewar techniques to determine exothermic onset temperatures

The heightened focus on safety in the chemical process industry has resulted in a growing trend to provide reliable thermochemical information before process scale-up. The exothermic onset temperature of compounds is of key concern. The small scale adiabatic dewar age (SSAD) test, developed at Merck Research laboratories, is used to determine exothermic onset temperatures. These tests are conducted under nearly adiabatic conditions using specially designed dewar sample cells and commercially available programmable calorimeters. The dewar cell method offers a rapid and accurate method to enhance standard test procedures and allows measurement of auto catalytic behavior using small samples.The SSAD technique is applicable to both solids and liquids and has the following advantages: 1) small sample size, 2) ability to handle compounds in which exothermic activity is accompanied by large pressure increases without damaging experimental equipment, 3) accurate determination of exothermic onset temperature, 4) ease of experiment set-up, 5) ease of data interpretation and 6) rapid experimental turn around time.The determination of the exothermic onset temperature using the SSAD technique will be presented and compared to data obtained using standard and isothermal age techniques.     

Heating of polymers during neck propagation

The heating of polyethylene terephthalate, polyamide-66, and polyamide-6 during tensile drawing at room temperature was studied theoretically and experimentally. At a low draw rate, the necking temperature was close to the temperature of the surrounding air. An increase in the rate results in the transition to the adiabatic conditions of drawing. A necking temperature of 140°C was experimentally recorded in polyethylene terephthalate at a draw rate of 1000 mm/min and during the approach to the adiabatic conditions of drawing. A formula describing the dependence of the necking temperature on the draw rate was derived. The resulting value agreed fairly well with the theoretical estimation of the temperature. The drawing (strain) ratio in the neck and the draw stress are the crucial parameters determining the temperature. The rate of the transition to the adiabatic conditions of drawing was determined. The temperatures of adiabatic heating for various polymers were calculated. The increases in the temperatures of polycarbonate and low- and high-density polyethylene are relatively low. The increases in temperature can be regarded as moderate for polypropylene and polyvinyl chloride, while they attain the highest values in polyamide-6 and polyethylene terephthalate owing to the high draw ratios in the neck and the high draw-stress values.     

Low-temperature heat capacities of 1-alkyl-3-methylimidazolium bis(oxalato)borate ionic liquids and the influence of anion structural characteristics on thermodynamic properties

Two chelated orthoborate ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(oxalato)borate ([Bmim][BOB]) and 1-hexyl-3-methylimidazolium bis(oxalato)borate ([Hmim][BOB]), were prepared and characterized. Their thermodynamic properties were studied using adiabatic calorimetry and differential scanning calorimetry (DSC). The thermodynamic properties of the two ILs were evaluated and compared with each other, and then with those of other [Bmim] type ILs. The results clearly indicate that for a given cation (or anion) and at a certain temperature, the more atoms in the anion (or cation), the higher the heat capacity; the higher glass-transition temperatures of [BOB] type ILs than others are mainly caused by the higher symmetry of the orthoborate anion structure. It is suggested that a high content of strong electronegative atoms and C(n) or C(nv) (n = 1,2,3,…,∞) point group symmetry in the anion are favorable for the design and synthesis of room temperature ILs with a wide liquid range.     

Inversion of the Temperature Coefficient of Internal Pressure and Structural Organization of Liquid Phase Systems

Inversion of the temperature coefficient of internal pressure in liquid phase systems is considered. Both temperature and concentration inversions of the coefficient are shown to be possible. Details of the temperature inversion are discussed. This type of inversion is characteristic of only associated hydrogen-bonded liquids and is due to weaker association of molecules.     

Effect of solvents on the cryogenic dynamic mechanical properties of polystyrene

Young's modulus and mechanical damping of 15 organic liquids in polystyrene have been measured from 4°K to 250°K. The concentration was generally in the range from 10 to 15%, but the polystyrene–toluene system was investigated over the range from 0 to 16%. Some liquids cause the 40°K damping peak of polystyrene to disappear, other liquids do not. Seven of the liquids which cause the disappearance of the 40°K peak give rise to new large damping peaks at the temperature expected for the secondary glass transition temperatures of the liquids, that is, at 0.77 T_g of the liquids. Some of the liquids produced large unexplained damping peaks at temperatures above the expected glass transition temperatures T_g of the liquids. It is suggested that the γ peak in polystyrene is caused by styrene monomer.     

Temperature dependence of the Landau-Placzek ratio in glass forming liquids

Here, we studied Rayleigh-Brillouin light scattering in ten different glass-forming liquids (α-picoline, toluene, o-toluidine, ethanol, salol, glycerol, dibutyl phthalate, o-terphenyl, propylene carbonate, and propylene glycol). For each of these liquids it was found that the Landau-Placzek ratio is in a good agreement with the theory at high temperatures and significantly exceeds the theoretical prediction below a certain temperature. Transition between the two temperature regimes occurs near T(A), where T(A) is crossover point from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature. Increase of the Landau-Placzek ratio relative to the theoretical prediction below T(A) seems to be the universal feature of glass-formers. We suggest that formation of locally favored structures in liquids below T(A) causes observed excess of the Landau-Placzek ratio.     

Adiabatic pulses in 1H-15N direct and long-range heteronuclear correlations

The application of adiabatic inversion pulses to the detection of (1)H-(15)N heteronuclear correlations is described. The pulse sequences studied were gHSQC, CRISIS-gHSQC, gHMBC and CRISIS-gHMBC. The poor inversion quality of rectangular 180 degrees X pulses can lead to a loss of signal at the peripheries of the spectrum. Replacing these pulses with adiabatic sweeps significantly improves sensitivity across the potentially large (15)N spectral window. Satellite spectrum profiles are shown to demonstrate the increase in sensitivity when employing adiabatic pulses on wide spectral widths. Additionally, the active pharmaceutical nizatidine was used as a model compound to demonstrate the improvements in the long-range correlation data.     

On the performance of thermostable electrowetting agents

Electrowetting of ionic liquids (ILs) in different alkane ambient and at different temperatures were studied under different AC voltages. The performance of IL‐based electrowetting at elevated temperatures and that at room temperature was compared. It turns out that the electrowetting of ILs at elevated temperatures resulted in faster response and lower operation voltage. Furthermore, the relationship between the structure of the ion and the contact angle change was specifically investigated and theoretically explained. Finally, the properties of IL‐based electrowetting in different alkane ambient were studied. It was demonstrated that the long length of the alkyl chain guaranteed a smaller contact angle under the same applied voltage. All the experimental results showed attractive features involving wide operating temperatures and in particular high stability, fast response, and good reversibility at elevated temperatures. The performance of IL‐based electrowetting could be further improved through an optimal choice of an ambient phase or a rational design and synthesis of ionic liquids. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular dynamics study of the surface tension of a binary immiscible fluid

The planar interface between two liquids having two degrees of affinity to mix has been studied by molecular dynamics simulations. The surface tension is calculated from the normal, PN, and transverse, PT, components of the pressure tensor P for a wide range of temperatures. An unusual increase in surface tension with increasing temperature is attributed to a pressure induced void transfer mechanism that is justified by basic thermodynamic arguments. This effect is diminished on the addition of a modest attractive potential between the two species, and there is a turnover point at higher temperatures beyond which the surface tension decreases with increasing temperature. An order parameter is identified as the gradient of the mole fraction distribution through the interfacial region. An additional effect is the dramatic inversion of the kinetic and potential contributions to the PN profile as the temperature is varied. It is found that a commonly used approximation for P, the Irving-Kirkwood 1 or IK1 method, results in a relatively modest unphysical variability in PN that weakly violates the condition of local mechanical stability. However, this artifact does not prevent the IK1 method from producing an interfacial tension which is nearly identical to that derived from the complete IK formula with no additional approximations.     

Temperature dependence of the electrical conductivity of imidazolium ionic liquids

The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10(7) Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion.     

Dynamics in supercooled ionic organic liquids and mode coupling theory analysis

Optically heterodyne-detected optical Kerr effect experiments are applied to study the orientational dynamics of the supercooled ionic organic liquids N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) and 1-ethyl-3-methylimidazolium tosylate (EMImTOS). The orientational dynamics are complex with relaxation involving several power law decays followed by a final exponential decay. A mode coupling theory (MCT) schematic model, the Sj?gren model, was able to reproduce the PMPIm data very successfully over a wide range of times from 1 ps to hundreds of ns for all temperatures studied. Over the temperature range from room temperature down to the critical temperature Tc of 231 K, the OHD-OKE signal of PMPIm is characterized by the intermediate power law t(-1.00+/-0.04) at short times, a von Schweidler power law t(-0.51+/-0.03) at intermediate times, and a highly temperature-dependent exponential (alpha relaxation) at long times. This form of the decay is identical to the form observed previously for a large number of organic van der Waals liquids. MCT analysis indicates that the theory can explain the experimental data very well for a range of temperatures above Tc, but as might be expected, there are some deviations from the theoretical modeling at temperatures close to Tc. For EMImTOS, the orientational dynamics were studied on the ps time scale in the deeply supercooled region near its glass transition temperature. The orientational relaxation of EMImTOS clearly displays the feature associated with the boson peak at approximately 2 ps, which is the first time domain evidence of the boson peak in ionic organic liquids. Overall, all the dynamical features observed earlier for organic van der Waals liquids using the same experimental technique are also observed for organic ionic liquids.     

Effect of surface on the thermodynamic properties of polar liquids and liquid crystals trapped in a porous medium

The heat capacity of some polar liquids (2,6-lutidine, aniline and nitrobenzene) and liquid crystals (nitrophenyldodecyloxybenzoate and dodecylcyanobiphenyl) trapped in glass and nickel porous matrices have been measured by using a high-resolution adiabatic calorimeter. A decrease in the molar heat capacity of substances trapped in porous media compared with the bulk molar heat capacity was observed. A new phenomenon caused by the effect of the glass surface on the confined fluid was revealed. This phenomenon was manifested as an unusual behaviour of the heat capacity versus temperature curve (a ‘crevasse’ in a curve). In addition, we observed the nematic phase of dodecylcyanobiphenyl liquid crystal induced by the variable curvature of the glass matrix surface.     

Energetics of left-handed helix formation of nucleic acids

The inversion of canonical right-handed double helical conformations of nucleic acids (B-DNA, A-RNA) into the left-handed form, due to changes in the polymer environment, can be followed by a variety of physical methods such as UV-, CD-, or NMR-spectroscopy. The only quantitative approach to the evaluation of the energy parameters in based on adiabatic scanning microcalorimetry. The results presented in this paper demonstrate that the inversion process is accompanied by an enthalpy change of about 5 KJ per mole basepairs and an entropy change of about 14 J per degree and mole basepairs. The inversion temperature for the thermally induced transition is about 45°C in almost all cases. This holds both for DNA- as well as RNA-sequences.     

The temperature dependence of the surface tension of monatomic liquids and the boiling transition

We have reviewed recent model theories of the surface tension and examined the data on the temperature dependence of the surface tension of elemental liquids. From this, we have been able to show that the surface tension of these liquids vary linearly with temperature with the linear coefficient being related to both the transition temperatures at melting and at boiling. We use this to show that the boiling transition temperature may be expressed in a form which was previously proposed by us in a general phenomenological theory of phase transitions involving quasi-particles.     

Effect of reaction type on TMRad,TD24 and other data obtained by adiabatic calorimetry

Journal of Thermal Analysis and Calorimetry - Time to maximum rate under adiabatic condition (TMRad) and the corresponding initial temperatures such as TD24 (the initial temperature when TMRad...     

Determination of the Phase Inversion Temperature of Orange Oil/Water Emulsions by Rheology and Microcalorimetry

Abstract

In low-energy emulsification processes, phase inversion occurs when the phases of a dispersion exchange, because of changes in the medium's properties. This paper reports experiments to determine the phase inversion temperature (PIT) of orange oil/water emulsions stabilized by nonionic surfactants. Two techniques were employed: rheology, which is already commonly used to obtain the PIT, and microcalorimetry, which has been proposed as a new technique. Continuous monitoring of the emulsions' viscosity permitted identifying different phenomena that occur while the temperature varies. For all the dispersions prepared, the rheological curves obtained showed two peaks, one attributed to the phase separation process and the other to the phase inversion phenomenon. The microcalorimetry technique showed two endothermic transitions as the dispersion's temperature increased. The initial temperatures were comparable to those obtained by rheology. The influence of the surfactant concentration and the hydrophilic-lipophilic balance (HLB) of the mixture of surfactants and the reduction in volume of the phases at the phase inversion temperature were also evaluated. In general, both methods used to evaluate the phase inversion of the orange oil/water systems (rheology and microcalorimetry) presented concordant results, both for the phase separation process and the phase inversion temperature.     

Thermochemical properties of 1-butyl-3-methylimidazolium nitrate

Heat capacity for 1-butyl-3-methylimidazolium nitrate [C₄mim][NO₃] in the temperature range (5–370) K has been measured by adiabatic calorimetry. Temperatures and enthalpies of its phase transitions have been determined. Thermodynamic functions have been calculated for the crystalline and the liquid states. Phase transition temperatures for set of nitrate salts have been compared. Enthalpy of combustion and enthalpy of formation for crystalline [C₄mim][NO₃] have been determined using a static-bomb isoperibol combustion calorimeter. A correlation scheme for the estimation of C_p of ionic liquids has been developed.     

Ferroelectric phase transitions near ionic liquid/vacuum interfaces

A simple theoretical model is developed describing ionic liquids as regular solutions. The separation of these ionic mixtures is studied on the base of the Cahn-Hilliard theory coupled with electrostatics. It is shown that the ionic liquids decompose to thin layers of oppositely charged liquids at low temperatures. At larger temperatures the separation occurs only near the ionic liquid/vacuum surface, thus explaining the oscillatory-decaying structure of the electric double layer observed via computer simulations. In contrast to noncharged liquids the ionic ones exhibit two critical temperatures, where the temperature coefficients of all characteristic lengths possess singularities. These second order ferroelectric phase transitions are possible explanations of the experimentally measured via light scattering peculiar temperature dependence of the interfacial dipole moment density on several ionic liquid/vacuum interfaces.