W. Van Dael

Experimental Evidence for a Nematic to Smectic A Tricritical Point in Alkylcyanobiphenyl Mixtures

Abstract An adiabatic scanning calorimeter has been used to investigate the temperature dependence of the enthalpy and the heat capacity near the smectic A (A) to nematic (N) phase transition in mixtures of alkylcyanobiphenyl (nCB) liquid crystals. Mixtures of 8CB + 10 CB with small N-ranges undergo first-order transitions, but mixtures with large N-ranges exhibit continuous ones. A tricritical point could be located for the 10CB mole fraction of 0.3135. For a nearly tricritical mixture, a critical exponent α = 0.50 ± 0.02 has been observed. For mixtures with continuous AN transitions it is seen that this specific heat exponent α decreases with the width of the nematic range.

Heat capacity of the binary liquid system triethylamine–water near the critical solution point

The specific heat Cpx has been measured for the binary system triethylamine–water as a function of temperature near the critical solution temperature at the critical concentration. Simple power law fits of the data resulted in rather large values (around 0.25) for the critical exponent α. For a fixed value of α equal to the Ising value αI=0.125 good fits could only be obtained if more complex expressions were used in fitting the data. For this case the best result was obtained by including a correction‐to‐scaling term. Combining our result for the amplitude of the specific heat singularity with the correlation length amplitude allowed us to test two‐scale‐factor universality. Our result strongly indicates that, in contrast to earlier findings, two‐scale‐factor universality is also valid for binary systems.

Heat Capacity and Enthalpy Behavior Near Phase Transitions in Some Alkylcyanobiphenyls

Abstract An adiabatic scanning calorimeter has been used in a calorimetric investigation of alkyl-cyanobiphenyl (nCB) liquid crystals. Measurements near solid to smectic (KA), smectic A to nematic (AN) and nematic to isotropic (NI) or smectic A to isotropic (AI) phase transitions have been carried out for scanning rates as low as a few μKs−1. Results for the temperature dependence of the heat capacity as well as for the enthalpy (including latent heats) have been obtained for 8CB, 9CB, 10CB and 11CB. Latent heat values are reported for the different KA, NI and AI transitions. For the two compounds 8CB and 9CB, which show a AN transition, it was found that, within the resolution of the experiments, these transitions are second-order. The anomalous heat capacity observed near the AN transition is much larger for 9CB than for 8CB. The critical exponent α obtained from power law fits to the heat capacity data is 0.31 ± 0.03 for 8CB and 0.50 ± 0.05 for 9CB. It is suggested that the AN transition in 9CB must be...

The specific-heat anomaly in triethylamine heavy-water near the critical solution point

The anomaly in the specific heat Cpx has been investigated by means of a constant heating rate method. Accurate results could be obtained for ‖T‐Tc‖/Tc values as small as 7×10−6. From a range shrinking procedure and nonlinear‐least‐squares fits to the data we obtained α=α′=0.107±0.002, which is close to the renormalization group (RG) value αRG=0.110±0.008 for the Ising model for the dimensionality d=3. For the universal amplitude ratio A′/A of the diverging part of the specific heat above and below Tc, a value of 1.75±0.03 is obtained. This is also in agreement with theoretical predictions for the Ising model. Combining the amplitude of the correlation length with the critical amplitude of the specific heat allowed us to test two‐scale‐factor universality. For the universal constant X=Avξ30/kB in this system, a value of (1.9±0.4)×10−2 is obtained, which is in agreement with theory and with results for other fluid systems.

Sound velocity measurements in liquid argon as a function of pressure and temperature

Abstract Measurements have been made of the velocity of ultrasonic pulses in liquid argon along seven isotherms at temperatures ranging from 100 K to 150 K and pressures up to 500 atm. Combining these results with available density data the following thermodynamic quantities are calculated at integer values of p and T : the adiabatic and isothermal compressibility, the thermal expansion coefficient, the specific heats C p and C v and their ratio γ.

Measurements of the thermal conductivity of gases: I. Experimental method. Data for pure gases

Abstract A description is given of a simple apparatus suitable for measuring relative values of the coefficient of thermal conductivity of gases. High sensitivity is obtained by measuring the variable heat input required in order to maintain a constant temperature gradient between a bead thermistor and the surrounding gas. Experimental data at room temperature are given for CO 2 , Ar, N 2 , CH 4 , Ne, D 2 , He and H 2 . The deviations from the most reliable data in the literature are smaller than 0.5%. For He and H 2 the actual data are systematically higher than some of the older results but they are in substantially better agreement with the theory.

Low-frequency dielectric-dispersion and electric-conductivity near the consolute point in some binary-liquid mixtures

We report on detailed results for the dielectric constant ϵ and the electric conductivity σ near the consolute point in the binary liquid mixtures benzonitrile-isooctane, nitroethane-cyclohexane and nitrobenzene-isooctane. The temperature dependence of the amplitude of the observed low-frequency Maxwell-Wagner critical dispersion can be understood on the basis of a droplet model for the critical fluctuations. A novel type of critical anomaly was observed for the conductivity σ in the homogeneous phase for some of the samples with sufficiently low ionic impurity conduction. Its temperature dependence can be explained qualitatively by invoking also the droplet picture for the critical fluctuations.

Sound velocity measurements in liquid argon, oxygen and nitrogen

Abstract Measurements are reported on the sound velocity in saturated liquid argon, oxygen and nitrogen in the temperature range between the triple point and the critical point. These data are reduced on a molecular scale and it is shown that the properties of liquid argon deviate from a generalized scheme suitable for liquid oxygen, nitrogen and methane.

Thermal diffusion in moderately dense gas mixtures and the pair-correlation function

Abstract A new method is worked out to calculate the pressure dependence of the thermal-diffusion factor α12 according to the thermodynamics of irreversible processes using an expression for the heat of transfer given by Bearman, Kirkwood and Fixman. A comparison of the calculated values with existing and some new experimental data shows much better agreement than obtained by earlier theories.

The specific heat anomaly in some ternary liquid mixtures near a critical solution point

The anomaly in the specific heat at constant pressure and constant overall concentration Cpx has been investigated near a critical solution point in a ternary system of triethylamine–water–heavy water and in two different mixtures of triethylamine–water–ethanol. Data have been obtained by means of a standard heat pulse technique and with a constant heating rate method. Accurate results could be obtained for ‖TC−T‖/Tc values as small as 10−5. A detailed analysis of the data has been carried out by means of a nonlinear least squares fitting procedure. No indication of exponent renormalization was observed for triethylamine–water–heavy water. Instead, the value for the critical exponent as well as for the universal amplitude ratio are in close agreement with recent values obtained for the binary system triethylamine–heavy water and with the Ising values. For the two critical mixtures of triethylamine–water–ethanol exponent renormalization effects were, however, clearly visible and increased with increasing e...