Excess compressibility in binary liquid mixtures

Brillouin scattering experiments have been carried out on some mixtures of molecular liquids. From the measurement of the hypersonic velocities we have evaluated the adiabatic compressibility as a function of the volume fraction. We show how the quadratic form of the excess compressibility dependence on the solute volume fraction can be derived by simple statistical effects and does not imply any interaction among the components of the system other than excluded volume effects. This idea is supported by the comparison of the experimental results with a well-established prototype model, consisting of a binary mixture of hard spheres with a nonadditive interaction potential. This naive model turns out to be able to produce a very wide spectrum of structural and thermodynamic features depending on values of its parameters. An attempt has made to understand what kind of structural information can be gained through the analysis of the volume fraction dependence of the compressibility.     

Surface tension of binary liquid mixtures

Experimental measurements of the liquid-air interfacial tension are reported for the systems benzene + n-hexane at 20°C and acetone + isooctane at 25°C. The excess surface tension for both systems is negative while the surface tension itself for the second system, when plotted against composition, exhibits a flat minimum. An attempt is made to interpret this behavior in terms of basic thermodynamic quantities of the mixtures such as the excess free enthalpy, the heat of mixing, the excess volume, and the isothermal compressibility.     

Thermal conductivity of binary liquid mixtures

A semi-empirical method is developed for the prediction of the thermal conductivity of binary liquid mixtures. The proposed method is tested by calculating the thermal conductivity of twelve binary liquid mixtures and an excellent agreement between the observed and calculated values is obtained.     

Investigation of benzene-hexafluorobenzene dynamics in liquid binary mixtures

The structure and microscopic dynamics of liquid mixtures of benzene and hexafluorobenzene at room temperature and several compositions have been studied by molecular-dynamics simulations. In this implementation we have rescaled the intermolecular H-F cross potential parameters obtained from the Lorentz-Berthelot combining rules, in order to avoid the substantial overestimation of the energy of mixing predicted by the model when the usual rules are employed. We found that a reduction in the strength of cross H-F interactions by 50% relative to the geometric mean is required in order to get a good agreement with experiments. Radial-angular pair-correlation functions between like and unlike species have been computed and analyzed, by comparing them with the correlations in the corresponding neat liquids. We have also studied the microscopic intermolecular momentum transfer, by computing the time correlation function between the initial velocity of a central molecule and later velocities of neighboring molecules. Structural and dynamical information extracted from the mentioned functions seem to be consistent with the picture of relatively long-lived benzene-hexafluorobenzene (Bz-Hf) complexes present in the mixtures, which would be responsible for the considerable perturbation of the structure in the first shell of like species, and would be moving within the liquid in a parallel face-to-face configuration. Using the tools developed originally to estimate hydrogen-bond lifetimes in liquids, we have computed the lifetimes of the Bz-Hf complexes as a function of the mixture composition, by two different methods: the direct time-averaging scheme and from the autocorrelation function of bond occupation numbers. The obtained lifetimes are strongly dependent on the scheme chosen to compute the characteristic times. We have obtained for the Bz-Hf dimer in solution, at room temperature, lifetimes in the range of 30-40 ps from averaging schemes and around 60-120 ps from autocorrelation function methods. In the latter case, the longest times correspond to the equimolar mixture.     

Sorption properties of celluloses in binary liquid mixtures

The adsorption capacity for methanol-benzene andn-propanol-water mixtures was measured on two kinds of microcrystalline celluloses and cotton linters. Excess adsorption isotherms were obtained; the free energy of displacement was calculated from the isotherms. The enthalpy of displacement for one crystalline cellulose was measured by flow calorimetry. The entropic function of displacement was derived from the enthalpy and free energy of displacement. The preferential adsorption of methanol andn-propanol was characterized by the adsorption equilibrium constant.On leave from the Department of Physical and Colloid Chemistry, Wageningen Agricultural University, The Netherlands     

Correlation of viscosity of binary liquid mixtures

A new two-parameter model presented in the previous work for correlating the viscosity of pure liquids is extended to correlate the viscosity of binary liquid mixtures. The correlation relates this non-equilibrium property to some equilibrium properties, the volume, the internal energy of vaporization and their excess properties. The viscosities of 47 binary liquid mixtures, including different kinds, which having no maximum or minimum viscosity, which having one maximum or one minimum viscosity, and which having both of one maximum and one minimum viscosities in the whole composition range, are correlated using the model and compared with data reported in the literature with the overall average absolute deviation of 1.05%. Further comparison with a good correlation, a free-volume type correlation, is also discussed. The results show that the new method is satisfactory.     

Nonmonotonic composition dependence of vibrational phase relaxation rate in binary mixtures

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time tau(c) at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation [] with tau(c) indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition chi(A) dependence of the dephasing time tau(v) is found to be primarily due to the nonmonotonic chi dependence of tau(c), rather than due to a similar dependence in the amplitude of Delta omega(2)(0). The probability distribution of Delta omega shows a markedly non-Gaussian behavior at intermediate composition (chi(A) approximately =0.5). We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity eta(*) with that of tau(v) and tau(c). It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.     

Thermal analysis of binary liquid crystalline mixtures

Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.     

Six-dimensional vibrational analysis of coupled intermolecular vibrations in a binary cluster

We report on full- (six-) dimensional calculations of the intermolecular vibrations of a binary aromatic-solvent cluster. An exact Hamiltonian for this kind of interaction is modified in a general manner in order to perform calculations of molecules without symmetry. The binary cluster phenol(H2O)1 is used as a test case since its intermolecular vibrations are anharmonic and highly coupled. The formulation of the Schrodinger equation leads to a complex-valued eigenvalue problem with a dimension larger than two million, which is solved by filter diagonalization to obtain both eigenvalues and eigenvectors. With the knowledge of the eigenvectors, an interpretation of all eigenvalues is possible by a characterization with pseudoquantum numbers that are related to the widely used nomenclature of intermolecular normal motions in aromatic(solvent) clusters.     

Critical resistivity in impurity doped binary liquid mixtures

The electrical resistance of the critical binary liquid system C₆H₁₂₊(CH₃CO)₂O is measured both in the pure form and when the system is doped with small amounts (≈ 100 ppm) of H₂O impurities. Near T_c, the resistance varies as dR/dT = A₁+A₂ (T-T_c)^-b with b ≈ 0.35. Neither the critical exponent b nor the amplitude ratio A₁/A₂ are affected by the impurities. A sign reversal of dR/dT is noticed at high temperatures T ? T_c.     

Calorimetric investigations of hydrogen-bonded liquid crystal binary mixtures

Journal of Thermal Analysis and Calorimetry - Double hydrogen-bonded liquid crystals formed between methyl malonic acid (MM) and p-n-alkyloxy benzoic acids (nBAO) are characterized. Variation in...     

Nematic-isotropic phase transition of binary liquid crystal mixtures

We experimentally studied the nematic-isotropic phase transition of (a) binary mixtures consisting of nematic and racemic liquid crystals and (b) binary mixtures consisting of positive and negative dielectric liquid crystals. We observed that the phase transition temperature is very sensitive to the chemical structures of the constituent components. We also used Maier-Saupe theory to calculate the transition temperature of binary mixtures. By fitting the experimental data, we obtained the interaction coupling constant between the constituent components.     

Microscopic theory for interface fluctuations in binary liquid mixtures

Thermally excited capillary waves at fluid interfaces in binary liquid mixtures exhibit simultaneously both density and composition fluctuations. Based on a density functional theory for inhomogeneous binary liquid mixtures we derive an effective wavelength dependent Hamiltonian for fluid interfaces in these systems beyond the standard capillary-wave model. Explicit expressions are obtained for the surface tension, the bending rigidities, and the coupling constants of compositional capillary waves in terms of the profiles of the two number densities characterizing the mixture. These results lead to predictions for grazing-incidence x-ray scattering experiments at such interfaces.     

Chirality-induced phase transitions in some liquid crystalline binary mixtures

Novel binary mixtures have been prepared between an optically active antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxy-biphenyl-4-carboxylate, and an optically active twin liquid crystal, (R)-3-methyladipic acid bis[4-(5-octyl-2-(pyrimidinyl)phenyl] ester, and the liquid crystalline properties investigated. The stability of each liquid crystal phase was found to decrease by mixing these two liquid crystalline materials. Furthermore, a phase diagram between these compounds showed a clear discontinuity in phase sequences. These results indicate that the liquid crystal phases are different in nature between these materials. The mixture consisting of the antiferroelectric material (40 per cent) and the twin material (60 per cent) shows an unusual liquid crystal phase, where the texture is similar to that reported for the twist grain boundary (TGB) phase. Related binary mixtures have been prepared between optically active or racemic materials, where the chirality of the system is expected to be altered systematically. The TGB phase was found to be induced only in the mixture between the optically active materials. Two kinds of effect on the appearance of the TGB phase, i.e. a strong helical structure induced by the optically active twin liquid crystal and a decrease of the smectic layer strength achieved by mixing between two types of liquid crystalline materials, are discussed.     

Molecular solvent/ionic liquid binary mixtures: designing solvents based on the determination of their microscopic properties

This work aims at analyzing the microscopic features of binary solvent systems formed by a molecular solvent (dimethylsulfoxide or acetonitrile or methanol) and an ionic liquid cosolvent (ethylammonium nitrate). The empirical solvatochromic solvent parameters E _T(30), π*, α and β were determined from the solvatochromic shifts of adequate indicators. The behavior of the solvent systems was analyzed according to their deviation from ideality. The study pays particular attention to the identification of solvent mixtures with relevant solvating properties. We have focused our attention on the most remarkable microscopic property of the explored systems, acidity, selecting three mixtures with particular hydrogen bond donating characteristics.     

Competitive hydrogenation of binary olefin mixtures in the liquid phase

The effect of a series of alcohol solvents on the selectivity of the competitive catalytic hydrogenation of 1-hexene and 2-methyl-3-butene-2-ol on Pt/SiO₂ catalyst has been investigated. With increasing molecular mass of the alcohol, the rate of hydrogenation of 1-hexene becomes relatively preferred; the same was observed also with branched alcohols compared with those having a straight chain. Systems were found in which the degree of coating of the platinum catalyst with copper affected (or did not affect) the selectivity of the competitive hydrogenation of pairs of olefinic substrates.

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— — 1- 2--3--2- Pt/SiO₂ . 1-, . , ( ) .

     

Controlling the formation of capillary bridges in binary liquid mixtures

We study the formation of capillary bridges between micrometer-sized glass spheres immersed in a binary liquid mixture using bright field and confocal microscopy. The bridges form upon heating due to the preferential wetting of the hydrophilic glass surface by the water-rich phase. If the system is cooled below the demixing temperature, the bridges disappear within a few seconds by intermolecular diffusion. Thus, this system offers the opportunity to switch the bridges on and off and to tune precisely the bridge volume by altering the temperature in a convenient range. We measure the bridge geometry as a function of the temperature from bright field images and calculate the cohesive force. We discuss the influence of the solvent composition on the bridge formation temperature, the strength of the capillary force, and the bridge volume growth rate. Furthermore, we find that the onset of bridge formation coincides with the water-lutidine bulk coexistence curve.