Perturbation calculations of the interaction energies between non-bonded hydrogen atoms - Part 1

This paper presents calculations of the interaction energies between non-bonded hydrogen atoms in the planar model systems A—H...H—B for RHH distances from 1.0 to 15.0 a.u. using a perturbation formalism including exchange. Trends in the interaction energy have been examined as a function of the parameters of the model. The analysis in terms of electrostatic concepts was attempted and the extension of such concepts to small H...H separations appears questionable. Finally, a convenient functional representation of the interaction energy was obtained for both linear and non-linear cases

Pretransitional effects near the convective instability in binary mixtures

The pretransitional fluctuations of the Rayleigh-Bénard or convective instability are obtained for a binary mixture and compared to the one component case. There are now enhanced concentration fluctuations and although the velocity fluctuations are critically enhanced, they are weaker by a factor of D/χ. The behaviour of the critical (soft) mode is compared to the fluctuations in the order parameter obtained for a classical analysis of a phase transition. The region around the instability where the classical treatment of the order parameter fluctuations is invalid is increased in the binary case by a factor of (χ/D)1/2 over that of the pure fluid Rayleigh-Bénard instability

Analysis of the approach to the convection instability point

A spectral analysis is presented of the fluctuations in a horizontal fluid layer subject to a downward directed temperature gradient, which, for a critical value, drives the system in a convective instability state. It is found that the external force resulting from the combination of the temperature gradient and the gravitation force gives rise to a coupling between the heat diffusion mode and a shear mode. As a result of this mode coupling the damping constant of the heat diffusion mode goes to zero when the temperature gradient increases towards its critical value, i.e. the heat diffusion mode behaves like a "soft mode". The implications of the mode coupling and of the ensuing softening of the heat diffusion mode on the light scattering spectrum are discussed

Spectral analysis of the light scattered by a ternary mixture

The normal mode analysis of the light scattered by a ternary mixture is presented. The treatment is based upon a method, previously formulated by the authors, which is valid under conditions for which the sound modes are decoupled from the modes associated with heat and mass diffusion. The expressions obtained for both the lorentzian and non-lorentzian contributions to the Brillouin peaks are straightforward extensions of those obtained for the one and two component cases. The damping and strength factors of the three lorentzian contributions to the Rayleigh peak are rather complicated functions of the transport coefficients and thermodynamic derivatives. As illustrative examples the situations where, (a) one species and (b) two species in the ternary mixture are present in low concentration, are considered in some detail

The Landau-Placzek ratio for multicomponent fluids

Under the assumption that the coupling between the sound modes and modes associated with heat and mass diffusion can be neglected, an expression for the Landau-Placzek ratio for multicomponent fluids is derived using thermodynamic fluctuation theory. Applications of the general formula to ternary systems arc discussed briefly

Phase behaviour of rod-like colloid + flexible polymer mixtures

The effect of non-adsorbing, flexible polymer on the isotropic-nematic transition in dispersions of rod-like colloids is investigated. A widening of the biphasic gap is observed, in combination with a marked polymer partitioning between the coexisting phases. Under certain conditions, areas of isotropicisotropic- nematic or isotropic-nematic-nematic three-phase coexistence appear in the phase diagram of rod-polymer mixtures

Phase transition in a solution of rodlike particles with different lengths

The Onsager theory for thé isotropic-nematic phase transition in a solution of rodlike particles is extented to the case of mixtures of such rods with different lengths. It is found that there is a significantly higher molefraction of long rods in the anisotropic phase than in the isotropic phase and a concomitant enhancement of the orientational order of these particles. This explains the fractionation that occurs between the isotropic and anisotropic phase of polydisperse solutions of rodlike polymers

Phase transitions in lyotropic colloidal and polymer liquid crystals

An overview is given of theory and experiments on liquid crystal phases which appear in solutions of elongated colloidal particles or stiff polymers. The Onsager (1949) virial theory for the isotropic-nematic transition of thin rodlike particles is treated comprehensively along with extensions to polydisperse solutions and soft interactions. Computer simulations of liquid crystal phases in hard particle fluids are summarized and used to assess the quality of statistical mechanical theories for stiff particles at higher volume fraction -like the inclusion of higher virial coefficients, y-expansion, scaled particle theory and density functional theory. Both computer simulations and density functional theory indicate formation of more highly ordered smectic phases.The range of experimental applicability is strongly widened by the extension of the virial theory to wormlike chains by Khokhlov and Semenov (1981, 1982). Finally, experimental results for a number of carefully studied, charged and uncharged colloids and polymers are summarized and compared to theoretical results. In many cases the agreement is semi-quantitative