Nematic liquid crystalline structures in dilute surfactant solutions

The nematic phase and the viscous isotropic (VI) phase formed by solutions of CTAB and SHNC are investigated using light scattering and viscosity techniques. It is argued that the turbidity in the nematic phase is due to orientational fluctuations in the director and therefore the intensity scales as Q^-2. It is shown that with dilution of the nematic phase with water the turbidity disappears with a simultaneous increase of viscosity.     

Ionic liquid salt bridge in dilute aqueous solutions.

A new type of salt bridge composed of a hydrophobic room-temperature ionic liquid, recently proposed (T. Kakiuchi and T. Yoshimatsu, Bull. Chem. Soc. Jpn., 2006, 79, 1017), has been shown to be satisfactorily usable in dilute aqueous solutions of submillimolar range. A stable phase-boundary potential has been demonstrated between an ionic liquid, 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(8)mim+][C(1)C(1)N-), and an aqueous KCl solution of submillimolar level, which is lower than the solubility of [C(8)mim+][C(1)C(1)N-] in water, 1.6 mmol dm(-3) at 25 degrees C. The phase-boundary potential between [C(8)mim+][C(1)C(1)N-] and water is maintained constant over more than four orders of magnitude change in the concentration of an aqueous electrolyte solution. The ionic-liquid salt bridge is a superior alternative to salt bridges based on equitransferent electrolytes in practical applications, particularly, the potentiometry of samples of low ionic strengths, such as potentiometric pH measurements of rainwater.     

Determination of caloric quantities of dilute liquid solutions

This article reviews recent advances in the experimental determination of caloric quantities, for instance heat capacities, of dilute liquid nonelectrolyte solutions. Direct methods, i.e. calorimetric methods, as well as indirect methods, i.e. vapor-liquid equilibrium studies as a function of temperature, are considered.     

Thermodynamic properties of dilute solutions of antimony in liquid lead

Various methods of assessing the limiting activity coeffi cient of antimony and the area of the Henry’s Law holding in liquid alloys of antimony-lead system were considered.     

Thermodynamic properties of dilute solutions of selenium in liquid lead

The activity coefficients and activities of selenium in liquid alloys of the Pb-Se system at low selenium content were estimated from the available experimental data.     

Thermodynamic properties of dilute solutions of bismuth in liquid lead

Value of the limiting activity coefficients in liquid alloys of bismuth with lead at 673, 773 and 873 K were assessed with application of various analytical expressions.     

On the nature of liquid junction and membrane potentials

Whenever a spatially inhomogeneous electrolyte, composed of ions with different mobilities, is allowed to diffuse, charge separation and an electric potential difference is created. Such potential differences across very thin membranes (e.g. biomembranes) are often interpreted using the steady state Goldman equation, which is usually derived by assuming a spatially constant electric field. Through the fundamental Poisson equation of electrostatics, this implies the absence of free charge density that must provide the source of any such field. A similarly paradoxical situation is encountered for thick membranes (e.g. in ion-selective electrodes) for which the diffusion potential is normally interpreted using the Henderson equation. Standard derivations of the Henderson equation appeal to local electroneutrality, which is also incompatible with sources of electric fields, as these require separated charges. We analyse self-consistent solutions of the Nernst-Planck-Poisson equations for a 1 : 1-univalent electrolyte to show that the Goldman and Henderson steady-state membrane potentials are artefacts of extraneous charges created in the reservoirs of electrolyte solution on either side of the membrane, due to the unphysical nature of the usual (Dirichlet) boundary conditions assumed to apply at the membrane-electrolyte interfaces. We also show, with the aid of numerical simulations, that a transient electric potential difference develops in any confined, but initially non-uniform, electrolyte solution. This potential difference ultimately decays to zero in the real steady state of the electrolyte, which corresponds to thermodynamic equilibrium. We explain the surprising fact that such transient potential differences are well described by the Henderson equation by using a computer algebra system to extend previous steady-state singular perturbation theories to the time-dependent case. Our work therefore accounts for the success of the Henderson equation in analysing experimental liquid-junction potentials.     

Ionic liquid salt bridge based on tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide for stable liquid junction potentials in highly diluted aqueous electrolyte solutions

A moderately hydrophobic ionic liquid, tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide ([TBMOEP⁺][C₂C₂N⁻]), shows a very stable liquid junction potential upon contact with an aqueous solution whose ionic strength is as low as 1 μmol dm⁻³. The stability with the maximum excursion of the potential within ±0.5 mV for 30 min is very promising for accurate determination of pH and other single ion activities potentiometrically.     

Ionic liquid-alkane association in dilute solutions

Enthalpies and entropies of transfer were measured by gas chromatography for dilute solutions of a homologous series of eight even n-alkanes (from octane to docosane) into six different ionic liquids (ILs) (namely, 1-butyl-3-methylimidazolium chloride, bromide, iodide, triflate and hexafluorophosphate; plus N-butylmethylpyridinium bis {(trifluoromethyl) sulfonyl}-imide) over the 80–150 °C temperature range, all ILs being in the liquid state. Over a narrow concentration range, the entropic change may be consistent with a solvophobic association model of n-alkanes in ILs. A very simple model is proposed to account for the thermodynamic data. This approach can be used to approximate interionic distances and possible dielectric constants for ILs. Although the model may have some use in dilute alkane-IL solutions, more sophisticated models, particularly for the enthalpic contributions, are desirable.     

Shear thinning in dilute polymer solutions

We use bead-spring models for a polymer coupled to a solvent described by multiparticle collision dynamics to investigate shear thinning effects in dilute polymer solutions. First, we consider the polymer motion and configuration in a shear flow. For flexible polymer models we find a sharp increase in the polymer radius of gyration and the fluctuations in the radius of gyration at a Weissenberg number approximately 1. We then consider the polymer viscosity and the effect of solvent quality, excluded volume, hydrodynamic coupling between the beads, and finite extensibility of the polymer bonds. We conclude that the excluded volume effect is the major cause of shear thinning in polymer solutions. Comparing the behavior of semiflexible chains, we find that the fluctuations in the radius of gyration are suppressed when compared to the flexible case. The shear thinning is greater and, as the rigidity is increased, the viscosity measurements tend to those for a multibead rod.     

Energy up-conversion in dilute polyfluorene solutions

Poly(9,9-dioctylfluorene) (PFO) shows highly efficient blue emission with photo excitation occurring between 340–400 nm. Here we show that PFO can in dilute solution emit at a wavelength well below that at which it is being exited. This, we propose is related to an energy transfer from conjugated parts of the polymer chain into more localised states which then emit at a lower wavelength. These localised states can be considered as defects in the conjugation of the polymer or as chain ends. These may produce quasi monomer or quasi dimer species within the chain, which will have a HOMO-LUMO gap of higher energy than the conjugated polymer. These then fluoresce at the lower wavelength; essentially causing, by energy transfer, a process of energy up-conversion.      

Jetting of dilute polymer solutions

Light emitting devices containing conjugated polymers are conveniently fabricated using ink-jet printing. A common problem in the processing of these materials is that the Newtonian viscosity of the polymer solution is not sufficient to describe the jetting performance because the molecular weights and concentrations employed are such that the resulting solutions are elastic. These differences in fluid elasticity levels cannot be measured using traditional techniques like dynamic mechanical experiments or the first normal stress difference in shear, but strongly impact the jetting behavior of the liquid. In this study, a variety of polystyrene solutions in DECALIN having a shear viscosity of ~5 mPa s but different elasticity levels were examined for their jetting behavior. The jetting behavior of these solutions was studied visually using drop-on-demand jetting equipment and their rheology was characterized using a custom extensional rheometer designed for measuring the elasticity of such low viscosity liquids. If elasticity effects are absent as in Newtonian liquids (corresponding to a Trouton ratio of 3) satellite drops are formed resulting in loss of liquid and poor positioning. On the other hand, if elasticity effects are very large (Trouton ratios ≫4) separation problems occur at the nozzle with undesirable “tailing.” The optimum range for stable, efficient jetting occurs at Trouton ratios in a narrow band between 3 and ~5. A very slight degree of elasticity corresponding to a Trouton Ratio around four thus seems to be optimum for the jetting process. This appears to be the first time that such a design criterion has been outlined for this process. Such an approach complements thermal techniques for elucidating the role of molecular and flow properties on the processing behavior of polymeric systems.     

The effective dipole moments of isobutyric acid in the liquid state and dilute solutions in methanol

The dipole moments of isobutyric acid (I) were determined in the liquid state (μ₁) and dilute solutions in methanol (μ₁) at 20–50°C. The permittivity of I in the liquid state was found to increase as the temperature grew, and the permittivity of solutions of I was lower than that of pure methanol; it decreased as the concentration of I and the temperature of solutions increased. The effective dipole moments of I were calculated using the Onsager polarization theory for the pure liquid and the Buckingham statistical polarization theory for solutions with various acid concentrations in methanol. The small μ₁ (～0.8 D) and higher μ₂ (～3.0 D) values compared with the dipole moment of I in the gas phase μ₀ (1.9 D) were analyzed as determined by the character of acid-acid, acid-solvent, and solvent-solvent intermolecular interactions, a key role in which was played by H-bonds. An analysis of the dipole moments of I in methanolic solutions led us to conclude that the μ₁ and μ₂ values corresponded to the dipole moments of associates and solvates comprising like and unlike molecules linked by intermolecular H-bonds. Their stoichiometry changed as the temperature increased.     

Morphologies of star-block copolymers in dilute solutions

The morphologies of star-block copolymer (AB)n and (BA)n in a selective solvent for A-block are investigated by using dissipative particle dynamics. For a star-block copolymer of (BA)n type with a large enough arm number n, since the solvophobic B-blocks are situated in the inner part of the star, it behaves as a unimolecular micelle with the B-block core and A-block hairy corona. These types of star copolymers repel each other, thus it is quite difficult to form multimolecular micelles. On the other hand, for a star-block copolymer of (AB)n type, a few aggregative domains develop on the outer rim of the molecule. As the length of B-blocks or the repulsive interaction between B-blocks and solvents is increased, the tendency of B-blocks to associate within the star increases and thus the average number of aggregative domains declines. Owing to the exposure of B-domains, (AB)n type star-blocks tend to form micelles with morphology different from typical micelles. Upon performing simulations for solutions with multiple stars, we have shown that the single molecular conformation may greatly affect the resulting morphology of the supramolecular structure, such as connected-star aggregate, multicore micelle, segmented worm, and core-lump micelle.