Morphology control of organic luminescent microcrystals and approach of their optical properties

Microcrystals of an organic fluorescent dye, 4-octylamino-7-nitrobenz-2-oxa-1,3-diazole, were prepared using a reprecipitation method, based on solvent-exchange. We show here that their shape and size can be nicely tuned by the presence of various macromolecules in the reprecipitation medium. To do so, poly(acrylic acid) and calf thymus DNA were used as additives. Homogeneous populations of microcrystals were thus obtained in a reproducible way. Their size and morphology varied with the additive used. The UV/vis absorption and luminescence properties of these microcrystals in suspension are briefly reported. To cite this article: M. Abyan et al., C. R. Chimie 8 (2005).     

Photoinduced phase transitions

Employing actinic light to alter/stabilise a particular thermodynamic phase via the photo-isomerisation of the constituent molecules is an interesting tool to investigate soft matter from a new dimension. This article focuses on our recent results on several aspects of these non-equilibrium phase transitions, which are isothermal in nature. We specifically discuss (i) the influence of different parameters, such as confinement, applied electric field, pressure etc., on the dynamics associated with both the photochemical transition driving the equilibrium nematic to the non-equilibrium isotropic phase and the thermal back relaxation recovering the nematic phase, (ii) unique light-driven disorder–order transition in a reentrant system, (iii) dynamic self-assembly of the smectic A phase, which is stabilised only in the presence of actinic light, (iv) novel temperature-intensity phase diagrams and an example of primary and secondary photo-ferroelectric effects in an antiferroelectric smectic C system. These results highlight the fact that the actinic light can be used as a new tool to study phase transitions and the associated critical phenomena that could also bring about effects that are not seen in equilibrium situations.     

Study of surface tension and surface properties of binary alcohol/n-alkyl acetate mixtures

The Butler equation is employed to describe quantitatively the nature, properties, and compositions of surface layers in binary liquid mixtures. Bulk mole fraction, surface molar area, and surface tension of pure components are necessary inputs for this equation. In addition, the UNIFAC group contribution method is applied to account for the nonideality of the bulk liquid as well as that of the surface layer. The average relative error obtained from the comparison of experimental and calculated surface tension values for 12 binary systems is less than 1%. Therefore, the model has good accuracy in comparison with other predictive equations. In addition to finding more information about the surface structure of binary mixtures, surface mole fraction was calculated using relative Gibbs adsorption values and an extended Langmuir model (EL). The obtained results show a good consistency between two models employed, i.e., the Gibbs adsorption model and EL model, based on the UNIFAC method.     

Surface order-disorder phase transitions and percolation

In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in the presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square, and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold theta(c) of the adlayer, which presents an interesting dependence with w/k(B)T (w, k(B), and T being the lateral interaction energy, the Boltzmann constant, and the temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a nonpercolating region is determined.     

Model for phase coexistence in phase transitions

We propose a general model for describing the phenomena of phase coexistence in relation to pressure induced phase transformations by means of the T–P distribution in statistical thermodynamics. Using the well‐known B1–B2 transition in NaCl as a prototype, we demonstrate how phase coexistence gives rise to the changes in the bulk modulus and the equation‐of‐state across the transition. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Toluene-induced phase transitions in blue phase liquid crystals

ABSTRACT

We report phase transitions in blue phase-forming liquid crystals (LCs) that are triggered by exposure to toluene vapours. Specifically, we reveal that room-temperature cholesteric phase mixtures of MLC-2142 and S-811 form blue phases (BP I, II and III) with increasing vapour pressure of toluene. To probe the mechanism underlying this observation, we investigated the phase behaviour of mixtures of BP-forming LCs containing a range of non-volatile aromatic compounds (e.g. pyrene). We interpret our observations to indicate that the principal effect of small aromatic compounds is to decrease the energy penalty associated with the formation of disclination lines in BPs. We also conclude that the absorption of toluene into the BP-forming LCs lowers the energy required for the formation of disclination cores in the BP phase, thus allowing the elastically favoured double-twist cylinders to form at lower temperatures. We demonstrate that BP-forming LCs containing pyrene can be used to detect toluene at concentrations below 200 ppm at room temperature. Overall, these results guide the design of LC-based materials that respond to VOCs at concentrations relevant to occupational settings.     

Enthalpy of phase transitions of lactams

The transpiration method is used to measure the temperature dependences of the vapors pressures of azacyclobutan-2-one (I, CAS 930-21-2) azacyclohexan-2-one (II, CAS 675-20-7); azacyclooctan-2-one (III, CAS 673-66-5); azacyclononan-2-one (IV, CAS 935-30-8) and azacyclotridecan-2-one (V, CAS 947-04-6). Enthalpies of sublimation and vaporisation are determined. The temperatures and enthalpies of fusion of compounds (I, III-V) are found by means of differential scanning calorimetry. The dependences of the enthalpies of vaporisation of lactones, lactams, cycloalkanes, cycloalkanones on the size of a cycle are analyzed.     

Surface phase transitions in foams and emulsions

Surface phase transitions in surfactant adsorption layers are known to affect the dynamic properties of foams and to induce surface nucleation in freezing emulsion drops. Recently, these transitions were found to play a role in several other phenomena, opening new opportunities for controlling foam and emulsion properties. This review presents a brief outlook of the emerging opportunities in this area. Three topics are emphasized: (1) the use of surfactant mixtures for inducing phase transitions on bubble surfaces in foams; (2) the peculiar properties of natural surfactants saponins, which form extremely viscoelastic surface layers; and (3) the main phenomena in emulsions, for which the surface phase transitions are important. The overall conclusion from the reviewed literature is that surface phase transitions could be used as a powerful tool to control many foam and emulsion properties, but we need deeper understanding of the underlying phenomena to fully explore these opportunities.     

Interactions and phase transitions in protein solutions

Understanding interactions and phase transitions in protein solutions is essential in order to develop systematic protein crystallisation strategies. Recent studies show that proteins near crystallisation behave as particles interacting via a very short-range attractive potential. The microscopic origin of this attractive term, and its strong dependence on the nature of the crystallisation agent, is, however, still obscure. The interplay of crystal nucleation with formation of weakly bonded amorphous aggregates also deserves further analysis.     

Magnetic phase transitions in nanoclusters and nanostructures

The review includes theoretical models and experimental results on magnetic phase transitions in magnetic nanoclusters and nanostructures. It is shown, that nanoclusters with the sizes of a few or tens nanometers possess the critical sizes (similarly to critical temperatures Curie or Neel), less of which cluster or cluster nanostructure loses magnetic ordering due to the first order magnetic phase transitions (by the jump). Change of the character of magnetic phase transitions (first-to-second order), the reduction of Curie or Neel points and the finding of critical sizes of nanoclusters has been observed on ferric oxides and hydroxides in solid state and matrix nanostructures.     

Positron annihilation and phase transitions in n-octadecanol

The positronium formation probability and the mean lifetime in n-octadecanol-1 were observed as a function of temperature. The changes of that probability are discussed in the terms of the spur reaction model, suggesting the residual effect of dielectric constant increase in the rotational phase.     

Thermodynamics of CaCO3 phase transitions

Thermodynamic quantities of the aragonite → calcite transition, were evaluated using results of calorimetric investigations. (1) Dissolution enthalpies of the CaCO₃ polymorphs aragonite and calcite measured near room temperature with different calorimeter, (2) the enthalpy of the spontaneous phase transformation obtained by differential scanning calorimetry, (3) heat capacities and heat capacity differences determined with a heat flux calorimeter as well as previously determined, (4)e.m.f. data on Gibbs-energies of the phase transition were processed simultaneously with an optimization routine developed recently. The optimized data set (25°C) given below corresponds reasonably with CODATA recommendations, however, the precision has markedly improved.     

On the phase transitions of cycloalkanes

Several cyclic alkanes melt in two steps. Thermograms from differential scanning calorimetry (DSC) are discussed with respect to the onset of molecular motion and the resulting degree of disorder. The cycloalkanes are clasified in three groups according to their phase transition behavior.     

Magnetic phase transitions in nanoclusters and nanostructures

Theoretical models and experimental data on magnetic phase transitions in magnetic nanoclusters and nanostructures were reviewed. It was shown that nanoclusters measuring from several nanometers to several tens of nanometers possess a critical size (which can be likened to the critical Curie or Neel temperatures). At undercritical cluster sizes the magnetic order in cluster and cluster nanostructure vanishes by first-order magnetic phase transitions (abruptly). In this context, a change-over from the first- to second-order magnetic phase transition, a decrease or increase in the Neel and Curie temperatures, and critical nanocluster size calculations were accomplished for a number of nanoobjects. These include ferric oxides and hydroxides in matrix nanostructures comprised of isolated nanoclusters, as well as in nanostructures including strongly interacting or organized clusters and in nanostructures induced by shear stress under high pressure loading.     

Morphology control of anglesite microcrystals with polyhedron: Synthesis, growth mechanism, and optical properties

The formation of novel structure polyhedron-shaped PbSO₄ single crystal by reaction of UV irradiation and hydrothermal treated method is described. The kinetics of crystallization of PbSO₄ assembly is studied as a function of aging time and reaction time. The possible mechanism is also discussed. The effect on the crystallization of PbSO₄ micro-polyhedron under different conditions is investigated. The optical properties are also studied. The results show that tungstocilicate acid (TSA) acting as catalysts and a new class of inorganic scaffolds for the synthesis of materials in crystal engineering and composites design for different applications is versatile reactor and may be extended to the creation of other highly novel inorganic structures with applications in catalysis, novel optical materials and other fields.     

Morphology controllable synthesis and luminescence properties of NaLa(WO4)2:Eu microcrystals

NaLa(WO₄)₂:Eu microcrystals with shapes of four-arris shuttle, quadrangled star, and quadrangled dendrite were hydrothermally synthesized at 180 °C for 16 h. The concentration of the reactants and cetyltrimethyl ammonium bromide (CTAB) influenced the morphologies of the products. As La(NO₃)₃, Na₂WO₄ and CTAB was 0.375, 1.0, and 1.0 mmol, respectively, four-arris shuttle was obtained. As the concentration of the reactants doubled and the amount of CTAB ranged from 0.4 to 2.0 mmol, quadrangled dendrite, quadrangled star and four-arris shuttle were prepared, respectively. Luminescence intensity measurement of the three morphologies of NaLa(WO₄)₂:Eu showed that quadrangled dendrite was the strongest and four-arris shuttle was the lowest.     

Size and surface effects on phase transitions

Size effects on liquid crystal phase transitions are investigated by scanning calorimetry of samples absorbed in porous silica. For pore diameter of order 100 Å, the liquid crystal transition depressions are of order 0.1°C and the solid melting points of order 10°C.