Showing papers by "Josep Salud published in 2006"

Dielectric relaxation in bulk and cylindrically confined octylcyanobiphenyl (8CB)

Abstract: The dielectric relaxation spectrum over the frequency range 102 to 1.8×109 Hz of 4‐octyl‐4′‐cyanobiphenyl, 8CB, in bulk and confined to 200 nm diameter cylindrical pores is reported. We used matrices with parallel cylindrical pores, obtaining different alignments of the molecular director depending on the treatment. Results show that there are two relaxations in the isotropic phase and in the mesophases for parallel alignment and three for perpendicular alignment. The molecular origin of theses modes and the effect of the confinement on their dynamics are discussed. To compare properly the results for bulk and confined 8CB, a re‐scaling of the experimental data is proposed.

Re-entrant Nematic Behavior in the 7OCB+9OCB Mixtures: Evidence for Multiple Nematic−Smectic Tricritical Points

Abstract: The metastable phase diagram of the two-component system heptyloxycyanobiphenyl (7OCB)+nonyloxycyanobiphenyl (9OCB) was determined by means of modulated differential scanning calorimetry (MDSC) and optical microscopy measurements. It was experimentally established that the 7OCB+9OCB two-component system exhibits a monotropic re-entrant nematic behavior. A complete quantitative thermodynamic analysis, through Oonk's equal G analysis, was performed, allowing for the calculation of the monotropic re-entrant behavior and the prediction of two tricritical points, one of them experimentally accessible for the SmAd-to-N transition and the other non-experimentally accessible for the RN-to-SmAd transition. From specific-heat measurements, latent heats were obtained for those mixtures displaying a first-order SmAd-to-N transition. Additionally, for some mixtures, the specific-heat critical exponents (α), through the second-order SmAd-to-N transition, were obtained. Both batches of data enable us to access to the ex...

Two-component system CCl4 + (CH3)3CBr: extrema in equilibria involving orientationally disordered phases.

Abstract: Phase equilibria involving orientationally disordered (OD) and liquid phases of the two-component system between carbon tetrachloride (CCl 4 ) and 2-methyl-2-bromomethane ((CH 3 ) 3 CBr) have been determined by means of X-ray powder diffraction and thermal analysis techniques from 210 K up to the liquid state. The isomorphism relation between the OD stable face-centered cubic (FCC) phase of (CH 3 ) 3 CBr and the metastable FCC phase of CCl 4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition, despite the monotropy of the FCC phase for the CCl 4 component with respect to its OD rhombohedral (R) stable phase. A continuous series of OD R mixed crystals is found, which confirms the R lattice symmetry of the OD phase II of (CH 3 ) 3 CBr, for which the crystallographic results have been long-time misinterpreted. X-ray patterns of such a phase were indexed according to the recent single-crystal results obtained by Rudman (Rudman, R. J. Mol. Struct. 2001, 569, 157). In addition, some experimental evidences are given to confirm the number of molecules per unit cell (Z = 21). The thermodynamic assessment reproduces coherently the phase diagram for the stable [R + L] and [R + FCC] two-phase equilibria as well as for the partially metastable [FCC + L] two-phase equilibrium and provides a set of data for the thermodynamic properties of nonexperimentally available phase transitions of pure components. Surprisingly, the phase equilibrium involving R and FCC OD phases appears as one of the very few showing a solid-solid equilibrium with two extremes.