Stefan Frunza

Temperature dependence of the density of calamitic and discotic nematic lyotropic liquid crystals containing sodium lauryl sulphate/decanol/water

Experimental results for the temperature dependence of density at normal pressure for two compositions of sodium lauryl sulphate/decanol/water solutions, exhibiting either a calamitic or a discotic lyotropic nematic phase at room temperature, are presented. Within the limits of experimental precision (±1 2 10−5 g cm−3), the systems show no jump in density at the nematic to isotropic phase transition. Over the studied temperature range, the mean thermal expansion coefficients were also evaluated.

Comment on “Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water, and Maxwell–Wagner processes” [J. Chem. Phys.128, 154503 (2008)]

This comment on the experimental methods and the conclusions of the paper “Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water, and Maxwell–Wagner processes” [J. Sjostrom et al., J. Chem. Phys.128, 154503 (2008)] exposes serious shortcomings in the understanding and the interpretation of the observed relaxation behavior of confined water presented in the article. There are contradictions between the suggested theory and the experimental observations that mislead the reader. In this comment we prove that the Maxwell–Wagner relaxation cannot explain the nonmonotonous temperature dependence for the process observed in confined water.

Nematic-isotropic interfacial tension anisotropy of a calamitic lyotropic liquid crystal

In a planar oriented sample of a calamitic nematic lyotropic system (mixture of sodium lauryl sulphate/water/decanol), isotropic pretransitional domains appear at the nematic to isotropic transition. The domains are oblong in shape with the long axis along the orientational direction. We show experimental evidence that this oblong shape is determined by the nematic–isotropic interfacial tension anisotropy. Two uniparametric models of simple angular dependences for the interfacial tension are tested. Using the differential system obtained from the Young–Laplace condition at the nematic–isotropic interface, the domain shape can be numerically calculated for each value of the interfacial tension anisotropy. By processing the values of the transmitted light through both an isolated isotropic domain and its surrounding nematic zone, we obtain the anisotropy of the interfacial tension as the main fitting parameter. An estimation of the ratio of the extreme values for the interfacial tension is given.

Dynamics of Cyanophenyl Alkylbenzoate Molecules in a Surface Layer Adsorbed onto Aerosil. I. Cyanophenyl Hexylbenzoate

A composite prepared from aerosil A380 and the liquid crystal (LC) 4-hexyl-4′-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel–Fulcher–Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration.

Confining Effects in Composites Containing Molecular Sieves

Confinement of 4n-octyl-4′-cyanobiphenyl to nanoporous molecular sieves with hexagonal structure of cylindrical pores or with cellular structure is studied. It was found by differential scanning calorimetry and broadband dielectric spectroscopy that composites with the pores completely filled with liquid crystal show phase transitions characteristic for bulk, while the partially filled samples do not show these transitions. Thermogravimetric, differential thermal analysis and in situ infrared spectroscopic investigations have indicated first time that the dealkylation and oxidation of the confined liquid crystal is quite possible inside the nanopores of these molecular sieves, at temperatures much lower than for the bulk.

Dynamic behavior of hybrid organic-inorganic composites: liquid crystals confined in SBA-15 mesoporous materials

Molecular dynamics of a nematic liquid crystal, 4n-octylcyanobiphenyl, confined to two types of SBA-type nanoporous molecular sieves were studied by broadband dielectric spectroscopy in order to obtain information about the liquid crystal-surface interactions. The temperature interval of the dielectric measurements was large, covering all the states of the bulk liquid crystal. It is found that there are several relaxation processes covering the large frequency range. A bulk-like relaxation comes from the liquid crystal molecules located in the center of the pores, which rather do not feel the effect of the pore walls. Another relaxation comes from the molecules located in the surface layer. The latter process is much slower than the bulk-like relaxation. The differences observed between the dynamic behavior of the liquid crystal confined to the two types of nanoporous materials are related to pore size, pore shape and liquid crystal-surface interactions.

Layered materials of LDH-type containing Zn ions: Dielectric measurements show rotational fluctuations of water molecules

Layered double hydroxide (LDH) materials containing Zn ions were studied by broadband dielectric spectroscopy to follow up rotational molecular dynamics of confined water. One main relaxation process was observed in the experimental window; it was assigned to fluctuations of water molecules forming a strongly adsorbed thin layer upon the oxide surface. The temperature dependence of the relaxation rates has an unusual shape characterized by the presence of a maximum. Despite the characteristic parameters, this non-monotonous dependence is shown to be a rather general feature for water confined to LDHs.

Pentylcyanobiphenyl as Test Molecule for the Acid Sites of Powdered Titanium(IV) Oxides: Sensitivity of Core Levels to the Local Structure

Pentylcyanobiphenyl was used successfully as a test molecule to show the acid sites upon some commercial nanoscaled titanium dioxide materials. The surface interactions were investigated in detail using methods sensitive to the surface or to the bulk material, e.g., X-ray photoelectron spectroscopy (XPS), infrared spectroscopy, and thermal analysis. Different oxidation states of surface Ti ions were revealed. Several species were found: the majority were bonded through the nitrogen lone pair of the cyano group, but bonding to the surface OH groups by benzene π electrons might also appear. The N 1s lines of adsorbed 5CB were able to distinguish between Lewis acid and relatively weak Brønsted acid sites.

Synthesis and Electro-Optical Studies on Composite Material: Functionalized Copolymer Particles/Nematic Liquid Crystal

We have synthesized new functionalized copolymers of the type divinyl benzene – maleic anhydride (DVB-MA) in order to ensure a better compatibility with the penthyl-phenol group of the liquid crystal. These particles have been mixed by stirring and ultrasound with nematic liquid crystals ZLI 1221 (Merck), concentration 5% b.w. (copolymer/LC). We applied cycles of temperature and electric field, corresponding to the TSDC method. We measured the depolarization currents and simultaneously the optical transmission as a function of the electric field and temperature. We obtained information regarding the conduction mechanism, the distribution of the activation energy and the influence of the polarizing voltage on the optical transmission.

Synthesis and Electro-Optical Studies on Composite Materials Polymer Particles/Nematic Liquid Crystal

We have synthesized polymer particles of the type alternated copolymers DVB- maleic anhydride. The dimensions of the obtained particles have been determined by electron microscopy and are in the range 1–2 μm. These particles have been mixed by stirring and ultrasound with nematic liquid crystals ZLI 1221 (Merck), in a concentration of 5% b.w.(polymer/LC). We applied cycles of temperature and electric field, corresponding to the TSDC method. We measured the depolarization currents and simultaneously the optical transmission as a function of the electric field and temperature. We obtained information regarding the conduction mechanism, the activation energy and the pre-exponential frequency factor in the Arrhenius diagrams. The dipolar current obtained is consequent to a superposition of a dipolar distribution with a wide spectrum of relaxation times.