D. Madhavi Latha

Phase Transitions and Thermodynamic Parameters of N-(p-n-octyloxybenzylidene)-p-n-alkoxyanilines—A Dilatometric Study

The variation of density with temperature in four compounds, N-(p-n-octyloxybenzylidene)-p-n-alkylanilines, 8O.Oms, viz. N-(p-n-octyloxybenzylidene)-p-n-propoxyaniline, 8O.O3, N-(p-n-octyloxybenzylidene)-p-n-butylaniline, 8O.O4, N-(p-n-octyloxybenzylidene)-p-n-hexylaniline, 8O.O6, and N-(p-n-octyloxybenzylidene)-p-n-heptylaniline, 8O.O7, is presented. The density and thermal expansion coefficient results reveal that the phase transitions present in these compounds are of first order. Using the thermal expansion coefficient, the temperature dependence of a number of thermodynamical (thermoacoustic and anharmonic) parameters of these compounds are estimated. These results reveal that all the parameters show characteristic change in the vicinity of the phase transformation. The results are discussed in light of the body of data available.

Computation of Molecular Free Length (L f ) and Molecular Radius (M r ) of p-n-Alkoxy Benzoic Acids

Dilatometric (density, ρ) and optics (refractive index, n) studies are utilized for the computation of thermoacoustic parameters, molecular radius, and intermolecular free length of p-n-alkoxy benzoic acids (nbas). For the estimation of Lf , the parameters like V 0, molar volume at absolute zero, and (Va ), the available volume is calculated from thermodynamic parameters. Further, V 0 can be obtained by the extrapolation of temperature variation of molar volume in isotropic phases to absolute zero. The molecular radius (Mr ) is obtained from the density and refractive index. The computed values using different methods are compared with one another as well as with the body of the data available. The temperature variation of different thermoacoustic parameters like Moelwyn-Hughes (C 1), reduced molar volume (V ∼), isochoric temperature coefficient of internal pressure (X), etc., calculated using different expressions are also presented. The increment in the molecular radius per methylene group in this homologous series is found to be 0.084 Å from density data and 0.054 Å from optics data. Further, it is observed that the Mr values obtained from optics data are found to be less compared to those obtained from density results. The Lf values estimated from different methods are found to show a similar type of variation with the chain length. However, the values differ slightly from one another and refinement is needed in the expressions to get concurrent results from different experimental data.

Estimation of order parameter from different models in symmetric dimeric liquid crystals

The birefringence measurements with the temperature are carried out employing the wedge technique at the wavelength λ = 5893Å on symmetric dimeric liquid crystalline compounds of α,ω-bis(4-alkylanilinebenzylidene-4′-oxy) alkanes which are popularly known as m.OnO.m’s with m = 3, 4 and 5 and spacer length n = 8, 9 and 10. The studied compounds are mono variant with long range of nematic phase. The birefringence data δn = (ne–no) along with the density results are employed to estimate the molecular polarisabilty anisotropy, δα = (αe–αo) assuming a particular local field (due to Vuks and Neugebauer) the nematic molecule experiences and the order parameter S is estimated from δα and Δα, i.e., polarisabilty anisotropy which is estimated employing different methods. Further, S can also be obtained from Kuczynski model and also using the effective geometry parameter, αg. Further, it is observed that from our analysis the S value obtained from Δn and αg are identical. The results are analysed and compared with the data available in the literature.

Dispersive power and crossover temperature, TCO, of two polar nematic liquid crystals in the visible spectral region

Refractive index studies are carried out on two highly polar liquid crystals: 1. N-(p-n-methoxy benzylidene)-p-amino benzonitrile, PmBAB, 2. N-(p-n-ethoxy benzylidene)-p-amino benzonitrile, PeBAB. The experimental investigations are carried out in the visible region at four different wavelengths, namely, 633, 589, 546 and 436 nm. The two compounds exhibit only the nematic liquid crystalline phase in between the isotropic and crystalline solid. The dispersive power ω is estimated for two consecutive wavelengths for the case of , ne and no for different wavelengths and found to be constant with temperature. Further the temperature gradients of ne and no are estimated, and the crossover temperature is obtained using dno/dT for all the wavelengths.

Phase transitions across different liquid crystalline modifications in compounds with octa decylaniline as one end group: a dilatometric study

The phase-transition studies such as the variation of density with temperature is carried in four newly synthesized compounds namely N-(p-n-decyloxybenzylidene)-p-n-octadecylaniline, 10O.18, N-(p-n-octadecyloxybenzylidene)-p-n-octadecyl aniline, 18O.18, (p-n-phenylbenzylidene)-p-n-octadecyl aniline, PB18A and terephthalyidene-bis-p-n-octadecylaniline, TB18A. These compounds exhibit a variety of phase variants such as isotropic-to-smectic-F (Iso–SmF) in 10O.18 and 18O.18, isotropic-to-smectic-A to-smectic-B (SmA–SmB) in PB18A and Isotropic-to-smectic-C to-smectic-I to-smectic-F (SmC–SmI–SmF) in TB18A. The density and thermal expansion coefficient results reveal that all the phase transitions studied in this article show first/second-order nature. The estimated values like density jumps and the thermal expansion coefficient maxima from the density data at the transitions exhibit characteristic changes. The results are discussed in light of the body of the data available.

Crossover temperature (TCO) and temperature gradient of refractive indices and length to breadth ratio of 4O.m mesomorphic compounds – an optical study

Refractive indices as a function of temperature are measured in a number of liquid crystals which belong to the N-(p-n-butoxy benzylidene)-p-n alkyl anilines, 4O.m compounds of the famous Schiff’s base nO.m liquid crystal compounds with m = 4 to 10 and 12. The temperature gradient of refractive indices, dne/dT and dno/dT, of these compounds are estimated. It is well known that in the case of dno/dT, there exists a crossover temperature, TCO, which exhibits an odd–even effect with the alkyl chain length as expected. Further, using the birefringence data, the length to breadth ratio, k, for these compounds is estimated, which shows not exactly even–odd effect but irregular change with the chain number. The results are discussed with the body of the data.

Phase Transition Studies and Thermodynamic Parameters of Four Members of nO.m Series—A Dilatometric Study

Density measurements are carried out on four alkoxy benzylidene alkyl anilines, viz., N-(p-n-methoxy/ethoxybenzylidene)-p-n-dodecyl and tetradecylanilines, 1O.12, 1O.14, 2O.12, and 2O.14. The first three compounds exhibit monovariant nematic phase while the last compound exhibits nematic and smectic-A phase with variable thermal ranges. The density measurements infer the first order nature for Isotropic to nematic (Iso-N) phase transformation while the nematic to smetic-A (N-SmA) transition as weak first order. Thermal expansion coefficient α estimated from density ρ is utilized for the computation of thermodynamic parameters of above liquid crystalline compounds. The temperature variation of different thermodynamic parameters like Moelwyn-Hughes parameter, reduced volume, reduced compressibility, isothermal, isochoric, and isobaric Gruneisan parameters, etc., are estimated. The results are compared with the available data in literature of other compounds.