Diffusion of hydrocarbons in confined media: translational and rotational motion

Diffusion of monatomic guest species within confined media has been understood to a good degree due to investigations carried out during the past decade and a half. Most guest species that are of industrial relevance are actually polyatomics such as, for example, hydrocarbons in zeolites. We attempt to investigate the influence of non-spherical nature of guest species on diffusion. Recent molecular dynamics (MD) simulations of motion of methane in NaCaA and NaY, benzene in NaY and one-dimensional channels AlPO4−5, VPI-5 and carbon nanotube indicate interesting insights into the influence of the host on rotational degrees of freedom and orientational properties. It is shown that benzene in one-dimensional channels where the levitation parameter is near unity exhibits translational motion opposite to what is expected on the basis of molecular anisotropy. Rotational motion of benzene also possesses rotational diffusivities aroundC6 and C2axes opposite to what is expected on the basis of molecular geometry. Methane shows orientational preference for 2+2 or 1+3 depending on the magnitude of the levitation parameter

Peyer's Patches Are Precocious to the Appendix in Human Development

PP are first visible at ∼15.5 wk gestation after which there is a rapid spurt in the development and maturation of lymphoid follicles so that at any given point of time new foci of PP development are continuously formed at a rapid rate. Addition of rows of follicles results in the formation of a PP. Immature PP of younger fetuses have a spongy structure in contrast with the compact lymphoid follicles of mature PP of older fetuses. Immunocytochemical studies reveal that there is a subtle gradation in the expression of lymphocyte surface markers with increasing fetal age. Expression of antigenic markers occurs in an ordered sequence viz. HLA – DR, CD19 (B cell population), CD9 (pre-B cells), CD3 T lymphocytes, CD4 helper / inducer lymphocytes, the CD8 suppressor / cytotoxic cells and lastly, the CD57 Natural Killer cells. The antigens are expressed first on lymphocytes of PP and thereafter in those of the appendix. Our findings clearly demonstrate that the ∼5 wk fetal period from 17.5 wk to 22 wk represents a major growth phase in the development of surface markers of lymphocytes in the mucosal immune system of the gut

Solid crystalline polymorphism in M-21

This paper reports on the different solid crystalline (SC) forms of 4-cyano-4'-heptyloxy biphenyl (M-21) as revealed through differential scanning calorimetric, polarizing microscopic and infrared spectroscopic investigations. There are three solid crystalline modifications, namely SCI,SCII,SCIII. The appearance and preponderance of the SC forms depend critically on the manner in which the liquid crystalline melt solidifies. Each SC phase has its CN stretching band split into two components because of Davydov or correlation splitting. The position, separation and relative intensities of the two components characterize each SC phase. The systematic study of the variations in these three features with temperature leads to interesting information about the intermolecular ordering forces and the spatial arrangement of the molecules in the unit cell. It is shown that the intermolecular interactions of the dipolar nature play a dominant role in the SC phases but contribute negligibly in stabilizing the nematic phase. The angle between the correlated molecules in the unit cell changes in the order SCI> SCII > SCIII > nematic = 0, resulting in parallel arrangement of molecules in the nematic phase

Study of line shape and angular variation of ESR spectra in two smectic a liquid crystals

Electron spin-resonance (ESR) measurement of the androstane nitroxide spin-probe is reported in isotropic, nematic and smectic A phases of the liquid crystals N(p-Octyloxy-benzylidine)-p-toludine [OBT] and p-nitrophenyl-p-n-octyloxy benzoate [NPOB]. Mono-domain samples in flat quartz tube were formed for the study of the angular variation of the spectra in smectic A phase. The analysis of the spectra using the theory of Polnaszek, Bruno and Freed (PBF) has resulted in the determination of the order parameter S, the rotational correlation time TR, and the anisotropic diffusion parameter N at different temperatures. The comparison between the two samples shows that the molecular geometry of the liquid crystals play important role in determining the value of the order parameter. The entire smectic A phase in OBT is characteristic of slow tumbling region (TR > 10-9 sec) and the anisotropic parameter N shows an anomalous increase in this phase. This indicates that the slowly relaxing local structure (SRLS) mechanism plays important role in the slow tumbling region. In NPOB the incipient slow tumbling region is almost at the end of smectic A phase and the SRLS mechanism does not seem to play as important role as in OBT

ESR line shape study of two nematic liquid crystals

Electron spin-resonance (ESR) measurements of the spin-probe17β-hydroxy-4,4'-dimethyl spiro[5α-androstane-3,2-oxazolidine]-3'-yloxyl in the liquid crystals 4-n-Amylacetophenon O-(4-n-heptyl benzoyl)-oxime [AAHBO] and 4-n-Butoxyphenyl hexyl benzoate [BPHB] are reported in isotropic and nematic phases. On the basis of the temperature variation of ESR line width, an extra, solid to solid, phase transition is observed in BPHB. The theory of Polnaszek, Bruno and Freed (PBF) appropriate for anisotropic viscosity diffusion is used to analyze the ESR spectra and thereby to determine the values of the order parameter, the rotational diffusion tensor and the correlation time at different temperatures for the two liquid crystals. The analysis shows that the slowly relaxing local structure mechanisms (SRLS) is active in slow tumbling region specially in AAHBO

Pulsed laser treatment at Fe/C<SUB>6</SUB>H<SUB>6</SUB> interface: a Mossbauer effect study

The pulsed ruby laser induced reactive-quenching process at Fe/C6H6 Ibenzenel has been investigated using conversion electron Mossbauer spectroscopy [CEMS]. It is shown that iron carbide phases can be synthesized when an iron foil immersed in benzene is treated with ruby Laser pulses [&#955;=694 nm, pulse width ~30 ns, energy density =15 J/cm2]. The results indicate the formation of &#949;-carbide and Fe5C2 phases in the as-treated sample and its transformation to Fe3C upon thermal treatment. The result of the CEMS measurements are supported by small angle X-ray diffractometry