Biomedical subjects

K A Suresh

Publications and source records attributed to K A Suresh.

6 recordsLinked to original sources

PubMed2005

Structure and dynamics in solvent-polarity-induced aggregates from a c60 fullerene-based dyad.

A novel methanofullerene dyad based on a hydrophobic (acceptor C60 moiety)-hydrophilic (bridge with benzene and ester functionalities)-hydrophobic (donor didodecyloxybenzene) network is designed and synthesized. Electronic absorption spectral features revealed the molecule to exhibit a strong tendency to self-aggregate in binary solvent mixtures at room temperature, where the dielectric constant exceeds a critical value, approximately 30. The dynamic structure factors of these spherical aggregates revealed stretched exponential decay with sizes varying between 110 and 250 nm with an increasing concentration, estimated from the dynamic light scattering experiments. However, a loss of shape selectivity of these aggregates was noted at lower water volume fractions in the binary solvent mixtures. The water-extracted spherical clusters were identified to be fractals with a dimension of 1.85, leading to diffusion-limited cluster aggregation as the mechanistic route for clusterization.

Fullerenes↗

PubMed2005

Thermotropic uniaxial and biaxial nematic and smectic phases in bent-core mesogens.

Two azo substituted achiral bent-core mesogens have been synthesized. Optical polarizing microscopy and synchrotron X-ray scattering studies of both compounds reveal the existence of the thermotropic uniaxial and biaxial nematic and three smectic phases at different temperatures in these single component small molecule systems. The transition from the uniaxial to biaxial nematic phase is confirmed to be second order. The transitions from the biaxial nematic to the underlying smectic phase and between the smectic phases have barely discernible heat capacity signatures and thus are also second order.

Journal Article↗

PubMed2004

Photoinduced phase separation and miscibility in the condensed phase of a mixed Langmuir monolayer.

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.

Azo Compounds↗

PubMed2004

AFM studies on Langmuir-Blodgett films of cholesterol.

The Langmuir monolayer of cholesterol at the air-water interface exhibits a condensed phase in which the cholesterol molecules are aligned normal to the water surface. We have transferred the monolayer from water surface to different substrates by Langmuir-Blodgett (LB) technique and have studied their assembly by atomic force microscope (AFM). Our studies reveal that the aggregation of cholesterol molecules on hydrophobic surfaces leads to interesting structures. The cholesterol molecules assemble into a uniform film, elongated domains and uniformly distributed torus-shaped domains (doughnuts) for one, two and four cycles of deposition, respectively. Beyond four cycles, the molecules adsorb and desorb by an equal amount resulting in no further deposition. The formation of uniformly distributed doughnuts can be attributed to the hydrophobic interaction and reorganization of the molecules due to successive adsorption and desorption during deposition cycles. Our studies on hydrophilic surfaces show that cholesterol cannot form more than one layer of deposition.

Adsorption↗

PubMed2003

Polar head group interactions in mixed Langmuir monolayers.

We have investigated the miscibility in the mixed monolayers of cholesterol (Ch)-octyl cyano biphenyl (8CB) and cholesteryl acetate (ChA)-8CB using surface manometry and epifluorescence microscopic techniques. The main skeleton is the same both in Ch and ChA, whereas the polar head group is alcohol in Ch and ester in ChA. The 8CB molecule has a polar cyano as a terminal group and we probe its interaction with the polar group of Ch or ChA molecules in the mixed monolayers. Both Ch-8CB and ChA-8CB mixed monolayers exhibit two collapse pressures. In the case of the Ch-8CB mixed monolayer, the lower collapse pressure varies after 0.6 mole fraction (MF) of Ch in 8CB and the higher collapse pressure is nearly independent of composition. In ChA-8CB mixed monolayer, the lower collapse pressure varies continuously with the composition of ChA while the higher collapse pressure is nearly independent of the composition of ChA. In both these mixed monolayers, above the lower collapse pressure, 8CB gets squeezed out of the monolayer and forms multilayers. We find that in the case of Ch-8CB there is a phase separation in the monolayer occurring in the range of 0.15-0.9 MF of Ch. However, in ChA-8CB, the monolayer phase is miscible in all the proportions (except at very high concentration of ChA) below the lower collapse pressure. We attribute this better miscibility in the ChA-8CB compared to the Ch-8CB to the role played by the ester and cyano polar head group interactions.

Journal Article↗

PubMed2002

Optical diffraction in nonuniform cholesteric liquid crystals: phase-grating mode.

We have worked out the diffraction pattern in the phase-grating mode of a cholesteric liquid crystal (cholesteric) with a pitch gradient. The pitch gradients considered are symmetric and asymmetric with respect to the sample center. For a uniform input beam, the intensity profile of each diffraction order becomes broad. Further, in the symmetric gradient, the profile of each order is irregular, while in the asymmetric gradient it is nearly flat. For a Gaussian input beam, for the symmetrically deformed structure the profile for each order is asymmetric, while for the asymmetrically deformed structure, the profiles just becomes broad. We find that even a 5% nonuniformity in the pitch can drastically alter the diffraction profiles.

Journal Article↗