Showing papers by "K. A. Suresh published in 2004"

AFM studies on Langmuir-Blodgett films of cholesterol.

Abstract: 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.

Monolayers and 3D films of cholesteryl derivatives at the air-water interface

Abstract: Cholesterol and its derivatives have drawn much attention because of their relevance to biology. They have also been studied at the air−water (A−W) interface. Here, we have systematically investigated the two-dimensional (2D) and three-dimensional (3D) phases of cholesteryl derivatives at the A−W interface employing surface manometry, epifluorescence, Brewster-angle and reflection microscopy techniques. We found that the short-chain ester cholesteryl acetate forms a stable monolayer. The higher homologues cholesteryl heptanoate and cholesteryl octanoate do not form monolayers; they yield 3D crystallites at very large area per molecule. Interestingly, we found cholesteryl nonanoate to be in the crossover regime in the homologous series of cholesteryl esters. It spontaneously forms a fluidlike bilayer at the A−W interface. The higher homologue cholesteryl laurate forms an unstable bilayer phase. The long-chain esters cholesteryl myristate, cholesteryl palmitate, and cholesteryl stearate exhibit crystalline ...

Photoinduced Phase Separation and Miscibility in the Condensed Phase of a Mixed Langmuir Monolayer

Abstract: 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 (∼5 mN/m). At higher surfac...