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B Bagchi

Publications and source records attributed to B Bagchi.

At least 19 recordsLinked to original sources
PubMed2002

Ionic self-diffusion in concentrated aqueous electrolyte solutions.

A self-consistent microscopic theory is developed to understand the anomalously weak concentration dependence of ionic self-diffusion coefficient D(ion) in electrolyte solutions. The self-consistent equations are solved by using the mean spherical approximation expressions of the static pair correlation functions for unequal sizes. The results are in excellent agreement both with the known experimental results for many binary electrolytes and also with the new Brownian dynamics simulation results. The calculated velocity time correlation functions also show quantitative agreement with simulations. The theory also explains the reason for observing different D(ion) in recent NMR and neutron scattering experiments.

Journal Article↗
PubMed2001

Reentrant behavior of relaxation time with viscosity at varying composition in binary mixtures.

In order to understand the long known anomalies in the composition dependence of diffusion and viscosity of binary mixtures, we introduce here two new models and carry out extensive molecular dynamics simulations. In these models, the two molecular species (A and B) have the same diameter and mass. In model I the interspecies interaction is more attractive than that between the pure components, while the reverse is true for model II. Simulations and mode coupling theory calculations reveal that the models can capture a wide variety of behavior observed in experiments, including the reentrant viscosity dependence of relaxation time.

Journal Article↗
PubMed2000

Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations

The optical, electronic and mechanical properties of synthetic and biological materials consisting of polymer chains depend sensitively on the conformation adopted by these chains. The range of conformations available to such systems has accordingly been of intense fundamental as well as practical interest, and distinct conformational classes have been predicted, depending on the stiffness of the polymer chains and the strength of attractive interactions between segments within a chain. For example, flexible polymers should adopt highly disordered conformations resembling either a random coil or, in the presence of strong intrachain attractions, a so-called 'molten globule'. Stiff polymers with strong intrachain interactions, in contrast, are expected to collapse into conformations with long-range order, in the shape of toroids or rod-like structures. Here we use computer simulations to show that the anisotropy distribution obtained from polarization spectroscopy measurements on individual poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] polymer molecules is consistent with this prototypical stiff conjugated polymer adopting a highly ordered, collapsed conformation that cannot be correlated with ideal toroid or rod structures. We find that the presence of so-called 'tetrahedral chemical defects', where conjugated carbon-carbon links are replaced by tetrahedral links, divides the polymer chain into structurally identifiable quasi-straight segments that allow the molecule to adopt cylindrical conformations. Indeed, highly ordered, cylindrical conformations may be a critical factor in dictating the extraordinary photophysical properties of conjugated polymers, including highly efficient intramolecular energy transfer and significant local optical anisotropy in thin films.

Journal Article↗
PubMed2000

Power law mass dependence of diffusion: A mode coupling theory analysis

The self-diffusion coefficient of a tagged molecule is known to exhibit a weak mass dependence, especially for solutes with size comparable to or larger than the size of the solvent molecules. Sometimes this mass dependence can be fitted to a power law, with a small exponent, less than 0.1. This weak mass dependence has often been considered as supportive of the hydrodynamic picture (that is, the Stokes-Einstein relation) of diffusion rather than the kinetic theory approach, which predicts a stronger mass dependence, for example, via the Enskog theory. Neither can explain the weak power-law mass dependence. In order to understand this, we have carried out a mode coupling theory (MCT) analysis of diffusion. It is found that a straightforward application of the existing mode coupling theory expressions lead to an inaccurate mass dependence-it predicts an increase of diffusion coefficient with an increase of the mass. We find that this is because of the inadequate description of the initial decay of the collective contributions to the friction. We have proposed a new prescription to accurately describe the short time dynamics of the density and the current term. In addition, we have modified the existing MCT by imposing the full self-consistency between the frequency-dependent friction and the mean square displacement over the whole time and frequency plane. Previously the self-consistency was performed only at the zero frequency level between the zero frequency friction and the diffusion coefficient. With these two generalizations, the mode coupling theory is found to provide a fairly accurate description of the mass dependence. In particular, the theory can correctly reproduce the power-law dependence of solvent-solute diffusion ratio on solute-solvent mass ratio, observed in computer simulations of Bearman and Jolly [Mol. Phys. 44, 665 (1981)]. Another important result is that the current mode is found to play no significant role in determining the diffusion. Thus the hydrodynamic argument of weak mass dependence has little validity for same size solute-solvent systems.

Journal Article↗
PubMed1992

Levinthal's paradox.

Levinthal's paradox is that finding the native folded state of a protein by a random search among all possible configurations can take an enormously long time. Yet proteins can fold in seconds or less. Mathematical analysis of a simple model shows that a small and physically reasonable energy bias against locally unfavorable configurations, of the order of a few kT, can reduce Levinthal's time to a biologically significant size.

Mathematics↗