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Energetic Interactions (energetic + interaction)

Distribution by Scientific Domains
Polymers and Materials Science66%
Chemistry33%

Selected Abstracts

Polymer Chain Collapse in Supercritical Fluids.

MACROMOLECULAR SYMPOSIA, Issue 1 2009

Abstract A few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T-x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end-to-end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper-critical solution temperature (UCST) suggesting the existence of a closed-immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent-to-monomer size ratio, the size of the size of the immiscibility loop can be fine-tuned. The relationship among size and the segment-solvent energetic interaction are correlated by a conformational parameter (,) for the first time. By monitoring the , behavior, it is possible to predict solution's phase behavior, transition zone from LCST-UCST in a closed-loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer-solvent mixtures. [source]

Theoretical reassessment of Whelk-O1 as an enantioselective receptor for 1-(4-halogeno-phenyl)-1-ethylamine derivatives

CHIRALITY, Issue S1 2004
Alberto Del Rio
Abstract A combination of molecular mechanics and first principles calculations was used to explore the enantioselectivity of receptors, taking into account experimental data from the CHIRBASE database. Interactions between the Whelk-O1 HPLC chiral stationary phase with the complete series of 1-(4-halogeno-phenyl)-1-ethylamine derivative racemates were studied. The objective was to extract information from the interactions between the chiral Whelk-O1 stationary phase and the enantiomers, hence probing the origin of the enantioselective behavior. Calculations correctly reproduce the elution orders and reasonably describe the experimental enantioselectivities and retention factors. Different binding modes were observed for the first eluted enantiomer complexes, whereas the second eluted show only one prevalent diastereomeric binding fashion. Natural bond orbital (NBO) analysis was used on the global minima bound-complexes to quantify donor-acceptor interactions among chiral stationary phase and ligand moieties. Intermolecular hydrogen bonding was found to be the essential energetic interaction for all systems studied. CH-,, aromatic stacking and various charge transfer interactions were found to be smaller in magnitude but still important for the global enantioselective behavior. The three-point interaction model is discussed, pointing out the difficulty of its application for the qualitative prediction of elution orders (absolute configurations). Chirality 16:S1,S11, 2004. © 2004 Wiley-Liss, Inc. [source]

Polymer Chain Collapse in Supercritical Fluids.

MACROMOLECULAR SYMPOSIA, Issue 1 2009

Abstract A few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T-x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end-to-end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper-critical solution temperature (UCST) suggesting the existence of a closed-immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent-to-monomer size ratio, the size of the size of the immiscibility loop can be fine-tuned. The relationship among size and the segment-solvent energetic interaction are correlated by a conformational parameter (,) for the first time. By monitoring the , behavior, it is possible to predict solution's phase behavior, transition zone from LCST-UCST in a closed-loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer-solvent mixtures. [source]

Statistical Mechanical Modeling of Protein Adsorption

MATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 12 2003
P. R. Van TasselArticle first published online: 5 JAN 200
Abstract We present rationale for and a derivation of a statistical mechanical model of protein adsorption. Proteins are modeled as rigid geometric objects adsorbing initially in a reversible manner and subsequently undergoing an irreversible change in shape to a permanently adsorbed state. Both adsorption and shape change occur subject to energetic interactions with previously adsorbed proteins. We evaluate the model quantitatively for proteins with disk-shaped projections within the scaled particle theory and compare the predictions to experimental measurements taken via optical waveguide lightmode spectroscopy. [source]

Spatial arrangement of molecules in homomolecular Z' = 2 structures

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2006
Elna Pidcock
The Box Model of crystal packing describes unit cells in terms of a limited number of arrangements of molecular building blocks. An analysis of Z,, 1 structures has shown that cell dimensions are related to molecular dimensions in a systematic way and that the spatial arrangement of molecules in crystal structures is very similar, irrespective of Z or space group. In this paper it is shown that the spatial arrangement of molecules in Z, = 2 structures are, within the context of the Box Model, very similar to that found for Z,, 1 structures. The absence of crystallographic symmetry does not appear to affect correlations between molecular dimensions and cell dimensions, or between the packing patterns and the positions of molecules in the unit cell, established from the analysis of Z,, 1 structures. The preference shown by Z, = 2 structures for low surface-area packing patterns and the observation that strong energetic interactions are most often found between the large faces of the independent molecules reaffirms the importance of molecular shape in crystal packing. [source]