dorsal/arxiv
View SchemaHydrophobic and Ionic Interactions in Nano-sized Water Droplets
| Authors | S. Vaitheeswaran, D. Thirumalai |
|---|---|
| Categories | |
| ArXiv ID | physics/0609103 |
| URL | https://arxiv.org/abs/physics/0609103 |
Abstract
We investigate the solvation of methane and methane decorated with charges in spherically confined water droplets. Free energy profiles for a single methane molecule in droplets, ranging in diameter D, from 1 to 4 nm, show that the droplet surfaces are strongly favorable as compared to the interior. From the temperature dependence of the free energy in D=3 nm, we show that this effect is entropically driven. The potentials of mean force (PMFs) between two methane molecules show that the solvent separated minimum in the bulk is completely absent in confined water, independent of the droplet size since the solute particles are primarily associated with the droplet surface. The tendency of methanes with charges (Mq+ and Mq- with q+ = q- = 0.4e, where e is the electronic charge) to be pinned at the surface depends dramatically on the size of the water droplet. When D=4 nm, the ions prefer the interior whereas for D<4 nm the ions are localized at the surface, but with much less tendency than for methanes. Increasing the ion charge to e makes the surface strongly unfavorable. Reflecting the charge asymmetry of the water molecule, negative ions have a stronger preference for the surface compared to positive ions of the same charge magnitude. With increasing droplet size, the PMFs between Mq+ and Mq- show decreasing influence of the boundary due to the reduced tendency for surface solvation. We also show that as the solute charge density decreases the surface becomes less unfavorable. The implications of our results for the folding of proteins in confined spaces are outlined.
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"abstract": "We investigate the solvation of methane and methane decorated with charges in\nspherically confined water droplets. Free energy profiles for a single methane\nmolecule in droplets, ranging in diameter D, from 1 to 4 nm, show that the\ndroplet surfaces are strongly favorable as compared to the interior. From the\ntemperature dependence of the free energy in D=3 nm, we show that this effect\nis entropically driven. The potentials of mean force (PMFs) between two methane\nmolecules show that the solvent separated minimum in the bulk is completely\nabsent in confined water, independent of the droplet size since the solute\nparticles are primarily associated with the droplet surface. The tendency of\nmethanes with charges (Mq+ and Mq- with q+ = q- = 0.4e, where e is the\nelectronic charge) to be pinned at the surface depends dramatically on the size\nof the water droplet. When D=4 nm, the ions prefer the interior whereas for D\u003c4\nnm the ions are localized at the surface, but with much less tendency than for\nmethanes. Increasing the ion charge to e makes the surface strongly\nunfavorable. Reflecting the charge asymmetry of the water molecule, negative\nions have a stronger preference for the surface compared to positive ions of\nthe same charge magnitude. With increasing droplet size, the PMFs between Mq+\nand Mq- show decreasing influence of the boundary due to the reduced tendency\nfor surface solvation. We also show that as the solute charge density decreases\nthe surface becomes less unfavorable. The implications of our results for the\nfolding of proteins in confined spaces are outlined.",
"arxiv_id": "physics/0609103",
"authors": [
"S. Vaitheeswaran",
"D. Thirumalai"
],
"categories": [
"physics.chem-ph",
"cond-mat.other",
"cond-mat.soft",
"physics.bio-ph"
],
"title": "Hydrophobic and Ionic Interactions in Nano-sized Water Droplets",
"url": "https://arxiv.org/abs/physics/0609103"
},
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