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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 29, Number 8
BKCSDE 29(8)
August 20, 2008 

Diffusion of Probe Molecule in Small Liquid n-Alkanes: A Molecular Dynamics Simulation Study
Choong-Do Yoo, Soon-Chul Kim, Song Hi Lee*
Molecular dynamics simulation, Probe molecule, n-Alkanes, Diffusion, Friction
The probe diffusion and friction constants of methyl yellow (MY) in liquid n-alkanes of increasing chain length were calculated by equilibrium molecular dynamics (MD) simulations at temperatures of 318, 418, 518 and 618 K. Lennard-Jones particles with masses of 225 and 114 g/mol are modeled for MY. We observed that the diffusion constant of the probe molecule follows a power law dependence on the molecular weight of nalkanes, DMY~M well. As the molecular weight of n-alkanes increases, the exponent γshows sharp transitions near n-dotriacontane (C32) for the large probe molecule (MY2) at low temperatures of 318 and 418 K. For the small probe molecule (MY1) DMY1 in C12 to C80 at all the temperatures are always larger than Dself of n-alkanes and longer chain n-alkanes offer a reduced friction relative to the shorter chain n-alkanes, but this reduction in the microscopic friction for MY1 is not large enough to cause a transition in the power law exponent in the log-log plot of DMY1 vs M of n-alkane. For the large probe molecule (MY2) at high temperatures, the situation is very similar to that for MY1. At low temperatures and at low molecular weights of n-alkanes, DMY2 are smaller than Dself of n-alkanes due to the relatively large molecular size of MY2, and MY2 experiences the full shear viscosity of the medium. As the molecular weight of n-alkane increases, Dself of n-alkanes decreases much faster than DMY2 and at the higher molecular weights of n-alkane, MY2 diffuses faster than the solvent fluctuations. Therefore there is a large reduction of friction in longer chains compared to the shorter chains, which enhances the diffusion of MY2. The calculated friction constants of MY1 and MY2 in liquid n-alkanes supported these observations. We deem that this is the origin of the so-called “solventoligomer” transition.
1554 - 1560
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