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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

Title	Diffusion of Probe Molecule in Small Liquid n-Alkanes: A Molecular Dynamics Simulation Study
Author	Choong-Do Yoo, Soon-Chul Kim, Song Hi Lee*
Keywords	Molecular dynamics simulation, Probe molecule, n-Alkanes, Diffusion, Friction
Abstract	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, D_MY~M^-γ 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) D_MY1 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 D_MY1 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, D_MY2 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 D_MY2 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.
Page	1554 - 1560
Full Text	PDF

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