Segmental dynamics in a blend of alkanes: Nuclear magnetic resonance experiments and molecular dynamics simulation

Abstract

The segmental dynamics of a model miscible blend, C 24 H 50 and C 6 D 14 , were investigated as a function of temperature and composition. The segmental dynamics of the C 24 H 50 component were measured with 13 C nuclear magnetic resonance T 1 and nuclear Overhauser effect measurements, while 2 H T 1 measurements were utilized for the C 6 D 14 component. Use of low molecular weight alkanes provides a monodisperse system in both components and allows differentiation of dynamics near the chain ends. From these measurements, correlation times can be calculated for the C-H and C-D bond reorientation as a function of component, position along the chain backbone, temperature, and composition. At 337 K, the segmental dynamics of both molecules change by a factor of 2 to 4 across the composition range, with the central C-H vectors of tetracosane showing a stronger composition dependence than other C-H or C-D vectors. Molecular simulations in the canonical and isobaric-isothermal ensembles were conducted with a united-atom force field that is known to reproduce the thermodynamic properties of pure alkanes and their mixtures with good accuracy. With a minor change to the torsion parameters, the correlation times for pure tetracosane are in good agreement with experiment. For pure hexane and its mixtures with tetracosane, the simulated dynamics are faster than experiment.

Relaxation time, diffusion, and viscosity analysis of model asphalt systems using molecular simulation

“…The corresponding dimethylnaphthalene relaxation times increased in the model asphalt systems: c = 1050 ps in the asphaltene2-based system and c = 1930 ps in the asphaltene1-based system. Budzien et al 29 found similar changes in relaxation time for C 6 and C 24 alkanes. The reason is that for a molecule placed in a more viscous solvent, its dynamics are slowed down by bigger surrounding molecules.…”

“…Similar behaviors ͑ 0 Ͼ KWW ͒ have been reported for other systems. 29 The decrease in ␤ indicates a broader range 194502…”

“…The P 2 function is related to NMR and Raman scattering experiments. 29,45,46 The P 3 function relates to polarized Raman spectra. 47 We chose the unit vector for analyzing local dynamics based on molecule structure and possible local motions.…”

“…For example, Budzien et al 29 analyzed the relationship of segmental dynamics with temperature and composition in a blend of alkanes. Using a united atom model, they found semiquantitative agreement between simulation and nuclear magnetic resonance ͑NMR͒ dynamics results for a moderate-size ͑C 6 ͒ alkane and very good agreement for a larger alkane ͑C 24 ͒.…”

Towards in Silico Liquid Crystals. Realistic Transition Temperatures and Physical Properties for n‐Cyanobiphenyls via Molecular Dynamics Simulations

“…It appears therefore, in agreement with the report by McDonald and Hanna for a simulation of 8CB with a similar UA FF, but devoid of electrostatic interactions, [25] that tumbling times for nCBs are faster than the ones found for cinnamates, [11] probably because of the higher flexibility and lower molecular weight of the former. It should be also pointed out that, at least for alkanes, UA FFs are known to speed up rotational dynamics [50] and we might expect a somewhat faster dynamics for our UA nCB as well when compared to the fully atomistic case.…”