The role of internal motions and molecular geometry on H NMR relaxation rates in liquid-state hydrocarbons is investigated using MD (molecular dynamics) simulations of the autocorrelation functions for intramolecular and intermolecularH-H dipole-dipole interactions. The effects of molecular geometry and internal motions on the functional form of the autocorrelation functions are studied by comparing symmetric molecules such as neopentane and benzene to corresponding straight-chain alkanes n-pentane and n-hexane, respectively. Comparison of rigid versus flexible molecules shows that internal motions cause the intramolecular and intermolecular correlation-times to get significantly shorter, and the corresponding relaxation rates to get significantly smaller, especially for longer-chain n-alkanes. Site-by-site simulations of H's across the chains indicate significant variations in correlation times and relaxation rates across the molecule, and comparison with measurements reveals insights into cross-relaxation effects. Furthermore, the simulations reveal new insights into the relative strength of intramolecular versus intermolecular relaxation as a function of internal motions, as a function of molecular geometry, and on a site-by-site basis across the chain.
The mechanism behind the NMR surface-relaxation
times (T
1S,2S) and the large T
1S/T
2S ratio of light hydrocarbons
confined in the nanopores of kerogen remains poorly understood and
consequently has engendered much debate. Toward bringing a molecular-scale
resolution to this problem, we present molecular dynamics (MD) simulations
of 1H NMR relaxation and diffusion of n-heptane in a polymer matrix. The high-viscosity polymer is a model
for kerogen and bitumen that provides an organic “surface”
for heptane. Diffusion of n-heptane shows a power-law
dependence on the concentration of n-heptane (ϕC7) in the polymer matrix, consistent with Archie’s
model of tortuosity. We calculate the autocorrelation function G(t) for 1H–1H dipole–dipole interactions of n-heptane
in the polymer matrix and use this to generate the NMR frequency (f
0) dependence of T
1S,2S as a function of ϕC7. We find that increasing molecular
confinement increases the correlation time, which decreases the surface-relaxation
times for n-heptane in the polymer matrix. For weak
confinement (ϕC7 > 50 vol %), we find that T
1S/T
2S ≃
1. Under strong confinement (ϕC7 ≲ 50 vol
%), we find that T
1S/T
2S ≳ 4 increases with decreasing ϕC7 and that the dispersion relation T
1S ∝ f
0 is consistent with previously
reported measurements of polydisperse polymers and bitumen. Such frequency
dependence in bitumen has been previously attributed to paramagnetism;
instead, our studies suggests that 1H–1H dipole–dipole interactions enhanced by organic nanopore
confinement dominate the NMR response in saturated organic-rich shales.
The mechanism behind the 1 H nuclear magnetic resonance (NMR) frequency dependence of T 1 and the viscosity dependence of T 2 for polydisperse polymers and bitumen remains elusive. We elucidate the matter through NMR relaxation measurements of polydisperse polymers over an extended range of frequencies ( f 0 = 0.01−400 MHz) and viscosities (η = 385−102 000 cP) using T 1 and T 2 in static fields, T 1 field-cycling relaxometry, and T 1ρ in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times T 1LM ∝ f 0 and T 2LM ∝ (η/T) −1/2 with a phenomenological model of 1 H− 1 H dipole−dipole relaxation, which includes a distribution in molecular correlation times and internal motions of the nonrigid polymer branches. We show that the model also accounts for the anomalous T 1LM and T 2LM in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the T 1 ∝ f 0 dispersion and T 2 of similar polymers simulated over a range of viscosities (η = 1−1000 cP) are in good agreement with measurements and the model. The T 1 ∝ f 0 dispersion at high viscosities agrees with previously reported MD simulations of heptane confined in a polymer matrix, which suggests a common NMR relaxation mechanism between viscous polydisperse fluids and fluids under nanoconfinement, without the need to invoke paramagnetism.
Atomistic molecular dynamics simulations are used to predict 1H NMR T1 relaxation of water from paramagnetic Gd3+ ions in solution at 25oC. Simulations of the T1 relaxivity dispersion function r1...
In this work, we present an efficient numerical algorithm for the solution of molecular density functional theory (DFT) in cylindrical geometry to facilitate the study of how curvature affects the microstructure and phase behavior of inhomogeneous fluids. The new solution algorithm is shown to have a better time scaling than the elliptic function method by Malijevskỳ[J. Chem. Phys. 2007, 126, 134710] and the transform method by Lado [J. Comput. Phys. 1971, 8, 417−433]. Convergence, performance, and stability of the numerical algorithm are discussed. We showcase two representative applications of the new method for modeling fluid adsorption and bottlebrush polymers using a specific DFT, interfacial statistical associating fluid theory (iSAFT). By comparing iSAFT with molecular simulation results, we found that iSAFT predicts layering transitions above the triple point for methane adsorption, and it captures power-law to parabolic transitions for polymer brush microstructure.
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