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Critical Role of Confinement in the NMR Surface Relaxation and Diffusion of n‑Heptane in a Polymer Matrix Revealed by MD Simulations

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journal contribution
posted on 2020-04-24, 12:41 authored by Arjun Valiya Parambathu, Philip M. Singer, George J. Hirasaki, Walter G. Chapman, Dilipkumar Asthagiri
The mechanism behind the NMR surface-relaxation times (T1S,2S) and the large T1S/T2S 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 (f0) dependence of T1S,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 T1S/T2S ≃ 1. Under strong confinement (ϕC7 ≲ 50 vol %), we find that T1S/T2S ≳ 4 increases with decreasing ϕC7 and that the dispersion relation T1Sf0 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.