NMR Analyses and Diffusion Coefficient Determination of Minor Constituents of Olive Oil: Combined Experimental and Theoretical Studies

Combined experimental and computational studies on biologically important minor constituents of olive oil, i.e., 1-eicosanol, squalene, α-tocopherol, erythrodiol, stigmasterol, β-sitosterol, campesterol, and cycloartenol, have been performed for analyzing their ¹H and ¹³C NMR spectra. The correlation equations for estimating experimental ¹H and ¹³C NMR chemical shifts from the calculated chemical shifts have been derived and tested for the molecules in which dispersion is effective. The presently obtained experimental NMR signals of these molecules were then assigned on the basis of quantum chemical calculations. The discriminative NMR signals of the minor constituents were determined and discussed. ¹H diffusion ordered spectroscopy (DOSY) NMR has been employed to discriminate the minor constituents and determine their diffusion coefficients in deuterated chloroform. We calculated some physicochemical parameters of these molecules involving their shapes and sizes as well as their interaction with solvent. Then, we found a regression equation that can be used in estimating diffusion coefficients of other compounds

Large-Scale Diffusion of Entangled Polymers along Nanochannels

Changes in large-scale polymer diffusivity along interfaces, arising from transient surface contacts at the nanometer scale, are not well understood. Using proton pulsed-gradient NMR, we here study the equilibrium micrometer-scale self-diffusion of poly­(butadiene) chains along ∼100 μm long, 20 and 60 nm wide channels in alumina, which is a system without confinement-related changes in segmental relaxation time. Unlike previous reports on nonequilibrium start-up diffusion normal to an interface or into particulate nanocomposites, we find a reduction of the diffusivity that appears to depend only upon the pore diameter but not on the molecular weight in a range between 2 and 24 kg/mol. We rationalize this by a simple volume-average model for the monomeric friction coefficient, which suggests a 10-fold surface-enhanced friction on the scale of a single molecular layer. Further support is provided by applying our model to the analysis of published data on large-scale diffusion in thin films

Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with ¹³C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observed ¹³C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the ¹³C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of ¹³C nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces