Self‐Diffusion in Viscous Liquids: Pulse NMR Measurements

Unexpected Diffusion Anisotropy of Carbon Dioxide in the Metal-Organic Framework Zn2(dobpdc)

Metal-organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is crucial for the practical application of these materials as adsorbents or in mixed-matrix membranes, since the rate of gas diffusion within the adsorbent pores impacts the required size (and therefore cost) of the adsorbent column or membrane. Here, we investigate the diffusion of CO2 within the pores of Zn2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. The residual chemical shift anisotropy for pore-confined CO2 allows PFG NMR measurements of self-diffusion in different crystallographic directions, and our analysis of the entire NMR line shape as a function of the applied field gradient provides a precise determination of the self-diffusion coefficients. In addition to observing CO2 diffusion through the channels parallel to the crystallographic c axis (self-diffusion coefficient D∥ = (5.8 ± 0.1) × 10-9 m2 s-1 at a pressure of 625 mbar CO2), we unexpectedly find that CO2 is also able to diffuse between the hexagonal channels in the crystallographic ab plane (D⊥ = (1.9 ± 0.2) × 10-10 m2 s-1), despite the walls of these channels appearing impermeable by single-crystal X-ray crystallography and flexible lattice MD simulations. Observation of such unexpected diffusion in the ab plane suggests the presence of defects that enable effective multidimensional CO2 transport in a metal-organic framework with nominally one-dimensional porosity.

A robust comparison of dynamical scenarios in a glass-forming liquid

We use Bayesian inference methods to provide fresh insights into the sub-nanosecond dynamics of glycerol, a prototypical glass-forming liquid.

High-pressure proton NMR study of lateral self-diffusion of phosphatidylcholines in sonicated unilamellar vesicles

Effects of pressure on the lateral diffusion of phospholipid molecules in sonicated pure 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles (15 wt%) in D2O were examined using the high-pressure proton NMR rotating frame spin-lattice relaxation time (T1rho) method. Proton T1rho were measured at pressures from 1 bar to 5000 bar and at temperatures of 50 degrees C to 70 degrees C for DPPC and 5 degrees C to 35 degrees C for POPC. The T(-1)1rho values were plotted as a function of the square root of the spin-locking field angular frequency (omega1(1/2) and the lateral diffusion coefficient (D) calculated from the slope. Pressure effects on lateral diffusion were observed in the liquid-crystalline (LC) phase. The lateral diffusion coefficient exhibited sharp decreases in response to the various pressure-induced phase transitions encountered. However, pressure had little, if any, effect on lateral diffusion in the pressure-induced gel I (GI) phase and pressure-induced interdigitated gel (Gi) phase. The activation volumes for diffusion were calculated from the slopes from plots of In D versus pressure for both DPPC (37 ml/mol at 50 degrees C, 34 ml/mol at 60 degrees C and 25 ml/mol at 70 degrees C) and POPC (16 ml/mol at 5 degrees C, 9 ml/mol at 20 degrees C and 6 ml/mol at 35 degrees C) sonicated vesicles in the LC phase. The activation energy for diffusion (Ea) was calculated using the slopes from plots of In D versus the inverse of the temperature (1/T) for both DPPC and POPC in the LC phase (3.5 kcal/mol and 3.9 kcal/mol, respectively) and for both DPPC and POPC in the GI phase (6.0 kcal/mol and 4.4 kcal/mol, respectively). From the lateral diffusion coefficient and line width data pressure-temperature phase diagrams for sonicated pure DPPC and POPC vesicles were constructed. The values of the temperature to pressure equivalence of DPPC (dTm/dP) were estimated to be 22.1 degrees C/kbar for the LC to GI phase transition and 28.6 degrees C/kbar for the GI to Gi phase transition. The value of the temperature to pressure equivalence of POPC for the LC to GI phase transition was estimated to be 19.0 degrees C/kbar.

Lateral diffusion of the phospholipid molecule in dipalmitoylphosphatidylcholine bilayers. An investigation using nuclear spin--lattice relaxation in the rotating frame

Measurements of the proton NMR spin--lattice relaxation time in the rotating frame (T 1rho) have permitted the explicit determination of the lateral diffusion coefficient of phospholipid molecules in the lamellar mesophase of dipalmitoylphosphatidylcholine at temperatures above the phase-transition temperature. The experimentally observed temperature and frequency dependence of T 1rho for the dipalmitoylphosphatidylcholine protons suggest that intermolecular dipole--dipole relaxation contributions are important. Proton T 1rho experiments involving dilution with deuterated dipalmitoylphosphatidylcholine support the premise that intermolecular dipolar interactions are significant and, concomitantly, that lateral diffusion is the motion modulating that interaction. The lateral diffusion coefficient is determined directly from the dependence of the rotating frame spin--lattice relaxation rate (1/T 1rho) on the strength of the applied radiofrequency field in the spin-locking experiment. A series of experiments with varying concentrations of dipalmitoylphosphatidylcholine in the lamellar mesophase indicates that the lateral diffusion coefficient varies as a function of phospholipid concentration.

Mobility of water bound to biological membranes. A proton NMR relaxation study

Water proton nuclear magnetic resonance relaxation measurements have been obtained for aqueous suspensions of red cell membranes. These data support a model in which water molecules are exchanging rapidly between a bound phase with restricted motions and a free phase with dynamic properties similar to liquid water. From this model and these data, estimates are obtained for the relaxation time for bound phase water. Possible relaxation mechanisms for bound phase water are discussed and some support is found for an intermolecular interaction modulated by translational motions characterized by a diffusion constant of 10(-9) cm2/s.