Temperature-dependent self-diffusion coefficients of water and six selected molecular liquids for calibration in accurate 1H NMR PFG measurements

TLDR

PFG NMR is a routine method for measuring self‑diffusion coefficients, but its precision can be improved by calibrating with samples of precisely known D values. The study aims to provide temperature‑dependent self‑diffusion reference data for water and six common solvents to serve as primary and secondary calibration standards for accurate PFG NMR measurements. Self‑diffusion coefficients were measured by 1H PFG NMR over 5–55 °C for water and six solvents, supplemented with literature tracer data up to 100 °C, and the results were fitted to a Speedy–Angell power law. The fitted Speedy–Angell power law shows excellent agreement between PFG NMR and tracer data, yielding reliable primary standard values with <1 % error and enabling temperature‑matched calibration, while the extensive dataset also provides a benchmark for testing theoretical models of molecular liquids.

Abstract

Pulsed magnetic field gradient (PFG) NMR is today a routine method for the determination of self-diffusion coefficients, D. However, a remaining goal is the improvement of the precision of the method. The best procedure for the determination of accurate diffusion coefficients by PFG NMR is a calibration with a sample of precisely known D-value. In continuation of our previous work on calibration at 25°C (M. Holz and H. Weingärtner, J. Magn. Reson., 1991, 92, 115) we present reference data as a function of temperature. Since H2O plays an outstanding role as liquid and as primary standard, we carefully measured self-diffusion coefficients of water by 1H PFG NMR in the temperature range from + 5 to + 55°C and added literature data obtained from tracer methods in the range between 0 and + 100°C. This comparatively large collection of data could then be fitted to a Speedy–Angell power law, showing the excellent congruence of the results of two completely different methods and proofing the certainty of the absolute values for water. In this manner reliable primary standard values with error limits of <1% were obtained, allowing us to adapt the temperature of the standard water to the sample temperature of interest. We further give 1H PFG NMR self-diffusion reference data in the range from + 5 to + 55°C for six easily accessible solvents, which we propose as secondary calibration standards, namely cyclohexane, dioxane, dodecane, DMSO, tetradecane and pentanol, covering a large range of absolute D-values and allowing us to match in addition the absolute D-values of calibration sample and sample under investigation. Furthermore, the gained accurate self-diffusion data are suited for an elaborate check of theoretical approaches in the physics of molecular liquids.