Estimation of solid-liquid interfacial potential enabled by quantitative analysis and relaxation observation of quadrupolar NMR

Abstract

Relaxation time measurements and quantitative analyses of quadrupolar nuclei via NMR were effective for the estimation of the interfacial potential at the solid-liquid interface. Moreover, quantitative nuclear magnetic resonance (qNMR) spectroscopy experiments for 23Na and 35Cl nuclei were performed. The calibration curve of the external standard aqueous NaCl solution showed excellent linearity over a considerably wide concentration range from 4 × 10−4 to 2 mol L−1 (an increase in concentration of 5000 times), with a simple experimental and analytical procedure. The relaxation time and detection rate of Cl– ions in the 35Cl NMR spectrum of a 0.10 mol L–1 NaCl aq. solution with 10 vol% α-Al2O3 drastically decreased as the zeta potential of the solid phase (ζs) of α-Al2O3 approximated a negative value. Such a result was due to the decrease in both the mobility of Cl– ions and the symmetry of the electrostatic field around 35Cl nuclei caused by the electrostatic interactions between Cl– ions and the solid surface. In the case of Na+ ions, which have a charge opposite to that of Cl– ions, the ζs dependences of the relaxation time and detection rate of Na+ ions of 23Na NMR were also opposite to those of 35Cl NMR. Particularly, such values were almost constant when ζs > 0 mV but decreased monotonically when ζs < 0 mV. It was also clarified that the counterions condensed around the vicinity of the solid phase wherein the concentration of the counterions was approximately several tens of times higher than that in the bulk phase. The estimation of electrostatic field intensities not only at solid-liquid interfaces but also at liquid-liquid interfaces and various interfaces, including those in non-aqueous solvents, can thus be performed through the proposed method.