Nuclear Magnetic Relaxation in Ionic Solution. I. Relaxation of 23Na in Aqueous Solutions of NaCl and NaClO4

Abstract

The relaxation time of the nuclei of sodium ions has been measured for aqueous solutions of sodium chloride and sodium perchlorate at 25°. The longitudinal and transverse relaxation times are equal and depend on solution composition according to the rate law T1−1=17.55+0.55cNaCl+11.95cNaClO4±0.30,with T1 in seconds and cX in moles of X per liter, for single and mixed electrolyte solutions at concentrations less than 1M. Deviations from the equation are small up to 3M total concentration, and also in D2O if the coefficients are first multiplied by the ratio of solvent viscosities. The limiting value of T1 at low concentration agrees roughly with the calculation of the effect of fluctuating electric-field gradients due to reorientational Brownian motion of water molecules coordinated to the sodium ion, but there are some very uncertain factors in the calculation. The coefficients of the rate law may be related to the distance of closest approach of the ions in various ways, with the result that this distance is much smaller for Na+ClO−4 than for Na+Cl−.