Magnetic relaxation of 23Na in heterogeneous systems

Abstract

In order to elucidate the origin of the nonexponential nature of the transverse magnetic relaxation of 23Na nuclei (spin 3 2 ) in heterogeneous systems, the 23Na resonance of model systems (sodium salt of higher fatty acid-related hydrophobic compound-NaCl in a sufficient amount of water) was studied. They are transparent solutions or semitransparent or milk-white emulsions (or jellylike gels), depending on the amount of the hydrophobic compound. 23Na in transparent micellar solutions and microemulsions possesses a single value of the transverse relaxation time T 2. In contrast, two different values of T 2 were always observed in turbid emulsions and gels. The condition for the occurrence of two T 2 values in heterogeneous systms is formulated on the basis of a two-state model formerly presented for the 23Na resonance of biological tissue (H. Monoi, Biophys. J. 14, 653 (1974)). A theoretical consideration indicates that the critical diameters of colloidal particles at which two T 2 values first appear are not less than 30 to 40 nm for samples containing sodium soap.