The NMR relaxation of hydrogen nuclei of a fluid in a porous material is generally interpreted in terms of the Brownstein and Tarr model, in which the relaxation rate of the signal is inversely proportional to the pore size. We have investigated whether this model can be applied to the relaxation of Na nuclei in a NaCl solution in a porous material.

A first complication is that the relaxation rate of bulk NaCl solution has a concentration dependence as can be seen in the figure 1.

Figure 1: The longitudinal (1/T1) and transverse (1/T2) relaxation rate of Na in 10 nm pores as a function
of the NaCl concentration (C).

Secondly, it was verified that the ions are in the fast modulation regime and therefore the quadrupolar character of the Na nucleus (I=3/2) can be neglected. Then the pore size dependence was studied, as can be seen in the figure 2.

Figure 2. The measured longitudinal and transverse relaxation rate for Na ions as a function of 1/(pore size).

The results indicate that the ion distribution over the pores can be obtained from an analysis of the Na NMR signal decay, if the pore sizes
are roughly below 1 mm. This information is very useful for studies of combined moisture and ion transport in porous building materials.An example is given in figure 3.

Figure 3 The T2 distributions of H (red line) and Na (black line) in two building materials. The dashed line is the bulk T2 for Na.