Nuclear Magnetic Relaxation in Ionic Solution. II. Relaxation of 23Na in Aqueous Solutions of Various Diamagnetic Salts

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Abstract
The β coefficients in the equation T1−1—T10−1=Σβscs, which represents the nuclear relaxation rate of 23Na+ in dilute aqueous solutions at 25° as a function of solute molarity, have been determined for a variety of ionic solutes and separated into cationic and anionic contributions. For many species β is simply expressed in terms of viscosity and volume effects. Large additional contributions to β are found for a group of symmetrical, singly charged oxy and fluoro anions of which ClO4=, studied in the previous paper, is typical. The same is true for ions such as SO4= and PO4−3 for which extra interaction (such as association) with Na+ has been deduced from other types of measurements. For some solutes, including sucrose, nonlinear contributions to T1−1 dominate at concentrations as low as 0.2M. In the case of SO4= the observed nonlinearity may be attributed to ion association, in which case a dissociation constant (molar scale) of 0.2, in agreement with the result of Davies may be deduced from the data if his activity‐coefficient approximation is used. The temperature dependence of T10−1 is found to be markedly different from that for a group of relaxation and transport coefficients (e.g., proton T1 and viscous flow), all of which have nearly common behavior. It is concluded from this that the strength of the 23Na+–H2O quadrupolar interaction that determines T10 is temperature dependent. By contrast, the temperature dependence of the β for the 23Na+–ClO4− interaction exhibits the common behavior. Approximate values of the β coefficients for the relaxation of 79Br− and 127I− are extracted from linewidth measurements on aqueous solutions of their salts reported by Hertz and Richards and co‐workers. With allowance for differences in nuclear properties they are of similar magnitude and range as those for 23Na+. Structure‐breaking cations give large β coefficients in these systems just as structure‐breaking anions do with 23Na+.