COUNTERION AFFINITY ORDERS IN AQUEOUS MICELLAR SOLUTIONS OF SODIUM DECYL PHOSPHATE AND SODIUM DODECYL-SULFATE DETERMINED BY CHANGES IN NA-23 NMR RELAXATION RATES - A SURPRISING DEPENDENCE ON HEAD GROUP CHARGE

Changes in quadrupole relaxation rates of Na-23 on addition of the Cl- salts of Na+, K+, Rb+, Cs+, TMA+, and TEA+ ions were used to determine the relative affinities of these cations for micelles composed of the decyl phosphate monoanion, DPH- (pH 5.3), the decyl phosphate dianion, DP2- (pH 12.6), and their 1:1 mixture (pH 7.8) at 35-degrees-C. Similar experiments were run in sodium dodecyl sulfate (SDS) micelles at 35 and 60-degrees-C for comparison. The affinity of alkali metal ions for decyl phosphate micelles clearly increases with cation size at all three pH's. The alkali metal cations show significantly less affinity than TMA+ and TEA+ for DPH- micelles and than TMA+ for SDS micelles. In DP2- micelles, the affinity order of the alkali metals remains the same, but surprisingly, TMA+ and TEA+ fail to displace Na+ from the micellar interface. The 1:1 mixture of DPH- and DP2- shows intermediate behavior. These changes in affinity order with head group charge can be interpreted qualitatively by assuming that alkali metal ions are hydrated at the surface of decyl phosphate micelles but that they interact much more strongly with divalent than monovalent phosphate head groups, perhaps by site binding to the dianionic phosphate head group through an intervening water molecule. These results are compared with affinity orders of monovalent cations in solutions of micelles, vesicles, polyelectrolytes, DNA, and ion-exchange resins.