The pseudophase ion exchange (PIE) model provides both a qualitative and quantitative interpretation of aqueous micellar effects on reaction rates and equilibria for a variety of thermal reactions between organic molecules and ions in the presence and absence of buffer. Our results show that the PIE model is also applicable to alkali-metal salts of decyl phosphate monoanion micelles, MDP (M = Na, K, Rb, and Cs). We measured the ratio of acid to base forms of the spectrophotometric indicator pyridine-2-azo-p-dimethylaniline, PADA, as a function of added salt MCl, up to 0.4 M in 0.08 M MDP, pH range 4-6, succinate buffer at 50 °C. Similar experiments were run in micellar solutions of sodium lauryl sulfate, NaLS, with added MCl under the same conditions to check for special effects caused by the phosphate monoanion head group. None were found. The assumptions of the PIE model are well obeyed provided we use the measured pH to calculate the activity of the proton at all salt concentrations and express the quantity of counterions in the aqueous phase as activities and not concentration units. Precise results also require correcting for specific salt effects on the response of the pH electrode. Our estimated value of pKAv, the acidity constant of micellar-bound PADA, is only slightly smaller than its value in water in both MDP and NaLS micelles, consistent with the commonly held view that reaction occurs at the water-rich micelle surface and not in the hydrocarbon core. The relative values of the ion-exchange constants between the alkali-metal cations and the proton, KHM, follows a Hofmeister series in both surfactants, Cs > Rb > K > Na, i.e., decreasing selectivity with increasing size of the hydrated cation. The alkali-metal cation selectivity order for several different phospholipid vesicles follows the opposite trend, suggesting that specific counterion interactions in micelles and vesicles with phosphate head groups are fundamentally different.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry