Ion binding to zwitterionic amphiphile interfaces, such as those constituting biological membranes, is an active field of research. The complex molecular composition of biological membranes has prompted the development of simpler models in order to analyze the molecular features of ion binding to interfaces and we have used zwitterionic detergents as lipid models. Here we present the relationship of interfacial anion concetration, [X^-]_b, and the entalpia of micellization, DH_mic, of the detegent N-dodecyl-N, N-dimethyl-3-ammonium-1-propane sulfonate, DPS, measured in solutions of different salts ( NaBr, NaCl, KBr and KCl) varying from 0.1 to 1.0M.
The thermodynamic parameters for DPS micellization, at 30^oC, were obtained by Isothermal Titration Calorimetry (ITC).The interfacial concentration of anions was obtained by chemical trapping method that is based in the quantitative analysis, by HPLC, from the products obtained after decomposition of an interfacial-incorporated diazonium salt probe.
The results show that [X ^-]_b are higher than the bulk salt concetration, [X^-]_f, and reaches a maximum value when [X^-]_f is ca. 0.8M, denoting a interfacial ion saturation. Comparing salts with the same cation, [Br^-]_b is higher than [Cl^-]_b which point out that a less hydrated anion binds better in a zwitterionic surface.
DH_mic of DPS is positive in salt free aqueous solution.Salt addition produces a decrease in DH_mic that reaches negative values at high salt concetration for NaBr and KBr solutions. The value of DH_mic in NaBr or KBr is more exothermic than in NaCl or KCl, indicating that the better binding of Br^- reflects in a larger effect in DH_mic, namely a larger enthalpic contribution for DPS micellization.
The plot of DH_mic in funcion of [X^-]_b shows a linear dependence between these two parameters until  [X ^-]_b reaches the interfacial saturation. The linear coefficient obtained is close to the value of DH_mic in salt free solution and DH_mic is more exothermic in the order:  NaBr< NaCl< KBr< KCl.
Theses results contribute to the thermodynamic characterization of the micellization process and show the importance of specific ion binding to zwitterionic micelles.
 Acknowledgements: FAPESP, CNPq, CAPES, Pró-Reitoria de Pesquisa da USP.