Ion pair formation in water. Association constants of bolaform, bisquaternary ammonium, electrolytes by chemical trapping

The first and second association constants, K₁ and K ₂, for ion pair formation in aqueous 0.02-3.5 M solutions of bis(trimethyl)-α,ω-alkanediammonium halides with variable spacer lengths, 1-n-1 2X (n = 2-4, X = Cl, Br) and bolaform salts and for tetramethylammonium halides (TMAX, X = Cl, Br), K_TMAX, were determined by the chemical trapping method. Values for K_TMAX are small, K_TMACl = 0.83 M^-1 and K_TMACl = 0.29 M^-1, in agreement with literature values. For the bolaform salts, K₁ depends on spacer length and counterion type, ranges from 0.4 to 17 M^-1, is 2-10 times larger than K₂, is larger for Br^- than Cl^-, and decreases by a factor of ∼3 for Cl^- and ∼10 for Br^- as n increases from 2 to 4. K ₂, for the formation of bolaform dihalide pair, is essentially the same as that for ion pair formation in TMAX solutions, i.e., K₂ ≈ K_TMAK- Values of K₁ and K_TMABr obtained from changes in ⁷⁹Br line widths are in good agreement with those obtained by chemical trapping. The results are consistent with a thermodynamic model in which the ion association depends on the balance of the ion specific hydration free energies of cations and anions and their ion specific and hydration interactions in ion pairs. Spacer length dependent ion pairing by bolaform electrolytes, which are analogues of the headgroups and counterions of gemini amphiphiles, suggests a new model for the spacer length dependent sphere-to-rod transitions of gemini micelles. Neutral, but polar, headgroup - counterion pairs have a lower demand for hydration that free headgroups and counterions, and headgroup-counterion pair formation releases interfacial water into the bulk aqueous phase, permitting tighter amphiphile packing in rodlike micelles.