| Abstract
| - The first and second association constants, K1 and K2, for ion pair formation in aqueous 0.02−3.5 M solutionsof 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), KTMAX, were determinedby the chemical trapping method. Values for KTMAX are small, KTMABr = 0.83 M-1 and KTMACl = 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 factorof ∼3 for Cl- and ∼10 for Br- as n increases from 2 to 4. K2, for the formation of bolaform dihalide pair,is essentially the same as that for ion pair formation in TMAX solutions, i.e., K2 ≈ KTMAX. Values of K1 andKTMABr obtained from changes in 79Br line widths are in good agreement with those obtained by chemicaltrapping. The results are consistent with a thermodynamic model in which the ion association depends on thebalance of the ion specific hydration free energies of cations and anions and their ion specific and hydrationinteractions in ion pairs. Spacer length dependent ion pairing by bolaform electrolytes, which are analoguesof the headgroups and counterions of gemini amphiphiles, suggests a new model for the spacer length dependentsphere-to-rod transitions of gemini micelles. Neutral, but polar, headgroup−counterion pairs have a lowerdemand for hydration that free headgroups and counterions, and headgroup−counterion pair formation releasesinterfacial water into the bulk aqueous phase, permitting tighter amphiphile packing in rodlike micelles.
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