Geng Yan, Romsted Laurence S
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Phys Chem B. 2005 Dec 15;109(49):23629-37. doi: 10.1021/jp0546195.
The first and second association constants, K1 and K2, for ion pair formation in aqueous 0.02-3.5 M solutions of bis(trimethyl)-alpha,omega-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(TMABr) = 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 approximately 3 for Cl- and approximately 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 approximately K(TMAX). 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.
采用化学捕获法测定了双（三甲基）-α，ω-链烷二铵卤化物（间隔长度可变，1-n-1 2X，n = 2-4，X = Cl、Br）、双季铵盐和四甲基铵卤化物（TMAX，X = Cl、Br）在0.02 - 3.5 M水溶液中形成离子对的第一和第二缔合常数K1和K2。K（TMAX）的值较小，K（TMABr） = 0.83 M⁻¹，K（TMACl） = 0.29 M⁻¹，与文献值一致。对于双季铵盐，K1取决于间隔长度和抗衡离子类型，范围为0.4至17 M⁻¹，比K2大2至10倍，对于Br⁻比Cl⁻大，并且随着n从2增加到4，对于Cl⁻下降约3倍，对于Br⁻下降约10倍。形成双季铵二卤化物对的K2与TMAX溶液中离子对形成的K2基本相同，即K2 ≈ K（TMAX）。由⁷⁹Br线宽变化得到的K1和K（TMABr）值与通过化学捕获得到的值高度一致。结果与一个热力学模型相符，在该模型中离子缔合取决于阳离子和阴离子的离子特异性水合自由能以及它们在离子对中的离子特异性和水合相互作用的平衡。双季铵电解质的间隔长度依赖性离子配对，其是双子表面活性剂头基和抗衡离子的类似物，为双子胶束的间隔长度依赖性球-棒转变提出了一个新模型。中性但极性的头基-抗衡离子对比游离的头基和抗衡离子对水合的需求更低，头基-抗衡离子对的形成将界面水释放到本体水相中，使得在棒状胶束中两亲分子能够更紧密地堆积。
