Geng Yan, Romsted Laurence S, Menger Fred
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, 19104, USA.
J Am Chem Soc. 2006 Jan 18;128(2):492-501. doi: 10.1021/ja056807e.
Results from chemical trapping experiments in micellar solutions containing 1.5-5 mM aqueous solutions of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths (12-n-12 2Br where n = 2-4 CH(2) groups) gave quantitative estimates of the molarities of interfacial bromide (Br(m)) and water (H(2)O(m)), the fractions of free and paired headgroups and counterions, and the net headgroup charge. These results are one of the most detailed compositional studies of an association colloid interface to date. Br(m) increases and H(2)O(m) decreases as n decreases and the two cationic charges are closer together. The 12-2-12 2Br gemini (the only one of the three geminis known to form threadlike micelles) shows a marked increase in Br(m) (from 2.3 to 3.6 M) and a decrease in H(2)O(m) (from 35 to 17 M) at the exceptionally low surfactant concentration in the vicinity of the previously reported sphere-to-rod transition or second cmc concentration. Rod formation occurs because of an increase in headgroup-counterion association and dehydration at the micelle surface that depend on both the free energies of hydration and specific ion interactions and surfactant and counterion concentrations. These and other recent chemical trapping results support a new model for the balance of forces controlling morphological transitions of association colloids. The hydrophobic effect drives the formation of headgroup-counterion pairs, which have a lower demand for water of hydration. Release of water permits tighter packing and formation of cylindrical aggregates.
在含有三种不同亚甲基间隔长度（12-n-12 2Br，其中n = 2 - 4个-CH₂-基团）的二癸基二阳离子二溴化物双子表面活性剂的1.5 - 5 mM水溶液的胶束溶液中进行化学捕获实验，得到了界面溴化物（Brₘ）和水（H₂Oₘ）的摩尔浓度、游离和配对头基及抗衡离子的分数以及净头基电荷的定量估计值。这些结果是迄今为止对缔合胶体界面进行的最详细的组成研究之一。随着n减小且两个阳离子电荷靠得更近，Brₘ增加而H₂Oₘ减小。12-2-12 2Br双子表面活性剂（已知的三种双子表面活性剂中唯一能形成线状胶束的）在先前报道的球-棒转变或第二临界胶束浓度附近的极低表面活性剂浓度下，Brₘ显著增加（从2.3 M增至3.6 M），H₂Oₘ减小（从35 M降至17 M）。棒状结构的形成是由于胶束表面头基-抗衡离子缔合增加和脱水，这取决于水合自由能、特定离子相互作用以及表面活性剂和抗衡离子浓度。这些以及其他最近的化学捕获结果支持了一种新的模型，用于解释控制缔合胶体形态转变的力的平衡。疏水效应驱动头基-抗衡离子对的形成，其对水合水的需求较低。水的释放允许更紧密的堆积并形成圆柱形聚集体。
