Rodríguez Amalia, del Mar Graciani María, Muñoz María, Robina Inmaculada, Moya María Luisa
Departamento de Química Física, Universidad de Sevilla, C/Profesor García Gonzalez 2, 41012 Sevilla. Spain.
Langmuir. 2006 Nov 7;22(23):9519-25. doi: 10.1021/la062225e.
Micellization of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths, 12-s-12,2Br- where s = 3-5 methylene groups, has been investigated in water-ethylene glycol, EG, mixtures with weight percentages of EG up to 50%. Subsequently, effects of the addition of the organic solvent on the micellar growth of these surfactants and on the surfactant concentration range where sphere-to-rod transitions occur were studied by means of steady-state and time-resolved fluorescence quenching and spectroscopic measurements. Results show that an increase in the weight percentage of ethylene glycol added to aqueous 12-s-12,2Br- (s = 3-5) micellar solutions causes the sphere-to-rod transition to occur at higher surfactant concentrations than in pure water. The diminution in the average aggregation number, N(agg), when wt % EG increases, provoked by the decrease in the interfacial Gibbs energy contribution to DeltaG degrees M, is the main factor responsible for this observation. The decrease in N(agg) is accompanied by a decrease in the ionic interactions and the extra packing contribution to the deformation of the surfactants tails, making formation of cylindrical micelles less favorable. Besides, an increase in the solvent content and polarity of the interfacial region does not favor formation of direct ion pairs, decreasing the tendency of micelles to grow.
研究了三种具有不同亚甲基间隔长度（12-s-12,2Br-，其中s = 3 - 5个亚甲基）的双十二烷基二价二溴化双子表面活性剂在水 - 乙二醇（EG）混合物中的胶束化行为，EG的重量百分比高达50%。随后，通过稳态和时间分辨荧光猝灭以及光谱测量，研究了添加有机溶剂对这些表面活性剂胶束生长以及球 - 棒转变发生的表面活性剂浓度范围的影响。结果表明，向12-s-12,2Br-（s = 3 - 5）胶束水溶液中添加乙二醇的重量百分比增加，会导致球 - 棒转变在比纯水中更高的表面活性剂浓度下发生。当EG重量百分比增加时，平均聚集数N(agg)减小，这是由界面吉布斯自由能对ΔG°M的贡献降低所引起的，这是导致该现象的主要因素。N(agg)的减小伴随着离子相互作用的降低以及表面活性剂尾部变形的额外堆积贡献的降低，使得圆柱形胶束的形成变得不那么有利。此外，溶剂含量和界面区域极性的增加不利于直接离子对的形成，降低了胶束生长的趋势。
