Behera Kamalakanta, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
Langmuir. 2008 Jun 1;24(13):6462-9. doi: 10.1021/la800141p. Epub 2008 May 30.
Modifying properties of aqueous surfactant solutions by addition of external additives is an important area of research. Unusual properties of ionic liquids (ILs) make them ideal candidates for this purpose. Changes in important physicochemical properties of aqueous zwitterionic N-dodecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate (SB-12) surfactant solution upon addition of hydrophilic IL 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF 4], are reported. Dynamic light scattering results indicate a dramatic reduction in the average micellar size in the presence of [bmim][BF 4]; micellar (or micelle-like) aggregation in the presence of as high as 30 wt % [bmim][BF 4] is confirmed. Responses from fluorescence probes are used to obtain critical micelle concentration (cmc), aggregation number ( N agg), and dipolarity and microfluidity of the micellar pseudophase of aqueous SB-12 in the presence of [bmim][BF 4]. In general, increasing the amount of [bmim][BF 4] to 30 wt % results in decrease in N agg and increase in cmc. Increase in the dipolarity and the microfluidity of the probe cybotactic region within the micellar pseudophase is observed on increasing [bmim][BF 4] concentration in the solution. It is attributed to increased water penetration into the micellar pseudophase as [bmim][BF 4] is added to aqueous SB-12. It is proposed that IL [bmim][BF 4] behaves similar to an electrolyte and/or a cosurfactant when present at low concentrations and as a polar cosolvent when present at high concentrations. Electrostatic attraction between cation of IL and anion of zwitterion, and anion of IL and cation of zwitterion at low concentrations of [bmim][BF 4] is evoked to explain the observed changes. Presence of IL as cosolvent appears to reduce the efficiency of micellization process by reducing the hydrophobic effect.
通过添加外部添加剂来改变水性表面活性剂溶液的性质是一个重要的研究领域。离子液体（ILs）的特殊性质使其成为实现这一目的的理想选择。本文报道了在两性离子型N-十二烷基-N,N-二甲基-3-铵基-1-丙烷磺酸盐（SB-12）水性表面活性剂溶液中添加亲水性离子液体1-丁基-3-甲基咪唑四氟硼酸盐（[bmim][BF₄]）后，其重要物理化学性质的变化。动态光散射结果表明，在[bmim][BF₄]存在下，胶束的平均尺寸显著减小；证实了在高达30 wt%的[bmim][BF₄]存在下会发生胶束（或类似胶束）聚集。利用荧光探针的响应来获得在[bmim][BF₄]存在下SB-12水溶液胶束假相的临界胶束浓度（cmc）、聚集数（Nagg）以及偶极矩和微流动性。一般来说，将[bmim][BF₄]的量增加到30 wt%会导致Nagg减小和cmc增加。随着溶液中[bmim][BF₄]浓度的增加，观察到胶束假相内探针环化区域的偶极矩和微流动性增加。这归因于在向SB-12水溶液中添加[bmim][BF₄]时，水向胶束假相中的渗透增加。有人提出，离子液体[bmim][BF₄]在低浓度时表现得类似于电解质和/或助表面活性剂，而在高浓度时表现为极性助溶剂。在低浓度的[bmim][BF₄]下，离子液体的阳离子与两性离子的阴离子之间以及离子液体的阴离子与两性离子的阳离子之间的静电吸引被用来解释观察到的变化。离子液体作为助溶剂的存在似乎通过降低疏水效应来降低胶束化过程的效率。
