Kumbhakar Manoj, Goel Teena, Mukherjee Tulsi, Pal Haridas
Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India.
J Phys Chem B. 2005 Jul 28;109(29):14168-74. doi: 10.1021/jp0520291.
The effect of added electrolytes on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation dynamics studies of C153 dye in the presence of different concentrations of NaCl, KCl, and CsCl salts. In all of the cases, the solvation dynamics is found to be biexponential in nature. It is seen that in the presence of added salts the solvation dynamics becomes slower. As previously reported (Charlton et al. J. Phys. Chem. B 2000, 104, 8327; Molina-Bolivar et al. J. Phys. Chem. B 2002, 106, 870), the presence of salt increases micellar hydration (and also size) for TX-100, mainly due to enhancement in the mechanically trapped water content in the palisade layer. Under normal circumstances, increased micellar hydration was expected to cause faster solvation dynamics (Kumbhakar et al. J. Phys. Chem. B 2004, 108, 19246), though in the present work, a reverse trend is in fact observed with the added salts. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microviscosity for the palisade layer of the TX-100 micelle with the added salts. The present results have been rationalized assuming that the ions reside in the palisade layer, and due to the hydration of the ions, especially the cations, the water molecules in the palisade layer undergo a kind of clustering, causing the microviscosity to in fact increase rather than decrease as expected due to increased micellar hydration. A partial collapse of the surfactant chains due to their dehydration as caused by the hydration of the ions in the palisade layer may also add to the increase in microviscosity and the consequent retardation in relaxation dynamics in the presence of salts.
通过在不同浓度的NaCl、KCl和CsCl盐存在下对C153染料进行溶剂化动力学研究，考察了添加电解质对Triton X - 100（TX - 100）胶束栅栏层中水分子性质的影响。在所有情况下，发现溶剂化动力学本质上是双指数的。可以看出，在添加盐的情况下，溶剂化动力学变得更慢。如先前报道（Charlton等人，《物理化学杂志B》2000年，104卷，8327页；Molina - Bolivar等人，《物理化学杂志B》2002年，106卷，870页），盐的存在增加了TX - 100的胶束水合作用（以及尺寸），这主要是由于栅栏层中机械截留的水含量增加。在正常情况下，胶束水合作用增加预计会导致更快的溶剂化动力学（Kumbhakar等人，《物理化学杂志B》2004年，108卷，19246页），然而在本研究中，实际上观察到添加盐后出现了相反的趋势。根据溶剂化动力学结果，荧光各向异性研究也表明，添加盐后TX - 100胶束栅栏层的微观粘度增加。假设离子存在于栅栏层中，并且由于离子的水合作用，特别是阳离子的水合作用，栅栏层中的水分子发生某种聚集，导致微观粘度实际上增加，而不是如预期的那样由于胶束水合作用增加而降低，从而对目前的结果进行了合理解释。栅栏层中离子水合作用导致表面活性剂链脱水，从而使其部分塌陷，这也可能导致微观粘度增加以及在盐存在下弛豫动力学相应减慢。
