Precision relative aggregation number determinations of SDS micelles using a spin probe. A model of micelle surface hydration
Precision relative aggregation number determinations of SDS micelles using a spin probe. A model of micelle surface hydration
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DOI：
10.1021/jp983364a
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发表时间：
1998-12-10
影响因子：
3.3
通讯作者：
Nascimento, OR
中科院分区：
文献类型：
--
作者：
Bales, BL;Messina, L;Nascimento, OR
描述了一种自旋探针方法，该方法可以检测SDS胶束中相对聚集数的变化，精度约为一个分子。该方法是基于这样的事实，即N-14超精细耦合常数是敏感的平均分数的区域中的氮氧化物部分，平均位于胶束表面附近的水所占据的体积。定义A（0）（N-A）为聚集数为N-A时的N-14超精细耦合常数，得到A（0）（N-A）= A（0）（0）+（偏导数A（0）/偏导数N-A）N-A，其中N-A受SDS或NaCl浓度的控制.对于自旋探针5-羟硬脂酸酯（5DSE），通过组合改变SDS和/或NaCl浓度的实验结果，线性最小二乘拟合得到A（0）（0）=（15.498 +/- 0.009）G和偏导数A（0）/偏导数N-A =-3.99 +/- 0.02 mG/分子（常数）。A（0）（N-A）仅取决于聚集数，尽管可以通过选择NaCl和SDS浓度的不同组合来制备给定值的N-A。这意味着，对于给定的胶束尺寸，胶束之间的相互作用和溶液的离子强度都不会影响A（0）的值。基于一个简单的模型，其中一个球形的碳氢化合物的核心是由一个同心的球形的极性壳包围的几何模型，开发预测的极性壳的体积分数的水所占据的N-A的函数。该水分数以每个表面活性剂分子的水合数N（H2O）表示。A（0）（N-A）通过采用Mukerjee等人引入的非经验极性指数与N（H2O）相关（Mukerjee，P.; Ramachandran，C.;皮特尔河A.物理化学杂志1982，86，3189）。N（H2O）的值随着胶束的增长而减小，因为极性壳层中每个表面活性剂分子的体积减小。一旦N（H2O）被指定为N-A的特定值，就没有进一步的可调参数进入模型。胶束大小的理论极性的变化是在很好的协议与实验值的N（H2O）是可比的那些发现从运输性能。观察到的球棒过渡作为一个相当尖锐的过渡，其中检测到的水的体积分数变得恒定。详细的未来测试的模型进行了概述。
A spin-probe method is described that can detect changes in the relative aggregation numbers in SDS micelles with a precision of about one molecule. The method is based on the fact that the N-14 hyperfine coupling constant is sensitive to the average fraction of the volume occupied by water in the region of the nitroxide moiety that is located on average near the micelle surface. Defining A(0)(N-A) to be the N-14 hyperfine coupling constant at an aggregation number N-A, We find A(0)(N-A) = A(0)(0) + (partial derivative A(0)/partial derivative N-A)N-A, where N-A is controlled by varying either the SDS or the NaCl concentrations. For the spin probe 5-doxylstearic acid ester(5DSE), by combination of the results of experiments in which the SDS and/or the NaCl concentrations were varied, linear least-squares fits gave A(0)(0) = (15.498 +/- 0.009) G and partial derivative A(0)/partial derivative N-A = - 3.99 +/- 0.02 mG/molecule (constant). A(0)(N-A) depends only on the aggregation number despite the fact that a given value of N-A may be prepared by choosing different combinations of NaCl and SDS concentrations. This means that, for a given micelle size, neither interactions between micelles nor the ionic strength of the solution influence the value of A(0). A geometric model, based on a simple model in which a spherical hydrocarbon core is surrounded by a concentric spherical polar shell, is developed to predict the volume fraction of the polar shell occupied by water as a function of N-A. This water fraction is written in terms of a hydration number per surfactant molecule, N(H2O). A(0)(N-A) is related to N(H2O) by employing the nonempirical polarity index introduced by Mukerjee et al. (Mukerjee, P.; Ramachandran, C.; Pyter, R. A. J. Phys. Chem. 1982, 86, 3189). The value of N(H2O) decreases as the micelle grows because the volume per surfactant molecule in the polar shell decreases. Once N(H2O) is specified at a particular value of N-A, no further adjustable parameters enter into the model. The variation with micelle size of the theoretical polarity is in excellent agreement with experiment for values of N(H2O) that are comparable to those found from transport properties. The sphere-rod transition is observed as a rather sharp transition in which the detected water volume fraction becomes constant. Detailed future tests of the model are outlined.