Experimental Method to Measure the Effect of Charge on Bimolecular Collision Rates in Electrolyte Solutions

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• 作者：Barney L Bales ; Kathleen M. Cadman ; Mirna Peric ; Robert N. Schwartz ; Miroslav Peric
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：October 13, 2011
• 年：2011
• 卷：115
• 期：40
• 页码：10903-10910
• 全文大小：867K
• 年卷期：v.115,no.40(October 13, 2011)
• ISSN：1520-5215

文摘

A stable, monoprotic nitroxide spin probe is utilized as a model to study molecular collisions in aqueous electrolyte solutions. The rate constants of bimolecular collisions, K_col for 2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-carboxylic acid (CP) when it is uncharged (at low pH) and K_col^{鈥?/sup> when it is charged (CP鈥?/sup>; at high pH), are measured as functions of temperature and ionic strength. The ratio f* 鈮?K_col鈥?/sup>/K_col is a direct measure of the effect of charge on the collision rate. Neglecting the small differences in size and diffusion coefficients of CP and CP鈥?/sup>, f* is the fractional change in collision rate due to Coulomb repulsion which was treated theoretically in Debye鈥檚 classic paper [Trans. Electr. Chem. Soc.1942, 82, 265]. K_col and K_col鈥?/sup> are determined from EPR spectral changes due to spin鈥搒pin interactions which are dominated by Heisenberg spin exchange under the conditions of these experiments. Values of f* vary linearly with values of 魏 路 d in the range 0.4 < 魏 路 d < 1.8, where 魏 and d are the inverse Debye screening length and the distance at closest approach, respectively. Values of d obtained in two independent ways, (1) from rotational correlation times measured by EPR and (2) by insisting that the experimental results be consistent with the Debye theory at infinite dilution, yield similar results. As the ionic strength is increased (魏 increased), the screening effect reduces the effect of the Coulomb barrier more slowly than predicted by the Debye theory. While values of K_col and K_col鈥?/sup> vary substantially with T, approximately following the Stokes鈥揈instein鈥揝moluchowski equation, values of f* depend only slightly on temperature at a given value of 魏 路 d, as is predicted by Debye鈥檚 theory.}