STATISTICAL DISTRIBUTIONS AND COLLISION RATES OF ADDITIVE MOLECULES IN COMPARTMENTALIZED LIQUIDS STUDIED BY EPR SPECTROSCOPY .1. SODIUM DODECYL-SULFATE MICELLES, 5-DOXYLSTEARIC ACID ESTER, AND COBALT(II)

A technique to study the statistical distribution and collision rates of additive molecules in compartmentalized liquids is developed and applied to Co(H2O)62+ in SDS micelles. The physical model employs a dilute solution of a nitroxide free radical, the indicator, in the presence and absence of a varying concentration of paramagnetic molecules, the broadeners, which are Co(H2O)62+ in this case. The EPR spectra of the indicator vary nonlinearly with the concentration of the broadener in a manner consistent with a hypothesis that the resident time of a Co(H2O)62+ ion on one micelle is long compared with 10(-7) s but inconsistent with a random distribution of the added divalent cations among the micelles. A simple theory is developed which takes into account interactions between broadeners residing in the same compartment and ignores such interactions if they reside in different compartments. Assuming a small electrostatic repulsion between the cations brings the experimental results into agreement with theory and allows the collision rate of the broadener with the indicator to be estimated. For Co(H2O)62+, this collision rate decreases linearly as a function of the inverse volume of the micelle as predicted by a simple random walk model. The polarity of SDS micelles, as estimated by the N-14 hyperfine coupling constant of the indicator, decreases linearly with micelle size, while the microviscosity, as estimated from the rotational correlation time of the indicator, increases linearly with this size.