STATISTICAL DISTRIBUTIONS AND COLLISION RATES OF ADDITIVE MOLECULES IN COMPARTMENTALIZED LIQUIDS STUDIED BY EPR .2. SODIUM DODECYL-SULFATE MICELLES, 5-DOXYLSTEARIC ACID ESTER, AND COPPER(II)

A technique to study the statistical distribution and collision rates of additive molecules in compartmentalized liquids introduced previously (J. Phys. Chem. 1993, 97, 3418) is applied to Cu2+ in SDS micelles in conjunction with the preceding fluorescence quenching paper (J. Phys. Chem., previous paper in this issue) which employed CU2+ as the quencher. The physical model employs a dilute solution of a hydrophobic nitroxide free radical, the indicator, in the presence and absence of a varying concentration of paramagnetic molecules, the broadeners, which are CU2+ in this case. The aggregation numbers found in the companion paper, computed under the assumption that the CU2+ ions are distributed among the micelles in conformance to a small electrostatic repulsion between CU2+ ions residing upon the same micelle, are fixed in this EPR study. Using these fixed sizes, it was only possible to bring the experimental observations into agreement with theory by assuming electrostatic repulsions in the EPR experiment as well. This electrostatic energy deduced from EPR measurements decreases from epsilon(2) = 0.18 to 0.06 kT (T = absolute temperature, k = Boltzmann constant) as the micelle size increases from 50 to 85 molecules and are therefore comparable to those found in the preceding paper. The spin exchange frequency between the nitroxide free radical and one CU2+ decreases linearly with the size of the micelle with a coefficient of correlation 0.997. The spin exchange frequencies are found to be quantitatively similar to fluorescence quenching rates, the ratio of the rates varying from about 1.5 to 1.2 depending upon the detergent concentration. A practical result of the present investigation is that the EPR spectra may be obtained using magnetic field modulation amplitudes up to 70% of the line width without affecting the experimentally determined electrostatic repulsion energies or the spin exchange frequencies. This reduces the necessary data acquision time by about a factor of 100. It is concluded that fluorescence quenching and EPR are an excellent complementary pair of techniques to study micelles, and a brief summary of the strengths and weaknesses of the two methods is given.