Characterization of mixed micelles of SDS and a sugar-based nonionic surfactant as a variable reaction medium

Time-resolved fluorescence quenching (TRFQ), electron paramagnetic resonanc e (EPR), and small-angle neutron scattering (SANS) are employed to characte rize mixed micelles of sodium dodecyl sulfate (SDS) and the nonionic sugar- based surfactant dodecylmalono-bis-N-methylglucamide (DBNMG) as a reaction medium. Interpretation of the results from the three methods are constraine d to fit the classical model of a hydrocarbon core surrounded by a polar sh ell. As measured by TRFQ, at 45 degreesC, the aggregation numbers increase from 48 for pure SDS to a maximum of 63 at 36 mole percent DBNMG and decrea se again to 49 for pure DBNMG. The aggregation numbers of the pure DBNMG in crease from 33 at 21 degreesC to 49 at 45 degreesC. SANS results of the pur e DBNMG are interpreted by fixing the aggregation number at 49 (from TRFQ) and allowing the polar shell thickness to vary as a fitting parameter, yiel ding a value of 5.8 Angstrom at 45 degreesC. EPR, utilizing a hydrophobic s pin probe, is used to measure the nonempirical polarity index, H(25 degrees C), of the polar shell. H(25 degreesC) is defined to be the ratio of molar concentration of OH dipoles in a solvent or solvent mixture to that in wate r at 25 degreesC; thus, both water and the sugar headgroups contribute. By fixing the volume in the polar shell inaccessible to water due to the prese nce of SDS (127 Angstrom (3)) and DBNMG (580 Angstrom (3)), theoretical val ues of H(25 degreesC) are computed, the result depending only on the number of OH bonds in DBNMG available to interact with the spin probe. A constant average value of 7.4 OH bonds out of a maximum possible number of 10 repro duces the measured values over the full range of mixed micelle compositions . At 45 degreesC, the microviscosity of the polar shell, as deduced from th e rotational correlation time of the spin probe, varies from 2.79 +/- 0.05 cP for pure SDS to 13.1 +/- 0.2 cP for pure DBNMG departing only slightly f rom a linear dependence on the mole fraction of DBNMG. The uncertainties in the viscosity are the standard deviations in 10 measurements and therefore represent the uncertainty in the relative values of the viscosity. The vis cosity decreases from 32 +/- 4 cP at 21 degreesC to 13.1 +/- 0.2 cP at 45 d egreesC for pure DBNMG micelles. These viscosities are used to show that th e quenching rate constant of pyrene by dimethyl benzophenone, measured by T RFQ, follows the Stokes-Einstein-Smolukhovsky equation with a quenching pro bability of P = 0.5 whether the mixed micelle composition is changed at con stant temperature or if the temperature is varied in pure DBNMG. The volume of the polar shell enters into the formulation because the effective conce ntration of the quencher depends on that volume; however, the quenching rat es are not correlated with volume alone.