Article

Characterization Of Micelles Of Quaternary Ammonium Surfactants As Reaction Media I: Dodeclytrimethylammonium Bromide And Chloride

Time-resolved fluorescence quenching (TRFQ) and electron paramagnetic resonance (EPR) were employed to characterize micelles of dodecyltrimethylammonium bromide and chloride (DTAB and DTAC) as reaction media. For DTAB, the aggregation numbers, N, and the quenching rate constant of pyrene by hexadecylpyridinium chloride, kq, were measured with TRFQ. Both these aggregation numbers for DTAB and those for DTAC taken from the literature depend only on the concentration of counterions in the aqueous phase, Caq, whether these counterions are supplied by the surfactant alone or by surfactant plus added salt. Both surfactants conform to the power law N = N0(Caq/cmc0)γ where N0 is the aggregation number at the critical micelle concentration in the absence of any additives (cmc0). N0 and γ differ for the two surfactants and vary with temperature in DTAB. EPR is employed to investigate the microviscosity and the hydration of the polar shell using a spin probe. The hydration is expressed by the nonempirical polarity parameter H, defined to be the ratio of molar concentration of OH dipoles in a solvent to that in water. For a solvent containing no other source of OH dipoles, H is the volume fraction occupied by water. This fraction decreases continuously with N from about 55% to 30% as the micelles grow from N = 48 to 73. Theoretical values of H are computed from a simple classical micelle model of a hydrocarbon core surrounded by a polar shell and compared with experiment. The model yields the number of water molecules per surfactant molecule, NH2O, which decreases continuously with N for DTA+ micelles independent of the counterion. These results suggest that cationic micelles are dryer at all values of N than their twelve carbon anionic counterpart, sodium dodecyl sulfate (SDS); moreover, they lose waters of hydration faster as a function of N. The microviscosity of the polar shell, as deduced from the rotational correlation time of the nitroxide moiety of the spin probe, shows a modest increase with the aggregation number, comparable to that found in SDS. These viscosities are used to show that the quenching rate constant of pyrene by hexadecylpyridinium chloride in DTAB and of 1-methylpyrene by tetradecylpyridinium chloride in DTAC follow a common Stokes−Einstein−Smolukhovsky equation with a quenching probability of P = 0.4. This is so whether the micelle aggregation number, the temperature, or the counterion is changed. The microviscosity of the polar shell of DTAB shows a normal liquid-state temperature activation energy that is comparable to that found in ethanol−water mixtures. DTAB and DTAC are the same medium with respect to their hydration and the collision rate of guest molecules. By employing different combinations of surfactant and salt concentrations leading to the same values of N, a value for the degree of counterion dissociation for DTAB at 25 °C was derived from both the TRFQ α = 0.23 ± 0.03 and EPR α = 0.257 ± 0.010 which are in good agreement with each other and with literature values. From EPR, at 10.1 °C, α = 0.190 ± 0.008 and at 45 °C, α = 0.273 ± 0.011. For DTAC, α = 0.365 ± 0.008, derived from EPR, is also in good agreement with literature values. With respect to the fraction of counterion concentrations associated with the micelle, DTAB and DTAC differ substantially.

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