Characterization of micelles of quaternary ammonium surfactants as reaction media I: Dodeclytrimethylammonium bromide and chloride

  • Bales B
  • Zana R
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Abstract

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)

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Authors

  • Barney L. Bales

  • Raoul Zana

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