E.S.R. study of aqueous sodium dodecyl sulphate solutions using di-t-butyl nitroxide as a probe

Abstract

The e.s.r. spectrum of di-t-butyl nitroxide in micelle-containing solutions of sodium dodecyl sulphate is a superposition of two spectra which can be associated with the radical in aqueous and micellar environments, the rate of exchange between the two environments being relatively slow. Quantitative measurements of the concentrations are interpreted phenomenologically in terms of the equilibria, nS ⇌S_n; K₁=[S_n]/[S]ⁿ, S_n+R[graphic omitted]S_nR; K₂=[S_nR]/[S_n][R], and we evaluate n= 60.5, K₁= 7.3 × 10¹³⁵ M^1–n, K₂= 4.58 × 10³ M^–1 at 296 K. The validity of the phenomenological description is discussed in terms of a multiple equilibrium model. Rotational correlation times for the radical in the two environments have been evaluated taking full account of unresolved proton hyperfine structure. At higher temperatures the spectra show line-broadening which is interpreted in terms of faster exchange of radical between the two environments. The exchange contribution to the linewidth for the radical in the micelles has been estimated to be k_–= 9.23 × 10¹¹ exp(–36 × 10³/RT) s^–1. Combining this value with K₂ gives k₊= 1.48 × 10⁹ M^–1 s^–1 at 296 K.