Perturbation of Micellar Size and Charge by Addition of t-Amyl Alcohol

We have investigated the influence of dilute tertiary amyl alcohol (t-AmOH) on micellar size and charge by dynamic light scattering using two related surfactant/salt systems: CTABr (C16H33N(CH3)3-Br) + NaBr, and CTAC1 (C16H33)N(CH3)3C1) + NaCl. These were chosen because of their contrasting sensitivity to added salt with respect to micellar growth. For CTABr there is substantial growth at 25°C for [NaBr] > 0.05M, whereas for CTAC1 there is negligible growth below [NaCl] ∼ 1M. We measured the micellar diffusion coefficient D as a function of [surfactant] and [salt] at 25°C over the range 0.25% ≤ [t-AmOH] ≤ 7% (w/w) , approaching the solubility limit of t-AmOH in water, ca. 11%. We focused our attention on relatively low salt concentrations, where interlicellar interactions are predominantly repulsive and micellar growth is either small-to-moderate (CTABr) or negligible (CTAC1). For the CTACl-related ternary system we applied linear interaction theory (with a DLVO-type pair potential) to estimate the hydrodynamic radius Rh and fractional ionization a, from fits to D vs [surfactant] for various [salt]. Over the range of t-AmOH investigated, in general Rh decreases and a increases somewhat with increasing [t-AmOH], consistent with previous findings in the literature.