Anomalous effects of concentrated salts on the equimolarly mixed cationic–anionic surfactant mixtures

Abstract

Effects of alkali metal halides (NaX, X = F⁻, Cl⁻, Br⁻, I⁻; KX, X = Cl⁻, Br⁻, I⁻; and CsCl) on the morphology of the aggregates of decyltriethylammonium bromide and sodium decylsulfonate (C₁₀NE–C₁₀SO₃) mixtures with equimolar ratios were studied by static/dynamic light scattering and viscosity measurements. The average hydrodynamic radii of the aggregates, the apparent aggregation number, and the relative viscosity as functions of both the concentrations of surfactants and added salts were investigated, respectively. It showed that the addition of a small amount of NaX (e.g., 0.1 M) had only small effects on the C₁₀NE–C₁₀SO₃ vesicles. However, striking anion specificity, which differed anomalously with the anions of salts varying from F⁻ to I⁻, was observed when large amounts of NaX (e.g., 1 M) were added. The size of the C₁₀NE–C₁₀SO₃ aggregates was normally increased and then a liquid–liquid phase separation occurred with the addition of NaF; meanwhile, the addition of NaCl had little effect on the C₁₀NE–C₁₀SO₃ aggregates under experimental conditions; however, the size of the C₁₀NE–C₁₀SO₃ aggregates was abnormally decreased when high concentrations of NaBr or NaI (up to 2 M) were added, implying a vesicle-to-micelle transition which might be due to a decrease in the intramicellar attraction between oppositely charged headgroups by electrical shielding, and the mixtures still remained homogeneous. A cation specificity of added salts was also observed with the increase of the ionic radii from Na⁺ to Cs⁺. However, it was noted that the influence of increasing the ionic radii of cations on the aggregate size was not as pronounced as that of anions in the presence of high concentrations of salts. The aggregates of C₁₀NE–C₁₀SO₃ became a little smaller when concentrated alkali metal halides with bigger cations were added into this homogeneous equimolar cationic–anionic surfactant mixture. Both the observed cation and anion specificities were consistent with the Hofmeister series.