Analytical study of highly condensed non ionic surfactants. Separation on cation exchangers

Summary This study was aimed at optimizing the separation of non-ionic surfactants, resulting from the condensation of ethylene oxide with natural fatty alcohols in the C₁₆ and C₁₈ range (saturated and unsaturated), and presenting a high degree of condensation, i.e. 20 and 25 ethylene oxide units (Brij 99 and KM 25). The cation exchange stationary phase is a partially ionized silica, conditioned in different ways. We have studied the influence of the nature of cations on the separation selectivity. Cations studied included alkali metals (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), alkaline earth metals (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), transition metals (Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Cu²⁺), and quaternary ammonium (NH₄⁺, NMe₄⁺) ions as well as a proton (acetic acid). The influences of ionic strength, pH and addition of cosolvent were examined. A study of the influence of temperature on the system selectivity evidenced a strong interdependance of these two parameters. The optimized conditions [mobile phase: CH₃CN/H₂O (92/8), pH 7.4, sodium acetate 5.10^–3 M; temperature gradient between 25 and 50°C] allowed for the first time the distributions of Brij 99 and KM 25 to be obtained.