ANALYTICAL STUDY OF HIGHLY CONDENSED NONIONIC SURFACTANTS - SEPARATION ON CATION EXCHANGERS

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 C-16 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 (M2+, 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=degrees-C] allowed for the first time the distributions of Brij 99 and KM 25 to be obtained.