Mixtures of anionic and cationic
surfactants exhibit synergistic behavior as evidenced by low critical
micelle concentrations (CMC) of the mixed system, increased surface activity, and improved detergency performance. The adsorption of a single-head anionic
surfactant,
sodium dodecyl sulfate (SDS), in mixture with a twin-head cationic
surfactant,
pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), showed synergism of adsorption onto
silica when present at a mixing ratio of 1:3 (cationic-rich), and also demonstrated lower
surfactant desorption with water
flushing of columns packed with the
surfactant-modified media. In addition, the proportion of the mixed
surfactants in the admicelles moved from the initial ratio of 1:3 towards equimolar after rinsing the
surfactant-modified
silica absorbent. The retardation of organic solutes passing through columns packed with modified-
silica adsorbent increased nominally three fold for
silica modified with mixed
surfactants versus single
surfactants (retardation factors increase from 4.0 to 12.8 for
styrene and from 32.1 to 90.2 for
ethylcyclohexane for single and mixed
surfactants, respectively). Thus, this study demonstrates that mixed
surfactant systems more effectively modified the
silica surface than single
surfactant systems both in terms of enhanced retardation of organic solutes and in terms of reduced
surfactant desorption.