The competitive adsorption of counter-ions at the surface of anionic surfactants solution

We have determined the surface excess of surface active anion and counter-ions in a non-aqueous polar solution of anionic surfactants blends, as well as their distributions near the solution surface. The blends of two anionic surfactants, sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CDS), with different contents were used as solutes to prepare the solutions. According to the isotherms that are separately fitted to the pure SDS and the pure CDS solutions (C. Wang and H. Morgner, Langmuir, 2010, 26, 3121), CDS has a slightly but significantly higher surface excess than SDS (CDS is 14.8% higher) at the concentration of 0.04 molal kg(-1) solvent. Therefore, in this work we chose 0.04 molal kg(-1) solvent as total anion concentration and varied the contents of surfactants. From present experimental results, we found that the surface excess of anion increases slightly with the CDS in the bulk content. Importantly, the fractions of Cs(+) in cationic surface excess are higher than its contents in the bulk for all three solutions. This demonstrates that Cs(+) is more competitive than Na(+) in the adsorption. The surface structure of the solutions have been characterized by concentration-depth profiles, of Cs(+), Na(+) and of sulfur which is used to identify dodecyl sulfate. Those profiles evidence that Cs ions penetrate deeper than sodium ions into the layer formed by the heads of the anions, reducing the electrical potential of the surface more efficiently. This can be used to explain the adsorption competition between those two counter-ions. The cause that makes Cs(+) more competitive than Na(+) in the adsorption can be attributed to its less tightly bound solvation shell, and thus, to its effectively smaller ion size.