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)...

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Bibliographic Details
Published in:Physical chemistry chemical physics : PCCP 2011-03, Vol.13 (9), p.3881-3885
Main Authors: Wang, Chuangye, Morgner, Harald
Format: Article
Language:English
Subjects:
Adsorption
Anions
Blends
Cadmium sulfides
Chemistry
Exact sciences and technology
General and physical chemistry
Sodium
Sulfates
Surface chemistry
Surface physical chemistry
Surfactants
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Summary: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. Cesium ions are more competitive and get closer to the adsorption layer of dodecyl sulfate ions, compared to sodium ions.
ISSN:1463-9076
1463-9084
DOI:10.1039/c0cp01600d