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

Abstract

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.

Thermodynamics and structure of liquid surfaces investigated directly with surface analytical tools**

“…Wang et al. found for the anionic surfactants sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CDS) in formamide solutions that the surface excess depends on the alkali cation . Keeping the total surfactant concentration constant Wang et al.…”

“…The left panel shows the surface excess of mixed SDS/CDS formamide solution. The total surfactant concentration was kept constant and the ratio between Na + and Cs + was changed . The right panel shows the position of the cation and the anion of both the SDS and the CDS solution .…”

“…Reproduced (a) from Ref. , with permission from the PCCP Owner Societies. Reprinted (b) from with permission of Springer.…”

Ion Scattering Studies of Molecular Structure at Liquid Surfaces with Applications in Industrial and Biological Systems

“…80 Pluis et al 5b have investigated melting at the surface of a cylindrically shaped Pb crystal via medium-energy ion scattering. They found that open faces like (112), (113), and (115) show surface melting while the closed face (111) did not, and that surface melting is controlled by excess free energy at the surface. Surface melting was also found at the Ge(111) surface in a similar way.…”

Molecular structure of ionic surfactant solution surface and effects of counter-ion therein—a joint investigation by simulation and experiment

“…However, so far, only few techniques are able to directly determine the concentration-depth profiles, i.e., the distributions of the species at liquid surfaces. Those techniques incorporate neutron reflectivity (NR) [18][19][20], X-ray reflectivity (XR) [21][22][23], angular resolve X-ray photoelectron spectroscopy (ARXPS) [24][25][26][27], and neutral impact collision ion scattering spectroscopy (NICISS) [28][29][30][31][32][33]. On the other hand, recently, the computer simulation, such as Monte Carlo (MC) and molecular dynamics (MD), has been widely employed to investigate the molecular structure of liquid surfaces, for the sake of overcoming those shortcomings of and as a complement of the experimental techniques.…”