Abstract
In the present paper, we aim to investigate the dependence of surface tension on the surface properties and reveal the counter-ion effects on the adsorption of ionic surfactants on the solution surface. The surface tension, surface excess and surface concentration (defined as the amount of surfactant adsorbed in the surface phase divided by the surface area) of two anionic surfactants, namely dodecyl sulfate sodium and dodecyl sulfate caesium, dissolved in non-aqueous polar solvent formamide have been separately measured at 6 °C through independent experiments. Then, the correlation of surface tension with surface concentration and that of surface tension with surface excess is inspected in detail. It was found that there is a linear relationship between the surface tension and the surface concentration for the pure solutions of each surfactant, but their surface tension and surface excess cannot be correlated linearly. It is striking that the same surface tension-surface concentration linearity holds for two different surfactants, although they have apparently distinct counter-ions. Based on this finding, it is derived that the surface tension is decided by surface concentration of the surface active ions. After analyzing the surface structure, it is concluded that the counter-ions affect the surface tension indirectly through modifying the adsorption amount of the surface active ions in the surface layer.
Highlights
The relationship between surface tension and surface excess of a solution containing multiple components is ref. 17 as follows: XnÀds 1⁄4 Gidmi (1)i1⁄41 where s is the surface tension of the solution and mi the chemical potential of the ith component in the system, while Gi is the amount of component i the value of which depends on the location of the dividing plane.23386 | Phys
In a previous study,[18] we have found that the surface excess, surface tension and the structure of the surface layer are much different if their bulk concentrations are identical, indicating that the counter-ions, namely, caesium ion and sodium ion, exert profound effects on the adsorption behaviour of those two ionic surfactants
We have investigated the dependence of surface tension on surface excess and surface concentration for
Summary
The relationship between surface tension and surface excess of a solution containing multiple components is ref. 17 as follows: XnÀds 1⁄4 Gidmi (1)i1⁄41 where s is the surface tension of the solution and mi the chemical potential of the ith component in the system, while Gi is the amount of component i the value of which depends on the location of the dividing plane.23386 | Phys. The relationship between surface tension and surface excess of a solution containing multiple components is ref. 17 as follows: Xn. i1⁄41 where s is the surface tension of the solution and mi the chemical potential of the ith component in the system, while Gi is the amount of component i the value of which depends on the location of the dividing plane. If the position z0 of the dividing plane is chosen to make. G1 = 0, G2 is the excess of solute per unit surface area. This surface excess, G2, is denoted by Ge2xcess, and it can be integrated with ðz[0] Ge2xcess 1⁄4 cðzÞdz þ ðzB
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