Abstract
In order to clarify the adsorption behavior of cationic surfactants on the air/aqueous electrolyte solution surface, we derived the theoretical equation for the surface tension. The equation includes the electrical work required for charging the air/water surface and the work attributable to the configurational entropy in the adsorbed film. By fitting the equation to the experimental data, we determined the binding constant between adsorbed surfactant ion and counterion, and found that the bromide ions, rather than the chloride ions, are preferentially adsorbed by the air/water surface. Furthermore, it was suggested that the contribution of configurational entropy to the surface tension is predominant in the presence of electrolytes because of the increase in the surface density of surfactant molecules associated with decreasing the repulsive interaction between their hydrophilic groups.
Highlights
The surface tension of pure water increases by adding an electrolyte [1,2,3,4,5]
The electrolyte ions are repelled from the air/water surface, which is known as negative adsorption
The presence of an adsorbed film of ionic surfactant attracts the electrolyte ions toward the air/water surface, because the adsorbed film provides the charged surface
Summary
The surface tension of pure water increases by adding an electrolyte [1,2,3,4,5]. This phenomenon has been theoretically explained by the concept of image charge tentatively existing in the air phase and an ion-free layer due to the hydration of electrolyte ions [6,7]. In a previous study [16], we derived the theoretical equation for the surface tension of aqueous solutions of ionic surfactant and electrolyte mixtures, which have a common ion, and evaluated the configurational entropy, the adsorption constant of surfactant ion, and the binding constant of counterion, by applying the theoretical equation into the experimental results of surface tension measurement in a lower concentration region. These parameters made it possible for us to discuss the adsorption behavior on the air/water surface. In the respect that two different kinds of counterion coexist in the aqueous solution, this makes it possible to discuss directly the preferential adsorption of counterion to the charged surface, as well as the contribution of configurational entropy to the surface tension
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