Abstract
In our latest communication, we proved experimentally that the ionic surfactant’s surface excess is exclusively determined by the size of the hydrated counterion.[Lunkenheimer, Langmuir,2017, 33, 10216−1022410.1021/acs.langmuir.7b00786]28925711. However, at this stage of research, we were unable to decide whether this does only hold for the two or three lightest ions of lithium, sodium, and potassium, respectively. Alternatively, we could also consider the surface excess of the heavier hydrated alkali ions of potassium, rubidium, and cesium, having practically identical ion size, as being determined by the cross-sectional area of the related anionic extended chain residue. The latter assumption has represented state of art. Searching for reliable experimental results on the effect of the heavier counterions on the boundary layer, we have extended investigations to the amphiphiles’ solutions of concentrations above the critical concentration of micelle formation (cmc).We provided evidence that the super-micellar solutions’ equilibrium surface tension will remain constant provided the required conditions are followed. The related σcmc-value represents a parameter characteristic of the ionic surfactant’s adsorption and micellization behavior. Evaluating the amphiphile’s surface excess obtained from adsorption as a function of the related amphiphile’s σcmc-value enables you to calculate the radius of the hydrated counterion valid in sub- and super-micellar solution likewise. The σcmc-value is directly proportional to the counterion’s diameter concerned. Taking additionally into account the radii of naked ions known from crystal research, we succeeded in exactly discriminating the hydrated alkali ions’ size from each other. There is a distinct sequence of hydration radii in absolute scale following the inequality, Li+ > Na+ > K+ > (NH4)+ > Rb+ > Cs+. Therefore, we have to extend our model of counterion effectiveness put forward in our previous communication. It represents a general principle of the counterion effect.
Highlights
In refs 1 and 2, we have reported on the behavior of counterions in adsorption layers of 1:1 ionic alkali perfluoro-nalkanoates at the air/water interface
Reminding the findings that the common adsorption parameters of the alkali perfluoro-n-octanoates are stable and reliable,[1,2] we asked what the detailed physicochemical meaning of the different σcmc-values is due to and whether it would be possible to determine the absolute size of the hydration sphere of the related alkali counterions solely by their surface tension data without regard to the hydration data received from other bulk methods
We proved that the absence of intermingling trace impurities at bulk concentrations at and above the critical concentration of micelle formation will result in a constant, characteristic equilibrium surface tension value σcmc
Summary
In refs 1 and 2, we have reported on the behavior of counterions in adsorption layers of 1:1 ionic alkali perfluoro-nalkanoates at the air/water interface. Reminding the findings that the common adsorption parameters of the alkali perfluoro-n-octanoates are stable and reliable,[1,2] we asked what the detailed physicochemical meaning of the different σcmc-values is due to and whether it would be possible to determine the absolute size of the hydration sphere of the related alkali counterions solely by their surface tension data without regard to the hydration data received from other bulk methods If this approach was successful, the resulting hydration radii would be the true ones encountered under experimental conditions in the boundary as well as in the bulk phase. The correct σcmc-value of the alkali perfluorooctanoate surfactant will be gained
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