Adsorption Properties of Surface Chemically Pure Sodium Perfluoro‑<i>n</i>‑alkanoates at the Air/Water Interface: Counterion Effects
within Homologous Series of 1:1 Ionic Surfactants
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Abstract
The
unusual behavior of saturation adsorption calculated from experimental
equilibrium surface tension (σ<sub>e</sub>) versus logarithm
of concentration (<i>c</i>) isotherms within the homologous
series of aqueous sodium perfluoro-<i>n</i>-alkanoate solutions
represents a particular problem in the adsorption of homologous ionic
1:1 amphiphiles at fluid interfaces. Special precautions were taken
to guarantee surface-chemical purity for all solutions, avoiding falsifying
effects by surface-active trace impurities. Surprisingly, all homologues’
adsorption isotherms reveal ideal surface behavior. The minimal surface
area demand per molecule adsorbed for shorter-chain homologues slightly
decreases with increasing chain lengths but then goes up steeply after
having passed a minimum. A similar feature has been observed with
the chemically quite different homologous series of the hydrocarbon
surfactants of sodium-<i>n</i>-alkylsulfates. Comparing
the corresponding 3D saturation concentrations in the boundary layer
and in the bulk, it becomes evident that at high bulk concentrations
when boundary layer and bulk concentrations are of the same order
of magnitude the adsorption behavior may be treated as that of a pseudononionic
surfactant. However, under conditions of the homologues’ strongest
surface activity, adsorption seems to become increasingly governed
by electrostatic repulsion, resulting in increasingly greater cross-sectional
areas. Deviation from pseudononionic behavior sets in when the Debye
length becomes distinctly greater than the adsorbent’s diameter
at saturation. Formerly available theories on ionic amphiphiles’
adsorption deal either with electrical conditions of surfactant ions
and counterions in the adsorption boundary layer or alternatively
with pseudononionic behavior neglecting the former theories completely.
Warszynski et al.’s novel theoretical model of the “surface
quasi-two-dimensional electrolyte” seems to be capable of describing
the adsorption of ionic amphiphiles at fluid interfaces in general.
We conclude that the conditions of the two alternative approaches
may be met within homologous series of ionic amphiphiles as limiting
cases only