Adsorption Properties of Surface Chemically Pure Sodium Perfluoro-n-alkanoates at the Air/Water Interface: Counterion Effects within Homologous Series of 1:1 Ionic Surfactants

Abstract

The unusual behavior of saturation adsorption calculated from experimental equilibrium surface tension (σ(e)) versus logarithm of concentration (c) isotherms within the homologous series of aqueous sodium perfluoro-n-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-n-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.

Assessing the potential contributions of additional retention processes to PFAS retardation in the subsurface

“…The enrichment factors measured for PFOS and PFOA were 158 and 63, respectively, demonstrating very large preferential adsorption at the interface. Others have reported similar measurements of strong air-water interface adsorption, measured in terms of surface excess, for PFOS and PFOA (Downes et al, 1995; Vecitis et al, 2008; Lunkenheimer et al, 2015). Recently, it has been hypothesized that adsorption to the air-water interfaces of air bubbles trapped on the surfaces of carbonaceous water-treatment sorbents is a primary source of the retention they afford (Meng et al, 2014).…”

The influence of molecular structure on the adsorption of PFAS to fluid-fluid interfaces: Using QSPR to predict interfacial adsorption coefficients

“…Thus, the Szyszkowski equation was applied to all of the measured data sets to provide a uniform means of data analysis. Numerous authors have demonstrated that the Szyszkowski equation provides accurate representation of surfactant surface-tension and interfacial-tension data (e.g., Adamson, 1982;Schick, 1987;Fainerman et al, 2001; Barnes and Gentle, 2005;Berg, 2010;Rosen and Kunjappu, 2012;Zhong et al, 2016), including for PFAS (e.g., Vecitis et al, 2008;Lunkenheimer et al, 2015). One form of the equation is given as (e.g., Adamson, 1982;Barnes and Gentle, 2005):…”

“…The training set consists of the homologous series of PFCAs and PFSAs, which represent the "standard" linear anionic PFAS. Prior research has shown that the surface activity of these PFAS is a function of chain length (e.g., Hendricks, 1953;Shinoda et al, 1972;Tamaki et al, 1989;Kissa, 2001;Lunkenheimer et al, 2015). The PFCAs and PFSAs comprise 10 and 5 compounds, respectively, for a total of 15 data points for the training set.…”

“…For another example, replacing a single fluorine with a hydrogen results in a ∼4-times smaller K i value for Na-9H hexadecafluorononanoate versus Na-PFNA. This latter phenomenon relates to the well-known significant difference in free energies of adsorption from solution between CF and CH groups (e.g., Shinoda et al, 1972;Mukerjee and Handa, 1981;Lunkenheimer et al, 2015).…”

“…Measured surface-tension data for the homologous Na-perfluorocarboxylate series. Data reproduced from Lunkenheimer et al (2015) and Tamaki et al (1989), except for C8-UAZ, which is from Lyu et al (2018). QSPR model for log K i versus molar volume for all data.…”