Arshadi et al., hereafter referred to as ARSH20, call into question results from our previous work, hereafter referred to as SCH19) that concluded that a Freundlich-based isotherm is more appropriate than a Langmuir-based isotherm for predicting the uptake of perfluorooctanesulfonate (PFOS) and pertluorooctane carboxylate (PFOA) at low concentrations to the air-water interface. First, the generalized form of the Gibbs Adsorption Equation used by SCH19 in eqs 2 and 3 accounts for changes in ionic strength, thus including data at the different NaCl concentrations are appropriate (the exception being deionized water, which is discussed in detail by SCH19). Second, although the Freundlich isotherm often is used empirically, a theoretical basis for the Freundlich isotherm applied to adsorption at the air-water interface has been rigorously developed, thus providing a basis for applying the Freundlich isotherm to describe PFOS and PFOA sorption at the air-water interface. It is noted that the theoretical basis for the Freundlich model applied to air-water interracial sorption assumes that concentrations are well below monolayer saturation. The PFOS and PFOA concentrations used in SCH19 were well below the critical micelle concentration, and the surface tension data were well described by the Freundlich model (Figures 1 and 2 in SCH19). We agree with ARSH20 that the Langmuir model is more appropriate when modeling PFOS and PFOA at concentrations approaching monolayer saturation, but this does not negate the appropriateness of a Freundlich-based isotherm for describing behavior at lower concentrations.
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