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. Author manuscript; available in PMC: 2020 Sep 15.
Published in final edited form as: Water Res. 2019 May 29;161:17–26. doi: 10.1016/j.watres.2019.05.095

The Influence of Surfactant and Solution Composition on PFAS Adsorption at Fluid-Fluid Interfaces

Mark L Brusseau 1,2,*, Sarah Van Glubt 1
PMCID: PMC7039257  NIHMSID: NIHMS1068180  PMID: 31174056

Abstract

The objective of this research is to examine the influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces. Surface tensions were measured for select PFAS, as well as a representative hydrocarbon surfactant. These data are supplemented with data sets collected from the literature. The influence of surfactant headgroup charge, specifically for zwitterionic PFAS, was investigated. The impacts of surfactant counterion for ionic PFAS and the influence of headgroup size for nonionic PFAS were also investigated. In addition, the influence of solution ion composition, ionic strength, and pH was examined. The impact of co-solutes, specifically ethanol, humic acid, and trichloroethene, was also examined, as well as the behavior of PFAS mixtures and fluorocarbon-hydrocarbon surfactant mixtures. The data were interpreted within the framework of a QSPR model recently developed to predict fluid-fluid interfacial adsorption coefficients (Ki) of PFAS. The results demonstrate that all of the factors investigated have some degree of impact on Ki values. Thus, the composition of soil-pore water and groundwater is likely to affect the magnitude of PFAS adsorption at air-water and organic liquid-water interfaces. However, the influence on Ki of most of the factors investigated is small for lower PFAS concentrations (less than ~1–10 mg/L). Hence, their impacts on fluid-fluid interfacial adsorption are likely to be relatively minor at the low PFAS concentrations representative of many environmental systems, especially compared to the impact of other factors such as fluid saturations, porous-medium properties, and PFAS molecular structure. The results of this study indicate that the revised QSPR model provides reasonable first-order predictions of Ki for a wide range of PFAS in environmental systems.

Keywords: Perfluoroalkyl, PFOA, PFOS, retardation, partitioning, air-water interfacial adsorption, QSPR

1. Introduction

Per-and poly-fluoroalkyl substances (PFAS) have become emerging contaminants of critical concern. Comprehensive understanding of the transport and fate of PFAS in the environment is necessary for accurate risk assessments, effective site characterization efforts, and successful planning and implementation of remediation projects. A major unresolved issue is the transport behavior of PFAS in source zones, including the types, magnitudes, and rates of retention, and the resultant mass flux to groundwater or surface water (SERDP, 2017).

Prior research has demonstrated that the vadose zone can serve as a significant source reservoir of PFAS (Shin et al., 2011; Xiao et al., 2015; Baduel et al., 2017; Weber et al., 2017; Anderson et al., 2019). Recent studies have demonstrated that adsorption of PFAS at air-water interfaces can be an important retention process for transport in variably saturated porous media (Brusseau, 2018; Lyu et al., 2018; Brusseau et al., 2019). The presence of organic immiscible liquids (OIL) such as chlorinated solvents and hydrocarbon fuels presents an additional complicating factor for PFAS transport in source zones. In such cases, adsorption of PFAS at OIL-water interfaces may be another relevant retention process (Brusseau, 2018; Brusseau et al., 2019).

Implementing transport modeling and conducting risk assessments of PFAS for systems wherein air-water or OIL-water interfacial adsorption is relevant requires knowledge of the fluid-fluid interfacial adsorption coefficient, Ki. Measuring Ki values for the many hundreds of individual PFAS that exist is impractical. As an alternative, quantitative-structure/property-relationship (QSPR) analysis methods can be used to provide empirical-based estimates. This approach is widely used for the prediction of many partition and adsorption coefficients. Brusseau (2019) recently conducted a QSPR analysis of air-water and OIL-water interfacial adsorption of 42 individual PFAS comprising homologous series of perfluorocarboxylates and perfluorosulfonates, branched perfluoroalkyls, polyfluoroalkyls, alcohol PFAS, and nonionic PFAS. The Ki values, determined from analysis of surface-tension and interfacial-tension data sets tabulated from the literature, varied across eight orders of magnitude and were a function of molecular structure. The results of the QSPR analysis demonstrated that a model employing molar volume (Vm) as a descriptor provides robust predictions of log Ki values (Figure 1).

Figure 1.

Figure 1.

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Molar volume (Vm) based QSPR model for fluid-fluid interfacial adsorption of PFAS and hydrocarbon surfactants. Modified from Brusseau (2019). The “original data” represent the measured data reported in Brusseau (2019) along with the original QSPR regression. The newly added data are referenced in Table 1. The dashed lines represent the lower and upper bounds of prediction uncertainty at the 95% confidence level for the original QSPR model. DIW is deionized water, SGW is synthetic groundwater, NaCl is 0.01 M NaCl solution. The new regression includes the new measured data and the original data for which Vm > 300 (i.e., wherein the effects of solution composition are minimal). The new regression equation is: log Ki = 0.0174(±0.0015)Vm – 7.05(±0.4), r2 = 0.95.

One potential limitation to the prior QSPR analysis is that the surface/interfacial-tension source data were measured for ideal solutions comprising deionized water. It is well established that the surface activity of surfactants is in general sensitive to solution composition, including ion composition, ionic strength, pH, and the presence of co-solutes. In addition, the ionic PFAS used in the QSPR analysis comprised alkali salts, primarily Na and K salts. However, PFAS also consist of acid forms and other counterion species including organics, and it is known that the nature of the counterion can influence the surface activity of surfactants. Finally, the data were measured for single-surfactant systems, whereas PFAS mixtures are prevalent for many environmental systems.

The objective of this research is to examine the influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces. Surface tensions are measured for select PFAS, as well as a representative hydrocarbon surfactant. These data are supplemented with data sets collected from the literature. The influence of surfactant headgroup charge, specifically for zwitterionic PFAS, is investigated. The impacts of surfactant counterion for ionic PFAS and the influence of headgroup size for nonionic PFAS are also investigated. In addition, the influence of solution ion composition, ionic strength, and pH are examined. The presence of co-solutes, specifically ethanol (representative miscible cosolvent), humic acid (representative dissolved organic carbon), and trichloroethene (representative contaminant), are also examined, as well as PFAS mixtures. The results are interpreted within the framework of the prior QSPR model.

Materials and Methods

Materials

Perfluorooctanoic acid (CAS#335–67-1; of 98% purity) was purchased from AIKE Reagent. Sodium perfluorooctanoate (CAS#335–95-5; 97%) was purchased from Manchester Organics. Sodium perfluoropentanoate (CAS#2706–89-0; >99%) was purchased from Synquest Laboratories. Potassium perfluorooctanesulfonate CAS# 2795–39-3; 98%) was purchased from Matrix Scientific. Perfluorotridecanoic acid (CAS# 72629–94-8; 97%), perfluorooctanesulfonic acid (CAS# 1763–23-1, 98%), sodium dodecylbenzene sulfate (CAS# 25155–30-0; technical grade), trichloroethene (CAS# 79–01-6; >99.5%), and humic acid (CAS#1415–93-6; technical grade) were purchased from Sigma Aldrich. Ethanol (CAS# 64–17-5; 99.9%) was purchased from Aaper Alcohol.

Different background electrolyte solutions were used to test the influence of solution ionic composition on surface tension. These included deionized water, 0.01 M NaCl, 0.1 M NaCl, 0.01 M KCl, 0.01 M CaCl2, and a synthetic groundwater (SGW). The SGW (concentration, mg/L) consists of the following cations: Na+1 (50), Ca+2 (36), Mg+2 (25), and anions: NO3−1 (6), Cl−1 (60), CO3−2/HCO3−1 (133), and SO4−2 (99). The pH and ionic strength of the groundwater solution are 7.7 and 0.01M, respectively. Solutions were prepared using distilled, deionized water.

Methods

The surface tensions of aqueous PFAS solutions were measured using a De Nouy ring tensiometer (Fisherscientific, Surface Tensiomat 21) following standard methods (ASTM D1331– 89). In addition, interfacial tensions between trichloroethene and aqueous solutions of PFOS were measured. The tensiometer was calibrated with a weight of known mass. Each sample was measured three times with the deviation between measurements less than 0.7%. All measurements were conducted at room temperature (25±1ºC).

Surface-tension data were also obtained from the literature to expand the investigation of surfactant and solution property effects. Data sets were available to test the influence of headgroup size, counterion type, pH, and the presence of surfactant mixtures. The open-source Engauge program was used to digitize the reported surface-tension data sets (Mitchell et al., 2017). The compounds and data sources are compiled in Table 1.

Table 1.

Sources for new data presented in Figure 1.

Acronym Formula Compound Notes Surface Tension Data Source
PFAS Acids in DI water
PFAA CF3CO2H Perfluoroacetic acid Hendricks 1953
PFPrA C2F5CO2H Perfluoropropionic acid Hendricks 1953
PFBA C3F7CO2H Perfluorobutanoic acid Hendricks 1953/Klevens 1954/Bernett 1959
PFHxA C5F11CO2H Perfluorohexanoic acid Hendricks 1953
PFOA C7F15CO2H Perfluorooctanoic acid Hendricks 1953/Klevens 1954/Bernett 1959/Shinoda 1972/Schild 1992/This Study
PFDA C9F19CO2H Perfluorodecanoic acid Hendricks 1953/Klevens 1954/Bernett 1959
PFTDA C12F25CO2H perfluorotridecanoic acid This Study
         
PFAS in SGW
Na-PFPeA C4F9CO2Na Na-Perfluoropentanoate This Study
Na-PFOA C7F15CO2Na Na-Perfluorooctanoate This Study
PFTDA C12F25CO2H Perfluorotridecanoic acid This Study
SDBS C18H29SO3Na Na-dodecyl benzenesulfonate Hydrocarbon This Study
PFOS C8F17SO3H Perfluorooctanesulfonic acid PCE-Water This Study
K-PFOS C8F17SO3K K-Perfluorooctanesulfonate This Study
         
PFAS Salts in 0.01 M NaCl
Na-PFOA C7F15CO2Na Na-Perfluorooctanoate This Study
Na-PFPeA C4F9CO2Na Na-Perfluoropentanoate This Study
SDBS C18H29SO3Na Na-Dodecyl benzenesulfonate Anwar 2000/Schaefer 2000/Araujo 2015
K-PFOS C8F17SO3K K-Perfluorooctanesulfonate This Study
         
PFAS Acids in 0.01 M NaCl
PFOA C7F15CO2H Perfluorooctanoic acid Lyu 2018
PFOS C8F17SO3H Perfluorooctanesulfonic acid PCE-Water This Study
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Fluid-fluid interfacial adsorption coefficients are determined from the surface/interfacial tension function. The surface excess Γ (mol/cm2) of a compound at the interface is related to the aqueous-phase concentration (C) using the Gibbs equation (e.g., Adamson, 1982; Hiemenz, 1986; Pashley and Karaman, 2004; Barnes and Gentle, 2005; Berg, 2010; Rosen and Kunjappu, 2012; Kronberg et al., 2014):

Γ=−1xRT∂γ∂lnC (1)

where γ is the interfacial tension (dyn/cm or mN/m), C is the aqueous phase concentration (mol/cm3), T is temperature (°K), R is the universal gas constant (dyne-cm/mol °K), and x is a coefficient equal to 1 for systems with nonionic surfactants (or ionic surfactants with excess electrolyte in solution), and equal to 2 for systems with ionic surfactants without excess electrolyte (e.g., deionized water).

The amount adsorbed at the interface as a function of aqueous concentration (adsorption isotherm) can be determined as (e.g., Fridrikhsberg, 1986; Hiemenz, 1986; Barnes and Gentle, 2005; Berg, 2010):

Ki=ΓC=−1xRTC∂γ∂lnC (2)

where Ki (cm) is the fluid-fluid interfacial adsorption coefficient for the fluid pair of interest (e.g., air-water interfacial adsorption coefficient for air-water systems, and OIL-water interfacial adsorption coefficient for OIL-water systems). Surfactant adsorption at the fluid-fluid interface is nonlinear. Ki can be determined for any given aqueous concentration by calculating the local slope of surface tension versus ln C (i.e., Γ in equation 1) through use of a tangent taken at the concentration of interest (e.g., Fridrikhsberg, 1986; Hiemenz, 1986; Kim et al., 1997; Brusseau, 2019), and dividing by the relevant C (equation 2). The results of recent studies have demonstrated that PFAS Ki values determined from surface-tension data in this manner are consistent with values determined from miscible-displacement transport experiments (Lyu et al., 2018; Brusseau et al., 2019).

A target concentration of 0.1 mg/L was used to calculate Ki values for this work, unless otherwise noted. The selected value is within the upper range of groundwater concentrations reported for PFAS at contaminated sites (e.g., Moody and Field, 1999; Ahrens, 2011; Schultz et al., 2004; McGuire et al., 2014; Cousins et al., 2016; Baduel et al., 2017). Brusseau (2019) demonstrated that, due to adsorption nonlinearity, the Ki values obtained for this target concentration represent essentially maximum values for most of the examined PFAS. Lower concentrations (e.g., 0.01 mg/L) are needed to determine maximum Ki values for larger PFAS due to their greater surface activity. The Ki values determined for this range of concentrations are anticipated to be representative for many environmental systems of interest.

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; Brusseau, 2019). One form of the equation is given as (e.g., Adamson, 1982; Barnes and Gentle, 2005):

γ=γ0[1−B ln(1+CA)] (3)

where γ0 is the interfacial tension at [PFAS] = 0 (e.g., the surface tension of pure water), and A and B are variables related to properties of the specific compound and of the homologous series, respectively. The best-fit functions were used to obtain the slope factors required for equation 1 for all data sets. Calculations and statistical analyses were conducted within excel.

The single-descriptor approach was used for the QSPR analysis, with molar volume (Vm, cm3/mol) as the descriptor. Whole-molecule molar volumes were calculated as the ratio of compound density and molecular weight. For the analysis of headgroup-size effect, the molar volumes of the respective headgroups and tails for each compound were calculated using a standard group-contribution analysis approach, employing the Schroeder method (e.g., Baum, 1998; Reinhard and Drefahl, 1999; Poling et al., 2000). More details on the QSPR methods are provided in Brusseau (2019).

Results and Discussion

Surface-tension Data

Studies reporting surface-tension data rarely if ever examine measurement reproducibility and uncertainty. Triplicate measurements of Na-PFOA surface tension are presented in Figure 2 for solutions of DI water and 0.01 M NaCl. Good reproducibility is observed overall. It is also important to consider data consistency when using data sets compiled from the literature. Brusseau (2019) examined consistency among nine surface-tension data sets reported for Na-PFOA in DI water. These data, along with two additional data sets, are presented in Figure 3A. As noted in the prior study, remarkable consistency is observed among the data sets, especially considering that the studies span several decades and involve the use of different measurement methods. Similar consistency is observed for the NH4-PFOA and PFOA acid (H-PFOA) surface-tension data sets presented in Figures 3B and 3C, respectively. The excellent reproducibility and consistency observed for these data sets supports the use of such data for assessing the impacts of surfactant and solution composition on surface activity and interfacial adsorption of PFAS.

Figure 2.

Figure 2.

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Replicate surface tensions measured for Na-perfluorooctanoate (PFOA) in solutions of deionized (DI) water and 0.01 M NaCl. Note that the data for 0.01 M NaCl #1 was originally reported in Lyu et al. (2018).

Figure 3.

Figure 3.

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Surface-tension data sets for perfluorooctanoate (PFOA) in deionized water compiled from the literature: (A) Na-PFOA (note that the two new data sets are listed below the simulation), (B) NH4-PFOA, (C) H-PFOA. The solid lines represent the best-fit simulation produced with the Szyszowski equation.

Influence of Surfactant Headgroup Charge

Brusseau (2019) demonstrated that a QSPR model employing molar volume was successful in predicting log Ki values for PFAS comprising a wide range of molecular structures (Figure 1). The model was effective for PFAS and hydrocarbon surfactants containing either anionic, cationic, or nonionic headgroups. However, no zwitterionic PFAS data sets were available to include in the analysis. Surface-tension data were recently reported for zwitterionic PFAS by Lin et al. (2018), Hill et al. (2018), and Shen et al. (2018). Lin et al. presented data for a synthesized PFAS with the formula CF3(CF2)2C(CF3)2CH2CONH(CH2)2N(CH3)2CH2CO2. Hill et al. presented data for a commercial PFAS, Capstone 1157, which has a molecular formula of CF3(CF2)5(CH2)2SO2NH(CH2)3N(CH3)2CH2CO2. Shen et al. presented data for a synthesized PFAS with the formula CF3(CF2)2OCFCF3CF2O CFCF3CONH(CH2)3N(CH3)2CH2CO2. A data set reported for cocamidopropyl betaine (Staszak et al., 2015), a zwitterionic hydrocarbon surfactant, is used for comparison. These data are used to test the effectiveness of the QSPR model for zwitterionic surfactants.

The air-water interfacial adsorption coefficients calculated herein from the reported surface-tension data are 0.02, 0.6, and 0.9 cm for the Lin, Hill, and Shen PFAS, respectively. The predicted Ki values obtained from the original QSPR model are 0.08 (0.15–0.4), 2 (0.4–10), and 4 (0.8–20) cm for the three compounds, respectively. The measured values are smaller than the predicted means, but are within the uncertainty ranges of the predictions. Measured and predicted values of 1.2 and 2.3 (0.5–11) cm are obtained for the zwitterionic hydrocarbon surfactant. These results suggest that the QSPR model is also effective for zwitterionic surfactants.

Influence of Surfactant Headgroup Size

The size of the headgroup for nonionic PFAS was noted to be a factor for the predictive capability of the QSPR model, wherein predicted log Ki values for compounds with very large headgroups were significantly larger than the corresponding measured values (Brusseau, 2019). This phenomenon results from the inability of the simple single-descriptor model to adequately represent the solution activity of the hydrophilic headgroups and the resultant impact on surface activity of the compound. The impact of headgroup size on prediction error was noted to depend upon the relative proportion of headgroup and tail components of the compound. This behavior is further investigated herein to quantify the impact of headgroup size.

Log Ki values were determined for 11 nonionic PFAS and 2 nonionic hydrocarbon surfactants (Triton) from reported surface-tension data. Predicted log Ki values were then obtained from the QSPR model of Brusseau (2019). The molar volumes of the respective headgroups and tails for each compound were calculated to determine the ratio of headgroup Vm to tail Vm (HG-T Vm). The HG-T Vm ranges from 0.5 to 4.6 for the 13 nonionic surfactants evaluated. For reference, the HG-T Vm ratio for PFOA is 0.14.

The deviation between the measured and predicted log Ki values is plotted as a function of the HG-T Vm ratio in Figure 4. It is observed that the predicted values match the measured values for HG-T Vm ratios less than approximately 1.3. The log-deviation increases progressively in a linear manner for greater ratios. As discussed by Brusseau (2019), improving the predictive capability of the QSPR model for compounds with larger headgroups would require a fragment-based multiple-descriptor model that characterizes separately the solution behavior of the hydrophobic and hydrophilic components, as for example the model developed by Huibers et al. (1996) to predict critical micelle concentrations. However, based on prior research (e.g., Soares et al., 2008; Fromel and Knepper, 2010), it may be anticipated that the long headgroups will be degraded relatively quickly in many environmental systems, and therefore that long headgroup surfactants would be much less prevalent than their shorter headgroup counterparts (Brusseau, 2019).

Figure 4.

Figure 4.

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Influence of headgroup size of nonionic PFAS and hydrocarbon (Triton) surfactants on prediction error for the QSPR model, with “log deviation” representing the difference between the measured and predicted log Ki values.

Influence of Counterion

The influence of alkali-metal counterion composition on surface activity was examined for PFOA by Lunkenheimer et al. (2017). They showed that surface activity varied as a function of the counterion, with greater surface activities measured for counterions of smaller hydrated radius. This effect was attributed to differences in the influence of the different counterions on activity and structure within the adsorbed layer. More surfactant monomers may reside within the adsorbed layer in the presence of smaller counterions, which will increase surface activity and result in a greater reduction in surface tension. Similar results have been observed for hydrocarbon surfactants, as reviewed by Gorodinsky and Efrima (1994).

The influence of alkali-metal counterion on measured PFOA surface tensions is demonstrated in Figure 5, wherein is presented data compiled from the literature. It is observed that surface activity at higher concentrations is lowest for Li-PFOA (with the largest hydrated radius) and greatest for NH4-PFOA (with a smaller hydrated radius). This is consistent with the results reported by Lunkenheimer et al. (2017). Ki values determined herein from the surface-tension data reported in Figure 5 are 0.00020 for Li-PFOA, 0.00023 for Na-PFOA, and 0.00021 cm for NH4-PFOA, for a concentration of 0.1 mg/L. The Ki values are identical within measurement uncertainty for the low concentrations representative of most environmental systems. Thus, the type of alkali-metal counterion present is generally expected to have minimal impact on the magnitude of fluid-fluid interfacial adsorption of PFAS in environmental systems.

Figure 5.

Figure 5.

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Compiled surface-tension data for perfluorooctanoate (PFOA) in DI water. The data for H-PFOA, NH4-PFOA, and Na-PFOA are those presented in Figure 3; the data for Li-PFOA are from Lunkenheimer et al. (2017) and the data for tripropylammonium-PFOA (TPNH4) are from Pottage et al. (2016). The solid lines represent the best-fit simulation produced with the Szyszowski equation.

As noted, the impact of different alkali-metal counterions is minimal. However, the Ki value for the PFOA acid (i.e., H+ as the counterion) in DI water is 0.0008 cm, approximately 4-times greater compared to the PFOA salts in DI water. The difference in surface activities between H-PFOA and the PFOA salts is influenced not only by counterion size differences, but also by the pH decrease that occurs at higher H-PFOA concentrations. The effect of pH on surface activity will be further discussed below.

Organic compounds are also used as counterions for PFAS. In particular, alkylammonium ions are frequently used as counterions to modify surface activity and aggregation behavior of PFAS. Pottage et al. (2016) examined the impact of a series of alkylammonium counterions on the surface activity of PFOA. Changing the counterion from ammonium to alkyl ammonium results in an increase in surface activity, as illustrated in Figure 5. Ki values for the series of alkylammonium-PFOAs along with NH4-PFOA for comparison are presented in Table 2. Ki is observed to increase as a function of the number of carbons comprising the counterion. For example, the Ki value for methylammonium-PFOA is 3.7-times larger than the NH4-PFOA value, versus a factor of 6.6 for propylammonium-PFOA. The structure of the alkylammonium counterion also influences its impact on the surface activity of PFOA. For example, the Ki value for propylammonium-PFOA is twice as large as the value for trimethylammonium-PFOA, both of which contain three carbons. The carbons are arranged as a single alkyl chain for the former, while they are distributed as single carbons for the latter. In addition to influencing electrostatic and structural interactions within the adsorbed layer, the organic counterions influence hydrophobic interactions in solution through the presence of the alkyl groups. With sufficient chain length, the counterions themselves behave as (cationic) surfactants. Similar results have been reported for PFOS (Li et al., 2004; Gao et al., 2014).

Table 2.

Air-water interfacial adsorption coefficients (Ki) for perfluorooctanoate (PFOA) with various alkylammonium counterions in DI water.

Counterion Ki (cm)a,b Factor Deviation from NH4-PFOA
NH4 0.00021 -
methyl-NH4 0.00077 3.7
ethyl-NH4 0.00093 4.4
propyl-NH4 0.0014 6.6
trimethyl-NH4 0.00069 3.3
triethyl-NH4 0.001 5.0
tripropyl-NH4 0.002 9.6
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a

Ki values determined from surface-tension data reported by Pottage et al. (2016)

b

Values determined for PFOA concentration of 0.1 mg/L

Influence of pH

The surface tensions measured for Na-PFOA and H-PFOA in DI water are presented in Figure 6. As previously noted, the Ki value for PFOA acid in DI water is approximately 4-times greater than the values for the alkali-metal PFOA salts in DI water. The greater surface activity observed for H-PFOA is due in part to the difference in counterion size, and to the influence of solution pH. For H-PFOA, pH decreased by approximately 3 units over the concentration range of 0.01 to 3000 mg/L. Conversely, pH decreased by only 0.2 units over a similar concentration range for Na-PFOA. Surface tension measured for H-PFOA in a pH-buffered solution is observed to exhibit reduced surface activity compared to the unbuffered solution (see Figure 6).

Figure 6.

Figure 6.

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Influence of pH on surface tension of perfluorooctanoate (PFOA) in DI water. Na-PFOA #1 originally reported by Lyu et al. (2018); H-PFOA Buffered data reported by Gorodinsky and Efrima (1994).

These results are consistent with the observed impact of acid addition upon surface tensions of solutions containing H-PFOA, Na-PFOA, or K-PFOS (Talbot, 1953, 1959). For example, the surface tension of a 0.001 M PFOS solution was observed to decrease linearly upon increasing acid addition, with an approximate decrease of 5 mN/m for each log increase in HCl or HNO3 concentration. The impact of pH was attributed to the influence of H+ on the dissociation status of the PFOS anion, wherein protonated PFOS, formed at lower pH, exhibits greater surface activity compared to the dissociated anionic PFOS (Talbot, 1959).

Log Ki values determined for a series of perfluorocarboxylic acids are presented in Figure 1 in the context of the QSPR model presented by Brusseau (2019). Recall that the original anionic PFAS data presented in the model comprised alkali-metal salts. The log Ki values for the PFAS acids are very similar to the regression-predicted values for the two larger compounds (perfluorodecanoic acid and perfluorotridecanoic acid, see Table 1), while they deviate for the smaller PFAS. The magnitude of the deviation increases approximately linearly as PFAS size decreases. This behavior indicates that the influence of headgroup and pH on PFAS activity in solution and at the interface is greater for smaller compounds, for which the headgroup represents a greater proportion of the entire molecule.

The differences between measured and predicted Ki values is at most one log, and the measured values fall within or near the upper prediction-uncertainty bound of the original QSPR model (Figure 1). Given that most subsurface environments have a natural pH buffering capacity, and considering the impact of solution buffering observed in Figure 6, it may be anticipated that deviations between measured and predicted Ki values for PFAS acids will be somewhat less than what is observed in Figure 1.

Influence of Electrolyte Composition

Surface tensions measured for Na-PFOA in DI water and various electrolyte solutions are presented in Figure 7. Comparison of the data for DI water versus 0.01 M NaCl reveals a significant reduction in surface tension in the presence of salt for PFOA concentrations greater than approximately 1 mg/L. An increase in ionic strength to 0.1 M NaCl further increases the reduction in surface tension. This increase in surface activity with an increase in ionic strength is consistent with prior observations for PFOA (Downes et al., 1995; López-Fontán et al., 2005; Shinoda and Nakayama, 1963; An et al., 1996; Hongtao et al., 2011) and ionic surfactants in general. The presence of electrolyte reduces electrostatic repulsion among the ionic headgroups at the interface and increases the activity of the hydrophobic tail in solution, with the latter effect resulting in an increase in the driving force for adsorption from solution.

Figure 7.

Figure 7.

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Surface tension measured for Na-perfluorooctanoate (PFOA) in various electrolyte solutions; SGW is synthetic groundwater.

The surface-tension curve for PFOA in the 0.01 M KCl solution is very similar to the curve for the 0.01 M NaCl solution. Conversely, the surface activity is much greater for the 0.01 M CaCl2 solution compared to the other two, illustrating the influence of the valence of the electrolyte. The surface activity of PFOA in the SGW, containing a mix of salts, is significantly greater than that in DI water. The surface-tension curve for PFOA in SGW, which has a total molarity of 0.01, resides between the curves for 0.01 M NaCl-KCl and 0.01 M CaCl2. This is consistent with the fact that the SGW contains both monovalent and divalent cations. In addition, it is observed that the surface tension for PFOA in SGW is similar to that of 0.1 M NaCl, reflecting again the presence of divalent cations in the SGW.

The Ki values for air-water interfacial adsorption of PFOA as a function of electrolyte composition are presented in Table 3. The values span two orders of magnitude, from 0.0002 to 0.017 cm. The values for the 0.01 and 0.1 M NaCl solutions are ~7 and 38 times larger than the value for DI water. The values for SGW and the 0.01 M CaCl2 solution are 26 and 74 times larger, respectively. These data illustrate the significant increase in interfacial adsorption of PFOA induced by the presence of salts in solution.

Table 3.

Air-water interfacial adsorption coefficients (Ki) for Na-perfluorooctanoate (PFOA) in various electrolyte solutions.

Solution Ki (cm)a Factor Deviation from DIW
Deionized water (DIW) 0.00023 -
0.01 M NaCl 0.0017 7.4
0.1 M NaCl 0.0087 38
0.01 M KCl 0.0015 6.5
0.01 M CaCl2 0.017 74
Synthetic groundwater (SGW) 0.006 26
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a

Values determined for PFOA concentration of 0.1 mg/L

Surface tensions measured for selected PFAS and the common hydrocarbon surfactant sodium dodecylbenzenesulfonate (SDBS) in 0.01 M NaCl and SGW solutions are presented in Figure 8. Note that the data in Figure 8 also include two points for TCE-water interfacial adsorption of PFOS. The log Ki values determined from these data are presented in Figure 1 in conjunction with the QSPR model. Similarly to the PFAS acids, the values match the regression-predicted values for the largest compounds and deviate for the smaller ones. The magnitudes of the deviations are at most approximately 1–1.5 logs for the electrolyte-addition data sets.

Figure 8.

Figure 8.

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Surface tensions for Na-perfluoropentanoate (PFPeA), K-perfluorooctanesulfonate (PFOS), perfluorotridecanoic acid (PFTDA), and Na-dodecylbenzenesulfonate (SDBS), and interfacial tension (TCE-water) for H-PFOS in solutions comprising 0.01 M NaCl and synthetic groundwater (SGW). The solid lines represent the best-fit simulation produced with the Szyszowski equation.

Influence of PFAS Mixtures

Based on literature reports, soil, surface water, and groundwater at PFAS-contaminated sites typically contain numerous individual PFAS. Hence, the fluid-fluid interfacial adsorption behavior of PFAS mixtures is of great interest. Surface tensions measured for two PFAS mixtures, one comprising an 8:2 molar mixture of Na-PFOA and PFTDA and the other an 8:2 molar mixture of PFTDA and Na-PFOA, are presented in Figure 9. Comparison of the surface tension curve for Na-PFOA in the presence of PFTDA compared to the curve for PFOA alone shows that the addition of PFTDA causes a significant increase in surface activity. This results in a large, factor of 38 increase in Ki, from 0.006 to 0.23 cm. Conversely, the addition of PFOA to the PFTDA solution causes a relatively minor increase in surface activity compared to PFTDA alone. The Ki increased in this case by a factor of ~2, from 0.15 to 0.34 cm.

Figure 9.

Figure 9.

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Surface tension for mixtures of Na- perfluorooctanoate (PFOA) and perfluorotridecanoic acid (PFTDA) in SGW. Data for single-surfactant systems (1:0) are presented for comparison.

These results demonstrate that the impact of PFAS mixtures on fluid-fluid interfacial adsorption depends upon the nature of the surfactants present. The compound with greater surface activity controls the overall surface activity of the mixed system. In this case, PFTDA, which has significantly greater surface activity compared to PFOA (compare surface tensions in Figures 7 and 8), controls the surface activity of the mixed system. Similar results were reported for a 1:1 PFOS-PFOA mixture wherein PFOS, the surfactant with greater surface activity, controlled the surface activity of the mixed system (Vecitis et al., 2008). Using their reported surface-tension data, Ki values of 0.00016 and 0.0023 cm are obtained for PFOA alone and with PFOS, respectively, a 14X increase. Conversely, a much smaller factor of 2.5 increase in Ki (0.001 vs. 0.0025 cm) is observed for PFOS with the addition of PFOA. This phenomenon reflects the preferential adsorption of the surfactant with greater surface activity (Zhao et al., 1984; Matsuki et al., 1992; Gorodinsky and Efrima, 1994).

Surface-tension measurements are typically conducted using research-grade compounds, which are in some cases further purified. Conversely, commercial products may contain multiple isomers of a given compound, in addition to multiple surfactants as well as other constituents. Gorodinsky and Efrima (1994) measured the surface tensions of PFOA and a commercial product, FC143, the latter comprising 83% NH4-PFOA, 9.3% NH4-perfluorononanoate (PFNA), and 7.7% NH4-perfluorodecanoate (PFDA), possibly with some proportion of branched isomers. They observed slightly greater surface activity for FC143 compared to PFOA at low surfactant concentrations. The Ki values determined herein from the two surface-tension data sets are 0.001 and 0.0025 cm for PFOA alone and in the commercial product, respectively. This result shows that the Ki for PFOA is not greatly different between the research grade and commercial products. Additional research should investigate this aspect for other commercial products.

Influence of Fluorocarbon-Hydrocarbon Surfactant Mixtures

Hydrocarbon surfactants are likely to be present in conjunction with PFAS at some sites. Several studies have investigated the behavior of mixtures of fluorocarbon and hydrocarbon surfactants. As noted above, interactions between surfactants in a mixture and the impact on overall surface activity depends in part upon the types of surfactants involved. The nature of the headgroups for the different surfactants is one important factor, with different behavior observed for systems with similar-charged versus dissimilar-charged headgroups. For example, Guo et al. (1992a) measured the surface tension of Na-PFOA in the presence of nonionic, zwitterionic, and cationic hydrocarbon surfactants. The critical micelle concentration (CMC) was reduced upon addition of the hydrocarbon surfactant in all three cases. However, the degree of surface-activity enhancement varied greatly. The maximum reduction in CMC was <20% for addition of the nonionic surfactant, versus slightly more than 50% for the addition of the zwitterionic surfactant. The CMC decrease was much greater, a maximum of ~100X, with addition of the cationic surfactant. Acosta et al. (2006) reported similar large decreases in CMC for a mixture of Na-PFOA and another cationic hydrocarbon surfactant.

The observed synergistic interaction among surfactants with oppositely charged headgroups and the resultant increase in surface activity stems from a reduction in the electrostatic repulsion between similarly charged headgroups caused by the presence of the oppositely charged headgroups. This interaction promotes greater concentrations of monomers at the interface compared to the concentrations associated with either surfactant alone (Guo et al., 1992a). The synergistic electrostatic interactions in effect counterbalance the repulsive forces that exist between the fluorocarbon and hydrocarbon chains (Mukerjee and Yang, 1976; Guo et al., 1992a; Holland and Rubingh, 1992; Matsuki et al., 1992).

Conversely, mixtures of fluorocarbon and hydrocarbon surfactants of similar headgroup charge exhibit antagonistic behavior because of the repulsive forces between the two sets of chains. As a result, the CMC of the mixed system increases. For example, the addition of anionic hydrocarbon surfactants increased the CMC by ~30 to ~60% for PFOS (Yoda et al., 1989; Matsuki et al., 1992) and PFOA (Mukerjee and Yang, 1976; Guo et al., 1992b). This antagonistic behavior observed for mixed fluorocarbon-hydrocarbon systems is in contrast to that of hydrocarbon-hydrocarbon or fluorocarbon-fluorocarbon mixtures of like-charge headgroups. Ideal mixing behavior is observed for these latter systems, and the CMC for the mixture is generally a smooth monotonic function of the relative proportions of the individual surfactants comprising the mixture (Mukerjee and Yang, 1976; Yoda et al., 1989; Guo et al., 1992a).

Based on the above, the impact of fluorocarbon-hydrocarbon surfactant mixtures on fluid-fluid interfacial adsorption of PFAS will depend upon the nature of the surfactants. To illustrate, the Ki value determined from the surface-tension data reported by Acosta et al. (2006) for Na-PFOA with addition of a cationic hydrocarbon surfactant (9:1 molar ratio) is ~10X greater than for Na-PFOA alone. Conversely, the Ki value determined from the surface-tension data reported by Zhao et al. (1984) for Na-PFOA with addition of an anionic hydrocarbon surfactant (10:1 molar ratio) is essentially the same as that for Na-PFOA alone. The behavior of mixed surfactant systems will also depend on other factors such as the relative concentrations of the different surfactants and properties of the solution.

Influence of Co-solutes

Surface tensions for Na-PFOA in SGW with addition of 10% ethanol, 10 mg/L humic acid, and 500 and 1000 mg/L TCE are presented in Figure 10. Normalized surface tensions are used to account for the varied impact of the co-solutes on surface tension of the background solution (no Na-PFOA). For example, the addition of 10% ethanol reduced the surface tension to 50 mN/m. Conversely, the addition of the humic acid reduced surface tension to ~67 mN/m, whereas the addition of TCE had no measurable impact. The data for the humic-acid solution are noisy compared to the other data sets reported herein. This may be a result of the multiple-component nature of the humic acid.

Figure 10.

Figure 10.

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Surface tensions for Na-perfluorooctanoate (PFOA) in synthetic groundwater (SGW) with addition of co-solutes; TCE is trichloroethene, with numbers representing concentration in mg/L.

The Ki values at [Na-PFOA] = 0.1 mg/L for the ethanol, humic acid, and TCE solutions are 0.004, 0.006, and 0.009, respectively. The Ki value for Na-PFOA in SGW with no co-solutes is 0.006 cm. The presence of the co-solutes is observed to have relatively minor impact on the interfacial adsorption of PFOA.

Revised QSPR Model

The results of this study indicate that several of the factors investigated have relatively minor impact on surface activities of PFAS. However, ionic strength was shown to have a significant impact on the magnitude of interfacial adsorption, with for example Ki values for PFAS in SGW increasing by up to an order of magnitude or more compared to DI water, depending on the individual PFAS. The QSPR model presented by Brusseau et al (2019), which employed data measured for DI water, can be revised to account for the impacts of solution composition by incorporating the Ki values reported herein.

As discussed, the impact of solution composition on the deviation of measured Ki values from those predicted with the original model was minimal for larger compounds (Vm > than ~300). Thus, the revised QSPR model was developed using all original data with Vm greater than ~300, plus all of the newly reported data referenced in Table 1. The resultant revised model is presented in Figure 1. This version can be used in place of the original to produce predictions that are representative of typical environmental conditions.

Conclusion

This work investigated the influence of PFAS headgroup charge, size, and counterion type, solution ion composition and ionic strength, solution pH, and the presence of co-solutes and surfactant mixtures on fluid-fluid interfacial adsorption of PFAS. The results showed that the QSPR model produces reasonable predictions of zwitterionic surfactants, in addition to anionic, cationic, and nonionic. It was also shown that Ki values for nonionic PFAS with very large headgroups deviate substantially from the predictions produced with the QSPR model. However, such large headgroup moieties may not be prevalent in many environmental systems due to degradation processes. The influence on Ki of counterion type for ionic PFAS was shown to be relatively small. Ki values were influenced by solution pH and acid forms of PFAS. The magnitude of Ki increased in the presence of electrolyte, with the magnitude of the increase dependent upon ion composition and ionic strength. The impacts of pH and ionic composition were greater for shorter-chain PFAS. The presence of representative co-solutes (cosolvent, dissolved organic matter, trichloroethene) likely present at many contaminated sites had minimal impact on Ki values. The impact of surfactant mixtures was shown to depend upon the nature of the individual surfactants present, with overall surface activity mediated by the stronger surfactant. To our knowledge, these results represent the first investigation of the impact of solution properties such as ion composition and co-solutes representative of contaminated sites on the adsorption of PFAS at air-water interfaces.

These results indicate that soil-pore water and groundwater composition is likely to influence the magnitude of air-water and OIL-water interfacial adsorption of PFAS. However, the impact on Ki of most of the factors investigated is small for PFAS concentrations less than ~1–10 mg/L, which represent the higher range typically observed in environmental systems. Hence, their impacts on fluid-fluid interfacial adsorption are likely to be relatively minor for typical environmental conditions, especially compared to other factors that have been demonstrated to be important such as fluid saturations, porous-medium properties, and PFAS molecular structure (Brusseau, 2018; Lyu et al., 2018; Brusseau et al., 2018; Brusseau, 2019). The results of this study indicate that the revised QSPR model provides reasonable first-order predictions of Ki for a wide range of PFAS in environmental systems.

Acknowledgements

This work was supported by the NIEHS Superfund Research Program (grant# P42 ES04940). We thank the reviewers for their constructive comments.

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