The overall objective of this proof-of-concept project is to measure and model key physical-chemical properties of the 40 per- and polyfluoroalkyl substances (PFAS) included in Environmental Protection Agency (EPA) Method 1633. The project team will accomplish this objective by completing the following specific research tasks: (1) critically evaluate existing literature data on conventional physical-chemical properties for PFAS to clarify inconsistencies, identify gaps, and determine the likely range of vital parameters; (2) experimentally measure key parameters that control PFAS partitioning, fate, and transport in the subsurface; and (3) rigorously develop fundamental chemistry models, including single- and poly-parameter linear free energy relationships (sp-LFER and pp-LFER, respectively), that inform PFAS distribution via accurate estimation of physical-chemical properties.

The following parameters have been identified for evaluation in this project: acid dissociation constant; air-water partition constant; boiling point; complexation constants with Ca²⁺, Mg²⁺, Fe³⁺, and Al³⁺; critical micelle concentration; melting point; octanol-air partition constant; octanol-water partition constant; Setschenow (or "salting") constants with sodium chloride; vapor pressure; and solubility. The project team will conduct a thorough literature review for all properties. Literature data and estimated parameters calculated with previously reported pp-LFER and sp-LFER will be aggregated into a shareable spreadsheet, which will eventually be scaled up into a searchable website on the physical chemical properties of PFAS. The proof-of-concept portion of the project will specifically focus on experimental measurement and modeling of acid dissociation, air-water partition, octanol-air partition, and octanol-water partition constants, along with Setschenow constants for PFAS with sodium chloride. The air-water and octanol-water partition constants will be evaluated as a function of pH to not only identify specific constants for the neutral and anionic PFAS species, but also to calculate acid dissociation constants from two different systems as an internal consistency control. Experiments will be conducted using batch reactors that enable sampling from both phases (for the corresponding partition constants) to ensure the accuracy of all calculated parameters. Solution pH and salt content (ionic strength) will be carefully considered to avoid convoluting effects. All physical-chemical properties will be evaluated at multiple temperatures to enable temperature-based corrections.

The literature contains conflicting information about the magnitude of key physical-chemical properties for select PFAS, such as perfluorooctanoic acid and perfluorooctane sulfonic acid. Few experimental data are available for the properties of other PFAS included in EPA Method 1633, which has been widely adopted by researchers and regulators. Ultimately, these knowledge gaps introduce major uncertainty into efforts to understand and model the distribution, fate, transport, and toxicity of PFAS in the environment. The objective and technical approach of this project help to (i) confirm the accuracy of previously reported physical-chemical properties for select PFAS, (ii) fill key knowledge gaps for other, under-studied PFAS, (iii) enhance confidence in the experimental measurements due to internal consistency checks based on the differential partitioning of neutral and anionic PFAS species in multiple systems, and (iv) report new models to estimate the physical-chemical properties of other PFAS. The corresponding data will have broad impacts on researchers evaluating the behavior of PFAS in natural and engineered systems, and will ultimately enhance the operational effectiveness and readiness of our forces by lessening the effects of these substances. (Anticipated Completion Date - 2026)