Despite their widespread use in products like nonstick pans and firefighting foams, per- and polyfluoroalkyl substances (PFAS)—a group of synthetic fluorinated organic compounds—have permeated the environment globally, particularly affecting water sources and being linked to numerous health issues. Foam fractionation offers a promising solution for separating PFAS from water by harnessing their unique surface-active properties for effective separation. Thus, a deep comprehension of PFAS's air-water interfacial properties becomes crucial for enhancing the feasibility of using foam fractionation for PFAS removal from PFAS-contaminated water in the future. However, the correlation between the interfacial properties of PFAS and their foam properties remains underexplored and not well systematically investigated. Moreover, the efficient removal of short-chain PFAS (i.e., four carbons, C4) through foam fractionation poses a significant challenge, highlighting a critical area for future research.

Hence, this dissertation initially delves into the air-water interfacial properties of PFAS, focusing on dynamic surface tension and interfacial dilatational rheology. It reveals that the PFAS with shorter chain length (C4) display lower adsorption capabilities, as indicated by the Gibbs and Langmuir isotherms, and a notable absence of interfacial dilatational modulus, likely contributing to their limited foaming capacity. In contrast, for the PFAS with longer chain length (C8), the dilatational elastic (storage) modulus is greater than the dilatational loss modulus, with both significantly influenced by the concentration and oscillation rate. A notable maximum in dilatational modulus relative to PFAS concentration aligns with predictions made using the Lucassen and van den Tempel model, highlighting the competition between molecular adsorption and molecular exchange between the air-water interface (subsurface) and the bulk for the PFAS with longer chain length (C8). In addition, according to the classic Ward-Tordai model, the diffusion coefficients of the PFAS with longer chain length (C8) are dependent on their bulk concentrations, showcasing that, at equal concentrations, the PFAS with shorter chain length (C4) demonstrate lower diffusion coefficients.

In this dissertation, foaming and defoaming properties of PFAS are studied, demonstrating that the PFAS with longer chain length (C8), particularly those with sulfonic acid head, exhibit superior foamability, foam stability, and higher liquid retention within the foam structure. Through dimensional analysis, connections between foaming behavior and various dimensionless numbers are established. For instance, our results show that the foaming kinetics for long-chain (C8) PFAS are strongly associated with parameters such as the Reynolds and Boussinesq numbers. However, accurately predicting the foaming kinetics of short-chain (C4) PFAS proves challenging without incorporating dimensionless numbers that account for the effects of interfacial dilatational rheology. Notably, the analysis reveals a peak in the rate of foam expansion correlating with the capillary number, coinciding with the highest observed dilatational modulus. Moreover, we find a significant correlation between the defoaming properties of long-chain (C8) PFAS and dimensionless numbers, emphasizing the role of the Boussinesq number in understanding PFAS defoaming behavior.

This research also investigates the impact of electrolyte addition. It finds that introducing electrolytes lowers PFAS surface tension, increases their maximum surface excess concentration, and shifts the critical micelle concentration (CMC) to lower PFAS concentrations. The type and valence of the salts play a significant role in altering the diffusion coefficients and dilatational modulus of PFAS. For example, the addition of NaCl results in a decrease and then an increase in both the dilatational elastic and loss moduli for long-chain (C8) PFAS with sulfonic acid head, indicating changes in interfacial viscoelasticity with increasing NaCl concentration. A similar trend but with different significance is observed, especially for dilatational elastic modulus, when CaCl2 is added. This study also shows that salt addition modifies PFAS foam characteristics; notably, CaCl2 enhances foam stability for long-chain (C8) sulfonic acid PFAS by affecting factors such as the Boussinesq number, the ratio of surface tension to interfacial elasticity, and the ratio of surface tension to interfacial loss modulus. Moreover, the presence of NaCl, in concentrations ranging from 5-50 mM, improves both foamability and stability for long-chain (C8) carboxylic acid PFAS, influenced by changes in the capillary number and the balance between surface tension and interfacial elasticity.

As ongoing research, to advance our understanding of how PFAS molecules interact with salt ions, preliminary investigations within the bulk phase have been conducted by using nuclear magnetic resonance (NMR) spectroscopy and small-angle X-ray scattering (SAXS). Findings from these methods suggest a preferential interaction of the sulfonic acid groups in C8 PFAS with Ca2+ ions, whereas the carboxylic acid groups show a tendency to interact with Na+ ions. Molecular dynamics (MD) simulations performed by collaborators reveal that introducing Ca2+ ions to long-chain (C8) sulfonic acid PFAS systems leads to the development of complex structures at the air-water interface.