Quantifying hydrophobicity of natural organic matter using partition coefficients in aqueous two-phase systems

The hydrophobicity of natural organic matter (NOM) is of great significance for its interfacial processes in natural and engineered systems. However, there is no well-accepted method for the routine determination of NOM hydrophobicity. In this study, the hydrophobicity of NOM spanning a wide range of origins and properties was quantified based on their partition coefficients (K_ATPS) in poly (ethylene glycol)/potassium citrate aqueous two-phase systems (ATPS). The LnK_ATPS of NOM correlated well with the elemental, structural, and thermodynamic indices commonly used to characterize NOM hydrophobicity, including (O + N)/C, O/C, aromatic and aliphatic carbon, and the free energy of interactions between molecules (ΔG_iwi). A simple linear model was developed to predict NOM hydrophobicity using K_ATPS. The model was validated using 20 natural water samples collected from rivers and lakes, which suggested good prediction power. ATPS scale system is simple, fast, low-cost, environmentally friendly, and requires little pretreatment and small sample volume. Overall, K_ATPS can be used as quantitative measure of NOM hydrophobicity that facilitates routinely characterizing the interfacial properties of NOM.