Development of an equilibrium headspace gas chromatographic method for the measurement of noncovalent association and partitioning of n-alkylbenzenes at infinite dilution in fulvic acid pseudophase

摘要

Fulvic acid (FA), the most important water soluble fraction of humic substances in nature, is known to form aggregate pseudophase and complexes with organic and inorganic species. Here, we report a novel equilibrium headspace gas chromatography (eHSGC) and a two-step reaction model to measure n-alkylbenzene-FA association constant (K-11) and n-alkylbenzene-pseudophase FA(n) association constant (K-n1) without solute concentration and response factor. The K-11 and K-n1, values were 2-3 orders of magnitude higher than those for sodium dodecylsulfate. Changes in peak area were used to calculate the critical FA-aggregation concentration (cfc), mole fraction based partition coefficients (K-x), activity coefficients of solute inside the aggregate pseudophase (gamma(infinity)(m)), and transfer free energies of alkyl-CH2- at infinite dilution. The cfc was found to be 10 +/- 0.5 mu M. The K-x values are of the order of 10(7) in the FA-aggregate pseudophase. The data shows that benzene has the lowest (0.0002) and n-butylbenzene has the highest (0.01) gamma(infinity)(m) values, which are seven orders of magnitude smaller than gamma(infinity)(w) in water. The transfer free energy of association of a-CH2-group, -155 cal/mol, compared to that of benzene, -9722 cal/mol, indicates that the FA-aggregate pseudophase is more polarizable benzene-like and less n-alkane aliphatic-like. (C) 2015 Elsevier B.V. All rights reserved.