title: 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.
abstract: 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 (K11) and n-alkylbenzene-pseudophase FAn association constant (Kn1) without solute concentration and response factor. The K11 and Kn1 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 (Kx), activity coefficients of solute inside the aggregate pseudophase (γm(∞)), and transfer free energies of alkyl CH2 at infinite dilution. The cfc was found to be 10±0.5μM. The Kx 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) γm(∞) values, which are seven orders of magnitude smaller than γw(∞) in water. The transfer free energy of association of a CH2 group, -155cal/mol, compared to that of benzene, -9722cal/mol, indicates that the FA-aggregate pseudophase is more polarizable benzene-like and less n-alkane aliphatic-like.
authors.affiliation: Center for Clean Water and Sustainable Technologies, Department of Chemistry and Biochemistry, George Mason University, Fairfax, VA 22030, USA.
authors.forename: Mahmoud D
authors.initials: MD
authors.lastname: Eljack
authors.affiliation: Center for Clean Water and Sustainable Technologies, Department of Chemistry and Biochemistry, George Mason University, Fairfax, VA 22030, USA.
authors.forename: Rachael E
authors.initials: RE
authors.lastname: Wilson
authors.affiliation: Center for Clean Water and Sustainable Technologies, Department of Chemistry and Biochemistry, George Mason University, Fairfax, VA 22030, USA. Electronic address: ahussam@gmu.edu.
authors.forename: Abul
authors.initials: A
authors.lastname: Hussam
authors.affiliation: Center for Clean Water and Sustainable Technologies, Department of Chemistry and Biochemistry, George Mason University, Fairfax, VA 22030, USA.
authors.forename: Shahamat U
authors.initials: SU
authors.lastname: Khan
mesh_terms: D001555:Benzene Derivatives / Q000032:analysis* / Q000737:chemistry; D001578:Benzopyrans / Q000737:chemistry*; D002623:Chemistry Techniques, Analytical / Q000379:methods*; D002849:Chromatography, Gas; D006812:Humic Substances / Q000032:analysis; D007700:Kinetics; D008962:Models, Theoretical; D014867:Water / Q000737:chemistry
chemical_list: D001555:Benzene Derivatives; D001578:Benzopyrans; D006812:Humic Substances; D014867:Water; C024268:n-propylbenzene; C004912:ethylbenzene; C053761:n-butylbenzene; C005023:fulvic acid
keywords: Equilibrium headspace gas chromatography; Fulvic acid; Infinite dilution activity coefficients; Pseudophase association constants; n-Alkylbenzenes