Probing the organization of fulvic acid using a cationic surfactant

Abstract

The organization of Suwannee River Fulvic Acid (SRFA) is investigated through the interaction of SRFA with a cationic surfactant molecule (Dodecyl-trimethylammonium chloride (DTAC)). Turbidity measurements, Dynamic light scattering, electrophoretic mobility, surface tension, and cryo-transmission electron microscopy are combined to describe the SRFA/DTAC molecular structures thus obtained. Increasing DTAC concentration, fulvic acid-rich unilamellar vesicles, globules, aggregates of globules, disks, DTAC-rich unilamellar vesicles, and loosely aggregated spheroidal structures, are successively observed. Such sequence of molecular structures is typically found in phase diagrams of catanionic systems (mixture of oppositely charged surfactants). The strong surface activity of fulvic acid/DTAC complexes and the geometrical constraints associated with the formation of vesicles imply that a major component of SRFA is an individual amphiphile negatively charged of molecular size similar to that of DTAC. This soft matter approach supports a supramolecular organization for SRFA.

Complexes of humic acid with cationic surfactants support the supramolecular view of extracted humic matter

“…Previous studies on the interactions between humic substances and cationic surfactants were interested in two main aspects: (i) the fate of the surfactant molecule introduced in the natural environment either as an organic contaminant through wastewater discharge (Koopal et al 2004;Ishiguro et al 2007;Hakim and Kobayashi 2019) or as a detergent for the remediation of aquifers and contaminated soils (Harwell et al 1999;Mao et al 2015); (ii) the chemical structure of the humic substances, the surfactant being used as a probe of amphiphilic properties of humic matter or as a titrant of the total amount of carboxyl functional groups (Tombacz et al 1988;De Nobili et al 1990;Yee et al 2006;Shang and Rice 2007;Chaaban et al 2016). Overall, the reports concur that the binding of the cationic surfactant to the humic substance initially proceeds through strong electrostatic interactions between the carboxylic groups of humic matter and the positively charged head groups of the surfactant.…”

“…A similar trend has been reported for the Suwannee River fulvic acid/DTAC system. In that case, the minimum is interpreted as the result of a rearrangement of fulvic acid upon neutralization with the cationic surfactant (Chaaban et al 2016). Indeed, the initial neutralization of external sites of humic matter by the cationic surfactant enhances the hydrophobicity of the complex, that then undergoes a structural rearrangement turning inside-out some previously unattainable negatively charged sites (Sieliechi et al 2008).…”

“…Likewise, aggregation studies of extracted humic matter and NOM do not directly identify a macromolecular or supramolecular structure (Kerner et al 2003;Wells and Stretz 2019). Chaaban et al (2016) recently proposed to probe the organization of extracted humic matter through its interaction with varying concentrations of a cationic surfactant. In the case of macromolecules, the association with the oppositely charged surfactant leads to the formation of complexes that start to aggregate at a Critical Aggregation Concentration (CAC) in surfactant (Fig.…”

“…In the case of an association between a cationic surfactant and anionic components such as humic matter, the assembly may contain both complexes and free molecules which leads to more complicated expressions for the packing parameter (Manohar and Narayanan 2012). Applying the above approach to the Suwannee River fulvic acid (SRFA), Chaaban et al (2016) showed that the addition of increasing amounts of Dodecyltrimethylammonium chloride (DTAC) successively leads to fulvic acid-rich unilamellar vesicles, aggregates, DTAC-rich unilamellar vesicles, and mixed micelles. Such a sequence, consistent with a supramolecular organization of fulvic matter also suggests that a major component of SRFA has an overall molecular size similar to that of DTAC.…”

Characterization of coal-based fulvic acid and the construction of a fulvic acid molecular model

“…Klencsaŕ and Köntös 15 completed the EPR analysis of Fe 3+ and Mn 2+ complex sites in FA, which can be used to track and characterize the structural changes in FA complexes under different conditions and chemical reaction conditions. Chaaban et al 16 showed that a major component of Suwannee River FA is an individual, negatively charged amphiphile with a molecular size similar to that of dodecyl trimethyl ammonium chloride.…”