Thermodynamic Characterization of Humic Acid-surfactant Interaction: New Insights into the Characteristics and Structure of Humic Acids

Leonardus Vergütz,Ivo Ribeiro Silva,Maria Do Carmo Hespanhol Da Silva,Luis Henrique Mendes Da Silva,Roberto Ferreira Novais,Aparecida Barbosa Mageste

doi:10.1590/01000683rbcs20140258

Leonardus Vergütz, Ivo Ribeiro Silva + Show 4 more

Open Access

https://doi.org/10.1590/01000683rbcs20140258

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Abstract

ABSTRACT Humic acids (HA) are a component of humic substances (HS), which are found in nearly all soils, sediments, and waters. They play a key role in many, if not most, chemical and physical properties in their environment. Despite the importance of HA, their high complexity makes them a poorly understood system. Therefore, understanding the physicochemical properties and interactions of HA is crucial for determining their fundamental role and obtaining structural details. Cationic surfactants are known to interact electrostatically and hydrophobically with HA. Because they are a very well-known and characterized system, they offer a good choice as molecular probes for studying HA. The objective of this study was to evaluate the interaction between cationic surfactants and HA through isothermal titration calorimetry in a thermodynamic manner, aiming to obtain information about the basic structure of HA, the nature of this interaction, and if HA from different origins show different basic structures. Contrary to what the supramolecular model asserts, HA structure is not loosely held, though it may separate depending on the conditions the HA are subjected to in their milieu. It did not show any division or conformational change when interacting with surfactants. The basic structure of the HA remains virtually the same regardless of the different sources and compositions of these HA.

Highlights

Humic acids (HA) are found in most soils, sediments, and natural waters
HA is a major fraction of humic substances (HS), which makes up the bulk of soil organic matter (SOM) and is very important for the global C cycle, with twice as much carbon stabilized as SOM than there is in the whole atmosphere (Amundson, 2001)
An example of the titration curves obtained in this study is shown in figure 1, where the observed enthalpy changes (∆Hobs) for each injection are plotted against the total surfactant concentration in the sample cell

Summary

Introduction

Humic acids (HA) are found in most soils, sediments, and natural waters. They are formed by chemical, physical, and biological transformation of plant, animal, and microorganism matter into relatively stable and polydisperse particles. After that, based on a series of Nuclear Magnetic Resonance (NMR) experiments, Simpson and his co-workers showed that even though hydrophobic associations, charge interactions, and hydrogen bonds are important for aggregation, metal ions play a crucial role in aggregate formation and stability of humic structures (Simpson et al, 2002) According to this view, a basic structure of HA does not exist, but rather they are complex mixtures of many components, which vary in ratio and structure according to sample origin, extraction procedure, maturity, degree of degradation, depositional environment, and formation processes (Lu et al, 2000; Simpson et al, 2002; Peña-Méndez et al, 2007)

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