Abstract
This study provides a comparative framework on the elucidation of analogies and differences in the interfacial protolytic processes and the associated colloidal behaviour of a typical humic acid (HA) and a set of single-layer graphene oxide (SLGO) and (multi-layered) graphite oxide samples in aqueous electrolyte media. The pH dependence of the surface charge densities of HA and SLGO was explored at three different salt concentrations by potentiometric acid-base titration, along with simultaneous determination of zeta potential and hydrodynamic sizes. Charging curves obtained in the pH range of 3 to 10 by cyclic titrations reveal the presence of a small hysteresis, proving the chemical stability of SLGO and graphite oxides in weakly acidic and alkaline solutions. HA and SLGO display a parallel shift of the pH-dependencies of their negative charge densities with increasing ionic strength, demonstrating a unique combination of particle and polyelectrolyte-like behaviour, which is absent for multi-layered graphite oxide exhibiting charging curves that resemble to “classical” colloidal particles. An accurate purification of SLGO results in inherent change in its surface properties; however, the salt tolerance of aqueous HA solutions is still superior to that of SLGO dispersions.
Highlights
Ubiquitous soil organic matter and the recently highlighted graphene-based materials constitute, according to the contemporary scientific consideration, very different classes of materials
Based on our long-standing experience on the colloidal behaviour of humic substances (HS) [11,12] and graphite oxide (GO) [13,14,15], we discovered that, by evaluating the pH-dependent charging and colloidal stability of a singlelayer graphene oxide (SLGO) sample, striking analogies and peculiarities exist between GO and humic acid (HA), with those of coal origin
This paper presents a novel investigation involving in-depth evaluation and comparative study of two, strikingly similar carbonaceous materials, humic acid and graphene oxide, mostly focusing on their colloid chemical behaviour
Summary
Ubiquitous soil organic matter and the recently highlighted graphene-based materials constitute, according to the contemporary scientific consideration, very different classes of materials. A remarkable linkage between them stems from their formation process, which is most often related to chemical oxidation. The oxidation of graphite by strong oxidants in harsh acidic environment (mainly Brodie [1] and Hummers [2] methods and their modifications) leads to the formation of graphite oxide (GO), which is the direct precursor of singlelayer graphene oxide (SLGO). Humic substances (HS) naturally form from coal by atmospheric oxidation [3] or from dead plant matter through oxidative microbial degradation of organic tissues to “monomers” of poorly definable structure, followed by the polymerization of these monomeric substances into high molecular weight
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