## 2 Models of the Structure of Humic Substances
### The Micellar Model  
Differently from orthodox micellar solutions, humic pseudo-micelles were initially considered as a type of aggregates characterized by a broad distribution of both size and functional groups. According to that vision, pseudo-micelles still retain the ability of traditional micelles to sequester hydrophobic species that partition into the nonpolar microdomains [44, 45, 46]. Using ultrasonic velocimetry [PERSON] et al. [47] demonstrated that the aggregation can be observed at concentrations as low as 1 mg\(\cdot\)L\({}^{-1}\) under various pH and ionic strength conditions. The micelle-like behavior of HS was also investigated by [PERSON] et al. [48] who verified through size exclusion chromatography (SEC), the effect of the concentration of organic acids on the HS dissociation into small-sized micellar aggregates. HS aggregation was also studied in aqueous solution by HPSEC [49]. The formation of aggregates in solutions of HS was studied through capillary zone electrophoresis (CZE) for the first time by [PERSON] et al. [50]. In the late 1990s, [PERSON] et al. [51] published a review in which they surveyed the major findings reported up to that time on the colloidal properties of HS, by emphasizing the aggregation behavior in both artificial and natural environments. By varying the composition of the mobile phase at fixed ionic strength of a SEC system, [PERSON] et al. [52] were able to monitor the conformational changes of different types of dissolved HA and FA. In particular, through high- and low-pressure chromatography, the authors verified that HS complied with the model describing the association of small heterogeneous molecules into larger dimensional structures. The work of [PERSON] et al. [53] on HA and FA offered further direct evidence more in favor of the model based on the reversible self-association of small molecules rather than the polymer random coil picture. The reductions in HS dimensions that occurred as a result of variation of composition of the mobile phase were attributed by the authors to a drastic attenuation of weak intermolecular associations giving rise to separate smaller molecules rather than to the compaction of macromolecular chains. Other studies have rejected the idea that the micellar structure of HS could only be inferred by changing the retention volumes following the addition of organic acids. According to [PERSON] et al. [54], secondary (ionic, hydrophobic) interactions should always be considered for the interpretation of SEC developed for HS. [PERSON] et al. [55] interpreted the SEC and fluorescence experiments on several type of HA aqueous solutions in terms of the formation of 'pseudo-micelles' assisted under the influence of cations in solution. The former concept of 'pseudo-macromolecularity' was proposed by [PERSON] [56] to suggest that the macromolecular properties of HS could be compatible with those produced by pseudo-micellar structures or by other molecular assemblies involving long-chain hydrocarbons, fatty acids, esters and suberin-like components. Subsequently, [PERSON] et al. [57] published a new model on the molecular structure of HS based on supramolecular associations of light molecular mass components, stabilized by hydrophobic interactions and H-bonds, which self-assemble into micellar aggregates in suitable aqueous milieu. This model was based on the use of several techniques, such as gel permeation chromatography (GPC), size exclusion chromatography at high pressure (HPSEC), and ultraviolet visible spectroscopy (UV-VIS). Indirect evidence in favor of the micellar model includes, (i) the extent of the reduction in surface tension of aqueous HS solutions observed with increasing HS concentration [58], (ii) the improved solubility of hydrophobic organic compounds at following dispersion in aqueous solutions of HS [37; 38; 39; 40; 41] and (iii) the preferred distribution of fluorescent probes such as pyrene, within the hydrophobic microdomain of HA aggregates in aqueous solutions [42; 45].