## 3 Selected Methods and Experimental Approaches for Analysis of HS Structure
### Liquid State Investigations
#### 3.3.1 Scattering Techniques
Indeed, it was found that the self-assembly of the 5-\(\mu\)m-sized particle formed, dissipated, and spontaneously re-formed over turbulent/quiescent cycles, indicating reversible self-assembly in all HS tested samples. In a systematic determination of both D\({}_{\text{h}}\) and zeta potential of HA sodium salt performed in aqueous dispersions in the concentration range of 3.2\(\cdot\)10\({}^{-4}\)-20 g/dm\({}^{3}\) at pH 6.0 and in absence of background electrolytes, it was found that the particle size varied from 30 nm to 5.6 \(\mu\)m, with PSD depending on the HA concentration [149]. The results were interpreted in terms of the existence of HA sodium salt in the associated state. The dependence on concentration, pH and time of the physicochemical properties of aqueous dispersions of HA, such as particle sizes, morphology, zeta potential, electrical conductance and surface tension, have been interpreted by considering aqueous solution of HS as self-organizing non-equilibrium systems [150]. DLS and zeta potential measurements have been extensively performed on aqueous solutions of HA and FA by [PERSON] et al. [151; 152; 153; 154; 155]. Two main processes were found responsible for the dynamics of HA and FA in solutions: the dissociation of acidic functional groups and the dissociation of humic particles into smaller molecular associations and/or single molecules. In particular, fulvic particles in water were bigger than in alkaline solution as a result of suppressed dissociation. The decrease in particle sizes with increasing concentration was interpreted in terms of enhanced strength of intermolecular interactions favoring HA and FA particles to adopt a more collapsed or coiled configuration. From the measurement of zeta potential, it was found that the colloidal stability was higher for HA than FA, probably due to the lower dissociation degree of the functional groups in FA compared to HA. In another study, DLS technique was used on soil extracted HA to evaluate the effect of H\({}^{+}\) on the aggregation kinetics in Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), and Ca\({}^{2+}\) nitrate electrolyte solutions with different ionic concentrations [156; 157]. The results showed that the specific ion effects in HA aggregation process could be related to the strong particle surface electric field and the polarization of adsorbed cations. In addition, the origin of the specific ion effect for H\({}^{+}\) was its chemical adsorption onto surfaces, while the effects for alkali cations were interpreted on the basis of non-classic polarization concept [158]. Light/X-ray/Neutron Scattering Studies and Fractal Architecture of HS Aggregates  
The concept of fractals, which are self-similar scale invariant geometric figures, can be applied to describe both the geometric properties of complex systems such as, e.g., the HS, and the aggregation mechanisms to which various natural compounds are subject. [74; 75; 76; 159; 160]. As described, for example, in the model developed by [PERSON] et al. [161], the fractal properties of an aggregate or a particle are described through: (i) its fractal dimension d\({}_{f}\), which gives information of a morphological type (i.e., whether the material is compact or loose), (ii) the particle radius (R\({}_{\text{p}}\)) and (iii) the particle-particle correlation length (\(\xi\)). Structural investigations of colloidal particles of submicron dimensions, dispersed in aqueous solutions, can be carried out with small angle scattering methods using light, neutrons or X-rays. Within the diffraction limit, accessible length scales are much less than the wavelength of the radiation being used. Thus, light scattering can only directly detect the larger end of the colloid size range (>400 nm) and is also limited to systems that are not too turbid whereas shorter length scales can only be probed by radiation of shorter wavelength than light, for example, by X-rays and neutrons.