## 3 Results and Discussion
### Study of the Physicochemical Properties of Aqueous IgG Colloid of Different Concentrations
It was previously shown that with prolonged contact of freshly distilled water with atmospheric gases, the water becomes saturated with oxygen and nitrogen molecules [35]. In this case, nano-sized clusters of gases, so-called bubstons, are formed in water [36]. The characteristic size of bubstons was determined earlier and is equal to 100-200 nm [37]. It can be assumed that the observed scatterers are most likely bubbles of dissolved gases--bubstons. It is obvious that when the size distribution of objects in a colloid changes, a change in many physicochemical properties of the solution should be observed. Figure 1c shows the dynamic viscosity of IgG colloids at different concentrations at different shear stresses. It is shown that the dynamic viscosity of a protein solution increases at low shear stresses, which indicates that the protein colloid is a non-Newtonian fluid. The dynamic viscosity of solutions with IgG concentration of 0.01 mg/mL is close to deionized water viscosity, and the minimum detectable concentration is \(\sim\)0.1 mg/mL. This observation is in good agreement with known data; it was previously shown that the minimum IgG concentration detectable with viscometry is \(\sim\)0.1 mg/mL [38]. It should be noted that the viscosity of colloids depends not only on concentration, but also on temperature [39], pH [40], ionic strength, cation-anion environment [41], and the like [42; 43; 44]. Progress in viscometry research can be found in a literature review [45]. Colloidal protein solutions are characterized by pseudoelasticity, a decrease in dynamic viscosity with increasing shear stress [46]. In our experiments, pseudoelasticity was also observed. Additionally, it should be noted that the tabulated value for water, 0.87 mPa \(\times\) s (25 \({}^{\circ}\)C), which does not depend on shear stress (Newtonian fluid), is not very different, but differs from what we measured. Water viscosity measurements in a rheometer can vary from 0.87 mPa \(\times\) s to 0.94 mPa \(\times\) s. Dynamic viscosity measurements are highly dependent on the geometry and volumes of the rotor and stator, especially when miniature attachments are used [47]. In such systems, there is a decrease in the absolute accuracy of measurements, while there is another significant advantage--a small volume--which is more important for measuring expensive protein solutions. To assess the density of solutions, aqueous antibody colloids were studied using differential ultrasonic spectroscopy. Figure 1d shows the relationship between the resonant frequencies of the measuring and reference cuvettes. It is shown that only when the concentration of IgG in the aqueous solution is 1 mg/mL or more are significant differences detected with the ultrasonic spectroscopy. Refractive indexes (RIs) of aqueous IgG colloids are shown in Figure 2a. Refractive index measurements of aqueous IgG solutions are shown in Figure 2a. It was shown that the RI of water and an aqueous colloid of antibodies (10 mg/mL) differs by more than \(10^{-3}\) RI. Differences compared to the control are observed in aqueous antibody colloids with a concentration of 0.1 mg/mL or more. A summary plot on a logarithmic scale, with the solvent contribution subtracted, is presented in Figure 2b. An increment of 0.19 g/mL is set aside as a \"theory\". This increment is typical for most proteins and varies slightly depending on their amino acid composition [48]. The graph shows that almost the entire curve for IgG dissolved in water lies below the increment line of 0.19 g/mL, which may, for example, indicate that the IgG molecules are poorly dissolved in water. There may be another explanation for the results obtained. For example, a discrepancy with theory may be caused by other factors: changes in the hydration shell of IgG or differences in the protein increment for a solvent from 0.19 g/mL. Previously, in [49], the relation of the increment of the RI of lysozyme in dependence with the chosen buffer was shown.