**4. Conclusions**

238 Nitroxides – Theory, Experiment and Applications

Molochnikov et al., 2007).

**Figure 17.** Scheme for the mechanism of binding Co2+ by hybrid systems

The elemental composition of the surface shows that chitosan does not cover it completely and some part of the functional groups remains on the support surface in free form. The more complex structure of the surface of hybrid system was characterized by investigating its acid–base properties via ESR spectroscopy of the NR used as pH probes. The titration curves of NR R1 on the surface of the Co2+-containing system are shown in Figs. 13-15. The shift in these curves relative to the calibration curve left or right allows us to determine whether the surface has a positive or negative charge, respectively (Kovaleva et al., 2000 ;

The technique for introducing Co2+ ions into the hybrid materials from the ethanol solutions could lead to the formation of sediments of basic cobalt chloride or chloride–alcoxide micelles on the surface of these materials. In the case of the MCC–chitosan (Fig. 15) and Al2O3–chitosan (Fig. 13) systems, the titration curves of the cobalt-containing materials at pH > 5 are shifted slightly to the left relative to the titration curves of the initial samples. Consequently, the occurrence of basic cobalt chloride on the surface of these materials does not lead to a change in the surface charge, but changes its value slightly. This is due to the effective neutralization of a positive charge of Co2+ ions by negative chloride and hydroxide ions. In order to confirm our hypothesis, we present the published data on studying the sorption of Co2+ ions from aqueous solutions and on the nature of the interaction between the metal center and amino groups of chitosan. According to (Minimisawa et al., 1999) maximum adsorption starts to decrease with increasing pH due to the formation of cobalt hydroxocomplexes. The maximum sorption of cobalt ions by chitosan found at pH 6–8 in (Silva et.al, 2008) is in good agreement with the data from (Minimisawa et al., 1999) and is determined by the formation of Со(ОН)2 phase or slightly soluble basic salts. No chemical

interaction with amino groups of chitosan occurs in this case (Zhao et al., 1998).

The SiO2–chitosan system behaves differently. Even at the highest pH values, the titration curve of the sample modified with cobalt (Fig. 17) was appreciably shifted to the left relative to the NR curve of the initial sample, although it remains to the right of the calibration curve. Accordingly, the formation of the chloride hydroxyl cobalt micelles immediately leads to a considerable reduction in the negative surface charge of the SiO2–chitosan system. The different behavior of the titration curve for system III (Table 2) is most likely associated pH-sensitive NRs gave reliable information on the local acidity of solutions in and the charge of a surface on pure and metal containing inorganic and organo-inorganic materials and systems and allowed to estimate an electric potential near the surface of TiO2 nanoparticles.

The differences between the acidities of external solutions (pHext) and inside pores (or near the surface) of all the studied materials and systems (pHint) were found.

The method of spin pH probes allowed to determine the ionization constants of characteristic functional groups of SiO2-based systems from the horizontal plateaus corresponding to the constant pHint in the samples.

An increase in concentration of H+ ions (a decrease in pHint) in solutions located inside - Al2O3, TiO2 hydrogel and near the surface of the BS-50 type SiO2, TiO2 and SiO2 xerogels ; the

related CMs and hybrid materials ; metal-containing systems, as compared to those of external solution can be explained by releasing H+ ions due to dissociation of acidic functional groups, exchange them with metal ions and the partial desruption of hydrogen bonds. It leads to negative charge of a surface of the above-mentioned objects. A decrease in concentration of H+ ions (an increase in pHint) as compared to those of external solution were characteristic for γ-Al2O3 and cellulose matrixes. This resulted from binding H+-ions by the surface of γ-Al2O3 and MCC and PC with basic functional groups such as -AlOH , –AlO and OH- , respectively. As a result, a surface gains a positive charge.

The sorption capacity of Cu2+ ions depends on a surface charge of the oxides gels, xerogels and the related CMs studied and decreases as a negative surface charge reduces. The sorption of Cu2+ ions on the surface of nanoparticles of nanostructural TiO2 increases the charge of the latter. An increase in a percentage of PC in the SiO2 –PC composites leads to an increase in the amount of silanol groups as a result of increasing in dispersivity of SiO2 particles and specific surface (Ssp) of the samples, and to reducing a negative surface charge up to zero, and even its reversing. It led to the formation of Cu(OH)2 .

The deposition of chitosan on the substrate always creates a negative charge on the surface. While the deposition of chitosan leads to relatively slight changes in the surface potential in the case of inorganic substrates such as Al2O3 and SiO2, these changes are so great in the case of MCC that they even lead to changes in the surface charge.

The charge of the surface of Co2+-modified organo-inorganic hybrid materials at different pHint was found to effect on the composition and structure of Co2+ -containing surface compounds.

The modification of a surface of powder cellulose with nanostructured SiO2 and TiO2 xerogels, aluminum oxides, silica and MCC with acidic functional groups and chitosan makes it possible to adjust the local acidity and surface charge over a wide range.

The study of the surface of organo–inorganic composites and hybrid materials and systems using pH-sensitive nitroxyl radicals allows also to reveal regularities in changing their properties during further modification. In addition, this method enables us to describe qualitatively the processes of structure formation in these systems and their effect on catalytic activity in different pH-dependent reactions.

The calculated φ value (31.7 mV) was found not to be the electric potential of TiO2 nanoparticle surface, but it only characterizes the electric eld generated by a nanoparticle at the site where the radical fragment –N–O• of NR is located. Once the anisotropic spectra of NR in nanostructured oxides are simulated, an electical potential of a surface can be determined. When measuring the SEP of solids, the knowledge of the distance between a radical and a surface is of principal importance. The xation of pH-sensitive NRs on the surface of nanoparticles with linkers of a known length can solve this problem. This will allow one to calculate the potential immediately on the surface of nanoparticles and to compare the calculation results with the experimental data on the electrokinetic potentials.
