The specific surface of the pure xerogels of TiO2 and SiO2 and the composites with different percentage of PC
Many inorganic materials are widely used as adsorbents and catalysts. For example, silica gels efﬁciently absorb vapors and gases in chemical reactors and are applied as ﬁlters for the puriﬁcation of mineral oils and water (Buyanov, 1998). Different aluminum oxide modiﬁcations show good adsorption and catalytic properties in many organic reactions due to the presence of active sites on their surface (Lisichkin et al., 2003). TiO2 gels are widely used in heterogeneous catalysis owing to their enhanced chemical stability, the accessibility of active sites on their surface throughout the reaction volume, simplicity of reaction product separation, and the feasibility of repeated regeneration (Petrov et al., 1998).
The efficiency of these materials mainly depends on the degree of surface development, texture and structural characteristics, availability of active centers, and possibly medium acidity near these centers. The specific surface diminishes on drying in any procedure for the preparation of xerogels. It can be maintained constant by a number of methods, including the use of additions (Ur’ev & Potanin, 1992), for example, powder cellulose (PC) (Shishmakov et al., 2007). Cellulose is a linear high molecular polysaccharide, which forms rigid chain structures due to the inter-molecular hydrogen bonding. It functions in nature as an agent that imparts high mechanical stability to plant tissues (Nikitin, 1962). The deposition of SiO2 and TiO2 xerogels on the PC surface affords composite materials (CMs) with a high dispersity of particles (Shishmakov et al., 2010).
The use of hybrid organo-inorganic materials as supports is a new area in the development of new metal-containing catalytic materials. It allows to obtain supports with specific surfaces that are capable of retaining the metallic component of a catalytic system more strongly.
Chitosan, poly-D-β-glucosamine, is a commercially available amino polymer that is a perfect complexing agent, due to the strong donor properties of both the amino and hydroxyl groups (Varma et al., 2004). Chitosan is thus widely used in obtaining various catalytic materials, including those containing Au0 that are used in the hydroamination of alkenes (Corma et al., 2007); Pd0 used for the reduction of ketones (Yin et al., 1999); the Pd0–Ni0 bimetallic system, used for carbonylation (Zhang & Xia, 2003); Os (VIII), used for hydroxylation (Huang et al., 2003); Co2+, used for hydration (Xue et al., 2004); and Cu2+, used for the oxidation of catecholamines (Paradossi et al., 1998).
SiO2 is usually used as the inorganic component for these systems. The obtained hybrid materials are used to create sorbents of 3d-metal ions (Liu et al., 2002); to immobilize enzymes (Airoldi & Monteiro, 2000); as a solid phase for the liquid chromatography of organic compounds (Budanova et al., 2001), including enantiomers (Senso et al., 1999); and to improve the mechanical properties of other polymers (Yeh et al., 2007). Other oxides in combination with chitosan allow us to obtain biosensors based on ZnO substrate (Khan et al., 2008), selective sorbents of fluoride ions based on Al2O3 substrate (Viswanathana & Meenakshib, 2010) and magnetic materials based on Fe3O4 substrate (Li et al., 2008). Using an organic polymer (e.g., cellulose) as a substrate also has advantages in the sorption of metal ions (Corma et al., 2007). Metal-containing hybrid organo–inorganic materials can also be used as antibacterial composites (Mei et al., 2009), as sorbents of proteins (Shi et al., 2003), and as pervaporation membranes (Varghese et al., 2010).
Nanostructured metal oxides, which are distinguished by extremely developed surface and porosity of particles, are new promising materials for different ﬁelds of science and technology, especially, for heterogeneous catalysis and chemistry of adsorption phenomena (Zakharova et al., 2005).
Many sorption and catalytic processes are pH-dependent. Therefore, the determination of acidity and other acid–base characteristics in pores of inorganic, organo-inorganic materials is of great practical interest, since the catalytic and adsorption properties of solid-phase objects are affected by not only the chemical nature of solutions, but also speciﬁc conditions inside pores and on the surface of these materials. The mobility of liquid molecules in pores of inorganic sorbents was investigated by some authors using the spin probe method (Borbat et al., 1990; Martini et al., 1985 ). Recently, a new method was developed for the determination of medium acidity in pores of solids (pHint) by means of pH-sensitive nitroxide radicals (NRs) as spin probes (Molochnikov et al., 1996 ; Zamaraev et al., 1995). In recent years, this method was used to measure pHint in micropores of various cross-linked organic polyelectrolytes (ion-exchange resins and ﬁlms) (Molochnikov et al., 1996, 2004) and in pores of some zeolites and kaolin (Zamaraev et al., 1995). We found that pHint inside sorbents differ from the pH of external solutions by 0.8–2.1 units (Molochnikov et al., 1996). The method developed allowed us to study the processes of sorption and hydrolysis in ion-exchange resins and the catalytic properties of Cu2+- containing carboxyl cation exchangers (Kovaleva et al., 2000), to determine ionization constants of functional groups and to give a critical estimation to the regularities previously found for the behavior of adsorbents in aqueous media.
pH-sensitive nitroxide radicals (NR) as labels were also used to determine surface electrical potential (SEP) of different biological objects like phospholipids (SLP) - derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE) (Voinov et al., 2009) and the mixed bilayers composed of dimyristoylphosphatidylglycerol and dimyristoylphosphatidilcholine (Khramtsov & Weiner, 1988).
This work is aimed to review the applications of pH-sensitive NR as probes and labels for determination of local acidic and electrochemical characteristics of inorganic and organo-inorganic materials and systems such as :
•Al2O3 (non-modified and modified with F- and SO42- ions) and TiO2 as hydrogels and nanopowders doped with Cu(II) (Molochnikov et al., 2007) ;
•pure and Cu2+-containing solid-phase composites based on nanostructured SiO2 and TiO2 and powder cellulose (Shishmakov et al., 2010; Parshina et al., 2011) ;
•pure and Co2+-containing hybrid organo-inorganic materials based on the chitosan–SiO2, chitosan– Al2O3, and –chitosan–cellulose systems (Mekhaev et al., 2011a, 2011b).
2.1. Objects of study
The technique of synthesis of
Composite materials (CMs) based on nanostructured TiO2 and PC called as TiO2–PC xerogels of 70, 53 and 43 % wt. TiO2 were prepared by diluting 3 g of tetrabutoxytitanium and 0.5 ; 1 and 1.5 g of PC, respectively, in 3 mL of methanol. The hydrolysis was pursued in 10 mL of water at 20°C under intensive agitation, resulting in the condension of TiO2 (PC didn’t participate in condensation). The TiO2 particles formed were deposited on a surface of PC.
The precipitates of the CMs prepared were washed out with hot water, filtered and dried at 100 ˚C during 24h until they attained a constant weight.
The specific surface (Ssp) of the synthesized samples was measured using a SORBIMS instrument (ZAO Meta, Novosibirsk) and calculated by the BET procedure. The data are given in Table 1 (Parshina et al., 2011).
Since the specific surface of powder cellulose did not exceed 1 m2/g, the growth of Ssp was caused by the fragmentation of TiO2 and SiO2 particles deposited on the PC surface during the synthesis of CMs. According to the absolute values of Ssp, the procedure used for the preparation of CMs afforded dioxides with a high degree of dispersity.
Microcrystalline cellulose (MCC) with an ash content of 0.16% and a humidity of 1.1% produced by JSC Polyex; Basic aluminum oxide; BS-50 silica and chitosan produced by JSC Sonat (Moscow) were used to obtain chitosan-containing ahybride organo-inorganic systems. The degree of deacetylation of chitosan (DD) determined by 1H NMR spectroscopy, its molecular weght as determined by viscosimetry and the ash content were found to be 0.84, 250 kDa and 0.19%, respectively (Mechaev et al., 2011a). The BS-50 type silica had a speciﬁc surface area of 45 m2/g and an average diameter of pores of 15 nm (Mekhaev et al., 2011a).
0.3 g (1.8 mmol) of citosan was dissolved in 14.5 ml of water containing 0.22 ml (3.84 mmol) of acetic acid with constant stirring. The substrate in quantities of 3 g was then added, and the solution was stirred for 30 min more.
1 M NaOH solution was added to the suspension under stirring until the pH value reached 13. The precipitate was filtered, washed until the pH value was 7, and dried at 60°С until it attained a constant weight.
CHN analysis was performed using an automatic analyzer PerkinElmer, Inc. The data are given in Table 1. IR spectra of diffuse reflection were recorded using the PerkinElmer Spectrum One spectrometer.
|MCC-Chitosan||43.08 (43.11)||6.42 (6.35)||0.37|
|-||-||10C6H10O5 0.5C6H11NO4 3H2O|
|MCC-Chitosan- Co2+ (I)||43.12|
|10C6H10O5 0.5C6H11NO4 Co0.4(OH)0.17Cl0.65|
|Al2O3-Chitosan- Co2+ (II)||3.64|
|16Al2O3 C6H11NO4 0.7CoCl2|
|SiO2-Chitosan- Co2+ (III)||4.02|
|16SiO2 C6H11NO4 Co0.88Cl0.96|
The surface area of the samples was determined by nitrogen adsorption in accordance with the BET method using a TriStar 3000 V.6.03A instrument. The instrumental error was 0.1 m2/g. The size of particles was estimated under the assumption that the particles were spherical.
The surface area (Ssp) and the diameter of particles (D) were found to be 28.9 m2/g and 47 nm ; 123.9 m2/g and 7 nm ; 2.4 m2/g and 818 nm for chitosan-SiO2, chitosan-Al2O3 and chitosan-MCC hybride systems, respectively.
2.2. Saturation of samples with Cu2+ and Co2+ ions
2.2.1. Saturation of samples with Cu2+ Ions
A 0.1 M NaNO3 solution (10 ml) was added into weighed samlpes (200 mg) of nanostructured TiO2, and the samples were kept for one week at a constant solution pH (5.5) held by adding dilute NaOH and HNO3 solutions. The sorption of Cu2+ ions on nanostructured TiO2 was performed by exposing samples in Cu(NO3)2 solutions (10 ml) with concentrations of 10–4, 10–3, and 10−2 mol/L and ionic strength (µ) of 0.1, which was adjusted using NaNO3. Solution pH equal to 4.3 was maintained by the titration with small volumes of NaOH and HNO3 solutions. After the equilibrium was established, the residual amount of Cu2+ ions the equilibrium solutions was measured to determine the amount of sorbed Cu2+. Then, TiO2 was separated from the solutions by centrifugation. The samples were washed twice with a 0.1 M NaNO3 solution (pH 4.3) to remove adsorbed Cu2+ -ions.
Cu2+ ions were sorbed on TiO2 hydrogel from CuCl2 and Cu(NO3)2 with subsequent its removal by ﬁltration and drying at 20°C for 3 days upto constant weights of the precipitates. A volume of solution and a mass of hydrogel were changed to vary the content of Cu2+ ions in the phase of the studied TiO2 hydrogel, which was determined by the atomic absorption method on a Perkin Elmer 403 spectrometer. As the ESR spectra of Cu2+-containing hydrated gels are difﬁcult to record, hydrogel samples ﬁltered and dried at room temperature were used. Preliminary experiments were performed to select the sample drying conditions preventing the structural changes of the complexes formed.
2.2.2. Synthesis and characterization of the Cobalt-Containing Chitosan hybrid systems
2.3. pH probes
The pH values of solutions inside pores and near the surface of the studied inorganic and organo-inorganic materials were determined using spin probes, namely, pH-sensitive NRs of the imidazoline (R1, R2) and imidazolidine (R3) types (Table 3), which were synthesized at the Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences (Volodarskii et al., 1988; Khramtsov et al., 1998; Kirilyuk et al., 2005).
2.4. Recording and processing of the ESR spectra of NR
The ESR spectra were recorded on a PS 100.X ESR spectrometer (ADANI, Belarus) in a three-centimeter (X) wavelength range at room temperature. Quartz sample holders with an internal diameter of 3.5 mm were used for solid samples. Solution spectra were recorded using quartz capillaries.
Figure 1 shows characteristic ESR spectra of the pH-sensitive NR in aqueous solutions. According to the ESR theory, isotropic signals are induced by the fast-motioned NR molecules (correlation times of 10–10 s and less) and present the triplet of fine lines. Depending on solution pH, NR can be in protonated (RH+), deprotonated (R), or intermediate (mixed) forms. Because hyperﬁne splitting constants aN for RH+ and R forms of the radicals are different (Table 3), in their ESR spectra, the distance
( 0.006 mT)
2.5. Determination of pH in the pore and near the sample surface using pH-sensitive spin probes
An aqueous KCl solution (10 ml) with an ionic strength of 0.1 was added to an oxide sample (200 mg) and the mixture was allowed to stand for a preset time. Then, the solution was thoroughly decanted and an NR solution (10–4 mol/L, µ = 0.1 ) was added to the sample. In some cases, required initial pH values of radical solution were obtained by preliminary mixing of HCl and KOH solutions. After the equilibrium was established, the suspension was titrated with HCl and KOH (HNO3 and NaOH) solutions to plot the titration curve for the NR present in the sample.
For chitosan cobalt-containing hybrid systems and solid-phase composites based on SiO2, TiO2 and cellulose powder the method of multiply batches was used: 0.05 g of sample was kept in 5 ml of buffer aqueous solution containing nitroxide radicals for 2 days (established experimentally). The solution was then decanted.
The pH values of the equilibrium solutions (pHext) over the samples were measured using a Mettler Toledo pH meter (Switzerland) with an accuracy of 0.01 units. The samples separated from the solutions by centrifugation or ﬁltration were placed into unsealed quartz ampules and their ESR spectra were recorded. After measuring the
As can be seen from Fig. 4, the ESR spectra of NR in the samples studied represent the superpositions of three components of an isotropic signal of the probes in aqueous solutions inside pores and a spectrum of the probes immobilized on the surface of the objects studied. For determination of pHint values, only the isotropic signals in the ESR spectra were used.
3. Results and discussion. Acid-base equilibria of inorganic and organo-inorganic materials and systems
3.1. Inorganic oxide materials
3.1.1. Pure oxides
The comparison of the titration curves for NR R1 radical in
Thus, the shift of the titration curve of R1 to the left observed for γ-Al2O3 indicates the binding of H+ ions with the surface (pHintr is higher than solution pH) and to a positive charge of γ-Al2O3 surface, while the right-hand side shift of the titration curve of this NR in SiO2 gel suggests the release of H+ ions and, consequently, a negative surface charge.
According to the data reported in (Hubbard, 2002) the acidic dissociation of hydrated aluminum oxide occurs in two stages, which can be described by the following equations:
The pKavalues of these equilibria (pKa1= 5.87 and pKa2 = 7.50) are given in (Lidin et al., 1987). Using NR, we can determine only pH values relevant to the ascending part of its S-shaped calibration curve (Fig. 2, 3 curve 1), i.e., the range, in which variations in
For example, as was shown previously, the PZC of an Al2O3 sample kept in water for a long time was equal to 9.2, and for Al2O3 treated at 1400°C, the PZC decreased to 6.7 (Robinson et al., 1964). The ﬁrst value corresponds to γ-Al2O3, and the second one, to -Al2O3. For hydrated Al2O3 the PZC was found to be equal to 8, and it decreases as a result of the modiﬁcation of Al2O3 surface with alkyl phosphate-based surfactants (Jeon et al., 1996). Moreover, these data suggest that, in the studied pH range, the surface of γ-Al2O3 has a positive charge that is in accordance with the shift of its titration curve to the left.
The pHint of the sorbents differ from the pHext by 0.5–1.5 units (Table 3). Smaller pHintr values compared to pH of the external solutions for all the studied samples (except for γ-Al2O3) are indicative of the shift in titration curves of NR R1 to the right relative to the calibration curve, similarly to that demonstrated in Fig. 2, 3 all the samples excepting γ-Al2O3.
It should be noted that the pHint value determined in this study for the hydrated -Al2O3 is in good agreement with the PZC value found in (Robinson et al., 1964). From the data given in Table 3 we notice that in all the studied samples excepting γ-Al2O3 acidic centers predominate. However, the acidity of these centers is rather weak; their pKa of dissociation are presumably close to 6. Only when strongly acidic sulfate residues appear in Al2O3, a drastic decrease in pHint is observed as a result of their dissociation. It should be noted that the same decrease in pHint of TiO2 hydrogel results from the sorption of Cu2+ ions by the hydrogel, which leads to the ion-exchange displacement of H+ ions (Fig. 5).
|Al2O3 – F-||6.9||6.4|
|Al2O3 – SO4 2-||4.9||3.8|
|BS-50 type SiO2||7.5||5.7|
The titration curves of radicals occurring in organic sorbents are characterized by the presence of horizontal plateaus corresponding to constant pHint (parameter
It should be noted that the titration curve of NR R1 in the BS-50 SiO2 contains two horizontal plateaus corresponding two constant pHint determined near the SiO2 surface when the pH of the external solution is changed (pHext) (Fig.2, curve 3). These plateaus are referred to as the titration process of functional groups on the surface (Golovkina et al., 2008). The рКа values for these groups were determined from the titration curves, and were рКа1 = 4.4–4.7 and рКа2 = 6.5–6.8. According to (Long et al., 1999 ; Zhao et al., 1997, 1998) amorphous SiO2 contains groups of three types: silanol, silandiol, and siloxane groups in a ratio of 59.2, 14.7, and 26.1%, respectively. We can therefore assume that the lower and longer horizontal plateau reflects the titration process of silanol functional groups on the SiO2 surface, and the upper plateau corresponds to silandiol groups. According to (Méndez et al., 2003), the intervals of changes in the dissociation constants are рК1 = 3.51–4.65 and рК2 = 6.17–6.84, respectively. The same values of constants of dissociation for silanol and silandiol groups of silica were measured and published in (Nawrocki, 1997 ; Neue, 2000). The presence of silanol and silandiol groups on the surface of the studied SiO2 was thus identified.
When studying acidity inside organic sorbents or hydrogels, pH probes are placed into water, which penetrates into a solid due to sample swelling. In this case, the probes occur in water environment far away from the chains of organic (synthetic ion-exchange resins) or inorganic (TiO2 and SiO2 hydrogel, xerogel) polymers.
The particles of nanostructured oxides in diameter of several nm possess extremely large surface (the ratio of the number of atoms on the surface of a nanoparticle to the number of atoms located inside the particle is equal to 1/7). Therefore, a pH-sensitive NR surrounded by water molecules can approach the hydrated surface of nanoparticles rather closely. It was expected that they as probes and labels would appear to be sensitive to SEP, which would make it possible to estimate its value.
The dependences of NR ionization constants on the surface electric potentials and the polarity of media were reported by Fromherz (Fromherz, 1989) and Khramtsov et al. (Khramtsov et al., 1992) for ﬂuorescent pH indicators and for pH-sensitive NRs, respectively, as follows:
where is the total shift in the pKa of an NR occurring in a sample relative to that of the NR in an aqueous solution and and are the shifts in the pKa of the NR occurring in the sample, which are due to th electric potential arising on the sample surface and to a change in the polarity of the medium relative to the polarity of the NR aqueous solution, respectively.
where is the electrostatic potential, e is the electron charge, is the Boltzmann constant, is absolute temperature, is a proportionality coefﬁcient dependent on the radical nature, ε is the medium permittivity, and εHOH is the permittivity of water.Figures 6 and 7 demonstrate the results of studying nano- structured TiO2. The measurements were performed using R3 probe at pH ≈ 5.5, which was close to the PZC of TiO2 (5.7) (Poznyak et al., 1999), and R2 probe at pH ≈ 4.3. Near the surface of TiO2 nanostructured particles, the titration curves of the radicals are shifted to the left, whereas, for TiO2 hydrogel and xerogel (Fig.3), a shift to the right was observed. At present, we can not find any explanation for such principal different shift in the titration curves for TiO2 –based samples. The TiO2 hydrogel obtained by hydrolysis was washed out with water, removing acidic components of the initial salts. Hereafter, the samples were prepared at different drying temperatures. The samples of xerogels and nanostructured oxides were dried at 100◦C and 200◦C, respectively.
As was mentioned above, the shift of the titration curve for γ-Al2O3 to the left corresponds to the positive charge of the particle surface that, in principle, correlates with the data obtained at pH below PZC. The left-shift (toward more acidic solutions) of the titration curve of the probe located near the surface of TiO2 nanoparticles indicates that the surface electric potential is also positive. No changes were found in the hyperﬁne splitting constants
3.1.2. Cu2+-containing samples
Hence, the explicit inconsistency between the determined ∆pKa values for NR R2 and NR R3 radicals and the theoretical predictions is established. We relate this difference to a larger distance of NR R2 from the surface compared to NR R3; this is possible because NR R2 has a substituent in position 4, which is presumably positioned toward the surface due to the tendency of its amino groups to protonation (or complexation with Cu2+ ions sorbed on the surface). Thus, the –N–O• fragment of the radical turns out to be removed from the surface of TiO2 by the chain length of this substituent. The electric potential induced on the −N–O• fragment will decrease with the distance from the surface and will be inversely proportional to the distance from this surface, when it is represented as, for example, a plane. The small NR R3 radical can approach the nanoparticle surface much closer and, therefore, it is affected more strongly by the electrostatic potential of the TiO2 surface. The sorption of Cu2+ ions on the surface of nanoparticles increases the charge of the latter. The observed shift of the titration curve (Fig. 7) and the broadening of the ESR spectra of these radicals attest to the orientation of the substituent in position 4 of NR R2 toward the surface and to the interaction between amino groups of the substituent and the Cu2+ ions sorbed on the nanoparticle surface.
3.2. Organic supports of composite and hybride materials
For PC and MCC, the titration curves of NR R1 were found to be shifted to the left of the calibration curve (Fig. 2, 3 ), indicating a posiive charge on the surface (Kovaleva et al., 2000 ; Molochnikov et al., 2007). Hydroxyl groups of celulose most likely play the role of surface bases whose protonation gives the surface a positive charge.
3.3. Solid-phase composites based on SiO2, TiO2 and cellulose powder
3.3.1. Pure systems
As shown in Fig. 8 and 9, the titration curves of NR R1 in the CMs TiO2 – PC with 70 (not shown) and 53 % wt. TiO2 and in the SiO2 – PC with 68 and 35 % wt. SiO2 (unprotonated parts) are shifted to the right relative to the calibration curve, as in the case of TiO2 and SiO2 xerogels. For CMs based on SiO2, TiO2 and cellulose powder ΔpH = pHext – pHint decreased (the curves are shifted to the left relative to those of pure xerogels) as the PC content in the samples increased; this corresponds to a decrease in the negative charge of the CM surface. This is because of the lower acidity of cellulose compared to the acidity of the solution (cellulose has basic alcohol functional groups in its structure) and the positive charge of its surface (Parshina, 2011).
This proves that a surface of the composites studied carries lesser negative charge as compared to that of pure xerogels. The left-shift of the curve of NR R1 in CM TiO2 – PC with 43% wt.TiO2 relative to the calibration curve can be explained by initial positive charge of the surface of this CM due to binding H+ ions. Hence, SEP of CMs based on TiO2 and PC is varied over a wide range and even changes its sign from negative to positive as an increase in PC content. From the curves plotted, we notice that a decrease in percentage of TiO2 xerogel in CMs from от 53% до 43% leads to a pHintr greater than a pHext. This fact can be explained by positive charge of a surface of cellulose due to the presence of alcohol groups in its structure.
For all the synthesized CMs based on SiO2 xerogel and PC after complete protonating silanol groups, the titration curves of NR R1 were shifted to the left relative to the calibration curve and, hence, a surface of the samples studied remained positively charged (Fig.9). Unlike the composites based on TiO2 and PC, an increase in a percentage of PC in the SiO2-PC composites leads to changing a surface charge from positive to negative with increasing pHext (above the horizontal plateau on the titration curves) due to dissociation of functional groups. Thus, the incorporation of PC into the samples doesn’t change acidity of silanol groups and doesn’t make a polarizing effect on the SiO-H bond. The length of the horizontal plateau slightly increases in the accordance with the amount of silanol groups and the percentage of PC in the CMs based on SiO2. This can be caused by the increased dispersity of SiO2 due to rising in Ssp of the CMs (Table 1).
By varying the cellulose percentage in the composites and pHext values, pHint values and SEP can be selected over a wide range. This information is needed for optimization of the conditions for pH-dependent adsorption and catalytic processes through a choice of CM with a certain pHint and SEP as catalyst substrate and adsorbent.
3.3.2. Cu2+-containing composites
The sorption of Cu2+ on the TiO2 and SiO2 xerogels and on the related CMs is accompanied by a change in the pH in their phases. However, the dependences of pHext(int) vs cCu (amount of sorbed Сu2+ ions) for the above- mentioned types of xerogels and the CMs based have some differences (Fig.10, 11).
From these figures we notice that :
•pHint is different from pHext, and with no Cu2+ in an external solution (cCu = 0), pHext is greater than pHint.
•With an increase in the amount of Сu2+ in the CMs, both the pHext and pHint descrease.
•There is a horizontal plateau on the dependences of pHint on cCu in the range of sorbed Cu2+ from 0.15 up to 0.3 mmol Cu2+/g. TiO2, SiO2 xerogel, within which pHint remains constant with increasing in cCu.
•At greater cCu pHint and pHext come closer and become almost equal.
The increase in acidity of external solution, first of all, is caused by the hydrolysis of Cu2+ at the initial pHext equal to 6-7 and is accompanied by intense release of H+ ions. The lesser pHint values as compared to pHext for both types of xerogels and the related CMs can be explained by a surface negative charge. A surface of the samples studied attracts H+ ions in its double electrical layer to compensate this charge. Also an increase in acidity of both external and internal (near the surface) solutions is caused by the fact that the sorption of Cu2+ ions is accompanied by intense release of H+ ions into them. The increase in the acidity of a medium due to the competition of H+ ions almost completely suppresses the ion exchange sorption of Cu2+. An analogous tendency was previously observed for organic ion-exchange resins (as grains) and TiO2 hydrogel (Kovaleva et al., 2000 ; Molochnikov et al., 2004).
The presence of the horizontal plateaus on the curves pHint vs cCu (Fig. 9) indicates the buffer properties of the TiO2 xerogel and the TiO2 : PC samples by analogy with the same properties of the mixed H+–Na+ form of the KB-2 × 4 cationite (Kovaleva et al., 2000). The greater is percentage of PC in a sample, the shorter is the horizontal plateau on the above-mentioned curves. The buffer effect of the TiO2 xerogel and the related CMs is caused by the existence of some amount of deprotonated functional groups in the samples before the sorption of Cu2+. The complexation of Cu2+ ions with these groups occurs first of all and does not lead to the release of H+ ions, which would reduce both pHext and pHint. In addition, releasing of hydroxide groups as a result of dissociation of molecules of Cu(OH)2 responsible for complexation with functional groups of a surface of xerogel can retard decreasing pH with increasing cCu.
Unlike TiO2 systems, for SiO2 and the related CMs pHint values were found to be more than pHext ones (Fig. 11). Apparently, this differance can be explained by the significant differences in pKa of functional groups of SiO2 and TiO2 oxides. pKa values of silanol groups (3.95 0.07) are significantly lower than pHext from which sorption of Cu2+ has been conducted. Therefore, silanol groups are completely dissociated and H+ ions don’t exchange with Cu2+ ions during complexation. Hence, no functional groups can neutralize OH—groups forming during breakdown of Cu(OH)2 molecules. As a result, pHint values (near a surface of samples) have high values.
As the titratuon curves for TiO2-based CMs don’t contain the horizontal plateaus as well as pure systems, hence, pKa values of active acidic centers of a surface of these materials are out of zone of sensitivity of the NR used (greater than 7) (Fig. 8). Therefore, these centers are mainly in the protonated form before sorption of Cu2+-ions. During complexation Cu2+-ions exchange with H+ ions of functional groups releasing them into external solution. Hence, pHint became less than pHext. It shows up in a general decrease pH (both pHint and pHext) with increasing cCu (Fig. 10 ).3.4. Hybrid organo-inorganic materials based on the chitosan– Al2O3, chitosan–SiO2, and cellulose –chitosan–systems
Hybrid systems were obtained in accordance with the scheme (Fig.12)
The compositions of the obtained hybrid systems were characterized by elemental analysis (Table 2). According to the obtained data, the reactivity of substrates was different and declined in the order MCC >Al2O3 > SiO2, since the compositions of the obtained hybrids in the case of inorganic oxides do not correspond to the molar ratios of the initial components.
The presence of chitosan molecules on the surface of the obtained systems was confirmed by the IR spectra, which contain characteristic absorption bands at 1652 and 1555 cm–1 corresponding to residual acetoamide groups of the polymer.
Since the obtained composites have to be electroneutral, we must assume that in the case of SiO2, the substrate, being a weak acid, forms an ionic bond with the chitosan’s amino group. In case of Al2O3, the same situation is possible, but some of the chitosan’s amino and hydroxyl groups are involved in complex-forming with aluminum. In the case of MCC, the interaction occurs through hydrogen bonds.
NR were used as pH-probes to obtain a more detailed characterization of the surface structure of hybrid systems, their acid–base properties.
A comparison of the ESR spectra of NR R1 in solution and in the samples studied showed that in all cases, there was an isotropic signal which indicated the lack of covalent bonds between the NR and the sample surface and the presence of this radical near the surfaces of particles (Fig. 4, spectrum 1).
3.4.1. Pure hybride systems
Analysis of the structure of the Al2O3–chitosan system according to the adsorbed probe molecules of the nitroxyl radical showed that coating the initial -Al2O3 substrate with chitosan leads to a slight increase in the surface negative charge, as was confirmed by the shift of the NR titration curve to the right (Fig. 13). The titration curves of -Al2O3 and Al2O3–chitosan are not parallel to the calibration curve at low pH values due to the gradual dissolving of -Al2O3 particles in weakly acidic media, as has been noted during investigations of samples containing aluminate ions (Iller, R., 1979 ; Golovkina, 2009).
For the SiO2–chitosan system, the NR titration curve overlaps the corresponding curve for SiO2 values within the horizontal section of the titration curve for silandiol groups in the range of high pH (Fig. 14). We can correspondingly claim that in this pH region, the surfaces of the samples all have the same charge, demonstrating that chitosan has no influence on the SiO2 surface. If pH is decreased below 6, the titration curve for the SiO2–chitosan system shifts to the right of the corresponding curve for SiO2, indicating the negative charge of the surface. The horizontal pleteau of the titration curve for silanol groups is lower in the case of the SiO2–chitosan system (pH 4.5–5), and the рКа value for silanol groups in the presence of chitosan falls slightly. A similar reduction in this parameter has been observed during modification of the surface of mesoporous molecular sieves based on SiO2 with aluminate and borate ions (Golovkina et al., 2009). Below the titration curve plateau for silanol groups (pH < 4.2), the titration curves of SiO2 and SiO2–chitosan overlap again, indicating a similar surface charge.
The deposition of chitosan on MCC causes the titration curve to shift to the right of to both the calibration curve and the NR titration curve of the initial MCC sample (Fig. 15), demonstrating the negative charge of the surface, as was observed for the Al2O3–chitosan and SiO2–chitosan systems. A peculiarity of this substrate is the recharging of the surface, from positive for MCC to negative for the MCC–chitosan system during chitosan deposition. In this case, the type of change in the surface charge occurring upon a reduction in pH is the same for both the MCC and the MCC–chitosan systems. Changing symbatically with the curve typical of the initial MCC sample, the titration curve for the MCC–chitosan system almost overlaps the calibration curve in the acidic region (pH ≤4.5). In this pH range, the potential of the MCC–chitosan system surface is therefore close to zero.
The deposition of chitosan on the substrate always causes the titration curve of the radical near the surface of hybrid material to shift to the right; i.e., it leads to 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, these changes are so great in the case of MCC that they even lead to changes in the surface charge.
Some of differences in the behavior of titration curves for chitosan-containing materials on the inorganic substrates that occur in the range of high pH values could be due to the close рКа values of the silandiol groups of the SiO2 substrate (рКа = 6.5–6.8) and of the chitosan (рКа = 6.42 (Skorik et al., 2003)). When the pH falls, amino groups of chitosan or silandiol groups become protonated and form hydrogen bonds with one another. As a result, the charge of the initial substrate surface and that of the hybrid material become similar. During further protonation of silandiol and amino groups at the same pH range, this interaction becomes impossible, and the surface charge of the SiO2–chitosan hybrid material becomes more negative than that of the SiO2.
This is in complete agreement with the relative positions of the radical titration curves for Al2O3 and Al2O3–chitosan. It must be emphasized that the рКа dissociation value of hydroxyl groups of Al2O3 is higher, and lies outside the radical sensitivity range.
Deposits of chitosan on the MCC substrate breaks it into smaller particles (Table 2), due most likely to the recharging of the surface from positive to negative. Chitosan molecules are appreciably smaller in size than MCC molecules, that is why a negative charge leads to increased repulsion inside large MCC particles, to their disintegration, and finally to an increase in the total surface area of the material. Chitosan thus plays the role of a disaggregating surfactant. In case of inorganic substrates, the deposition of chitosan leads to an increase in the particle size, and consequently to a decrease in their surface area (Table 1). Chitosan is therefore a weakly aggregating surfactant as to inorganic substrates.
The рКа value for amino groups of chitosan is 6.42 (Skorik et.al, 2003). In the investigated pH range (pH ≤ 6), polymer molecules must consequently be positively charged due to the formation of NH3+ ammonium groups. If chitosan molecules were adsorbed on the surface of inorganic oxides so that the protonated amino groups were on the material surface, this kind of adsorption would be reflected in the titration curves of the modified samples and would cause the titration curve to shift to the left of the titration curves of the initial oxides. Since this did not happen, we can assume that during the formation of hybrid material, chitosan amino groups form weak hydrogen bonds or Van der Waals bonds with functional groups of substrates (organic or inorganic), leading to an arrangement of chitosan molecules in which glucosamine rings are turned in the direction of the substrate.
3.4.2. Co2+ -containing hybride systems
The presence on the support surface of a polymer that is capable of acquiring metal ions of high coordination numbers is needed to fix these ions more firmly on metal-containing hybride materials. This role was played by chitosan on the surfaces of the inorganic (SiO2, Al2O3) and organic (MCC) supports. Co2+ ions were sorbed from an aqueous ethanol solution (Fig. 16).
The composition of the studied systems was characterized by elemental analysis. According to the data, the sorption capacity of the hybrid systems relative to Co2+ ions is different and declines in the following order: SiO2 >Al2O3 > MCC. We should note that in this case, there is a difference between the mechanisms of binding ions for different supports, since their surfaces have the same coating influencing the process.
As follows from the elemental analysis, chitosan plays the role of a complexing agent. In this case, all amino groups are involved in the coordination by cobalt ions (ratio Co:NH2 = 1:1-1.5).
In the case of the Al2O3–chitosan system, some of the amino groups do not participate in the complexation, but the Co : Cl ratio corresponds to the composition of the initial salt that provides the coordination mechanism for binding cobalt ions (Fig.17). In the case of the SiO2–chitosan system, the Co : Cl ratio is ~1. The need to obey the law of electroneutrality for the obtained hybrid systems requires assuming that the support, in addition to being a weak acid, also plays a coordinating role with respect to Co2+ ions, which accords with the covalent mechanism of binding Сo2+ ions (Fig. 17). In the case of the MCC–chitosan system, metal ions are sorbed as a basic salt, and hydroxyl groups on the surface provide weaker coordination binding than Al2O3.
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 ; Molochnikov et al., 2007).
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 with the initially lower amount of Cl– ions in the cobalt micelles (Co : Cl = 1 : 1), i.e., to its higher amount of ОН– ions. Acid sites on the SiO2 surface (silanol groups) also likely interact with the basic cobalt chloride particles precipitating on the surface, thereby replacing the Cl– ions. In both cases, the titration curve must shift left due to the neutralization of the negative surface charge. In the study of the samples in the aqueous medium at high pH values, the transformation for systems I–III (Table 2) thus occurs; hydration of the surface, followed by hydrolysis leading to the formation of colloidal particles based on chloro-hydroxo complexes of Co2+ ions, is observed.
These micelles of Co2+ chloro-hydroxocomplexes begin to dissolve at pH ≤ 5 for all investigated systems. The dependence of sorption of cobalt ions on pH (Minimisawa et al., 1999) shows that the maximum adsorption starts to decline at pH < 5, and Co2+ ions in the solution are in the form of aqua complexes (Zhao et al., 1998). The latter are sorbed by the chitosan primary amino groups, thereby charging the surface positively; in this case, the titration curves of systems I–III (Table 2) (Figs.13-15) are shifted left relative to the corresponding curves for the initial samples and to the calibration curve. A further reduction in pH leads to the neutralization of ОН– groups, and no further changes in pH near the particle surface are observed upon a change in pHext.
This indicates the presence of a horizontal plateau on the titration curve. The value of the horizontal section is lower for system III than for systems I and II (Table 2), since some of the ОН– groups are replaced by residues of silicic acid. At high pH values, Co2+ ions initially cause a substantial decrease in the negative surface charge for the case of system III; their transformation into the form coordinated by the chitosan - NH2 groups thus influences the surface charge to a lesser degree upon declining pH. Our results indicate the participation of chitosan amino groups in the complexation with Co2+ ions. During the interaction of the hybrid materials with the Co2+-containing solution, at least a part of glucosamine rings are consequently turned outward and are capable of becoming ligands.
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 value (31.7 mV) was found not to be the electric potential of TiO2 nanoparticle surface, but it only characterizes the electric ﬁeld 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 ﬁxation 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.
The authors are very grateful to :
Prof. A.M. Volodin and Prof. A.I. Kulak for synthesizing and supplying aluminum oxides and nanostructured TiO2 both original and modiﬁed with F– and SO42– groups,
Dr. I.A. Kirilyuk and Prof. I.A. Grigor’ev for synthesis of pH-sensitive nitroxide radicals,
Dr. A.B. Shishmakov, Mrs. Yu. V. Mikushina and Dr. E. V. Parshina for synthesizing TiO2 and SiO2 hydrogels and xerogels, pure and Cu2+-containing solid-phase composites based on nanostructured SiO2 and TiO2 and powder cellulose,
Mr. A.V. Mechaev for synthesis and characteization of pure and Co2+-containing hybrid organo-inorganic materials based on the chitosan–SiO2, chitosan– Al2O3, and –chitosan–cellulose systems,
Prof. A.I. Kokorin, Dr. A.V. Pestov and Prof. Yu.G. Yatluk for fruitful discussion of the study results.
The studies presented in this manuscript were financially supported by the Ministry of the Education of the Russian Federation (2007-2011, Theme 01.2.007-06425).
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