A Research of Chemical Nature and Surface Properties of Plant Disperse Fillers

Chemical nature and surface properties of plant disperse fillers are investigated: buckwheat (BH) and oat (OH) husk, wood (WF) and conifer flour (CF). Using IR spectroscopy, it was found that oxygen-containing atomic groups –OH, –C–O–, –C=O prevail in the filler components. It was found that a hydroxyl-hydrate layer of functional groups is present on the surface of air-dry fillers. By potentiometric titration of aqueous suspensions using the Parks–Bobyrenko method, it was determined that all fillers are of the «polyfunctional solid» type. It is shown that the hydroxyl-hydrate surface layer consists of functional groups with similar values of acid-base characteristics. Functional groups of acidic nature were additionally found on the surface of the fillers: groups with pKa≈4.37−5.66 on the BH surface, groups with pKa≈4.49−4.90 on the CF surface and groups with pKa≈3.91−4.30 on the WF surface. As a result of potentiometric titration, it was shown that the surface acidity of the fillers decreases in the WF&gt;CF&gt;BH&gt;ОH series, which coincides with the one in which the total cellulose and lignin content decreases, and the resistance of fillers to thermal-oxidative breakdown increases. It was found that the rate of hydrolytic processes in aqueous suspensions at the interface decreases in the ОH&gt;CF&gt;BH&gt;WF series and inversely depends on the concentration of functional groups on the surface of the fillers, and also that the change in the rate of hydrolytic processes at the interface depending on the content of fillers is described by step functions. It is revealed that for the effective use of the studied disperse waste in composite materials and as adsorbents for the extraction of pollutants, dispersion media with the following ranges of the hydrogen index are required: for BH − pH&gt;4.4; OH − pH&gt;6.4; WF − pH&gt;3.9; CF − pH&gt;4.5. The results obtained make it possible to predict and control acid-base interfacial interactions, as well as reasonably approach the development of new effective technologies.

It is known that the influence on physical-mechanical, rheological, operational and other characteristics of composite materials is related to many properties of plant disperse fillers. The most important of them are the chemical composition, particle surface size and morphology, pre-treatment method, storage conditions, content and method of addition to the composite. The influence of these factors on the characteristics of filled composites has been studied by many researchers [1,9,12]. However, the chemical nature and acid-base characteristics of surface functional groups of particles of plant disperse fillers, which directly affect the interfacial interactions in the compo site material and give rise to most of the properties, have not been studied enough. In turn, the chemical nature and characteristics, including acid-base ones, of functional groups of the surface layer of particles are obviously determined by the chemical composition, storage conditions, method of preliminary chemical, physical or mechanical treatment of disperse fillers [2,9,12,18].
Plant disperse fillers are multicomponent amorphous materials, have a complex chemical composition and, obviously, unstable structure and surface properties. Particular attention is needed to study the acid-base properties of the surface. Acidbase characteristics are important because acid-base interactions occur between the functional groups of the filler surface and the matrix during the formation of composites at the interface. This is confirmed by theoretical and experimental studies on the acid-base nature of interfacial interactions in composites [19,20]. Information on the acid-base characteristics of surface functional groups allows predicting interfacial interactions and controlling the properties of filled composites [21].
The study of the properties of the surface layer of promising plant disperse fillers is one of unresolved chemical and technological problems. In most cases, chemical and physicochemical methods are used for determining the chemical nature and preliminary assessment of the surface properties of filler particles. In order to interpret and find the nature of the particle surface, it is especially important to study the qualitative and quantitative chemical composition of fillers and determine correlations in the «chemical composition -surface properties» system. In this case, it is possible to determine the nature of certain individual atoms or atomic groups that are part of molecules and can potentially be located on the surface and cause both local and general acid-base characte ristics. For example, in the composition of functional groups, the individual atoms of oxygen :О =, nitrogen :N≡, sulfur :S = predetermine alkalinity, and the atoms of metals Ме n+ , carbon ≡С + , silicon ≡Si + predetermine the Lewis surface acidity. At the same time, some functional groups: hydroxyl -OH, carboxyl -COOH, aldehyde -СОН cause the Brønsted surface acidity or alkalinity. In addition, by analogy with inorganic fillers [19][20][21], water molecules can be adsorbed on the surface, which form a hydroxyl-hydrate layer of the Brønsted functional groups (OH groups) with a wide range of acid-base characteristics. Therefore, in the study of surface properties, the first priority is to determine the qualitative and quantitative chemical composition of plant disperse fillers.
Thus, chromatographic analysis of aqueous, oxalate and alkaline extraction solutions found that buckwheat waste (husk and straw) contains hetero-chain natural polymers − polysaccharides, as well as uronic acids and some metals [22]. The total yield of polysaccharides ranges within 3.2−6.3 % for husk and 7.6−12.2 % for straw. Polysaccharides found in alkaline extracts have a complex monosaccharide structure and contain rhamnose, arabinose, xylose, mannose, glucose and galactose residues, depending on the type of waste. Uronic acids are contained in all the polysaccharides obtained and are mainly represented by galacturonic acid.
As a result of the chromatographic analysis of water-alcohol extraction solutions, the amino acid composition of rice, buckwheat and sunflower husk was investigated [23]. The total amount of free amino acids in terms of absolutely dry matter was: in the husk of rice -0.06 wt. %, buckwheat -0.05 wt. %, sunflower -0.08 wt. %. The qualitative composition of essential amino acids of the husk is represented by valine and leucine in rice and sunflower, leucine in buckwheat. The qualitative composition of nonessential amino acids of the husk is represented by aspartic and glutamic acids in rice, serine, phosphoserine and aspartic acid in buckwheat, asparagine, glutamic and γ -aminobutyric acids in sunflower. In addition, the analysis of the results shows that the husk of all the studied crops also contains urea in the amount of 4•10 -5 ,1•10 -5 , 8•10 -5 g/kg in the rice, buckwheat and sunflower husk, respectively.
The authors [24,25] investigated the qualitative and quantitative chemical composition of inorganic components of husk and straw of rice, oat, sunflower and buckwheat. The total amount of ash obtained after waste firing at a temperature of 400-650 °C is: oat -3.5-5.1 wt. %, rice -6-22 wt. %, buckwheat -1.7-5.8 wt. %. The authors note that the main component of waste ash of rice and oat is SiO 2 , and waste ash of buckwheat and sunflower consists mainly of carbonates of alkali and alkaline earth metals. In terms of acid-base characteristics, the content of oxides of different chemical nature in waste ash is interesting (Table 1). Table 1 shows that in terms of the content of inorganic oxides in the ash, the highest acidity is characteristic of the husk and straw of rice and oat, buckwheat waste has alkaline properties, and sunflower husk and straw have amphoteric or slightly alkaline character.
The authors of [26] found soluble amino acids, ascorbic and folic acids, riboflavin, nicotinamide in aqueous extracts of oat straw.
It is known that buckwheat waste contains organic dyes of complex chemical structure, which obviously have a significant effect on the acid-base nature of the particle surface. The authors of [27] have developed an effective complex technology for extracting organic dyes and biologically active substances by complex aqueous acid extractants from buckwheat husk using liquefied carbon dioxide. It was found that the rate of extraction significantly increases with increasing temperature and degree of waste dispersion.
The main components of crop waste are cellulose polysaccharide (30-50 %) and a mixture of aromatic lignin polymers (20-30 %) [9,18,28,29]. Using simple technologies, natural polymer materials are obtained -pulp, hydrolyzed lignin, which can be used as fillers and adsorbents [11,12,30,31]. Cellulose and lignin do not dissolve or hydrolyze in water due to high molecular weight and formation of numerous intra-and intermolecular bonds. Polymer molecules contain a large number of different functional groups with a wide range of acidity, which upon reaching the surface determine the acid-base characteristic.
The information indirect method of surface characterization is the study of the sorption properties of disperse filler particles with respect to substances of different chemical nature [32][33][34][35][36]. The presence of functional groups and the acid-base nature of the surface of plant disperse waste were found in [32], which investigated the sorption properties of buckwheat and sunflower husk in relation to various organic and inorganic reagents. Pre-treatment by the acid-alkaline method has shown to increase the adsorption capacity of husk particles of both species 2-3 times. In [33,34], kinetic patterns of adsorption on the surface of disperse materials are considered. All the above works studied the chemical nature of organic and inorganic compounds, which are part of many popular plant wastes. And thanks to these studies, it is possible to get a fairly complete picture of the chemical composition of plant disperse materials. However, when using waste as fillers for composite materials, information about the surface properties of disperse particles, namely, the acid-base nature of surface functional groups that can interact with the material matrix, is essential. However, this issue is not considered in the presented studies. This may be due to the complexity of interpreting the results of experimental studies of the above disperse fillers by known physicochemical me thods. This is because due to the natural origin of plant waste, the chemical composition and surface properties of filler particles are significantly dependent on many factors such as climatic conditions of plant cultivation, chemical treatment methods (pesticides, fertilizers, herbicides, etc.), features of processing technology to obtain the target product, storage conditions and methods of waste preparation for reuse. Also, the absence of the above studies can be explained by the fact that non-chemical interfacial interactions, which are considered to significantly affect the characteristics of composites are often neglected in the development of filled composites.
Earlier, the work [18] investigated the chemical composition, physicochemical, morphological and surface properties of disperse fillers on the basis of plant raw materials: buckwheat and oat husk, wood and conifer flour. The acid-base properties of the particle surface were studied using the potentiometric method of A. P. Nechiporenko. Patterns of changes in surface acid-base properties from the chemical composition and physicochemical characteristics of fillers are revealed. But the results provide only a generalized rough assessment of the acid-base properties of the filler surface and do not give an idea of the chemical structure and properties of functional surface groups. In addition, the results do not take into account the specific surface area of the fillers, i. e. do not give an idea of the concentration of functional groups per unit surface area of the disperse particle. Oxides of amphoteric elements Electronic copy available at: https://ssrn.com/abstract=3702229 Thus, the issue of studying the chemical nature and acidbase characteristics of surface functional groups of plant disperse fillers remains unresolved. There is also little knowledge of the relationship between the acid-base characteristics of surface functional groups, chemical nature and quantitative content of organic and inorganic components of fillers.

The aim and objectives of the study
The aim of the study is to investigate the chemical nature and acid-base characteristics of surface functional groups of disperse fillers on the basis of plant raw materials of buckwheat (BH) and oat (OH) husk, wood (WF) and conifer (CF) flour. This will allow predicting and regulating the acid-base interactions between the filler surface and the dispersion medium.
To achieve the aim, the following objectives are formulated: -using the IR spectroscopy method, to determine the chemical nature of functional groups and using potentiometric titration of aqueous suspensions to investigate the acid-base properties of the surface of plant disperse fillers; -to investigate the relationship between the acid-base properties of surface functional groups, chemical nature and quantitative content of organic and inorganic components in the fillers; -to formulate practical recommendations for the effective use of plant disperse fillers.

Materials and methods of the study
As materials for the study, disperse waste from the processing of multi-tonnage agricultural products and woodworking industry were selected. This is buckwheat (BH) and oat (OH) husk. For comparative characterization, wood (WF) and conifer (CF) flour is selected. The materials were milled and dried under the same conditions in an oven at 105 °C.
IR spectroscopy was used to study the chemical structure of filler components. Spectrograms were obtained on a SPECORD 75 UR spectrophotometer (Germany) at a temperature of 293−298 K in the wavenumber range of 4,000-500 cm -1 with the following settings: monochromator slit width -3 nm, recording time -13.2 minutes, and time constant of recorder pen deviation -1 second. Samples for IR spectra were prepared as pellets of filler powder and KBr with a filler weight content of about 1 %.
Potentiometric titration of aqueous suspensions of fillers according to the Parks-Bobyrenko method [37] was carried out at room temperature of 293-298 K using a portable SX 711 pH meter (China) with the measurement accuracy of hydrogen index ±0.001 рН. In accordance with the method, the pattern of changes in the suspension pH during titration was investigated. Given that the investigated fillers are characterized by an acidic or weakly acidic surface [18], an alkaline solution of KCl is chosen as the electrolyte, and a 0.1 M HCl solution is chosen as the titrant. First, a blank experiment was conducted. 25•10 -5 dm 3 of original alkaline electrolyte (0.1 M aqueous KCl solution with pH 0 ≈10, the value of which was regulated by the addition of 0.1 M NaOH solution) were added to the potentiometric cell with glass and chloride silver electrodes. After stabilizing the potential of the glass electrode (after 2-3 minutes with continuous stirring with a magnetic stirrer), the solution was titrated with 0.1 M HCl solution to reach the titration end point. The titrant was added in small portions of 2•10 -5 dm 3 every 2-3 minutes after stabilizing the pH of the suspension. After that, a working titration was carried out. 25•10 -5 dm 3 of 0.1 M aqueous KCl solution and a sample of the studied filler (0.10; 0.25; 0.50 g) were added to the potentiometric cell. The suspension was stirred with a magnetic stirrer until the pH was stabilized and the рН susp value was recorded. The suspensions were then titrated as in the blank experiment.
According to the titration results, the amount of excessively adsorbed OH ions on the surface of the fillers (ΔG, mol/m 2 ) was calculated using the improved formula: where ΔpH = pH 0 -pH susp is the change in the suspension pH as a result of hydrolytic adsorption; ΔV is the volume of titrant added, dm 3 ; C is the concentration of HCl (titrant) solution, mol/dm 3 ; m is the filler sample, g; S is the specific surface area of the filler, m 2 /g (the factor added to account for the concentration of functional groups per unit surface area of the filler); specific surface area values of the fillers are 0.68; 0.75; 1.20 and 0.92 m 2 /g for BH, OH, WF and CF, respectively [18]. According to the constructed graphs pH susp = f(V) and ΔG = f(pH 0 ), the following characteristics and values were determined: 1) by the nature of the graph ΔG = f(pH 0 ), the chemical nature of surface functional groups was evaluated according to the classification [37]: polyfunctional solid, monofunctional strong alkali, bifunctional surface with strong and weak alkaline groups; solid with a weakly alkaline surface; 2) pH eq -the pH of the suspension, at which adsorption of an equal number of H + and OH − ions on the filler surface occurs; defined as the intersection point of the potentiometric titration curves of the electrolyte solution (blank experiment) and the suspension (working titration) on the graph pH susp = f(V); 3) рН ІІP -the pH of the suspension at the isoadsorption (isoionic) point, which indicates the preference of surface functional groups with a certain acid-base characteristic (рK а ); defined as the intersection point of three graphs pH susp = f(V) for different filler samples and, to obtain a more accurate value, as the intersection point of the graph ΔG = f(pH 0 ) with the abscissa axis, that is, at ΔG = 0; pH ІIP was thought to indicate the preference of surface functional groups with pK a , that is, pH ІІP ≈pK a ; 4) v -the value of the exchange capacity of surface functional groups of the filler (characteristic of the speed of hydrolytic processes at the interface), defined as the slope of the straight line ΔG = f(pH 0 ), i. e. the value of the factor k in the equation ΔG = k•pH 0 ± b.

Experimental results of the study of the chemical nature of components and acid-base surface properties of plant disperse fillers by IR spectroscopy and potentiometric titration of aqueous suspensions
Using the IR spectroscopy method, the chemical structure of functional groups included in the compounds − filler components before and after heat treatment at T = 523 K for 2 hours was investigated. The resulting filler spectrograms are presented in Fig. 1.
As noted earlier [18], to estimate the degree of integral (total) acidity of a solid surface, it is quite convenient to use the theory of «surface isostate». Surface isostate is defined as the equilibrium steady state of inertia of a solid surface in a suspension, which is characterized by the absence of mass transfer of charged ions at the interface [37]. The isostate is quantified by the isopoint in which the equilibrium between the solid surface ions and the liquid phase ions is maintained for some time. From the point of view of mass, the isoadsorption state (point of zero charge PZC) arises, and from the point of view of charge -the isoelectric state (isoelectric point IEP). The pH value in the isopoint indicates the preference of surface functional groups with a certain acid-base characteristic, which is reflected by the pK a index.
In [18], the isoadsorption state and pH of aqueous suspensions at the isoionic point for buckwheat husk (BH), oat husk (OH), wood flour (WF) and conifer flour (CF) were in-vestigated by the potentiometric method of A.P. Nechiporenko. Two types of functional groups are found on the surface of GL: weakly acidic and neutral. The OH surface is close to neutral. The surfaces of WF and CF have a general weakly acidic nature. It was found that the integral acidity of the filler surface decreases in the CF>WF>BH>OH series. It was found that, with the exception of CF, the surface acidity is directly proportional to the total content of cellulose and lignin in the fillers.
In order to expand the understanding of the properties of the isoadsorption state of aqueous suspensions and determine the acid-base characteristics of surface functional groups of the above-mentioned fillers, studies were performed by the Parks-Bobyrenko potentiometric titration method. Using the improved formula (1), the amount of excessively adsorbed OH ions on the surface was calculated.  The graphs pH susp = f(V ) (Fig. 2-5, a) are titration curves that reflect the change in the hydrogen index of aqueous suspensions as a portion of the titrant is added. The curves shown are based on the average рН susp values calculated from the results of three parallel experiments. It is found that after heat treatment, there is a decrease in the intensity of the bands 3,600-3,100 cm -1 , which are responsible for −OH valence vibrations with a maximum at 3330 cm -1 , involved in the hydrogen bonding system and associated with the presence of moisture on the surface of all fillers. Thus, the presence of a hydroxyl-hydrate layer on the surface of the filler particles is obvious. It is also apparent that the functional groups of the hydroxyl-hydrate surface layer determine the acid-base interactions at the interface.
The differences in the chemical structure of BH and OH in comparison with WF and CF include the presence of a broad band of skeletal vibrations of functional groups with aromatic fragments in the region of 1,614-1,570 cm -1 , which cause an increase in the thermal stability of these fillers. At the same time, the total amount of aromatic component for BH is higher compared to OH, which in turn may lead to increased thermal stability: thermal breakdown of BH begins only after T > 493 K, and OH -after T > 473 K. At the same time, thermal breakdown of WF and CF begins already at T > 433−443 K [18].
Thus, the analysis of the IR spectra of the fillers leads to the following conclusions: -spectral analysis revealed that among the atomic groups included in the components of the fillers and, apparently, determining the acid-base properties, the oxygen-containing groups -OH, -C-O, and -C = O predominate; these atomic groups may in turn be within more complex functional groups such as carboxyl, complex ether or aldehyde; -it is found that BH and OH contain aromatic compounds that increase the thermal stability of these fillers; -the decrease in the intensity of the bands responsible for valence vibrations of -OH groups with hydrogen bonds after heat treatment indicates the presence of a considerable amount of adsorption water on the surface of air-dry fillers, which forms the hydroxyl-hydrate surface layer of the Br nsted functional groups; -given the diverse chemical composition of plant fillers, it can be assumed that the Brønsted functional groups with a wide range of acid-base characteristics are present on the surface of the particles.
By the nature of the graphs ΔG = f(pH 0 ) ( Fig. 2-5, b) and according to the classification [37], all the investigated fillers are of the «polyfunctional solid» type. This is indicated by the nature of the graph with two sections: the linear section of positive values of ΔG and the nonlinear section of negative values of ΔG. The region of positive values characterizes the process of adsorption of ОН − ions on the surface of fillers in suspensions with a certain hydrogen index pH 0 . The number of adsorbed ions increases in direct proportion with increasing pH 0 , that is, with increasing concentration of ОН − ions in the solution. The linearity of this section of the graph is due to the fact that at these pH 0 values, the adsorption of ОН − ions mainly occurs in the suspensions, the rate of which is the highest in comparison with other hydrolytic processes. The approximation was carried out for these sections of the graphs. After crossing the abscissa axis at the isoadsorption point (pH ІІP ), the graph loses linearity and the nonlinear section of negative values of ΔG appears. Thus, it can be argued that in the area of positive values of ΔG, the adsorption process on the surface of the fillers in aqueous suspensions is dominant and actively occurs in the range of the hydrogen suspension index pH 0 >pH ІІP . The nonlinearity of the graph in the region of negative values of ΔG is obviously due to the fact that in addition to the adsorption of ОН − ions, other hydrolytic processes are intensified. The adsorption processes of ОН − ions slow down. As the concentration of Н + ions increases, that is, the hydrogen index decreases, processes of desorption of ОН − ions and competitive adsorption of H + ions occur. In addition, with increasing acidity of the me dium, extraction processes may occur, the possibility of which is noted in [26,27].
On solid surfaces of the «polyfunctional solid» there are no adsorbed impurities, the dissociation degree of which exceeds the dissociation degree of surface functional groups of the filler itself and can change the chemical equilibrium at the interface [37]. This means that all hydrolytic processes that are observed during the titration of suspensions occur at the interface and reflect the acid-base characteristics of surface functional groups of the fillers. It is also apparent that the definition of the investigated fillers as «polyfunctional solids» indicates that there is a hydroxyl-hydrate layer of functional groups with close values of acid dissociation constants pK a on the surface of the particles. Table 2 presents the results of the calculation of the characteristics of aqueous suspensions of fillers according to the obtained graphs of potentiometric titration рН susp = f(V) and ΔG = f(pH 0 ). Electronic copy available at: https://ssrn.com/abstract=3702229 Table 2 shows that the use of the Parks-Bobyrenko potentiometric titration method in comparison with the Nechiporenko method provides refined рН eq and рН ІІP values. The method further revealed acidic functional groups on the surface of the fillers. So, groups with pK a ≈4.37−5.66 on the BH surface, groups with pK a ≈4.49−4.90 on the CF surface, and groups with pK a ≈3.91−4.30 on the WF surface are detected. This is due to the fact that, first, the Nechiporenko method provides a general (integral) acid-base characteristic, which indicates the acidity of the functional groups that prevail on the surface. Thus, it can be argued that the number of acidic groups detected on the surface of the fillers is small and the effect of these groups on the overall acid-base character is negligible. secondly, only one sample of the fillers (~0.2 g) was investigated by the Nechiporenko method, and three samples (0.1; 0.25 and 0.5 g) were used in the potentiometric titration method, which yielded refined results. In general, according to the results of potentiometric titration, the surface acidity of the fillers decreases in the WF>CF>BH>OH series [18]. This series completely coincides with the one in which the total cellulose and lignin content in the investigated fillers decreases and the resistance of the fillers to thermal-oxidative breakdown increases [18]. Therefore, on the basis of the above, it can be concluded that the Parks-Bobyrenko potentiometric titration method is more informative for investigating the acid-base characteristics of the surface of plant disperse fillers.
Potentiometric titration of the three filler samples made it possible to determine the exchange capacity of the surface functional groups of the fillers, that is, the velocity characteristic of the hydrolytic processes on the surface v. This value was defined as the slope of the straight line ΔG = f(pH 0 ) to the abscissa axis or as the value of the k factor in the equation ΔG = k •рН 0 ±b ( Table 2). The results show that the rate of hydrolytic processes at the interface in aqueous phase suspensions decreases in the OH>CF>BH>WF series and depends on the concentration of functional groups on the surface of the fillers. It is shown that the decrease in the concentration of functional groups increases the rate of interfacial hydrolytic processes. These data correlate with the specific surface area values of the fillers and with the time of equilibrium establishment in the suspensions [18]. The specific surface area and the time of equilibrium establishment in the suspensions of the fillers are reduced in the WF>CF>OH>BH series.
It is known that the rate of adsorption processes in aqueous suspensions of inorganic adsorbents, depending on the content of the disperse phase, varies by step functions [34,35,38]. Approximation of the obtained dependencies, even with limited data, revealed that on the surface of plant disperse fillers, this dependency is also described by equations of step functions (Fig. 6).
The obtained graphs show that in the aqueous suspensions of all studied plant disperse fillers, hydrolytic processes occur at the interface according to the same patterns. The rate of hydrolytic processes in aqueous suspensions decreases with increasing weight (content) of disperse fillers, which does not contradict the results of studies by other authors [35,38].
Thus, the results obtained provide more complete information about the acid-base properties of the surface of the investigated fillers, which in turn allows predicting and re gulating the interfacial interactions in disperse systems. Plant disperse waste is actively used to create composite materials, both in water-based concretes [4] and in organicbased polymers [1][2][3][4][5][6][7][12][13][14][15][16]. These wastes can also be used as adsorbents for removing contaminants from aqueous and organic liquid media [32][33][34][35][36]. In both cases, reliable information on the possible limits of using a particular disperse material is needed to create effective technologies. This is due to the need to provide the most intense acid-base interfacial interactions, which in the first case provides improved properties of composites, and in the second -effective removal of contaminants. It is found that the following ranges of the hydrogen index of dispersion media are required for the effective application of the investigated disperse waste in composites and as adsorbents: for BH − pH>4.4; OH − pH>6.4; WF − pH>3.9; CF − pH>4.5. It should be noted that the higher the hydrogen index of the dispersion medium, the more intense the interface processes are. Also, as the graphs show, these limits may vary somewhat depending on the content of fillers in suspensions and further studies are needed to determine these limits. Considering the peculiarities of the experiments conducted (in aqueous suspensions), these recommendations are more suitable to apply in aqueous dispersion media. With some assumptions, they can also be used in organic low-polar media, but in this case additional experimental studies are required for effective use.  1. Experimental studies of the chemical nature and surface properties of plant disperse fillers -buckwheat husk, oat husk, wood flour and conifer flour were carried out. IR spectroscopy studies revealed absorption bands characteristic of cellulose-containing materials. It is found that among the atomic groups, which are part of the filler components and determine the acid-base surface properties, the oxygen-containing groups −OH, −C−O− and −C = O predominate. The atomic groups may in turn be within more complex functional groups such as carboxyl, complex ether or aldehyde. It is found that on the surface of air-dry fillers there is a hydroxylhydrate surface layer of the Brønsted functional groups with a wide range of acid-base properties.
2. Using the method of potentiometric titration of aqueous suspensions by the Parks-Bobyrenko method, the acidbase characteristics of surface functional groups of fillers were investigated. It was found that all the investigated fil lers are of the «polyfunctional solid» type. Hydrolytic adsorption processes observed during titration of suspensions occur at the interface and reflect the acid-base characteristics of surface functional groups. The hydroxyl-hydrate surface layer of the fillers is shown to consist of functional groups with close values of acid dissociation constants. Functional groups of acid character were additionally found on the surface of the fillers: groups with pK a ≈4.37−5.66 on the BH surface, groups with pK a ≈4.49−4.90 on the CF surface, and groups with pK a ≈3.91−4.30 on the WF surface. In general, the potentiometric titration results show that the acidity of the filler surface decreases in the WF>CF>BH>OH series. This series completely coincides with the one in which the total cellulose and lignin content decreases and the resistance of fillers to thermal-oxidative breakdown increases. It is found that the change in the rate of hydrolytic processes in suspensions at the interface depending on the content (weight) of fillers is described by step functions. It is found that the rate of hydrolytic processes decreases in the OH>CF>BH>WF series and inversely depends on the concentration of functional groups on the surface of the fillers.
3. It is found that for the effective use of the investigated disperse waste in composite materials and as adsorbents for the removal of contaminants from liquids, dispersion media with the following ranges of hydrogen index are required: for BH − pH>4.4; OH − pH>6.4; WF − pH>3.9; CF − pH>4.5. In this case, it is advisable to use dispersion media with the largest possible values of the hydrogen index.