Design of Composite Systems Based on Hydrophilic Silica and Organic Acids: Gallic, Glycyrrhizic and Its Salts

Abstract

The process of formation of composite systems based on nanosilica A-300 and biologically active substances (BAS), namely gallic acid (GA), glycyrrhizic acid (GLA), and its salts, was studied using a set of physicochemical methods. It was shown that when BAS are immobilized on the silica surface by the method of joint grinding in a porcelain mortar, they pass into a nanosized X-ray amorphous state. Water adsorbed on the surface of such composite systems is also in a clustered state, and the radius of adsorbed water clusters is in the range of 0.4–50 nm. The chloroform environment has a complex effect on the size of water clusters. In general, there is a tendency for the radius of water clusters to increase when air is replaced by a chloroform environment. However, this does not always lead to a decrease in the interfacial energy. The possibility of the existence of metastable ice in the temperature range up to 287 K, stabilized by the surface of composite systems, was discovered. The amount of such ice can reach 20% of the total water content in the sample. The possibility of using complex viscosity measurements for hydrated silica powders and silica containing immobilized biologically active substances was shown. These measurements allow recording changes in the phase state of complex mixtures during the formation of compact composite forms under the influence of periodic mechanical loading.

1. Introduction

Gallic acid (3,4,5-trihydroxybenzoic, C₇H₆O₅) is a polyphenol of plant origin. It is found in red wine, grape leaves, oak bark, and many other plants [1,2]. It has high biological activity—antioxidant [3,4,5], anti-inflammatory [6,7,8], anti-cancer [9,10,11,12,13,14,15], and tonic [16,17,18]. There is extensive information on the activity of gallic acid (GA) in the treatment and restoration of cartilage tissue [19,20]. However, since GA is poorly soluble in cold water, its bioavailability is low, and special methods are required to increase it. Promising methods in this direction include changing the phase state of GA by immobilizing it on the surface of a mineral carrier (highly dispersed silica) in the form of nanosized clusters and using mediator substances capable of simultaneously forming complexes with GA and specific to phospholipids, which form the basis of cell membrane receptors.

Glycyrrhizic acid (20β-Carboxy-11-oxo-30-norolean-12-en-3β-yl-2-O-β-D-glucopyranuronosyl-α-D-glucopyranosideuronic acid) (gross formula C₄₂H₆₂O₁₆), which is a tribasic saponin consisting of a triterpenoid aglycone, glycyrrhizic acid (GLA), combined with a disaccharide of glucuronic acid, can become such a mediator. GLA is contained in licorice roots (Glycyrrhiza glabra). It is used as a food sweetener and in licorice preparations and as a drug with antiviral and anti-inflammatory activity [21]. Recently, GLA has been actively considered as a substance that increases the biological activity of other drugs [22,23,24]. This effect may be based on the ability of GLA to integrate into cell membranes and change their physical and functional properties. One of the possible mechanisms of the antiviral action of GA is considered to be the prevention of the fusion of the virus envelope with the plasma membrane of the host cell.

One of the promising areas of development of modern pharmacology is the creation of complex composite systems based on highly dispersed silicas and medicinal or biologically active substances immobilized on their surface [25,26,27,28,29]. In this case, the mineral component can play the role of not only a means of delivering medicinal substances to the intestinal mucosa but can also regulate the rate of their release into biological fluids. In addition, in some cases, highly dispersed silicas are used as effective enterosorbents to cleanse the body of toxic substances and metabolic products.

The aim of this work was to create composite systems based on highly dispersed silica with GA, GLA, and GLA salts in which the organic phase is in a clustered state in order to study the structure of the bound water layer in composites and their differences from bound water in silica.

2. Materials and Methods

2.1. Materials

Highly dispersed silica A-300, produced by the Kalush Experimental Plant of the O.O. Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, was used. Its specific surface area (by nitrogen at 76 K) was 295 m²/g, and the total pore volume was about 1 cm³/g. It should be noted that the structure of nanosilica particles (unlike silica gels) is not rigid and strongly depends on preliminary preparation. In particular, if its initial bulk density after production is 50 mg/mL, various methods of mechanical or hydraulic compaction can increase it to 200 mg/cm³ [30]. This occurs mainly due to changes in the structure of aggregates and does not affect the structure of primary silica particles [31,32].

GA (I), GLA (II) (Figure 1) and its salts, namely monoammonium (MonAm) and monopotassium (MonK), were of chemical grade (ch) and produced by Ningxian in Gansu, China.

To prepare the samples, 10 g of silica A-300 with a bulk density of C_d = 50 mg/mL, containing 50 mg/g of residual water (absorbed from the air), were ground in a porcelain mortar and compacted to C_d = 200 mg/mL. This material was used as the starting material for the preparation of composite systems with GA, GLA, and its salts. For this, the weighed portions of acids (or salts) were mixed with A-300 in a proportion of 1/10 and additionally ground for 10 min until a homogeneous composite system was formed. These samples contained 50 mg/g of water. To increase the amount of water, 100 mg/g of distilled water was added to the samples, and they were additionally ground for 1–2 min, after which they were kept in a closed ampoule for 1–2 h.

2.2. X-Ray Diffraction (XRD)

X-ray diffraction patterns were recorded over 2θ = 5–70° range using a DRON-3M (Burevestnik, St. Petersburg, Russia) diffractometer with Cu K_α (λ = 0.15418 nm) radiation and a Ni filter. The XRD data could be used for simple estimation of average sizes of crystallites using Scherrer or Debye–Scherrer equations [33,34].

2.3. Infrared Spectroscopy

Infrared spectra of the studied samples were recorded using a Fourier transform infrared (FTIR) IRTracer-100 (Shimadzu, Kyoto, Japan) spectrophotometer with a diffuse reflectance (DR) attachment DRS-8000A (Shimadzu, Kyoto, Japan). To record the IR spectra, samples with A-300 or AM-1 alone or with KBr (1:300) were pressed into thin pellets (~20 mg).

2.4. Thermal Analysis

Thermograms (thermogravimetry (TG) at the average weight errors ± 0.1 mg, differential TG, DTG, and differential thermal analysis (DTA) data) were recorded using a Derivatograph Q-1500 D apparatus (Paulik, Paulik & Erdey, MOM, Budapest, Hungary) upon heating of samples (~0.2 g) in air at a heating rate of 10 °C/min from 20 °C to 1000 °C. The TG curves show that the amounts of water in samples under measurement are smaller than the initial ones due to water evaporation upon sample preparation.

2.5. NMR Spectroscopy and Cryoporometry

Low-temperature ¹H NMR spectra of static samples were recorded using a high-resolution NMR spectrometer (Varian 400 “Mercury”, Agilent Technologies, Santa Clara, CA, USA) with an operating frequency of 400 MHz. Eight 60° probe pulses were used with a duration of 1 μs and a bandwidth of 20 kHz. The temperature was using a Bruker VT-000 thermal attachment (Billerica, MA, USA) with an accuracy of ± 1 degree. Signal intensities were determined by measuring peak areas using a signal decomposition procedure assuming a Gaussian waveform and optimizing the zero line and phase with an accuracy of at least 5% for well-resolved signals and ± 10% for overlapping signals. To prevent overcooling of water in the objects under study, measurements of the concentration of unfrozen water were carried out by heating of samples pre-cooled to 210–215 K. Temperature dependences of the intensity of ¹H NMR signals were carried out in an automated cycle, where the sample was kept at a constant temperature for 5 min, and the measurement time was 1 min.

The process of freezing (thawing) of water bound to solids occurs with changes in the Gibbs free energy caused by the influence of a solid surface, confined space effects, solutes, colligative properties of solutions (cryoscopic effects), and co-solvents. Frozen water does not contribute the recorded ¹H NMR signals due to a narrow bandwidth of 20 kHz and a large difference in the transverse relaxation time of mobile (liquid water and solutions) and immobile (frozen water and solids) phases [30]. Water (or other liquids) can be frozen in narrower pores (or voids between nanoparticles) at lower temperatures, as described by the Gibbs–Thomson relationship for the freezing point depression for liquids confined in cylindrical pores at radius R [35,36,37,38]. This relationship is the base of cryoporometry, giving information of the size distributions of pores (voids) infilled by unfrozen water.

The area under ΔG(C_uw) curve determines the modulus of the total changes in the Gibbs free energy ΔG vs. the amount of unfrozen water (C_uw) that are caused by interactions of water with silica and NaCl crystallite surfaces [35]:

$${\gamma }_S=-A{\displaystyle \underset{0}{\overset{C_{uw}^{\max }}{\int }}\Delta G(C_{uw})d{C}_{uw}}$$

(1)

where $C_{uw}^{max}$ is the total amount of water unfrozen at T = 273 K, and A (>0) is a constant dependent on the type of units used in this equation.

Water can be frozen in narrower pores (or voids between nanoparticles) at lower temperatures as described by the Gibbs–Thomson relationship for the freezing point depression for liquids confined in cylindrical pores at radius R [35,36,37,38]:

$$\Delta T_m=T_{m,\infty }-T_m(R)=-{\displaystyle \frac{2{\sigma }_{sl}{T}_{m,\infty }}{\Delta H_f\rho R}}={\displaystyle \frac{k_{GT}}{R}}$$

(2)

where T_m(R) is the melting temperature of ice in cylindrical pores of radius R, T_m,∞ the bulk melting temperature, ΔH_f the bulk enthalpy of fusion, ρ the density of the solid, σ_sl the energy of solid–liquid–air interaction, and k_GT is the Gibbs–Thomson constant (here, k_GT = 50 K nm for water).

2.6. Rheometry

Rheometric measurements (in air at 20 °C) were carried out using an MCR 92 (Anton Paar, Graz, Austria) rheometer in a rotational mode with a rotated disk of 25.5 mm in diameter and RheoCompass™ software version 1.19.335. A starting value of shear strain was 0.05% increased to 60% (which corresponds to shear rate $\dot{\gamma }$ = 0.0498 − 100.0 s⁻¹) and then reduced to 0.05%.

3. Results

Figure 2 shows thermograms of the studied composite systems prepared on the basis of gallic acid (a), glycyrrhizic acid (b), and its monoammonium (c) and monopotassium (d) salts. Several inflections on the weight loss curves are observed on the thermograms, which manifest themselves as maxima on the DTG curves. Peaks in the region of T = 50–150 °C are related to physically adsorbed water. This water can be associated with both the surface of silica and the organic phase of the composite systems. Peaks with maxima in the region of T = 200 °C can be attributed to the destruction of acid crystal hydrates. At a higher temperature, the destruction of organic acids occurs, which occurs in several stages—resinification and carbonization. Accordingly, additional peaks appear on the DTG curves.

Electron micrographs of the initial gallic acid (GA), glycyrrhizic acid (GLA) powders, and composite systems prepared by mechanical treatment of acid mixtures (or salts) with A-300 nanosilica powder are shown in Figure 3. The initial acids have a pronounced microcrystalline structure, with crystals of 5–30 μm in size. After mechanical treatment, a composite structure is formed in which only silica aggregates are visually observed. It can be concluded that for all samples, mechanical treatment results in crushing of acid crystals (or salts) and uniform distribution of the organic phase in the interparticle gaps of silica.

The phase state of acids (salts) immobilized on the surface of nanosilica, initially and after immobilization on the silica surface, can be determined using X-ray phase analysis data (Figure 3). Figure 4a,b show X-ray diffraction patterns of the initial acids (salts). All samples, with the exception of MonAmGLA, have a sparse crystalline structure. After immobilization on the surface of silica A-300 (Figure 4c,d), crystallinity is retained only for the A-300/GA sample. In the case of GA, against the background of an intense signal of the amorphous phase (which applies to both silica and GA), low-intensity peaks of GA nanocrystals are observed. Based on the signal width (Figure 4d), it is possible to estimate the size of crystallites, which, according to the Debye–Scherrer formula, is about 40 nm.

The state of water adsorbed on the surface at the same (h = 100 mg/g) hydration of the samples can be determined using IR spectroscopy data (Figure 5). Water is observed as several merged signals ν⁻¹ = 2750–3800 cm⁻¹. In all composite systems, there is also a low-intensity signal of silanol groups, which participates in the formation of hydrogen bonds, at ν⁻¹ = 3750 cm⁻¹. Since the interaction of water with the surface of composite materials can be carried out both by forming water clusters on the primary adsorption centers of silica (silanol groups) and on the hydrophilic (acidic) centers of organic compounds immobilized on its surface, one could expect significant differences in the shape of the water signal for composites (Figure 5) and the original silica [35]. However, a comparison of the shape of the signals of water adsorbed on the surface of composite systems that differ in the type of organic phase fixed on their surface (Figure 5) and the spectra given in the literature for hydrated silicas [25,31] show the similarity of the IR spectra of water. Then, it can be assumed that under conditions of relatively weak hydration of the surface, the formation of hydrogen-bonded water clusters occurs predominantly on the areas of the silica surface free of organic acids (or their salts). The frequency bands of the stretching vibrations of CH bonds of composite systems of glycyrrhizic acid and its salts have maxima at 2974–2874 cm⁻¹: the CH₃ and CH₂ groups signal in the regions at 2974, 2948, and 2874 cm⁻¹. There are bands related to the stretching vibrations of C=O bonds in the composition of carbonyl groups (1723 cm⁻¹). The stretching vibrations of carboxylate groups (COO-) are observed with an average intensity in the region of 1615 cm⁻¹ and the deformation vibrations of CH₃ and CH₂ at 1453 and 1431 cm⁻¹. In the region of 973 cm⁻¹, deformation vibrations of the =CH group are present. In addition, in the spectra of MonAmGLA/SiO₂, in contrast to GLA/SiO₂, deformation vibrations are present at (NH₄) 1387 cm⁻¹. Also, in the IR spectra of composite systems based on glycyrrhizic acid and its salts in the region of 3500–3400 cm⁻¹, stretching vibrations of hydroxyl groups (OH⁻) are observed in the form of a wide band. This may indicate that hydroxyl groups of the carbohydrate part of the molecule participate in the formation of intermolecular bonds. The stretching vibrations of the CO bonds in the C–O–C and C–OH groups of the carbohydrate part of glycyrrhizic acid and its salts should appear in the region of 1200–1000 cm⁻¹, but this region is overlapped by signals from silica.

The ¹H NMR spectra of water in the studied samples of hydrated silica A-300 and composite systems created on its basis, taken at different temperatures, are shown in Figure 6. The measurements were carried out in air and in a weakly polar organic solvent—chloroform. In order to prevent the appearance of an intense signal of liquid-phase chloroform protons in the spectra, its deuterated form (CDCl₃) was used, in which the content of non-deuterated chloroform was less than 0.5%. The signal of residual amounts of CHCl₃ is observed in some samples containing chloroform as a weakly intense signal with a chemical shift of δ_H = 7.2–7.5 ppm (Figure 6).

The main signal of water (Figure 6) is observed in the spectra in the temperature range of 215–287 K at δ_H = 4–6 ppm. The chemical shift of water corresponds to strongly associated water (SAW), each molecule of which takes part in the formation of 2.5–3 hydrogen bonds with neighboring molecules [35]. In addition to this, in a chloroform medium (and in some samples in air), the spectra contain a relatively weak signal of weakly associated water (WAW, δ_H = 1–1.5 ppm), corresponding to water molecules either not participating in the formation of hydrogen bonds or included in clusters in which there are few such bonds (on average, less than one hydrogen bond per water molecule). The fact that the SAW and WAW signals are observed separately indicates that these clusters exchange water molecules (or protons) slowly (on the NMR time scale) [39]. They are probably spaced apart or separated from each other by a layer of chloroform.

A significant decrease in the freezing temperature of interfacial water is due to adsorption interactions, which, in nanostructured systems, are caused by the effects of water clustering and changes in the structure of the hydrogen bond network [35,40]. With decreasing temperature, the intensity of the water signal decreases as smaller and smaller water clusters freeze. It should be noted that Figure 6 records the reverse process—an increase in temperature from the minimum value (T = 215 K) up to T = 287 K; i.e., the process of melting of water clusters from the smallest (at low temperature) to large (at high temperature) is observed. The large width of the SAW signal can be associated with the presence of several types of water clusters with slightly different chemical shift values, which overlap. With decreasing temperature, the chemical shift of the SAW signal increases due to the increase in the ordering of the hydrogen bond network in the SAW clusters.

If we assume that at T = 287 K, all the water is in a liquid state and is equal to the total hydration of the sample (h = 50 or 150 mg/g), then the amount of non-freezing water (C_uw = h·I/I₂₈₇) can be easily calculated from the intensity of the water signal at an arbitrary temperature (I). For the studied systems, the corresponding dependencies are shown in Figure 6a,b. Since there is a directly proportional dependence [41] between the free energy of ice and temperature in a wide temperature range (T < 273 K), and under equilibrium conditions, the free energy of ice is equal to the free energy of non-freezing water, the dependences C_uw(T) can be transformed into dependences of the change in the Gibbs free energy on the concentration of non-freezing water (Figure 7c,d). From the data in Figure 6, it follows that during the heating of the samples, not all interfacial water passes from the solid to the liquid state. Some of it remains solid up to T = 287 K. That is, the surface of composite systems is capable of stabilizing the metastable state of ice (MsIce) in nanosized pores in a wide temperature range. The amount of such metastable ice is quite large and can reach 10–15% of the total water content in the samples.

The part of water that freezes at T > 265 K (ΔG < 0.5 kJ/mol) is weakly bound (WBW), and the rest is strongly bound (SBW) [30,31,32,33]. The amounts of strongly bound (C_uw^S) and weakly bound (C_uw^W) water and metastable ice (C_ms^Ice) for all the studied systems are given in

Table 1. Thermodynamic characteristics of interfacial water in hydrated silica A-300 and composite systems based on it in air and CDCl₃.

System	h (mg/g)	CuwS (mg/g)	CuwW (mg/g)	CmsIce (mg/g)	ΔGS (kJ/mol)	γS (J/g)	γSIce (J/g)
A-300, Air	150	125	15	10	−2.5	7.8	0.15
A-300, CDCl3	150	140	10	0	−2.8	9.8	-
A-300/GA, Air	150	110	15	25	−3	7.1	2.1
A-300/GA, CDCl3	150	145	5	0	−2.5	8.2	-
A-300/GLA, Air	150	75	20	55	−2.8	7.2	1.1
A-300/GLA, CDCl3	150	95	15	40	−2.6	7.9	0.68
A-300/MonAmGLA, Air	50	28	5	17	−3.5	3.2	0.29
A-300/MonAmGLA, CDCl3	50	30	11	9	−3	3.3	0.14
A-300/MonAmGLA, Air	150	100	15	35	−3	9.2	0.49
A-300/MonAmGLA, CDCl3	150	105	15	30	−2.8	8.1	0.44
A-300/MonKGLA, Air	50	22.5	10	17.5	−3	2.7	0.27
A-300/MonKGLA, CDCl3	50	22.5	7.5	10	−2.8	2.2	0.21
A-300/MonKGLA, Air	150	100	30	20	−2.5	6.4	0.23
A-300/MonKGLA, CDCl3	150	75	35	40	−2.1	4.6	0.6

. In addition, Table 1 presents the values of the maximum decrease in the Gibbs free energy in the layer of strongly bound water (ΔG^S) and the interfacial energy (γ_S), which is determined in accordance with Equation (1) and is equal to the total decrease in the free energy of interfacial water caused by adsorption interactions. For clarity, the γ_S values of the studied systems are presented in the form of diagrams in Figure 7.

Among the general regularities in the dependence of the interfacial energy on the solid component and the observation environment, it should be noted that in an air environment, immobilization of GA or GLA on the silica surface has little effect on the γ_S value, although the amount of strongly bound water decreases noticeably. Probably, the effect of decreasing C_uw^S is compensated by some increase in the amount of WBW and MsIce (Table 1). Unlike many systems described earlier [35], a weakly polar organic environment leads to an increase in the interfacial energy (Figure 8). This tendency is not observed for GLA salts immobilized on the surface of A-300 silica. For the monopotassium salt of GLA, a significant decrease in the γ_S value is recorded compared to the original silica both in an air environment and in a CDCl₃ environment.

The formation patterns of metastable ice should be particularly discussed. In the initial silica, superheated ice is practically not formed (Figure 6a,c; Table 1). Immobilization of organic acids, regardless of their chemical nature, promotes the stabilization of metastable ice. The chloroform environment in all systems reduces its amount.

The described regularities can be largely determined by the influence of the heterogeneous system surface on the structure of adsorbed water clusters, calculated in accordance with Equation (2) (Figure 9). In the initial silica (h = 150 mg/g), two maxima are observed in the distributions at R = 1.5 and 2.5 nm. In a chloroform medium, the average radius of water clusters increases slightly due to the fact that the maximum at R = 2.5 disappears, and one maximum is formed instead at R = 2 nm. Immobilization of GA or GLA on the silica surface leads to the formation of a large number of small water clusters on the surface, but at the same time, the shape of the distribution curves changes, and some of the interfacial water is in the form of metastable ice. A chloroform medium stabilizes the formation of relatively large water clusters, but this does not lead to a decrease in the interfacial energy. In composite systems based on GLA salts, the type of distributions becomes more complicated. A significant number of both small and large clusters of adsorbed water are present.

Rheological measurements are usually used for viscous continuous media consisting of concentrated solutions of low-molecular-weight substances, polymer solutions, suspensions of solid particles in aqueous or organic liquids, and emulsions formed by liquids that are insoluble in each other [42,43,44,45]. In rheological studies, several terms are used to describe the viscoelastic properties of the object under study. These are viscosity or internal friction, which characterizes the ability of liquid substances to resist the movement of one part relative to another; dynamic viscosity, which is equal to the resistance force when moving one layer of liquid relative to another; and complex viscosity, which contains information about the method of dissipation of energy obtained when shifting some layers of matter relative to others. In this case, the viscoelastic properties are primarily influenced by the interphase interactions of the substances that make up the heterogeneous system. Based on the dependences of viscosity on shear load, the ratio of the concentrations of the solid and liquid phases, temperature, and other parameters, it is possible to draw reasonable conclusions regarding the phase state of the system. The general principles used in the mechanics of liquid fluids can also be transferred to studies of the properties of hydrated powders or solid suspensions. In this case, the rheometer should be classified as a destructive method, which can lead to a partial or complete irreversible change in the sample during the measurement process.

For hydrated powders of the original silica and silica with GA, GLA, and its salts immobilized on its surface, measurements were made of the complex viscosity as a function of the magnitude of shear deformation (Figure 9a,b) and the viscosity as a function of the shear rate (Figure 10c,d).

For different types of dispersed systems, the type of dependences η(γ) differs greatly. This is due to the complex nature of the dependence of the complex viscosity on the composition of the colloidal system and the molecular interactions occurring in it during the mechanical action of the rheometer rotor on the hydrated silica powder with substances immobilized on its surface. Unlike aqueous suspensions, in hydrated powders in the interparticle gaps, there can be not only organic substances and water adsorbed to the surface but also air, the volume of which can be comparable to the volume of water. Under the influence of the mechanical load created by the rheometer, there can be a reorganization of silica aggregates, the movement of organic substances along its surface, and partial removal of air from the interparticle gaps (which is accompanied by an increase in the bulk density of the material). Accordingly, significant changes in the dynamic viscosity of the samples under study will occur.

The initial silica after its preliminary compaction has a bulk density of C_d = 200 mg/cm³. After it absorbs 1 g/g of water, the bulk density increases approximately twofold. However, a significant part of the interparticle gaps continues to be filled with air. At the initial section of the η(γ) curve for the initial silica (Figure 10a), some decrease in the complex viscosity with increasing shear strain is observed. Then, at γ = 7%, a sharp jump in the η value occurs, indicating a fundamental change in the state of the sample. The probable cause is the partial (or complete) removal of air from the interparticle gaps of the silica, which is accompanied by the formation of a continuous (or almost continuous) water film in them, which ensures easy displacement of the silica particles relative to each other under load. In fact, a phase transition of the solid–water–air system into the solid–water system takes place. This system remains stable even in the case of a decrease in the dynamic load.

The dependence η(γ) for silica with glycerol acid immobilized on its surface has a similar appearance (Figure 10a). In the case of gallic acid, the process of air removal from the interparticle gaps, and the formation of a continuous water film in them does not have a stepwise nature but is extended over a wide range of shear load changes. As expected, an increase in the amount of adsorbed water reduces the amount of air in the interparticle gaps, and the dependences η(γ) become smoother (Figure 10a). It should be noted that an increase in the amount of water in the sample can lead to both a decrease in the dynamic viscosity in the case of GA (relative to the initial one) and to its significant increase in the case of GLA (Figure 10a). A probable cause of the increase may be the formation of a viscous GLA hydrate in the interparticle gaps of silica, which sharply increases the magnitude of interparticle interactions.

For the GLA salts immobilized on the silica surface, the presence of a large amount of air in the interparticle gaps, which is removed during the growth of the mechanical load, is observed for the A-300/MonAmGLA sample containing 1 g/g of adsorbed water (Figure 10b), and for A-300/MonKGLA, the process of air removal is extended over a wide range of shear load. For the samples containing 2 g/g of adsorbed water, a strong increase in viscosity is observed, caused by the “gluing” of silica particles by hydrated acid salts.

For comparison, Figure 10c,d shows the viscosity measurements from the shear rate for all the studied systems. In contrast to the data on the complex viscosity measurements, these measurements turned out to be significantly less informative since they do not explicitly contain information regarding the ratio of the shares of mechanical energy spent on plastic deformation and the reorganization of the internal structure of the particles of the sample.

4. Conclusions

When immobilized by the method of joint grinding in a porcelain mortar on the surface of pyrogenic silica A-300 organic acids, which differ greatly in chemical nature (GA, GLA, and its salts), they pass into a nanosized X-ray amorphous state.

Water adsorbed on the surface of such composite systems is also in a clustered state, and the radius of the adsorbed water clusters is in the range of 0.4–50 nm. The chloroform environment has a complex effect on the size of the water clusters. In general, there is a tendency for the radius of water clusters to increase when air is replaced by a chloroform environment. However, this does not always lead to a decrease in the interfacial energy.

The possibility of the existence of metastable ice in the temperature range up to 287 K, stabilized by the surface of composite systems, was discovered. The amount of such ice can reach 20% of the total water content in the sample.

The advantages of using the measurement of the complex viscosity of hydrated silica powders containing immobilized GA, GLA, or their salts for studying the processes of interparticle and interphase interactions in complex systems containing solid, liquid, and gaseous phases as well as their changes under the influence of periodic mechanical loading are shown. In this case, for samples containing 1 g/g of adsorbed water, under the influence of periodic loading, part of the air is removed from the interparticle gaps, which manifests itself in the form of spontaneous or gradual rearrangement of composite aggregates (the complex viscosity can irreversibly decrease by 2–3 orders of magnitude). With an increase in the amount of water to 2 g/g, the systems become more reversible, and a sharp change in the complex viscosity with an increase in shear strain no longer occurs.