Mechanocomposites
organic substance – layered silicate
N.Z. Lyakhov, T.F. Grigorieva, I.A. Vorsina, A.P. Barinova
Institute of Solid State Chemistry and Mechanochemistry of SB RAS, Kutateladze str., 18, Novosibirsk 630128, Russia.
E-mail:
The mechanochemical approach is very efficient for the surface chemical reactions, e.g for grafting organic compounds to the surface of other substances, either organic or inorganic ones.
Mechanical activation of organic compounds causes rupture of intermolecular and intramolecular bonds; radicals are formed at break points, and the substance becomes able to interact with other compounds forming composite structures with chemical bonding between components.
Mechanochemical preparation of composites
It is known that the molecules of organic compounds containing OH-, NH- or SH groups are bound to each other through hydrogen bonds. Hydrogen bonds are broken during mechanical activation of individual compounds but they recover very soon after the load is removed. When one carries out activation of mixtures with other compounds in which mechanical activation liberates functional groups able to react with the functional groups of organics, chemical reaction takes place on the contact surface.
Mechanochemical formation of the composites of organic acids with layered silicates was investigated.
It is known that organic acids are dimeric and form cyclic pairs with very strong hydrogen bonds between carbonyl and hydroxide groups of both molecules. The cyclic pairs of monocarboxylic acids contact each other be means of electrostatic interaction between oxygen atoms of hydroxide groups forming planar nets with motif (1) (Scheme 1). Packets of these nets are hold together due to interplanar bonds of two types, (2 a) and (2 b).
In the plane, aromatic acids have a packing motif in which carboxylic groups of the dimers are located close to the phenyl ring with the С-НО distance equal to 3,5 Å; so, the molecules are bound not only in cycles through hydrogen bonds of the С-О-НО type but also form chains due to the side С-НО bonds; because of this, acids have layered crystal structure. These bonds result in close packing. Interlayer contacts of the types (З a) and (3 b) are typical for these acids (Scheme 1).
Scheme 1.
Mechanical activation of individual higher carboxylic acids (including aromatic ones) does not result in any changes in the crystal and molecular structure [1, 2]. Such a behaviour was also observed for dicarboxylic, unsaturated and hydroxo acids [3, 4]. The situation for low-molecular amino acids is somewhat different; it was considered for -glycine as an example [5]. Its molecules exist in the form of a zwitter ion; they are bound to each other with two relatively short and therefore rather strong N-НО hydrogen bonds to form netlike layers paralleled to the ac plane. The changes in IR spectra of -glycine after activation are attributable to the presence of -glycine in -glycine. However also possible is disordering of hydrogen bonds between the double layers [6].
These acids exhibit quite a different behaviour in the presence of natural silicates. For instance, mechanical activation of stearic acid with kaolinite (Fig. 1) causes rather fast decrease in the intensity of bands related to the carboxyl group: ОН 27502450 cm–1, 33003000 cm-1; С=О 1690 cm-1, 1710 cm-1 (shoulder), and the bands related to the vibrations of hydroxo group: 1430 cm-1, 1300 cm-1 – deformation vibrations (ОН + СО). At the same time, a broad diffuse band related to ОН of water molecules appears within the range 35503300 cm-1 with the deformation vibration in the form of a weakly exhibited shoulder 1630 cm-1 of the 1590 cm-1 band. The bands at 1590 and 1460 cm-1 appear (the latter one appears as a shoulder of the 1475 cm-1 band); taking into account a decrease in the intensity of OH group vibraiton bands, these bands can be assigned to antisymmetric and symmetric stretching vibrations of carboxylate ion, respectively. Similar changes occur in the IR spectra of the mixtures of kaolinite with other solid-phase organic acids. It should also be noted that after a definite time of
Fig. 1. a- IR spectra of mixture kaolinite with stearic acid, b – these spectra in region of absorption of carboxylate ion. 1 – initial; 2 – 3 min MA; 3 – 10 min MA.
activation of the mixture of kaolinite with the acid (up to 20 wt.% of the organic acid) the bands related to the vibrations of carboxyl group disappear almost completely; clear bands ОН, ОН of water molecules (35503300, 1630 cm-1) and carboxylate ion as, s 1590, 1460 cm–1 appear. Since mechanical activation of kaolinite causes the formation of surface centres of basic character on the newly liberated surfaces, and mechanical activation of organic acids may result in their depolymerization, then, the joint mechanical treatment of these compounds results in neutralization of the surface hydroxo groups of kaolinite by the protons of acids, and the ionized acid anion gets grafted to the kaolinite surface. Mechanocomposite kaolinite – organic acid anion is formed. A similar behaviour of the solid-phase organic acids was observed during their joint activation with talc, pyrophyllite and other layered silicates [1-4, 6-9].
An evidence of the possible mechanochemical grafting of the anion to the silicate surface can be the fact that after the joint activation the crystal structure of a silicate is conserved (Fig. 1), while in the IR spectra there are bands not of organic acid, but of carboxylate ion. The diffraction patterns of a mixture of succinic acid with talc and the samples after activation for different time are shown in Fig. 2.
After treatment for 1 min, only the reflections of talc are observed. The IR spectra of the same products reveal the presence of absorption related to carboxylate ions (Fig. 3).
High-resolution electron microscopy provides evidence of similar morphology of talc and the products of its mechanochemical inter-action with succinic acid (Fig. 4) [3].
Mechanocomposites of this kind are formed also between layered silicates and higher alcohols. Alcohols, similarly to organic acids, form polymeric associations due to hydrogen bonds which are destroyed during grinding and restored after the load is eliminated. This is confirmed by the IR spectra and X-ray diffraction patterns that remain almost unchanged. During mechanical activation of a mixture of kaolinite with cetyl alcohol, the intensity of X-ray reflections of the latter decreases sharply, while the IR spectra exhibit the formation of a new chemical bond of the type of the alcoholate one 4, that is, not only carboxylic but also hydroxyl groups of the organic compound can interact with active surface centres formed on kaolinite. It was noticed for the joint activation of kaolinite with hydroxy acids, for example with tartaric acid, which contains both the carboxylic and hydroxyl functional groups, that chemical interaction of kaolinite with the acid is realized only through the carboxylic group 4.
A similar mechanochemical interaction of layered silicates as an example of talc was also observed with the salts of organic acids [10] including those with complicated cations: substituted ammonium – tert-butylamine succinate and polycation – natural mucopolysaccharide chitosan (chitosan succinate).
The X-ray diffraction patterns of the mixtures of talc with sodium benzoate, succinate, oxalate and stearate, unlike for individual compounds, even after activation for a short time (а = 10 s) all the reflections of salts are absent; the intensity of talc reflections decreases.
1. Sodium succinate + talc
A comparison of the IR spectra of the initial and activated (а > 3 min) mixtures of sodium succinate with talc (Fig. 5a) reveals that: 1) in the region of antisymmetric stretching vibrations of carboxylate ions (asСОО-) a band with the maximum at 1590 cm–1 appears instead of the band with the maximum at 1560 cm-1 (for sodium oxalate); 2) the shape and position of the band of deformation vibrations of СН2 groups (СН2) 13701500 cm-1 change under the action of changes in the adjacent СО groups. The position and shape of these bands are fully identical to those in the IR spectra of the activated mixture (а > 3 min) of succinic acid with talc (Fig. 5b). At the same time, we observe: 1) a decrease in the intensity of the bands of stretching vibrations of ОН groups of talc (ОН, 3670 cm-1) till their complete disappearance for а > 3 min; 2) broadening of the bands related to as Si-O- and Si-O-Si bonds (1200850 cm-1) and a decrease in the intensity of s Si-O-Si (670 cm-1), for а > 3 min the band disappears. The shape of the band of deformation vibrations of the silicon-oxygen layer of talc (the region below 500 cm-1) changes substantially, which is an evidence of the deformation of talc lattice, because these vibrations are modified by the lattice vibrations.
The data obtained, including the similarity of the IR spectra of activated mixtures of sodium succinate with talc and succinic acid with talc within the region of asСОО- and СН2 vibrations allow assuming that the mechanochemical activation of sodium succinate with talc involves substitution of hydroxyl groups of talc with the succinate anion with the formation of a product similar to that formed in the interaction of succinic acid with talc. In addition, the character of changes in the parameters of IR bands related to talc in a mixture of the salt with talc activated for 3 min allows us assuming that a chemical bond is formed between the surface active centres of acidic type in talc and Na+ cations, similar to the interaction between natural silicates and a basic oxide [6]. Water is formed as a result of interaction. We discovered similar mechanochemical substitution reactions resulting in the formation of chemical bonds between talc and not only acid carboxylate ion but also Na+ cation when investigating the joint mechanochemical activation of talc with sodium benzoate, oxalate and stearate. The IR spectra of the mixtures of talc with sodium benzoate, the initial one and that activated for 7 min, are shown in Fig. 6; for comparison, the IR spectra of the initial and activated (а = 5 min) mixtures of talc with benzoic acid are shown in the same Figure. One can clearly see in Fig. 6 that the spectra of activated mixtures are identical within the regions of as and s (15701500 and 15001450 cm-1) and within the region of benzene ring vibrations 17001570 cm-1.
2.Chitosan succinate + talc
It follows from the analysis of the IR spectra of the initial and activated mixtures of chitosan succinate with talc (Fig. 7) and their comparison with the IR spectra of activated mixtures of succinic acid and sodium succinate with talc (Fig. 5) that mechanochemical interaction of chitosan succinate with talc occurs during mechanochemical activation. The IR spectra of activated mixtures (а 30 s) exhibit: 1) gradual shift of СН2 bands 1400 1450 (for а = 3 min) and asСОО- band (carboxylate ion of succinic acid) 1560 1580 cm-1; 2) a decrease in the intensity of all the bands related to talc, especially the bands of ОН groups (37003650 cm-1); 3) changes in the shape of the band of deformation vibrations of talc lattice within the range 550400 cm-1 for а 1 min. The IR spectrum of a mixture of chitosan succinate with talc, activated for 3 min, within the spectra regions 37503000, 18001400 cm-1, and talc vibration bands below 1200 cm-1 is almost identical with the IR spectrum of a mixture of succinic acid with talc (а = 1 min) and sodium succinate with talc (а = 3 min) (Fig. 5). The X-ray diffraction patterns of activated mixtures of succinic acid with talc and chitosan succinate with talc are almost identical, too, they differ only by an amorphous halo with a maximum about 6, which is likely to be due to amorphized chitosan. These data allow assuming that the joint mechanical activation of chitosan succinate with talc involves substitution of hydroxyl groups of talc with the carboxylate ions of succinic acid. According to the data of high-resolution electron microscopy, the product of this interaction is composed of agglomerates of plate-like particles, that is, the product conserves the structure of initial talc, similarly to the case of the product of interaction of succinic acid with talc.
Chitosan bands in the IR spectra (Fig. 7a) of the activated mixtures of chitosan succinate with talc (а 3 min) are not observed, since, according to the data of XPA, chitosan succinate, being initially poorly crystallized, amorphizes very rapidly during activation (а 30 s).
3.Tert-butylamine succinate + talc
The possibility for substitution reaction to proceed during mechanochemical activation of the salts of organic acids with talc is confirmed by the data of analysis of the IR spectra of activated mixtures of tert-butylamine (TBA) succinate with talc. It follows from Fig. 7b that after the activation of the mixture for 3 min, instead of the bands of initial TBA succinate in the region 18001300 cm-1 there are two broad bands with the maxima at 1580 and 1450 cm–1, similarly to the spectra of activated mixtures of talc with sodium, chitosan succinates and with succinic acid (compare Fig. 7 and Fig. 5).
Capacity of silicates in the reaction with organic acids
Thus obtained nanocomposites can be used as nutrition additives and active components in curative cosmetics. In this case, the question concerning the capacity of silicates becomes urgent; silicate capacity is here understood as the silicate : acid molar ratio (m.r.) with which the mechanochemical neutralization proceeds till completeness. This ratio defines maximal possible concentration of the active substance in the composite. It is also important to study the return of grafted biologically active substances.
Mechanochemical interaction of organic acids with talc
It follows from the analysis of spectra (Fig. 8) that the capacity of talc in mechanochemical reactions with monocarboxylic acids with the talc to the acid m.r. 1 : 1 is defined by the nature of acids. This means that only 50 % or less of hydroxyl groups of talc can participate in mechanochemical interaction with the protons of carboxylic groups of saturated and aromatic acids. For unsaturated crotonic acid, the capacity of talc is maximal, m.r. talc : acid = 1 : 2, that is, almost all the hydroxyl groups of talc participate in neutralization with the protons of carboxylic groups. Indeed, bands related to the vibrations of carboxylic groups of acids are absent from the IR spectra: ОН - 30002450 cm-1; С=О - 18001650 cm –1, but there are the bands related to carboxylate ions: as - 16501550 cm-1; s - 14001300 cm-11-3, 6, 10. At the same time, the intensity of ОН band of talc at 3670 cm-1 decreases. The parameters of characteristic bands of talc do not exhibit substantial changes. Clear changes in the shape of the band of deformation vibrations of the layer at 550440cm-1 provide evidence that definite deformation of lattice occurs in the reaction product. The x-ray diffraction patterns of these mixtures do not contain reflections corresponding to acids; reflections of talc are broadened and shifted but no more than by 5 % with respect to the position in the X-ray diffraction patterns of the initial talc. According to the data of electron microscopy, the mechanocomposite formed as the reaction product is composed of plate-like nanometer-sized particles in which Ме ions of the silicate are chemically bound with carboxylate ions of the organic acid 1-3, 6.
The IR spectra of activated mixtures of talc with monocarboxylic saturated and aromatic acids at the m.r. > 1 : 1 and 1 : 2, along with vibration bands of carboxylate ion , as, s COO-, contain the bands characteristic of carboxylic group: С=О and ОН, as well as ОН of talc (Fig. 8, b, d). The X-ray diffraction patterns of these samples contain reflections belonging not only to talc and to mechanocomposite but also rather intensive reflections of unreacted acid. These facts provide evidence that mechanochemical reaction in these mixtures proceeds not till completion.
Fig. 8. IR spectra of mixtures of talc with acids: stearic (a), lauric (b), crotonic (c), salicylic (d). M.r. talc : acid 1 : 0,5 (a), 1 : 2 (b), 1 : 2 (c), 1 : 1 and 1 : 2 (d). Time MA, min: 1 – 0, 2 – 5, 3 – 7, 4 – 9.
Dicarboxylic acids, independently of their nature (saturated, unsaturated, hydroxy), interact with talc for m.r. = 1 : 1, that is, for the molar ratio of such a kind with which the number of hydroxyl groups of
Fig. 9. IR spectra of mixtures of talc with acids: succinic (a), suberic (b), fumaric (c), citric (d). M.r. talc : acid 1 : 1; for citric acid 3 : 2. Time MA, min: 1 – 0, 2 – 3, 3 – 5, 4 – 13.
Mg3(Si4O10)(OH)2 silicate is equal to the number of carboxyl groups of R(COOH)2. For tricarboxylic citric acid (OH)C(CH)2(COOH)3 such an interaction with talc occurs for m.r. = 3 : 2. Indeed, the IR spectra of activated mixtures (Fig. 9) contain no bands characterizing the vibrations of carboxyl group of the acid ОН - 32502450 cm-1, С=О – 18001650 cm-1; the band of stretching vibrations of talc hydroxyl groups disappears almost completely: ОН – 3670 cm-1. The IR spectrum of the product of mechanochemical interaction of talc with multicarboxylic acid contains the bands of carboxylate ion: asСОО- - 16501550 cm-1, sСОО- - 14001300 cm-1; the bands corresponding to the stretching vibrations of Si-O- and Si-O-Si bonds of talc, respectively, - 1020, shoulder at 1040 and a band at 670 cm-1 somewhat broaden; while their maxima shift by 10-15 cm-1. The participation of weakly acidic Si-OH groups of talc along with Mg-OH groups in neutralization with multicarboxylic acids is likely to cause some strengthening of Si-O- bonds and weakening of Si-O-Si bonds with the corresponding obvious shift of Si-O and as Si-O-Si bands to higher-, and s Si-O-Si to lower-frequency regions of the spectrum.
Since deformation vibrations of the silicon-oxygen layer are modified by lattice vibrations, changes in the parameters of the band corresponding to these vibrations (the region within 550400 cm-1) are more important in this case since they provide evidence of the deformation of talc lattice in the mechanocomposite.
In the diffraction patterns of these samples, the reflections related to acids are absent, while the reflections of talc are substantially broadened and shifted (depending on the nature of an acid) but not more than by 4 % with respect to the position in the initial talc. A similar picture is observed for a mixture of talc with crotonic acid for m.r. = 1 : 2. The data obtained allow us to assume that a mechanocomposite retains the type of crystal structure of the initial talc (Fig. 9). This assumption is confirmed by the data of high resolution electron microscopy according to which the composite of talc with succinic acid is composed of agglomerates of plate-like particles 10.
The differences in the interaction of talc with mono- and multicarboxylic acids during their joint activation can be understood if we take into account the following considerations. Talc belongs to layered silicates of 2 : 1 type having the only kind of Oh groups which are interlayer hydroxy groups: weakly acidic Si-OH and basic Mg-OH, which are situated only at the side faces and edges of plate-like particles. It is necessary to note that under definite conditions, namely, for рН < 2, Si-OH groups can play the part of basic ones. Because of this, it may be assumed that the acidity of reaction medium becomes pH < 2 during mechanical activation of multicarboxylic acids and monocarboxylic unsaturated acid due to dissociation of the acids. As a result, Si-OH groups can participate in neutralization as basic groups. Alcohol hydroxyl groups of acids do not participate in chemical reactions of neutralization with talc as they are typical ol-groups. The duration of mechanochemical interaction а of talc with carboxylic acids is determined by the nature of an acid.