THE INTERNAL AND EXTERNAL SOURCES OF SELF-ORGANIZING PROCESSES IN CHEMICAL REACTIONS
V. A. Zhabrev, S. V. Chuppina, V. I. Margolin
Grebenshchikov Institute of Silicate Chemistry, RussianAcademy of Sciences,
nab. Makarova 2, St. Petersburg, 199034 Russia
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ABSTRACT
The internal and external components of information potential are considered to be the main sources of self-organizing processes in chemical reactions. The dependence of the information potential on the number of particles in the ensemble is the most important function of self-organization of nanoparticles. The notion of the information potential or the information component can be associated with the notion of the reception of information, which is used in the theory of dynamic systems.
INTRODUCTION
The purpose of this paper is to attempt to give the definition of internal and external sources of self-organizing process in chemical reactions.
In our opinion, the determination of the internal and external sources of self-organizing process in chemical reactions allows onе to understand the logicof the synthesis of nanocomposites and nanomaterials.
NATURE OF CHEMICAL BONDINGIN INORGANIC, ORGANIC, AND ORGANOELEMENT COMPOUNDS
The first question arising in consideration of thenanostate and nanoparticles lies in a variety of forms ofcarbon-containing nanoparticles and carbon compounds, a large variety of forms of nanoparticles inbiology, and a considerably smaller number of forms ofnanoparticles (recently known) for other chemical elements.
An almost unlimited variety of organic compounds, as a rule, is attributed to the ability of the carbon chemical element to form homochain molecules. This circumstance is responsible for the homology and isomerism phenomena. Furthermore, carbon atoms can exist in the sp3, sp2, and sp hybridization states and, correspondingly, form ordinary, double, and triple bonds thatdiffer in length, energy, and reactivity, as well as cyclic compounds and compounds conjugated multiple bonds.
The presence of labile π bonds in molecules of organic compounds is responsible for the pronounced tendency of these molecules toward addition reactions. We can assume that carbon nanoparticles (fullerenes, astralenes, or other structural forms) are formed as a result of the presence of labile bonds in their precursors. Possibly, a similarity of the metallic bond is formed in carbon nanophase formations, when electrons are collectivized into an “electron gas,” which binds the structural state.
These concepts enable us to assume that the formation of individually structured nanoparticles is more probable for metals or complex organic associates with a large number of π bonds (in cycles, helices, rings, islands).
In silicon, the valence electrons are also representedby sp3 electrons. However, unlike carbon, silicon has avacant 3d orbital, which can affect the character ofchemical bonds in silicon compounds. Compared tocarbon, silicon does not form stable atomic chains:silanes differ from hydrocarbons by instability of−Si(H2)–Si(H2)– polysilane chains. The highest knownmember of the homologous series is hexasilane Si6H14,which slowly decomposes even at room temperature.Organo-substituted polysilanes are also characterizedby a low chemical and thermal stability. Germanium forms chains consisting of two, four, and sixatoms. Tin forms chains composed of two, three, four,and five atoms. Lead forms only dimers. Other transition elements of the Group IV do not form chains.
Academician Andrianov emphasized that “in organoelement chemistry, stable compounds with multiplebonds have not been prepared at all. These bonds canparticipate in the formation of macromolecules only inthe case where they are involved in structural groupssurrounding an organoelement chain”. In organosilicon compounds, an important role is played by thecovalent bond dπ–pπ (when silicon atoms are electronacceptors due to the presence of vacant orbitals) arisingbetween silicon and oxygen (or halogen, sulfur, nitrogen, and carbon of an aromatic nucleus) atoms.
The possibilities of constructing heterochain organoelement high-molecular compounds are wide andvarious. The character of bonds in heterochains can bespecific for each individual element. The B, Al, C, Si,Ti, Ge, P, and As electropositive elements in combination with the O, N, and S electronegative elements areused for forming heterochains with weakly ionogenicbonds. Heterochain organoelement polymers with ionogenic bonds in many cases can be formed from alkaline-earth metals, some nontransition elements (Tl, P,B), and transition metals (Cr, W, etc.) in combinationwith polyfunctional organic and organoelement compounds. Polyorganoelementsiloxanes with chainsconstructed from Si and O atoms and elements E = Al,B, Ti, Sn, Ge, Pb, P, Sb, As, Ni, and Co have been synthesized to date. It is known that many GroupIV elements (Si, Ti, Ge, Sn, Pb), B and Al (Group III),and P (Group V) in combination with oxygen and nitrogen can form inorganic chains of polymer moleculessurrounded by organic and organosiloxane groups.
In inorganic compounds SiO2, SiC, Si3N4, and others, multiple bonds do not substantially affect association processes. For these compounds, nanosized structural formations are known only for silica. The casewhen silicon forms Si–O–Si homopolar bonds is morecharacteristic of silicates. This leads to the formation of silicates with chain, framework, or island structures with different degrees of short- and long-range order.
The Si–O bond is resonant in character, and the ionic component in this bond is considerably smaller than the covalent component. Moreover, the Si–O bonds have adouble bond character (of the order of 30%) as a resultof the presence of the additional pπ–dπ donor–acceptorbond due to the overlap between the vacant 3d orbitalsof silicon and 2p orbitals of oxygen having lone electron pairs (1, 2). The “mean” length of the bondbetween silicon and oxygen due to the partial doublebond character is equal to 1.63 Ǻ(3).The character of the Si–O bond is determined by theelectron acceptor properties of silicon, which, in turn,depend on the atoms bonded to silicon. Strong electronrepulsive substituents can be represented, for example,by nonbridging oxygen ions (Scheme 1). The presenceof a negative charge in these ions suppresses the electron-acceptor properties of silicon. As a consequence,the degree of covalence (double bond character) in thebond of silicon with nonbridging oxygen decreaseswith an increase in the degree of double bond characterin the neighboring bond of silicon with nonbridging oxygen. Owing to the competitive character of thepπ–dπinteraction in the –O–Si–O–Si–O– bond, an enhancement of the double bond character of the –Si–O– bondsis accompanied by a decrease in the degree of doublebond character of one of the neighboring Si–O–(Si–O–)bonds. It is known that the stronger the double bondcharacter of the –Si–O– bonds, the larger their strengthand the force constant and the lower their ionicity and the smaller the bond length (3). As a result, thelength of the Si–O– bond with a stronger double bondcharacter should be shorter than a “mean” length andthe length of the bond with a weaker double bond character can approach the length of a “pure” σ bond (1.75Ǻ). These notions are described in the literature as theMills–Nixon effect (1, 2). A strong electron-repulsive interaction of nonbridging oxygen not only canlead to a change in the “own” Si–O bonds but also canbreak the symmetry in the neighboringbonds –O–Si–O–Si–O–. This effect is called the conjugation effect (1).In Scheme 1, the symbol↔ indicates a “shortened”bond and the symbol← represents an “elongated”bond.
– O Si O Si O¯Na+
– O Si O – Al – O Si O –
– O Si O – B – O Si O –
Sheme 1. Character of chemical bonds between silicon and oxygen in silicates
If a particular element is isomorphically introduced into the silicon-oxygen chain, this element breaks the symmetry of the bonds. We assume that aluminum or boron isomorphically replace silicon in the spatial structure of silicates (Scheme 1). It is known that the larger the difference between the electronegativities of elements A and B forming the bond, the higher the ionicity of the A–B bond.
Since the electronegativity of aluminum is higherthan that of silicon, the ionicity of the Al–O bond ishigher than the ionicity of the Si–O bond. According tothe conjugation effect, the degree of covalence of theneighboring bond increases and it becomes shorter. Bycontrast, the degree of covalence of the B–O bond ishigher than that of the Si–O bonds. This also leads to anincrease in the degree of ionicity of the neighboringSi−O bond (Scheme 1).
Therefore, the introduction of any other elements(able to replace isomorphically silicon atoms) into thesilicon–oxygen network results in a redistribution ofthe type of bonds and their parameters in the space(bond lengths) and breaking of the symmetry in thestructure of these associates (4).
The above considerations can be generalized as follows.
(i) The presence of a labileπ bond (especially in closed or nonlinear chains) leads to the formation of nanoparticles with a particular structural composition (C60, astralenes, icosahedral diamond, etc.). This fact and a high specific surface area (the specific surface area is considered to mean the surface area–volume ratio characterizing the degree of nanodimensionality of a particle) can explain the high reactivity of these nanoparticles. The presence of independent isolated nanoparticles of this type can be expected in metal clusters or complexorganimolecular formations. Organic–inorganic hybrids of the “nanocarbon–organic molecule” or “metal nanocluster–organic molecule” type can be formed as a result of the presence of a large number of collectivized π electrons.
(ii) Isolated structured nanoparticles of a specificcomposition cannot be formed in silicate systems dueto the absence of labilepπ–pπ bonds (electron gas).Nanoparticles and organic–inorganic hybrid composites can be formed only through the addition to dangling silicon–oxygen bonds. The incorporation oforganic molecules into pores, channels, cavities, orholes of the silicon–oxygen network results in the formation not of an organic–inorganic hybrid but of anorganic–inorganic composite, in which the organic andinorganic components are linked together only by van der Waals bonds or there is chemisorption on silicate surfaces rich by reactive silanol groups.
These are the first inferences that follow from consideration of the nature of chemical bonding in organic,inorganic, and organoelement compounds.
CHEMICAL REACTIONSAND SELF-ORGANIZING PROCESSES
If we consider a compound or a composite from the standpoint of the size factor, it turns out that their properties have long been known. For example, a sodium carbonate solution is a combination of hydrated ions and has a pH higher than seven, because one of the ions is hydrolyzed by water molecules. From the viewpoint of the formal scale, we are dealing with a typical nanocomposite.
If the chemical reaction is considered a self-organizing process, we see nothing new. Let the following reactions to proceed:
AgNO3 + KCl → AgCl↓ + KNO3,
CH3COOH + CH3OH → CH3–COOCH3 + H2O,
6HCOH + 4NH4OH → (CH2)6N4 + 10H2O.
In the first reaction, at will, the observer can find isolated nanoparticles of silver chloride with the solubility product equal to 10–10 mol/l (i.e., one silver chloride particles per ten trillions of water molecules per cm3) and chlorine and silver ions in the solution.
The second reaction represents the synthesis of methyl acetate and can be interpreted as the reaction of synthesizing ester–water nanoparticles.
The third reaction describes the preparation of hexamethylenetetramine (urotropin) from formaldehyde and an ammonia solution and can be treated as the reaction for preparing the organic–inorganic hybrid.
Let us consider further several reactions, for example, 3C2H2→ C6H6. In this case, three acetylene molecules above the activated carbon catalyst at a temperature of 600oC are “self-organized” or, more precisely, are cyclically trimerized into benzene according to the well-known Zelinsky reaction.
Polymerization reactions lead to the formation of high-molecular compounds, for example poly(styrene) from the vinylbenzene monomer. Depending on the polymerization procedure, the resulting homopolymer can be amorphous (the preparation in the presence of organic peroxides, for example, dicumyl peroxide or tert-butyl perbenzoate, through the mechanism of radical polymerization) or crystallized stereoregular (syndiotactic) in the case of cationic polymerization under the catalytic action of HBF4. The polymer is termed syndiotactic if, in the chain segment involving four units connected in a “head–tail” manner, the Ph substituents (phenyl radicals) in the main chain of the macromolecule (–CH2–CHR—)n are orderly arranged and located on different sides of the plane of the main chain.The cationic polymerization of styrene proceedsaccording to the following scheme:
(1) chain initiation CH2=CHPh + HBF4→CH3C(+)HPhB F4(-)
(2) chain growth CH3C(+)HPh + CH2=CHPh→CH3CHPhCH2C(+)HPh,
(3) chain termination R–CH2C(+)HPh + BF4–→R–CH=CHPh + HBF4.
Is this reaction a prototype of the self-organizing process? Most likely, the case in point is not this nanotechnology.
Let us consider a more complex reaction, i.e., the chlorination of polyorganosiloxanes in the presence of oligoazines with a system of conjugated multiple bonds –C=N–N=C–. In this case, there can occur several conjugated concurrent or sequential reactions. In the dark at a temperature of 20oC, the interaction of molecular chlorine with poly(dimethylphenylsiloxane) (PDMPS) and poly(dimethylsiloxane) in the presence of oligoazine with terminal carbonyl groups (5, 6) (synthesized from diacetyl and hydrazine) proceeds as a substitution reaction. In this case, oligoazine is initially chlorinated through the stage of forming a π–σ donor–acceptor complex of the 1 : 1 composition (7). Then, the homolytic decomposition of this complex initiates the radical chain process of chlorination of the polymer.
Can we equate the set of these reactions and the essence of the self-organizing process? Does this complex multistage process lead to new properties in the nanostate under consideration? Would it be a substitution of notions in this case?
Actually, this complex multistage process can be described in detail using the known regularities. Moreover, a technology has been proposed for preparing new products, i.e., chlorinated polyorganosiloxanes, which are of practical interest for the design of anticorrosive heat-resistant organosilicate coatings with improved physicomechanical properties (8, 9).
It is evident that, as applied to macromolecular (carbochain, organoelement) compounds, the notion of self-assembly (self-organization) can be interpreted in terms of chemistry and physics of high-molecular compounds. Most likely, it is not always possible to equate chemical reactions and self-organization processes. It is clear that notevery chemical reaction (even though it can be represented as a multistage process by researchers) is a result of self-organization of atoms and molecules into ensembles that bring the system into the nanostate. However, it cannot be ruled out that a closer examination of the process can reveal that its particular stages bear indications of self-organization; the reaction is accompanied by a spontaneous search for the stable structure; and the nanostate of reactants, activators, and other elements of the system has a special effect.
The chlorination of PDMPS in the presence of oligoazines leads to an anomalous order of the reactive sequence: chlorine bound by the polymer is predominantly in the form of chlorine-substituted benzene rings.
It is believed that a change in the reactivity of a substratum RH with a variation in R is associated with the three main structural factors, i.e., the energy, polar, and spatial effects. The degree of manifestation of these effects and their relative significance depend on the substratum structure.
As a rule, chlorination reactions with the use of molecular chlorine are characterized by a low sensitivity to a variation in the energy effect with variation in R in substratum RH (10).
At close thermal effects of the reaction, as a rule, the larger the difference between the electronegativities of a radical and an attacking particle, the lower the activation energy. Taking into account the electron-acceptor properties of the phenyl group, the chlorination rate of this group should be lower than the chlorination rate of the methyl group. Very frequently, the polar factors for chlorination reactions occurring through the mechanism of free-radical substitution appears to be dominant (10).
Spatial effects usually play a secondary role.
However, experiments demonstrate that, among structural factors determining the reactivity of groups in PDMPS in the course of chlorination, the main role is played by spatial effects. Most likely, this is associated with the direct screening of methyl groups by more bulky phenyl groups or the high accessibility of phenyl groups for the reaction due to the special packing (configuration) of linear dimethylsiloxane fragments of PDMPS in a carbon tetrachloride solution. The special packing of chains is possible, because the chlorination proceeds although in a dilute solution (no more than 10 wt %) but in a “bad” solvent (carbon tetrachloride) for polyorganosiloxanes (aromatic hydrocarbons, such as toluene and xylene would be ideal; however, they under these conditions are chlorinated themselves).
Is it possible to believe that, the correlated self-organization of different particles (participants of the chlorination process) of this multicomponent system “oligoazine + Cl2 + PDMPS + CCl4” under the given conditions (the absence of illumination, 20oC) resulted in the choice of the direction of the chlorination reaction that led to the aforementioned unexpected result? It seems likely that the answer is yes. At present, the point of view that polymers are self-organizing systems and self-organizing processes participate in specific reaction stages has been universally accepted in chemistry of high-molecular compounds.
Now, we dwell on one more circumstance: the formal analysis of the complexation of oligoazines with halogens shows that, hypothetically, three coordination sites, i.e., lone electron pairs of nitrogen atoms, terminal oxygen atoms, and the system of conjugated bonds –C=N–, are possible in diacetyl azine. However, no band of halogen in the charge-transfer complex that is characteristic of a complex of the n–σ type (the complex would be formed by the antibonding lone electron pair of the nitrogen or oxygen atom and the σ bond of the halogen molecule) was revealed in the long-wave length range of the UV electronic spectrum. Further-more, no new bands were found for the “acetone–bromine” model system even at acetone concentrations that are one order of magnitude higher than the concentration of terminal groups in the azine. It was established that a strong π–σ donor–acceptor complex of the stoichiometric composition, which is stable at temperatures of no higher than –20oC, is formed in the “diacetyl azine–bromine” system (7). It is evident that, if complexes of then–σ type would be formed, they would turn out to be considerably less stable than complexes of the π–σ type, because the donor ability ofthe lone electron pairs of nitrogen and oxygen atoms issignificantly lower than that in the case of conjugatedbonds. To put it differently, we observe the formation ofthe stronger complex; i.e., the direction of complexation that leads to the formation of the most stable complex is chosen in the “oligoazine–molecular chlorine”system. When the direction of the process is chosen, itsfurther course obeys specific laws: the equilibrium constants and the extinction coefficients of the complexincrease with an increase in the length of the conjugation chain in the oligoazine.
Let us consider the synthesis of barium titanatenanoparticles (11). It is known that barium titanate hasbeen widely used for the design of systems intended forwriting large amounts of information and that nanoparticles substantially improve the properties of the finalcomposite. The reaction of barium nitrates with thehydrated form of titanium dioxide proceeds in moltensalt KNO3 as a reaction medium. A comparison of theX-ray powder diffraction data for polycrystallinebarium titanate and those for the nanosized product(Fig. 1) demonstrates that the reflections of nanoparticles are smeared in the X-ray powder diffractionpatterns, whereas the corresponding reflections are pronounced in the X-ray powder diffraction patterns of thepolycrystalline samples. Only atomic-force microscopy (AFM) allows one to reveal the dimension of particles (Fig. 2). This example illustrates that zero-dimensional nanoparticles impart radically new properties toa material.