Physico-Chemical Correlations |
The principle of the stability of a chemical substance is a set of qualititaive regularities according to which the relatively low chemical (ch) thermodynamic stability of a compound in a state of ideal gas or solution () causes relatively high supra-molecular (intermolecular, im) thermodynamic stability of condensed phases formed by that compound (). Conversely, the higher the chemical thermodynamic stability of a substance, the lower its supra-molecular thermodynaic stability in a condensed state. This regularity can be expressed through values of DH_{comb} , DH_{form }and other functions. The principle was applied by the author to various hierarchies as part of the theory of the evolution of life. It is in agreement with the principle of structural stabilization. The principle of the stability of a chemical substance can be applied with a number of qualifications to multi-component systems ( Figures 4 and 7) and reflects the tendency of atoms of different elements to condense around other atoms through chemical and non-chemical (intermolecular, im) ties. The above regularities can be identified for isoatomic substances, several homologous series, lyophilic, lyophobic and biophilic chemical compounds by building the folowing dependencies: where , - coeffcients and the subscript j relates to the substance. The available data demonstrate that the above relationships cover most simple substances soluble in water. Precise mathematical formulation of the principle may be impeded, because the measurement of the absolute values of thermodynaic functions is not possible. Principle of structure stabilisation - the principle stating that in hierarchic systems each higher-level partial evolution or, in other words, higher i-th partial process, which is a component of the general evolution - the process of hierarchic structure formation - stabilises, due to the aggregation, the products of the lower partial evolution, or (i-1)-th partial process. For instance, in a non-stationary open system where various chemical reactions and other processes take place, structure stabilization “chooses” supramolecular structures that are most stable thermodynamically.
Gases and Water - H_{2}, N_{2 }, O_{2},CO_{2}, H_{2}O
Tissues - Fats (Lipids), Proteins, Proteids, Sugars, RNA, DNA
For instance, the molecularly (chemically) stable substances such as H_{2 }, N_{2 }, O_{2 }, CO_{2 }, H_{2}O have relatively low melting and boiling points that indicates the low thermodynamic stability of their condensed phases. On the other hand, such energy-intensive substances (with low molecular thermodynamic stability) as sugars, peptides, nucleic acids melt at relatively high temperatures and decompose during melting and “boiling”. The aggregated phases of these substances are highly stable!
The principle defined here is in agreement with the experimental data and calculations done for the Gibbs - Helmholtz equation: (4) where is an averaged measure of the stability of supramolecular structure, - specific enthalpy (variation of specific enthalpy) of melting - denaturation (), T_{o }- standard temperature, i.e., 25 ^{0}С, - melting point of i-th substance, and - variation of specific heat during melting. It is very important to bear in mind that the principle of the stability of chemical substance is a thermodynamic principle. According to it the tendency of biological system during evolution (ontogenesis and phylogenesis) to generate relatively highly stable structures of higher hierarchies lead to the selection of relatively less stable structures of lower hierarchies. That evolutionary tendency of the biological system “rejuvenates” the lower hierarchical structures and causes unbounded evolution of the biological world.
For closed systems, the dependence of the Gibbs function variation on temperature T is determined by the Gibbs-Helmholtz equation: where is the enthalpy variation during the process, T and p are temperature and pressure. An equation analogous to the Gibbs-Helmholtz equation can be also written for systems whose composition is not identical. (Note that T and p are constant.) This assumption is precise only for substances (i) that have the same values of thermodynamic parameters. Nevertheless, it proved reasonable for a certain range of the variation of thermodynamic characteristics. Indeed, the experimental data revealed the correlation (predicted in [9]) between - the specific Gibbs function of the structure formation - and D T=T_{m}-298, 298 being the standard temperature, K. The correlation has been obtained for non-equilibrium phase transitions “overcooled liquid - solid” for a wide range of organic compounds with varying within a broad temperature interval. (The index relates to the melting point of the i-th substance.) Several results of calculation by means of Eq. 7 relating to fatty acids are shown in Fig.10. One can see that natural substances with high (i.e., with high ), which have been studied by us, indeed have a tendency to form thermodynamically stable phases - solid structures (with more negative values of , see Eq. 7, since condensation (crystallisation) processes of pure substances can be considered as enthalpy-controlled processes, from the viewpoint of the summarised effect. If more precise calculations are necessary, corrections for the specific heat variation of the substances at their melting, , are to be made, and so on. Note that the chemical energy capacity of a series of similar natural compounds () grows as their increases ([27, 28], see also Table 1).
In the case of entropy-controlled processes, such as aggregation of cells during the self-assembly of cellular structures, denaturation of some proteins, etc., the rise in the thermal stability of the structures is accompanied by a drop of . However, apparently, the relative share of such entropy-controlled processes is rather small during the structure formation in some biotissues, and also some organells. Thus, the data of Fig.10 are in agreement with the conclusion that it is thermodynamically beneficial for living organisms to accumulate substances with high chemical energy capacity, which oust water from the biotissue. Let us make some comments. In fact, the author has studied the correlation between the values of (25° C) calculated for individual closed systems (substances), and . The existence of such correlations enables one to estimate the differences between the values of the Gibbs function of the structure formation () of various systems (for instance, supramolecular ones). These differences are characterised by the values calculated for constant temperature (T_{0}). The values of are determined by the differences in the chemical (supramolecular) composition of the systems under study (= ). The Gibbs-Helmholtz equation is given by the relation for a closed system. As to the correlation equations suggested by the author, they are given by relations of the type or , and so on. The Gibbs-Helmholtz equation is a general strict thermodynamic equation, whereas the correlation equations written by the author describe particular cases. They are well satisfied only within a limited temperature interval for uniform systems (substances) of variable (different) composition. Thus, despite the limited scope of the correlations considered by the author, they can be used for predicting the thermodynamical tendency of the open systems evolution. The equation of the type , suggested by the author, where is the specific enthalpy variation due to crystallisation, are variables and is a constant, is an analogue of the Gibbs-Helmholtz equation [9, 14-16]. The same equation considered for constant , and variable is the approximate Gibbs-Helmholtz equation. |