Yu. A. Mikheev

The hydration of hydrophobic substances

A model of the hydration of hydrophobic substances in water is suggested. The models of fluctuation formation of empty cavities in water as a stage of hydration extensively used in the literature were shown to be at variance with experiment. The fundamental role played by the interphase boundary surface was emphasized. On this surface, the successive addition of water molecules with the formation of capsules around hydrophobic molecules occurred. The physical meaning of the Ostwald equation was revealed. This equation characterized the distribution of hydrophobic volatile substances between the gas and aqueous phases. The method of optical probes (hydrophobic aromatic molecules) was used to reveal the synergistic character of autocorrelation of dispersion interactions between water and hydrophobic substance molecules. This synergism was at variance with the Lennard-Jones potential. The synergism (superadditivity) of dispersion attraction forces, which strengthened their directional character, caused the self-organization and enhanced stability of hydration capsules with encapsulated hydrophobic molecules. Computer models were used to show that the spatially directional character of dispersion interactions necessary for the self-organization of hydrated aggregates could be simulated by the molecular mechanics method on the basis of orientational correlation of water molecules and hydrophobic substances in the starting system.

Structural and entropic effects of blending polymers with liquids

The absence of spontaneous blending of an amorphous polymer with a liquid, when their solubility parameters coincide, is explained by a specific feature of the polymer supermolecular skeleton. The process of spontaneous blending requires dissolving the supermolecular skeleton and takes place only under the sufficient entropy–donor activity of the liquid. The lack of this activity rules out the spontaneous process. However, the compatibility does occur via indirect pathway when stimulating a proper excitation or disintegration of a supermolecular carcass.

A supramolecular model of hydrophobic interactions in aqueous solutions of acenes

The optical spectra of aqueous suspensions of tetracene (obtained by heating crystals in water) and 9,10-bromoanthracene (obtained by mixing alcoholic solutions of the compound with water) were studied. Both optical pictures were turbidity spectra related to the formation of comparatively large colloidal light scatterers superimposed upon the absorption bands of light hydrophobic ensembles; the absorption bands of free acene molecules were absent. An analysis of the optical spectra and the construction of models with the use of molecular mechanics programs led us to conclude that light hydrophobic ensembles were molecular dimers comparatively stable in the aqueous phase because of the formation of hydration capsules around them. A model of spontaneous dispersion of crystalline tetracene in water under heating was suggested. The model included the stages of the emergence of tetracene molecules from the surface of crystals and their hydration in water layers adjacent to crystals, formation of molecular dimers on the surface of crystals, and diffusion of dimers into the bulk phase, where they experienced clusterization with the formation and sedimentation of fairly large light scattering colloidal particles.

Origin of the coloration and structure of azobenzene chromogen

Enhancement of the visible (VIS) absorption band intensity of trans-azobenzene (t-AB) in solutions containing water and hydrogen ions is established. This contradicts the current belief that it is part of the n → π* transition. At the same time, it qualitatively reflects the properties of the π → π* bands of protonated azobenzene (ABH+). It is concluded that t-AB molecules display an autopolarization property and exist in the form of two individual electronic (e) tautomers. One of these is nonpolar and has the canonical chemical structure; its content considerably exceeds that of the polar e-tautomer. The polar e-tautomer forms as a result of the reversible transfer of an electron from the nonbonding donor sp2 orbital of nitrogen to the nonbonding acceptor Rydberg’s 3S orbital (R3S) of the local N=N chromophore within the molecule. The positively charged chromogen corresponding to it displays a π → π* transition in the visible spectral region, and the π→π* transition (not the traditionally postulated n → π* transition) is clearly responsible for the orange color of AB. A model of transient e-configurations with the participation of R3S and explaining the previous poorly understood experimental results from optical absorption, fluorescent, raman spectroscopic, and photoionization femtosecond kinetic studies is considered. It is shown that famous ideas about the violation of Kasha’s rule in t-AB fluorescence and photoisomerization processes are incorrect. The reasons for the increased intensity of the VIS band of cis-azobenzene (c-AB) are explained. It is concluded that there is an equilibrium between nonpolar and polarized e-tautomers in cis-azobenzene as well, but it is shifted more toward the polar tautomer in c-AB due to its structural features, making the VIS band more intense.

Nature of chromogens of protonated azobenzene

Changes in the absorption spectra of azobenzene in UV-Vis light are studied with respect to its protonation in sulfuric acid solutions and interaction with gaseous HCl in cellulose triacetate films. In both processes, the emergence of an intense orange color is shown to represent the electronic structure of forming monocations where the positive charge of NH+ groups shifts electrons from the o- and p-positions of Nphfragment phenyls. It was found that the charge distribution and color was the same as observed for the cations of the benzyl and phenylaminyl types. The mechanism of azobenzene monocation photocyclization in sulfuric acid is presented, and the origin of the red color of azobenzene dications formed in oleum is explained.

Mechanism of hydrostimulated displacement of aromatic compounds from cellulose triacetate films

The effect of temperature on the rate of desorption into water of naphthalene, diphenyl, benzophenone, para-terphenyl, α-naphthol, stilbene, anthracene, and dibutyl phthalate introduced into cellulose triacetate films was studied. It was shown that in water, the removal rate of these compounds from the films increases sharply as compared to desorption into air. The relation between the activation energies and pre-exponential factors characterizing the desorption rate was found in the form of a compensatory effect. The nature of the compensatory effect is explained by osmotic phenomena due to hydrophobic hydration of the polymer. We conclude that mobile polymeric chains of non-crystalline polymer are structured around water and Ar-compound molecules as sponge-like structures forming fringed nanoporous capsules. The simultaneous presence of molecules of different compounds in the polymer was found to cause osmotic competition for a place in the sponge; water absorbed by the chain sponge enhances the volume pulsations of nanocapsules. It was revealed that diffusion occurs because of thermal fluctuations inducing the reorganization of nanoporous capsules and their movement together with Ar-molecules in the matrix by the principle of peristalsis.

Heterogeneous features of autooxidation of hydrocarbon polymers

The advantage of a colloidal-heterogeneous reaction model of autooxidation of non-crystalline polymeric hydrocarbons compared to a homogeneous-liquid phase one is demonstrated by a kinetic analysis. The model considers the supermolecular organization of polymer chains into a type of spongy micelle, including a non-uniform porous zone of nanostructural scale. The mechanism of oxidation of such a micellar hydrocarbon sponge takes into account the distribution of the conjugated reacting chains in non-uniform nanophases, distinguished by the sizes and thermal fluctuational dynamics of their nanopores. Kinetic equations of macroscopic stages of the process are given.

The nature of chromaticity of triphenylmethane, xanthene, phthalocyanine, and thiazine dyes

Works concerned with the origin of coloration of organic compounds are reviewed. Proofs are given that individual triphenylmethane, xanthene, phthalocyanine, and thiazine dye molecules do not absorb light in the visible range and are not chromogens, that is, do not determine compound chromaticity. Individual molecules of these dyes should be considered chromophoric particles, necessary but insufficient for coloration generation. Elementary chromogens of the dyes under consideration are dimers (supramolecular particles). The blue coloration of aromatic compound azulene has a similar origin.

The optical properties of triphenylmethane dye molecules and chromogens

Industrial dye monomers, including malachite green, crystal violet, brilliant green, and methyl violet, were isolated by extraction with the use of heptane. UV light absorption bands characteristic of pure molecules were determined. The molecules of the dyes studied, which were ion pairs (formed by dye cations and oxalate or chlorine anions), did not absorb light in the visible range; that is, they were not chromogens. The conclusion was drawn that chromogen particles responsible for chromaticity were supramolecular dimers of nonchromogenic triphenylmethane series molecules. This conclusion was substantiated by trends in spectral transformations with the participation of immonium hydroxides obtained from dyes and side products of the synthesis of industrial dyes with quinoid molecular structures.

UV–Vis spectra and structure of acid-base forms of dimethylamino- and aminoazobenzene

The changes in the UV–Vis absorption spectra of dimethylaminoazobenzene (DAB) in the cellulose triacetate (CTA) matrix during the absorption of gaseous hydrogen chloride and protonation of DAB in weakly and strongly acid solutions of hydrochloric and sulfuric acids were studied. The yellow-orange color of DAB and its analog aminoazobenzene (AAB) was shown to be due to the dimers, whose structure involves phenylaminyl type cations formed due to the promotion of the sp2 electrons of the azo groups to the Rydberg R3s orbitals of the azo groups and possessing Vis bands at 400–417 nm. The cations exist immanently as the dimers due to the Rydberg intermonomer bonds; the Vis bands of the dimers shifted toward 500–520 nm under the action of HCl (in CTA) and weakly acid media as a result of the formation of phenyl aminyl type pair cations, whose π* levels are split by Simpson’s mechanism, during the protonation of interrelated monomers. In the concentrated sulfuric acid, the Vis bands at 500–520 nm vanished because of the decomposition of the dimers into diprotonated monomers possessing Vis bands at 400–417 nm. The mechanism of transformations of chromogens responsible for the spectral transformation was given.