Polarity, microenvironment

Tautomerism on polymer should be quite sensitive to neighbouring group effects (composition and unit distribution, steric hindrance and tacticity) and to the microenvironment polarity in solution (copolymer-solvent interactions, critical concentration c of coil interpenetration). The determination of the tautomerism constant KT=(total conjugated forms)/(keto form) in dilute (csemi-dilute (c>c ) solution from H-NMR at 250 MHz and from UV spectroscopy has been reported elsewhere (39,43). The following spectrometric data related to keto-2-picolyl and keto-qui-naldyl structures are quite illustrative ... 

Szentirmay et al. studied the microchemical environments of Nafion 117 in the acid and Na+ forms using Py and Ru(bpy)32+ probes in fully hydrated ( 40%) samples in various cation forms.Ru(bpy)32+ emission spectra cannot be interpreted in terms of environmental polarity in as straightforward a fashion as in the case of Py, but blue shifts can reflect this aspect. One of the results of this study was that the microenvironment polarities were such that Lj//i values for Py are between those for fluorocarbon and aqueous environments, and this conclusion was strengthened by the results of Ru(bpy)3 + probe studies, as well as the similar conclusion of Lee and Meisel. Another conclusion that was reached was that the SOa" clusters are chemically heterogeneous, an idea that was in line with the view of Yeager and Steck, who spoke of mixed interfacial regions. ... 

Z values have been widely used to correlate other solvent-sensitive processes with solvent polarity, e.g. the a absorption of haloalkanes [61], the n n and n n absorption of 4-methyl-3-penten-2-one [62], the n n absorption of phenol blue [62], the CT absorption of tropylium iodide [63], as well as many kinetic data (Menschutkin reactions, Finkelstein reactions, etc. [62]). Copol5mierized pyridinium iodides, embedded in the polymer chain, have also been used as solvatochromic reporter molecules for the determination of microenvironment polarities in synthetic polymers [173]. No correlation was observed between Z values and the relative permittivity e, or functions thereof [317]. Measurement of solvent polarities using empirical parameters such as Z values has already found favour in textbooks for practical courses in physical organic chemistry [64]. 

Model calculations of interface-solute electrostatic interactions reproduce well the view of microenvironment polarities of micelles and bilayers obtained from experimental data [57]. According to molecular dynamics simulations, at 1.2 nm from a bilayer interface, water has the properties of bulk water. At shorter distances, water movement slows as individual water molecules become attracted to the interface. At the true interface, which is a region containing both H2O molecules and the surfactant polar head groups, the water molecules are oriented with... 

The curvature in the log k vs. n plots, observal with micellar and hybrid mobile phases, was first attributed to the different locations (with different microenvironment polarities) in the micelle, for different members of a homologous series [6, 7], Methylene selectivity decreases as the difference between mobile and stationary phase polarities is reduced. For a given mobile phase composition, the larger and more hydrophobic homologous compounds are located in a less polar environment of micelles, it is then conceivable to assume that these compounds experience a smaller change in their microenvironment polarity upon being transferred from the micellar pseudo-phase to the bonded alkyl stationary phase. The a(CH2) value between n-pentylbenzene and n-butylbenzene is smaller than it is between ethylbenzene and toluene (Table 9.1), because the former pair is located in a more nonpolar environment than the latter. 

For measuring the microenvironment polarity of synthetic polymers it is of great importance to know to what extent specific inter-... 

In Table I, the semiempirical parameter of the solvent polarity and the polymer microenvironment polarity in the same solvents are compared. In all the cases, the microenvironment polarity of a polymer in solution was lower than that of the solvent. In polymers with a partially nonpolar character, such as poly(4-vinylpyridine), poIy(2-vinyl-pyridine), poly(methyl methacrylate), as well as poly(2-hydroxyethyl methacrylate), part of the interactions (dipole-dipole, dipole-induced dipole, multipole, charge-dipole, specific association such as hydrogen bonding, etc.)38 are shielded by the nonpolar backbone of the polymer chain and by the side chains. Solvation of the polymer polar group differs from the solvation of the low-molecular analogue also in other respects. In spite of a relative polarity of the polymer units, the orientation of their dipoles to a bound polar reporter or reactive residues is not as free as for a solvent molecule so that a much wider dispersion of orienting electric dipoles and energy interactions may be encountered (see p. 21h. 

Lower solvatochromism in the polymer spectrum implies that merocyanine is solvated by ester groups of the same polymer chain and are shielded from the bulk solvent. This observation fits in with the foregoing conclusions on microenvironment polarity as estimated from the shift of the absorption band of the solvatochromic reporter SB. 

Although molecular probes are extremely useful for monitoring microenvironment polarity, it is important to be careful in interpreting the results obtained. Each probe responds differently depending on the nature of the emissive state, the temperature and the molecular conformation. It is therefore important to perform detailed calibration experiments on the probe in isotropic media (e.g. in solution), before attempting to obtain meaningful and quantitative information from more complex anisotropic biological systems. 

Molecular probes are used extensively to investigate the microenvironment polarity of lipid membranes. Synthetic liposomes and vesicles that mimic the... 

The study of the influence of the chain on the reaction medium led to the development of a new concept microenvironment polarity this has been studied through various experimental approaches by mainly spectroscopic methods. " ... 

The effect of the microenvironment (polarity, local interactions) can modify the reactivity of a group even in a denatured polypeptide chain compared to that of a model compound. 

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