Molecular association, influence

It is well known the tendency of polysaccharides to associate in aqueous solution. These molecular associations can deeply affect their function in a particular application due to their influence on molecular weight, shape and size, which determines how molecules interact with other molecules and water. There are several factors such as hydrogen bonding, hydrophobic association, an association mediated by ions, electrostatic interactions, which depend on the concentration and the presence of protein components that affect the ability to form supramolecular complexes. 

Phthalocyanine-based dyes are especially useful for CD-R, as the chromophore absorption band falls in the desirable spectral range, and they are noted for excellent photostability. Unlike cyanine dyes, phthalocyanines tend to have very poor solubility, particularly in solvents such as alcohols and aliphatic hydrocarbons (which do not attack polycarbonate and are therefore used for spin coating). Therefore, the main barrier to the wider use of these dyes is the relatively high cost of synthesizing soluble derivatives. Suitable modifications to the Pc core which have been developed, notably by Mitsui Toatsu, are shown in Scheme 7. The bulky R groups reduce undesirable molecular association (which in turn lower the extinction coefficient and hence reflectivity), whereas partial bromination allows fine-tuning of the film absorbance and reflectivity. The metal atom influences the position of the absorption band, the photostability, and the efficiency of the radiationless transition from the excited state.199 This material is marketed by Ciba as Supergreen.204... 

As mentioned in Sect. 2.2.3, the biodistribution of HPMA copolymers depends on many factors. Molecular weight influences the uptake in the isolated tissue of yolk sac [266] as well as the elimination in vivo [124, 125,267,268]. Nonspecific increase in the rate of polymer uptake can be achieved by incorporation of positively charged or hydrophobic comonomers into the HPMA copolymer structure, such as methacryloyloxyethyltrimethylammonium chloride [22], N-methacryloyltyrosinamide [21], or N-[2-(4-hydroxyphenyl)ethyl]acrylamide [267]. The incorporation of hydrophobic moieties may influence the solution properties of the HPMA copolymer conjugates [132,134,269]. The interaction with the cellular surface may depend on the association number and the stability of the micelles. 

In 1975 Mauret et al. (75BSF1675) carried out a detailed dipolarimetric study of the azole series, on the basis of the various, and sometimes conflicting, values of the dipolar moments reported in the literature. For pyrazole and imidazole, measurements were performed with the solvents dioxane and benzene at different concentrations and at 25°C, with the aim of determining the influence of concentration and solvent on the value of the dipolar moments, and, at the same time, the involvement of the different molecular associations owing to the formation of intermolecular hydrogen bonds. Thus, for imidazole (linear polymers), the dipolar moment increased with a rise in concentration, whereas for pyrazole (cyclic dimers), the dipolar moment decreased with higher concentrations, and consequently the dielectric permittivity fell. 

The propagating anion and its counterion exist in relatively nonpolar solvents mainly in the form of associated ion pairs. Different kinds of ion pairs can be envisaged, depending on the extent of solvation of the ions. As a minimum, an equilibrium can be conceived between intimate (contact) ion pairs, solvent-separated ion pairs, and solvated unassociated ions. The nature of the reaction medium and counterion strongly influences the intimacy of ion association and the course of the polymerization. In some cases the niicrostructure of the polymer that is produced from a given monomer is also influenced by these variables. In hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates. Such inter-molecular association is not important in more polar media where the ion pairs can be solvated and perhaps even dissociated to some extent. 

The influence of structure on molecular association in the alcohols was shown by Smyth (24), who determined the dielectric constant and molar polarization of 22 isomeric octanols in the pure state. Only general conclusions can be drawn from these results. As Smyth says, associations in which the dipoles reinforce each other, giving high P and e, seem to occur when the OH group is at the end of a long C chain and remote from a branch in the chain—i.e., when linear multimer formation is most favored. When the OH group is in the middle of the chain and there is also branching at that point as in 4-methyl-4-heptanol, P is approximately half the value for 1-octanol, as would be expected if cyclic multimers predominate. 

Solvents vary in their dissolving power, so that the line of demarcation between solvents, latent solvents and nonsolvents is difficult to define. Some of the factors which influence solvency are atmospheric conditions, purity and molecular association. Molecular aggregation is the explanation for increased, attenuated, or decreased solvent power or, more concisely, eccentric solvency. Any substance that will dissolve another is called a solvent. Thus, we have a gaseous solution when a liquid or a solid is dissolved in a gas a liquid solution when any one of these is dissolved in a liquid, and a solid solution when any one of them is dissolved in a solid. 

Typically, in specific solvents, the process of monomer formation [9.57] is characterized by considerably lower dimerization constants, as compared with those in universal media. In fact, acetic acid dimerization constant in water-dioxane binary solvent, the components of which are solvation-active in respect to the acid, vary in the 0.05-1.2 range. Replacing the solvent is often the only method to vary the molecular association state of dissolved compound. To achieve dimer concentration in 0.1 M solution of phenol in n-hexane equal to dimer concentration in nitrobenzene solution (50% at 25 C), it would be necessary to heat the solution to 480 C, but it is impossible under ordinary experimental conditions. 9.4.3 MIXED SOLVENT INFLUENCE ON THE CONFORMER EQUILIBRIUM Equilibrium took place in solutions... 

Moreover, the number of nitro groups has a very important influence, as shown by Table 48, on the bond strength of the resulting molecular association recent measurements of Halban and Zimpelmann and of Briegleb and Schachowsky have proved this. 

The nature and the extent of molecular association in liquid hydrogen fluoride is influenced by the presence of ionizing impurities, particularly water. Density, acidity and H-n.m.r. behaviour suggest drastic changes in size and arrangement of the (HF)n-polymers. 

Since molecular crystals are essentially assemblies of molecules in a void, the atom-atom FFs determined in the crystalline state should be substantially transferable to molecular associations in the gas phase. Less is known on the role they may play in the formation of molecular complexes in nonpolar or polar solvents, though their influence on the intermolecular association constants in these environments may perhaps be predicted to be small in view of internal compensation in the solvation of both reagents and products. 

Influence of Molecular Association.—The variability of c is even more marked when the molecules of one of the substances are associated in the liquid state. This is the... 

Figure 27 presents the lnT (p) dependencies obtained for 8CB. The activation enthalpies A f/ and activation volumes V for the nematic and smectic phases are presented in Figure 28, both for 8CB and 80CB. The activation enthalpies obtained for the smectic phases increase for 8CB but decrease for 80CB with rising pressure, whereas it always decreases in the nematic phase. Also the plots for the activation volumes (Fig. 28b) exhibit similar trends. The opposite pressure dependencies observed for the nematic and smectic A phase of 8CB have been discussed in terms of a peculiar pressure influence on the molecular associations. The existence of dimers in the smectic layers enlarges the free volume that facilitates the molecular reorientations thus the activation enthalpy for the smectic phase is reduced, which has also been noted for other liquid crystals. ... 

The marked increase in splay and twist elastic constants due to the presence of a cyano-group is thought to be a consequence of the dramatic increase in molecular association that accompanies the increased longitudinal dipole moment. Thus the apparently strong influence of the dipole moment on the elastic constants is really a consequence of the enhanced intermolecular interactions [38]. 

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