Hydrogen bonding effect viscosity

The urea usually is added to the finished PF-resin and causes a distinct decrease of the viscosity due to disruption of hydrogen bonds [95] and due to dilution effects. There is obviously no co-condensation of this post-added urea with the phenolic resin. Urea only reacts with the free formaldehyde of the resin forming methylols, which, however, do not react further due to the high pH [19]. Only at the higher temperatures of the hot-press does some phenol-urea co-condensation occur [93,94,96]. 

Other considerations aside, the use of dilute reagents minimizes effects of nonideality. This allows the use of concentrations in place of activities. Of course, the time scale, the sensitivity of the analytical method at different concentrations, and the use of other reaction components introduce additional considerations. Tied closely to this decision is the choice of solvent. Reaction rates may (or may not) be affected by such variables as polarity, dielectric constant, hydrogen-bonding ability, donor capacity, and viscosity. A change in solvent may change not only the rate but also the mechanism and possibly even the products. One cannot even assume that the net reaction is the... 

In fact this "unhydrolyzed" polyacrylamide sample is slightly charged and its low polyectrolyte character is confirmed by a slight difference of red values at pH 7 and 5, for salt free solutions. A really neutral polymer should be necessary to differentiate low effects of electrostatic interactions from non ionic interactions. coordination binding at low pH and hydrogen bonds at pH 7. Nevertheless, at this pH, the adsorption of the chain on Al(0H)3 aggregates can probably be considered as the main origin of the loss of viscosity. 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

In conclusion, lifetimes and quantum yields are characteristics of major importance. Obviously, the larger the fluorescence quantum yield, the easier it is to observe a fluorescent compound, especially a fluorescent probe. It should be emphasized that, in the condensed phase, many parameters can affect the quantum yields and lifetimes temperature, pH, polarity, viscosity, hydrogen bonding, presence of quenchers, etc. Attention should be paid to possible erroneous interpretation arising from the simultaneous effects of several factors (for instance, changes in viscosity due to a variation in temperature). 

Hydroxylic solvents are capable of solvating anions through hydrogen bonding, and so halide mobilities are relatively low in alcohols, with chloride the least mobile. The mobility decreases observed for all the halides upon going up the homologous series of aliphatic alcohols may be the result of the increased size and mass of the alkyl group. A similar mass effect may be seen in the lowered mobility of the halides in dimethylacetamide compared to dimethylformamide. Here, as in the alcohol series, dipole moments and viscosities of the two solvents do not appear to be sufficiently different to explain the mobility differences. 

With Increased proton character, the H has a tendency to hydrogen bond. Below pH 7.5, the appearance of the cationic form has a marked condensing effect In association with unionized amine oxide and results In the production of elongated structures replacing progressively the small and spherical micelles of LDAO In the nonlonlc form. Below pH 4.5, the elongated structures are progressively converted Into low viscosity, small spherical micelles made of LDAOH. 

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