Important characteristics of solvents

Some important physical and chemical characteristics of solvents from the standpoint of their use in paints and coatings are discussed below. 

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An important characteristic of solvents is rate of evaporation. Rates of solvent loss are controUed by the vapor pressure of the solvent(s) and temperature, partial pressure of the solvent over the surface, and thus the air-flow rate over the surface, and the ratio of surface area to volume. Tables of relative evaporation rates, in which -butyl acetate is the standard, are widely used in selecting solvents. These relative rates are deterrnined experimentally by comparing the times required to evaporate 90% of a weighed amount of solvent from filter paper under standard conditions as compared to the time for -butyl acetate. The rates are dependent on the standard conditions selected (6). Most tables of relative evaporation rates are said to be at 25°C. This, however, means that the air temperature was 25°C, not that the temperature of the evaporating solvent was 25°C. As solvents evaporate, temperature drops and the drop in temperature is greatest for solvents that evaporate most rapidly. 

Solvents are introduced into the paint formulation to dissolve the binder and to provide adequate fluidity of the paint to allow it to be applied. The most important characteristics of solvents are their capacity to dissolve the resins and their volatility, which controls the speed of evaporation. Solvents employed in solvent-based products are hydrocarbons (both aliphatic and aromatic compounds), oxygenated solvents (frequently alcohols, esters, ethers, or ketones), and terpenic solvents in general, mixtures of solvents are used in paint manufacture. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

The use of organic solvents in nonaqueous capillary electrophoresis not only increases the solubility of the solutes, but also allows one to control important characteristics of the separation. For instance, the solvent properties affect the acid-base behavior of the analytes on a wider scale than... 

Ultraviolet (UV) spectroscopy does not tend to be the method of choice for structure determination, but a list of UV absorptions was given in the review by Knowles <1996CHEC-II(7)489>. Fluorescence properties and triplet yields of [l,2,3]triazolo[4,5-r/ pyridazines in various solvents have been reported <2002JPH83>. These heterocyclic systems were found to be photochemically very stable. In a recent paper, Wierzchowski et al. studied the fluorescence emission properties of 8-azaxanthine ([l,2,3]triazolo[4,5-r/ pyrimidine-5,7-dione) and its A -alkyl derivatives at various pH s <2006JPH276>. For the 8-azaxanthines, an important characteristic of emission spectra in aqueous solutions was the unusually large Stokes shift. Since 8-azaxanthine is a substrate for purine nucleoside phosphorylase II from Escherichia coli, the reaction is now monitored fluorimetrically. The fluorescence properties of [l,2,3]triazolo[4,5-r/ -pyrimidine ribonucleosides were earlier described by Seela et al. <2005HCA751>. 

Monolayer Films of Phthalocyanine Derivatives. A series of organic derivatives of phthalocyanines were prepared that have two important characteristics of materials to be deposited by the Langmuir-Blodgett technique (1) they are soluble in volatile organic solvents, and (2) they form monomolecular films on the surface of water. Further study of deposited films of these phthalocyanine derivatives will be necessary in order to determine the exact orientations on the surface, but regardless of their orientations, they offer interesting possibilities for construction of thin films of ordered arrays of molecules on the surface of gas sensors. 

The election of the appropriate reference electrode for a given electrochemical system is conditioned by different factors (like the solvent or the temperature). The most important characteristic of a reference electrode is that it should provide a constant and reproducible potential difference when connected to the other semicell unit. For a detailed list of different reference electrodes, see [15-17]. 

Method validation is important to ensure that the analytical method is in statistical control. A method may be validated by the so-called method evaluation function (MEF) (Christensen et al., 1993), which is obtained by linear regression analysis of the measured concentrations versus the true concentrations. A true concentration in a solution can be obtained by use of a high purity standard obtained from another manufacturer or batch than the one used for calibration. Both the high purity standard and the solvent are weighed using a traceable calibrated balance. If certified reference material is available this is preferred. The method evaluation includes the most important characteristics of the method as the following elements (see Figure 2.7) ... 

The density, d, of a solvent depends on both the temperature and the pressure and its value at ambient conditions is an important characteristic. Most solvents at 25°C and 0.1 MPa have densities between those of w-pentane (0.62319 g cm-3) and of tetrachloromethane... 

Solvent dyes [1] cannot be classified according to a specific chemical type of dyes. Solvent dyes can be found among the azo, disperse, anthraquinone, metal-complex, cationic, and phthalocyanine dyes. The only common characteristic is a chemical structure devoid of sulfonic and carboxylic groups, except for cationic dyes as salts with an organic base as anion. Solvent dyes are basically insoluble in water, but soluble in the different types of solvents. Organic dye salts represent an important type of solvent dyes. Solvent dyes also function as dyes for certain polymers, such as polyacrylonitrile, polystyrene, polymethacrylates, and polyester, in which they are soluble. Polyester dyes are principally disperse dyes (see Section 3.2). 

Such hydrophobic interactions are important characteristics of many biological systems. Many biomolecules (e.g., proteins) possess both hydrophilic and hydro-phobic features in the same molecule and are therefore called amphiphilic. Hydro-phobic amino acids of proteins tend to be buried in the interior of the protein. Hydrophilic amino acids are at the surface of the protein, exposed to solvent water. 

Unlike the thermosetting resins, the thermoplastic resins will soften on heating or on contact with solvents. They will then harden on cooling or on evaporation of the solvent from the material. This is a result of the noncrosslinked chemical structure of thermoplastic molecules. The following are important characteristics of thermoplastic resins that can affect their joining capability. 

An important characteristic of correlations based on eqn (5) is the tendency of different pairs or groups of solvents to form separate correlation lines (Fig. 19). Whilst some of this behaviour may be attributed to the choice of t-butyl chloride as model compound, there are probably a variety of more subtle solvation effects causing slight differences in behaviour even for solvents as similar as... 

A dilute ceramic suspension has Newtonian rheoU. Thus an important characteristic of a dilute suspension is its viscosity. The viscosity of a dilute suspension, rj, is alwa5re higher than that of the pure solvent, Tjj. Using pure hydrodynamics Einstein [10,11] derived an expression relating the viscosity to the volume fraction, , of the dispersed phase ... 

An important characteristic of liquid ionic systems is that they lack an inert solvent they are pure electrolytes. Owing to this characteristic, some aspects of transport phenomena in pure molten salts are simpler than similar phenomena in aqueous solutions. 

Reference to the hydrogen ion, H" ", is also common. However, in the presence of solvent, the extremely small size of the proton (radius approximately 1.5 X 10" pm) requires that it be associated with solvent molecules or other dissolved species. In aqueous solution, a more correct description is H30 (a ), although larger species such as H9O4 are also likely. Another important characteristic of H" " that is a consequence of its small size is its ability to form hydrogen bonds. 

Initially, some relevant thermodynamic and molecular properties of polar solvents are considered. Then, their dielectric properties are considered in detail. Ion solvation in these solvents is also discussed with emphasis on some non-thermo-dynamic methods of dividing experimentally measured data for electrolytes into contributions for the cation and anion. Finally, the important characteristics of the solvent in its direct interaction with the solute, namely, its acidity and basicity, are also described. 

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