Classification of Solvents using Physical Constants

The following physical constants can be used to characterize the properties of a solvent melting and boiling point, vapour pressure, heat of vapourization, index of refraction, density, viscosity, surface tension, dipole moment, relative permittivity, polarizability, specific conductivity, etc. A compilation of data of usual organic solvents is given in 

The degree of association of molecules in a liquid can be estimated by means of its Trouton constant [117]. At the normal boiling temperature, Tbp, vapourization proceeds with standard molar changes of enthalpy, and entropy, A5(]p, from which Trou- 

This rule works best for apolar, quasi-spherical molecules. Large deviations occur when chemical association is involved [e.g. carboxylic acids), from molecular dipolarity [e.g. dimethyl sulfoxide), and from molecular asphericity e.g. neopentane/n-pentane). Strongly associating solvents e.g. HF, H2O, NH3, alcohols, carboxylic acids) have Trouton constants which are higher than the average value of 88 J mol K found for non-associating solvents such as diethyl ether and benzene. 

In addition to Trouton s rule, some other parameters for measuring the structuredness of solvents have been recommended, for example a solvent dipole orientation correlation parameter [175, 200], the solvent s heat capacity density [175, 200], and a so-called Ap parameter derived from the solvent s enthalpy of vapourization minus EPD/ EPA and van der Waals interactions [201], According to these parameters, solvents can be classified as highly structured e.g. water, formamide), weakly structured e.g. DMSO, DMF), and practically non-structured e.g. -hexane and other hydrocarbons) [200, 201]. 

In this connection, two other physical solvent properties are important the cohesive pressure c (also called cohesive energy density) and the internal pressure r of a solvent [98-100, 175]. 

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Detectors can be broadly classified into two types. Bulk Property detectors which function by measuring some bulk physical property of the column eluent (e.g. dielectric constant or refractive index) and Solute Property detectors which function by measuring a physical and/or chemical property that is characteristic of the solute only (e.g. UV absorption). This classification is not completely precise, for example, the UV detector, which is usually classed as a solute property detector, when used with an ethyl acetate-heptane solvent mixture as the mobile phase will give a constant background signal due to UV absorption by the ethyl acetate. Furthermore, any fluctuation in ethyl acetate content of the mobile phase will appear as noise on the detector output. It follows that the UV detector, although a solute property detector, behaves as a hybrid between a bulk property detector and a solute property detector under some conditions of use. 

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