Molecular volume difference method surface

A quite different means for the experimental determination of surface excess quantities is ellipsometry. The technique is discussed in Section IV-3D, and it is sufficient to note here that the method allows the calculation of the thickness of an adsorbed film from the ellipticity produced in light reflected from the film covered surface. If this thickness, t, is known, F may be calculated from the relationship F = t/V, where V is the molecular volume. This last may be estimated either from molecular models or from the bulk liquid density. 

Immersion of dry samples in liquids of different molecular size This method is designed to take advantage of molecular sieving. The basic data are simply in the form of a curve of the specific energy of immersion versus the molecular size of the immersion liquid. This provides immediate information on the micropore size distribution. For room-temperature experiments one can use the liquids listed in Table 8.1, which are well suited for the study of carbons. Because of the various ways of expressing the critical dimension of a molecular probe or its molecular size , one must be careful to use a consistent set of data (hence the two separate lists in Table 8.1). Again, one can process the microcalori-metric data to compare either the micropore volumes accessible to the various molecules (see Stoeckli et a ., 1996), or the micropore surface areas, as illustrated in Figure 8.5. 

To what extent helium is adsorbed has been of major concern in adsorption studies for both volumetric and gravimetric methods. Until recently, the experimental error was often attributed to the finite adsorption of helium at high pressures, and different remedial methods were suggested [38-40]. The effect of helium adsorption on the gravimetric technique is clearly shown in Eq. (8). The volume difference, AF, will be overestimated if the adsorption of heUum is not negligible. Its effect on the volumetric technique ean be explained in terms of Fig. 1. The volume of the solid phase of adsorbent, F, is experimentally determined by heUum. This volume is sometimes called dead space or hehum volume of the adsorption cell, which is, indeed, the volume of adsorbent inaccessible to the hehum molecules. However, this value is usually taken for the volume of adsorbent inaccessible to the adsorbate molecules. The difference in molecular dynamic size and shape between helium and adsorbate is logically a source of error. The irregular solid surface and/or the complex strueture of micropores inevitably render uncertainty in the determination of P. As a eonsequenee of helium adsorption, the dead volume is underestimated. 

More and more publications have reported the physicochemical properties of some ILs, but the overall amount of property data measured by experimental methods are still not fulfilling the requirements for their broad apphcation, especially, due to the lack of data of IL homologues which would be helpful to improve the selection of more appropriate test candidates for different applications. A recently developed technical approach- based on the experimental data of densities and surface tensions of small number of ionic liquids -enables estimation and prediction of density, surface tension, molecular volume, molar volume, parachor, interstice volume, interstice fractions, thermal expansion coefficient, standard entropy, lattice energy and molar enthalpy of vaporization of their homologues. 

The volume of the solid phase Vp is usually measured by a pycnometric technique, which measures the excluded volume of a pycnometric fluid, whose molecules cannot penetrate the solid phase of PS. A simple example of a pycnometric fluid is helium [55], The pycnometric fluid fill in all void space (pores) accessible to it, and presumably do not adsorb on the surface of PS. In the case of microporous PSs, measurement of the volume accessible for guests with various sizes allows the determination of a distribution of micropores volume vs. the characteristic size of guest molecules. This approach lays the basis of the method of molecular probes. The essence of this method is in the following we have a series of probe molecules with different mean sizes (dl>d2>d3>---). The pycnometric measurements of the excluded volume will give a series The difference A V=Vpi-Vpi(i>j) corresponds to the volume of micropores with pycnometric sizes of d in a range dt[Pg.283]

QSAR methods can be divided into several categories dependent on the nature of descriptors chosen. In classical one-dimensional (ID) and two-dimensional (2D) QSAR analyses, scalar, indicator, or topological variables are examples of descriptors used to explain differences in the dependent variables. 3D-QSAR involves the usage of descriptors dependent on the configuration, conformation, and shape of the molecules under consideration. These descriptors can range from volume or surface descriptors to HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy values obtained from quantum mechanics (QM) calculations. 

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