Molecular sizes, critical, hydrocarbons

The critical loci of binary systems composed of normal paraffin hydrocarbons are shown in Figure 2-16.2 Obviously, the critical pressures of mixtures are considerably higher than the critical pressures of the components of the mixtures. In fact, a larger difference in molecular size of the components causes the mixtures to have very large critical pressures. 

Particulate soils arise from dust, dirt, soot, hydrocarbons, metal oxides and even from hair products based on materials such as silicas or aluminas from about 1pm to less than 0.1-pm particle size see Figure 5-3. The removal of particulate soil is not controlled by the hydrophilicity of the fiber surface. Particulate soil removal depends on the bonding of the particle to the surface, the location of the particle [14], and the size of the particle. Particle size is perhaps the most critical variable for the removal of particulates. As the particle size decreases, the area of contact with the fiber increases, making it more difficult to remove from the hair. At particle sizes of less than 0.1 pm, it is very difficult to remove material from hair surfaces by ordinary shampooing [15]. When the soil particle consists of nonpolar components, its adhesion depends mainly on Van der Waals forces (e.g., waxes or polymeric resins and dimethicone polymers and the molecular size and shape are critical to their removal). Unless very high molecular weights are involved, the removal of such soils is oftentimes easier than for cationic polymers where adhesive binding includes a combination of ionic and Van der Waals forces. 

Bansal, studied the adsorption desorption isotherms of benzene, toluene and o-xylene on sugar charcoal associated with varying amounts of the carbon-oxygen surface groups and observed that the area of the hysteresis loop decreases as the molecular dimensions of the adsorbate increase from benzene to o-xylene (Table 2.7). The point of inception of the hysteresis loop was also found to shift to lower relative vapor pressures as we move up the series of hydrocarbons, which is due to an increase in the molecular size of the adsorbate. The point of inception of hysteresis loop was calculated using Cohn postulates and compared with the values read from the experimental curves (Table 2.8). It is seen that the two values agree closely for all the adsorbate-adsorbent systems. Higute, from thermodynamic considerations, also proposed that the critical radius for the inception of capillary condensation is equal to four times the molecular radius of the adsorbate. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

When the critical properties of the two mixture components differ substantially, type-III phase behavior is usually observed. The critical properties of a given substance are a function of the molecular weight, structure, and intermolecular forces between the molecules. For binary mixtures comprised of normal hydrocarbons, type-III behavior occurs when the size difference between the components reaches a certain value. The occurrence of three phases in this instance is an entropically driven phenomenon since the enthalpic interactions between two different normal hydrocarbons should be indistinguishable from the interactions between two of the same hydrocarbons. 

The current information on size, structure and chemistry of diamond nuclei is primarily speculative, with a small number of conclusive results. It has been proposed that diamond nuclei may be multiple twinned particles, likely containing some of the structures related to the boat-boat conformer of bicyclodecane (10 carbon atoms) or boat-chair-chair-boat tetracyclo octadecane (18 carbon atoms) within higher molecular weight compounds formed by the partial hydrogenation of graphitic or polyaromatic hydrocarbons. The diameter of a critical nucleus of diamond is presumably around 3 nm. 

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