Molecular occupied area

The cross-sectional area of the hydrophobic chain or of the hydrophilic head group, whichever is greater, determines the molecular occupied area. Since the adsorbing species forming the Gibbs mono-layer has a sufficiently large solubility in the solvent, when the area of the saturated surface is more expanded, the surface-active molecules dissolved in the solution will fill in the created area. 

Fig. 2 (a) II-A isotherms of pure Cig, C24 and of (C16/C24) mixed monolayers with molar fractions of75/25,50/50 and 25/75 on the water surface at Tgp of 293 K, (b) composition dependence of the collapse, transition and plateau pressures on the monolayer, (c) composition dependence of molecular occupied area at constant surface pressures of 5, 10, 15 and 20 mN m ... 

A distinction between the restrictions of the translation and the rotation could be made by comparing various gases, including the noble gases which have no rotations. In drawing the conclusions we found that the area occupied per molecule also proved to be an important entity. Rotating molecules show a molecular surface area which is approximately equal to the value of the two-dimensional van der Waals b( 2) 185,186). 

The quantity A/ /L does not involve the translational free energy change of a surfactant molecule since the latter was included in the mixing entropy contribution. The change from an LC crystallized solid to a disordered liquid is considered to occur in two steps the first is from the LC crystallized solid (called below LC solid) to a liquid (called LC liquid), in which the molecules become somewhat disordered but occupy the same surface area Ac and the second step is from the LC liquid to the LE liquid, in which the molecular surface area varies from Ac to Ad- The former is a kind of melting process and has a melting free energy... 

Figure 8 illustrates the occupied cells reported [11, 13] for the reference databases using a six dimensional BCUT chemistry space and six bins per axis (charge, electronegativity, H-bond donor, two types of polarizability, molecular surface area) derived using DiverseSolutions. 

In the derivation of Equation 5.6, some simpUfying and contestable assumptions have been made [50,51]. However, the main conclusions that may be inferred from this equation remain valid R depends on the alcohol interfacial concentration via Xgthanoi and on the type of alcohol through fl " and flc jT - Clearly, ethanol IS related to the (total) ethanol concentration in solution water-ethanol, and to its chemical structure, as these parameters determine the partition of the alcohol between the aqueous, oleic, and interfacial regions. Increasing the added alcohol concentration does not entail enhanced water solubilization. The ratios fl /fl jT and Ms/Athanoi (where 5, denotes the molecular surface area occupied by the component i at the interface) determine whether water solubilization (and consequently Aj) is increased, is decreased, or remains constant with increasing Aithanol [71,72]. 

In addition to the electrostatic contribution to the free energy, there are also components from van der Waals and hydrogen bonding interactions with the solvent as well as the energy necessary to create the solvent cavity occupied by the macromolecule. These free-energy terms are often estimated on the basis of molecular surface area " ... 

This observation that the length of the hydrocarbon chain could be varied from 16 to 26 carbon atoms without affecting the limiting area could only mean that at this point the molecules were oriented vertically. From the molecular weight and density of palmitic acid, one computes a molecular volume of 495 A a molecule occupying only 21 A on the surface could then be about 4.5 A on the side but must be about 23 A long. In this way one begins to obtain information about the shape and orientation as well as the size of molecules. 

In their pioneer work, Brunauer and Emmett adopted the value a (Ar) = 13-8 for the molecular area of argon, by insertion of the liquid density Pi in the standard equation (2.27). The same figure was recommended by McClellan and Harnsberger " as a result of their comprehensive survey of the literature, already referred to. These workers noted that the recorded values of a (based on a (N2) = 16 2 A ) extended over the wide range 10-19 A, and concluded that the area occupied per molecule of argon in the completed monolayer varied from one adsorbent to another. 

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