Mean area per molecule

Static Involving Use of Adsorption Isotherms BRUNAUER, EMMETT, AND TELLER (B.E.T.). In this method tire surface area is not measured directly, but the number of molecules of the adsorbed substance required to give a monolayer (N) is determined. If the mean area per molecule (a) of the adsorbed substance is known by other means, the area of the solid may... 

All the normal acids show practically the same value of jB at a water-air interface, a value which gives the mean area per molecule of 24-3 0 3 Jl. This value is also given by i-butyric acid at a water-benzene interface. The values of B for the iso-acids are consistently a little larger than those for the normal acids. The values of x show that these acids resemble gases whose critical temperatures increase as the carbon chain lengthens. The iso-... 

Figure 6. (a) Interfacial pressure vs mean area per molecule... 

Figure 8. Mean area per molecule vs mole fraction of...

Recording surface pressure/area isotherms is a relatively simple method for describing the miscibility behavior of a two component lipid mixture. The mean area per molecule of a binary mixture can be calculated using the following equation ... 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

The tt-A isotherms at the air-water interface provide information not only on the orientation of the molecules but also on the interactions between the constituents of the mixed monolayer. The mean area per molecule for a mixed two-component system has been calculated using the additivity... 

Upon illumination of a mixed monolayer of chlorophyll with ferridoxine in the presence of ascorbate, Brody and Owens [22] detected a shght change in the isotherm, which in their opinion is an indication of a reaction between these components. Upon illumination of mixed films of chlorophyll and oxidized cytochrome c, a reduction in the mean area per molecule by about 18% is observed. On the other hand, illumination of films of chlorophyll with reduced cytochrome increases the area per molecule significantly (by about 10%). The difficulty of interpreting very small effects lies in the fact that in the dark the area per molecule increases both for the films with reduced cytochrome c and for the mixture containing reduced cytochrome. At any rate, for the case with reduced cytochrome c, Brody and Owens believe that they observed an electron-transfer reaction stimulated by light ... 

A common problem in the evaluation of binary and ternary systems to which a solute has been added is the basic distinction between the mean area per molecule and the partial molecular surface. When areas per molecules are measured, the link between total area and molar fraction may be highly nonlinear, in particular for the case of solutes denominated as cosurfactants which are solubilized in the palisade layer. The partial molar area is the increase of surface per surfactant due to the addition of a solute at a constant density of surfactant, while the area is the observed average. This problem has been discussed in detail by Boden et al. (5). 

The relevant geometrical parameters controlled by the experiment [22,36-40] are the area and volume of the vesicles. Given a mean area per molecule, the membrane area can be considered fixed at constant temperature and a constant number of molecules. The volume is controlled by the osmolarities of the inner and outer solutions due to the high water permeability of the membrane. Any osmolarity differences would lead to large osmotic forces inducing water flow through the membrane. It is thus the permeability of the membrane to water which keeps the enclosed water volume constant at given external osmolarity. 

It is quite noticeable that these figures are very close to those known for melted paraffins of similar chain lengths. Then the development of NMR studies of deuterated moleoules and of NMR permitted to show that the amplitude of the deformations are about the same in all structures, ordered liquid crystals as well as in disordered micellar solutions. Thus there does not appear any definite correlation between one structure and the molecular behavior in it. The chains are about equally disordered in all structures, in spite of important changes of interfacial curvature and mean area per molecule at the interface. 

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