Molecular surface area determination

The molecular surface area (Area) descriptor is a 3D spatial descriptor that describes the van der Waals area of a molecule. The molecular surface area determines the extent to which a molecule exposes itself to the external environment. This descriptor is related to the binding, transport properties and the solubility of compounds. 

Molecular area, a (Ar) of argon at 77 K on graphitized carbon blacks (Argon BET plots constructed with p (liquid) surface areas determined by BET-nitrogen, with a (Na)... 

Benzene has enjoyed some popularity as an adsorbate for surface area determination over a number of years. It can be used conveniently at temperatures around ambient, but assignment of a value to its molecular... 

Kelder, J., Grootenhuis, P. D. J.. Bayada, D. M., Delbressine, L. P. C., Ploeman, J.-P. Polar molecular surface area as a dominating determinant for oral absorption and brain permeation of drugs. Pharm. Res., 1999, 16, 1514-1519. 

Pearlman, R. S. (1986) Molecular surface area and volume Their calculation and use in predicting solubilities and free energies of desolvation. In Partition coefficient, Determination and Estimation. Dunn, III, W. J., Block, J. H., Pearlman R. S., Eds., pp. 3-20, Pergamon Press, New York. 

Traube s rule accommodates the balance between hydrophobicity and hydro-philicity. It has been extended somewhat and formalized with the development of quantitative methods to estimate the surface area of molecules based on their structures [19, 237]. The molecular surface area approach suggests that the number of water molecules that can be packed around the solute molecule plays an important role in the theoretical calculation of the thermodynamic properties of the solution. Hence, the molecular surface area of the solute is an important parameter in the theory. In compounds other than simple normal alkanes, the functional groups will tend to be more or less polar and thus relatively compatible with the polar water matrix [227,240]. Hence, the total surface area of the molecule can be subdivided into functional group surface area and hydro carbonaceous surface area . These quantities maybe determined for simple compounds as an additive function of constituent groups with subtractions made for the areas where intramolecular contact is made and thus no external surface is presented. 

In the literature, many different approaches have been proposed for estimating the surface area of a solid. Surface areas may be estimated from the exclusion of like charged ions from a charged interface. This method is intriguing in that no estimation of either site or molecular area is needed. In general, however, surface area determination by means of solution adsorption studies, while convenient experimentally, may not provide the most correct information. Nonetheless, if a solution adsorption procedure has been standardized for a given system by means of independent checks, it can be very useful determining relative areas of a series of similar materials. In all cases, it is also more real as it is what happens in real life. 

According to Eq. (25), the equilibrium radius R of the island is the product of two factors one of them is the equilibrium radius in the absence of other islands (C = 0), and the other one increases exponentially with the area fraction of the islands. R is determined by the line tension (2) and the difference Ap between the two-dimensional dipole moment densities of the LC islands and the LE molecules. Both X and Ap depend on the molecular surface area difference (jAd — Ac) between the LC and LE phases. Since Ac is assumed to be constant and is considered to... 

For. a number of reasons, nitrogen (at 77 K) is generally considered to be the most suitable adsorptive for standard surface area determination and for this purpose it is usually assumed that the BET monolayer is close-packed (with the molecular area taken as 0.162 nm2). One advantage of nitrogen is that the path of its multilayer isotherm is not very sensitive to differences in adsorbent structure. A useful check on the validity of nm is that the value of C(BET) should be neither too low nor too high if C(BET) < 50, Point B is not sufficiently sharp if C(BET) > 200, there is either a significant micropore filling contribution or localized adsorption on specific sites. 

A concomitant benefit of this technique is that estimates of the molecular surface area and volume are generated as by-products of the contouring routines, whether the surface is being drawn around one or several molecules. Additionally, the generated surfaces and volumes are readily susceptible to logical operations, such as union, intersection, or subtraction, enablingthe rapid determination of, for example, union or difference volumes among a series of molecules. 

The Universal Quasi-chemical Theory or UNIQUAC method of Abrams and Prausnitz divides the excess Gibbs free energy into two parts. The dominant entropic contribution is described by a combinatorial part ( ). Intermolecular forces responsible for the enthalpy of mixing are described by a residual part ( ). The sizes and shapes of the molecule determine the combinatorial part, which is thus dependent on the compositions and requires only pure component data. Since the residual part depends on the intermolecular forces, two adjustable binary parameters are used to better describe the intermolecular forces. As the UNIQUAC equations are about as simple for multi-component solutions as for binary solutions, the UNIQUAC equations for multicomponent solutions are given below. Species are identified by subscript i, subscript j is a dummy index. Here, is a relative molecular surface area and r, is a relative molecular volume. Both of these quantities are pure-species parameters. 

A number of different methods are available for obtaining prefractal pore shape characteristics (Sahimi, 1993 Russ, 1994). We will focus on adsorption and image analysis, since these are the most direct and widely used methods. Avnir et al. (1983) and Pfeifer and Avnir (1983) pioneered the development of adsorption techniques to characterize pore surface properties. Their original idea was that different-sized molecules could be used as yardsticks to measure the area of a prefractal surface as a function of the size of the yardstick. Monolayer coverage (that is typically determined from an adsorption isotherm) for various species with different molecular surface areas, co, can then be shown to satisfy the relation,... 

R. S. Pearlman, in Partition Coefficient Determination and Estimation, W. J. Dunn, J. H. Block, and R. S. Pearlman, Eds., Pergamon Press, New York, 1986. Molecular Surface Area and Volume Their Calculation and Use in Predicting Solubilities and Free Energies of Desolvation. J. S. Murray, P. Lane, T. Brinck, K. Paulsen, M. E. Grice, and P. Politzer, J. Phys. Chem., 97, 9369 (1993). Relationships of Critical Constants and Boiling Points to Computed Molecular Surface Properties. 

Adsorption from the gas phase is commonly applied in determining the specific surface area of finely dispersed materials. For that purpose, assumptions have to be made concerning the dimensions of the gas molecules and the structure of the adsorbed layer under saturation conditions (fully packed monolayer, multilayer, etc.). Small gas molecules may enter pores and capillaries in porous materials. Hence, by comparing the surface area determined by gas adsorption with the outer surface area obtained from, for example, electron microscopy, the porosity of the material can be estimated. Moreover, by using different types of gas having different molecular dimensions, an impression of the pore size distribution may be obtained. 

The lipid layer is highly compressible with almost zero compressibility modulus at 24°C until the molecular surface area reaches the value of 75 A /molecule (Figure 3). From that point the modulus rises intensely while increases further as the film got into the condensed state. Contrary to that the lipid layer presents a significant compressibility modulus at 36°C from the beginning of the isotherm which is getting higher approx, at the same density as the layer at the lower temperature. The moduli increase almost parallel in the condensed range of the isotherm but the value determined at 36°C remains smaller e.g. the layer is more fluid than that was observed at the lower temperature. 

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]. 

For many years it was established practice to use dyestuffs in aqueous solution for surface area determination. Methylene blue was often employed, but it has been shown that the apparent molecular area is not constant from one type of surface to another and that it cannot be predicted . Giles recommended the use of p-nitrophenol as the solute. This has the advantage that it is fairly small and is soluble in both water and organic solvents (e.g. n-hexane or xylene), but it is evident that its application is dependent on calibration by means of a set of reference materials. 

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