Molecular sized cavity

G. Tovar describes one of the novel chemical applications of modern colloidal systems by using such miniemulsions (in addition to classical suspension polymerization) for molecular imprinting. Here, the stable nanoreactor situation is used to synthesize particle surfaces with molecular sized cavities for biomedically relevant species or species to be separated from each other. Such receptor sites are nowadays preferentially made by the pathways of modern colloid chemistry. 

The most important aspect concerning the catalytic application of zeolites is not this range of potential acid-base properties since that is also available with the amorphous aluminosilicates. Instead it is the presence in these crystalline materials of molecular sized cavities and pores that make the zeolites effective as shape selective catalysts for a wide range of reactions. >53-59... 

Here we make use of Uhlig s model, writing eq. [5.5.14] as the energy required to create a molecular-sized cavity in the solvent. 

It should be realized that the hole theory assumes the existence of atomic or molecular size cavities in a liquid which otherwise has the properties of a free surface. 

Hydrates are solid structures composed of water molecules joined as crystals that have a system of cavities. The structure is stable only if at least one part of the cavities contains molecules of small molecular size. These molecules interact weakly with water molecules. Hydrates are not chemical compounds rather, they are clathrates . 

An alternative explanation for the lower sampling rates of very hydrophobic compounds is that the membrane may become rate limiting again for compounds with large molar volumes and low conformational freedom, dne to the fact that molecular size may be too large to fit into the transient cavities in the LDPE. Combining Eqs. 3.36 and 3.38 gives the condition for membrane controlled uptake... 

Additional experiments were performed to examine the role of the molecular size of the host enantioselectivity. Since the methyl groups in methylated jS-CD orient themselves toward the center of the cavity, it is expected that decreasing the extent of methyl derivatization in jS-CD from 21 (the permethylated jS-CD) to 14 methyl groups increases the effective size of the cavity." " As a matter of fact, the enantioselectivity of valine (kolkL = 0.32 with 21-methyl j8-CD) increases to W l = 0.18 with 14-methyl jS-CD. No significant effect of the cavity size is observed with the smaller alanine. 

When an ion (or a molecule) is dissolved, a cavity must be formed in the solvent to accommodate it. By the increase in the ionic (or molecular) size and by the strengthening of the interaction between solvent molecules, the energy needed for cavity formation increases. Water molecules are strongly bound to each other by... 

The correlation between aqueous solubility and molar volume discussed by McAuliffe [5] for hydrocarbons, and the importance of the cavity term in the solvatochromic approach, indicates a significant solubility dependence on the molecular size and shape of solutes. Molecular size and shape parameters frequently used in quantitative structure-water solubility relationships (QSWSRs) are molecular volume and molecular connectivity indices. Moriguchi et al. [33] evaluated the following relationship to estimate Cw of apolar compounds and a variety of derivatives with hydrophilic groups ... 

Since its discovery, isolation, and purification in the early twentieth century, insulin has been administered to diabetic patients exclusively by injection until the recent introduction of inhaled insulin. Insulin possesses certain physiochemical properties that contribute to its limited absorption from the gastrointestinal tract, and requires subcutaneous injection to achieve clinically relevant bioavailability. With a molecular size of 5.7 kDa, insulin is a moderately sized polypeptide composed of two distinct peptide chains designated the A chain (21 amino acid residues) and the B chain (30 amino acid residues) and joined by two disulfide bonds. Like all polypeptides, insulin is a charged molecule that cannot easily penetrate the phospholipid membrane of the epithelial cells that line the nasal cavity. Furthermore, insulin monomers self-associate into hexameric units with a molecular mass greater than 30 kDa, which can further limit its passive absorption. Despite these constraints, successful delivery of insulin via the nasal route has been reported in humans and animals when an absorption enhancer was added to the formulation. 

The molecular size of a solvent can be characterized in several ways. One of them is to assign the solvent a molecular diameter, as if its molecules were spherical. From a different aspect, this diameter characterizes the cavity occupied by a solvent molecule in the liquid solvent. From a still further aspect, this is the mean distance between the centers of mass of two adjacent molecules in the liquid. The diameter plays a role in many theories pertaining to the liquid state, not least to those treating solvent molecules as hard spheres, such as the scaled particle theory (SPT, see below). Similar quantities are the collision diameters a of gaseous molecules of the solvent, or the distance characterizing the minimum in the potential energy curve for the interaction of two solvent molecules. The latter quantity may be described, e.g., according to the Lennard-Jones potential (Marcus 1977)... 

U/V, equalling the square of the solubility parameters reported in Table 3.1. The work that must be done against this stiffness in order to create cavities that are able to accommodate a solute of given size in a series of solvents is proportional to this quantity. This work is also given by the product of surface tension a of the solvent (Table 3.9) and the surface area of this cavity. (This holds strictly for macroscopic cavities, but is apparently extendable also to molecular sized ones.) For non-associated solvents another measure of their stiffness is the internal pressure, Pl (see Table 4.2), that equals... 

The molecule is often represented as a polarizable point dipole. A few attempts have been performed with finite size models, such as dielectric spheres [64], To the best of our knowledge, the first model that joined a quantum mechanical description of the molecule with a continuum description of the metal was that by Hilton and Oxtoby [72], They considered an hydrogen atom in front of a perfect conductor plate, and they calculated the static polarizability aeff to demonstrate that the effect of the image potential on aeff could not justify SERS enhancement. In recent years, PCM has been extended to systems composed of a molecule, a metal specimen and possibly a solvent or a matrix embedding the metal-molecule system in a molecularly shaped cavity [62,73-78], In particular, the molecule was treated at the Hartree-Fock, DFT or ZINDO level, while for the metal different models have been explored for SERS and luminescence calculations, metal aggregates composed of several spherical particles, characterized by the experimental frequency-dependent dielectric constant. For luminescence, the effects of the surface roughness and the nonlocal response of the metal (at the Lindhard level) for planar metal surfaces have been also explored. The calculation of static and dynamic electrostatic interactions between the molecule, the complex shaped metal body and the solvent or matrix was done by using a BEM coupled, in some versions of the model, with an IEF approach. 

If the standard BET procedure is to be used, it should be established that monolayer-multilayer formation is operative and is not accompanied by micropore filling (Section 11.2.1.8.C), which is usually associated with an increase in the value of C (>200, say). It should be appreciated that the BET analysis does not take into account the possibility of micropore filling or penetration into cavities of molecular size. These effects can thus falsify the BET surface areas and in case of doubt their absence should be checked by means of an empirical method of isotherm analysis or by using surface area reference samples (see Section 11.2.1.6.B). 

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