Types of Molecular Interactions

Equation (1) can be viewed in an over-simplistic manner and it might be assumed that it would be relatively easy to calculate the retention volume of a solute from the distribution coefficient, which, in turn, could be calculated from a knowledge of the standard enthalpy and standard entropy of distribution. Unfortunately, these properties of a distribution system are bulk properties. They represent, in a single measurement, the net effect of a large number of different types of molecular interactions which, individually, are almost impossible to separately identify and assess quantitatively. 

The standard free energy can be divided up in two ways to explain the mechanism of retention. First, the portions of free energy can be allotted to specific types of molecular interaction that can occur between the solute molecules and the two phases. This approach will be considered later after the subject of molecular interactions has been discussed. The second requires that the molecule is divided into different parts and each part allotted a portion of the standard free energy. With this approach, the contributions made by different parts of the solvent molecule to retention can often be explained. This concept was suggested by Martin [4] many years ago, and can be used to relate molecular structure to solute retention. Initially, it is necessary to choose a molecular group that would be fairly ubiquitous and that could be used as the first building block to develop the correlation. The methylene group (CH2) is the... 

The Distribution of Standard Free Energy Between Different Types of Molecular Interactions... 

It was explained in the previous chapter that solute retention and, consequently, solute selectivity is accomplished in an LC column by exploiting three basic and different types of molecular interactions in the stationary phase those interactions were described as ionic, polar and dispersive. 

Thus, by careful choice of solvents, evoked by an understanding of the essential role played by the different types of molecular interactions in the chromatographic process, the solutes of interest cannot only be separated, but also eluted in a reasonable time. 

Interactive LC systems are those where solute retention is predominantly controlled by the relative strengths of the molecular interactions between solute molecules with those of the two phases. In such systems, exclusion and entropically driven interactions will be minor contributions to retention. The three basically different types of molecular interaction, dispersive, polar and ionic give rise to three subgroups, each subgroup representing a separation where one specific type of interaction dominates in the stationary phase and thus governs solute retention. The subgroups are as follows ... 

One important parameter in the solvent characterization is the solvent polarity, which can be defined in different ways [24]. Polarity is connected with solute-solvent interactions, and these depend on the solute. Thus, there are five types of molecular interactions ... 

Dimension of mechanism space also the number of steps (types of molecular interaction) in chemical system. 

Owing to the diverse chemical nature of functional groups in proteins and polysaccharides, they are prone to a variety of types of molecular interactions, both in bulk aqueous media and at air-water or oil-water interfaces. To a first approximation one may consider an adsorbed layer of biopolymers at the interface as simply a special type of highly concentrated biopolymer solution. Thus, the same variety of interactions that are typically found for biopolymers in a bulk aqueous media also occur in biopolymer adsorbed layers at the interfaces in food colloids. Moreover, these same molecular interactions are also involved in the close encounters between pairs of colloidal particles covered by adsorbed biopolymer layers. In the rest of this chapter we shall briefly remind ourselves of the main basic types of intermolecular interactions readers requiring more detailed background information are directed to other sources (Cantor and Schimmel, 1980 Lehninger, 1982 Israelachvili, 1992 Dickinson, 1998 Finkelstein and Ptitsyn, 2002 McClements, 2005, 2006 Min et al., 2008). 

Schreiber and co-workers have noted very persistent history effects in linear polyethylenes (69). Fractions which have been crystallized from dilute solution required times of the order of hours in the melt state at 190° C in order to attain a constant die swell behavior upon subsequent extrusion. The viscosity on the other hand reached its ultimate value almost immediately. The authors concluded from this result that different types of molecular interactions were responsible for elastic and viscous response. However, other less specific explanations might also suffice, since apparent viscosity might be relatively intensitive to the presence of incompletely healed domain surfaces, while die swell, requiring a coordinated motion of the entire extrudate, might be affected by planes of weakness. It would... 

Various types of molecular interactions which occur in mixed mono-layers can be distinguished by simultaneous measurements of the surface pressure, potential, and fluidity of monolayers. Limitations of Goodrich s thermodynamic treatment of mixed monolayers are mentioned. Surface properties of cholesterol have been correlated with its function in biomembranes. 

One limitation of the one-solubility parameter model is that it assumes that the solute can only interact with the organic matter through London forces. Although this assumption may be reasonable for SOM, DOM is typically more polar and can participate in other types of van der Waals interactions. These include permanent dipole-induced dipole (Debye) and permanent dipole-permanent dipole (Keesom) interactions in which the degree of binding that occurs depends on the polarizability of the DOM (Gauthier et al., 1987 Uhle et al., 1999). To account for these types of interactions Chin and Weber (1989) segregated the solubility parameter terms into three components to account for all these different types of molecular interactions to... 

In this section, we describe the application of the GBPM method to different selected complexes. The resulting pharmacophore models have been tested extensively for their capacity to retrieve known ligands for the target. The application examples were selected to be representative of a certain type of molecular interaction, including protein-protein, and DNA-ligand interactions. More application examples will be described elsewhere [13]. 

The adsorbates and adsorbents in this study were chosen to reflect a range of different types of molecular interactions as well as to observe whether the retention volume data trends could be generalized. In addition, solutes were picked which would rapidly equilibrate with the chosen adsorbents (no hysteresis) and whose distribution coefficients could be measured conveniently over as wide a pressure range as possible. As shown in Table I, the adsorbents corresponded to two distinctly different chemical types as classified by the criterion of Kiselev (40). The alumina represented an adsorbent capable of specific interactions with sorbates having localized peripheral... 

The classification of pores is also based on the difference in the types of molecular interactions controlling adsorption in the different groups. That is, in micropores, the overlapping surface forces of opposing pore walls are predominant in mesopores, surface forces and capillary forces are significant, while for the macropores, the contribution of the pore walls to surface forces is very small [15,19],... 

Four general types of molecular interaction lead to a potential energy for adsorption ... 

It has been shown that the free energy of adhesion can be positive, negative, or zero, implying that van der Waals interactions can be attractive as well as repulsive [130,133,134]. While Eq. (14) can, strictly speaking, be expected to hold only for systems that interact by means of dispersion forces only, there are no restrictions on Eq. (15). Since this equation describes very well the fundamental patterns of the behavior of particles, including macromolecules, independent of the type of molecular interactions present, it was found to be convenient to define an "effective Hamaker coefficient that reflects the free energy of adhesion [130],... 

The four main types of HPLC techniques are NP, RP, lEX, and SEC (Section 1.2). The principal characteristic defining the identity of each technique is the dominant type of molecular interactions employed. There are three basic types of molecular forces ionic forces, polar forces, and dispersive forces. Each specific technique capitalizes on each of these specific forces ... 

Polar forces are the dominant type of molecular interactions employed in normal-phase HPLC (see Chapter 5). 

Chiral separations generally rely on the formation of transient diastereomeric complexes with differing stabilities. Complexes are defined as two or more compounds bound to one another in a definite structural relationship by forces such as hydrogen bonding, ion pairing, metal-ion-to-ligand attraction, n-acid/ n-base interactions, van der Waals attractions, and entropic component desolvation. In the following sections, the most important types of molecular interactions in chiral separations are discussed. 

Figure 2.23 shows one example of an acyclic host that can recognize a guest molecule through the cooperative effects of different types of molecular interaction. The recognized guest in this case is L-tryptophan, and three different... 

The term hydrogen bond has been introduced to cover a type of molecular interaction of considerable significance, the qualitative effects of which have been extensively observed. 

This means that the interaction curves for the various types of molecular interaction all have minima at the same value of r, but the depth of the minimum varies from case to case (c/. fig. 25.3). 

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