Effective ionic forces

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

The polar groups in ionomers are suppressing the tendency of crystallization. Moreover, a ionic crosslinking is effected. Thus, both secondary valency forces and ionic forces are active. The special types of bonds effect a special toughness of the materials. However, ionomers are true thermoplastic materials. 

The forces involved in chemistry are essentially electrostatic. They are variants on the Coulomb force. We can distinguish two orders primary forces and secondary forces. Primary forces are those which hold the atoms together in molecules, and the oppositely charged ions in crystalline salts. Respectively, they are known as covalency and electrovalency (or, sometimes, the ionic force). The latter is directly electrostatic, the mutual attraction between Na+ and Cl" in common salt, for example. The former is usually figured as the sharing of an electron-pair between two atoms— Cl-Cl in the chlorine molecule, where the bond stands for a shared pair of electrons. We need quantum mechanics to understand why, in certain circumstances, electron density builds up in the region between the two chlorine atoms. Granted that it does so, we can explain the covalent bond as due to a resultant electrostatic effect. 

Ionic forces can be very pH dependent. For example, carboxylates (9.5) are in equilibrium with their conjugate acid (9.6), a carboxylic acid (Scheme 9.1). Carboxylic acids have a pKa of approximately 5. If the pH is above 5, the carboxylate form predominates. Below pH 5, the acid is the major form. Therefore, below pH 5, the ability of this functional group to participate in ionic intermolecular forces is greatly diminished. Cations are also subject to pH effects. Especially common in pharmaceuticals are protonated amines (9.7). Alkyl ammonium ions have a pKa of approximately 10. 

The decrease of the concentration of the electroactive species with increasing potential has to be attributed to double layer effects. As first pointed out by Frumkin [58], in dilute solutions the electron transfer rate is affected by variations of the potential in the double layer in two ways. The potential in the outer Helmholtz plane, fa, is due to the extension of the double layer not identical to the potential in the solution (at the end of the double layer), so that the effective driving force of the reaction is DL — fa. Furthermore, the concentration of ionic reactants in the reaction plane, c, is influenced by electrostatic effects and differs from the concentration just outside the double layer, c0, by a Boltzmann term ... 

In addition to above phenomena, there is the sizable contribution of hydrophobic effects in our nanoparticulate system. As the opposite charges of interacting molecules are neutralized, and hydrophobicity rises, the particle is instantaneously formed. This is supported by our observations on the effect of ionic strength (Fig. 18), leading to an enhancement of release when the salt concentration is lowered. Hydrophobic interactions, in addition to ionic forces, were identified as an important mechanism for drug release from ion exchange resin [64], from the CT/TPP complex [62] and from theoretical calculations of forces involved in the assembly of polyelectrolytes [65]. 

Before proceeding with the review of known effects of ionic forces on poljmiers, it is advisable to discuss the various ways in which ions can... 

With regard to the network poljuners specifically, is there a uniform and predictable way in which the introduction of purely ionic forces (in the form of network modifiers) affects the glass transition or do the effects depend much more on the nature of the network ... 

Dipole-dipole forces are generally stronger than London dispersion forces. However, both of these forces between molecules are usually much weaker than ionic forces in crystals. There are exceptions. One major factor is the size of the atoms, ions, or molecules. The larger the particles are, the farther apart they are and the smaller the effects of the attraction are. If an ionic substance has very large ions—especially if the ions are not symmetrical—the ionic substance s melting point can be very low. A few ionic compounds are even liquid at room temperature, such as 1-butylpyridinium nitrate, shown in Figure 15. 

However, there are systems with gross deviations from these predictions. Liquid-liquid immiscibility was observed with some of these salts even in aqueous solutions [37, 71]. In such cases, the ionic forces are not expected to drive the phase separation. Rather, solvophobic effects of salts with large ions in solvents of high cohesive energy density may be responsible for these transitions [37],... 

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