Non-specific Electrostatic Interactions

In a simple model a neutral molecule can be described through two properties related to its electron distribution, the permanent dipole moment i and the average polarizability oc. There are therefore four electrostatic interactions between a solute molecule and the surrounding solvent molecules, as shown in Table 3.1. 

For practical purposes the solvent is described as a continuum, so that the dimensions of the solvent molecules do not appear explicitly in the interaction energy equations the permanent dipole moments and the polarizabilities of the solvent are expressed as functions of macroscopic properties which are the dielectric constant D and the refractive index n the interaction 

1 Separation of Electronic and Nuclear Motions. The polarizabilities of the ground state and the excited state can follow an electronic transition, and the same is true of the induced dipole moments in the solvent since these involve the motions of electrons only. However, the solvent dipoles cannot reorganize during such a transition and the electric field which acts on the solute remains unchanged. It is therefore necessary to separate the solvent polarity functions into an orientation polarization and an induction polarization. The total polarization depends on the static dielectric constant Z), the induction polarization depends on the square of the refractive index n2, and the orientation polarization depends on the difference between the relevant functions of D and of n2 this separation between electronic and nuclear motions will appear in the equations of solvation energies and solvatochromic shifts. 

2 Dipole-Dipole Interaction. The first of the four terms in the total electrostatic energy depends on the permanent dipole moment of the solute molecule of radius a (assuming a spherical shape) immersed in a liquid solvent of static dielectric constant D. The function f(D) = 2(D - l)/(2D + 1) is known as the Onsager polarity function. The function used here is [f(D) — f(n2)] so that it is restricted to the orientational polarity of the solvent molecules to the exclusion of the induction polarity which depends on the polarizability as of the solvent molecules, related to the slightly different Debye polarity function q (n2) according to 

3 Solute Dipole-Solvent Polarization. In a solvent of refractive index n, a solute molecule of permanent dipole moment /xM is stabilized by interaction with the dipoles induced in the solvent this interaction energy is 

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For an understanding of the various phenomena characteristic of solutions, a quantitative description of the intermolecular forces would be necessary. The possibility of this arises primarily if the non-specific electrostatic interactions are regarded as predominating. Intermolecular forces of such a type between the solvent and the dissolved molecules (ions) are less sensitive to the distance between the particles and to their orientation than are explicit chemical bonds. To a first approximation, the magnitude of these forces can be regarded as independent of the chemical natures of the participating species. 

Besides calcium carbonate and phosphate, only a few minerals have been studied with bio-inspired monolayers. Letellier et al. investigated the formation of calcium oxalate monohydrate at phospholipid monolayers [161]. The authors claim that lattice matching and hydrogen bonding cannot be a dominant factor in the control of crystal orientation. Rather, non-specific electrostatic interactions, similar to those reported for calcium carbonate [162], seem to control the crystallization of calcium oxalate monohydrate. 

Here Vij denotes the distance between atoms i and j and g(i) the type of the amino acid i. The Leonard-Jones parameters Vij,Rij for potential depths and equilibrium distance) depend on the type of the atom pair and were adjusted to satisfy constraints derived from as a set of 138 proteins of the PDB database [18, 17, 19]. The non-trivial electrostatic interactions in proteins are represented via group-specific dielectric constants ig(i),g(j) depending on the amino-acid to which atom i belongs). The partial charges qi and the dielectric constants were derived in a potential-of-mean-force approach [20]. Interactions with the solvent were first fit in a minimal solvent accessible surface model [21] parameterized by free energies per unit area (7j to reproduce the enthalpies of solvation of the Gly-X-Gly family of peptides [22]. Ai corresponds to the area of atom i that is in contact with a ficticious solvent. Hydrogen bonds are described via dipole-dipole interactions included in the electrostatic terms... 

The toxicological profiles of dendrimers and dendritic nanoparticles can be also controlled by their size and surface modifications. Importantly, the charge-based non-specific interaction between positively charged dendrimers and cell membranes are typically associated with toxicity.[84,85] As discussed earlier (Section 2.A non-specific cell interaction), the electrostatic interactions of cationic dendrimers with cell membrane induced membrane disruption, which could be a reason for the apparent toxicity observed in animal studies.[86] Surface modification of terminal amine groups to different groups (charge neutral or... 

The interactions between the solvent and solute, as discussed above, are the result of a number of different specific (coordination, hydrogen bonding) and non-specific (electrostatic) factors therefore, it is not possible to find a single physical parameter characterizing the solvent, which in itself could rationalize the solvation process. Accordingly, it was necessary to introduce empirical parameters serving to characterize the solvent effect. 

The adsorption of anionic, cationic and non-ionic surfactants on to hydrophilic and hydrophobic surfaces in aqueous continuous phases has been considered in Chapter 1. Adsorption of surfactants on to non-polar surfaces occurs via hydrophobic interactions, the hydrocarbon chain adsorbing and lying close to the solid surface adsorption on to polar surfaces can occur by specific electrostatic interactions in which the surface is converted from a hydrophilic surface to a hydrophobic surface by the orientation of the alkyl chains of the surfactants outward into the water (Fig. 9.3). Adsorption of surfactants in this way frequently gives rise to multilayer adsorption by hydrophobic interactions between the primary and secondary monolayers, as shown in Fig. 9.3. Non-ionic surfactants based on polyoxyethylene ethers may also adsorb on to hydrophilic surfaces such as silica in this way. A representation of the orientation of non-ionic surfactants at a silica surface is shown in Fig. 9.3b. Adsorption isotherms for polar and nonpolar systems reflect these different possibilities as has been discussed previously (section 1.4). 

The non-specific electrostatic forces caused by the surface charge attract ions of opposite sign (counterions) and repel ions of same charge (co-ions). Depending on the nature of the counterions, their interaction with the surface will be more or less strong. 

Some ions or molecules are attracted to the surface by non-specific electrostatic attractions and are able to penetrate the Stern layer and bind chemically on surface sites. Most often, these ions are complexing anions, easily hydrolyzable cations or neutral molecules forming true coordination complexes with surface groups. Depending on the strength of the interaction between the adsorbed species and the surface, inner sphere or outer sphere complexes ate formed. These species are called physisorbed or chemisorbed respectively. This phenomenon is known as specific adsorption, or surface complexatioii, and a chemical term appears in the free enthalpy of adsorption. Specific adsorption is further discussed in Chapter 9. 

Independent molecules and atoms interact through non-bonded forces, which also play an important role in determining the structure of individual molecular species. The non-bonded interactions do not depend upon a specific bonding relationship between atoms, they are through-space interactions and are usually modelled as a function of some inverse power of the distance. The non-bonded terms in a force field are usually considered in two groups, one comprising electrostatic interactions and the other van der Waals interactions. 

In some cases, e.g., the Hg/NaF q interface, Q is charge dependent but concentration independent. Then it is said that there is no specific ionic adsorption. In order to interpret the charge dependence of Q a standard explanation consists in assuming that Q is related to the existence of a solvent monolayer in contact with the wall [16]. From a theoretical point of view this monolayer is postulated as a subsystem coupled with the metal and the solution via electrostatic and non-electrostatic interactions. The specific shape of Q versus a results from the competition between these interactions and the interactions between solvent molecules in the mono-layer. This description of the electrical double layer has been revisited by... 

Ionic surfactants also participate in specific chemical and electrostatic interactions with sorbents that do not occur with non-ionic compounds. In fact, recent research has shown that hydrophobic... 

As the electrostatic interaction between the solvated ions and the metal is indirect, it is virtually independent of the chemical nature of the ions these latter are said to be non-specifically adsorbed. 

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