Nonelectrostatic interactions

The commonly used method for the determination of association constants is by conductivity measurements on symmetrical electrolytes at low salt concentrations. The evaluation may advantageously be based on the low-concentration chemical model (lcCM), which is a Hamiltonian model at the McMillan-Mayer level including short-range nonelectrostatic interactions of cations and anions [89]. It is a feature of the lcCM that the association constants do not depend on the physical... 

Therefore, the activity coefficients in solutions are determined primarily by the energy of electrostatic interaction w j between the ions. It is only in concentrated solutions when solvation conditions may change, that changes in (but not the existence of) solvation energy must be included, and that nonelectrostatic interactions between ions must be accounted for. 

Fig. 7.4, curve 4). By analogy with this behavior, it has been proposed as a way of accounting for nonelectrostatic interaction forces between ions, to supplement Eq. (7.43) with an additional term b c or bl ... 

Another special feature of ionic liquids is the lack of a foreign ( inert ) molecular medium, particularly a solvent, between the ions. Hence, they lack ion-molecule and the many types of nonelectrostatic interactions. 

The Debye-Hiickel term, which is the dominant term in the expression for the activity coefficients in dilute solution, accounts for electrostatic, nonspecific long-range interactions. At higher concentrations, short-range, nonelectrostatic interactions have to be taken into account. This is usually done by adding ionic strength dependent terms to the Debye-Hiickel expression. This method was first outlined by Bronsted [5,6], and elaborated... 

In the following, the modelling of solute-solvent interactions at the interface will be discussed in Section Electrostatic and in Section Nonelectrostatic Interactions (cavitation, dispersion, and repulsion). Applications of the methods developed to the modelling of molecular properties at liquid surfaces will be described in the Section Energetics and Properties at Liquid-Gas and Liquid-Liquid Interface. 

The sharp dielectric surface was implemented for the Polarizable Continuum Model (PCM) for the first time by Bonaccorsi et al. [9] and further developed by Hoshi etal. [10] The only requirements for the employment of the PCM is a knowledge of the constitutive parameters of the system geometry of the dielectrics and corresponding dielectric constants. The same model has been subsequently revisited in 2000 [11] supplemented with the modelling of nonelectrostatic interactions (see later). 

The first attempt to describe nonelectrostatic interactions at the interface has been done by extending the semiempirical models [19,20] of such interactions to a sharp interface. In order to illustrate how semiempirical models are extended to the interface let us consider the dispersion interaction. In bulk solution dispersion is modelled through a sum of interatomic interactions between the solute atoms and the solvent atoms. The assumption of a uniform distribution of the solvent atoms allows us to obtain a closed... 

Because of their crucial role in the comparison between theoretical modelling and experiments, nonelectrostatic interactions have recently been reconsidered for diffuse interfaces. In particular, the introduction of repulsion proved to be the key to obtaining agreement between the CM and experimental findings. In particular, it allowed the surfactant behaviour of halides [17] and [18] to be modelled. 

Inside the sphere where the interactions take place, the use of statistical mechanics is required. To represent a dielectric with dielectric constant , consisting of polarizable molecules with a permanent dipole moment, Frohlich [6] introduced a continuum with dielectric constant s X, in which point dipoles with a moment id are embedded. In this model, id has the same nonelectrostatic interactions with the other point dipoles as the molecule had, while the polarizability of the molecules can be imagined to be smeared out to form a continuum with dielectric constant 00 [7]. 

NaF). The bromide ion is no longer adsorbed when the potential is about - 0.25 V versus E in the same solution. Adsorption of an anion at a negative rational potential is a clear indication that "chemical" or some other "nonelectrostatic" interactions are involved. The negative potential needed to drive an anion out of the interphase is a quantitative measure of the strength of the chemical bond holding it there. 

Our ability to understand the structure and properties of water in all its forms has been dramatically enhanced by the use of computer simulation. Early studies of the liquid used simple representations of the potential surface. These were often three or four point-charge distributions, adjusted to fit dipole and quadrupole moments, embedded in a simple spherical nonelectrostatic interaction. The simulations used classical Monte Carlo (MC) or molecular dynamic (MD) calculations, and the water molecules were assumed to be rigid. Recently, more advanced calculations have been based on quantum simulations, the introduction of intramolecular degrees of freedom, and accurate potential surfaces. As one side benefit... 

This calculation relies on the prediction of the dissociation constant of water, K, at elevated temperatures. The prediction involves the dielectric constant of water and a power series consistent with non-electrostatic interactions in the absence of a dielectric medium (12). The dissociation constant of water was calculated based upon the assumption that the entropy of the dissociation of water is the sum of the contributions of the electrostatic and nonelectrostatic interactions. The deviation between the predicted and the experimental (12) values of pK is smaller than 0.01. 

Electrostatic interactions appear when the adsorptive is an electrolyte that is dissociated or protonated in aqueous solution under the experimental conditions used. These interactions, which can be either attractive or repulsive, strongly depend on the charge densities of both the carbon surface and the adsorptive molecule and on the ionic strength of the solution. The nonelectrostatic interactions are always attractive and can include van der Waals forces and hydrophobic interactions. The factors that influence the adsorption process are the characteristics of the adsorbent and the adsorptive, the solution chemistry, and the adsorption temperature. 

The adsorption of organic electrolytes is a more complicated process than that of nonelectrolytes because it is a complex interplay between electrostatic and nonelectrostatic interactions. In this section, I present results obtained with three representative types of organic electrolytes phenol and its derivatives, dyes, and surfactants. These observations demonstrate the importance of the surface chemistry of carbons on the adsorption processes. 

Adsorption of a complex mixture such as NOM, in which polyelectrolytes predominate, involves both electrostatic and nonelectrostatic interactions as weU as molecular sieving effects. Several researchers [53—55] reported that the adsorption of humic substances altered the surface properties of carbons because their... 

The influence of the pore size distribution of carbon on NOM uptake has been recognized by several researchers [57, 63]. Likewise, Karanfil and coworken [64] and Kilduffand coworkers [65] concluded that the adsorption of humic substances was largely governed by molecular size distribution in relation to pore size. Moreover, a good linear relationship (Fig. 25.6) was foundbetween the amount adsorbed by different carbons and their pore volume between 0.8 and 50 nm [61, 66] when the adsorption was carried out at pH 3. This is because electrostatic effects are minimized under these experimental conditions and nonelectrostatic interactions predominate. The adsorption mechanism would be due to hydrophobic and/or TT-TT-electron interactions, and in this case as with other electrolytes (see above). 

This chapter shows that a unified explanation can be given of the adsorption from dilute aqueous solutions of different organic solutes, from nonelectrolytes to electrolytes, polyelectrolytes, and bacteria. Thus, the adsorption process is a complex interplay between electrostatic and nonelectrostatic interactions. Electrostatic interactions depend on the solution pH and ionic strength. The former controls the charge on the carbon surface and on the adsorptive... 

Adsorption will take place into pores that are appropriate to the molecular size of the adsorptive when nonelectrostatic interactions are the driving forces that control the adsorption process. This condition can be attained by controlling the surface chemistry of the carbon and the pH and ionic strength of the solution. 

Sedlak M. On the possible role of nonelectrostatic interactions in the mechanism of the slow polyelectrolyte mode observed by dynamic light scattering. J Chem Phys 1994 101 10140-10144. 

Saveiyev, A., Materese, C. K., and Papoian, G. A. (2011a). Is DNA s rigidity dominated by eiectrostatic or nonelectrostatic interactions Journal of the American Chemical Society 133,48, pp. 19290-19293. 

All these factors combined are of great importance when considering PE adsorption to charged surfaces. We distinguish between physical adsorption, where chain monomers are attracted to surfaces via electrostatic or nonelectrostatic interactions, and chemical adsorption, where a part of the PE (usually the chain end) is chemically bound (grafted) to the surface. In all cases, the long-ranged repulsion of the dense layer of adsorbed PEs and the entropy associated with the counterion distribution are important factors in the theoretical description. 

Here, t is proportional to the still-unknown area per chain, s Rcote), whereas u specifies the relative strength of non-ionic and ionic interactions. When electrostatic interactions are weak compared to nonelectrostatic interactions, m In the opposite limit, i.e., when electrostatic interactions dominate, m 0. 

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