Forces in ionic solutions

Patey, G. N. Valleau, J. P., A Monte Carlo method for obtaining the interionic potential of mean force in ionic solution, J. Chem. Phys. 1975, 63, 2334-2339... 

Obtaining the Interionic Potential of Mean Force in Ionic Solution. 

It is a well-known fact that bubbles produced by mechanical force in electrolyte solutions are much smaller than those in pure water. This can be explained by reduction of the rate of bubble coalescence due to an electrostatic potential at the surface of aqueous electrolyte solutions. Thus, k a values in aerated stirred tanks obtained by the sulfite oxidation method are larger than those obtained by physical absorption into pure water, in the same apparatus, and at the same gas rate and stirrer speed [3]. Quantitative relationships between k a values and the ionic strength are available [4]. Recently published data on were obtained mostly by physical absorption or desorption with pure water. 

A useful descriptor of the overall strength of ionic forces in electrolyte solutions is given by the ionic strength I of a solution, defined as... 

Hydration stabilisation. Due to the polarisability of the water molecules, even in a deionised aqueous solution, stabilisation may occur. Positively charged alumina particles, for example, bind preferentially to the negative oxygen of the water molecule. As a result, a double layer is formed, similar to ionic electrostatic repulsion. In ionic solutions, the hydration repulsive force occurs simultaneously with the electrostatic force, while the proportion of the two forces depends on the ionic concentration [22],... 

Fuzzy spheres. Radially varying dielectric response, 79 "Point-particle" interactions, 79 Point-particle substrate interactions, 85 Particles in a dilute gas, 86 Screening of "zero-frequency" fluctuations in ionic solutions, 89 Forces created by fluctuations in local concentrations of ions, 90 Small-sphere ionic-fluctuation forces, 91... 

The CN- problem deserves reinvestigation, not only to establish dynamically the involvement of the named solvent molecule combination bands and to confirm in this context the Coulomb force dominance for the VET, but also to explore the possible role in the VET of any counter ions in the CN- first solvation shell at higher concentrations of the solute (17). The latter issue, which is in principle ubiquitous in ionic solute VET studies, has yet to receive any theoretical attention. Some improvement is... 

From an interpretation of peaks in the far IR spectral region, one can obtain knowledge of hindered translations among water molecules in ionic solutions. Somewhat surprisingly, the force constants associated with such movements are loweredhy the presence of ions because ions free some water molecules from the surrounding solvent structures. Thus, force constants are given by where U is the potential... 

Rau DC, Lee B, Parsegian VA. Measurement of the repulsive force between polyelectrolyte molecules in ionic solution hydration forces between parallel DNA double helices. Proc. Natl. Acad. Sci. U.S.A. 1984 81 2621-2625. 

Ion-dipole forces act between ions and molecules with permanent dipole moments and are next strongest after ionic forces. They are relatively long ranged and are important in ionic solutions. 

Solubility is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. Not all ionic compounds dissolve in water. Whether or not an ionic compound is water soluble depends on the relative magnitudes of the water molecules attraction to the ions, and the ions attraction for each other. We will learn more about the magnitudes of attractive forces in ionic compounds in Chapter 8, but for now it is useful to learn some... 

In this lecture, I will discuss a few of the theories of electrolytes developed since the classic work of Debye and Huckel (1923). Rapid progress in this field came after it was realized that many of the methods used since 1960 to study fluids could also be applied, with some modifications for the long-range coulomb forces, to ionic solutions. 

P. Richmond, /. Chem. Soc. Faraday Trans. 2, 70,1066 (1973). Electrical Forces between Particles with Arbitrary Fixed Surface Charge Distributions in Ionic Solution. 

With the electrostatic approach, ions or charged molecules are attracted to or dissociated from the particle surfaces to produce a system of similarly charged particles. When the repulsive double-lsiyer electrostatic forces between the particles are greater than the attractive Van der Waals forces, the particles repel to produce a dispersed system. The net Interparticle force (In aqueous solutions) can be altered by changing the type of concentration of the ionic species as well as the pH. When the particle charge approaches zero, the particles floe and eventually produce a very open network of touching particles. The zeta potential provides a convenient experimental measure of such forces. 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

Electro-osmosis has been defined in the literature in many indirect ways, but the simplest definition comes from the Oxford English Dictionary, which defines it as the effect of an external electric held on a system undergoing osmosis or reverse osmosis. Electro-osmosis is not a well-understood phenomenon, and this especially apphes to polar non-ionic solutions. Recent hterature and many standard text and reference books present a rather confused picture, and some imply directly or indirectly that it cannot take place in uniform electric fields [31-35]. This assumption is perhaps based on the fact that the interaction of an external electric held on a polar molecule can produce only a net torque, but no net force. This therefore appears to be an ideal problem for molecular simulation to address, and we will describe here how molecular simulation has helped to understand this phenomenon [26]. Electro-osmosis has many important applications in both the hfe and physical sciences, including processes as diverse as water desahnation, soil purification, and drug delivery. 

We may say then that in each of those processes the change AF in the free energy consists of two parts, a unitary part and a communal part. When an ionic solution is not extremely dilute, the free energy of the solution receives a contribution from the interionic forces this quantity depends on the concentration of the solute and is a communal quantity. When, however, the solution is extremely dilute, the interionic contribu-... 

In Sec. 41 it was pointed out that, when we are dealing with a solution that is not formed by a process of one-for-one substitution, this is, by itself, sufficient to make the solution a non-ideal solution—that is to say, is sufficient, by itself, to introduce a communal term that is different from the simple cratic term. Nevertheless, in an ionic solution at any concentration this deviation is small compared with the deviation caused by the electrostatic forces between the ions. In this book it will therefore be sufficient to mention only the interionic forces when speaking of the difference between a communal term and a eratic term. 

The Disparity of a Solution. We may begin to use the word disparity in a technical sense, for the quantity defined above, and to speak of d as the disparity of the solution when the mole fraction of the solute is x. In dilute ionic solutions the sign of d is always negative. The effect of the interionic forces is that ions added to a dilute solution always lose more free energy than they would when added to the corresponding ideal solution hence the total communal term is less than the cratic term. 

The Electrostatic Energy. In Chapter 2 we drew attention to the fact that, when a proton transfer (117) has been carried out in a solvent, the electrostatic fields of two ions have been created and work must have been done to supply the amount of energy associated with these ionic fields. Let us now compare (117) with the process (123), both in aqueous solution at the same temperature. In both cases an (HaO)+ ion will be formed but in (123), when the proton is removed from the (IIS04)-ion, we have to separate the particles against the mutual attraction of the proton and the doubly charged ion (S04)". Consequently, more work must be done against the electrostatic forces of attraction than in the removal of a proton from a neutral particle. 

Properties of Different Solvents. In discussing molecular dipoles in Sec. 25, we estimated the force of attraction between an atomic ion and a dipole having the most favorable orientation and found this attraction to be very strong. In any ionic co-sphere those molecular dipoles which have a favorable orientation will bo attracted, while those that have the opposite orientation will be repelled. Since the former are more numerous the solvent in the co-sphere is, on the whole, attracted toward the ion. Since the liquid is not incompressible, we must expect that this will lead to a contraction in each co-sphere. In any ionic solution the sum of the contractions that have taken place in the co-spheres of the positive and negative ions will be apparent if we measure accurately the volume of the solution. 

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