Foreword:
The connection between chemistry and electricity is a very old one, going back to Al discovery in 1793 [1][2], that electricity could be produced by placing two
sides of moistened paper. In 1800 Nicholson and Carlisle using Volta’s primitive battery as a demonstrated that electric current could decompose water into oxygen and hydro
considered one of the most significant experiments in the history of chemistry [and f transport], because it implied that atoms of hydrogen and oxygen were associated with negative electric charges, which must be the source of the bonding forces b
Swedish chemist Berzelius [1] who was experimenting with electrolysis speculated that when an ltage was applied across defined electrodes; this a
vo
opposite charges, pulling the electrified atoms apart in the form of ions (named by Berzelius f Greek for “travellers”). It would take almost a hundred years to the turn of the
behaviours of ions in solution and in electrolytes could be with relative confidence explained. Such advances of understanding were a achieved with theories in the form of the shared electron pair theory of G. N. Lewis [2], Maxwell’s theory of “molecular electricity” , and Debye & Hückel’s theory describing the distribution of charges around an ion in solution; and the distribution of charges about the ion.
inter-ionic fields are distance depende lation density will in turn depend on the nature of the electrolyte itself, i.e. on the e which the electrolyte gives rise to ions in solutions. The Debye-Hückel theory [4] olvents by their bulk properties, namely
lative permittivity
nt. The ionic popu xtent to recognises s ε, viscosityη
re , and density ρ of the pure solvent. In addition, the range of alidity of this theory is restricted to very low concentrations only, and is experimentally
in m s [6]. The importance of the bulk properties
as t eases. At the molecular scale, solvents may be
lassified according to soft/hard donor and acceptor properties of both solvent and the solute.
e and Hückel [4] is a classical image of ion-ion interactions as compared to the ore modern, theoretical approaches of the Born-Oppenheimer internal charge distributions
of one
mu
the nd
polym
2.1 Th
me rse
be f a
liquid together, but their kinetic energies are comparable to their potential energies [1]. As a result, the molecules are free to escape completely from the bulk; hence, the overall structure is very mobile. The best description of the average locations of particles in a liquid v
unattainable any organic solvent system decreases he electrolyte concentration incr c
Although Deby m
solvent molecules. As every good historian knows in order to understand the future, st first understand the past. The Debye-Hückel is still a valuable and viable platform for explanation of the current view of ion-ion interaction behaviours ionic liquids a
er-gels that are of great interest, which was alluded to in section {1}.
.2.
e Debye-Hückel Theory of Ion-Ion Interactions:
In order to understand the ion-ion interactions one must develop a quantitative asure of the interactions of ions [5]. The starting point for this discussion should of cou a short examination of the liquidus state itself. Intermolecular forces hold the particles o
Intial State
is in terms of the pair distribution function - g. In a crystal, g is a periodic array of sharp spikes, representing the certainty that defined particles lie at definite locations. This regularity continues out to large distances and therefore a material is defined as having long-range order. When the crystal melts to a liquid state, the long-range order is lost and is replaced by
No
Ion-Ion Interactions
Work of Ion-Ion
Final State Interaction ∆GI - I
Ion-Ion Interactions
Fig. (2.1.1)
short-range order. This means that for any reference ion within the liquid state; its nearest neighbours might still adopt approximately their original positions, even if new comers displace them and these newcomers might adopt their vacated positions.
Therefore, the basic postulates of the Debye-Hückel theory elude:
• The central ion relates to the surrounding ions in the form of a smoothed-out charge density and not as discrete charges.
• All the ions in the electrolytic solution are free to contribute to the charge density and there is, fo instance, no pairing up of positive and negative ions to form any electrically neutral couple.
• Only short-range Coulombic forces such as dispersion forces play a negligible role.
• The solution is sufficiently dilute to warrant the linearisation of the Boltzmann equation.
r
• The only role of the solvent is to provide a dielectric medium for the operation of inter-ionic forces; i.e. the removal of a number of ions from the solvent that
tral ion is neglected.
Debye and Hückel proposed in their formulation a very simple but powerful model for the tim
solvated ions and water molecules. The first step in the De arbitarily any one ion out of the assembly and call it a refere molecules are looked upon as a continuous dielectric med solution fall back into anonymity, their charges being unde distribution of the opposite sign. It is recognised that there w in the particular region under consideration. The charges of
cling more or less permanently to ions other than the cen
e-averaged distribution of ions in very dilute solutions of electrolytes. From this modelled distribution, they were able to obtain the electrostatic potential contribution from the surrounding ions as depicted schematically in Fig. (2.1.2) and hence the chemical potential change arising from the ion-ion interactions. The electrolyte solution consists of bye-Hückel approach is to select nce ion or central ion. The water ium. The remaining ions of the r-footed into a continuous spatial ill be a rise a net or excess charge the discrete ions that populate the surroundings of the central ion; are thought of as smoothed out and contribute to the continuum dielectric a net charge density. In this way, water enters the analysis in the guise of a dielectric constant ε and the ions except the specific one chosen as the central ion, in the form of an excess charge density ρ Fig. (2.1.3).
Charging Work
Discharged Ions Charged Ions
∆GI - I
Fig. (2.1.2)