Energetics of ionic solvation : Non-structural Treatment.
Born Model (1920)
Born Model (1920)
We consider the following situation.
Initial state: no ion/solvent interactions.
This corresponds to ion in vacuum
Vacuum
Ion
This corresponds to ion in vacuum
.
Final State: ion/solvent interactions operative.
This corresponds to ion in solution.
Thermodynamics states that for reversible
Process taking place at const T & P
Vacuum
Free energy change
GS
Process taking place at const
.
T & P
,
Free Energy change G equals net work
(other than PV work) done on system.
We need to calculate the work done in
Transferring an ion from vacuum into a solvent
Solvent
Charged
Transferring an ion from vacuum into a solvent
.
A solvent such as water has a complex structure.
To simplify our analysis we dispense with structure
Charged
sphere
nucleus
Electron
cloud
ii
qze
i
r
i
r
Net charge
and regard the solvent as a structurless dielectric
continuum. This medium has a dielectric constant
given by SThe ion also has structure (ion +
associated hydration sheath). We represent the ion
hd h f h
d d
nucleus
as
a
c
h
arge
d
sp
h
ere
o
f
net
c
h
arge
qi=
ziean
d
ra
d
ius
ri.
l
Dielectric constant S
Gibbs energy of solvation computed via
Structured solvent Structur
l
ess
Dielectric
continuum
a thermodynamic cycle.
