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Chapter VII Reactivity at Dislocation….
and possess hydrophobic nature. It can be conjectured that the closed structure
with attached branch of L-histidine offers steric hindrance and exhibits lower
reactivity with etchant containing acetic acid and offer lower reactivity with
etchant containing acetic acid and offer high activation energy among all the
three amino acids studied. The hydrophobic nature and non-polar side chain of
DL-methionine has offered higher reactivity compared to its linear counter part
with polar side chain- L- threonine to the reacting etchant.
7.8 Thermodynamic Parameters
The thermodynamic parameters can be estimated for the motions of ledges of
etch pits. Various thermodynamic parameters are obtained from the following
standard relations [81]. The standard enthalpy of activation (∆ # H o ) was calculated
by using the following relation,
∆ # H o = E – 2RT (7.17)
The standard change in entropy of activation (∆ # S o ) could be calculated by
using the following formula,
272
kT
# #
(7.18)
h
0 0
W x exp S / R x exp H / RT
Where, W = Average width of etch pit, k = Boltzmann constant, h = Planck’s
constant, T = Absolute temperature, R = Gas constant
The standard change in Gibbs free energy of activation (∆ # G o ) is possible to
estimate from the following equation
∆ # G o = ∆ # H o – T∆ # S o
(7.19)