
202 Perturbation Theories for Fluids and Solids
F
Nk
B
T
=
F
0
Nk
B
T
+2πρ
i,j
x
i
x
j
σ
0
ij
3
g
(
0
)
ij
σ
0
ij
ij
. (7.102)
For the particular case of symmetric binary mixtures, σ
1
= σ
2
= σ, the reference
AHS mixture is a single-component HS fluid and, using for the latter the expression of
the free energy and the contact RDF obtained from the CS equation (5.21), expression
(7.102) reduces to
F
Nk
B
T
= 3ln +ln ρ − 1 +
i
x
i
ln x
i
+
(
4 −3η
)
η
(
1 −η
)
2
+12x
1
x
2
(
2 −η
)
η
(
1 −η
)
3
,
(7.103)
which is denoted as MIX1 approximation by Melnyk and Sawford (1975).
Henderson et al. (1972) developed the second-order perturbation theory for mix-
tures with multicomponent reference fluid and Melnyk and ...