Thermodynamics and Chemistry

CHAPTER 12 EQUILIBRIUM CONDITIONS IN MULTICOMPONENT SYSTEMS

12.6 LIQUID–LIQUIDEQUILIBRIA 392

We can define the solubility of B in A as the maximum amount of B that can dissolve
without phase separation in a given amount of A at the given temperature and pressure.
Treating B as a solute, we can express its solubility as the mole fraction of B in the phase
at the point of phase separation. The addition of any more B to the system will result in
two coexisting liquid phases of fixed composition, one of which will have mole fractionxB
equal to its solubility.^10
Consider a system with two coexisting liquid phasesíandìcontaining components A
and B. Letíbe the A-rich phase andìbe the B-rich phase. For example, A could be water
and B could be benzene, a hydrophobic substance. Phaseíwould then be an aqueous phase
polluted with a low concentration of dissolved benzene, and phaseìwould be wet benzene.
xíBwould be the solubility of the benzene in water, expressed as a mole fraction.
Below, relations are derived for this kind of system using both choices of standard state
or reference state.

Solute standard state

Assume that the two components have low mutual solubilities, so that B has a low mole
fraction in phaseíand a mole fraction close to 1 in phaseì. It is then appropriate to treat
B as a solute in phaseíand as a constituent of a liquid mixture in phaseì. The value ofxBí
is the solubility of liquid B in liquid A.
The equilibrium when two liquid phases are present is B(ì)ïB(í), and the expression
for the thermodynamic equilibrium constant, with the solute standard state based on mole
fraction, is

KD

aíx;B

aìB

D

x;íB (^) x;íBxíB
Bì (^) BìxìB

(12.6.1)

The solubility of B is then given by

xíBD

Bì (^) BìxBì
x;íB (^) x;íB

K (12.6.2)

The values of the pressure factors and activity coefficients are all close to 1 , so that the
solubility of B in A is given byxíBK. The temperature dependence of the solubility is
given by
d lnxBí
dT



d lnK

dT

D

Åsol,BH

RT^2

(12.6.3)

whereÅsol,BHis the molar enthalpy change for the transfer at pressurepof pure liquid
solute to the solution at infinite dilution.
H 2 O andn-butylbenzene are two liquids with very small mutual solubilities. Figure
12.8on the next page shows that the solubility ofn-butylbenzene in water exhibits a mini-
mum at about 12 C. Equation12.6.3allows us to deduce from this behavior thatÅsol,BH
is negative below this temperature, and positive above.

(^10) Experimentally, the solubility of B in A can be determined from thecloud point, the point during titration of
A with B at which persistent turbidity is observed.