SECOND LECTURE
Systems containing two liquid components.
Two Component Systems Containing Liquid
Phases:
miscible
partially miscible
immiscible
ethyl alcohol and water
phenol and water
water and mercury
Phenol and water system:
miscible
Partially miscible
Two factors affecting
misciblity
1- Concentration of
phenol in water.
2- Temperature.
Two component systems:
Phenol
Phenol / water
water
The curve g b h c i shows limits of temperature and
concentration within which two liquid phases exist in
equilibrium.
2 phases
1 phase
Point A
100% water
10 % phenol
11 % phenol
water rich phase
contains water+ phenol(11%)
Phenol rich phase
contains Phenol (63%)+ water
> 63 % phenol
1 phase
24% phenol
The curve g b h c i shows limits of temperature &
concentration within which two liquid phases exist in
equilibrium.
Point A
100% water (pure water)
Phenol
Point B (11 % phenol)
2 phases
water rich phase& phenol rich phase
More Phenol
Point C ( >63% phenol)
1 phase
Completely miscible
The Tie Line
It is always parallel to the base line
in two component systems.
All systems prepared on a tie line,
at equilibrium, will separate into
phases of constant composition.
known as conjugate phases.
Any system represented by a point
on the line bc , at 50oC.
separates to give a pair of
conjugate phases whose
composition is 11% phenol
in water rich phase (A) & 63 %
phenol in phenol rich phase.
Importance of Tie line:

Calculation of the composition of each phase.

Determination of the weight of each phases.
(calculation of the distribution of phenol (or
water) throughout the system as a whole.
F = 2 - 1 +1
2
F= 2 - 2 +1
1
The use of Tie line in calculations:






As an example, let us suppose that we mixed 24 g of
phenol with 76 g of water, warmed the mixture to 50oC,
and allowed it to reach equilibrium at this temperature.
Weight phase A/weight of phase B = dc/bd =6324/24 -11=39/13 =3/1
Weight of A= ¾ x 100= 75, wt. of B = ¼ x 100= 25
Phase A=75 gm , phase B =25 gm.
Amount of phenol in A=75 x 11/100= 8.25 gm
Amount of phenol in B= 25 x 63/100= 15.75 gm
24
gm
Application of Tie line:


To formulate systems containing more than one
component where it may be advantageous to
achieve a single-phase product.
Handling of solid phenol, a necrotic agent
(caustic agent), is facilitated in the pharmacy if a
solution of phenol and water is used. The most
convenient formulation of a single liquid phase
solution was 80% w /v, equivalent to about 76%
w / w. This mixture has a freezing point of
about 3.5oC
The Critical Solution Temperature: CST


Is the maximum temperature at which the 2-phase
region exists (or upper consolute temperature). In the
case of the phenol-water system, this is 66.8oC (point h)
All combinations of phenol and water > CST are
completely miscible and yield 1-phase liquid systems.
Systems Showing a Decrease in Miscibility
with Rise in Temperature:

A few mixtures, exhibit a lower
critical solution temperature
(low CST), e.g. triethylamine
plus water. The miscibility
with
in temperature.

In the preparation of paraldehyde
enemas, (consist of a solution of
paraldehyde in normal saline).

Cooling the mixture during
preparation allows more rapid
solution, and storage of enema in
a cool place is recommended.
Systems Showing Upper and Lower CSTs
The miscibility with
temp. in
systems having a lower CST is not
indefinite.
> a certain temperature miscibility
starts to again with further in
temperature.
Closed-phase diagram, i.e. nicotinewater system.
Blending :
The
in miscibility of two liquids due to the
addition of a third substance.
Example : the formulation of solution of cresol
with soap BP 1968, which contains 50% cresol.
Cresol is only partially miscible with water, but the
soap in this preparation decreases the upper CST
and produces complete miscibility at ordinary
temperature.
The Effects of Added Substances on CST:
Type of
CST
Solubility of additive in
each component
Effect on
CST
Effect on
miscibility
Upper
Approx. equally soluble in
both components
Lowered
Increased
Raised
Decreased
Raised
Increased
Lowered
Decreased
Upper
Lower
Lower
Readily soluble in one
component but not in the
other
Approx. equally soluble in
both components
Readily soluble in one
component but not in the
other
Addition of substance
That equally miscible
In 2 phases.