Phase Rule and Phase Phase Rule and Phase EquilibriaEquilibria

Two-component Systems Two-component Systems Containing Solid and Liquid Containing Solid and Liquid PhasesPhases::

Solid- liquid mixtures in which 2 Solid- liquid mixtures in which 2 components are completely miscible components are completely miscible in the liquid state and completely in the liquid state and completely immiscible as solid. immiscible as solid.

Examples of such systems are:Examples of such systems are: Salol & thymol. Salol & thymol. Salol & camphor.Salol & camphor.

Solid thymol + solid Salol

a4 b4 X4

Increasing the % of thymol in the mixture till reaching 100 %.

100%100% thymolthymol100%100% salolsalol

The phase diagram for the salolThe phase diagram for the salol

thymol systemthymol system::

(i) Single liquid phase, (ii) Region containing solid salol and a

conjugate liquid phase,

(iii) Region in which solid thymol is in equilibrium with

a conjugate liquid phase.(iv) Region in which both components are

present as pure solid phases.

Those regions containing two phases (ii,

iii, and iv) are comparable to the two-phase region of the phenol-water system.

Solid thymol + solid Salol

a4 b4 X4

F=2-2+1=1F=2-2+1=1

System is represented by point X (60% by weight of thymol in salol) temperature (50 o C)

On cooling the system, the following sequence of the phase occurs: The system remains as a single liquid until 29oC. At 29oC a minute amount of solid thymol At 25oC, (system X1) a liquid phase, a1 (53% thymol in salol) and b1 (pure solid thymol). At 20oC, (system X2) the liquid phase is a2 (45%. by weight of thymol in salol), b2 (pure solid thymol). At 15oC, (system X3) the liquid phase a3 is 37 % thymol in salol and b1 (pure solid thymol).

Below 13 Below 13 o o C the liquid phase disappears C the liquid phase disappears altogether and the system contains two altogether and the system contains two solid phases of pure salol and pure solid phases of pure salol and pure thymol. thymol.

At 10At 10ooC (point X4), the system contains C (point X4), the system contains an equilibrium of an equilibrium of a4 & b4 (pure solid a4 & b4 (pure solid thymol + pure solid salol). thymol + pure solid salol).

The lowest temperature at which The lowest temperature at which liquid phase coexists is known as liquid phase coexists is known as eutectic point.eutectic point.

In case of thymol / salol system the In case of thymol / salol system the eutectic point is 13 eutectic point is 13 o o CC ( 3 phases ( 3 phases liquid, solid salol & solid thymol)liquid, solid salol & solid thymol)

The eutectic point therefore denotes an The eutectic point therefore denotes an invariant system for, in a condensed invariant system for, in a condensed system system

F = 2 - 3 + 1 = 0. F = 2 - 3 + 1 = 0. Substances forming eutectic mixtures Substances forming eutectic mixtures

(e.g., camphor, chloral hydrate, (e.g., camphor, chloral hydrate, menthol, and betanaphthol).menthol, and betanaphthol).

If such combinations is dispensed as If such combinations is dispensed as dry powder, drying may be achieved by dry powder, drying may be achieved by the addition of an absorbent powder the addition of an absorbent powder such as kaolin or light magnesium such as kaolin or light magnesium oxide.oxide.

Phase Equilibria in Three-Phase Equilibria in Three-Component SystemComponent System In systems containing three components In systems containing three components

but only one phase,but only one phase,

F = 3 - 1 + 2 = 4F = 3 - 1 + 2 = 4

For non-condensed system. The four For non-condensed system. The four degrees of freedom are temperature, degrees of freedom are temperature, pressure & the concentration of 2 of the pressure & the concentration of 2 of the 3 components. 3 components.

For condensed & the temperature is keptFor condensed & the temperature is kept

constant, then F = 2 .constant, then F = 2 .

TT

PP

C2C2C 1C 1

constantconstant

condensedcondensed44

Ternary System with One Pair of Ternary System with One Pair of Partially Miscible LiquidsPartially Miscible Liquids::

Water and benzene are partially Water and benzene are partially miscible systemmiscible system

two-phase systemtwo-phase system . .

water saturated with water saturated with benzenebenzene

benzene saturated with benzene saturated with waterwater22 – – phasephase

systemsystem

11 - -phase systemphase system

Addition of alcohol Addition of alcohol (solvent effect(solvent effect))

Mixture = 60% B, 20%A, 20% C.

A, B & C represent water, alcohol & benzene, respectively. AC binary mixture of A and C. a & c are the limits of solubility of C in A and A in C.

waterwater benzenebenzene

AlcoholAlcohol

System (g) after reaching equilibrium, will separate into two phases, (f ) and ( i).

weight of phase f /weight of phase I = gi / fg. Mixture h, mid point of the tie line, will contain equal weights of

the two phases at equilibrium.

The curve a f d e i c, a binodal curve (the extent

of the two-phase region). The remainder of the triangle contains

one liquid phase.

The directions of the tie lines are related to the shape

of binodal, (depends on the relative solubility of 3rd

component (alcohol) in the other 2 components).

when the added component acts equally on the other

two components to bring them into solution

binodal be symmetric & the tie lines are parallel

to the base line.

Effect of Temperature:

Changes in temperature will cause the area of immiscibility, (the binodal curve) to change.

Area of the binodal as the temperature is & miscibility is

A point is reached at which complete miscibility is obtained and the binodal vanishes.

Ternary Systems with Two or Three Ternary Systems with Two or Three Pairs of Partially Miscible LiquidsPairs of Partially Miscible Liquids::

A & C , B & C show partial miscibility. A and B are completely miscible at the temperature used.

Temperature gradually leads to a reduction in the areas of the two

binodal curves & their eventual disappearance. (c) Temperature expands the binodal curves.

At a sufficiently low temperature, they meet and fuse to form a single band of immiscibility as shown in (a).

Systems containing three pairs of Systems containing three pairs of

partially miscible liquidspartially miscible liquids

3 binodal curves meet, a central region appears in which 3 conjugate liquid phases exist in equilibrium. In this region, D, which is triangular, F = 0 ( condensed system under isothermal conditions). All systems lying within this region consist of 3 phases whose composition are always given by the points x, y & z. The only quantity that varies is the relative amounts of these 3 conjugate phases.

One phaseOne phase

33 phasesphases

22 phasesphases

XX

YYZZ

A, B, C

Arrangement of three phases:Arrangement of three phases: ItIt depends on the composition of the phasesdepends on the composition of the phases

At point D , F = 0 ??????At point D , F = 0 ??????